JP2011153336A - High strength cold rolled steel sheet having excellent formability, and method for producing the same - Google Patents

Abstract

The present invention provides a steel plate that is less likely to break during forming by suppressing precipitation of coarse carbides, or dissolves carbides, and has improved strength-ductility balance due to an increase in the amount of retained austenite, and a method for producing the same.
SOLUTION: In mass%, C: 0.10 to 0.40%, Mn: 0.5 to 3.0%, Si: 0.005% or more, Al: 0.005% or more, provided that Si + Al: 0.8 to 2.5%, balance: Fe and inevitable impurities, among the above inevitable impurities, P: 0.05% or less, S: 0.02% or less, N: 0.006% or less It has a limited chemical composition, and the microstructure is composed of 10-60% ferrite by area ratio, 2-30% retained austenite, 10% or less martensite, and the balance bainite, and the weight density of cementite is 5 mg / A high-strength steel sheet excellent in formability, characterized by being cm ³ or less. This steel sheet manufacturing method obtains the above microstructure by limiting the heating temperature and cooling conditions for annealing.
[Selection] Figure 1

Description

  The present invention relates to a high-strength steel sheet excellent in formability and a method for producing the same.

  In order to achieve both weight reduction and safety of automobile bodies, parts, etc., the strength of steel plates as materials is being increased. In general, when the strength of a steel plate is increased, ductility, hole expansibility and the like are lowered, and formability is impaired. Therefore, in order to use a high-strength steel sheet as a member for automobiles, balances such as strength, ductility and hole expansibility are necessary.

  In response to such demands, so-called TRIP steel sheets utilizing transformation-induced plasticity of retained austenite have been proposed (Patent Documents 1 and 2). These aim to stabilize and increase the strength of retained austenite by increasing the amount of C and Si. As C concentrates in austenite, austenite is stabilized and remains stably even at room temperature.

  However, the added C is not only concentrated in austenite but also precipitated as coarse carbides. Then, problems such as material deterioration due to a decrease in the amount of retained austenite and generation of cracks at the time of hole expansion starting from carbides arise.

  Further, if the amount of C is further increased in order to compensate for the decrease in the amount of retained austenite due to the precipitation of carbides, a decrease in weldability becomes a problem.

Japanese Patent Laid-Open No. 61-217529 Japanese Patent Laid-Open No. 5-59429

  The present invention suppresses the precipitation of coarse carbides or dissolves carbides, so that there is no cracking at the time of hole expansion starting from carbides, and the strength-ductility balance is improved by increasing the amount of retained austenite. Succeeded in manufacturing steel sheets.

  The present inventors have succeeded in producing a steel sheet excellent in strength, ductility, and hole expandability by optimizing the components and production conditions of TRIP steel and controlling the size of carbides during annealing. The summary is as follows.

(1) In mass%,
C: 0.10 to 0.40%,
Mn: 0.5 to 3.0%
Si: 0.005% or more Al: 0.005% or more However, Si + Al: 0.8 to 2.5%,
The balance: Fe and inevitable impurities, and
Of the above inevitable impurities
P: 0.05% or less,
S: 0.02% or less,
N: having a chemical composition limited to 0.006% or less,
The microstructure is composed of 10-60% ferrite in area ratio, 2-30% retained austenite, 10% or less martensite, and the balance bainite, and the weight density of cementite is 5 mg / cm ³ or less. High strength steel plate with excellent formability.

(2) In mass%,
Cr: 0.01 to 0.8%
Mo: 0.01 to 0.3%,
Ni: 0.01 to 5%,
Cu: 0.01 to 5%
1 type or 2 types or more of these are included, The high strength steel plate excellent in the formability as described in said (1) characterized by the above-mentioned.

(3) In mass%,
Nb: 0.001 to 0.10%
Ti: 0.001 to 0.10%
V: 0.001 to 0.10%
W: 0.001% to 0.10%
A high-strength steel sheet excellent in formability according to any one of the above (1) to (2), comprising one or more of the above.

(4) In mass%,
B: A high-strength steel sheet excellent in formability as described in any one of (1) to (3) above, containing 0.0003 to 0.003% or less.

(5) Contains one or more of Ca, REM, Mg, Zr,
The high-strength steel sheet having excellent formability according to any one of the above (1) to (4), wherein the mass% is Ca + REM + Mg + Zr: 0.0005% to 0.05%.

(6) The high-strength steel sheet having excellent formability according to any one of claims 1 to 5, which is an alloyed hot-dip galvanized steel sheet.

(7) A method for producing a high-strength steel sheet according to any one of (1) to (5) above, wherein the chemical composition according to any one of (1) to (5) above is provided. The slab is hot-rolled at a finishing temperature of ₃ or more points of Ar, cooled to 350 to 600 ° C., wound, then cold-rolled by 40 to 85%, heated to 750 to 900 ° C., annealed, 10 Cooling at ~ 200 ° C / s, holding in a temperature range of 350-450 ° C for 60-900s, further cooling, average area A (µm ² ) of pearlite in the structure after hot rolling, and average of annealing The manufacturing method of the high strength steel plate excellent in the formability characterized by heating temperature T (degreeC) and the heating time t (s) of annealing satisfy | filling the relationship of the following Formula (1).

T × log (t) / α> 110 Formula (1)
Where α = (1 + 0.3Si + 0.5Al + Cr + 0.5A)

(8) The method for producing a high-strength steel sheet according to (6) above, wherein the slab having the chemical composition according to any one of (1) to (5) is described in (7) above. After hot rolling, cooling, winding, cold rolling,
After holding at a temperature range of Ac1 to Ac3 for 10 to 300 s, cooling at 3 to 200 ° C./s, holding at 15 to 1200 s at a temperature range of 350 to 550 ° C., hot dip galvanizing, 470 to Alloying is performed at 600 ° C., further cooling, the average area A (μm ² ) of pearlite in the structure after the hot rolling, the average heating temperature T (° C.) for annealing, and the heating time t (s for annealing). ) Satisfies the relationship of the following formula (1): A method for producing a high-strength steel sheet having excellent formability.

T × log (t) / α> 110 Formula (1)
Where α = (1 + 0.3Si + 0.5Al + Cr + 0.5A)

  According to the present invention, by optimizing the chemical composition, securing a predetermined amount of retained austenite, and reducing the amount of cementite, the strength and ductility and hole expandability are combined to provide excellent formability. A steel plate is obtained.

FIG. 1 is a graph showing the effect of T × log (t) / (1 + 0.3Si + 0.5Al + Cr + 0.5A) on the amount of cementite. FIG. 2 is a graph showing the effect of T × log (t) / (1 + 0.3Si + 0.5Al + Cr + 0.5A) on the product of strength, ductility and hole expansibility. FIG. 3 is a graph showing the effect of T × log (t) / (1 + 0.3Si + 0.5Al + Cr + 0.5A) on the product of strength and hole expansibility.

  The present inventors have found that when cementite generated during hot rolling is dissolved during annealing and the particle size is reduced, the balance of strength, ductility and hole expansibility is excellent. The reason will be described below.

  In the TRIP steel, tensile properties are improved by increasing the amount of retained austenite by concentrating C in the austenite during the annealing process and by increasing the amount of C in the austenite itself. However, when cementite during hot rolling remains even after cold rolling and annealing, a part of the added C exists as carbide, so the amount of austenite and the amount of C in the austenite are reduced, and the strength and ductility are reduced. The balance is reduced. Moreover, it becomes a starting point of a crack at the time of a hole expansion test, and a moldability deteriorates.

<The amount of cementite is 5 mg / cm ³ or less>
Therefore, the present inventors investigated a method for reducing the amount of cementite. The present inventors examined the relationship between the amount of cementite of the hot-rolled sheet, the heating temperature T and time t during annealing, the tensile properties, and the hole expandability. As a result, by satisfying the following formula (1), the amount of cementite after annealing was 5 mg / cm ³ or less, and the knowledge that the balance of strength, ductility and hole expansibility was improved was obtained.

T × log (t) / α> 110 Formula (1)
Where α = (1 + 0.3Si + 0.5Al + Cr + 0.5A)
More preferably,
T × log (t) / α> 160 Formula (2)
It is.

  The pearlite transformed during hot rolling is spheroidized during annealing and becomes coarse cementite. When this coarse cementite is annealed at a temperature equal to or higher than Ac1, the dissolution progresses, and if the formula (1), preferably the formula (2) is satisfied, the cementite becomes sufficiently small and does not become a starting point of fracture when the hole is expanded. The total amount of C to be concentrated is increased, the retained austenite is increased, and the balance between strength and ductility is improved.

  Here, the physical meaning of Formula (1) is demonstrated below.

  T × log (t) in the formula is considered to be related to the diffusion rate of carbon and iron. This is because the reverse transformation from cementite to austenite proceeds by the diffusion of atoms.

  Α in the formula increases when Si, Al, Cr are large and / or when the area of pearlite deposited during hot rolling is large. When α is large, in order to satisfy the formula (1), preferably (2), it is necessary to set the annealing conditions so as to increase T × log (t).

  The reason why α in the formula varies depending on the area ratio of Si, Al, Cr, and pearlite during hot rolling is as follows.

  Since Si and Al are elements that suppress cementite precipitation, at the time of hot rolling, the transformation of bainite with a small amount of ferrite or carbide proceeds first, and carbon is concentrated in austenite. Thereafter, perlite transformation occurs from austenite enriched in carbon. Such pearlite has a large proportion of cementite, and is easily spheroidized during the subsequent annealing and heating, and tends to be coarse cementite that is difficult to dissolve.

  Since Cr is an element that makes cementite difficult to dissolve, it is considered that the value of α in the formula is increased.

  The reason why α in the equation increases as the area A of pearlite during hot rolling increases is that the amount of cementite correspondingly increases as the area ratio of the pearlite increases.

  Next, the reason for limiting the chemical composition of the steel sheet of the present invention will be described. In the following, “%” indicating the content of component elements means “% by mass”.

  First, essential components will be described.

<C: 0.10 to 0.40%>
C is an extremely important element for increasing the strength of the steel and securing retained austenite. In order to obtain a sufficient amount of retained austenite, a C amount of 0.10% or more is required. On the other hand, when C is contained excessively, weldability is impaired, so the upper limit of the C content is set to 0.40% or less.

<Mn: 0.5 to 3.0%>
Mn is an element that stabilizes austenite and improves hardenability. In order to ensure sufficient hardenability, it is necessary to add 0.5% or more of Mn. On the other hand, if Mn is added excessively, ductility is impaired, so the upper limit of the amount of Mn is made 3.0%.

<Si: 0.005% or more>
<Al: 0.005% or more>
<Si + Al: 0.8 to 2.5%>
Si and Al are deoxidizers and need to be added in an amount of 0.005% or more.
Further, Si and Al are elements that stabilize ferrite during annealing, and contribute to securing retained austenite by increasing the C concentration of austenite to suppress cementite precipitation during bainite transformation. In order to obtain the effect, Si + Al needs to be 0.8% or more. On the other hand, the higher the Si and Al, the greater the effect. However, excessive addition of Si or Al leads to deterioration of surface properties, paintability, weldability, etc., so the upper limit of Si + Al is 2.5% or less. .

  Next, inevitable impurities will be described.

<P: 0.05% or less>
P is an impurity, and if contained excessively, ductility and weldability are impaired. Therefore, the upper limit of the P content is 0.05% or less.

<S: 0.02% or less>
S is an impurity, and if contained excessively, MnS stretched by hot rolling is generated, which causes deterioration of formability such as ductility and hole expansibility. Therefore, the upper limit of the S amount is 0.02% or less.

<N: 0.006% or less>
N is an impurity. When it exceeds 0.006%, ductility is deteriorated. Therefore, the upper limit of the N amount is set to 0.006% or less.

  Below, an arbitrary component is demonstrated.

<Cr, Mo, Ni, Cu>
Furthermore, you may add 1 type, or 2 or more types of Cr, Mo, Ni, Cu. Mo, Cr, Ni, and Cu are elements that improve the strength of the steel sheet. In order to obtain this effect, addition of 0.01% or more is necessary. However, when these elements are added excessively, the strength increases and ductility may be impaired. Therefore, it is preferable that the upper limit is Mo: 0.3% or less, Cr: 0.8% or less, Ni: 5% or less, and Cu: 5% or less, respectively.

<Nb, Ti, V, W>
Nb, Ti, V, and W are elements that generate fine carbides, nitrides, or carbonitrides, and are effective in securing strength. Therefore, it is possible to add one or more as necessary. is there. To achieve this, 0.010% addition is required. On the other hand, excessive addition increases the strength too much and lowers the ductility, so the upper limit was made 0.10%.

<B>
Steel can contain B: 0.0003% or more. B can delay the transformation and increase the strength of the steel, but if B is less than 0.0003%, the hardenability is weak, and the required strength cannot be obtained to promote ferrite formation at high temperatures. On the other hand, if the addition exceeds this range, the hardenability becomes too strong, and the ferrite and bainite transformations are slowed, so the C concentration to the residual austenite phase is delayed, so the upper limit is 0.003%. did.

<Ca, REM, Mg, Zr>
The steel can further contain one or more of Ca, REM (rare earth element), Mg, and Zr alone or in total of 0.0005% or more and 0.05% or less. Ca, REM, Mg and Zr improve the local ductility and hole expansibility by controlling the shape of sulfides and oxides. For this purpose, it is necessary to add one or more of these elements alone or in total of 0.0005% or more. However, excessive addition deteriorates workability, so the upper limit was made 0.05%.

  Next, the reason for limiting the microstructure of the steel sheet of the present invention will be described.

  The microstructure of the steel sheet of the present invention is composed of ferrite, retained austenite, martensite, and the balance is bainite. “%” Representing the amount of each phase is an area ratio.

<Ferrite: 10-60%>
Ferrite is a structure with excellent ductility, but if it is too much, the strength decreases. The strength level may be a target level of development, but by setting the strength level to 10 to 60%, an excellent balance between strength and ductility can be obtained.

<Residual austenite: 2 to 30%>
Residual austenite is a structure that increases ductility, particularly uniform elongation, by transformation-induced plasticity, and an area ratio of 2% or more is required. Moreover, since it transforms into martensite by processing, it contributes to the improvement of strength. The higher the area of retained austenite, the better. However, in order to ensure retained austenite with an area ratio of more than 30%, it is necessary to increase the amount of C and Si, which impairs weldability and surface properties. Therefore, the upper limit of the area ratio of retained austenite is set to 30% or less.

<Martensite: 10% or less>
Martensite is a hard structure and is effective in securing strength. However, in the present invention, in order to ensure ductility, the upper limit is 10% in area ratio.

<Balance Bay Night>
Further, bainite or bainitic ferrite is included. These tissues are necessary for concentrating C in γ, and need to be contained by 10% or more. However, if the amount is large, the amount of ferrite having high work-hardening properties decreases and the uniform elongation decreases, so it is necessary to make it 75% or less.

  The average crystal grain size of ferrite is preferably 10 μm or less. If the average crystal grain size of ferrite is 10 μm or less, the strength can be increased without impairing total elongation and uniform elongation. Further, it is considered that the microstructure becomes uniform when it is made fine, so that strain introduced during molding is uniformly dispersed, strain concentration is small, and it is difficult to break.

  Next, the reason for limitation of the manufacturing method of the steel plate of this invention is demonstrated.

  The steel sheet of the present invention is produced by hot rolling a steel piece obtained by melting and casting steel in a conventional manner, and subjecting the hot-rolled steel sheet to pickling, cold rolling, and annealing. Hot rolling is performed with a normal continuous hot rolling line, and annealing after cold rolling is performed with a continuous annealing line. Further, skin pass rolling may be performed on the cold-rolled steel sheet.

<Slab manufacturing>
The molten steel may be one produced by a normal blast furnace method or one using a large amount of scrap as in the electric furnace method. The slab may be manufactured by a normal continuous casting process or may be manufactured by thin slab casting.

<Hot rolling>
When the hot rolling finishing temperature is too high, scale formation is promoted, and the surface quality and corrosion resistance of the product are adversely affected. Therefore, it is desirable that the finishing temperature of hot rolling be 1000 ° C. or less. Further, if the finishing temperature of the hot rolling is less than 850 ° C., ferrite-austenite two-phase region rolling is performed, and the shape of the plate may be deteriorated. Therefore, the finishing temperature is set to 850 ° C. or more.

<Rewinding after cooling>
After hot rolling, it is cooled, wound, and coiled. In order to make the structure of the cold-rolled steel sheet fine, it is necessary to set the coiling temperature within a range of 350 to 600 ° C. When the coiling temperature is less than 350 ° C., the structure of the hot-rolled steel sheet is mainly martensite, and the cold rolling load increases. On the other hand, when the coiling temperature exceeds 600 ° C., pearlite increases, the average grain size of ferrite in the cold-rolled steel sheet exceeds 10 μm, and the balance between strength and hole expansibility is lowered.

<Cold rolling>
In cold rolling, the reduction ratio is set to 40% or more in order to refine the microstructure after annealing. On the other hand, if the rolling reduction of cold rolling exceeds 85%, the load increases due to work hardening, and the productivity is impaired. Therefore, the rolling reduction of cold rolling is 40 to 85%.

<Annealing>
After cold rolling, annealing is performed. In the present invention, in order to control the microstructure of the steel sheet, the heating temperature and cooling conditions for annealing are extremely important.

  The purpose of annealing heating is to recrystallize the work structure formed by cold rolling and to concentrate austenite stabilizing elements such as C into austenite. In the present invention, the heating temperature for annealing is a temperature at which ferrite and austenite coexist.

  If the heating temperature for annealing is less than 750 ° C., recrystallization is insufficient and sufficient ductility cannot be obtained. On the other hand, when the heating temperature for annealing exceeds 900 ° C., austenite increases and agricultural production of C and the like becomes insufficient. As a result, the stability of austenite is impaired, and it becomes difficult to secure retained austenite after cooling. Therefore, the heating temperature of annealing shall be 750-900 degreeC.

  The holding time for annealing needs to be a condition satisfying the formula (1) or the formula (2) in order to sufficiently dissolve the carbide and secure the C amount of austenite.

  It cools at 10-200 degreeC / s to the temperature range of 350-450 degreeC after the heating of annealing. When the cooling rate is less than 10 ° C./s, pearlite is generated. On the other hand, when the cooling rate exceeds 200 ° C./s, it becomes difficult to control the stop temperature. The upper limit of the cooling rate of cooling is preferably 100 ° C./s or less.

  After cooling, hold at 350 to 450 ° C. for 60 to 900 s and cool. By holding at 350 to 450 ° C., bainite is generated, precipitation of cementite is prevented, and a decrease in the amount of solute C is suppressed. Therefore, when the bainite transformation is promoted, retained austenite can be secured. When the holding temperature is higher than 450 ° C., pearlite is generated. On the other hand, when the holding temperature is less than 350 ° C., the bainite transformation becomes insufficient. Further, if the holding time is less than 60 s, the bainite transformation becomes insufficient, and it becomes difficult to secure retained austenite. On the other hand, when the holding time exceeds 900 s, productivity is impaired.

  The steel sheet of the present invention can also be produced as an galvannealed steel sheet. The manufacturing method performs the same steps as described above until cold rolling, and changes the subsequent steps to the following steps including hot dip galvanizing and alloying.

[Change process] (After cold rolling)
After holding in the temperature range of Ac1 to Ac3 for a time satisfying formula (1) or formula (2), cooling is performed at 3 to 200 ° C./s, and 15 to 1200 s is held in the temperature range of 350 to 550 ° C. After hot dip galvanization, alloying is performed at 470 to 600 ° C., and cooling is further performed.

  The purpose of each process and the reason for limitation will be described.

<Hold in the temperature range of Ac1 to Ac3>
The purpose of annealing heating is to recrystallize the work hardening formed by cold rolling and to concentrate austenite stabilizing elements such as C into austenite. When the heating temperature for annealing is less than Ac1, the amount of austenite obtained by annealing is small, and sufficient austenite cannot be left in the steel sheet. Moreover, since the high temperature of annealing causes the coarsening of crystal grains, the upper limit of the annealing temperature is set to Ac3.

<Cooling at 3 to 200 ° C./s>
Cooling after annealing is important for promoting the transformation from the austenite phase to the ferrite phase and concentrating C in the untransformed austenite phase to stabilize the austenite. When the cooling rate is less than 3 ° C./s, pearlite is generated, and the strength-ductility is significantly deteriorated. On the other hand, when the cooling rate is 200 ° C./s, the ferrite transformation cannot be sufficiently advanced. Therefore, the cooling rate after annealing was set to 3 to 200 ° C./s.

<Hold for 15 to 1200 seconds in a temperature range of 350 to 550 ° C.>
Cooling temperature shall be 300-550 degreeC. If the temperature is lower than 300 ° C., martensite is likely to be generated. If the temperature exceeds 550 ° C., it is difficult to advance the bainite transformation, and it is easy to generate cementite during the bainite transformation.
It is necessary to hold for 15 to 1200 s at the above temperature. If it is less than 15 s, bainite cannot be generated sufficiently. On the other hand, if it exceeds 1200 s, carbides are generated from austenite, and the strength-ductility balance deteriorates.

<Hot galvanized>
The cold-rolled steel sheet produced as described above is immersed in a hot dip zinc plating bath to perform plating, and the bath temperature is 450 to 475 ° C. When the temperature is lower than 450 ° C., the viscosity of the molten zinc is high, and there is a problem that it is not suitable for wiping by wiping, and bottom dross is likely to occur. On the other hand, when the temperature is higher than 475 ° C., problems such as an increase in the production of zinc oxide and an increase in zinc are caused.

<Alloying at 470 to 600 ° C.>
Subsequently, an alloying process is performed at a temperature of 470 to 600 ° C. When the alloying treatment temperature is less than 470 ° C., alloying does not proceed, or alloying is not progressed sufficiently, and an alloyed hot-dip galvanized layer cannot be formed, and the surface of the steel sheet is workable. This is because it is covered with the inferior η phase and ζ phase. In addition, when the processing temperature is higher than 600 ° C., alloying proceeds too much, the plating adhesion at the time of processing decreases, and austenite before alloying during alloying becomes bainite or pearlite containing carbide. This is because it transforms and the tensile properties deteriorate.

  Using steel strips having the chemical composition shown in Table 1, hot-rolled steel sheets and cold-rolled steel sheets were produced under the conditions shown in Table 2. In Table 2, treatment numbers 1 to 18 are invention steels, and treatment numbers 19 to 46 are comparative steels. It should be noted that only the treatment number 18 of the inventive steel is an alloyed hot dip galvanized steel sheet subjected to hot dip galvanizing and alloying treatment. The structure of the obtained steel sheet was observed with an optical microscope, and the area ratio of pearlite in the hot-rolled steel sheet and the area ratio of ferrite, martensite, and bainite in the cold-rolled steel sheet were measured.

  The volume fraction of retained austenite and its carbon concentration were determined by X-ray diffraction as described in JP-A-11-193435. That is, the volume fraction Vγ of retained austenite can be calculated from the data obtained using the Mo—Kα ray by the following equation.

Vγ = (2/3) [100 / (0.7 × α (111) / γ (200) +1)] + (1/3) [100 / (0.78 × α (211) / γ (311) +1) ]
However, α (211), γ (200), α (211), and γ (311) are X-ray diffraction intensities from each surface.

  The carbon concentration Cγ of retained austenite is obtained by calculating the lattice constant (unit: angstrom) from the reflection angles of the (200), (220), and (311) surfaces of austenite by X-ray analysis using Cu-Kα ray. Can be calculated.

Cγ = (Lattice constant-3.572) /0.033
The particle diameter of the ferrite can be obtained, for example, by observing an arbitrary portion of the steel sheet using an optical microscope, measuring the number of ferrite grains in a range of 1000 μm ² or more, and obtaining an average equivalent circle diameter.

  Tensile test pieces defined in JIS No. 5 were collected from each steel plate and their mechanical properties were measured. In addition, the amount of cementite in the cold-rolled steel sheet was determined by extracting the extraction residue by electrolytic extraction, and the amount of carbide precipitation by obtaining the amount of the extraction residue by fluorescent X-ray diffraction. For the extraction of the precipitate by electrolysis, a 10% acetylacetone-1% tetramethylammonium chloride-methanol electrolyte was used.

  Table 3 shows the area ratio of the microstructure, the amount of cementite, and the results of the tensile test and the hole expansion test.

Process No. In all the comparative steels 19 to 43, the amount of cementite is larger than 5 mg / cm ^3. The product of strength (TS) and ductility (u-EL, t-EL), strength (TS) and hole expandability. It can be seen that the product of (λ) is low, the product of strength (TS), ductility (u-EL), and hole expansibility (λ) is low, and the moldability is poor.

  In the comparative steels with treatment numbers No. 44 to 46, the component range is out of the range of the present invention, so the amount of retained austenite is out of the range of the present invention, and the product of strength and ductility and the product of strength and hole expandability are low. Also, the product of strength, ductility and hole expansibility is low and the moldability is poor.

On the other hand, the invention steels of treatment numbers 1 to 18 have an appropriate chemical composition and appropriate conditions from hot rolling to annealing, so the amount of cementite in the cold-rolled steel sheet is 5 mg / cm ^3. Less and less melted cementite, resulting in higher retained austenite and higher product of strength and ductility, while less cementite reduces the starting point of rupture in the hole expansion test and tensile strength. It can be seen that the product of the hole expandability is high, the product of strength, ductility and hole expandability is also high, and the moldability is excellent.

The results are collectively shown in FIGS. By satisfy | filling Formula (1), as shown in FIG. 1, the quantity of the coarse cementite after cold rolling-annealing will be 5 mg / cm < ³ > or less. When the amount of cementite is 5 mg / cm ³ or less, as shown in FIG. 2, improvement in hole expansibility and improvement in the balance between strength and ductility are confirmed as shown in FIG. The reason why the material is improved when the amount is 5 mg / cm ³ or less is not clear, but is considered to be determined by the relationship with the particle size of the base material.

  ADVANTAGE OF THE INVENTION According to this invention, the high strength steel plate excellent in ductility and hole expansibility can be provided. If this steel plate is used, industrial contributions are particularly remarkable, such as making it possible to achieve both weight reduction and safety of automobiles.

Claims (8)

% By mass
C: 0.10 to 0.40%,
Mn: 0.5 to 3.0%
Si: 0.005% or more,
Al: 0.005% or more,
However, Si + Al: 0.8-2.5%,
The balance: Fe and inevitable impurities, and among the above inevitable impurities,
P: 0.05% or less,
S: 0.02% or less,
N: having a chemical composition limited to 0.006% or less,
The microstructure is composed of 10-60% ferrite in area ratio, 2-30% retained austenite, 10% or less martensite, and the balance bainite, and the weight density of cementite is 5 mg / cm ³ or less. High strength steel plate with excellent formability. % By mass
Cr: 0.01 to 0.8%
Mo: 0.01 to 0.3%,
Ni: 0.01 to 5%,
Cu: 0.01 to 5%,
The high-strength steel sheet having excellent formability according to claim 1, comprising one or more of the following. % By mass
Nb: 0.001 to 0.10%,
Ti: 0.001 to 0.10%,
V: 0.001 to 0.10%,
W: 0.001 to 0.10%,
The high-strength steel sheet excellent in formability according to claim 1 or 2, characterized by containing one or more of the following. % By mass
The high-strength steel sheet having excellent formability according to any one of claims 1 to 3, wherein B: 0.0003 to 0.003% or less is contained. Contains one or more of Ca, REM, Mg, Zr,
% By mass
Ca + REM + Mg + Zr: 0.0005% to 0.05%, The high-strength steel sheet with excellent formability according to any one of claims 1 to 4.   The high-strength steel sheet having excellent formability according to any one of claims 1 to 5, which is an alloyed hot-dip galvanized steel sheet. It is a manufacturing method of the high strength steel plate of any one of Claims 1-5, Comprising: The slab which has the chemical composition of any one of Claims 1-5 at a finishing temperature of 850 degreeC or more. Hot-rolled, cooled to 350 to 600 ° C. and wound, then subjected to 40 to 85% cold rolling, heated to Ac 1 to Ac 3 and annealed, cooled at 10 to 200 ° C./s, 350 Hold for 60 to 900 s in a temperature range of ˜450 ° C., further cool down, average area A (μm ² ) of pearlite in the structure after hot rolling, average heating temperature T (° C.) of annealing, and heating of annealing A method for producing a high-strength steel sheet having excellent ductility, characterized in that the time t (s) satisfies the relationship of the following formula (1).
T × log (t) / α> 110 Formula (1)
Where α = (1 + 0.3Si + 0.5Al + Cr + 0.5A) It is a manufacturing method of the high strength steel plate of Claim 6, Comprising: The hot rolling, cooling, and winding which have the chemical composition of any one of Claims 1-5 as described in Claim 7 After cold rolling,
After holding at a temperature range of Ac1 to Ac3 for 10 to 300 s, cooling at 3 to 200 ° C./s, holding at 15 to 1200 s at a temperature range of 350 to 550 ° C., hot dip galvanizing, 470 to Alloying is performed at 600 ° C., further cooling, the average area A (μm ² ) of pearlite in the structure after the hot rolling, the average heating temperature T (° C.) for annealing, and the heating time t (s for annealing). ) Satisfies the relationship of the following formula (1): A method for producing a high-strength steel sheet having excellent formability.
T × log (t) / α> 110 Formula (1)
Where α = (1 + 0.3Si + 0.5Al + Cr + 0.5A)

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