JP5521444B2 - High-strength cold-rolled steel sheet with excellent workability and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength cold-rolled steel sheet suitable for use in automobile parts and the like that are press-formed into a strict shape and a method for producing the same, reducing the content of C that hinders workability and weldability, Cu, Ni, Without actively containing expensive elements such as Cr and Mo, it is intended to achieve both excellent workability and high strength such as yield ratio: 0.50 or more and 0.80 or less and tensile strength (TS): 1180 MPa or more. is there.

  In recent years, the application of high-strength steel sheets in automobile parts has been expanding from the viewpoint of improving fuel efficiency and ensuring collision safety by reducing the weight of the vehicle body. However, many automobile parts are manufactured through processing steps such as press molding and burring, and steel sheets tend to have low elongation (ductility) and stretch flangeability as their strength increases. Therefore, when a high-strength steel plate is applied to a part that requires complicated press forming or burring, cracking during processing becomes a problem. Further, when the yield point increases with the increase in strength of the steel sheet, there arises a problem that the shape freezing property after press forming is lowered. Furthermore, in steel plates of 1180 MPa class or higher, it is usually possible to increase the strength by increasing the amount of additive elements such as C. However, as the amount of added elements such as C increases, the weldability decreases, so automobile parts are welded together. It becomes a problem when integrating. Therefore, in order to apply a high-strength steel sheet to automobile parts, it is essential to develop a high-strength steel sheet having not only desired strength but also elongation, stretch flangeability, low yield ratio, and weldability.

  For example, Patent Documents 1 to 5 describe, as conventional technologies related to high-strength steel sheets with excellent workability, high elongation and stretch flangeability by limiting steel components and structures, optimizing hot rolling conditions, and annealing conditions. A technique for obtaining a strength cold-rolled steel sheet is disclosed.

JP 2003-193193 A JP 2004-332099 A JP 2005-163055 A JP 2007-9269 A JP 2008-297592 A

  However, since the steel sheet described in Patent Document 1 has a structure having bainite or bainitic ferrite as a main phase, in order to achieve a tensile strength of 1180 MPa class or more, a large amount of expensive C, Mo, Nb, etc. It is necessary to contain alloying elements. Moreover, since the steel sheet described in Patent Document 2 also has a structure having bainite or bainitic ferrite as a main phase, expensive alloy elements such as Ni, Cu, Cr, and Mo are essential for achieving high strength. To do. The steel sheets described in Patent Document 3, Patent Document 4 and Patent Document 5 do not require an expensive alloy additive element, but require a tempering process, so that the tempering process takes time and labor, and the manufacturing cost increases.

The present invention advantageously solves the above-mentioned problems found in the prior art, reduces the content of C that impairs weldability, and eliminates expensive alloy elements such as Cu, Ni, Cr, and Mo. High strength cold rolling with low yield ratio and high strength of yield ratio: 0.50 or more and 0.80 or less, tensile strength (TS): 1180 MPa or more, and excellent elongation and stretch flangeability The object is to provide a steel sheet together with its advantageous production method.
In the present invention, “excellent in elongation and stretch flangeability” means that TS × El ≧ 16000 MPa ·% and TS × λ ≧ 25000 MPa ·% are satisfied.

  The present inventors diligently studied to solve the above problems. As a result, even if the steel sheet has a component composition that reduces the C content from the viewpoint of workability and weldability and does not actively add expensive alloy elements such as Ni, Cu, Cr, Mo, etc. It was found that by optimizing, desired properties including tensile strength of 1180 MPa or more can be obtained.

The present invention has been made based on the above findings, and the gist thereof is as follows.
(1) By mass%,
C: 0.10% to 0.15%,
Si: 1.0% to 2.0%,
Mn: 2.0% to 3.0%,
P: 0.030% or less,
S: 0.0050% or less,
Al: 0.005% to 0.1%,
N: 0.01% or less,
Ti: 0.005% to 0.050% and
B: Ferrite phase containing 0.0001% or more and 0.0050% or less, with the balance being composed of Fe and inevitable impurities, volume fraction: 35% or more and 65% or less, and average grain size: 1 μm or more and 10 μm or less Volume fraction: 15% to 45% and average crystal grain size: 1 to 10 μm bainite phase and volume fraction: 5% to 25% and average crystal grain size: 0.5 to 5 μm martensite phase A high-strength cold-rolled steel sheet excellent in workability, characterized in that it has a structure consisting of : Yield ratio: 0.50 or more and 0.80 or less and Tensile strength: 1180 MPa or more.

(2) A method for producing a high-strength cold-rolled steel sheet according to (1) above, wherein a steel slab having the component composition described in (1) above is hot-rolled, pickled, and rolling rate: 20 The steel sheet is cold-rolled by cold rolling at a rate of not less than 50% and not more than 50%. The cold-rolled steel sheet is annealed in a temperature range of 780 ° C. or more and less than _AC3 point, and then a cooling rate: 10 ° C./second or more and 80 ° C. It is cooled to a cooling stop temperature range of Ms point above 550 ° C or less at / second or less, and the temperature drop from the cooling stop temperature is kept in the temperature range of 0 to 100 ° C for 100 seconds or more and 1000 seconds or less. The manufacturing method of the high-strength cold-rolled steel plate excellent in workability.

  According to the present invention, a high-strength cold-rolled steel sheet having a yield ratio of 0.50 or more and 0.80 or less and a tensile strength of 1180 MPa or more, which is excellent in elongation and stretch flangeability without impairing weldability, is manufactured without incurring high costs. can do. Therefore, the high-strength cold-rolled steel sheet obtained by the present invention is suitable as an automobile part that is press-formed and welded to a particularly severe shape, and is extremely useful for reducing the weight of an automobile body.

 The reasons for limiting the component composition and structure of the present invention will be described below. The unit of the element content in the steel sheet is “% by mass”, but hereinafter, it is simply indicated by “%” unless otherwise specified.

First, the appropriate range of the chemical component (composition) of steel in the present invention and the reasons for its limitation are as follows.
C: 0.10% to 0.15%
C is an essential element for forming the bainite phase and the martensite phase, which are low-temperature transformation phases, and contributes to increasing the strength of the steel sheet.In the present invention, this C amount is from the viewpoint of workability and weldability. Less than before. That is, if the C content exceeds 0.15%, the steel sheet becomes excessively hard, and it becomes difficult to achieve excellent elongation and stretch flangeability. Moreover, if welding is performed on a steel plate with a large amount of C, the welded part and the weld heat-affected zone become excessively hard, causing problems such as weld cracking. However, if the C content is less than 0.10%, it is difficult to ensure the strength. Therefore, the C content is in the range of 0.10% to 0.15%. Preferably, it is 0.11% or more and 0.14% or less of range.

Si: 1.0% to 2.0%
Si is a ferrite forming element and is an essential element for generating a desired amount of ferrite. Si is an element that reduces the solid solution C in the ferrite phase and contributes to the improvement of elongation. Further, Si is an element effective in improving stretch flangeability because it has a solid solution strengthening ability of the ferrite phase and reduces the difference in hardness between the ferrite and the low temperature transformation phase. In order to obtain these effects, the Si content needs to be 1.0% or more. On the other hand, when the Si content exceeds 2.0%, the steel sheet becomes brittle. Therefore, when a member is manufactured by forming the steel sheet into a desired shape, a crack at the time of forming is caused. For this reason, the Si content is in the range of 1.0% to 2.0%. Preferably, it is 1.1% or more and 1.6% or less of range.

Mn: 2.0% to 3.0%
Mn is an element that enhances hardenability and contributes to increasing the strength of the steel sheet, and this effect is recognized when the Mn content is 2.0% or more. On the other hand, if the amount of Mn exceeds 3.0%, the hardenability is excessively increased, it becomes difficult to secure a desired ferrite phase, and the elongation decreases. Therefore, the Mn content is in the range of 2.0% to 3.0%. Preferably it is 2.4 to 2.9% of range.

P: 0.030% or less
P adversely affects weldability, so it is preferable to reduce it as much as possible in the present invention, but up to 0.030% is acceptable. Preferably it is less than 0.015%. In addition, excessively reducing the amount of P leads to an increase in melting cost, which is economically disadvantageous. Therefore, the lower limit of the amount of P is preferably about 0.001%.

S: 0.0050% or less
S forms MnS in steel to form plate-like inclusions, and reduces stretch flangeability. Therefore, in the present invention, it is preferable to reduce the amount of S as much as possible. However, the above problem does not become apparent if the amount of S is 0.0050% or less. Preferably it is 0.0030% or less. In addition, excessive reduction of the amount of S causes an increase in desulfurization cost in the steel making process, so the lower limit of the amount of S is preferably about 0.0001%.

Al: 0.005% to 0.1%
Al is used as a deoxidizer. In order to obtain a deoxidizing action, the Al amount needs to be 0.005% or more. However, if the Al amount exceeds 0.1%, the component cost increases. Therefore, the Al content is in the range of 0.005% to 0.1%. Preferably it is 0.02% or more and 0.06% or less of range.

N: 0.01% or less In the present invention, N is an impurity and is preferably reduced as much as possible, but it is acceptable up to 0.01%. Preferably it is 0.0050% or less. Note that, if the N amount is excessively reduced, the melting cost increases, which is economically disadvantageous. Therefore, the lower limit of the N amount is preferably about 0.0001%.

Ti: 0.005% to 0.050%
Ti combines with C and N in steel to form fine carbides, fine nitrides, and fine carbonitrides, thereby suppressing coarsening of crystal grains during heating. It works effectively for uniformizing the fine grain of the steel sheet structure. It is also effective in suppressing the formation of B nitride by forming nitride and ensuring the hardenability by adding B described later. In order to exhibit these effects, the Ti content needs to be 0.005% or more. However, when the Ti content exceeds 0.050%, these effects tend to be saturated. Moreover, when Ti is contained excessively, precipitates are generated excessively in the ferrite phase, the ductility of the ferrite phase is lowered, and further, the rolling load at the time of hot rolling and cold rolling increases due to the hardening of the steel sheet. . Therefore, the Ti content is in the range of 0.005% to 0.050%. Preferably it is 0.010% or more and 0.040% or less of range.

B: 0.0001% or more and 0.0050% or less
B enhances hardenability, suppresses excessive formation of a ferrite phase in the cooling process after annealing, and contributes to obtaining a desired amount of bainite and martensite. In order to exhibit such an effect, the B amount needs to be 0.0001% or more. On the other hand, when the amount of B exceeds 0.0050%, the above effect is saturated. Therefore, the B content is in the range of 0.0001% to 0.0050%. Preferably it is 0.0005% or more and 0.0020% or less of range.
In the steel sheet of the present invention, components other than those described above are Fe and inevitable impurities. However, as long as the effects of the present invention are not impaired, the inclusion of components other than those described above is not rejected.

Next, the steel structure, which is one of the important requirements for the present invention, and the reason for the limitation will be described.
Volume fraction of ferrite phase: 35% or more and 65% or less Since the soft ferrite phase contributes to the improvement of the elongation of the steel sheet, in the present invention, the ferrite phase is contained in a volume fraction of 35% or more. On the other hand, if the ferrite phase exceeds 65% in volume fraction, it may be difficult to ensure a tensile strength of 1180 MPa, depending on the combination with the type of low-temperature transformation phase. Therefore, the ferrite phase is 35% or more and 65% or less in volume fraction. Preferably they are 40% or more and 60% or less.

Average grain size of ferrite phase: 1 μm or more and 10 μm or less When the average grain size of ferrite phase is less than 1 μm, hard low-temperature transformation phases (bainite phase and martensite phase) exist in close proximity, and the desired steel plate When forming into a shape, the contribution of the low-temperature transformation phase to the deformability at the time of molding (during processing) increases, so it becomes difficult to ensure excellent moldability (workability). On the other hand, if the average grain size of the ferrite phase exceeds 10 μm and becomes too coarse, a non-uniform structure is formed, resulting in non-uniform deformation during molding (processing) and ensuring excellent moldability. It becomes difficult. Therefore, the average crystal grain size of the ferrite phase is 1 μm or more and 10 μm or less.

The volume fraction of the bainite phase: 15% or more and 45% or less The bainite phase and martensite phase, which are low-temperature transformation phases from the austenite phase, need to contain predetermined amounts to ensure strength, but these amounts are excessive. When it becomes, it will become high strength too much and workability (elongation, stretch flangeability) will fall. In addition, in order to balance the elongation and stretch flangeability with the tensile strength, the martensite phase should not be excessively contained, and the low-temperature transformation phase should be mainly composed of the bainite phase, that is, the volume fraction of the bainite phase should be martensite. It is preferable to make it larger than the volume fraction of the phase.
The bainite phase, which is a low temperature transformation phase from the austenite phase, is softer than the martensite phase, which is also a low temperature transformation phase, and is smaller than the martensite phase in terms of the hardness difference from the ferrite phase. Therefore, when the bainite phase is positively used, when the steel sheet is formed into a desired shape, the whole steel sheet is uniformly stretched during forming (during processing), and it is advantageous for stretch flangeability. In order to express these effects, in the present invention, the bainite phase is contained in a volume fraction of 15% or more. On the other hand, if the bainite phase exceeds 45% in volume fraction, the steel sheet becomes excessively hard and it becomes difficult to ensure elongation. Therefore, the bainite phase is 15% to 45% in terms of volume fraction. Preferably, it is 20% or more and 40% or less.

Average crystal grain size of bainite phase: 1μm or more and 10μm or less When the average crystal grain size of bainite phase is less than 1μm, the harder bainite phase is finely dispersed than the ferrite phase, which affects the deformability during processing. The contribution of the bainite phase is increased, the strength is excessively increased due to the fine graining, and it becomes difficult to ensure excellent moldability (workability). On the other hand, when the average crystal grain size of the bainite phase exceeds 10 μm and becomes excessively coarse, a non-uniform structure is formed, and it becomes a factor that inhibits uniform deformation during molding, so that it is difficult to ensure excellent moldability. Therefore, the average crystal grain size of the bainite phase is 1 μm or more and 10 μm or less.

Martensite volume fraction: 5% or more and 25% or less Strength and workability (elongation, stretch flangeability) by making the structure containing martensite phase within the range of volume fraction: 5% or more and 25% or less ) And a good material balance. When the martensite phase is less than 5% in volume fraction, it becomes difficult to secure a tensile strength (TS) of 1180 MPa. On the other hand, when the martensite phase is more than 25% in volume fraction, the steel sheet becomes excessively strong, the elongation is remarkably reduced, and it is difficult to ensure excellent stretch flangeability due to the hardness difference from the ferrite phase. It becomes. Therefore, the martensite phase is 5% to 25% in terms of volume fraction. Preferably, it is 10% or more and 20% or less.

Average grain size of martensite phase: 0.5 μm or more and 5 μm or less When the average grain size of the martensite phase is less than 0.5 μm, the hard martensite phase is finely dispersed in the ferrite matrix or adjacent to the bainite phase For this reason, voids are frequently generated at the interface between the ferrite phase or bainite phase and the martensite phase during molding, and elongation and stretch flangeability are deteriorated. In addition, it becomes difficult to secure an excellent moldability by excessively increasing the strength due to the finer particles.
On the other hand, if the average crystal grain size of the martensite phase is excessively coarsened exceeding 5 μm, it becomes a non-uniform structure, and it becomes difficult to ensure excellent formability as a factor that inhibits uniform deformation during molding. . Therefore, the average crystal grain size of the martensite phase is 0.5 μm or more and 5 μm or less. Preferably they are 1 micrometer or more and 3 micrometers or less.
As for the remaining structure other than the ferrite phase, bainite phase, and martensite phase, if the residual austenite phase and cementite that are inevitably generated are 3% or less in total volume fraction, the effect of the present invention is not impaired. Absent.

Next, the manufacturing method of the high-strength cold-rolled steel sheet of this invention is demonstrated.
The steel slab having the above component composition, hot rolling, pickling, rolling rate: 20% to 50% of the subjected to cold rolling and cold-rolled steel sheet, the cold-rolled steel sheet 780 ° C. or higher A _C3 After annealing in a temperature range below the point, the cooling rate is 10 ° C / second to 80 ° C / second, cooling to the cooling stop temperature range above the Ms point and below 550 ° C, and the temperature drop from the cooling stop temperature The amount is kept in the temperature range of 0 to 100 ° C. for not less than 100 seconds and not more than 1000 seconds. Although the high-strength cold-rolled steel sheet that is the object of the present invention is obtained by such a manufacturing method, the steel sheet may be subjected to skin pass rolling.
Hereinafter, the appropriate range of manufacturing conditions and the reason for limitation will be described.

  In the present invention, the process before cold rolling may be carried out in accordance with a conventional method. For example, a steel slab obtained by melting and casting steel prepared in the above component composition range is cast or reheated. After that, finish rolling is performed in a temperature range of 850 ° C. or higher and 950 ° C. or lower, and then rolled into a hot rolled steel sheet in a temperature range of 450 ° C. or higher and 650 ° C. or lower. The reheating temperature need not be specified, but it must be reheated under conditions that can secure the finish rolling temperature, and is generally 1100 ° C. or higher and 1300 ° C. or lower. Next, the hot-rolled steel sheet is pickled and then subjected to cold rolling.

Cold rolling rate: 20% or more and 50% or less The rolling rate of cold rolling is important for controlling the low temperature transformation phase (bainite phase and martensite phase) and controlling the crystal grain size. When the rolling rate is less than 20%, strain is not uniformly introduced into the steel sheet, resulting in variations in the progress of recrystallization in the steel sheet, resulting in a mixed grain structure in which coarse grains and fine grains are present. The crystal grain size becomes coarse. On the other hand, when the rolling rate exceeds 50%, recrystallization proceeds rapidly and grain growth is promoted, so that the crystal grain size becomes coarse. Therefore, the cold rolling rate is in the range of 20% to 50%.

Annealing Temperature: If the annealing temperature lower than 780 ° C. or higher A _C3 point is lower than 780 ° C., due to the ferrite fraction during annealing increases, finally obtained ferrite phase becomes excessive after annealing, the tensile strength : It is difficult to secure 1180MPa or more. Moreover, in the steel plate finally obtained, the strain introduced by cold working is present without being recovered, or non-recrystallized ferrite is present, and the workability (elongation, stretch flangeability) of the steel plate is deteriorated. Show the trend. On the other hand, when heated to a temperature range of A _C3 point or more austenite single phase, and austenite grain size is too coarse, the amount of ferrite phase formed in the subsequent cooling process is reduced, elongation decreases. Further, the crystal grain size of the ferrite phase and the low-temperature transformation phase (bainite phase and martensite phase) becomes coarse, and the stretch flangeability deteriorates. Therefore, the annealing temperature is in the range of 780 ° C. or more and less than _{AC 3} point. The annealing treatment time is preferably about 20 seconds to 200 seconds.
The _AC3 point may be actually measured, but may be calculated using the following equation (1). The element symbol in the formula (1) is the content (mass%) of each element.
A _C3 (℃) = 910-203√C + 44.7Si-30Mn + 700P + 400Al + 400Ti… (1)

Cooling rate: 10 ° C / second or more and 80 ° C / second or less The cooling rate after annealing controls the volume fraction of the soft ferrite phase and the hard low-temperature transformation phase, ensuring a tensile strength and workability of 1180 MPa class or higher. It is important to do. If the average cooling rate exceeds 80 ° C / second, ferrite formation during cooling is suppressed, and the volume fraction of the low-temperature transformation phase, the bainite phase and the martensite phase, increases. Although easy, moldability (workability) is deteriorated. On the other hand, if it is less than 10 ° C./second, the amount of the ferrite phase generated during the cooling process becomes excessive, and it becomes difficult to ensure the tensile strength. Therefore, the cooling rate after annealing is set to 10 ° C./second or more and 80 ° C./second or less. Preferably, it is 10 ° C./second or more and 40 ° C./second or less. In order to control the cooling rate, it is preferable to cool the steel plate after the annealing treatment by gas cooling, but it is also possible to cool by combining furnace cooling, mist cooling, roll cooling, water cooling, etc. .

Cooling stop temperature: Ms point above 550 ° C. or less In the present invention steel, a large amount of Si, which is a ferrite stable element, is added, so that a ferrite phase is easily generated during cooling under the cooling rate control. Here, when the cooling stop temperature is equal to or lower than the Ms point, the cooling time under the cooling rate control is longer than that in the case where the cooling is stopped after the Ms point is exceeded, and the amount of ferrite phase generated tends to increase. Further, when the cooling stop temperature is equal to or lower than the Ms point, the low temperature transformation phase from the austenite phase is mainly the martensite phase. For this reason, if the cooling stop temperature is below the Ms point, the strength tends to be insufficient due to an increase in the amount of ferrite phase produced, and stretch flangeability due to the hardness difference between the ferrite phase and the martensite phase. Decreases. On the other hand, when the cooling stop temperature exceeds 550 ° C., the cooling time under the cooling rate control is relatively short, the amount of ferrite phase generated is decreased, and the amount of bainite phase generated is likely to be increased. For this reason, it becomes too strong and it becomes difficult to ensure elongation.
For the above reasons, in the present invention, the volume fraction of the bainite phase and the martensite phase is controlled to ensure a tensile strength of 1180 MPa class or higher and to achieve excellent workability, so that the cooling stop temperature exceeds the Ms point. The range is 550 ° C or less.
The value of Ms point is obtained by creating a continuous cooling transformation diagram (CCT diagram) showing the state of transformation that occurs in the process of continuous cooling from the austenite state at a cooling rate of 0.3 to 100 ° C./second, for example. However, it may be obtained by the following equation (2) which is an empirical equation. The element symbol in the formula (2) is the content (mass%) of each element.
Ms (℃) = 550-360C-40Mn (2)

Steel plate residence temperature: Temperature range of 0 to 100 ° C. temperature drop from cooling stop temperature The residence temperature of the steel plate after cooling stop is important in controlling the volume fraction of the bainite phase and martensite phase. When the steel sheet after cooling stop is retained for a predetermined time, the untransformed austenite phase transforms into a bainite phase and a martensite phase, which are low temperature transformation phases, but when the steel sheet temperature during retention falls below 100 ° C, the martensite phase May form excessively. Moreover, the residence temperature of the steel plate after cooling is stopped is important for ensuring the homogeneity of the bainite phase. When the residence temperature fluctuates significantly, the hardness of the bainite phase to be generated varies, and when the steel sheet is formed into a desired shape, there is a risk of causing non-uniform deformation. Further, there is a concern that part of the bainite phase is excessively cured and the workability is deteriorated. Therefore, the steel plate residence temperature is set to a temperature range of 0 to 100 ° C. from the cooling stop temperature.

Steel plate residence time: 100 seconds or more and 1000 seconds or less The residence time after stopping cooling is important in controlling the fraction of the bainite phase and the martensite phase. When the residence time is less than 100 seconds, the bainite transformation during residence becomes insufficient. For this reason, the untransformed austenite phase becomes a martensite phase after retention and the martensite phase becomes excessive. As a result, the steel sheet becomes excessively strong and the formability is lowered. On the other hand, if the residence time exceeds 1000 seconds, the bainite phase becomes excessive and the amount of martensite phase produced becomes insufficient. As a result, the steel sheet becomes soft and it becomes difficult to ensure the tensile strength. In order to secure a tensile strength of 1180 MPa or higher and achieve excellent workability, the residence time needs to be in the range of 100 seconds to 1000 seconds. Preferably, it is 150 seconds or more and 750 seconds or less.

  Examples of the means for holding the steel plate after stopping the cooling at the above residence temperature include a means for adjusting the temperature of the steel plate to the above residence temperature by providing a heat retaining device or the like in the lower process of the cooling equipment after annealing. However, if the steel plate temperature drop from the cooling stop temperature falls within the temperature range of 0 to 100 ° C even if the steel plate after the cooling stop is allowed to cool, the special means such as the heat retaining device is not provided. May be. In addition, the steel plate after residence is cooled to desired temperature by arbitrary methods.

  After the above annealing, the cold-rolled steel sheet finally obtained may be subjected to temper rolling (skin pass rolling) for the purpose of shape correction or surface roughness adjustment. However, excessive skin pass rolling introduces strain. Since the crystal grains are stretched to form a rolled structure and the ductility is lowered, the reduction rate of the skin pass rolling is preferably about 0.05% to 0.5%.

  According to the present invention, a low yield ratio of 0.50 or more and 0.80 or less can be obtained despite the high tensile strength of 1180 MPa or more. The reason is not clear, but is presumed to be due to the optimization of the volume fraction of the bainite phase and the martensite phase. In the case of a structure composed of a ferrite phase and a martensite phase, when transforming from the austenite phase to the martensite phase, dislocations are easily introduced into the ferrite phase, and as a result of increasing the number of movable dislocations, the yield ratio tends to decrease. . On the other hand, in the case of a structure composed of a ferrite phase and a bainite phase, when transforming from an austenite phase to a bainite phase, it is difficult for dislocations to be introduced into the ferrite phase, and as a result of less mobile dislocations, the yield ratio tends to increase. . In the present invention, it is presumed that a desired yield ratio is achieved by controlling the volume fractions of the bainite phase and the martensite phase within an appropriate range. More preferably, the yield ratio is in the range of 0.55 to 0.70.

Example 1
A steel having the composition shown in Table 1 is melted to form a slab, which is subjected to hot rolling at a slab heating temperature of 1200 ° C., a finish rolling temperature of 900 ° C. and a winding temperature of 550 ° C., and subsequently pickled with hydrochloric acid. Cold rolling and annealing were performed under the conditions shown in Table 2 to produce a cold rolled steel sheet having a thickness of 1.6 mm. About the obtained cold-rolled steel plate, the material characteristic was investigated by the material test shown below. The _AC3 point and Ms point shown in Table 1 were obtained from the above formulas (1) and (2), respectively.
The results obtained are shown in Table 3.

(1) Structure of steel sheet: Investigation was made by observing the cross section in the rolling direction and the position of the 1/4 thickness with an optical microscope or a scanning electron microscope (SEM). Observation was performed at a magnification of 1000 to 3000 times and N = 5 (5 observation fields). The crystal grain size of the ferrite phase was calculated according to the method (cutting method) defined in JIS G 0552 (1998) and converted to an average grain size. It measured similarly about the bainite phase and the martensite phase. The volume fraction of the ferrite phase, bainite phase, and martensite phase is the area occupied by each phase existing in a 100 mm x 100 mm square area arbitrarily set by image analysis using a cross-sectional structure photograph at a magnification of 1000 times. The volume fraction of each phase was determined. The distinction between the bainite phase and the martensite phase is made using a cross-sectional structure photograph at a magnification of 3000 times. Was determined to be a martensitic phase.

(2) Tensile properties: Using No. 5 test piece described in JIS Z 2201, with the rolling direction and 90 ° as the longitudinal direction (tensile direction), a tensile test based on JIS Z 2241 was performed and evaluated. The evaluation criteria for tensile properties were TS × El ≧ 16000 MPa ·% or more (TS: tensile strength (MPa), El: total elongation (%)).

(3) Stretch flangeability (hole expansion rate): Executed based on the Japan Iron and Steel Federation standard JFS T 1001. When a hole with an initial diameter d ₀ = 10 mm was punched and the 60 ° conical punch was raised to widen the hole, the punch was stopped when the crack penetrated the plate thickness, and the punched hole diameter d after crack penetration was measured, hole expansion _{ratio: ((d- d 0) /} d 0) was calculated as × 100 (%). The same number of steel sheets was tested three times, and the average value (λ) of the hole expansion rate was obtained. The evaluation criteria for stretch flangeability (hole expansion rate) was TS × λ ≧ 25000 MPa ·% or more.

From Table 3, it can be seen that in the present invention example, a high-strength cold-rolled steel sheet excellent in workability that satisfies TS × El ≧ 16000 MPa ·% and TS × λ ≧ 25000 MPa ·% or more is obtained.
On the other hand, J, K, and L whose steel components are outside the scope of the present invention cannot control the desired structure, and are inferior in workability (elongation). M and N, which have a cold rolling rate outside the range of the present invention, were inferior in elongation (ductility) and stretch flangeability due to the coarse average crystal grain size. The annealing temperature is less than O of the present invention, the cooling rate is less than Q of the present invention, the cooling stop temperature is lower than the lower limit of the present invention, and the volume fraction of the ferrite phase is high, and the residence time exceeds the present invention. W had a low volume fraction of martensite phase, and none of them satisfied the tensile strength of 1180 MPa.
The annealing temperature exceeds the present invention P, the cooling rate exceeds the present invention R, the cooling stop temperature exceeds the present invention T, the temperature drop from the cooling stop temperature exceeds the present invention U, the residence time satisfies the present invention. No V had a high volume fraction of the bainite phase or martensite phase and a good tensile strength, but was inferior in workability (elongation and stretch flangeability).

  According to the present invention, the volume fraction and average crystal of each of the ferrite phase, the bainite phase, and the martensite phase are reduced without actively containing expensive elements such as Cu, Ni, Cr, and Mo in the steel sheet. By specifying the grain size, it is possible to obtain a high-strength cold-rolled steel sheet with excellent workability at a yield ratio of 0.50 or more and 0.80 or less, and a tensile strength (TS) of 1180 MPa or more under low cost and excellent weldability. it can. The high-strength cold-rolled steel sheet according to the present invention is particularly suitable for automobile parts that are press-formed into a severe shape and welded. However, in addition to automobile parts, severe dimensional accuracy and workability are required in the fields of architecture and home appliances. It is also suitable for the intended use.

Claims (2)

% By mass
C: 0.10% to 0.15%,
Si: 1.0% to 2.0%,
Mn: 2.0% to 3.0%,
P: 0.030% or less,
S: 0.0050% or less,
Al: 0.005% to 0.1%,
N: 0.01% or less,
Ti: 0.005% to 0.050% and
B: Ferrite phase containing 0.0001% or more and 0.0050% or less, with the balance being composed of Fe and inevitable impurities, volume fraction: 35% or more and 65% or less, and average grain size: 1 μm or more and 10 μm or less Volume fraction: 15% to 45% and average crystal grain size: 1 to 10 μm bainite phase and volume fraction: 5% to 25% and average crystal grain size: 0.5 to 5 μm martensite phase A high-strength cold-rolled steel sheet excellent in workability, characterized in that it has a structure consisting of : Yield ratio: 0.50 or more and 0.80 or less and Tensile strength: 1180 MPa or more. A method for producing a high-strength cold-rolled steel sheet according to claim 1, wherein the steel slab having the component composition according to claim 1 is hot-rolled, pickled, and rolling rate: 20% to 50% After cold-rolling to obtain a cold-rolled steel sheet, the cold-rolled steel sheet was annealed at a temperature range of 780 ° C or higher and less than _AC3 point, and then cooled at a cooling rate of 10 ° C / second or higher and 80 ° C / second or lower. Cooling to a cooling stop temperature range of over 550 ° C or less above the point, and the temperature drop from the cooling stop temperature is retained in the temperature range of 0 to 100 ° C for 100 seconds or more and 1000 seconds or less, and excellent workability A method for producing high strength cold-rolled steel sheets.