JP2013249501A - High-strength cold-rolled steel plate with minimized dispersion of mechanical characteristics, and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength cold-rolled steel plate with minimized dispersion of characteristics, and to provide a method for manufacturing the same.SOLUTION: A high-strength cold-rolled steel plate has a composition including, by mass, C: 0.05-0.30%, Si: 3.0% or less, Mn: 0.1-5.0%, P: 0.1% or less, S: 0.02% or less, Al: 0.01-1.0%, N: 0.01% or less, and the balance iron and unavoidable impurities; and has a structure composed of tempered martensite and/or tempered bainite, which includes 20-50% of ferrite that is a soft first phase by area ratio with the remainder being a hard second phase. In the steel plate, a difference ΔVα=Vαs-Vαc between the area ratio Vαs of ferrite in a steel plate surface layer portion from the steel plate surface to a depth of 100 μm and the area ratio Vαc of a central portion of t/4-3t/4 (t is the plate thickness) is less than 10%, and a ratio RHv=Hvs/Hvc of the hardness Hvs of the steel surface layer portion to the hardness Hvc of the central portion is 0.75-1.0.

Description

  The present invention relates to a high-strength steel sheet having a small variation in mechanical properties used for automobile parts and the like, and a method for producing the same.

  In recent years, there has been an increasing need for high-strength steel sheets having a tensile strength of 590 MPa or more as a material for structural parts in order to achieve both fuel efficiency improvement and collision safety of automobiles, and the application range has been expanded. However, high-strength steel sheets have larger variations in mechanical properties such as yield strength, tensile strength, work hardening index, etc. compared to mild steel, so the amount of springback during press forming changes the dimensional accuracy of the press-formed product. It is difficult to secure the press mold, and even if the strength varies, it is necessary to set the average strength of the steel sheet higher in order to ensure the required strength of the press-formed product. There are challenges.

  In order to solve such problems, various efforts have been made for suppressing variation in mechanical properties of high-strength steel sheets. The cause of the variation in mechanical properties as described above in high-strength steel sheets can be found in the variation in chemical composition and the variation in manufacturing conditions. The following proposal has been made as a method for reducing the variation in mechanical properties. Yes.

[Prior art 1]
For example, in Patent Document 1, when A is defined as A = Si + 9 × Al, a ferrite and martensite dual phase steel satisfying 6.0 ≦ A ≦ 20.0, and this steel plate is manufactured. The recrystallization annealing / tempering treatment is held at a temperature of Ac1 or higher and Ac3 or lower for 10 s or more, slowly cooled to 500 to 750 ° C. at a cooling rate of 20 ° C. or lower, and then 100 ° C. or lower to 100 ° C. / The above two-phase structure when the rapid cooling start temperature, which is the temperature at the end of the slow cooling, fluctuates by raising the A3 point of the steel by quenching at a cooling rate of s or more and tempering at 300 to 500 ° C. A method for improving the stability of the material and reducing the variation in mechanical properties is disclosed.

[Prior Art 2]
In Patent Document 2, the thickness of the steel sheet, the carbon content, the phosphorus content, the quenching start temperature, the quenching stop temperature, the tempering temperature after quenching and the relationship between the tensile strength and the tensile strength are obtained in advance. Considering the carbon content, phosphorus content, quenching stop temperature, and tempering temperature after quenching, calculate the quenching start temperature according to the target tensile strength, and quenching at the obtained quenching start temperature, the variation in strength A method for reducing the above is disclosed.

[Prior Art 3]
Moreover, in patent document 3, in manufacturing the steel plate which has a structure | tissue containing 3% or more of retained austenite, in the annealing process after cold-rolling a hot-rolled steel plate, more than 800 degreeC and less than Ac3 point, 30 seconds-5 After soaking for 1 minute, primary cooling is performed to a temperature range of 450 to 550 ° C., then secondary cooling is performed at a cooling rate smaller than the primary cooling rate to 450 to 400 ° C., and further at 450 to 400 ° C. A method for improving variation in elongation characteristics in the plate width direction by holding for 1 minute or more is disclosed.

[Prior Art 4]
Patent Document 4 includes a ferrite phase having an average crystal grain size of 10 μm or less and a martensite phase having a volume fraction of 30 to 90%, and the ratio of sheet thickness surface layer hardness to sheet thickness center hardness is 0.6 to 1. There is a structure in which the maximum depth of cracks and recesses extending from the interface between the plating layer and the steel plate to the steel plate side is 0 to 20 μm, and the area ratio of the smooth part other than the cracks and recesses is 60% to 100%. Thus, a method for improving the drawability of a high-strength hot-dip galvanized steel sheet is disclosed.

  The above prior art 1 increases the Ac3 point by increasing the amount of Al added, thereby expanding the two-phase temperature range of Ac1 to Ac3, and reducing the temperature dependence in the two-phase temperature range, thereby reducing the annealing temperature. It is characterized by suppressing changes in the tissue fraction due to fluctuations in the size. On the other hand, the present invention is characterized by suppressing fluctuations in mechanical properties due to changes in heat treatment conditions by aligning the fraction and hardness of the steel sheet surface layer portion and the internal hard-soft phase. . Therefore, the prior art 1 does not suggest the technical idea of the present invention. Furthermore, since the prior art 1 needs to increase the amount of Al added, there is also a problem that the manufacturing cost of the steel sheet increases.

  In addition, since the prior art 2 changes the quenching temperature in accordance with the change in the chemical composition, even if the variation in strength can be reduced, the tissue fraction varies between the coils. It cannot be reduced.

  Moreover, although the said prior art 3 is mentioned about the reduction | decrease of the dispersion | variation in elongation, it is not suggested about the reduction | decrease of the dispersion | variation in stretch flangeability.

  Moreover, the said prior art 4 prescribes | regulates the average crystal grain diameter of a ferrite phase as 10 micrometers or less, and the steel sheet surface layer and center hardness ratio to 0.6-1 for the purpose of improving press moldability. . However, since the crystal grain size of the ferrite phase is defined only by the average value, if there is a large variation in the size of individual ferrite grains, improvement in press formability cannot be expected. Moreover, although the steel sheet surface layer and the center hardness ratio are prescribed | regulated, it cannot be said that hardness and the deformability of a hard soft phase correspond. For example, when the fraction of the tempered hard phase, which is inferior in deformability, is high, and when the fraction of the soft phase, which is excellent in deformability, is high, the press formability is different even if the hardness is the same. Is effective, but it is assumed that the degree of improvement will vary. On the other hand, the present invention is not intended to improve the press formability itself, but by reducing the difference between the structural fraction and hardness of the steel sheet surface layer part and the inside (center part), mechanical properties are reduced. It is characterized by suppressing variations in the above. Therefore, the prior art 4 does not suggest the technical idea of the present invention.

JP 2007-138262 A JP 2003-277832 A JP 2000-212684 A JP 2008-156734 A

  The present invention has been made to solve the above-described problems, and it is an object of the present invention to provide a high-strength cold-rolled steel sheet having a small variation in mechanical properties and a method for producing the same.

The invention described in claim 1
% By mass (hereinafter the same for chemical components)
C: 0.05 to 0.30%
Si: 3.0% or less (excluding 0%),
Mn: 0.1 to 5.0%,
P: 0.1% or less (excluding 0%),
S: 0.02% or less (excluding 0%),
Al: 0.01 to 1.0%,
N: 0.01% or less (excluding 0%)
Each having a component composition consisting of iron and inevitable impurities,
Including ferrite, which is a soft first phase, in an area ratio of 20 to 50%,
The balance is a hard second phase, and has a structure composed of tempered martensite and / or tempered bainite,
The difference ΔVα = Vαs−Vαc between the ferrite area ratio Vαs in the surface layer portion of the steel sheet from the steel sheet surface to the depth of 100 μm and the ferrite area ratio Vαc in the central portion of t / 4 to 3t / 4 (t is the plate thickness) is 10 The ratio RHv = Hvs / Hvc between the hardness Hvs of the steel sheet surface layer part and the hardness Hvc of the central part is 0.75 to 1.0. A small high-strength cold-rolled steel sheet.

The invention described in claim 2
Ingredient composition further
Cr: 0.01 to 1.0%
The high-strength cold-rolled steel sheet having a small variation in mechanical properties according to claim 1.

The invention according to claim 3
Ingredient composition further
Mo: 0.01 to 1.0%,
Cu: 0.05 to 1.0%,
The high-strength cold-rolled steel sheet having small variations in mechanical properties according to claim 1 or 2, comprising Ni: 0.05 to 1.0%, or one or more.

The invention according to claim 4
Ingredient composition further
Ca: 0.0001 to 0.01%,
Mg: 0.0001 to 0.01%
Li: 0.0001 to 0.01%
The high-strength cold-rolled steel sheet according to any one of claims 1 to 3, which includes one or more of REM: 0.0001 to 0.01%.

The invention described in claim 5
A steel material having the composition shown in any one of claims 1 to 4 is hot-rolled under the conditions shown in the following (1) to (4), then cold-rolled, and then annealed. A method for producing a high-strength cold-rolled steel sheet with small variations in mechanical properties, characterized by tempering.
(1) Hot rolling conditions Finish rolling finish temperature: Ar ₃ points or more Winding temperature: Over 600 ° C and 750 ° C or less
(2) Cold rolling conditions Cold rolling rate: Over 50% and below 80%
(3) Annealing conditions After holding at an annealing temperature of Ac1 or more and less than (Ac1 + Ac3) / 2 for an annealing holding time of 3600 s or less, the temperature from the annealing temperature to the first cooling end temperature of 730 ° C. or less to 500 ° C. or more is 1 ° C. After slow cooling at a first cooling rate of not less than / s and less than 50 ° C./s, rapid cooling is performed at a second cooling rate of not less than 50 ° C./s to a second cooling end temperature below the Ms point.
(4) Tempering conditions Tempering temperature: 300-500 ° C
Tempering holding time: 60 to 1200 s in the temperature range of 300 ° C. to tempering temperature

  According to the present invention, in a multiphase steel composed of ferrite, which is a soft first phase, and tempered martensite and / or tempered bainite, which is a hard second phase, the ferrite area ratio of the steel sheet surface layer portion and the central portion is By controlling both the difference and the hardness ratio within a predetermined range, it has become possible to provide a high-strength steel sheet having a small variation in mechanical characteristics and a method for producing the same.

It is a cross-sectional structure photograph of an invention steel plate and a comparative steel plate.

  In order to solve the above problems, the inventors of the present application collectively refer to ferrite as a soft first phase and tempered martensite and / or tempered bainite (hereinafter referred to as “tempered martensite etc.”) as a hard second phase. Focusing on a high-strength steel sheet having a multi-phase structure composed of Hereinafter, “mechanical characteristics” may be referred to as “characteristics” and “variations in mechanical characteristics” may be referred to as “characteristic variations”.

  In order to suppress variation in characteristics, from a microscopic viewpoint, the difference in hardness between the soft first phase (also simply referred to as “soft phase”) and the hard second phase (also simply referred to as “hard phase”) is obtained. It is effective to make it smaller. On the other hand, from a macro perspective, it is effective to reduce the difference in characteristics in the thickness direction of the steel sheet, that is, the difference in material.

  However, only by reducing the difference in hardness between the hard and soft phases from a microscopic viewpoint, the fraction of both phases changed due to the difference in deformability between the two phases as described in the prior art 4 above. In this case, characteristic variation occurs.

  Therefore, the inventors of the present application consider that it is more effective to suppress variation in characteristics by reducing the difference in material in the thickness direction of the steel sheet, that is, the difference in material in the thickness direction of the steel sheet. We proceeded with investigations on how to make it smaller.

  As specific means, it is effective to make the hardness of the hard and soft phases constituting the surface layer part and the inside (center part) and the hardness of the surface layer part and the inside (center part) as much as possible.

  By adopting such a structure, the same characteristics can always be exhibited when the characteristic evaluation method and the actual processing method are the same.

  However, it is difficult to obtain the above-described structure by a conventional general manufacturing method.

  For example, the following method can be considered to create the above-described organization form. That is, a combination of hot rolling in hot rolling, a high cold rolling rate, and annealing on the low temperature side of the two-phase region is effective. First, by increasing the coiling temperature after hot rolling, the size of the structure can be made large and uniform as a whole, and it is also effective for making a structure of only two phases of ferrite + pearlite (α + P). . Next, the amount of strain introduced into the surface layer portion and the inside can be made substantially equal by increasing the cold rolling rate during cold rolling and applying strong processing. If the cold rolling rate is low, the strain of the surface layer portion tends to increase compared to the inside, and the strain amount tends to be inclined in the thickness direction of the steel sheet. Even if the cold rolling rate is increased, the strain amount is inclined in the thickness direction of the steel sheet, but the influence can be minimized. In addition, high strain acts effectively in the subsequent annealing. That is, by applying high strain to the entire thickness direction of the steel sheet by cold rolling during annealing, nucleation of austenite is activated during heating, and a fine austenite structure is obtained. During soaking, ferrite precipitates from the grain boundary triple point of the fine austenite. Here, by setting the soaking temperature to the low temperature side of the two-phase region, a structure composed of relatively large ferrites and fine austenite having uniform sizes is formed. From there, by dropping the shoulder and cooling, the ferrite grows and becomes large, and new ferrite precipitates from the grain boundary triple point of the fine austenite. By making the structure before annealing fine in this way, both the surface layer and inside have different temperature histories, but nucleation is activated in both ferrite and austenite, so the same nucleation and growth behavior are shown. become. As a result, the fractions of the surface layer portion and the internal hard-soft phase are substantially equal, and the surface layer portion and the interior have the same structure size depending on the formation process of the structure, so the hardness is also approximately the same.

  The formability of the steel sheet having such a structure is almost the same under the same strain condition in the surface layer portion and inside, and exhibits excellent characteristic stability.

  And, as a result of conducting a verification test described in the following [Example] based on the above thought experiment, confirmation was obtained, so further investigation was made and the present invention was completed.

  Hereinafter, the structure characterizing the invention steel sheet will be described first.

[Invention steel sheet structure]
As described above, the invention steel plate is based on a multiphase structure composed of ferrite, which is a soft first phase, and tempered martensite, which is a hard second phase. The difference in ferrite fraction and the hardness ratio are controlled.

<Ferrite as soft first phase: 20 to 50% in area ratio>
In a multiphase steel such as ferrite-tempered martensite, deformation is mainly handled by ferrite having high deformability. Therefore, the elongation of the duplex steel such as ferrite-tempered martensite is mainly determined by the area ratio of ferrite.

  In order to ensure the target elongation, the area ratio of ferrite needs to be 20% or more (preferably 25% or more, more preferably 30% or more). However, since the strength cannot be secured when the ferrite is excessive, the area ratio of the ferrite is 50% or less (preferably 45% or less, more preferably 40% or less).

<Difference between the ferrite area ratio Vαs in the surface layer portion of the steel sheet from the steel sheet surface to a depth of 100 μm and the ferrite area ratio Vαc in the central portion of t / 4 to 3t / 4 (t is the plate thickness) ΔVα = Vαs−Vαc: <10%>
By aligning the ferrite fraction of the steel sheet surface layer and the internal as much as possible, in combination with the following steel sheet surface layer and the internal hardness, the material in the thickness direction of the steel sheet is made macroscopic and the characteristic variation is suppressed. It is to do. In order to obtain the above effect, the difference ΔVα in the area ratio between the steel sheet surface layer portion and the central portion of the ferrite needs to be less than 10% (preferably 8% or less, more preferably 6% or less).
Here, the reason why the steel plate surface layer portion is limited to a portion from the steel plate surface to a depth of 100 μm is that the structure is particularly easily changed by a general manufacturing method.

<Ratio of hardness Hvs of the steel sheet surface layer part and hardness Hvc of the central part RHv = Hvs / Hvc: 0.75 to 1.0>
By aligning the hardness of the steel sheet surface layer and the central part as much as possible, in combination with the above-mentioned steel sheet surface layer and the internal ferrite fraction, the material in the thickness direction of the steel sheet is made macroscopically uniform, resulting in characteristic variations. It is for suppressing. In order to acquire the said effect, hardness ratio RHv needs to be 0.75 or more (preferably 0.77 or more, more preferably 0.79 or more). However, when the hardness ratio RHv exceeds 1.0, for example, when the surface layer portion becomes harder than the inside as in the case of carburizing treatment, the characteristic variation increases.

  Hereinafter, the area ratio of each phase in the entire thickness of the steel sheet, the area ratio of ferrite in the steel sheet surface layer part and the center part, and each measuring method of the hardness in the steel sheet surface layer part and the center part will be described.

[Measurement method of area ratio of each phase in the whole steel sheet thickness]
First, regarding the area ratio of each phase in the entire thickness of the steel sheet, each test steel sheet was mirror-polished and corroded with 3% nital solution to reveal a metal structure, and then a magnification of 2000 for a field of view of approximately 40 μm × 30 μm. A double scanning electron microscope (SEM) image was observed, 100 points per field of view were measured by a point calculation method to determine the area of each ferrite grain, and these were summed to determine the area of ferrite. In addition, a region containing cementite was determined as a hard second phase by image analysis, and the remaining region was retained austenite, martensite, and a mixed structure of retained austenite and martensite. And the area ratio of each phase was computed from the area ratio of each area | region.

[Area ratio of ferrite in steel sheet surface layer and center]
The ferrite area ratio in the central portion is in the range of t / 4 to 3t / 4 (t is the plate thickness), in the same manner as in the above [Method for measuring the area ratio of each phase in the entire steel plate thickness]. The area ratio was determined.
On the other hand, about the area ratio of the ferrite in the steel sheet surface layer portion, in the range from the steel sheet surface to a depth of 30 μm, the same as in the above [Measurement method of area ratio of each phase in the entire thickness of the steel sheet] for 5 fields of about 30 μm × 40 μm region. Thus, the area ratio of ferrite was obtained.

[Method for measuring hardness in steel sheet surface layer and center]
Further, regarding the hardness in the steel plate surface layer portion and the center portion, the steel plate surface layer portion has a depth of 0.05 mm from the steel plate surface in a plate thickness section parallel to the rolling direction under the condition of a load of 100 g using a Vickers hardness tester. The center part is at a position of t / 4 (t: plate thickness), and the hardness of 5 points is measured in the direction perpendicular to the plate thickness direction, and the measured values of these 5 points are arithmetically averaged. Asked.

  Next, the component composition which comprises the invention steel plate of this application is demonstrated. Hereinafter, all the units of chemical components are mass%.

[Ingredient composition of invention steel plate]
C: 0.05-0.30%
C is an important element that affects the area ratio of the hard second phase, and consequently the area ratio of ferrite, and affects the strength, elongation, and stretch flangeability. If it is less than 0.05%, the strength cannot be secured. On the other hand, if it exceeds 0.30%, the weldability deteriorates. The range of C content is preferably 0.10 to 0.25%, more preferably 0.14 to 0.20%.

Si: 3.0% or less (excluding 0%),
Si has an effect of suppressing the coarsening of cementite particles during tempering, and is a useful element that contributes to both elongation and stretch flangeability. If it exceeds 3.0%, the formation of austenite at the time of heating is inhibited, so that the area ratio of the hard second phase cannot be ensured and stretch flangeability cannot be ensured. The range of Si content becomes like this. Preferably it is 0.50 to 2.5%, More preferably, it is 1.0 to 2.2%.

Mn: 0.1 to 5.0%
Mn contributes to both elongation and stretch flangeability by increasing the deformability of the hard second phase, in addition to having the effect of suppressing coarsening of cementite during tempering, similar to Si. Moreover, there exists an effect which expands the range of the manufacturing conditions from which a hard 2nd phase is obtained by improving hardenability. If the content is less than 0.1%, the above effects cannot be sufficiently exhibited, so that it is impossible to achieve both elongation and stretch flangeability. On the other hand, if it exceeds 5.0%, the reverse transformation temperature becomes too low and recrystallization becomes impossible. And the balance of growth cannot be secured. The range of Mn content is preferably 0.5 to 2.5%, more preferably 1.2 to 2.2%.

P: 0.1% or less (excluding 0%)
P is unavoidably present as an impurity element, and contributes to an increase in strength by solid solution strengthening, but segregates at the prior austenite grain boundaries and causes the brittleness of the grain boundaries to deteriorate the stretch flangeability. % Or less. Preferably it is 0.05% or less, More preferably, it is 0.03% or less.

S: 0.02% or less (excluding 0%)
S is also unavoidably present as an impurity element, forms MnS inclusions, and becomes a starting point of a crack at the time of hole expansion, thereby reducing stretch flangeability. Therefore, the content is made 0.02% or less. Preferably it is 0.018% or less, More preferably, it is 0.016% or less.

Al: 0.01 to 1.0%
Al is added as a deoxidizing element and has the effect of making inclusions finer. Moreover, it combines with N to form AlN and reduces the solid solution N that contributes to the occurrence of strain aging, thereby preventing elongation and stretch flangeability from being deteriorated. If it is less than 0.01%, solute N remains in the steel, so strain aging occurs, and elongation and stretch flangeability cannot be ensured. On the other hand, if it exceeds 1.0%, austenite formation during heating is inhibited. The area ratio of the hard second phase cannot be secured, and the stretch flangeability cannot be secured.

N: 0.01% or less (excluding 0%)
N is also unavoidably present as an impurity element and lowers the elongation and stretch flangeability by strain aging, so the lower one is preferable, and the content is made 0.01% or less.

  The steel of the present invention basically contains the above components and the balance is substantially iron and impurities, but the following allowable components can be added as long as the effects of the present invention are not impaired.

Cr: 0.01 to 1.0%
Cr is a useful element that can improve stretch flangeability by suppressing the growth of cementite. If the addition is less than 0.01%, the above-described effects cannot be exhibited effectively. On the other hand, if the addition exceeds 1.0%, coarse Cr ₇ C ₃ is formed, and the stretch flangeability deteriorates. Resulting in.

Mo: 0.01 to 1.0%,
Cu: 0.05 to 1.0%,
Ni: One or more of 0.05 to 1.0% These elements are useful elements for improving strength without degrading formability by solid solution strengthening. If each element is added below the lower limit, the above-described effects cannot be exhibited effectively. On the other hand, if each element exceeds 1.0%, the cost becomes too high.

Ca: 0.0001 to 0.01%,
Mg: 0.0001 to 0.01%
Li: 0.0001 to 0.01%
REM: One or more of 0.0001 to 0.01% These elements are useful elements for improving stretch flangeability by making inclusions finer and reducing the starting point of fracture. . If less than 0.0001% of each element is added, the above effect cannot be exhibited effectively. On the other hand, if more than 0.01% of each element is added, inclusions are coarsened and stretch flangeability is deteriorated. To do.

  Note that REM refers to a rare earth element, that is, a group 3A element in the periodic table.

  Next, the preferable manufacturing method for obtaining the said invention steel plate is demonstrated below.

[Preferred production method of invention steel plate]
In order to manufacture the cold-rolled steel sheet as described above, first, the steel having the above composition is melted, slab is formed by ingot casting or continuous casting, hot-rolled, and pickled and then cold-rolled. Do.

[Hot rolling conditions]
As the hot rolling conditions, it is preferable to set the finish rolling finishing temperature to Ar ₃ or higher, appropriately cool, and then wind up in the range of more than 600 and 750 ° C. or less.

<Winding temperature: more than 600 and 750 ° C. or less>
By making the coiling temperature higher than 600 ° C. (more preferably 620 ° C. or more, particularly preferably 640 ° C. or more), the size of the structure can be made large and uniform as a whole, and ferrite + pearlite (α + P ) Of two phases only. However, if the coiling temperature is too high, the structure size of the hot-rolled sheet becomes too large, so the temperature is set to 750 ° C. or lower (more preferably 730 ° C. or lower, particularly preferably 710 ° C. or lower).

[Cold rolling conditions]
As the cold rolling conditions, it is preferable that the cold rolling rate (hereinafter also referred to as “cold rolling rate”) be in the range of more than 50% and 80% or less.

<Cold rolling ratio: Over 50% and below 80%>
By setting the cold rolling rate to be more than 50% (more preferably 55% or more), the strain amount introduced into the surface layer portion and the inside can be made substantially equal by performing strong processing during cold rolling. However, if the cold rolling rate is too high, the deformation resistance at the time of cold rolling becomes too high, and the productivity is extremely deteriorated due to the reduction in rolling speed, so 80% or less (more preferably 75% or less). .

  Then, after the cold rolling, annealing and further tempering are performed.

[Annealing conditions]
As annealing conditions, after holding for an annealing holding time of 3600 s or less at an annealing temperature of Ac1 or more and less than (Ac1 + Ac3) / 2, the first cooling end temperature (slow cooling end) is 730 ° C. or less and 500 ° C. or more from the annealing temperature. Temperature) at a first cooling rate (slow cooling rate) of 1 ° C./s or more and less than 50 ° C./s and then to a second cooling end temperature (quenching end temperature) below the Ms point at 50 ° C./s. It is preferable to perform rapid cooling at the second cooling rate (rapid cooling rate).

<Holding for an annealing holding time of 3600 s or less at an annealing temperature of Ac1 or more and less than (Ac1 + Ac3) / 2>
This is because soaking is performed on the low temperature side of the two-phase region to form a structure composed of relatively large ferrites and fine austenite having a uniform size.

If the annealing temperature is less than Ac1, it will not be transformed into austenite and a predetermined two-phase structure will not be obtained. On the other hand, if the annealing temperature is (Ac1 + Ac3) / 2 or more , the ferrite in the surface layer will grow too much, The difference in internal ferrite fraction and hardness becomes excessive, and the variation in characteristics increases.

  Further, if the annealing holding time exceeds 3600 s, productivity is extremely deteriorated, which is not preferable. A more preferable lower limit of the annealing holding time is 60 s. By increasing the heating time, strain in the ferrite can be further removed.

<Slow cooling to a first cooling end temperature of 730 ° C. or lower and 500 ° C. or higher at a first cooling rate of 1 ° C./s or higher and lower than 50 ° C./s>
Stretch flangeability by reducing the size of ferrite that nucleates when cooling from the shoulder and making it approximately the same size as the ferrite formed in the above two-phase region, and forming a ferrite structure with an area ratio of 20 to 50% by combining them. This is because the elongation can be improved while securing the above.

  If the temperature is less than 500 ° C. or the cooling rate is less than 1 ° C./s, ferrite is excessively formed, and the strength and stretch flangeability cannot be ensured.

<Rapid cooling to the second cooling end temperature below the Ms point at the second cooling rate of 50 ° C./s or higher>
This is to suppress the formation of ferrite from austenite during cooling and obtain a hard second phase.

  When quenching is terminated at a temperature higher than the Ms point or when the cooling rate is less than 50 ° C./s, bainite is excessively formed, and the strength of the steel sheet cannot be secured.

[Tempering conditions]
As the tempering conditions, the temperature after the annealing cooling is heated from the tempering temperature: 300 to 500 ° C., the tempering holding time is kept in the temperature range of 300 ° C. to the tempering temperature: 60 to 1200 s, and then cooled.

  While the solid solution C concentrated in the ferrite at the time of annealing is left in the ferrite as it is after tempering, the hardness of the ferrite is increased, while the reaction of the concentration of the solid solution C in the ferrite at the time of annealing is as follows. This is because the hardness of the hard second phase is lowered by further precipitating C as cementite by tempering or coarsening fine cementite particles from the hard second phase having a reduced C content.

  When the tempering temperature is less than 300 ° C. or the tempering time is less than 60 s, the heating state between the surface and the inside becomes non-uniform, and the difference in hardness between the surface and the inside becomes large, resulting in a large variation in characteristics. On the other hand, if the tempering temperature exceeds 500 ° C., the hard second phase becomes too soft and the strength cannot be secured, or the cementite becomes too coarse and the stretch flangeability deteriorates. Moreover, since tempering time exceeds 1200 s, productivity will fall and it is unpreferable.

  A more preferable range of the tempering temperature is 320 to 480 ° C., and a more preferable range of the tempering holding time is 120 to 600 s.

  As shown in Table 1 and Table 2 below, steels of various components were melted to form an ingot having a thickness of 120 mm. After this was hot-rolled to a thickness of 25 mm, it was hot-rolled again to 3.2 mm under various production conditions shown in Tables 3 to 5 below. Cold rolled to 6 mm and then heat treated.

  In addition, Ac1 and Ac3 in Table 1 were calculated | required using the following formula 1 and formula 2 (translated by Shigeyasu Koda, "Leslie Steel Materials Science", Maruzen Co., 1985, p. 273).

Formula 1: Ac1 (degreeC) = 723 + 29.1 [Si] -10.7 [Mn] +16.9 [Cr] -16.9 [Ni]
Formula 2: Ac3 (° C.) = 910−203√ [C] +44.7 [Si] +31.5 [Mo] −15.2 [Ni]
However, [] shows content (mass%) of each element.

  For each steel plate after heat treatment, by the measurement method described in the above section [Mode for Carrying Out the Invention], the area ratio of each phase in the entire thickness of the steel sheet, the area ratio of ferrite in the steel sheet surface layer portion and the center portion, and The hardness in the steel plate surface layer part and the center part was measured.

  Moreover, about each steel plate after the said heat processing, the tensile strength TS, elongation EL, and stretch flangeability (lambda) were measured, and the characteristic of each steel plate was evaluated.

  Specifically, the properties of the steel plate after the heat treatment are those that satisfy all of TS ≧ 980 MPa, EL ≧ 13%, and λ ≧ 40%, which are acceptable (◯), and the others that are not acceptable (×). .

  In addition, the stability of the characteristics of the steel sheet after the heat treatment is such that the test condition of the same steel type is heat-treated by changing the production conditions within the maximum fluctuation range of the production conditions of the actual machine, and the TS change width ΔTS ≦ 200 MPa. Those satisfying all of the change width ΔEL ≦ 2% of EL and the change width Δλ ≦ 20% of λ were determined to be acceptable (◯), and the others were determined to be unacceptable (×).

  The tensile strength TS and elongation EL were measured in accordance with JIS Z 2241 by preparing No. 5 test piece described in JIS Z 2201 with the long axis in the direction perpendicular to the rolling direction.

  Moreover, stretch flangeability (lambda) performed the hole expansion test according to the iron continuous standard JFST1001, and measured the hole expansion rate, and made this the stretch flangeability.

  The measurement results are shown in Tables 6-9.

  From these tables, steel no. 1-2, 6-9, 32-35, 37-50, 54-60 are invention steels that satisfy all the requirements of the present invention. It can be seen that any of the invention examples is not only excellent in the absolute value of the mechanical properties but also obtained a homogeneous cold-rolled steel sheet in which variations in the mechanical properties are suppressed.

  Steel No. 14, 15, 17, 18, 20, 23, 25, 27, 29, 30, 61-80 also satisfy all the requirements of the present invention. These steel sheets have been confirmed to be excellent in the absolute value of mechanical properties, but have not yet been evaluated for variations in mechanical properties. However, it can be inferred that the variation in mechanical properties is also at an acceptable level as in the case of the above invention steel.

  In contrast, comparative steels that do not satisfy any of the requirements of the present invention have the following problems.

  Steel No. In Nos. 3 to 5, since the winding temperature is too low, bainite is easily generated in the hot-rolled sheet structure after winding. In addition, since the cold rolling rate is higher than usual, the bainite in the surface layer portion is easily decomposed during annealing and the ferrite fraction is likely to change. As a result, the difference in ferrite fraction and hardness from the inside (center portion) becomes large and the characteristics are satisfied, but the variation in tensile strength TS becomes large and the acceptance standard is not reached.

  Steel No. 10 and 11, since the annealing temperature is too high, the ferrite fraction of the surface layer portion accompanying decarburization increases, and the difference between the ferrite fraction of the surface layer portion and the inside becomes large and the characteristics are satisfied, but the variation in elongation EL Does not reach the acceptance criteria.

  Steel No. No. 12 is steel no. Contrary to 3-5, since the coiling temperature is too high, the ferrite of the surface layer part grows too much. As a result, the difference in ferrite fraction and hardness between the inside (center portion) and the hardness become large and the characteristics are satisfied, but the variation in the elongation EL becomes large and does not reach the acceptance standard.

  Steel No. Since the cold rolling rate of No. 13 is too low, the difference between the surface layer portion and the internal ferrite fraction and the hardness is large, and the characteristics are satisfied, but the variation in the elongation EL becomes large and does not reach the acceptance standard.

  Steel No. In No. 16, since the slow cooling rate is too low, ferrite grows too much in the surface layer portion and inside, the ferrite fraction of the entire structure of the steel sheet becomes excessive, and the tensile strength TS cannot be secured.

  Steel No. In No. 19, since the annealing end temperature is too low, ferrite is generated too much, the ferrite fraction becomes excessive, and the tensile strength TS cannot be secured.

  On the other hand, Steel No. In No. 21, since the annealing end temperature is too high, ferrite is not sufficiently generated, the ferrite fraction of the entire steel sheet structure is insufficient, and the elongation EL cannot be secured.

  Steel No. In No. 22, since the rapid cooling rate is too low, other structures (mainly retained austenite) are generated, and the stretch flangeability λ cannot be secured.

  Steel No. In No. 24, since the quenching end temperature is too high, other structures (mainly retained austenite) are generated, and the stretch flangeability λ cannot be secured.

  Steel No. In No. 26, since the tempering temperature is too low, the hardness of the hard second phase becomes high, the entire structure of the steel sheet becomes too hard, the non-uniformity of strength in the structure increases, and the elongation EL and stretch flangeability λ cannot be secured.

  On the other hand, Steel No. In No. 28, since the tempering temperature is too high, the hard second phase of the surface layer portion is particularly softened, and the tensile strength TS cannot be secured.

  Steel No. In No. 31, since the amount of Si is too large, the ferrite is strengthened by solid solution and ductility is impaired, and the elongation EL and the stretch flangeability λ cannot be ensured.

  Steel No. Since the amount of C is too large, the ferrite fraction is insufficient due to suppression of ferrite transformation and increase in hardenability, and the elongation EL and stretch flangeability λ cannot be ensured.

  Steel No. No. 51 has too little Mn, so that the solid solution strengthening of ferrite is insufficient, and the tensile strength TS cannot be secured.

  On the other hand, Steel No. In No. 52, since the amount of Mn is too large, the ferrite fraction is insufficient due to suppression of ferrite transformation and increase in hardenability, and the elongation EL and stretch flangeability λ cannot be ensured.

  Steel No. 53 is a steel no. Contrary to 36, since the amount of C is too small, the ferrite fraction becomes excessive and the tensile strength TS cannot be secured.

  Incidentally, the difference in structure between the surface layer portion and the central portion of the inventive steel (steel No. 6) and the comparative steel (steel No. 10) is illustrated in FIG. The figure shows the result of observation with an optical microscope. The plain whitish area is ferrite and the dark area is the hard second phase. As is clear from the figure, in the comparative steel, the ferrite fraction in the surface layer is much higher than that in the center, whereas in the invention steel, the ferrite fraction in the surface layer is almost the same as that in the center. Is accepted.

Claims (5)

% By mass (hereinafter the same for chemical components)
C: 0.05 to 0.30%
Si: 3.0% or less (excluding 0%),
Mn: 0.1 to 5.0%,
P: 0.1% or less (excluding 0%),
S: 0.02% or less (excluding 0%),
Al: 0.01 to 1.0%,
N: 0.01% or less (excluding 0%)
Each having a component composition consisting of iron and inevitable impurities,
Including ferrite, which is a soft first phase, in an area ratio of 20 to 50%,
The balance is a hard second phase, and has a structure composed of tempered martensite and / or tempered bainite,
The difference ΔVα = Vαs−Vαc between the ferrite area ratio Vαs in the surface layer portion of the steel sheet from the steel sheet surface to the depth of 100 μm and the ferrite area ratio Vαc in the central portion of t / 4 to 3t / 4 (t is the plate thickness) is 10 The ratio RHv = Hvs / Hvc between the hardness Hvs of the steel sheet surface layer part and the hardness Hvc of the central part is 0.75 to 1.0. Small high-strength cold-rolled steel sheet. Ingredient composition further
Cr: 0.01 to 1.0%
The high-strength cold-rolled steel sheet having a small variation in mechanical properties according to claim 1. Ingredient composition further
Mo: 0.01 to 1.0%,
Cu: 0.05 to 1.0%,
The high-strength cold-rolled steel sheet with a small variation in mechanical properties according to claim 1 or 2, wherein Ni: 0.05 to 1.0%, or one or more of them is included. Ingredient composition further
Ca: 0.0001 to 0.01%,
Mg: 0.0001 to 0.01%
Li: 0.0001 to 0.01%
The high-strength cold-rolled steel sheet according to any one of claims 1 to 3, which contains one or more of REM: 0.0001 to 0.01%. A steel material having the composition shown in any one of claims 1 to 4 is hot-rolled under the conditions shown in the following (1) to (4), then cold-rolled, and then annealed. A method for producing a high-strength cold-rolled steel sheet with small variations in mechanical properties, characterized by tempering.
(1) Hot rolling conditions Finish rolling finish temperature: Ar ₃ points or more Winding temperature: Over 600 ° C and 750 ° C or less
(2) Cold rolling conditions Cold rolling rate: Over 50% and below 80%
(3) Annealing conditions After holding at an annealing temperature of Ac1 or more and less than (Ac1 + Ac3) / 2 for an annealing holding time of 3600 s or less, the temperature from the annealing temperature to the first cooling end temperature of 730 ° C. or less to 500 ° C. or more is 1 ° C. After slow cooling at a first cooling rate of not less than / s and less than 50 ° C./s, rapid cooling is performed at a second cooling rate of not less than 50 ° C./s to a second cooling end temperature below the Ms point.
(4) Tempering conditions Tempering temperature: 300-500 ° C
Tempering holding time: 60 to 1200 s in the temperature range of 300 ° C. to tempering temperature