JP4091894B2 - High-strength steel sheet excellent in hydrogen embrittlement resistance, weldability, hole expansibility and ductility, and method for producing the same - Google Patents

Description

  The present invention is excellent in weldability, hole expansibility and ductility suitable for building materials, home appliances, automobiles, etc., and is particularly problematic in high strength steel plates with a tensile strength of 800 MPa or more, such as hydrogen embrittlement, set cracking and delayed fracture. The present invention relates to a suppressed high-strength thin steel sheet and a method for producing the same.

  Conventionally, high-strength steel is often used for applications such as bolts, PC steel wires, and line pipes, and it is known that when the tensile strength exceeds 980 MPa, delayed fracture occurs due to the penetration of hydrogen into the steel. Yes. On the other hand, since the thin steel plate is thin, hydrogen is released in a short time even if hydrogen enters, so it can be said that the awareness of problems with so-called delayed fracture was low. Recently, however, because of the need to reduce the weight of automobiles and improve collision safety, bumpers and impacts have been applied to ultra-high-strength steel sheets of 980 MPa or higher by press forming, pipe forming, bending, end face processing, hole expansion processing, etc. The number of cases where it is used for a reinforcing material such as a beam or a seat rail is rapidly increasing. Therefore, there is an urgent need to develop ultra high strength thin steel sheets with delayed fracture resistance.

  Until now, most techniques for improving delayed fracture resistance have been developed for steel products such as bolts, strips, and thick plates that are often used as products and are often used below yield strength or yield stress. For example, in the steel for steel bars and bolts, development has been conducted mainly on tempered martensite. Non-patent document 1 {"New development of delayed fracture elucidation" (Japan Steel Association, published in January 1997)} describes Cr, Mo. It has been reported that additive elements exhibiting resistance to temper softening such as V and V are effective in improving delayed fracture resistance. This is a technique for precipitating alloy carbides and using them as hydrogen trap sites to shift the delayed fracture mode from grain boundaries to intragranular fracture. However, these steels have a C content of 0.4% or more and contain a large amount of alloy elements, so the workability and weldability required for thin steel sheets are inferior, and the precipitation of alloy carbide takes several hours or more. Therefore, there is a problem in manufacturability.

  In Non-Patent Document 1, it is considered that an oxide mainly composed of Ti and Mg is effective in preventing hydrogen defects. However, this is intended for thick steel plates, and is especially considered for delayed fracture after welding with high heat input. However, there is no thin steel plate car that fully considers the usage environment in parts. On the other hand, regarding delayed fracture of thin steel sheets, for example, Non-Patent Document 2 (CAMP-ISIJ vol. 5 (1992) pages 1839 to 1842) reports on the promotion of delayed fracture due to work-induced transformation of the amount of retained austenite. Yes. This is in consideration of the forming process of a thin steel sheet, but describes the regulation of the amount of retained austenite which does not deteriorate the delayed fracture resistance. That is, it relates to a high-strength thin steel sheet having a specific structure, and is not a fundamental countermeasure for improving delayed fracture resistance.

  Furthermore, when assembling members using such high-strength materials, ductility, bendability, hole expansibility, weldability, etc. become a major problem over high-strength steel sheets with a tensile strength of up to about 590 MPa. Measures against are necessary. The following measures are taken for each characteristic. For example, as to the hole expandability, as described in Non-Patent Document 3 (CAMP-ISIJ vol. 13 (2000) page 395), the main phase is bainite and the hole expandability is improved. In addition, it is disclosed that the retained austenite is produced in the second phase, thereby exhibiting the same stretchability as that of the current retained austenitic steel. Furthermore, it is also shown that when retained austenite having an area ratio of 2 to 3% is generated by austempering at a temperature equal to or lower than the Ms temperature, the tensile strength × the hole expansion ratio is maximized. However, the weldability that manifests above 800 MPa and the softening behavior in the weld heat affected zone are not considered.

As for weldability, there are many cases where the softening behavior (HAZ softening behavior) in the weld heat affected zone is regarded as a problem. On the other hand, for example, as disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 2000-87175), it is shown that the HAZ softening behavior is suppressed by precipitation of carbides (Nb, Mo) C of Nb and Mo. However, although this technique is considered with respect to fatigue strength, it does not sufficiently consider workability such as hole expansibility. Moreover, HAZ softening elevation Dosomosomo system effects the strength level is low enough not to be said about the weldability and workability in the above very high-strength materials 800 MPa. In particular, when the tensile strength is 800 MPa or more, welding itself becomes difficult, and becomes more remarkable at 980 MPa or more. For this reason, in addition to the conventional welding methods such as spot welding, there is an example in which laser welding or the like is partially applied. However, the material variation in the weld and heat affected zone in high strength late JP becomes very significant compared to the high strength material of 590MPa class. In addition, the use of martensite for increasing the strength promotes hole expandability and ductility degradation.

"New development of delayed fracture elucidation" (Iron Japan Society, published in January 1997) CAMP-ISIJ vol. 5 (1992) 1839-1842 CAMP-ISIJ vol. 13 (2000) 395 JP 2000-87175 A Japanese Patent Laid-Open No. 11-293383

  As described above, development examples that take into account the hydrogen embrittlement-type delayed fracture taking into account the usage environment of thin steel sheets for automobiles, and also taking into account the use characteristics such as weldability, hole expansibility, ductility, etc. There is almost no. An object of the present invention is to solve the above-mentioned problems and to provide a high-strength steel sheet that simultaneously improves hydrogen embrittlement resistance, weldability, hole expansibility and ductility of a high-strength steel sheet having a tensile strength of 800 MPa or more, and a method for producing the same. And

  From the above background, the inventors have found a method for improving delayed fracture resistance while ensuring weldability and workability in consideration of the use environment in thin steel sheets. That is, it has been found that delayed fracture resistance due to hydrogen can be improved by controlling the trap site in the steel sheet and reducing the amount of hydrogen that can enter from the environment in addition to the structure and precipitate control of the steel sheet. Details are as follows.

(1) In mass%, C: 0.05 to 0.3%, Si: 0.01 to 3.0%, Mn: 0.01 to 4.0%, P: 0.0001 to 0.020 %, S: 0.0001 to 0.020%, Al: 0.01 to 0.23% , N: 0.0001 to 0.01 % , Ni: 0.001 to 5.5%, Cu: 0.001 to 3.0%, Cr: 0.001 to 5.0%, Mo: contain at least one of 0.005 to 5%, the balance consists of iron and unavoidable impurities, micro The structure contains one or both of bainite and bainitic ferrite as the main phase in a total area of 34 to 97%, the austenite area ratio (Vγ) as the second phase is 3 to 30%, and the balance is ferrite And / or martensite, and the tensile strength (TS) is 800 MPa or more. A high-strength steel sheet excellent in hydrogen embrittlement resistance, weldability, hole expansibility and ductility, characterized by satisfying the formulas 1-1) and (1-2).
0 ≦ 0.8 × {2Cu + 20Mo + 3Ni + Cr} − {0.1−3.5 × 10 ⁷ × (TS) ^−3.1 } −0.3 Vγ (1-1)
0 ≦ Si + Al + 7.67C-1.78 (1-2)
Here, TS: tensile strength (MPa), and the element symbol indicates mass% of each element contained in the steel.

(2) By mass%, C: 0.05% to 0.3%, Si: 0.01 to 3.0%, Mn: 0.01 to 4.0%, P: 0.0001 to 0.00. 020%, S: 0.0001 to 0.020%, Al: 0.01 to 0.23% , N: 0.0001 to 0.01 % , Ni: 0.001 to 5.5% Cu: 0.001 to 3.0%, Cr: 0.001 to 5.0%, Mo: One or more of 0.005 to 5%, V: 0.005 to 1% The balance is composed of iron and inevitable impurities, and the microstructure contains one or both of bainite and bainitic ferrite as the main phase in a total area of 34 to 97%, and the area of austenite as the second phase The rate (Vγ) is 3 to 30%, the balance is made of ferrite or martensite, and the tensile strength (TS) is A high-strength thin steel sheet excellent in hydrogen embrittlement resistance, weldability, hole expansibility and ductility, which is 800 MPa or more and further satisfies the following formulas (2-1) and (2-2).
0 ≦ 0.8 × {2Cu + 20Mo + 3Ni + Cr + 20V} − {0.1−V / 5−3.5 × 10 ⁷ × (TS) ^−3.1 } −0.3Vγ (2-1)
0 ≦ Si + Al + 7.67C-1.78 (2-2)
Here, TS: tensile strength (MPa), and the element symbol indicates mass% of each element contained in the steel.

(3) Further, in mass%, Se: 0.0002 to 0.05%, As: 0.0002 to 0.05%, Sb: 0.0002 to 0.05% , Pb: 0.0002 to 0.05%, Bi: 0.0002 to 0.05%, or one or two or more thereof, and the total thereof satisfies 0.05% or less (1) or (2) A high strength thin steel sheet excellent in hydrogen embrittlement resistance, weldability, hole expansibility and ductility.
(4) Furthermore, it contains Nb: 0.001-1.0% by the mass% in the range with which the following (3) Formula is satisfy | filled, Any one of said (1)-(3) characterized by the above-mentioned. High strength thin steel sheet with excellent hydrogen embrittlement resistance, weldability, hole expansibility and ductility.
(3.0Nb + 2.5Mo + 2 / 3Si + Mn) − (2.3C ^0.5 +1.80)> 0 (3)

(5) Further, in mass%, REM: 0.0002 to 0.10%, Ca: 0.0002 to 0.10%, Y: 0.0002 to 0.10%, Mg: 0.0002 to 0 . Highly excellent in hydrogen embrittlement resistance, weldability, hole expansibility and ductility according to any one of the above (1) to (4), comprising 10% or more Strength thin steel plate.
(6) Further, in mass%, Ti: 0.002 to 1%, Zr: 0.005 to 1%, Hf: 0.005 to 1%, Ta: 0.005 to 1%, or The high-strength thin steel sheet excellent in hydrogen embrittlement resistance, weldability, hole expansibility and ductility according to any one of the above (1) to (5), comprising two or more types.

(7) Further, in terms of% by mass, one or two of W: 0.005 to 5% and Co: 0.005 to 2.0% are contained (1) to (6 High strength thin steel sheet excellent in hydrogen embrittlement resistance, weldability, hole expandability and ductility.
(8) Hydrogen embrittlement resistance and welding as described in any one of (1) to (7) above, further comprising B: 0.0002 to 0.1% by mass% High-strength thin steel sheet with excellent properties, hole expansibility and ductility.
(9) the main phase, characterized in that it consists of bainite, and one or both of bainitic ferrite, martensite is 600 or less carbon atoms is 0.3% or less, or the Vickers hardness in mass% the ( A high-strength steel sheet excellent in hydrogen embrittlement resistance, weldability, hole expansibility and ductility according to any one of 1) to (8).

(10) A slab comprising the composition according to any one of (1) to (8) is heated to 1100 ° C or higher, hot-rolled at a finishing temperature of Ar ₃ or higher, and scraped at 400 to 800 ° C. Then, after pickling, after the cold rolling with a rolling reduction of 10 to 80%, the maximum temperature during the subsequent annealing is 0.8 × (Ac ₃ -Ac ₁ ) + Ac ₁ (° C.) or higher, Ac ₃ +30 (° C. ) Hydrogen embrittlement resistance after annealing in the following, cooling to a temperature range of 200 to 450 ° C. at a cooling rate of 3 to 150 ° C./second, and subsequently holding in the same temperature range for 1 to 3000 seconds This is a method for producing a high-strength thin steel sheet excellent in weldability, hole expansibility and ductility.

(11) After annealing, cooling to a temperature range of Mf + 10 ° C. to 450 ° C. at a cooling rate of 3 to 150 ° C./second, hydrogen embrittlement resistance, weldability, hole expansibility and A method for producing a high strength thin steel sheet with excellent ductility.
However, Mf (° C.) = 361-474 × C (mass%) − 33 × Mn (mass%) − 17 × Ni (mass%) − 17 × Cr (mass%) − 21 × Mo (mass%)

  As described above, according to the present invention, a high-strength steel sheet having improved hydrogen embrittlement-type delayed fracture characteristics, weldability, hole expansibility and ductility of a high-strength steel sheet having a tensile strength of 800 MPa or more and a method for producing the same are obtained. be able to.

  In conventional tempered martensite steel, which is a high strength steel material, delayed fracture caused by hydrogen accumulates in the old austenite grain boundaries, etc., forming voids, etc. It is thought to occur. Therefore, if the hydrogen trap sites are dispersed evenly and finely and hydrogen is trapped there, the concentration of diffusible hydrogen is lowered and the susceptibility to delayed fracture is lowered. As described in the above-mentioned Patent Document 2 (Japanese Patent Laid-Open No. 11-293383), the delayed fracture resistance due to hydrogen can be improved by controlling the oxide dispersion in the thick steel sheet to which Mg and Ti are added in combination. I know it.

  However, considering the case where the amount of hydrogen coming from the environment is large even locally, delayed fracture due to hydrogen will inevitably occur no matter how many hydrogen trap sites are dispersed in the steel. Furthermore, retained austenite must be used to some extent from the viewpoint of ensuring sufficient ductility. For this reason, (b) in addition to keeping trapped sites within the steel and controlling the retained austenite in balance with ductility to increase the allowable hydrogen amount of the steel itself (b) It is important to reduce the amount of hydrogen that can enter from.

In light of the above-mentioned background, the present inventors ensured and improved delayed fracture resistance in the environment where thin steel sheets are used, in order to influence the dispersion of various crystallized substances, precipitate trap sites and the strength of the steel sheets. In addition to the above, reduction of the amount of hydrogen that could enter from the environment was studied.
As a result, the inventors have found a technique for improving and securing the resistance to delayed fracture due to hydrogen under the usage environment of thin steel sheets (for example, with the addition of design stress after pressing). That is,
(A) Control of precipitates and retained austenite amount by strength and components of steel sheet.
(B) Control of intrusion-resistant hydrogen characteristics by steel plate components.
By performing each of the above, it is possible to improve hydrogen embrittlement resistance under the usage environment of the thin steel sheet for automobiles. Formulas (1-1), (1-2), (2-1), and (2-2) were defined as conditions for satisfying this.

By satisfying this formula, the delayed fracture property of the high strength thin steel sheet can be secured.
Next, (b) control of hydrogen intrusion characteristics by steel components will be described. During the hydrogen intrusion process, when a reduction reaction of water molecules (in a neutral or alkaline environment) or hydrogen ions (in an acidic environment) occurs on the steel sheet surface due to corrosion or pickling, hydrogen atoms are generated on the steel sheet surface. Adsorb. The adsorbed hydrogen atoms (1) recombine and gasify as hydrogen molecules or enter the steel plate. As a result of intensive studies on these processes, the present inventors have found that in order to reduce the hydrogen penetration rate, in addition to improving the corrosion resistance, (1) reducing the pH (hydrogen ion concentration) of the environment as the corrosion reaction progresses as much as possible. It was found that it is effective to suppress the concentration of adsorbed hydrogen atoms on the surface and to accelerate the recombination reaction (hydrogen generation reaction).

About (1), it discovered that REM, Ca, and Mg addition to steel were effective. Here, REM is an abbreviation for Rare Earth Metal and is a general term for lanthanoid elements starting from La. As industrial addition, it is often added in the form of misch metal. In this case, the amount of La and Ce added is increased. When REM, Ca, and Mg are eluted in the corrosion reaction, the atmosphere is alkalized by the equilibrium reaction of the hydroxide, that is, a decrease in pH due to the corrosion reaction is suppressed. Two methods have been found for (2).

  The first method is a method of increasing the exchange current density in the reduction reaction of hydrogen ions or water. Cu, Ni, Cr, and Mo are effective, and when 0.1 ≦ 2Cu + 20Mo + 3Ni + Cr + 20V is satisfied, the hydrogen permeation rate is remarkably suppressed. The second method is a method of limiting impurity elements that lower the exchange current density or significantly increase the hydrogen generation overvoltage. By limiting Se, As, Sb, Pb, and Bi as corresponding impurity elements, an increase in hydrogen permeation rate can be suppressed.

  In the use of automotive steel sheets, hydrogen intrusion occurs in the following process. The first is a processing step such as press working, the second is an anticorrosion coating step such as pickling, degreasing, water washing, and painting, and the third is corrosion in the use environment. In any environment, it is effective to control the hydrogen penetration characteristics by the steel components described above. In order to improve the bare corrosion resistance of automobile steel plates and suppress hydrogen intrusion, it is necessary to add a large amount of expensive elements. In these methods (1) and (2), both are remarkable by adding a small amount. There is an advantage that an advantageous effect can be obtained.

  Furthermore, regarding the securing of weldability, hole expansion and ductility, the inventors, in mass%, C: 0.01 to 0.3%, Si: 0.005 to 2.5%, Mn: 0.01 -3%, P: 0.0010 to 0.1%, S: 0.0010 to 0.005%, Al: 0.005 to 2%, based on a steel sheet consisting of the balance Fe and inevitable impurities, Each alloy was melted, cast, or once cooled and then heated again. The hot-rolled steel sheet wound after hot rolling was pickled, cold-rolled and then annealed to prepare a cold-rolled annealed sheet. The steel sheet was subjected to microstructural observation, hole enlargement test specified by the Federation of Steels, JIS-compliant tensile test, laser welding with the steel sheet, followed by ball head overhang test and comparative evaluation of each characteristic. As a result, it has been found that a tensile strength of 800 MPa or more can be obtained by the microstructure control finally obtained, and a high-strength steel sheet excellent in weldability, hole expansibility and ductility can be produced.

Next, a preferable microstructure of the base steel sheet will be described.
In order to ensure sufficient hole expansibility, it is effective that the main structure is one or both of bainite and bainitic ferrite, and the total area ratio is 34% or more. On the other hand, in order to ensure ductility, it is 97% or less. Moreover, the bainite said here includes both the so-called upper bainite in which carbide is generated at the lath boundary and the lower bainite in which fine carbide is generated in the lath. Bainitic ferrite means bainite without carbides, for example, acicular ferrite is one example. In order to improve the hole expansibility, it is desirable that the main phase is composed of lower bainite in which carbide is finely dispersed or bainitic ferrite without carbide.

  However, in this case, securing ductility and weldability, particularly prevention of softening in the heat affected zone is a problem. In addition, when hard martensite coexists, it becomes difficult to ensure ductility, resistance to softening resistance during welding, etc., but the carbon content in the martensite is 0.3% or less by mass% or Vickers hardness is low. When it is 600 or less, it is not large in terms of the reduction in ductility and weldability (however, low carbon and low hardness are desirable from the viewpoint of weldability). Therefore, martensite with low carbon or relatively low hardness functions sufficiently as the main phase. In general, it is difficult to distinguish this type of low temperature transformation product.

  However, it can be distinguished by collecting etching and expansion / contraction curves with a repeller solution. For example, it can be distinguished by the difference of inflection points observed in the shrinkage curve during cooling. Specifically, Ms (° C.) = 561-474 × C (mass%) − 33 × Mn (mass%) − 17 × Ni (mass%) − 17 × Cr (mass%) − 21 × Mo (mass%) ), The low-temperature transformation product is martensite, and Ms or more is Bs (° C.) = 830-270 × C (mass%) − 90 × Mn (mass%). The transformation product observed in the temperature range below −37 × Ni (mass%) − 70 × Cr (mass%) − 83 × Mo (mass%) is bainite. Further, the hardness can be obtained by micro Vickers hardness measurement with a load of 100 g or less. Normally martensite is composed of very fine structural units, but here the hardness of the martensite phase as the main phase is specified, and it is desirable to obtain the indentation size under measurement conditions of about several tens of microns or less. .

  When aiming for high ductility, it is effective to leave the austenite phase as an area ratio of 3% or more as the second phase. On the other hand, it is 30% or less to ensure hole expandability. Further, the remaining structure may be ferrite and / or martensite, and even if polygonal ferrite is partially included in the range of 40% or less, the tensile strength may be able to ensure 800 MPa or more, and this case also falls within the scope of the present invention. The second phase is not polygonal ferrite but retained austenite.

Further, the second phase as a residual austenite, in order to secure an amount appropriate to achieve both hole expandability and ductility are required to satisfy the equation (1-2) and (2-2). Moreover, about weldability, the weldability of a high strength material was ensured by satisfy | filling (3) Formula which prescribed | regulated the relationship of the component so that it may mention later. From the viewpoint of increasing the strength, martensite may be included in addition to austenite. However, when martensite or stabilized austenite is included, the hardness ratio between the main phase and the second phase (the second phase) tends to promote the hole expandability and the softening behavior of the weld heat affected zone. Hardness / hardness of the main phase) is desirably in the range of 0.5 to 1.5.

  Further, from the viewpoint of suppressing the softening behavior of the weld heat-affected zone, the steel material component satisfies the formula (3), and further, the area ratio of the hard martensite is as small as possible with respect to the area ratio of the second phase in the microstructure. Desirable from. When the hardness ratio is less than 0.5 or exceeds 1.5, hole expandability and ductility are lowered, and softening of the weld heat-affected portion becomes remarkable. The hardness was measured using a micro Vickers hardness meter and using a load of 1 to 100 g according to the size of the structure.

Moreover, when not satisfy | filling Formula (3), 800 MPa or more cannot be ensured by tensile strength, and it becomes difficult to ensure hole expansibility and ductility in addition to not being able to suppress softening of a welding heat affected part. On the other hand, when the amount of austenite is small and the amount of martensite is large, the hole expandability and ductility are lowered although the strength is increased. In particular, when the amount of martensite is increased, the tendency of the hole expansibility and ductility to decrease becomes remarkable, and softening of the weld heat-affected portion cannot be suppressed.
(3.0Nb + 2.5Mo + 2 / 3Si + Mn) − (2.3C ^0.5 +1.80)> 0 (3)

  In addition to the above, the case where one or more of carbides, nitrides, sulfides, oxides, and the like are contained as the remaining microstructure of the microstructure with an area fraction of 1% or less can also be used in the present invention. Further, when the main phase contains martensite, it may contain ferrite as the remaining structure. In addition, each phase of the above microstructure, ferrite (bainitic ferrite), bainite, austenite, martensite, interfacial oxidation phase and remaining structure, identification of the existing position, and measurement of the space factor are measured by Nital reagent and JP The steel plate rolling direction cross section or the rolling perpendicular direction cross section is corroded with the reagent disclosed in Japanese Patent Publication No. 59-219473, and observed with an optical microscope of 500 to 1000 times and an electron microscope (scanning type and transmission type) of 1000 to 100,000 times. Quantification is possible. It is possible to obtain an area ratio of each tissue by observing 20 fields of view or more and using a point counting method or image analysis. The austenite amount can be determined by X-ray diffraction. The total of each phase of the microstructure is 100%, but phases that cannot be observed and identified with an optical microscope such as carbides, oxides, and sulfides are included in the area ratio of the main phase.

The present invention is described in further detail below. First, the reasons for limiting the chemical components of steel in the present invention will be described.
C is an element that can increase the strength of the steel sheet. In particular, it is an essential element for generating a hard phase such as martensite and austenite and increasing the strength, and 0.05% or more is necessary to obtain a strength of 980 MPa or more. In order to increase the cementite that is the starting point of fracture, hydrogen embrittlement is likely to occur. Therefore, the upper limit was made 0.3%. Further, from the viewpoint of securing retained austenite, the ranges of the formulas (1-2) and (2-2) are satisfied.

Si is a substitutional solid solution strengthening element that hardens the material greatly, and is an element that suppresses cementite precipitation in addition to being effective in increasing the strength of the steel sheet by containing 0.01% or more. If it exceeds 0%, scale formation by hot rolling becomes remarkable, and scratches are costly to remove and economically disadvantageous, so 3.0% is made the upper limit. Moreover, since Si is a ferrite forming element, it was decided to satisfy the ranges of formulas (1-2) and (2-2) from the viewpoint of securing retained austenite.

Mn is an element effective for increasing the strength of the steel sheet. However, since this effect cannot be obtained if the content is less than 0.01%, the lower limit is set to 0.01%. On the contrary, if the amount is large, segregation becomes prominent and the workability may be deteriorated.
P is an element that promotes grain boundary fracture due to grain boundary segregation, and a lower value is desirable, but an extremely low reduction is not preferable in terms of manufacturing cost, so the lower limit was made 0.0001%. Moreover, since it is an element which degrades corrosion resistance, the upper limit is made 0.020%.

S is an element that promotes hydrogen absorption in a corrosive environment, and is preferably as low as possible especially when there are few sulfide-forming elements added, so the upper limit is made 0.020%. On the other hand, since the extreme reduction is not preferable in terms of manufacturing cost, the lower limit was made 0.0001%.
Al is added in an amount of 0.01% or more for deoxidation, but inclusion increases such as alumina and the workability deteriorates as the addition amount increases, so 3.0% is made the upper limit. Moreover, since Al is also a ferrite forming element like Si, it was decided to satisfy the ranges of formulas (1-2) and (2-2) from the viewpoint of securing retained austenite. In addition, the upper limit of the Al amount is the steel type No. in Table 1 of the examples of the present invention. Based on 0.2 of 20, it was set to 0.23% or less.

N is better because it contributes to workability deterioration and blowhole generation during welding. If it exceeds 0.01%, workability deteriorates, so 0.01% is made the upper limit. Moreover, since the extreme drop is economically disadvantageous, the lower limit is made 0.0001%.
Ni has an effect of suppressing hydrogen penetration and improving delayed fracture characteristics, and an effect of ensuring the strength of the steel sheet by enhancing the hardenability of the steel sheet. However, if less than 0.001%, these effects cannot be obtained, so the lower limit was set to 0.001%. On the other hand, if it exceeds 5.5%, the workability deteriorates, so the upper limit was set to 5.5%.

Cu is effective for suppressing hydrogen intrusion and improving delayed fracture characteristics, and is effective for strengthening. Further, since fine precipitation of confidence contributes to the improvement of delayed fracture, it is added in an amount of 0.001% or more. Moreover, since excessive addition causes deterioration of workability, the upper limit was made 3.0%.
Cr is an element that is effective for suppressing hydrogen intrusion and improving delayed fracture characteristics, and for increasing the strength of the steel sheet. However, since these effects cannot be obtained at less than 0.001%, the lower limit is set to 0.001%. On the other hand, if the content exceeds 5%, the workability deteriorates, so the upper limit was made 5%.

  Mo is not only an effective element to suppress hydrogen intrusion and improve delayed fracture characteristics, but also to increase the hardenability of the steel sheet and to obtain austenite and martensite stably in a continuous annealing facility. It has the effect of strengthening and suppressing the occurrence of hydrogen embrittlement. Furthermore, since it is also effective in preventing softening of the weld heat affected zone, the lower limit was made 0.005%. Further, if it exceeds 5%, these effects are saturated and ductility is reduced, so the upper limit is set to 5%.

  V can also be used as a hydrogen trap site by controlling the form of carbonitride in addition to the effects of suppressing hydrogen intrusion and improving delayed fracture characteristics, increasing the strength of steel sheets, and reducing the grain size. It is an important additive element for improving hydrogen embrittlement. However, since this effect cannot be obtained at less than 0.005%, the lower limit is set to 0.005%. On the other hand, when the content exceeds 1%, the precipitation of carbonitrides becomes remarkable, and the ductility decreases remarkably. For this reason, the upper limit is set to 1%.

When Se, As, Sb, Sn, Pb, Bi is contained alone or in total exceeding 0.05%, the delayed fracture resistance is remarkably inhibited. Therefore, the upper limit is set to 0.05% for each element, and The upper limit for the total of one or more of these elements was 0.05%. On the other hand, for the extremely low reduction, 0.0002% was set as the lower limit for each element because the limitation on recycling was narrowed.
Nb is an element effective for increasing the strength and refining of the steel sheet. Furthermore, since it is also effective in suppressing softening of the weld heat affected zone, the lower limit is set to 0.001%. On the other hand, when the content exceeds 1%, precipitation of carbonitride increases, resulting in deterioration of workability and delayed fracture resistance. Therefore, the upper limit is set to 1%.

REM, Ca, and Mg are effective elements for suppressing the increase in the hydrogen ion concentration in the interface atmosphere accompanying the corrosion of the steel sheet surface, that is, for suppressing the decrease in pH. However, since these effects cannot be obtained if the content is less than 0.0002%, the lower limit is set to 0.0002%. On the other hand, if the content exceeds 0.1%, the workability deteriorates, so the upper limit was set to 0.1%.
Y is effective for controlling the form of inclusions and contributes to delayed fracture resistance, so 0.0002% or more was added. On the other hand, excessive addition deteriorates hot workability, so 0.1% or less was added.

Ti is an element necessary for generating precipitates and inclusions. However, if the content is less than 0.002%, the precipitate cannot be used, so the lower limit is set to 0.002%. On the other hand, if it exceeds 1%, coarse precipitates or crystallized products are formed, so that workability and delayed fracture resistance deteriorate. For this reason, the upper limit is set to 1%.
Zr is an element effective for increasing the strength and refining of the steel sheet. However, since these effects cannot be obtained at less than 0.005%, the lower limit is set to 0.005%. On the other hand, when the content exceeds 1%, precipitation of carbonitride increases, resulting in deterioration of workability and delayed fracture resistance. Therefore, the upper limit is set to 1%.

Hf is an element effective for increasing the strength and refining of the steel sheet. However, since these effects cannot be obtained at less than 0.005%, the lower limit is set to 0.005%. On the other hand, when the content exceeds 1%, precipitation of carbonitride increases, resulting in deterioration of workability and delayed fracture resistance. Therefore, the upper limit is set to 1%.
Ta is an element effective for increasing the strength and refining of the steel sheet. However, since these effects cannot be obtained at less than 0.005%, the lower limit is set to 0.005%. On the other hand, when the content exceeds 1%, precipitation of carbonitride increases, resulting in deterioration of workability and delayed fracture resistance. Therefore, the upper limit is set to 1%.

W is an element effective for increasing the strength of the steel sheet. However, since these effects cannot be obtained at less than 0.005%, the lower limit is set to 0.005%. On the other hand, if the content exceeds 5%, the workability deteriorates, so the upper limit was made 5%.
Co is effective for strengthening, so 0.005% or more was added. Moreover, since excessive addition causes deterioration of workability, the upper limit was made 2.0%.
B is an element effective for increasing the strength of the steel sheet. However, since these effects cannot be obtained if the content is less than 0.0002%, the lower limit is set to 0.0002%. On the other hand, if the content exceeds 0.1%, the hot workability deteriorates, so the upper limit was made 0.1%.

Next, a manufacturing method will be described. In particular, in order to ensure the surface state on the product plate, the following production method is desirable from the viewpoint of sufficiently forming oxide scale and deske in the production process.
First, the heating temperature at the time of hot rolling is set to 1100 ° C. or more from the viewpoint of deformation resistance, and if it is too high, there are problems such as grain coarsening and increased scale formation. In hot rolling, hot rolling is performed at Ar ₃ or higher in order to prevent excessive strain on ferrite grains and deterioration of workability, and the recrystallized grain size after annealing becomes larger than necessary even if the temperature is too high. Therefore, the finishing temperature is desirably 940 ° C. or lower.

  With regard to the coiling temperature, recrystallization and grain growth are promoted at a high temperature, and improvement in workability is desired. However, scale formation that occurs during hot rolling is also promoted and pickling properties are reduced. And On the other hand, since it hardens | cures when it becomes low temperature, the load at the time of cold rolling becomes high. For this reason, it shall be 400 degreeC or more. Here, in order to positively precipitate fine precipitates as trap sites during winding, a winding process at 400 to 800 ° C., preferably 550 to 650 ° C. is desirable. In cold rolling after pickling, if the rolling reduction is low, it becomes difficult to correct the shape of the steel sheet, so the lower limit is set to 10%. Further, when rolling at a rolling reduction exceeding 80%, the upper limit is set to 80% because of the occurrence of cracks and the disorder of the shape of the edge of the steel sheet.

When annealing after cold rolling, the annealing temperature is expressed by the temperature Ac ₁ and Ac ₃ temperature determined by the chemical composition of the steel (for example, “Steel Material Science” by W.C. Leslie, translated by Kosei Naruse, Maruzen P273). Less than 0.8 × (Ac ₃ −Ac ₁ ) + Ac ₁ (° C.), the amount of austenite obtained at the annealing temperature is small, so mainly bainite or bainitic ferrite is produced in the final steel sheet. I can't let you. Further, since a sufficient amount of retained austenite phase or martensite phase cannot be left as the second phase, this was set as the lower limit of the annealing temperature. In addition, as the annealing temperature becomes higher, the coarsening of the crystal grains and the surface oxidation are promoted, and the upper limit of the annealing temperature is set to Ac ₃ +30 (° C.) in order to increase the manufacturing cost. The annealing time in this temperature range requires 10 seconds or more to make the temperature of the steel plate uniform and to secure austenite. However, if it exceeds 30 minutes, the generation of a grain boundary oxidation phase is promoted and the cost is increased.

  Subsequent primary cooling suppresses the transformation from the austenite phase to the ferrite phase to some extent, forms bainite or bainitic ferrite, and further stabilizes the austenite by concentrating C in the untransformed austenite phase. Is important to. Setting this cooling rate to less than 3 ° C./second may promote the formation of ferrite and pearlite and cause a decrease in strength, so the lower limit of the cooling rate was set to 3 ° C./second. On the other hand, when the cooling rate is higher than 150 ° C./sec, a hard phase such as a martensite phase in the final steel sheet becomes large, and it is difficult to operate, so this was set as the upper limit.

  When this primary cooling is performed to less than 200 ° C., a large amount of martensite is generated during cooling to promote hole expansibility and delayed fracture, so the cooling stop temperature is set to 200 ° C. or higher. In addition, if the cooling stop temperature exceeds 450 ° C., carbides are generated in a short time during subsequent holding, leading to a decrease in strength. Next, in order to stabilize austenite and lower the hardness of martensite, the temperature is maintained. If the retention time is long, it is not preferable from the viewpoint of productivity, and carbides are generated. Further, in order to stabilize the austenite phase remaining in the steel sheet at room temperature, it is essential to further increase the carbon concentration in the austenite by transforming a part of the austenite phase into the bainite phase. It is desirable to hold the above, preferably 15 seconds to 20 minutes. If it is less than 200 ° C., bainite transformation hardly occurs, and if it exceeds 450 ° C., carbides are generated and it is difficult to leave a sufficient residual austenite phase.

  Further, when the martensite phase having a relatively low carbon or low hardness is used as the main phase or used as a part of the main phase, it is not particularly necessary to promote the bainite transformation. Therefore, the lower limit of the primary cooling stop temperature is set to Mf + 10 ° C. When it becomes Mf + 10 degrees C or less, it will become a 100% martensite and it will become difficult to ensure the retained austenite required for ductility ensuring. Here, the Mf temperature is Mf (° C.) = 361−474 × C (mass%) − 33 × Mn (mass%) − 17 × Ni (mass%) − 17 × Cr (mass%) − 21 × Mo ( mass%) and is expressed empirically. Further, regarding the welding method, it is within the scope of the present invention to perform a usual welding method such as arc, TIG, MIG, mash and laser welding.

Next, this invention is demonstrated based on an Example.
Steels having the components shown in Table 1 were melted and slabs were obtained by continuous casting according to a conventional method. No. Nos. 1 to 22 are steels of the components according to the present invention, No. 23-No. 27 is a component deviating. These steels were heated in a heating furnace at a temperature of 1160 ° C to 1250 ° C, hot rolled at a finishing temperature of 870 ° C to 900 ° C, and wound up at 400 ° C to 800 ° C. This is followed by pickling, cold rolling at a rolling reduction of 10 to 80%, followed by recrystallization annealing at 800 to 900 ° C., followed by 0.4% temper rolling and a sheet thickness of 1.2 mm. It became a cold-rolled steel sheet. Table 2 shows the material characteristics of each steel plate. The evaluation results of the delayed fracture resistance of the steel sheet and the values of the formula (1-1) or (2-1), formula (1-2) or (2-2), and formula (3) of each steel are shown. Details of the evaluation method are as follows.

(1) After temper rolling , 2% strain is applied to the steel sheet for the purpose of simulating strain during pressing.
(2) A notched plate-like tensile test piece having a stress concentration rate of 3.2 is taken from the steel plate.
(3) A constant current cathodic charge is applied at 0.01 to 0.025 mA / cm ² in a 3% NaCl 3 g / 1 NH ₄ SCN aqueous solution.
(4) Cd plating is performed.
(5) A constant load of 0.7 times the tensile strength is applied.
(6) The test is performed up to 100h, and it is judged whether the rupture or not.

  As shown in Table 1 and Table 2, those satisfying the formula (1-1) or (2-1) and the formula (1-2) or (2-2) in the claims of the present invention example are delayed. Not broken in the destructive test. On the other hand, Comparative Steel No. No. 23 was broken in the above-mentioned delayed fracture test, although the strength level was equal or low. In addition, JIS No. 5 tensile test specimens were collected from these steel plates and measured for mechanical properties. In addition, a hole expansion test was performed in accordance with the Steel Federation standard, and the hole expansion rate was obtained. As for weldability, laser welding was performed with steel sheets attached together, a ball head overhang test was performed with Teflon (registered trademark) lubrication, and the overhang height and fracture position of the base material were measured.

  As shown in Table 2, it can be seen that the inventive steel that satisfies the requirements of the present invention is excellent in weldability, ductility, strength (tensile strength of 800 MPa or more), and hole expansibility. On the other hand, Comparative Example No. Since 25 does not satisfy the microstructure and the formula (3), the weldability is inferior. Table 3 shows the steel type No. 1, 7 and no. 15 shows the influence of manufacturing conditions on each of the 15 materials. If the production conditions are not satisfied, the specified structure is not obtained, the strength is 800 MPa or less, and the weldability and delayed fracture resistance are inferior. The conditions other than those shown in Table 3 were the same as those in the experiment of Table 2.

  Table 4 shows the steel type No. The influence of manufacturing conditions on each material of 19-22 is shown. The conditions other than those shown in Table 4 were the same as those in the experiment of Table 2. It can be seen that the four steel types 19 to 22 mainly composed of low carbon and low hardness martensite also have excellent weldability, ductility, strength (tensile strength of 800 MPa or more), and hole expansibility. On the other hand, for those that do not satisfy the conditions of the manufacturing method and do not have a prescribed structure, for example, No. 1 In No. 10, the weldability was No. 10. 14 shows that the hole expandability is low despite the low strength.

特許出願人 新日本製鐵株式会社
代理人 弁理士 椎名 彊 他１

Patent applicant: Nippon Steel Corporation
Attorney Attorney Shiina and others 1

Claims (11)

In mass%
C: 0.05 to 0.3%
Si: 0.01-3.0%,
Mn: 0.01 to 4.0%,
P: 0.0001 to 0.020%,
S: 0.0001 to 0.020%,
Al: 0.01 to 0.23% ,
N: 0.0001 to 0.01%
Containing
Ni: 0.001 to 5.5%,
Cu: 0.001 to 3.0%,
Cr: 0.001 to 5.0%,
Mo: 0.005 to 5%,
1 or more of them, the balance consists of iron and inevitable impurities, the microstructure contains one or both of bainite and bainitic ferrite as the main phase in a total area of 34 to 97%, As a phase, the area ratio (Vγ) of austenite is 3 to 30%, the balance is made of ferrite and / or martensite, the tensile strength (TS) is 800 MPa or more, and the following (1-1) and (1 -2) A high-strength thin steel sheet excellent in hydrogen embrittlement resistance, weldability, hole expansibility and ductility, characterized by satisfying the formula.
0 ≦ 0.8 × {2Cu + 20Mo + 3Ni + Cr} − {0.1−3.5 × 10 ⁷ × (TS) ^−3.1 } −0.3 Vγ (1-1)
0 ≦ Si + Al + 7.67C-1.78 (1-2)
Where TS: tensile strength (MPa),
The element symbol indicates mass% of each element contained in the steel. In mass%
C: 0.05% to 0.3%,
Si: 0.01-3.0%,
Mn: 0.01 to 4.0%,
P: 0.0001 to 0.020%,
S: 0.0001 to 0.020%,
Al: 0.01 to 0.23% ,
N: 0.0001 to 0.01%
Containing
Ni: 0.001 to 5.5%,
Cu: 0.001 to 3.0%,
Cr: 0.001 to 5.0%,
Mo: 0.005 to 5%,
Containing one or more of
Furthermore,
V: 0.005 to 1%
The balance is composed of iron and inevitable impurities, and the microstructure contains one or both of bainite and bainitic ferrite as the main phase in a total area of 34 to 97%, and the area of austenite as the second phase The rate (Vγ) is 3 to 30%, the balance is made of ferrite or martensite, the tensile strength (TS) is 800 MPa or more, and the following formulas (2-1) and (2-2) are satisfied. A high-strength thin steel sheet with excellent hydrogen embrittlement resistance, weldability, hole expansibility and ductility.
0 ≦ 0.8 × {2Cu + 20Mo + 3Ni + Cr + 20V} − {0.1−V / 5−3.5 × 10 ⁷ × (TS) ^−3.1 } −0.3Vγ (2-1)
0 ≦ Si + Al + 7.67C-1.78 (2-2)
Where TS: tensile strength (MPa),
The element symbol indicates mass% of each element contained in the steel. Furthermore, in mass%,
Se: 0.0002 to 0.05%,
As: 0.0002 to 0.05%,
Sb: 0.0002~0.05%,
P b: 0.0002~0.05%,
Bi: 0.0002 to 0.05%,
The hydrogen embrittlement resistance, weldability, hole expansibility, and ductility according to claim 1 or 2, characterized in that one or more of the above are contained and the total thereof satisfies 0.05% or less. Excellent high-strength thin steel sheet. The hydrogen embrittlement resistance according to any one of claims 1 to 3, further comprising, in mass%, Nb: 0.001 to 1.0% within a range satisfying the following formula (3): High-strength thin steel sheet with excellent resistance, weldability, hole expansibility and ductility.
(3.0Nb + 2.5Mo + 2 / 3Si + Mn) − (2.3C ^0.5 +1.80)> 0 (3) Furthermore, in mass%,
REM: 0.0002 to 0.10%,
Ca: 0.0002 to 0.10%,
Y: 0.0002 to 0.10%,
Mg: 0.0002 to 0.10%
The high-strength thin steel sheet excellent in hydrogen embrittlement resistance, weldability, hole expansibility and ductility according to any one of claims 1 to 4, characterized by comprising one or more of the following. Furthermore, in mass%,
Ti: 0.002 to 1%,
Zr: 0.005 to 1%,
Hf: 0.005 to 1%,
Ta: 0.005 to 1%,
The high-strength thin steel sheet excellent in hydrogen embrittlement resistance, weldability, hole expansibility, and ductility according to any one of claims 1 to 5, characterized by containing at least one of the following. Furthermore, in mass%,
W: 0.005 to 5%,
Co: 0.005 to 2.0%,
The high-strength thin steel sheet excellent in hydrogen embrittlement resistance, weldability, hole expansibility and ductility according to any one of claims 1 to 6, characterized in that it contains one or two of the following. Furthermore, in mass%,
B: 0.0002 to 0.1%
The high-strength thin steel sheet excellent in hydrogen embrittlement resistance, weldability, hole expansibility, and ductility according to any one of claims 1 to 7. Main phase, bainite, claims and one or both of bainitic ferrite and carbon in weight percent, characterized in that it consists of martensite is 600 or less at 0.3% or Vickers hardness 1-8 A high-strength thin steel sheet excellent in hydrogen embrittlement resistance, weldability, hole expansibility and ductility according to any one of the above.
A slab comprising the composition according to any one of claims 1 to 8 is heated to 1100 ° C or higher, hot-rolled at a finishing temperature of Ar ₃ or higher, scraped at 400 to 800 ° C, and then pickled. Thereafter, after cold rolling with a reduction ratio of 10 to 80%, after annealing, the maximum temperature during annealing is 0.8 × (Ac ₃ −Ac ₁ ) + Ac ₁ (° C.) or more and Ac ₃ +30 (° C.) or less. Cooling to a temperature range of 200 to 450 ° C. at a cooling rate of 3 to 150 ° C./second, and subsequently maintaining the temperature in the same temperature range for 1 to 3000 seconds. For producing a high-strength thin steel sheet having excellent properties and ductility. It is excellent in hydrogen embrittlement resistance, weldability, hole expansibility, and ductility according to claim 10, characterized by cooling to a temperature range of Mf + 10 ° C. to 450 ° C. at a cooling rate of 3 to 150 ° C./second after annealing. Manufacturing method of high strength thin steel sheet.
However, Mf (° C.) = 361-474 × C (mass%) − 33 × Mn (mass%) − 17 × Ni (mass%) − 17 × Cr (mass%) − 21 × Mo (mass%)