JP2012017500A - Method for production of high-strength steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for production of a high-strength steel sheet capable of reducing to the utmost, variation in composition and characteristics of the finally-produced high-strength steel sheet even in such a case that the composition before continuous annealing varies, or that the temperature condition or the like when continuously annealing fluctuates.SOLUTION: The method includes: elevating the temperature of a steel material having desired component composition to 600°C at an average temperature rising rate of 3°C/sec or more and to 600-750°C at the rising rate of 0.2-2.5°C/sec; performing a heating process of elevating the temperature to the range of Ac3 to (Ac3+50°C); performing a soaking process of keeping the above temperature range for ≤600 sec; and then quenching and tempering.

Description

  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, thereby ensuring ductility (elongation) with soft ferrite and securing strength with hard martensite. Therefore, this DP steel has been widely adopted in recent years as a high-strength automobile steel sheet and the like that require excellent formability since 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 stringent, 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 that are already the previous process of the continuous annealing process, there are often variations in the structure due to variations in the process conditions, etc.If there is a variation 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 has not been regarded as a problem in the past. It can be said that the current situation is that the reduction of variation is becoming an issue.

  Furthermore, various process conditions may vary even in a continuous annealing line. In such a case, it is even more necessary to control the structure and characteristics of the high-strength steel sheet that is finally produced. It's getting harder. In this way, when variations occur in the structure and characteristics of the high-strength steel sheet that is finally produced, there is a high possibility that it will lead to problems during the manufacture of automobiles and the like that are produced using the high-strength steel sheet as a material. Become.

  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) and the like. However, high-strength automobile steel sheets and the like are also required to have small variations in structure and characteristics. However, at present, it is very difficult to reduce variations in the structure and characteristics of DP steel by conventional methods.

JP 2009-215571 A JP 2009-215572 A JP 2004-18911 A

  The present invention has been made as a solution to the above-described conventional problems, and even if the structure before continuous annealing is varied or the temperature conditions during continuous annealing vary, the final It is an object of the present invention to provide a method for producing a high-strength steel sheet that can reduce variations in the structure and characteristics of the high-strength steel sheet produced as much as possible.

  Invention of Claim 1 is the mass%, C: 0.05-0.3%, Si: 0.7-3.0%, Mn: 0.5-3.0%, Al: 0.01 A method for producing a high-strength steel sheet, which contains 0.1% and a high-strength steel sheet having a structure mainly composed of ferrite and martensite by continuous annealing, and has an average temperature increase rate up to 600 ° C. of 3 ° C. / Sec or more, an average heating rate of 600 ° C. to 750 ° C. was increased at 0.2 to 2.5 ° C./sec, and then a heating step was performed to increase the temperature to Ac3− (Ac3 + 50 ° C.). Then, it is the manufacturing method of the high-strength steel plate characterized by implementing the soaking process which hold | maintains 600 sec or less in the temperature range of Ac3- (Ac3 + 50 degreeC), and then performing hardening and tempering.

  According to the method for producing a high-strength steel sheet of the present invention, even if the structure before continuous annealing is varied or the temperature conditions during continuous annealing vary, the final manufactured high Variations in the structure and characteristics of the high strength steel sheet can be reduced.

It is a graph which shows the relationship between the temperature and time of a basic heat treatment process in case DP steel is manufactured with a continuous annealing line. It is a graph which shows the relationship between the temperature and time of the heating process and soaking process of the manufacturing method of the high strength steel plate of this invention.

  When manufacturing the high-strength steel sheet by continuous annealing, the present inventors have a final structure even if the structure before the continuous annealing varies or the temperature conditions during continuous annealing vary. In order to find a method capable of reducing the variation in characteristics of the high-strength steel sheet manufactured in an efficient manner, the inventors have made extensive studies. As a result, it has been found that stabilizing the γ grain size at the end of continuous annealing without variation is an effective method for reducing variation in characteristics of the finally produced high-strength steel sheet.

  When producing a composite structure steel (DP steel) having a structure mainly composed of ferrite and martensite in a continuous annealing line, as shown in FIG. 1, heating, soaking, slow cooling, rapid cooling, reheating, holding, It goes through a heat treatment process called cooling.

  The structure of the steel material when DP steel is manufactured through this heat treatment process changes as follows. First, in the heating process and the soaking process, the steel material has a γ single-phase structure. In the slow cooling process up to the next quenching start temperature, ferrite is precipitated in the structure, and the remaining austenite is transformed into martensite in the subsequent rapid cooling process. As a result, the steel material that has finished the rapid cooling step has a structure mainly composed of ferrite and martensite. Furthermore, the strength of martensite is adjusted by a tempering process including reheating, holding, and cooling.

  At this time, depending on the component composition of the steel material, heat treatment conditions, etc., it may contain other structures such as bainite, pearlite, and cementite in addition to ferrite and martensite. The effects of the present invention are intended, and therefore these are included in the scope 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.

  Previously, the mechanical properties required for steel sheets for automobiles, etc. were explained as yield strength (YP), tensile strength (TS), elongation (EL), stretch flangeability (λ), etc. In the case of DP steel mainly composed of ferrite and martensite, the ferrite fraction and the hardness of martensite can be cited as the structure factor that governs the above characteristics. Among these, the ferrite fraction is determined from the temperature history until the start of quenching, and the hardness of martensite is determined from the temperature history of reheating (tempering).

  Here, if there is a large variation in the γ grain size at the end of continuous annealing due to variations in the structure and soaking temperature before continuous annealing, even if the subsequent thermal history variation is minimized, the final The variation in characteristics of high strength steel sheets that are manufactured in a conventional manner cannot be reduced.

  As described above, DP steel is retained in the γ single phase region in the soaking process at the time of production, and then ferrite is precipitated in the α + γ2 phase region in the subsequent slow cooling step. Changes depending on the γ particle size at the end of the soaking step. When the γ grain size at the end of the soaking step becomes coarse, the frequency of ferrite nucleation decreases. On the other hand, when the γ grain size at the time when the soaking process is completed becomes fine, the frequency of nucleation of ferrite increases.

  Therefore, if there is a large variation in the γ grain size at the time when the soaking process is completed, even if it is attempted to control the variation in the subsequent slow cooling step, the final ferrite fraction will vary. . Conversely, if the variation in γ grain size at the end of the soaking process can be reduced, even if there is some variation in the subsequent heat history, the desired ferrite fraction and the desired It can be said that the martensite hardness after tempering can be obtained, and it becomes possible to reduce variations in characteristics of the finally produced high-strength steel sheet.

  Therefore, the degree of variation in the γ grain size at the end of the soaking process can specifically reduce the variation in the structure and properties of the high-strength steel sheet that is finally produced. Preliminary tests were conducted and evaluated.

  In this preliminary test, first, after the soaking step, a heat treatment for quenching was performed immediately to measure the austenite grain size. At that time, various austenite grain sizes were prepared by changing the soaking temperature and holding time. The same heat treatment was performed three times, and the average value of N = 3 clarified the heat treatment conditions of the soaking step, and the relationship between the austenite and the particle size that changed accordingly.

  Next, after performing various soaking process heat treatments in which the austenite grain size at the end of the soaking process is known by the above-described method, the following explanation will be given which can be considered in the production of ordinary DP steel as shown in FIG. Various heat treatments in the range of heat treatment conditions were applied. After completion of the heat treatment, a JIS No. 5 tensile test piece was collected and subjected to a tensile test to measure the tensile strength. For one soaking step (i.e., austenite particle size), the heat treatment and the tensile test were performed five times under the same conditions thereafter, and the tensile strength was determined based on the average value.

  In these preliminary tests, the DP steel that was finally produced had a strength variation within ± 3%, and was accepted. As a result, it was confirmed that if the variation in γ grain size at the end of the soaking process was 4 μm or less, the variation in the structure and characteristics of the finally produced high-strength steel sheet could be reduced.

  The heat treatment conditions considered in the production of ordinary DP steel are as follows: annealing rate R11 in FIG. 1 is 3 to 20 ° C./sec, quenching start temperature T11 is 500 to 700 ° C., and quenching rate R12 is 50 ° C./sec or more. The reheating degree T12 is 250 to 550 ° C., and the holding time t12 is 30 to 1200 sec.

  In addition, it can be said that the variation in the γ grain size at the time when the soaking process is completed increases due to the variation in the structure and soaking temperature before the continuous annealing, the results of various studies by the present inventors. By defining various conditions in the heating process and soaking process in continuous annealing, even if there is a variation in the structure before soaking and soaking temperature, It has been found that variation in characteristics can be reduced.

  Specifically, as shown in FIG. 2, first, it is important to divide the heating process into a plurality of stages and perform heating at different heating rates, and the average heating rate up to 600 ° C. is 3 ° C. The important point is that the average rate of temperature increase from 600 ° C. to 750 ° C. is from 0.2 to 2.5 ° C./sec. It is also important to carry out a heating step for raising the temperature to a temperature range of Ac3 to (Ac3 + 50 ° C.), and then a soaking step for 600 sec or less in the temperature range of Ac3 to (Ac3 + 50 ° C.). .

  The reason for setting the average temperature rising rate up to 600 ° C. to 3 ° C./sec or higher is that there is almost no recrystallization at temperatures up to 600 ° C., but it is not necessary to slow down the temperature rising rate. It was set to ° C / sec or more.

  On the other hand, 600 ° C. to 750 ° C. is a temperature region in which recrystallization behavior occurs abruptly. It was decided that the recrystallization was sufficiently promoted by slowing it up compared to the temperature rising rate until. The reason for setting the upper limit of the heating rate to 2.5 ° C./sec is that sufficient recrystallization cannot be achieved when the heating rate exceeds 2.5 ° C./sec. On the other hand, the reason for setting the lower limit of the heating rate to 0.2 ° C./sec is that when the heating rate is less than 0.2 ° C./sec, precipitation and coarsening of AlN become remarkable, and in the next soaking step. This is because the pinning effect of the γ grain size becomes dilute.

  On the other hand, the rate of temperature rise until reaching the soaking temperature in the temperature range of Ac3 to (Ac3 + 50 ° C) exceeding 750 ° C has very little effect on the structure change. Although it is not necessary to take this into consideration, it is desirable that the temperature is appropriately determined within a range of a temperature increase rate that is normally performed at 0.3 to 10 ° C./sec in consideration of the temperature increase capability and productivity of the equipment.

  In addition, in the temperature rising process in ordinary DP steel, there is no particular change in the temperature rising rate during the process. For example, as shown in the example of Patent Document 3, heating is performed to a target soaking temperature at a temperature rising rate of about 20 ° C./second without changing the temperature rising rate on the way.

  In addition, the soaking step for holding 600 sec or less in the temperature range of Ac3 to (Ac3 + 50 ° C.) is performed, but the holding time in this soaking step also affects the variation in γ particle size. When this holding time exceeds 600 seconds, not only has the disadvantage of inhibiting productivity, but also the growth of γ grains is promoted, resulting in variations in γ grain sizes. On the other hand, the lower limit of the holding time in the soaking step is not limited. However, if the holding time is too short, the reverse transformation may become insufficient, and thus it is desirable that the holding time be 60 seconds or longer.

  Next, the chemical component composition in the high-strength steel plate manufactured by the manufacturing method of the present invention will be described. Even if the production method described above is appropriate, the high-strength steel plate produced by the production method of the present invention has a desired effect as long as the content of each chemical component (element) is not within an appropriate range. I can't. Therefore, the high-strength steel sheet produced by the production method of the present invention also requires that the content of each chemical component is within the range described below. In addition, all the content (%) of the following chemical component shows the mass%.

C: 0.05-0.3%
C is an important element that affects the area ratio of martensite and its hardness and affects the yield strength and stretch flangeability. If it is less than 0.05%, the area ratio of martensite is insufficient and sufficient yield strength cannot be ensured. On the other hand, if it exceeds 0.3%, the martensite becomes too hard and stretch flangeability cannot be secured. Therefore, the C content needs to be 0.05 to 0.3%. The minimum with preferable content of C is 0.07%, and a preferable upper limit is 0.2%.

Si: 0.7-3.0%
Si, as a solid solution strengthening element, increases the yield strength without degrading the elongation, and also has the effect of suppressing the coarsening of the cementite particles present in the martensite during tempering. It is a useful element that also has the effect of improving stretch flangeability by suppressing its formation. If it is less than 0.7%, such an effect cannot be exhibited effectively. On the other hand, if it exceeds 3.0%, the formation of austenite at the time of heating is hindered, so the area ratio of martensite cannot be secured, and the yield strength and stretch flangeability cannot be secured. Therefore, the Si content needs to be 0.05 to 3.0%. The preferable lower limit of the Si content is 1.0%, and the preferable upper limit is 2.0%.

Mn: 0.5 to 3.0%
Similar to Si, Mn, as a solid solution strengthening element, increases the yield strength without deteriorating the elongation, and also has the effect of suppressing the coarsening of cementite particles present in the martensite during tempering. It is a useful element that also has the effect of improving stretch flangeability by suppressing the generation of particles. Moreover, it has the effect of improving yield strength and stretch flangeability by increasing the hardenability and contributing to securing the area ratio of martensite. If it is less than 0.5%, the solid solution strengthening action and the cementite coarsening suppressing action cannot be exhibited effectively, and a large amount of bainite is formed during quenching for quenching, resulting in insufficient martensite area ratio. Therefore, the yield strength and stretch flangeability cannot be secured. Therefore, the Mn content is set to 0.5 to 3.0%. The preferable lower limit of the Mn content is 1.0%, and the preferable upper limit is 2.5%.

Al: 0.01 to 0.1%
Al combines with the inevitable impurity N to form AlN, reducing the solid solution N that contributes to the occurrence of strain aging, thereby preventing the stretch flangeability from deteriorating and contributing to improving the strength by solid solution strengthening. To do. If it is less than 0.01%, solute N remains in the steel, strain aging occurs, and it becomes impossible to ensure elongation and stretch flangeability. On the other hand, if it exceeds 0.1%, the formation of austenite at the time of heating is inhibited, so the area ratio of martensite cannot be ensured and stretch flangeability cannot be ensured. Therefore, the Al content is set to 0.01 to 0.1%. The preferable lower limit of the Al content is 0.03%, and the preferable upper limit is 0.08%.

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

  It is effective for the high-strength steel plate manufactured by the manufacturing method of the present invention to contain at least one of Ti, Nb, V, and Zr in a total 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.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and the present invention is implemented with appropriate modifications within a range that can meet the gist of the present invention. These are all included in the technical scope of the present invention.

  In Examples of the present invention, first, steels having respective component compositions shown in Table 1 were melted, and various steel sheets having various structures were produced through a hot rolling process, a pickling process, and a cold rolling process. Then, after performing the heat treatment simulating the heating process and the soaking process in the continuous annealing line under the manufacturing conditions shown in Table 2 and FIG. 2, finally quenching as shown in FIG. The γ particle size of the tissue was measured. In the heat-rolled structure in Table 2, B indicates bainite, F indicates pearlite, P indicates pearlite, and M indicates martensite.

  The measurement of the γ particle size was performed according to the method described in JIS G0551, and the evaluation was performed by converting the particle size number into a particle size.

  The test results are shown in Table 2. In this test, heat treatment was performed simulating the four conditions of temperature equalization process and holding time, and the γ grain size of each steel sheet structure after rapid cooling was measured, and the difference between the maximum and minimum values was measured. It was set as the dispersion | variation in (gamma) particle size at the time of completion | finish of continuous annealing. In this test, the variation was within 4 μm.

  No. Nos. 1 to 6 are invention examples satisfying the requirements of the present invention, and the variations in the γ particle diameter were all within 4 μm, and the test results were acceptable. In contrast, no. Nos. 7 to 9 are comparative examples in which the average temperature rising rate of 600 ° C. to 750 ° C. or the soaking temperature condition to be maintained is inappropriate. As a result, the variation in the γ particle size becomes larger than 4 μm, and the test results are not good. It was a pass.

Claims (1)

In mass%, C: 0.05-0.3%, Si: 0.7-3.0%, Mn: 0.5-3.0%, Al: 0.01-0.1% A manufacturing method of a high-strength steel sheet for manufacturing a high-strength steel sheet having a structure mainly composed of ferrite and martensite by continuous annealing,
The average temperature increase rate up to 600 ° C. is 3 ° C./sec or more, the average temperature increase rate from 600 ° C. to 750 ° C. is increased from 0.2 to 2.5 ° C./sec, and then Ac3— (Ac3 + 50 ° C.) After carrying out the heating process to raise the temperature to the temperature range,
The manufacturing method of the high strength steel plate characterized by implementing the soaking process which hold | maintains 600 sec or less in the temperature range of the Ac3- (Ac3 + 50 degreeC), and then quenching and tempering.

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