JP5280795B2 - Method for producing high-strength cold-rolled steel sheet with excellent mechanical property stability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength cold rolled steel sheet having excellent stability in mechanical properties, which can be produced without being influenced by variation in annealing-heating holding conditions such as annealing-heating temperature and holding time therefor, and to provide a method of producing the same. <P>SOLUTION: The high strength cold rolled steel sheet having excellent stability in mechanical properties has a composition comprising, by mass, 0.02 to 0.30% C, &le;3.0% (including 0%) Si, 0.2 to &lt;3.0% Mn, &le;0.1% (including 0%) P, &le;0.01% (including 0%) S, 0.002 to 0.030% N and &ge;0.002% Al, and in which the contents of N and Al satisfy -0.0005&le;[N]-(14.01/26.98)&times;[Al]&le;0.0020 (wherein, [ ] denotes the content (mass%) of each element), and the balance iron with inevitable impurities. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a high-strength steel sheet excellent in workability used for, for example, an automotive structural component.

  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 more variations in mechanical properties such as yield strength, tensile strength, work hardening index, etc. than mild steel, so the amount of springback changes during press forming, resulting in 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 to suppress variations 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 proposals have 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 up to 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, the quenching start temperature is calculated according to the target tensile strength, and quenching is performed at the obtained quenching start temperature. A method for reducing the above is disclosed.

[Prior Art 3]
Further, in Patent Document 3, in the annealing process after cold rolling a hot-rolled steel sheet, after soaking at a temperature of over 800 ° C. and less than Ac 3 for 30 seconds to 5 minutes, primary cooling is performed to a temperature range of 450 to 550 ° C. Next, after performing secondary cooling at a cooling rate smaller than the primary cooling rate up to 450 to 400 ° C., holding at 450 to 400 ° C. for 1 minute or longer further reduces the variation in elongation characteristics in the plate width direction. A method for improving is disclosed.

  However, the prior art 1 has the following problems. In other words, in actual operation, the cooling rate for slow cooling from the annealing temperature to the rapid cooling start temperature can be adjusted by adjusting the flow rate of the coolant (gas, mist, etc.) sprayed onto the steel plate, so the rapid cooling start temperature is set to the target value. It is easy to achieve the target, and the fluctuation of the rapid cooling start temperature hardly causes a problem in actual operation. Rather than that, the annealing heating temperature and its holding time (hereinafter collectively referred to as “annealing heating holding conditions”) need to be adjusted only by the furnace atmosphere temperature and the plate feed speed, making control much more difficult. It is. The prior art 1 cannot cope with variations in mechanical properties due to a change in the ratio of ferrite and austenite immediately after annealing, which occurs when the annealing and heating conditions change.

  Moreover, although the said prior art 2 considers the quenching start temperature (rapid cooling start temperature) and tempering temperature at the time of an annealing process, just like the said prior art 1, immediately after the annealing which arises when an annealing heating holding condition fluctuates. It cannot cope with variations in strength due to changes in the ratio of ferrite and austenite and austenite grain size.

  Further, in the above prior art 3, the cooling during the annealing process is divided into the rapid cooling primary cooling and the slow cooling secondary cooling, thereby stabilizing the holding temperature after cooling, thereby varying the elongation characteristics in the plate width direction. However, as in the case of the prior arts 1 and 2, it cannot cope with the variation in elongation characteristics due to the change in the ratio of ferrite and austenite immediately after annealing, which occurs when the annealing and holding conditions fluctuate.

As described above, the effect of fluctuations in chemical components can be compensated by controlling the cooling conditions after annealing, the effects of fluctuations in cooling conditions after annealing can be reduced by adjusting chemical components, Although various efforts have been made such as changing the cooling conditions themselves to a method with less fluctuation, in actual operation, the ratio of ferrite and austenite due to fluctuations in annealing heating holding conditions such as annealing heating temperature and holding time and austenite Since the change in the particle size greatly affects the transformation behavior during the subsequent cooling, the above-mentioned conventional techniques 1 to 3 in which the influence due to the change in the annealing and heating conditions is not taken into account stabilize the true mechanical characteristics. It is the present condition that has not been achieved.
JP 2007-138262 A JP 2003-277832 A JP 2000-212684 A

  Accordingly, an object of the present invention is to produce a high-strength cold-rolled steel sheet excellent in stability of mechanical properties, which can be produced without being affected by fluctuations in annealing heating holding conditions such as annealing heating temperature and holding time, and a manufacturing method thereof. Is to provide.

The invention described in claim 1
% By mass (hereinafter the same for chemical components)
C: 0.02 to 0.30%,
Si: 3.0% or less (including 0%),
Mn: 0.2% or more and less than 3.0%,
P: 0.1% or less (including 0%),
S: 0.01% or less (including 0%),
N: 0.002 to 0.030%,
Al: containing 0.002% or more, and further the content of N and Al,
−0.0005 ≦ [N] − (14.01 / 26.98) × [Al] ≦ 0.0020 (where [] represents the element content (mass%)).
When the steel material having a component composition consisting of iron and inevitable impurities as a balance is hot-rolled and then cold-rolled and then subjected to heat treatment,
After the heat treatment is held at Ac 3 to 950 ° C. for 1000 s or less, 450 to 750 ° C. is slowly cooled at an average cooling rate of less than 20 ° C./s, and then to 200 ° C. or less to an average cooling rate of 100 ° C./s or more. It is a method for producing a high-strength cold-rolled steel sheet excellent in mechanical property stability, characterized in that it is quenched and annealed at 550, and then tempered at 150 to 600C for 1000 seconds or less .

The invention described in claim 2
Ingredient composition further
Cr: 0.01 to 3.0%,
Mo: 0.01 to 1.0%,
Cu: 0.01 to 2.0%,
Ni: 0.01 to 2.0%,
The method for producing a high-strength cold-rolled steel sheet having excellent mechanical property stability according to claim 1, comprising one or more of the above.

The invention according to claim 3
Ingredient composition further
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
REM: 0.0001 to 0.01%
The method for producing a high-strength cold-rolled steel sheet having excellent mechanical property stability according to claim 1 or 2, comprising one or more of the above.

The invention according to claim 4
When the steel material having the component composition according to any one of claims 1 to 3 is hot-rolled and then cold-rolled, and then heat-treated,
After the heat treatment is held at Ac 3 to 950 ° C. for 1000 s or less and then cooled to 300 to 550 ° C. at an average cooling rate of 20 ° C./s or more, and then retained in a temperature range of 300 to 600 ° C. for 10 to 1000 s. This is a method for producing a high-strength cold-rolled steel sheet having excellent mechanical property stability, which is performed by cooling.

The invention described in claim 5
When the steel material having the component composition according to any one of claims 1 to 3 is hot-rolled and then cold-rolled, and then heat-treated,
After the heat treatment is held at Ac 3 to 950 ° C. for 1000 s or less, 450 to 750 ° C. is slowly cooled at an average cooling rate of less than 20 ° C./s, and then 300 to 550 ° C. is average cooling rate of 20 ° C./s or more. The method is a method for producing a high-strength cold-rolled steel sheet having excellent mechanical property stability, characterized in that it is cooled after cooling for 10 to 1000 seconds and then cooled for 10 to 1000 seconds.

  According to the present invention, by controlling the content of N and Al in the steel so as to satisfy a specific relationship, even if the annealing heating holding conditions such as the annealing temperature and the holding time that are difficult to control change, As a result of the pinning action, coarsening of the austenite grains is suppressed, and as a result, the transformation behavior during cooling is stabilized, and the mechanical properties of the steel sheet after heat treatment can be stabilized.

  In order to stabilize the mechanical properties of the steel sheet after the heat treatment, the present inventors suppress the variation in the structure state immediately after annealing, that is, before transformation occurs due to cooling, caused by fluctuations in the annealing heating temperature and its holding time. I thought it was important.

  For this purpose, if the annealing is performed in the two-phase region where ferrite and austenite are generated as in the above-described prior art 1, the ratio of the ferrite and austenite inevitably changes due to fluctuations in the annealing heating temperature and the holding time. Since the structure after the transformation due to subsequent cooling remains affected, austenite single-phase heating in which only austenite is generated was adopted. And, it is known that the grain size of austenite produced by austenite single-phase heating changes depending on the annealing heating temperature and the variation of its holding time, but by suppressing the grain size change, the structure after transformation AlN particles that have a pinning effect on the growth of austenite grains (Steel Handbook 4th Edition, Volume 3 (1), Japan Iron and Steel Institute, July 2002, Chapter 7) I came up with the idea of using 9) and 1).

  Therefore, when the influence of the AlN particles on the growth behavior of the austenite grains was examined, it was found that the coarsening behavior of the AlN grains was an important factor for the growth of the austenite grains. Also, the coarsening rate of alloy nitride particles such as AlN depends on the concentration of the solid solution alloy element, and the increase in the solid solution amount of the alloy element increases the coarsening rate of the alloy nitride particles. It is said that the effect of suppressing the growth of austenite grains due to the pinning effect is reduced (see Sakuma: Journal of the Japan Institute of Metals, 20 (1981), p.254, formula (18)). Therefore, in order to utilize AlN as pinning particles, sol. The content ratio of Al and N is important. When Al is excessively contained in a mole fraction with respect to N, the AlN particles are coarse due to the presence of a large amount of Al in a solid solution state when heated to around 900 ° C. which is the austenite single-phase heating temperature. Therefore, when the annealing heating temperature and the holding time thereof fluctuate, the change in the prior austenite grain size becomes remarkable. In general super high tension, sol. Al: 0.020-0.030%, N: 0.003-0.006% and sol. Al is contained in a supersaturated state and changes in the austenite grain size are promoted during annealing and heating.

  Therefore, sol. Based on this knowledge, it is found that AlN content can be prevented from being coarsened by making the content ratio of Al and N substantially the same or in excess of N, and coarsening of the austenite grain size during annealing can be suppressed. Further studies have been made and the present invention has been completed.

  Below, the component composition which comprises this invention steel plate is demonstrated. Hereinafter, all the units of chemical components are mass%. The steel sheet of the present invention does not need to be special in its structure, and includes ferrite-martensite dual phase steel (DP steel), ferrite, bainite and retained austenite, which are common as high-strength steel sheets. Composite steel such as TRIP steel made of a structure can be used.

[Component composition of the steel sheet of the present invention]
N: 0.002 to 0.030%
As described above, N is sol. By combining with Al to form AlN, it is an important element that suppresses the growth of austenite grains during annealing and contributes to stabilization of mechanical properties. The higher the N content, the greater the amount of AlN formed and the greater the effect of suppressing austenite grain coarsening, but there is a limit to the amount of N that can be dissolved in steel during melting. That is, if it is less than 0.002%, the amount of AlN formed is insufficient, and the austenite grain coarsening suppressing action cannot be exhibited effectively. On the other hand, if it exceeds 0.030%, it is difficult to industrially contain it in steel.

Al: 0.002% or more,
−0.0005 ≦ [N] − (14.01 / 26.98) × [Al] ≦ 0.0020 (where [] represents the element content (mass%)) Formula (1)
As described above, Al combines with N to form AlN, thereby suppressing the growth of austenite grains during annealing heating and is an important element contributing to stabilization of mechanical properties. The higher the Al content, the greater the amount of AlN formation. Al increases, the coarsening of AlN grains is promoted, and the austenite grain coarsening suppression effect is diminished. Therefore, the Al content is 0.002% or more, preferably 0.003 or more, and serves as an index of the AlN formation amount represented by the above formula (1) {[N] − (14.01 / 26. 98) The value of [Al]} needs to be controlled in the range of -0.0005 to 0.0020%, and it is particularly preferable to control it as high as possible within this range. The upper limit of {[% N]-(14.01 / 26.98) × [% Al]} is set to 0.0020. If this value is exceeded, excessive solute N exists in the steel. In addition, strain aging is likely to occur, and elongation cannot be secured.

C: 0.02 to 0.30%
In the case of DP steel, C contributes to an increase in the fraction of martensite. In the case of TRIP steel, C affects the amount of retained austenite and the carbon concentration in retained austenite. It is an important element that affects the balance between strength and elongation. If the content is less than 0.02%, the strength cannot be ensured. On the other hand, if it exceeds 0.30%, the weldability that is a necessary characteristic of the thin steel sheet cannot be secured. The range of C content is preferably 0.05 to 0.25%, more preferably 0.07 to 0.20%.

Si: 3.0% or less (including 0%)
Si is a useful element that can increase strength without significantly degrading elongation by solid solution strengthening. If it exceeds 3.0%, the Ac1 point becomes too high, and the temperature that can be maintained in the austenite single phase becomes high, so that the effect of suppressing austenite grain coarsening by AlN particles cannot be obtained. The range of Si content becomes like this. Preferably it is 0.1-2.5%, More preferably, it is 0.5-2.0%.

Mn: 0.2 or more and less than 3.0% Mn is an element useful for securing the martensite fraction and improving the balance between strength and elongation by enhancing the hardenability of the steel sheet. If it is less than 0.2%, sufficient hardenability cannot be secured, and a sufficient martensite area ratio cannot be secured during rapid cooling, so that strength cannot be obtained. On the other hand, if it exceeds 3.0%, in the case of DP steel, the ferrite transformation during cooling is too suppressed, so that it does not become a dual phase steel, while in the case of TRIP steel, the ferrite transformation during cooling or Since the bainite transformation is excessively suppressed, carbon cannot be concentrated in the retained austenite, and in any case, elongation cannot be secured. The range of the Mn content is preferably 0.3 to 2.9%, more preferably 0.5 to 2.8%.

P: 0.1% or less (including 0%)
P is unavoidably present as an impurity element, and contributes to an increase in tensile strength by solid solution strengthening, but segregates at the prior austenite grain boundaries and deteriorates the elongation by embrittlement of the grain boundaries, so 0.1% The following. Preferably it is 0.05% or less, More preferably, it is 0.03% or less.

S: 0.01% or less (including 0%)
S is also unavoidably present as an impurity element, forms MnS inclusions, and becomes a starting point of cracks at the time of deformation, thereby reducing stretch flangeability. More preferably, it is 0.003% or less.

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

Cr: 0.01 to 3.0%,
Mo: 0.01 to 1.0%,
Cu: 0.01 to 2.0%,
Ni: 0.01 to 2.0%,
These elements are useful elements that enhance the hardenability of the steel, contribute to an increase in the martensite fraction, and improve the balance between strength and elongation. When each element is added below the lower limit value, the above-described effects cannot be exhibited effectively. On the other hand, when each element exceeds the upper limit value, austenite remains at the time of quenching and deteriorates elongation.

Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
REM: 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 during deformation. When each element is added below the above lower limit value, the above-described action cannot be exhibited effectively. On the other hand, when the element exceeds the above upper limit value, inclusions are coarsened and the elongation deteriorates.

  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 this invention steel plate is demonstrated below.

[Preferred production method of the steel sheet of the present invention]
In order to manufacture the cold-rolled steel sheet as described above, first, steel having the above composition is melted and formed into a slab by ingot forming or continuous casting and then hot-rolled. After hot rolling is completed, pickling is performed and then cold rolling is performed. The cold rolling rate is preferably about 30% or more. And after the said cold rolling, it heat-processes, ie, annealing, and also tempering as needed. Hereinafter, the heat treatment conditions will be described taking the case where the steel sheet of the present invention is DP steel and the case of TRIP steel as examples. The steel sheet of the present invention includes not only cold-rolled steel sheets but also hot-dip galvanized steel sheets and galvannealed steel sheets.

[Heat treatment conditions (1)]
This heat treatment condition is a preferable heat treatment condition when the steel of the present invention is DP steel. Annealing heating temperature: Heated to Ac 3 to 950 ° C., and held for annealing holding time: 1000 s or less, then, from annealing heating temperature to first cooling end temperature: 450 to 750 ° C. First cooling rate: average less than 20 ° C./s Slow cooling is performed at a cooling rate, and then the second cooling end temperature: 200 ° C. or less is rapidly cooled at an average cooling rate of second cooling rate: 100 ° C./s or more, followed by annealing, and then tempering heating temperature: 150 to 600 ° C. Tempering holding time: 1000 s or less.

<Annealing heating temperature: Ac3 to 950 ° C., annealing holding time: 1000 s or less>
When heating is less than 3 points, it is in the process of transition from the two-phase state of ferrite and cementite to the two-phase state of ferrite and austenite, so when the heating temperature and holding temperature change, the fraction of ferrite and austenite changes, Since the structure is not stabilized, the final structure after heat treatment is not stabilized, and as a result, the mechanical properties of the steel sheet are not dispersed and stabilized. For this reason, the annealing heating temperature is set to be Ac3 point or higher at which the austenite single phase can be obtained. On the other hand, when heated to over 950 ° C., the AlN particles become prominent even if the ratio of Al and N contents is controlled, so that austenite grain growth can be effectively prevented, resulting in variations in mechanical properties. Can not be sufficiently suppressed. Therefore, the annealing heating temperature needs to be 950 ° C. or lower.

  Further, if the annealing holding time exceeds 1000 s, productivity is extremely deteriorated, which is not preferable.

<First cooling end temperature: 450 to 750 ° C. First cooling rate: slow cooling at an average cooling rate of less than 20 ° C./s>
Since it is necessary to form ferrite to obtain a DP steel structure, the first cooling rate is slowly cooled to an average cooling rate of less than 20 ° C./s up to 450 to 750 ° C., which is a temperature range in which ferrite transformation can occur. .

  If the first cooling end temperature is less than 450 ° C., bainite is formed during cooling, while if the first cooling end temperature is higher than 750 ° C., ferrite is not sufficiently formed, and in either case, the DP steel structure is Since it cannot be obtained, the balance between strength and elongation cannot be secured.

  On the other hand, if the first cooling rate is 20 ° C./s or more, the ferrite transformation does not proceed sufficiently, so that the balance between strength and elongation cannot be ensured.

<Second cooling end temperature: up to 200 ° C. or less, second cooling rate: rapid cooling at an average cooling rate of 100 ° C./s or more>
This is to suppress the bainite transformation and build a DP steel structure.

  If the second cooling end temperature is higher than 200 ° C. or the second cooling rate is lower than 100 ° C./s, bainite is formed, so that a balance between strength and elongation cannot be secured.

<Tempering heating temperature: Tempering holding time: 1000 s or less at a temperature of 150 to 600 ° C.>
Ductility can be enhanced while securing strength by tempering hard martensite and softening.

  If the tempering heating temperature is less than 150 ° C., the martensite is not sufficiently softened, so that the elongation cannot be secured. On the other hand, if the tempering heating temperature is higher than 600 ° C., the martensite becomes too soft and the strength cannot be secured.

  Further, if the tempering holding time exceeds 1000 s, productivity is lowered, which is not preferable.

[Heat treatment conditions (2)]
This heat treatment condition is a preferable heat treatment condition when the steel of the present invention is TRIP steel and the strength is more important than the elongation in the balance between strength and elongation. Annealing heating temperature: Heated to Ac 3 to 950 ° C., held for annealing holding time: 1000 s or less, and then anneal annealing temperature to first cooling end temperature: 300 to 550 ° C. First cooling rate: average of 20 ° C./s or more Cooling is performed at a cooling rate, and then the residence time is kept in a temperature range of 300 to 600 ° C. for a residence time of 10 to 1000 seconds, followed by cooling.

<Annealing heating temperature: Ac3 to 950 ° C., annealing holding time: 1000 s or less>
The reason is the same as the above [heat treatment condition (1)].

<First cooling end temperature: cooling from 300 to 550 ° C. at an average cooling rate of the first cooling rate: 20 ° C./s or more>
This is because, unlike the above [heat treatment condition (1)], the ferrite transformation during the cooling is suppressed and the bainite transformation is promoted to increase the strength.

  When the first cooling end temperature is less than 300 ° C., martensitic transformation occurs. On the other hand, when the first cooling end temperature exceeds 550 ° C., ferrite transformation occurs. Therefore, in any case, a balance between strength and elongation is ensured. become unable.

  On the other hand, if the first cooling rate is less than 20 ° C./s, ferrite transformation occurs during cooling, so that the balance between strength and elongation cannot be ensured.

<Residence temperature: residence time: 10 to 1000 s residence in a temperature range of 300 to 600 ° C.>
This is to promote the formation of bainitic ferrite and promote the concentration of carbon to retained austenite. The residence temperature does not need to be constant, and heating and cooling may be performed within the above temperature range as necessary. Even if there is a temperature change, if it is within the above temperature range, it is regarded as staying, and the total time is defined as staying time. For example, in the case of a manufacturing process of an alloyed hot-dip galvanized steel sheet, since it is immersed in a galvanizing bath, it is once held at 450 ° C. for 10 s, and then reheated to 550 ° C. over 10 s at 10 ° C./s for alloying. After holding at that temperature for 10 s, it is cooled to room temperature at 25 ° C./s (it takes 10 s to 300 ° C.). The residence time at that time is 10 s for holding at 450 ° C. and 10 s for reheating for alloying. The total is 40 s including 10 s holding at 550 ° C. and 10 s cooling to 300 ° C.

If the residence temperature is less than 300 ° C., martensitic transformation occurs. On the other hand, if the residence temperature exceeds 600 ° C., ferrite transformation occurs. Therefore, in any case, the balance between strength and elongation cannot be ensured.

  Further, if the residence time is less than 10 s, bainitic ferrite is not sufficiently formed, and carbon concentration to the retained austenite becomes insufficient. On the other hand, if the residence time exceeds 1000 s, the retained austenite decomposes into ferrite and cementite. Therefore, in any case, the elongation decreases.

[Heat treatment conditions (Part 3)]
This heat treatment condition is the case where the steel according to the present invention is TRIP steel as in the case of the heat treatment condition (No. 2). However, unlike the heat treatment condition (No. 2), the emphasis is on elongation rather than strength in the balance between strength and elongation. This is a preferable heat treatment condition. Annealing heating temperature: Heated to Ac 3 to 950 ° C., and held for annealing holding time: 1000 s or less, and then averaged from annealing heating temperature to first cooling end temperature: 450 to 750 ° C., first cooling rate: less than 20 ° C./s Cool slowly at the cooling rate, then cool down to the second cooling end temperature: 300 to 550 ° C. at the second cooling rate: average cooling rate of 10 ° C./s or more, and then the residence temperature: 300 to 600 ° C. Dwell time: 10 to 1000 s.

<Annealing heating temperature: Ac3 to 950 ° C., annealing holding time: 1000 s or less>
The reason is the same as the above [heat treatment condition (1)].

<First cooling end temperature: 450 to 750 ° C. First cooling rate: slow cooling at an average cooling rate of less than 20 ° C./s,
Second cooling end temperature: 300 to 550 ° C. Second cooling rate: cooling at an average cooling rate of 20 ° C./s or higher>
Unlike the above [heat treatment condition (2)], the ferrite transformation is advanced to some extent by slow cooling in the first half of the cooling to ensure elongation while sacrificing the strength, and the ferrite transformation is suppressed by rapid cooling in the second half of the cooling. This is because the strength sacrificed in the first half of the cooling is recovered by promoting the above.

  If the first cooling end temperature is less than 450 ° C., bainite is formed during cooling, whereas if the first cooling end temperature is higher than 750 ° C., ferrite is not sufficiently formed, and the first cooling rate is 20 If it is higher than or equal to ° C./s, the ferrite transformation does not proceed sufficiently, and in any case, it becomes impossible to ensure elongation.

  When the second cooling end temperature is less than 300 ° C., martensitic transformation occurs. On the other hand, when the second cooling end temperature exceeds 550 ° C., ferrite transformation occurs, and the second cooling rate is 20 ° C./s. If it is less than 1, ferrite transformation occurs during cooling, and in any case, it is impossible to secure a balance between strength and elongation.

<Residence temperature: residence time: 10 to 1000 s residence in a temperature range of 300 to 600 ° C.>
This is because the formation of bainitic ferrite is promoted and the concentration of carbon to retained austenite is promoted in the same manner as in the above [heat treatment condition (2)].

  As mentioned above, the manufacturing method illustrated above is applicable to each manufacturing method of a cold-rolled steel plate, a hot-dip galvanized steel plate, and an alloyed hot-dip galvanized steel plate.

  A steel made of various components is melted to prepare an ingot having a thickness of 120 mm, and this is hot rolled to a thickness of 25 mm, and then hot rolled again to a thickness of 3.2 mm. After washing, it was cold-rolled to a thickness of 1.6 mm to obtain a test material.

  Then, each test material is subjected to heat treatment under the heat treatment conditions exemplified in the above-mentioned [Preferred production method of steel plate of the present invention], and the mechanical properties of each steel plate after the heat treatment are measured. The stability of mechanical properties was evaluated.

  As the mechanical properties, tensile strength TS and elongation EL were measured. In these measurements, a No. 5 test piece described in JIS Z 2201 was prepared by taking a long axis in a direction perpendicular to the rolling direction. Measurements were performed according to Z 2241.

  Hereinafter, the test results are shown separately for each heat treatment condition.

[Test 1]: When heat treatment conditions (part 1) are applied Table 1 shows the components of the test material, Table 2 shows the heat treatment conditions (see the heat treatment pattern in FIG. 1) and the mechanical properties of the steel plate after heat treatment. Each is shown.

[Test 2]: When heat treatment conditions (No. 2) are applied Table 3 shows the components of the test material, Table 4 shows the heat treatment conditions (see the heat treatment pattern in FIG. 2) and the mechanical properties of the steel plate after the heat treatment. Each is shown.

[Test 3 ]: When heat treatment conditions (Part 3) are applied Table 5 shows the components of the test material, and Table 6 shows the heat treatment conditions (see heat treatment pattern [case where residence temperature changes] in FIG. 3 ) and after heat treatment. The mechanical properties of each steel sheet are shown below.

In each of the above tests 1 to 3 , as shown in Tables 2, 4, and 6, heat treatment was performed by changing the annealing heating temperature by 30 ° C., and the amount of change in the tensile strength TS (variation). The stability of the mechanical properties was evaluated.

As shown in Tables 1 to 6 above, all of the inventive steels (judgment: ◯) have a tensile strength TS variation ΔTS of 20 KPa or less, and are high-strength cold-rolled steel plates with excellent mechanical property stability. Obtained.

  On the other hand, the comparative steel (determination: x) is tensile because the N and Al contents do not satisfy the relationship defined by the present invention, or the annealing heating temperature is out of the recommended range. The variation ΔTS of the strength TS exceeds 20 KPa, and the stability of the mechanical properties is inferior.

It is a figure which shows the heat processing pattern of Test 1 typically. It is a figure which shows the heat processing pattern of Test 2 typically. It is a figure which shows the heat processing pattern of Test 3 typically.

Claims (5)

% By mass (hereinafter the same for chemical components)
C: 0.02 to 0.30%,
Si: 3.0% or less (including 0%),
Mn: 0.2% or more and less than 3.0%,
P: 0.1% or less (including 0%),
S: 0.01% or less (including 0%),
N: 0.002 to 0.030%,
Al: containing 0.002% or more, and further the content of N and Al,
−0.0005 ≦ [N] − (14.01 / 26.98) × [Al] ≦ 0.0020 (where [] represents the element content (mass%)).
When the steel material having a component composition consisting of iron and inevitable impurities as a balance is hot-rolled and then cold-rolled and then subjected to heat treatment,
After the heat treatment is held at Ac 3 to 950 ° C. for 1000 s or less, 450 to 750 ° C. is slowly cooled at an average cooling rate of less than 20 ° C./s, and then to 200 ° C. or less to an average cooling rate of 100 ° C./s or more. A method for producing a high-strength cold-rolled steel sheet having excellent mechanical property stability, characterized in that it is rapidly cooled and annealed at a temperature of 150 ° C. to 600 ° C. for 1000 s or less . Ingredient composition further
Cr: 0.01 to 3.0%,
Mo: 0.01 to 1.0%,
Cu: 0.01 to 2.0%,
Ni: 0.01 to 2.0%,
The method for producing a high-strength cold-rolled steel sheet having excellent mechanical property stability according to claim 1, comprising one or more of the following. Ingredient composition further
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
REM: 0.0001 to 0.01%
The method for producing a high-strength cold-rolled steel sheet having excellent mechanical property stability according to claim 1 or 2, comprising one or more of the following. When the steel material having the component composition according to any one of claims 1 to 3 is hot-rolled and then cold-rolled, and then heat-treated,
After the heat treatment is held at Ac 3 to 950 ° C. for 1000 s or less and then cooled to 300 to 550 ° C. at an average cooling rate of 20 ° C./s or more, and then retained in a temperature range of 300 to 600 ° C. for 10 to 1000 s. A method for producing a high-strength cold-rolled steel sheet having excellent mechanical property stability, which is performed by cooling. When the steel material having the component composition according to any one of claims 1 to 3 is hot-rolled and then cold-rolled, and then heat-treated,
After the heat treatment is held at Ac 3 to 950 ° C. for 1000 s or less, 450 to 750 ° C. is slowly cooled at an average cooling rate of less than 20 ° C./s, and then 300 to 550 ° C. is average cooling rate of 20 ° C./s or more. A method for producing a high-strength cold-rolled steel sheet having excellent mechanical property stability, characterized in that it is cooled after cooling for 10 to 1000 s in a temperature range of 300 to 600 ° C. and then cooled.

Images