JPH06145891A - High-strength cold-rolled steel sheet excellent in ductility and delayed fracture resistance and method for producing the same - Google Patents

Abstract

(57) [Summary] [Purpose] Having sufficient formability to withstand press working,
Further, the present invention provides a high-strength cold-rolled steel sheet that does not cause delayed fracture in the subsequent use environment and has low notch sensitivity and high reliability as a component. [Composition] C: 0.05 to 0.20%, Mn: 1.80 to 5.00%, P:
0.02-0.08%, Al: 0.10% or less, Cu: 0.10-1.00%, C
r: 0.05 to 1.00%, B: 0.0010 to 0.0050%, Nb: 0.005
~ 0.040%, S: 0.0050% or less and Ni: 0.05-1.00%
And the balance becomes the composition of Fe and inevitable impurities,
The area ratio of the steel structure is bainite ≧ 50%, (bainite + tempered martensite) ≧ 90%, and retained austenite ≦ 4%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength cold-rolled steel sheet excellent in ductility and delayed fracture resistance, which is suitable for use mainly as an automobile member, especially for use as a high-strength material in relatively light processing. The present invention relates to a manufacturing method thereof.

In recent years, there is a strong demand for the use of high-strength steel sheets in order to improve the safety of automobiles and reduce the weight of vehicle bodies.
The door impact beam, bumper, and other parts targeted by this invention are conventionally 60 kgf / mm ² in tensile strength (TS).
However, steel sheets of 100 kgf / mm ² have been used, but recently, high-strength steel sheets of TS ≧ 120 kgf / mm ² class with further improved strength are required.

[0003]

^2. Description of the Related Art The following methods are known as methods for producing a steel sheet having a TS exceeding 120 kfg / mm ² , but each has its own problems. (1) Work strengthened steel, recovery annealed steel This seed steel is not only unstable in material because it depends on work hardening for strength increase, but also softened when welding is performed in the assembly process of parts. There is a problem that is remarkable. Further, this type of steel sheet has a high notch sensitivity, and there is a problem from the viewpoint of safety.

(2) Hardened (tempered) steel (steel having martensitic structure) When martensite is mainly used (for example, JP-A-58)
-61219), high strength can be obtained relatively easily, but a tempering step is essential to improve workability. At that time, if the temperature and time are not strictly adjusted, not only the target material cannot be obtained, but also the variation is large. Moreover, if appropriate conditions are not selected, there is a large risk of so-called temper embrittlement. Furthermore, with the composition of conventional steel, it was difficult to prevent hydrogen embrittlement and delayed fracture due to hydrogen that enters and accumulates in the steel as corrosion progresses in the actual use environment. Further, a steel sheet having a mixed structure of ferrite and martensite has good ductility, but has a strong notch sensitivity due to the mixture of hard phase and soft phase, which is a problem in terms of safety.

(3) Precipitation-strengthened steel It is difficult to raise TS sufficiently with a steel in which carbonitride-forming elements such as Nb and Ti are added to the steel for precipitation-strengthening, and it is about 100 kgf / mm ^2. Was the limit.

[0006]

Since the residual stress remaining in steel is one of the main causes of delayed fracture, if sufficient stress relief treatment is carried out after press forming, this problem can be solved in almost all steel types. It can be resolved. However, since such a process not only leads to the redundancy of the process but also causes a significant decrease in the strength of the parts, it cannot be used for actual use. There is also a proposal to perform quenching and tempering by forming it into a pipe shape, but this method not only leads to a decrease in productivity due to complicated steps, but it does not have flexibility in the shape for subsequent use. There's a problem.

The present invention was developed in view of the above-mentioned current situation and has sufficient formability to withstand press working,
Another object of the present invention is to propose a high-strength cold-rolled steel sheet that does not cause delayed fracture in the subsequent use environment and has low notch sensitivity and high reliability as a component together with its advantageous manufacturing method.

[0008]

The present invention has been completed by optimizing not only the composition range of steel components but also the microstructure. That is, the gist of the present invention is as follows. 1. C: 0.05 to 0.20 wt% (hereinafter simply indicated as%), Mn: 1.
80-5.00%, P: 0.02-0.08%, Al: 0.10% or less,
Cu: 0.10-1.00%, Cr: 0.05-1.00%, B:
0.0010 to 0.0050%, Nb: 0.005 to 0.040%, S: 0.0050
% Or less, Ni: 0.05 to 1.00%, with the balance being Fe and inevitable impurities,
And the area ratio of the steel structure is bainite ≧ 50%, (bainite + tempered martensite) ≧ 90%, retained austenite ≦ 4%, tensile strength ≧ 120 kgf / mm ² , yield ratio ≦ 0.
High-strength cold-rolled steel sheet satisfying 70 and excellent in delayed fracture resistance (first invention).

2. C: 0.05 to 0.20%, Mn: 1.80 to 5.
00%, P: 0.02-0.08%, Al: 0.10% or less, Cu: 0.
10-1.00%, Cr: 0.05-1.00%, B: 0.0010-0.00
50%, Nb: 0.005 to 0.040%, S: 0.0050% or less,
Ni: contains 0.05 to 1.00%, and Ti: 0.005 to 0.100%,
Contains one or more selected from Si: 0.05 to 0.50% and V: 0.010 to 0.100%, with the balance being
The composition of Fe and unavoidable impurities, and the area ratio of the steel structure is bainite ≧ 50%, (bainite + tempered martensite) ≧ 90%, retained austenite ≦ 4%, tensile strength ≧ 120 kgf / mm ² , High-strength cold-rolled steel sheet (second invention) excellent in ductility and delayed fracture resistance satisfying a yield ratio ≤ 0.70.

3. C: 0.05 to 0.20%, Mn: 1.80 to 5.
00%, P: 0.02-0.08%, Al: 0.10% or less, Cu: 0.
10-1.00%, Cr: 0.05-1.00%, B: 0.0010-0.00
50%, Nb: 0.005 to 0.040%, S: 0.0050% or less,
A steel slab with a composition containing Ni: 0.05 to 1.00% is soaked at a temperature of 1100 ° C or higher,
Finish rolling temperature: hot rolling under the condition of 800-950 ℃,
After winding at a temperature of 500 to 650 ℃, after pickling, cold rolling at a reduction rate of 10% or more, then soaking temperature: 800 to 90
0 ℃, soaking time: 20 ~ 120 s, average cooling rate from 800 ℃ to 400 ℃ or less quenching stop temperature: 10 ~ 150 ℃ / s, cooling rate from quenching stop temperature to 200 ℃: 3 ℃ / s A method for producing a high-strength cold-rolled steel sheet excellent in ductility and delayed fracture resistance, which comprises continuous annealing under the above conditions (third invention).

[0011]

The reason for limiting the steel composition to the above range in the present invention will be described. C: 0.05 to 0.20% C is an essential element for strengthening steel by utilizing a low temperature transformation phase, and at least 0.05% addition is necessary to obtain TS ≧ 120 kgf / mm ^2. However, if the content exceeds 0.20%, high strength is obtained, but the weldability is significantly deteriorated, so the C content is limited to the range of 0.05 to 0.20%.

Mn: 1.80 to 5.00% Mn is an essential element for suppressing ferrite transformation and pearlite transformation and obtaining a preferable structure mainly composed of bainite, but if the content is less than 1.80%, it is stable and mainly composed of bainite. However, the addition of more than 5.00% not only saturates the effect, but also causes an increase in alloy cost and also has the disadvantage of deteriorating cold workability in the cold rolling process after hot rolling. . Therefore, the Mn content is set in the range of 1.80 to 5.00%.

P: 0.02 to 0.08% Although the detailed mechanism of P is not clear, it is effectively contributed to the improvement of delayed fracture resistance by being contained in combination with Cu, Cr and the like. However, if the amount of P added is less than 0.02%, the effect of improving delayed fracture resistance is insufficient, while 0.08%
The above effect reaches saturation even if added in excess of 10%, and rather the layering of the structure due to the segregation of P itself easily occurs,
This causes deterioration of local ductility. Therefore, the P content is 0.02-0.08
It was limited to the range of%.

Al: 0.10% or less Non-metallic inclusions such as oxides in steel lead to deterioration of local ductility, so it is necessary to sufficiently reduce it. For that purpose, addition of Al in the manufacturing process is advantageous. . However, if the amount of addition is excessive and exceeds 0.10%, the surface properties are deteriorated, which further leads to deterioration of bending workability and local ductility. Therefore, Al should be contained at 0.10% or less.
The lower limit is not particularly limited, but it is preferably 0.020% or more.

Cu: 0.10 to 1.00% Cu is a useful element that contributes to the improvement of delayed fracture resistance together with P and Cr. However, if the amount of Cu is less than 0.10%, the above effect is not sufficiently exhibited, while even if added over 1.00%, the effect reaches saturation, rather leading to an increase in the cost of alloying elements, so Cu is 0.10 to The content was set to be 1.00%.

Cr: 0.05 to 1.00% As described above, Cr effectively contributes to the improvement of delayed fracture resistance by the combined action of P and Cu. However,
If the Cr content is less than 0.05%, the above effect is not sufficiently exhibited, while if it exceeds 1.00%, the effect reaches saturation, and rather the steel hardens and the cold workability deteriorates. Therefore, Cr
The content was in the range of 0.05 to 1.00%.

B: 0.0010 to 0.0050% B is effective in improving the strength, and particularly has the effect of facilitating the formation of a bainite-based structure by suppressing the formation of ferrite during rapid cooling during continuous annealing. Further, it also has an effect of suppressing the tendency of the local ductility to deteriorate due to the addition of P. However, at least 0.0010 is required to achieve such an effect.
%, More than 0.0050%, these effects reach saturation, and rather the hot-rolled base plate becomes hard and interferes with the cold-rolling process. Therefore, the B content is 0.0010-
It was set to 0.0050%.

Nb: 0.005 to 0.040% Nb is a useful element that effectively contributes not only to grain refinement of the structure but also to formation of a structure mainly composed of bainite during continuous annealing. These effects become remarkable when added in an amount of 0.005% or more, but when it exceeds 0.040%, the hot-rolled base plate becomes hard and interferes with the cold-rolling process, and the delayed fracture resistance tends to deteriorate slightly. Therefore, Nb is included in the range of 0.005 to 0.040%.

S: 0.0050% or less Limiting the amount of S is important from the viewpoint of improving the local ductility and improving the delayed fracture resistance. That is, since S exists as a non-metallic inclusion in steel and serves as a stress concentration source, its reduction is advantageous for various mechanical properties.
The allowable upper limit of S depends on the microstructure and strength level of the steel sheet.
Good properties can be obtained if the content is suppressed to 0.0050% or less.

Ni: 0.05 to 1.00% Ni, although the detailed mechanism is unknown, effectively contributes to the improvement of delayed fracture resistance of steel. This effect is exhibited when the content is 0.05% or more, but when it exceeds 1.00%, the effect of addition reaches saturation and rather the alloy cost rises. Therefore, the Ni content is set to 0.05 to 1.00%.

Although the basic components have been described above, the present invention may further contain the following elements in order to improve mechanical properties. Ti: 0.005 to 0.100% Ti is effective not only for preventing cracking during slab casting, but is also effective for improving local ductility, although the detailed mechanism is unknown in terms of material. In order to obtain such effects, it is necessary to add at least 0.005%, but if added over 0.100%, not only the surface properties deteriorate, but also the carbides increase excessively and the delayed fracture resistance deteriorates. Invite.
Therefore, Ti is included in the range of 0.005 to 0.100%.

Si: 0.05 to 0.50% Si is a useful element that improves the local ductility by effectively reducing the inclusions in the steel. It also contributes effectively to the reduction of the yield stress to tensile strength ratio, that is, the yield ratio (YR), and is also useful for improving delayed fracture resistance and local ductility. However, if the content is less than 0.05%, the effect of addition is poor, while if it exceeds 0.50%, the chemical conversion treatability deteriorates. Therefore, Si is included in the range of 0.05 to 0.50%.

V: 0.010 to 0.100% V, like Ti and Si, is useful for improving the local ductility and also effectively contributes to the reduction of the yield ratio. However, if the content is less than 0.010%, its addition effect is poor, while if it is added over 0.100%, the yield ratio rises, and
Material improvement effect decreases. Therefore, V is 0.010 to 0.100
The content is set to be in the range of%. The above three elements,
Ti, Si, and V are effective even if added alone, but the effect is not offset even if they are added in combination.

Although the proper component composition range has been described above, in order to obtain the intended effect of the present invention, it is not sufficient to limit the component composition to the above-mentioned proper range, and the steel structure is reduced in area ratio. It is important to regulate within the following range. Bainite fraction ≥50% The bainite fraction must be 50% or more. This is because if it is less than 50%, improvement in local ductility cannot be expected. In addition to bainite alone, the total fraction with martensite described below is also important.

[0025] Fraction of bainite + tempered martensite ≥ 90% If the fraction of these hard low-temperature transformation phases is less than 90%, the remaining soft phases are spatially partially connected, resulting in void connection. It becomes easy, and bending workability, local ductility, and delayed fracture resistance are all deteriorated.

Retained austenite fraction ≤4% If the retained austenite fraction exceeds 3%, the ductility is improved, but the delayed fracture resistance is greatly deteriorated to a level that cannot be practically used.

By making the composition and composition ranges and the steel structure described above, tensile strength ≧ 120 kgf / mm ² , yield ratio ≦ 0.70
It is possible to obtain a high-strength cold-rolled steel sheet having excellent local ductility and delayed fracture resistance satisfying the above conditions. Note that the yield ratio is particularly important in mechanical properties, and setting the yield ratio to 0.70 or less facilitates propagation of strain during press molding, and as a result, harmful residual tensile stress can be reduced. Further, since the amount of springback during press molding can be reduced, it is also effective for improving the accuracy of press molding.

Next, the production suitability according to the present invention will be specifically described. Slab manufacturing method Since continuous casting has less segregation of components and a uniform material can be obtained, it is desirable to manufacture a slab by continuous casting, but it is naturally applicable to an agglomeration method with less segregation.

Slab soaking temperature: 1100 ° C or higher If the slab soaking temperature is less than 1100 ° C, the addition Nb is not sufficiently melted, the strength is lowered, and the detailed reason is unknown, but delayed fracture resistance The characteristics also deteriorate.

Finishing rolling temperature: 800 to 950 ° C. When the finishing rolling temperature is less than 800 ° C., the structure after cold rolling and annealing becomes coarse, and hot rolling becomes difficult because the plate shape is disturbed. On the other hand, when the temperature exceeds 950 ° C, the unevenness of the steel sheet in the thickness direction increases, which is not preferable in terms of local ductility. Therefore, the finish rolling temperature was set to 800 to 950 ° C.

Hot rolling coiling temperature: 500 to 650 ° C. When coiled at a temperature of less than 500 ° C., the steel sheet remarkably hardens and cold rolling becomes difficult, and the shape of the steel sheet also significantly deteriorates. To do. On the other hand, when wound at a temperature higher than 650 ° C, the final steel sheet structure also becomes nonuniform as the microstructure of the hot rolled base plate becomes nonuniform, and the local ductility deteriorates.

Cold reduction rate: 10% or more By setting the cold reduction rate to 10% or more, it is possible to eliminate the non-uniformity of the shape of the steel sheet surface, which is effective for improving delayed fracture resistance. . The upper limit of the cold reduction is not particularly limited, and can be adjusted to the required final product thickness.

Soaking temperature during continuous annealing: 800 to 900 ° C. The soaking temperature is important for adjusting the fraction of the austenite phase during soaking. If the temperature is less than 800 ° C, a sufficient amount of austenite phase cannot be obtained, and in addition, a layered structure remains in the steel sheet, and sufficient local ductility cannot be obtained. On the other hand, 950 ℃
If it exceeds, the structure becomes coarse, and the workability and the secondary work embrittlement resistance also deteriorate.

Soaking time during continuous annealing: 20 to 120 s If less than 20 s , finally stable material cannot be obtained. On the other hand, when it exceeds 120 s, the element concentration on the surface of the steel sheet becomes remarkable, which not only deteriorates the chemical conversion treatability, but also increases hydrogen absorption from the atmosphere during annealing and deteriorates the delayed fracture resistance.

Quenching rate: 10 to 150 ° C./s If the cooling rate from 800 to 400 ° C. is less than 10 ° C./s, a soft phase precipitates, not only lowering the strength but also degrading local ductility. On the other hand, when the cooling rate exceeds 150 ° C / s, the fraction of bainite is remarkably reduced and the structure is mainly composed of martensite. As a result, the strength is slightly increased but the delayed fracture resistance is remarkably deteriorated. In addition, material variations due to uneven cooling in the plate width direction also increase.

Cooling rate after rapid cooling: After rapid cooling from 3 ° C./s to 400 ° C., it is necessary to continuously cool to 200 ° C. at a rate of 3 ° C./s or more. The reason is that if the material stays in the high temperature range again in this temperature range, the yield ratio increases as a result of the tempering of the low temperature transformation phase increasing, and the precipitates in the steel also increase, resulting in a marked deterioration of the delayed fracture resistance. Is.

[0037]

【Example】

Example 1 Steels having various compositional compositions shown in Table 1 were melted in a converter,
After being continuously cast into a slab, it was processed under the conditions shown in Table 2 to obtain a cold rolled steel sheet. Table 3 shows the results obtained by examining the mechanical properties and delayed fracture resistance of each of the obtained cold-rolled steel sheets. Among the mechanical properties, the tensile properties were evaluated by an ordinary measuring method using ordinary JIS No. 5 test pieces. Regarding the bending test, a 180 ° U-bend test using a punch die was performed using a test piece having a width of 40 mm and a length of 100 mm, and the critical bending radius at which bending was possible without cracks was evaluated. Regarding local ductility, a standard JIS No. 5 test piece with a 2 mm V notch machined in the center of the parallel part was prepared and
JIS No. 5 tensile properties, especially changes in TS were compared. When notch sensitivity is high, the presence of notches causes so-called low stress fracture. In the delayed fracture test, a steel plate was deep-drawn into a cylinder having a drawing ratio of 1.80 with a punch having a diameter of 33 mm, immersed in pure water, and evaluated for the presence or absence of cracking after a lapse of about one week.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

[Table 3]

As is clear from Table 3, the steel sheet obtained according to the present invention has a high strength of TS of 120 kgf / mm ² or more, a low yield ratio, a small critical bending radius and good bend formability. is doing. Further, it can be seen that it has good delayed fracture resistance without causing delayed fracture. Further, it can be seen that in the invention steel having various properties, the microstructure satisfies the conditions of bainite ≧ 50%, (bainite + tempered martensite) ≧ 90%, and retained austenite ≦ 4%. It should be noted that when the notch is present, it is desirable that the tensile strength be slightly increased as compared with the smooth material, but in the steel of the present invention, it is 5 to 7 kgf / mm ²
It is increasing, and this condition is also satisfied.

On the other hand, in the comparative examples which do not satisfy the requirements of the present invention, the strength is greatly reduced, the notch sensitivity is also large, and there is a high risk of causing low stress fracture when some defects are present as parts. Shows.

Example 2 Steels having the chemical compositions shown in Table 4 were used and cold rolled steel sheets were prepared under the production conditions shown in Table 5. Table 6 shows the final microstructure of each of the obtained cold-rolled steel sheets and the results of investigations on tensile properties, bending properties, and delayed fracture resistance. Each test method is the same as in the case of Example 1.

[0044]

[Table 4]

[0045]

[Table 5]

[0046]

[Table 6]

As is clear from Table 6, all the steel sheets obtained according to the present invention have high strength of TS of 120 kgf / mm ² or more, and have good bending workability and delayed fracture resistance. There is.

Example 3 Cold-rolled steel sheets were manufactured by using three steel types having substantially the same composition as in Table 4 and various manufacturing conditions centered on continuous annealing conditions. Table 7 shows the results of the investigation on the relationship with the delayed fracture characteristics. The tensile strength is 138-
It has been confirmed that the range is 151 kgf / mm ² .

[0049]

[Table 7]

As is clear from Table 7, bainite ≧ 50
%, (Bainite + tempered martensite) ≧ 90% and retained austenite ≦ 4%, particularly good delayed fracture resistance is obtained.

[0051]

As described above, according to the present invention, a high-strength steel sheet excellent in ductility and delayed fracture resistance can be stably obtained, and its industrial value is extremely large. The steel of the present invention is suitable mainly for strength members of automobiles, has extremely high strength and sufficient formability, and at the same time has high local ductility, and therefore has high reliability as a part. Further, since it is also excellent in delayed fracture resistance, by using this steel sheet, it is possible to reduce the thickness of the steel sheet and thus the weight of the automobile body.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshiyuki Kato 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Technical Research Division, Kawasaki Steel Corporation (72) Hideki Emoto 1 Kawasaki-cho, Chuo-ku, Chiba Kawasaki Steel Corporation Technical Research Division

Claims (3)

[Claims] 1. C: 0.05 to 0.20 wt%, Mn: 1.80 to 5.00
wt%, P: 0.02-0.08 wt%, Al: 0.10 wt% or less, Cu: 0.10-1.00 wt%, Cr: 0.05-1.00 wt%, B: 0.0010-0.0050 wt%, Nb: 0.005-0.040 wt%, S: 0.0050 wt% or less, Ni: 0.05-1.00 wt%, the balance is Fe and inevitable impurities,
And the area ratio of the steel structure is bainite ≧ 50%, (bainite + tempered martensite) ≧ 90%, retained austenite ≦ 4%, tensile strength ≧ 120 kgf / mm ² , yield ratio ≦ 0.
A high-strength cold-rolled steel sheet that satisfies 70 and has excellent delayed fracture resistance. 2. C: 0.05 to 0.20 wt%, Mn: 1.80 to 5.00
wt%, P: 0.02-0.08 wt%, Al: 0.10 wt% or less, Cu: 0.10-1.00 wt%, Cr: 0.05-1.00 wt%, B: 0.0010-0.0050 wt%, Nb: 0.005-0.040 wt%, S: 0.0050 wt% or less, Ni: 0.05 to 1.00 wt%, Ti: 0.005 to 0.100 wt%, Si: 0.05 to 0.50 wt%, V: 0.010 to 0.100 wt%, selected from 1 or 2 Contains more than one seed, the balance is
The composition of Fe and unavoidable impurities, and the area ratio of the steel structure is bainite ≧ 50%, (bainite + tempered martensite) ≧ 90%, retained austenite ≦ 4%, tensile strength ≧ 120 kgf / mm ² , A high-strength cold-rolled steel sheet with a yield ratio ≤ 0.70 and excellent ductility and delayed fracture resistance. 3. C: 0.05 to 0.20 wt%, Mn: 1.80 to 5.00
wt%, P: 0.02-0.08 wt%, Al: 0.10 wt% or less, Cu: 0.10-1.00 wt%, Cr: 0.05-1.00 wt%, B: 0.0010-0.0050 wt%, Nb: 0.005-0.040 wt%, S: 0.0050 wt% or less, Ni: 0.05 to 1.00 wt% steel slab having a composition, soaking temperature: 1100 ℃ or more,
Finish rolling temperature: hot rolling under the condition of 800-950 ℃,
After winding at a temperature of 500 to 650 ℃, after pickling, cold rolling at a reduction rate of 10% or more, then soaking temperature: 800 to 90
0 ℃, soaking time: 20 ~ 120 s, average cooling rate from 800 ℃ to 400 ℃ or less quenching stop temperature: 10 ~ 150 ℃ / s, cooling rate from quenching stop temperature to 200 ℃: 3 ℃ / s A method for producing a high-strength cold-rolled steel sheet excellent in ductility and delayed fracture resistance, characterized by continuous annealing under the above conditions.