JP2007138262A - High strength cold rolled steel sheet reduced in dispersion in mechanical characteristic, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength cold rolled steel sheet reduced in dispersion in mechanical properties and its manufacturing method. <P>SOLUTION: The high strength cold rolled steel sheet has a composition which consists of 0.06 to 0.15% C, 0.5 to 1.5% Si, 1.5 to 3.0% Mn, ≤0.05% P, ≤0.01% S, 0.5 to 1.5% Al and the balance Fe with inevitable impurities and in which the value of A, defined by A=Si+9×Al, satisfies 6.0≤A≤20.0. Moreover, the steel sheet has a structure which is composed of a dual-phase structure of ferrite and martensite and in which the area ratio of the ferrite is 40 to 90%. When manufacturing this steel sheet, recrystallization annealing/tempering treatment is carried out by carrying out: holding at a temperature ranging from the Ac<SB>1</SB>point and the Ac<SB>3</SB>point for ≥10 sec; cooling down to 500 to 750°C at ≤20°C/sec cooling rate; rapid cooling down to ≤100°C at ≥100°C/sec cooling rate; and tempering at 300 to 500°C. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a steel plate suitable for, for example, a strength member of an automotive part that requires excellent bending workability and stretch flangeability, and a method for manufacturing the same.

  In recent years, lighter weight parts are required for structural parts of automobiles from the standpoint of resource saving and energy saving. On the other hand, high-strength steel sheets with a tensile strength exceeding 590 MPa have begun to be applied in order to improve collision safety. Further, the structural member is often used by being press-molded into a complicated shape. For this reason, when the mechanical characteristics vary, the amount of return by the spring bag or the like is different, and the shape defect becomes a problem.

In contrast to the current situation, it is known that variations in mechanical properties occur due to variations in chemical components and variations in manufacturing conditions, and several methods for reducing such variations have been proposed.
For example, in Patent Document 1, the thickness of the steel sheet, the carbon content, the phosphorus content, the quenching start temperature, the quenching stop temperature and the relationship between the tempering temperature after quenching and the tensile strength, Considering the carbon content, phosphorus content, quenching stop temperature, and tempering temperature after quenching, calculate the quenching start temperature according to the target tensile strength, and quenching at the obtained quenching start temperature, the strength variation A method of reducing is disclosed.

  In Patent Document 2, in producing a structure containing 3% or more of residual γ, primary cooling is performed to a temperature range of 450 to 550 ° C after soaking, and the temperature range of 450 to 400 ° C is compared with the primary cooling rate. By adopting a two-stage cooling method in which secondary cooling is performed at a low cooling rate, a homogeneous steel sheet with little variation in elongation in the sheet width direction is obtained.

Patent Document 3 discloses that Si and Mn are added in a larger amount than that of the conventional steel sheet, thereby reducing variation in elongation characteristics in the sheet width direction of the steel sheet.
Japanese Patent Laid-Open No. 2003-277832 JP 2000-212684 A Japanese Patent Laid-Open No. 2004-256872

  However, in Patent Document 1, since the quenching temperature is changed according to the chemical component, even if the variation in tensile strength is reduced, the martensite area ratio is different between coils, so the variations in YP, El, and λ are not reduced. .

  The technique described in Patent Document 2 is intended for steel containing ferrite, bainite, and residual γ, and cannot be applied to a dual-phase steel whose main structure is ferrite and martensite. That is, the manufacturing conditions are significantly different from the case of obtaining a steel sheet having a structure composed of ferrite and martensite. Further, although reduction in elongation variation is mentioned, it does not suggest that a high-strength steel sheet with small variation in stretch flangeability can be produced.

  Patent Document 3 is a technique relating to a steel sheet having a ferrite and tempered martensite structure. However, since Si is added in a large amount of 1.6 to 3.0%, deterioration of surface properties becomes a problem due to grain boundary oxidation of Si.

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

  As a result of intensive studies to solve such problems, the present inventors have obtained the following knowledge. That is, when Al is added in an amount of 0.5 to 1.5%, a change in structure due to fluctuations in annealing conditions is reduced, and a high workability and high strength cold-rolled steel sheet with little variation in mechanical properties such as elongation and stretch flangeability can be obtained.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] mass%, C: 0.06 to 0.15, Si: 0.5 to 1.5, Mn: 1.5 to 3.0, P: ≦ 0.05, S: ≦ 0.01, Al: 0.5 to 1.5, and A = Si + 9 × Al defined by Al satisfies 6.0 ≦ A ≦ 20.0, the balance is composed of iron and inevitable impurities, and a two-phase structure of ferrite and martensite, and the area ratio of ferrite is 40% or more and 90% or less A high-strength cold-rolled steel sheet having a small mechanical property variation, characterized by having a structure of
[2] The high-strength cold-rolled steel sheet with small variation in mechanical properties, characterized in that in [1], the material further contains one or two of mass%, Cr: 1% or less, Mo: 1% or less. .
[3] In the above [1] or [2], further, in mass%, V: 0.05 to 0.2%, B: 0.0002 to 0.002%, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.1% or A high-strength cold-rolled steel sheet having small variations in mechanical properties, characterized by containing two or more.
[4] The steel having the composition according to any one of [1] to [3] is hot-rolled and then cold-rolled, and then subjected to recrystallization annealing / tempering treatment. Annealing and tempering treatment is held at a temperature of Ac1 or more and Ac3 or less for 10s or more, and cooled to 500 to 750 ° C at a cooling rate of 20 ° C / s or less. A method for producing a high-strength cold-rolled steel sheet with small variations in mechanical properties, characterized by quenching at a cooling rate and tempering at 300 to 500 ° C.

  In addition, in this specification,% which shows the component (composition) of steel is all mass%.

  In the present invention, the high-strength cold-rolled steel sheet is a cold-rolled steel sheet having a tensile strength (TS) suitable for machine structural components and having a tensile strength (TS) of 590 Mpa or more.

  According to the present invention, it is possible to obtain a high workability and high strength cold-rolled steel sheet with small variations in mechanical properties. Since the steel sheet of the present invention has high strength, and is excellent in elongation and stretch flangeability, and its variation is small, conventionally, it is difficult to apply high-strength steel sheets, such as difficult-to-form members such as automobile structural members. It becomes possible to apply. Furthermore, when the high-strength cold-rolled steel sheet of the present invention is used as an automobile structural member, it contributes to reducing the weight of the automobile, improving safety, and the like, and can be said to be an extremely useful invention in industry.

  The reason for limiting the chemical composition (composition) of steel in the present invention will be described below.

C: 0.06-0.15%
C is an important element for strengthening the martensite in the quenched structure. If it is less than 0.06%, the effect of increasing the strength becomes insufficient. On the other hand, when C exceeds 0.15%, weldability deteriorates. From the above, C is 0.06% or more and 0.15% or less.

Si: 0.5-1.5%
Si is an effective element for obtaining a steel plate having high strength and high elongation, and is extremely important in promoting variation reduction. In order to exhibit such an effect effectively, it is preferable to add 0.5% or more. If it is less than 0.5, the effect of the present invention cannot be exhibited effectively. On the other hand, when a large amount of Si is added, a large amount of Si oxide is formed on the surface of the steel sheet, the chemical conversion treatment property deteriorates, and the surface properties deteriorate due to grain boundary oxidation. Therefore, Si is 0.5% or more and 1.5% or less.

Mn: 1.5-3.0%
Mn is an element useful for ensuring high strength stably. In order to obtain such an effect, 1.5% or more is added. On the other hand, if it exceeds 3.0%, slab cracking occurs in the continuous casting process. From the above, Mn is set to 0.5% or more and 3.0% or less.

P: 0.05% or less
P segregates at the prior austenite grain boundaries to deteriorate the low temperature toughness and has a strong tendency to segregate in the steel, thereby reducing the workability of the steel sheet. Therefore, P is preferably reduced as much as possible. For this reason, P is made 0.05% or less.

S: 0.01% or less
S is an impurity in the steel of the present invention, and when it segregates in the prior austenite grain boundaries or contains a large amount of MnS, it lowers the low-temperature toughness and lowers the workability of the steel sheet. For this reason, S is made 0.01% or less.

Al: 0.5-1.5%
Al is the most important element in the present invention. It is useful as a deoxidizer and has the effect of suppressing the formation of carbides. In addition, there is also an effect of greatly increasing the Ac3 point, and this effect is extremely important for promoting the reduction of variation in the present invention. In order to exhibit such an effect effectively, it is preferable to add 0.5% or more. On the other hand, when it is added excessively exceeding 1.5%, not only the production cost of steel increases, but also the surface properties deteriorate. From the above, Al is 0.5% to 1.5%.

A = Si + 9 × Al: 6.0-20.0
A is defined as A = Si + 9 × Al, which is an important requirement in the present invention. When A is less than 6.0, strength stability decreases. Further, if it exceeds 20.0, not only the alloy cost increases, but also the surface properties of the steel deteriorate. Therefore, A is set to 6.0 or more and 20.0 or less.

  With the above essential additive elements, the steel of the present invention can achieve the desired characteristics. In addition to the above essential additive elements, the steel according to the present invention can contain the following elements according to desired characteristics.

Cr: 1% or less, Mo: 1% or less
Cr: 1% or less
Cr is added as necessary to increase the strength of the steel sheet. In some cases, such as improved corrosion resistance. In order to obtain the above effect without impairing the effect of the present invention, Cr is preferably added in a content of 1% or less.

Mo: 1% or less
Mo is a precipitation strengthening element, but if it is too much, the ductility is lowered and the price is also expensive. Therefore, when added, Mo is made 1% or less.

V: 0.05-0.2%, B: 0.0002-0.002%, Nb: 0.005-0.1%, Ti: 0.005-0.1%
V, B, Nb, Ti nitride-forming elements can be contained within a range that does not deteriorate the properties and manufacturability for the purpose of adjusting the strength. In this case, V: 0.05% or more,% 0.2% or less, B : 0.0002% or more and 0.002% or less, Nb: 0.005% or more and 0.1% or less, Ti: 0.005% or more and 0.1% or less, preferably 1 type or 2 types or more.

  The balance other than the above consists of iron and inevitable impurities. As an unavoidable impurity, for example, O forms non-metallic inclusions and adversely affects quality, so it is desirable to reduce it to 0.003% or less. In the present invention, Cu, Ni, W, Zr, Sn, and Sb may be contained in the range of 0.1% or less as impurity elements that do not impair the effects of the present invention.

Next, the reasons for limiting the metal structure of the high-strength cold-rolled steel sheet of the present invention will be described.
First, the high-strength cold-rolled steel sheet of the present invention has a two-phase structure of ferrite and martensite that is advantageous for simultaneously improving strength and workability.

  The area ratio of ferrite is 40 to 90%. When the area ratio of ferrite is less than 40%, the elongation characteristics are inferior. On the other hand, when it exceeds 90%, it becomes difficult to achieve a tensile strength of 590 MPa. By setting the area ratio of ferrite to 40% or more and 90% or less, high strength can be secured and elongation and stretch flangeability are improved. The balance is basically martensite, but it is allowed to contain, for example, 10% or less of retained austenite, bainite, etc. within a range that does not impair the effects of the present invention.

  Next, the manufacturing method of the high-strength cold-rolled steel sheet of this invention is demonstrated.

  From the molten steel adjusted to the above chemical composition range, a slab is melted by continuous casting or ingot forming. Subsequently, the obtained slab is cooled and then reheated or hot rolled as it is. The final rolling temperature in hot rolling is preferably Ar3 or higher in order to improve elongation and stretch flangeability. If the final rolling temperature is lower than the Ar3 point, the ferrite grains become extremely coarse due to the two-phase structure at the final rolling stage, and a steel sheet with good workability may not be obtained even if cold rolling and annealing are performed.

  Next, the hot-rolled sheet obtained as described above is cooled and wound. The winding temperature is preferably 620 ° C. or lower in order to improve elongation and stretch flangeability.

  Next, pickling and cold rolling are performed to obtain a desired thickness. The cold rolling rate at this time is preferably 50% or more in order to improve elongation and stretch flangeability.

Next, a recrystallization annealing / tempering treatment is performed on the cold-rolled steel sheet obtained as described above.
The present invention is characterized in that it has a two-phase structure of a ferrite phase and a martensite phase, and the area ratio of the ferrite is 40% or more and 90% or less. Therefore, recrystallization annealing and tempering treatment are performed at a temperature of Ac1 point or more and Ac3 point or less for 10 s or more to cool to 500 to 750 ° C at a cooling rate of 20 ° C / s or less in order to obtain the above structure. Then, the temperature is rapidly cooled to 100 ° C. or lower at a cooling rate of 100 ° C./s or higher, and tempering is performed at 300 to 500 ° C. This is the most important requirement as the manufacturing condition of the present invention. Hereinafter, the details will be described in 1) to 3).

1) Hold for 10 s or more at a temperature of Ac1 or more and Ac3 or less, then cool to 500 to 750 ° C (quick start temperature) at a cooling rate of 20 ° C / s or less. If the recrystallization annealing temperature is less than Ac1, an austenite phase cannot be obtained while maintaining a high temperature, and therefore a martensite phase cannot be obtained after rapid cooling, and high strength cannot be achieved. On the other hand, if the Ac3 point is exceeded, coarse martensite is mixed in the structure after rapid cooling, and the characteristics deteriorate. In addition, when the holding time is less than 10 seconds, there is a high possibility that undissolved carbides are present, and there is a possibility that the amount of austenite phase is reduced. Also, if the cooling rate from 500 to 750 ° C (quenching start temperature) exceeds 20 ° C / s, it will be difficult to control the amount of ferrite phase generated, and the variation in the mechanical properties of the final product will increase. And In addition, after holding | maintenance, means, such as a gas jet, can be used for cooling to the rapid cooling start temperature, for example, and can be performed by a normal method.
Conventionally, the rapid cooling start temperature is greatly changed when there is a variation in plate thickness between coils, and is one of the variation factors of mechanical characteristics. And since the mechanical characteristic of the site | part in which the rapid cooling start temperature changed greatly differs from the average mechanical characteristic value in a coil, the trim of the site | part was needed and the yield was reduced. However, in the steel of the present invention, by adding appropriate amounts of Al and Si, the Ac1 temperature and Ac3 temperature ranges are increased, and even when the quenching start temperature changes, the structural change is small, and the mechanical property variation can be reduced. Become.

2) Rapid cooling at a cooling rate of 100 ° C / s or higher from the rapid cooling start temperature to 100 ° C or lower. If the cooling rate is less than 100 ° C / s, precipitation of pearlite and bainite and martensite transformation may not occur, and new problems such as increasing the amount of alloy added to obtain high strength have occurred. End up. In order to obtain a sufficient martensite phase, rapid cooling is performed to 100 ° C. or less.
The rapid cooling start temperature is 500 ° C. or higher and 750 ° C. or lower. If it is less than 500 degreeC, a 2nd phase may not fully martensite-transform and a bainite may be mixed. On the other hand, if the temperature exceeds 750 ° C., the martensite area ratio increases and the workability decreases, so the temperature is set to 750 ° C.
As a rapid cooling method, water cooling is preferable in that temperature unevenness in the plate width direction is reduced and a cooling rate can be easily secured. However, the rapid cooling method is not limited to water cooling, and gas jet cooling, mist cooling, roll cooling, etc. can be used alone or in combination.

3) Perform tempering treatment. The tempering temperature at this time is 300 ° C. or more and 500 ° C. or less. When the tempering temperature is less than 300 ° C., the martensite phase is not tempered sufficiently and the ductility is low. On the other hand, if the tempering temperature exceeds 500 ° C., the strength rapidly decreases, and it becomes difficult to stably obtain high strength.
By performing recrystallization annealing and tempering treatment within the range specified as described above, the structure of the high-strength cold-rolled steel sheet of the present invention is a two-phase structure of ferrite and martensite, and the area ratio of ferrite is 40 to 90. %, The variation in mechanical properties is small, and a high-strength steel sheet excellent in workability can be obtained.

  Steel slabs having the chemical composition shown in Table 1 were produced by continuous casting, and the slabs were reheated to 1250 ° C. and hot-rolled to a thickness of 2.8 mm. Hot rolling was performed at a finishing temperature of 850 to 900 ° C and a winding temperature of 500 to 600 ° C. Next, pickling and cold rolling were performed to obtain a cold-rolled steel sheet having a thickness of 1.2 mm. The obtained cold-rolled steel sheet was recrystallized and annealed by a continuous annealing furnace, quenched by water quenching to obtain a composite structure steel having a ferrite phase, and then tempered. Table 2 shows the conditions at this time. Subsequently, about the cold-rolled annealing board obtained by the above, the structure | tissue, the tensile characteristic, and the hole expansion rate were investigated. Each measuring method and conditions are as follows. The obtained results are shown in Table 2 together with the conditions.

  The structure was polished and subjected to Nital etching at a position of 1/4 of the plate thickness, followed by SEM observation, and the ferrite fraction was measured by image processing.

  The tensile test was performed in accordance with JIS 2241 using JIS No. 5 test pieces taken with the rolling direction as the longitudinal direction. The hole expansion test was carried out in accordance with the iron standard JFST1001.

  From Table 2, No. Examples 3 to 4, 7 to 16, 23 to 26, and 33 to 34 are examples that satisfy the requirements of the present invention. The difference in mechanical properties between the rapid cooling start temperatures of 700 ° C and 600 ° C is specified in Table 2. In all examples, ΔTS is 100MPa or less, and ΔEl is within 3.0%. It can be seen that a homogeneous cold-rolled steel sheet is obtained. In this example, a comparison is made using the case where the rapid cooling start temperature is 700 ° C. and 600 ° C., but in the actual operation, the fluctuation range of the rapid cooling start temperature is ± 30 ° C. or less with respect to the target temperature. It can be said that the mechanical characteristic fluctuation in the coil at the time is even smaller.

On the other hand, the comparative examples that do not satisfy the requirements of the present invention have the following problems. Comparative Example No. In 1-2, 5-6, and 17-18, when the rapid cooling start temperature is 700 ° C., the ferrite fraction is less than 40%, and the ductility is inferior. Further, ΔTS is over 100 Mpa, and ΔEl is over 3.0%, resulting in large variations in mechanical properties.
In Comparative Examples 19 to 20, 27 to 28, 29 to 30, and 35 to 36, ΔTS is over 100 Mpa, and ΔEl is over 3.0%, resulting in large variations in mechanical properties.
In Comparative Examples 21 to 22 and 31 to 32, a large amount of Al is added, and the hole-expanding property that is considered to be caused by a high residual γ fraction is inferior.

  The ultra-high-strength cold-rolled steel sheet of the present invention has excellent elongation and stretch flangeability, and also has a small variation in properties, so that it can be applied to severely stretched and stretched flanged parts. It is preferably used in fields where severe workability is required, such as architecture.

Claims (4)

In mass%, C: 0.06-0.15, Si: 0.5-1.5, Mn: 1.5-3.0, P: ≦ 0.05, S: ≦ 0.01, Al: 0.5-1.5, and A = Si + 9 × Al A defined to satisfy 6.0 ≦ A ≦ 20.0, the balance consisting of iron and inevitable impurities,
A high-strength cold-rolled steel sheet having a small mechanical property variation, characterized in that it has a two-phase structure of ferrite and martensite and a structure in which the area ratio of ferrite is 40% or more and 90% or less.   2. The high-strength cold-rolled steel sheet according to claim 1, wherein the high-strength cold-rolled steel sheet has a small variation in mechanical properties according to claim 1, characterized by containing at least one of Cr: 1% or less and Mo: 1% or less.   Furthermore, in mass%, one or more of V: 0.05 to 0.2%, B: 0.0002 to 0.002%, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.1% are contained. A high-strength cold-rolled steel sheet having a small variation in mechanical properties as described in 1 or 2. When steel having the composition according to any one of claims 1 to 3 is subjected to hot rolling and then cold rolling, and then subjected to recrystallization annealing and tempering treatment,
The recrystallization annealing / tempering treatment is maintained at a temperature of Ac1 or higher and Ac3 or lower for 10 seconds or more, and is cooled to 500 to 750 ° C. at a cooling rate of 20 ° C./s or lower. A method for producing a high-strength cold-rolled steel sheet with small variations in mechanical properties, characterized by quenching at a cooling rate of s or more and tempering at 300 to 500 ° C.