JP5462742B2 - Method for producing high-strength steel sheet with excellent mechanical property stability - Google Patents

Description

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

  The present invention relates to a method for producing a high-strength steel sheet for producing a high-strength steel sheet used for automobiles or the like by continuous annealing, and more specifically, it is possible to reduce structural variations and mechanical property variations caused by the structural variations. The present invention relates to a method for producing a high-strength steel plate.

  As a representative example of a high-strength steel plate used for automobiles and the like, a composite structure steel (DP steel) having a structure composed of ferrite and martensite has been conventionally known. This DP steel has a structure in which soft ferrite and hard martensite are mixed to ensure ductility (elongation; EL) with soft ferrite and strength (tensile strength; TS) with hard martensite. It is to try. Therefore, this DP steel has been widely adopted in recent years as a high-strength automobile steel sheet or the like that requires excellent formability because both strength and elongation can be achieved. Properties are listed.

  However, due to recent technological development and improvement in appearance design, the shape of automobiles and the like has become complicated every year, and in order to produce such complicated shaped automobile members and the like by press molding without molding defects, Automotive steel sheets and the like are required to have better mechanical properties. Examples of mechanical properties required for such steel sheets for automobiles include yield strength (YP), tensile strength (TS), elongation (EL), stretch flangeability (λ), and the like. As described above, in recent years, higher performance has been demanded with respect to these mechanical characteristics, and coupled with the combination of these characteristics, the manufacturing conditions for the high-strength steel sheet have become more severe, and the manufacturing itself has become difficult. The current situation is that it is becoming difficult.

  In DP steel, which is a composite structure steel in which soft ferrite and hard martensite coexist, in order to coexist with the properties described above, the phase fraction and hardness of ferrite and martensite that occupy most of the structure. Must be accurately controlled to a desired value. However, since the phase fraction and hardness of ferrite and martensite vary greatly depending on temperature and time conditions during production, it is considered very difficult to control various characteristics to desired values with high accuracy.

  Moreover, when manufacturing DP steel, the present condition is controlling the structure | tissue and the characteristic of the high strength steel plate finally manufactured by heat-processing in a continuous annealing line. However, in the hot rolling process and the cold rolling process, which are the previous processes of the continuous annealing process, there are often variations in the structure due to variations in the process conditions, etc., and if there are variations in this previous process, It is not possible to suppress the occurrence of variations in the structure and characteristics of the finally produced high-strength steel sheet. That is, this variation in the previous process is a factor that further increases the variation in the structure and characteristics of the high-strength steel sheet that is finally produced.

  Conventionally, it is said that if the temperature is raised to the γ single phase region in the soaking process, the variation in the structure that occurs in the preceding process of the continuous annealing process is made harmless, but in practice, in the preceding process However, in the situation where high and low characteristic variations are required in recent years, it is not considered as a problem in the past. It can be said that the current situation is that the reduction of variation is becoming an issue.

  In addition, various process conditions such as temperature patterns and sheet feeding speeds may vary within the continuous annealing line. In such cases, the structure and characteristics of the high-strength steel sheets that are finally produced It is even more difficult to control. In this way, when variations occur in the structure and characteristics of the high-strength steel sheet that is finally produced, there is a problem during the manufacture of automobiles or the like that are produced using the high-strength steel sheet as a material (for example, the final part shape). When press forming, the amount of springback is not constant, and the dimensional accuracy of the final part is deteriorated, and if it is severe, there is a high possibility that the crack will occur.

  Of course, the properties required for high-strength automotive steel sheets include mechanical properties such as the above-mentioned yield strength (YP), tensile strength (TS), elongation (EL), stretch flangeability (λ), etc. However, high-strength automobile steel sheets and the like are also required to have small variations in characteristics, particularly tensile strength (TS) and elongation (EL).

  In order to reduce the variation in the characteristics of DP steel, the present applicant has conducted intensive research in order to meet the above-mentioned demands. ) To suppress the coarsening of the austenite particles, stabilize the transformation behavior to ferrite during subsequent cooling, and stabilize the microstructure after transformation, and made the following proposals (See Patent Documents 4 and 5).

[Prior art 1]
In Patent Document 4, Al and N are contained as components of a steel sheet at a molar ratio of approximately 1: 1, thereby suppressing the growth of AlN grains and stabilizing the number of AlN particles effective for pinning. By ensuring this, a technique was proposed that can reduce the variation in austenite grain size during heating in the annealing process and reduce the variation in characteristics of the high-strength steel sheet, which is the final product after heat treatment.

[Prior Art 2]
Further, in Patent Document 5, the average temperature increase rate of 600 to 750 ° C. in the temperature increasing process of the annealing process is limited to 0.2 to 2.5 ° C./s, so that austenite grain growth (coarsening) during heating is performed. ) While suppressing the growth (coarse) of AlN grains that exhibit a pinning effect against the austenite, it promotes recrystallization to austenite, thereby reducing the variation in austenite grain size during heating, and in the final product after heat treatment We proposed a technology that can reduce the variation in characteristics of a high-strength steel sheet.

  However, since the prior art 1 needs to contain Al and N in substantially the same amount by mole fraction, if the amount of Al added to the molten steel is increased during the melting of steel, N is required accordingly. Although the amount of addition must be increased, it is technically difficult because bubbles are likely to remain in the steel. On the other hand, if the amount of N added to the molten steel is reduced, the amount of Al added to the molten steel becomes very small. However, this is also technically difficult, and in any case, it is difficult to adjust the steel composition.

  Moreover, since the said prior art 2 needs to reduce a temperature increase rate in the middle of temperature rising in an annealing process, there exists a problem that productivity falls.

JP 2009-215571 A JP 2009-215572 A JP 2004-18911 A JP 2010-100956 A Japanese Patent Application No. 2010-155669

  Accordingly, an object of the present invention is to provide a method for producing a high-strength steel sheet having excellent mechanical property stability without requiring difficult component adjustment and without reducing productivity during heat treatment. is there.

The invention described in claim 1
In mass%, C: 0.05 to 0.3%, Si: 0.7 to 3.0%, Mn: 0.5 to 3.0%, Al: 0.01 to 0.1%, N: The content of N and Al is 0.16 [Al] − [N] + 0.001 ≧ 0 (where [] is the element content (mass%). A steel material having a component consisting of the remaining iron and inevitable impurities,
Heating temperature: 1150 to 1250 ° C., rolling end temperature: 800 to 900 ° C., rolling reduction: 90 to 99.5%, cooling rate after rolling end: 35 ° C./s or more, coiling temperature: 650 ° C. or less After hot rolling,
Cold rolled,
Annealing heating temperature: Heated to Ac3 to Ac3 + 100 ° C. and held for annealing holding time: 1000 s or less, and then, from this annealing heating temperature to the first cooling end temperature: 500 to 750 ° C., the first cooling rate: less than 20 ° C./s Slow cooling is performed at an average cooling rate, and then the second cooling end temperature: 100 ° C. or less is rapidly cooled and annealed at an average cooling rate of 30 ° C./s or more, and then tempering heating temperature: 300 to 600 Tempering holding time at 10 ° C .: Continuous annealing is performed under the condition that tempering is performed for 10 to 1000 seconds,
It has a structure containing 90% or more of ferrite and martensite in total in area ratio, and has a variation width of each of tensile strength and elongation [where variation width = (maximum value−minimum value) / (average value) × 100 (%). ] Is a method for producing a high-strength steel sheet having excellent mechanical property stability, characterized by producing a high-strength steel sheet having both of 3% or less .
The invention described in claim 2
The high-strength steel sheet having excellent mechanical property stability according to claim 1, wherein the component further contains at least one of Ti, Nb, V, and Zr in a total amount of 0.01 to 0.1%. It is a manufacturing method.
The invention according to claim 3
The method for producing a high-strength steel sheet having excellent mechanical property stability according to claim 1 or 2, wherein the component further contains 1% or less of Ni and / or Cu in total.
The invention according to claim 4
The method for producing a high-strength steel sheet having excellent mechanical property stability according to any one of claims 1 to 3, wherein the component further contains Cr: 2% or less and / or Mo: 1% or less. .
The invention described in claim 5
The component is a method for producing a high-strength steel sheet excellent in mechanical property stability according to any one of claims 1 to 4, further comprising 0.0001 to 0.005% of B.
The invention described in claim 6
The component is a method for producing a high-strength steel sheet having excellent mechanical property stability according to any one of claims 1 to 5, further comprising 0.003% or less of Ca and / or REM in total. .

  According to the present invention, the N and Al contents in the steel are controlled within a specific range on the low N content / high Al content side than the prior art 1, and the rolling end temperature is set to a specific temperature range. By controlling, the precipitation of AlN is stabilized and the austenite grains are coarsened by effectively exerting the pinning effect of AlN particles even when the annealing heating holding conditions such as the annealing temperature and the holding time are difficult to control. As a result, the transformation behavior during subsequent cooling is stabilized, and the mechanical properties of the steel sheet after heat treatment can be stabilized.

  In addition, since the N and Al contents in the steel can be controlled within a specific range on the low N content / high Al content side than the prior art 1, it is easy to adjust the steel composition and increase the temperature in the annealing process. Since it is not necessary to reduce the temperature increase rate during the temperature, the mechanical properties of the steel sheet after the heat treatment can be stabilized while maintaining the productivity.

It is a figure which shows typically the time change from the hot rolling process to the heat treatment process, and the time change of the amount of AlN precipitation when manufacturing DP steel by heat-treating with a continuous annealing line (CAL). It is a graph which shows the influence of the temperature which acts on the precipitation amount of AlN in steel by thermodynamic equilibrium calculation. It is a graph which shows the influence of Al content in steel, and N content on the difference (AlN precipitation amount maximum fluctuation width) of AlN precipitation in steel in 800 ° C and 1000 ° C by thermodynamic equilibrium calculation. It is a graph which shows the appropriate range of Al content and N content in steel in this invention and the prior art 1. FIG. It is a graph which shows typically the temperature pattern of the test in an Example.

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

  For that purpose, if the annealing is a two-phase region heating where ferrite and austenite are generated, the ratio of ferrite and austenite inevitably changes due to fluctuations in the annealing heating temperature and its holding time, and after the transformation by the subsequent cooling Since the structure 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 having a pinning effect on the growth of austenite grains (Steel Handbook 4th Edition, Volume 3 (1), Japan Iron and Steel Institute, July 2002, 7 (See Chapter 9, Section 1).

  And in order to stabilize the austenite grain size, it is important to stabilize the number of AlN particles present in the steel during the heat treatment (that is, to suppress variation in the amount of precipitated AlN). As schematically shown in FIG. 1, the variation in the AlN precipitation amount in the heat treatment process is the amount of AlN precipitation generated due to the influence of fluctuations in the heating temperature and rolling end temperature (FDT) in the upper hot rolling process. It is considered that the variation is caused by bringing the heat treatment process almost as it is. Therefore, it was considered that the variation in the AlN precipitation amount in the heat treatment step can be more reliably reduced by reducing the variation in the AlN precipitation amount in the hot rolling step.

  As a result of further investigation based on the above consideration, the present inventors have determined that the steel component composition is in mass% (hereinafter the same as the chemical composition), Al: 0.01 to 0.1%, N: 0. 0.001 to 0.015%, 0.16 × [Al] − [N] + 0.001 ≧ 0 (where [] represents the element content (mass%). The same shall apply hereinafter). In the hot rolling process, the heating temperature is set to 1150 ° C. or higher and Al and N are sufficiently dissolved, and the rolling end temperature (FDT) is set to 800 to 900 ° C., thereby sufficiently reducing the variation in the precipitation amount of AlN. I found out that I can do it.

The invention (present invention) relating to the method for producing a high-strength steel sheet completed based on the above knowledge is as follows:
In mass%, C: 0.05 to 0.3%, Si: 0.7 to 3.0%, Mn: 0.5 to 3.0%, Al: 0.01 to 0.1%, N: The content of N and Al is 0.16 [Al] − [N] + 0.001 ≧ 0 (where [] is the element content (mass%). A steel material having a component satisfying
Heating temperature: 1150 to 1250 ° C., rolling end temperature: 800 to 900 ° C., reduction ratio: 90 to 99.5%, cooling rate after completion of rolling: 20 ° C. or higher, coiling temperature: 650 ° C. or lower After rolling
It is characterized by producing a high-strength steel sheet having a structure mainly composed of ferrite and martensite by cold rolling and continuous annealing.

  According to the present invention, unlike the prior art 2, in the continuous annealing process, it is possible to reduce variations in mechanical properties without lowering the temperature rising rate. Further, unlike the prior art 1, it is no longer necessary to increase the amount of N added even when the Al content is high.

  Hereinafter, the grounds for setting the requirements characterizing the present invention will be sequentially described.

  First, the component composition of the steel used in the production method of the present invention will be described. Hereinafter, all the units of chemical components are mass%.

[Component composition of steel]
Al: 0.01 to 0.1%
Al combines with N to form AlN, thereby suppressing the growth of austenite grains during annealing and contributing to the stabilization of mechanical properties. If it is less than 0.01%, the amount of AlN formed is insufficient, and the austenite grain coarsening suppressing action cannot be exhibited effectively. On the other hand, even if the content exceeds 0.1%, the effect of suppressing the growth of austenite grains by AlN is saturated, so 0.1% is made the upper limit.

N: 0.001 to 0.015%
N is an important element that combines with Al to form AlN, thereby suppressing the growth of austenite grains during annealing and contributing to the stabilization of mechanical properties. If it is less than 0.001%, 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 0.015%, it becomes difficult to produce such as blow holes during casting, so 0.015% is made the upper limit.

0.16 [Al]-[N] + 0.001 ≧ 0
In order to quantitatively predict the variation in the AlN precipitation amount in the hot rolling process, first, a typical component composition (in mass% [hereinafter the same for chemical components]), C: 0.17, by thermodynamic equilibrium calculation. %, Si: 1.3%, Mn: 2.0%) was estimated. The calculation result at N: 0.005% is illustrated in FIG. As shown in the figure, although the AlN precipitation level varies depending on the Al content level, the AlN precipitation amount increases rapidly when cooling from the high temperature side and reaches an almost constant amount at 800 ° C. or lower ( Saturation). Here, the Al particles precipitated in a temperature range exceeding 1000 ° C. are coarsened because the precipitation temperature is high, and the number of particles decreases. Therefore, when the Al content level increases, the AlN precipitation amount precipitated in this temperature range is Although more, it can be ignored as having virtually no pinning action. That is, the AlN particles that effectively exhibit the pinning action can be regarded as only those precipitated between 1000 ° C. and 800 ° C.

  Therefore, the maximum width of variation (variation) in the AlN precipitation amount in the hot rolling process can be defined by the difference between the AlN precipitation amount at 800 ° C. and the AlN precipitation amount at 1000 ° C. by the thermodynamic equilibrium calculation. . Therefore, from FIG. 2, for each Al content level, the difference between the AlN precipitation amount at 800 ° C. and the AlN precipitation amount at 1000 ° C. by the thermodynamic equilibrium calculation (hereinafter referred to as “AlN precipitation amount maximum fluctuation range”). ) And this AlN precipitation amount maximum fluctuation range was plotted in FIG. 3 as a relationship with the Al content (mark ◆ + curve a). In the same figure, the AlN precipitation maximum fluctuation range obtained in the same manner as described above is plotted together with the Al content at different N content levels (0.010%, 0.020%). (N: 0.010% is mark □ + curve b, N: 0.020% is mark ▲ + curve c).

  Here, it has been confirmed by an experiment separately that when the maximum fluctuation range of the AlN precipitation amount is 40 ppm (0.004%) or less, the variation in mechanical characteristics is sufficiently reduced. The upper limit of the allowable range of the maximum fluctuation amount of AlN precipitation is indicated by a straight line m.

  And in the range of 0.01 to 0.1% which is an appropriate range of Al content of the high-strength steel sheet according to the present invention, the X coordinates of the intersections A, B and C of the curves a, b and c and the straight line m When the values of (Al content) were determined, they were 0.023%, 0.056%, and 0.067%, respectively. From this, for example, when the N content is 0.005% (curve a), if the Al content is 0.023% or more, the AlN precipitation maximum fluctuation range is 40 ppm (0.004%) or less. Thus, it can be seen that the variation in mechanical characteristics is sufficiently small.

  Therefore, based on the results obtained above, FIG. 4 shows a range of combinations of Al content and N content in which variation in mechanical properties is sufficiently small. The X and Y coordinate values of the mark ● in FIG. 4 are the X coordinate values (Al content) and N content of the intersections A, B, and C in FIG.

  Therefore, in FIG. 4, if the combination of the Al content and the N content is selected on the curve connecting the three points D, E, and F of the mark ● and in the lower right range, the AlN precipitation amount maximum fluctuation range is 0. It can be expected that the variation in mechanical characteristics will be sufficiently small.

  However, as described above, while increasing the N content becomes difficult to control, it is relatively easy to increase the Al content. Therefore, the N content (Al content is also low) A range on the straight line n and its lower right side (0.16 × [Al ]-[N] + 0.001 ≧ 0) was set to an appropriate range.

  If a combination of Al content and N content is selected in the range on the straight line n and the lower right side thereof (the range of the present invention), the maximum fluctuation range of the AlN precipitation amount becomes 0.004% or less, and the mechanical characteristics It is clear that the variation of the is sufficiently small.

  In addition, in FIG. 4, although the suitable range of the combination of Al content in steel and N content in the prior art 1 was shown, the suitable range is narrow, and also on the side with high Al content, N content However, in the present invention, even if the Al content is high, the variation in mechanical properties can be reduced with a significantly lower N content than that of the prior art 1. Thus, unlike the prior art 1, it is not necessary to increase the N content, and there is an effect that the component adjustment is facilitated.

C: 0.05-0.3%
C is an important element that contributes to an increase in the martensite fraction and affects the balance between strength and elongation. If it is less than 0.05%, the strength cannot be ensured. On the other hand, if it exceeds 0.3%, the weldability, which is a necessary characteristic of the thin steel sheet, cannot be ensured. The range of C content is preferably 0.07 to 0.2%.

Si: 0.7-3.0%
Si is a useful element that can increase strength without significantly degrading elongation by solid solution strengthening. If it is less than 0.7%, such an effect cannot be exhibited effectively. On the other hand, if it is excessively contained, the strength becomes too high and cold working becomes difficult, so the upper limit is made 3.0%. The range of Si content is preferably 1.0 to 2.0%.

Mn: 0.5 to 3.0%
Similar to Si, Mn is a useful element that can increase strength without significantly degrading elongation by solid solution strengthening. Moreover, by improving the hardenability of the steel sheet, the martensite fraction is ensured, and the balance between strength and elongation is improved. If it is less than 0.5%, the solid solution strengthening effect cannot be exhibited effectively, and sufficient hardenability cannot be ensured, and sufficient martensite area ratio cannot be ensured during rapid cooling, so that strength cannot be obtained. On the other hand, if it is excessively contained, the strength becomes too high and cold working becomes difficult, so the upper limit is made 3.0%. The range of Mn content is preferably 1.0 to 2.5%.

  The above are the essential contained elements that define the steel material used in the production method of the present invention, and the balance is iron and inevitable impurities. Further, it is also effective to positively contain the following elements, and the mechanical properties of the high-strength steel sheet produced by the production method of the present invention are further improved depending on the type of chemical component (element) contained. The

  It is effective that the steel material used in the production method of the present invention contains at least one of Ti, Nb, V, and Zr in a total amount of 0.01 to 0.1%. Furthermore, it is effective to contain 1% or less of Ni and / or Cu in total. Furthermore, it is effective to contain Cr: 2% or less and / or Mo: 1% or less. Furthermore, it is effective to contain B in an amount of 0.0001 to 0.005%. Furthermore, it is effective to contain a total of 0.003% or less of elements selected from Ca and REM. Note that REM refers to a rare earth element, that is, a group 3A element in the periodic table.

  Next, manufacturing conditions in the manufacturing method of the present invention will be described below.

[Production conditions]
In order to manufacture a cold-rolled steel sheet, first, steel having the above composition is melted and slab (steel material) is formed by continuous casting or ingot forming, and then hot rolling is performed. 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. The steel sheet produced by the production method of the present invention includes not only cold-rolled steel sheets but also hot-dip galvanized steel sheets and galvannealed steel sheets. Hereinafter, hot rolling conditions and heat treatment conditions will be described.

[Hot rolling conditions]
Heating temperature: 1150 to 1250 ° C., rolling end temperature: 800 to 900 ° C., rolling reduction: 90 to 99.5%, cooling rate after rolling end: 35 ° C./s or more, winding temperature: 650 ° C. or less Hot rolling is performed.

<Heating temperature: 1150 to 1250 ° C.>
In order to sufficiently dissolve the coarse AlN produced during the production of the ingot, it is heated to 1150 ° C. or higher. However, if heated to an excessively high temperature, the solid solution effect is saturated, but problems such as weight loss of the steel material due to surface oxidation of the slab occur, so the temperature is set to 1250 ° C. or lower.

<Rolling end temperature: 800 to 900 ° C.>
In order to stabilize the precipitation of AlN, the temperature at which AlN sufficiently precipitates is set to 900 ° C. or less. However, the rolling load increases at an excessively low temperature, or the rolling load becomes unstable due to two-phase rolling. Since it becomes difficult, the temperature is set to 800 ° C or higher.

<Rolling ratio: 90 to 99.5%>
This is because the steel material is heated by rolling to maintain the temperature within a temperature range where AlN can be precipitated, and further, the precipitation of AlN is further promoted by the dislocations introduced by the rolling. If the rolling reduction is less than 90%, the above effect is insufficient. On the other hand, excessive rolling increases the load and makes the rolling itself difficult, so the rolling reduction is set to 99.5% or less.

<Cooling rate after rolling: 35 ° C./s or more>
If a large amount of scale is formed during cooling after the end of rolling, it becomes difficult to remove even in the subsequent pickling process.For example, in cold-rolled steel sheets, oxides remain on the surface, and the chemical conversion for coating the steel sheets after press forming is performed. Processability deteriorates. In order to suppress the amount of scale generation to such an extent that it can be removed by pickling, the cooling rate after rolling is set to 35 ° C./s or more.

<Winding temperature: 650 ° C. or less>
As with the above cooling rate, a large amount of scale is generated when coiled at a high temperature, so the coiling temperature is 650 ° C. or less.

[Preferred heat treatment conditions]
This heat treatment condition is a preferable heat treatment condition for producing DP steel by the production method of the present invention. Annealing heating temperature: Heated to Ac3 to Ac3 + 100 ° C. and held for annealing holding time: 1000 s or less, and then, from this annealing heating temperature to the first cooling end temperature: 500 to 750 ° C., the first cooling rate: less than 20 ° C./s Slow cooling is performed at an average cooling rate, and then the second cooling end temperature: 100 ° C. or less is rapidly cooled and annealed at an average cooling rate of 30 ° C./s or more, and then tempering heating temperature: 300 to 600 Tempering holding time at 10 ° C .: 10 to 1000 s.

<Annealing heating _{temperature: Ac 3 ~Ac 3 + 100 ℃} , annealing holding time: 1000 s or less>
Ac Heating less than ₃ points 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 initial structure is not stabilized, the final structure after the heat treatment is not stabilized, and as a result, the mechanical properties of the steel sheet are not dispersed and stabilized. Therefore, the annealing heating temperature is set to ₃ points or more of Ac that can be converted to austenite single phase. On the other hand, when heating is performed at a temperature exceeding Ac ₃ + 100 ° C., coarsening of the AlN particles becomes remarkable, so that austenite grain growth can be effectively prevented, and variations in mechanical properties cannot be sufficiently suppressed. Therefore, the annealing heating temperature needs to be Ac ₃ + 100 ° C. or lower. Here, Ac ₃ is Ac ₃ (° C.) = 910−203 · √ [C] + 29.1 · [Si] − (30 · [Mn] −700 · [P] −400 [Al]) (Leslie Steel) The value calculated by material science, Maruzen (1985) p.273) may be used.

  Further, if the annealing holding time is too long, the productivity is extremely deteriorated, so the annealing holding time is set to 1000 s or less.

<First cooling end temperature: 500 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 in order 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 500 to 750 ° C., which is a temperature range where ferrite transformation can occur. .

  When the first cooling end temperature is less than 500 ° C., bainite is formed during cooling, while when 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 100 ° C. or less, second cooling rate: rapid cooling at an average cooling rate of 30 ° 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 100 ° C. or the second cooling rate is lower than 30 ° C./s, bainite is formed, so that a balance between strength and elongation cannot be secured.

<Tempering heating temperature: Tempering holding time: 10 to 1000 s tempering at a temperature of 300 to 600 ° C.>
Ductility can be enhanced while securing strength by tempering hard martensite and softening.

  When the tempering heating temperature is less than 300 ° C., the martensite is not sufficiently softened, so that 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 is less than 10 s, the martensite is not sufficiently softened, so that the elongation cannot be secured. On the other hand, when the tempering holding time exceeds 1000 s, productivity is lowered, which is not preferable.

  The steel sheet manufactured under the above manufacturing conditions has a structure mainly composed of ferrite and martensite. Depending on the component composition and heat treatment conditions of the steel material, it may contain other structures such as bainite, pearlite, and cementite in addition to ferrite and martensite, but the effects of the present invention can be achieved even when these structures are included in a small amount. Therefore, these are included in the category of the present invention. The allowable range of the structure other than ferrite and martensite is 10% or less in area ratio, preferably 5% or less, more preferably 3% or less.

  Moreover, the manufacturing method by said manufacturing conditions is applicable to each manufacturing method of a cold-rolled steel plate, a hot dip galvanized steel plate, and an galvannealed steel plate.

  Steels composed of various components shown in Table 1 below were melted to produce 120 mm thick ingots, which were hot rolled under the hot rolling conditions shown in Table 2 below (see the temperature pattern in FIG. 5). Thereafter, this was further pickled to remove the scale, and cold-rolled to a thickness of 1.6 mm to obtain a test material.

  And each sample material was heat-treated under the heat treatment conditions shown in Table 3 and Table 4 below (see the temperature pattern in FIG. 5), and the mechanical properties of each steel plate (product steel plate) after the heat treatment were measured. The stability of the mechanical properties was evaluated from the degree of variation of the results, and the results are also shown in Table 3 and Table 4.

  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.

  In order to evaluate the influence of variations in hot rolling conditions and / or heat treatment conditions on the degree of variation in the mechanical properties of the product steel sheet, TS and The variation width of each EL: (maximum value−minimum value) / (average value) × 100 (%) was calculated, and the variation width of these TS and EL satisfying 3% or less was regarded as acceptable.

[Test 1] When only the heat treatment conditions are changed First, Table 3 below summarizes the results of tests conducted to investigate the effect on the degree of variation in the mechanical properties of product steel plates due only to changes in the heat treatment conditions. It is. That is, about the thing which hot-rolled the slab which has each component composition of following Table 1 on the conditions of hot rolling condition b1 of following Table 2, heating temperature fluctuates as a typical thing of the fluctuation | variation of the process conditions of an annealing process. Assuming the case, heat treatment was performed at two levels of 900 ° C. and 925 ° C. to prepare a set of two points, and the variation widths of TS and EL of the product steel plate were obtained.

[Test 2] When both hot rolling conditions and heat treatment conditions are changed Next, Tables 4 and 5 below show the product steel plate machine when both hot rolling conditions and heat treatment conditions change. This is a summary of the results of tests conducted to investigate the effect on the degree of variation in the physical characteristics. That is, the slab having the composition of steel type A shown in Table 1 below was hot-rolled under the hot rolling conditions shown in Table 2 below, as in Test 1 above, annealing temperatures of 900 ° C. and 925 ° C. 2 A set of four samples were prepared by heat treatment at a standard, and the variation widths of TS and EL of the product steel plate were determined.

  As is apparent from the results shown in Tables 3 to 5 below, when a high-strength steel sheet is manufactured under conditions satisfying both the component composition of the steel material and the hot rolling conditions specified in the present invention (steel No. 1-1) -1-10, Steel No. 2-1 to 2-5), and the variation width of TS and EL of the product steel plate are both 3% or less, satisfying the acceptance criteria (judgment: ◯), It was confirmed that the variation was reliably reduced.

  On the other hand, when manufacturing a high strength steel plate on the conditions which do not satisfy at least any one of the component composition of steel materials and hot rolling conditions prescribed | regulated by this invention (steel No.1-11-1-14, steel No.1). 2-6 to 2-10), depending on the conditions, it may happen that the dispersion of mechanical properties may satisfy the acceptance criteria (Steel Nos. 2-6, 2-7), but overall it does not satisfy the acceptance criteria. It can be seen that there are many cases (judgment: x).

Claims (6)

In mass%, C: 0.05 to 0.3%, Si: 0.7 to 3.0%, Mn: 0.5 to 3.0%, Al: 0.01 to 0.1%, N: The content of N and Al is 0.16 [Al] − [N] + 0.001 ≧ 0 (where [] is the element content (mass%). A steel material having a component consisting of the remaining iron and inevitable impurities,
Heating temperature: 1150 to 1250 ° C., rolling end temperature: 800 to 900 ° C., rolling reduction: 90 to 99.5%, cooling rate after rolling end: 35 ° C./s or more, coiling temperature: 650 ° C. or less After hot rolling,
Cold rolled,
Annealing heating temperature: Heated to Ac3 to Ac3 + 100 ° C. and held for annealing holding time: 1000 s or less, and then, from this annealing heating temperature to the first cooling end temperature: 500 to 750 ° C., the first cooling rate: less than 20 ° C./s Slow cooling is performed at an average cooling rate, and then the second cooling end temperature: 100 ° C. or less is rapidly cooled and annealed at an average cooling rate of 30 ° C./s or more, and then tempering heating temperature: 300 to 600 Tempering holding time at 10 ° C .: Continuous annealing is performed under the condition that tempering is performed for 10 to 1000 seconds,
It has a structure containing 90% or more of ferrite and martensite in total in area ratio, and has a variation width of each of tensile strength and elongation [where variation width = (maximum value−minimum value) / (average value) × 100 (%). ] Is a method for producing a high-strength steel sheet excellent in mechanical property stability, characterized by producing a high-strength steel sheet having 3% or less .   The high-strength steel sheet having excellent mechanical property stability according to claim 1, wherein the component further contains at least one of Ti, Nb, V, and Zr in a total amount of 0.01 to 0.1%. Production method.   The method for producing a high-strength steel sheet having excellent mechanical property stability according to claim 1 or 2, wherein the components further contain 1% or less of Ni and / or Cu in total.   The method for producing a high-strength steel sheet having excellent mechanical property stability according to any one of claims 1 to 3, wherein the component further contains Cr: 2% or less and / or Mo: 1% or less.   The method for producing a high-strength steel sheet having excellent mechanical property stability according to any one of claims 1 to 4, wherein the component further contains B in an amount of 0.0001 to 0.005%.   The method for producing a high-strength steel sheet having excellent mechanical property stability according to any one of claims 1 to 5, wherein the component further contains 0.003% or less of Ca and / or REM in total.

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