JP7276366B2 - High-strength steel plate and its manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a high-strength steel sheet and a method for manufacturing the same.

In recent years, from the standpoint of global environmental conservation, improving the fuel efficiency of automobiles has become an important issue. Therefore, there is an active movement to reduce the weight of the vehicle body itself by increasing the strength of the vehicle body material such as steel plate to reduce the thickness of the vehicle body parts.
In order to increase the strength of a steel sheet, for example, the ratio of hard structures such as bainite may be increased (Patent Document 1).

JP-A-4-235253

For example, steel plates used as materials for vehicle body parts that require particularly high strength, such as door impact beams and bumper reinforcements that suppress deformation during automobile collisions, will be required to have a tensile strength (TS) of 1300 MPa or more in the future. it is conceivable that.
However, for example, increasing the strength of a steel sheet by increasing the proportion of hard structures such as bainite tends to result in deterioration of workability.
Therefore, development of a steel sheet having both high strength and excellent workability (in particular, ductility and stretch-flangeability) is desired.

By the way, conventionally, various composite structure steel sheets such as ferrite-martensite dual phase steel (DP steel) and TRIP (Transformation Induced Plasticity) steel utilizing transformation induced plasticity of retained austenite have been developed.

In a composite structure steel sheet, when the ratio of the hard structure is increased, the workability of the steel plate is strongly affected by the workability of the hard structure.
That is, when the proportion of hard structure is small and soft polygonal ferrite is high, the deformability of polygonal ferrite dominates the workability of the steel sheet. Workability such as is ensured.
On the other hand, when the ratio of the hard structure is large, the deformability of the hard structure itself, not the deformability of polygonal ferrite, directly affects the formability of the steel sheet.

For this reason, conventionally, for example, in the case of a cold-rolled steel sheet, heat treatment is performed to adjust the amount of polygonal ferrite generated by heating and subsequent cooling, and then the steel sheet is water-quenched to generate martensite, and the steel sheet is manufactured again. The temperature is raised and maintained at a high temperature. As a result, martensite is tempered to form carbides in martensite, which is a hard structure, and to improve workability of martensite.
However, in equipment for water quenching, the temperature after quenching is inevitably close to the water temperature. Therefore, most of the untransformed austenite transforms into martensite, making it difficult to utilize retained austenite and other low-temperature transformed structures.
Therefore, the improvement of the workability of the hard structure is limited to the effect of tempering martensite, and as a result, the improvement of the workability of the steel sheet is also limited.

The present invention has been made in view of the above points, and an object of the present invention is to provide a high-strength steel sheet having a tensile strength of 1300 MPa or more and excellent workability, and a method for producing the same.

The present inventors have made intensive studies to achieve the above object. As a result, C and Mn, which are strengthening elements, are used to increase the strength, and by performing martensite nucleation that promotes bainite transformation twice, a sufficient amount of retained austenite is generated and martensite ( (Fresh martensite) was suppressed. In this way, the inventors have found that a high-strength steel sheet having excellent workability (especially balance between strength and ductility and balance between strength and stretch-flangeability) and tensile strength of 1300 MPa or more can be obtained. Thus, the present invention was completed.

That is, the present invention provides the following [1] to [5].
[1] % by mass, C: 0.20% to 0.50%, Si: 0.5% to 2.5%, Mn: 2.5% to 5.0%, P: 0.5% to 2.5%; 100% or less, S: 0.0500% or less, Al: 0.01% or more and 0.50% or less, and N: 0.010% or less, with the balance being Fe and unavoidable impurities. A steel slab having the above is hot-rolled and then cold-rolled to obtain a cold-rolled steel sheet, the cold-rolled steel sheet is heated at a temperature T1 in the austenite single phase region for 15 seconds or more and 1000 seconds or less, and then , Ms -50 ° C. or higher and cooled to a temperature T2 lower than Ms ° C., then raised to a temperature T3 of 500 ° C. or lower and held for 15 seconds or more and 1000 seconds or less, and then Ms -200 ° C. or higher and the above temperature A method for producing a high-strength steel sheet, comprising cooling to a temperature T4 that is lower than T2, then raising the temperature to a temperature T5 of 500° C. or less and holding the temperature for 15 seconds or more and 1000 seconds or less.
Here, when the content of the component X in the above component composition is set to [X%] in units of mass %, Ms is determined in units of °C by the following formula.
Ms=550−35×[Mn%]−13×[Si%]−10×[Cr%]−12×[Mo%]−600×{1−exp (−0.96×[C%])}
[2] The above component composition further includes Ti: 0.100% or less, Nb: 0.100% or less, V: 0.100% or less, Ta: 0.100% or less, and W: 0.100% or less. 500% or less, Cu: 2.00% or less, Ni: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Cr: 1.000% or less, Mo: 1.000% At least selected from the group consisting of B: 0.0050% or less, Mg: 0.0050% or less, Zr: 0.100% or less, REM: 0.0050% or less, and Ca: 0.0050% or less The method for producing a high-strength steel sheet according to [1] above, which contains 1 type.
[3] In mass %, C: 0.20% to 0.50%, Si: 0.5% to 2.5%, Mn: 2.5% to 5.0%, P: 0.5% to 5.0%. 100% or less, S: 0.0500% or less, Al: 0.01% or more and 0.50% or less, and N: 0.010% or less, with the balance being Fe and unavoidable impurities. , the area ratio of ferrite is 0% or more and 10% or less, the area ratio of tempered martensite is 40% or more and less than 80%, the area ratio of bainite is 5% or more and less than 20%, and the area ratio of martensite is is 0% or more and 10% or less, and the area ratio of retained austenite is 10% or more and 20% or less.
[4] The high-strength steel sheet according to [3] above, wherein the ratio of the area ratio of martensite having an aspect ratio of less than 2.0 to the area ratio of all martensite is 0.5 or less.
[5] The above component composition further includes, in % by mass, Ti: 0.100% or less, Nb: 0.100% or less, V: 0.100% or less, Ta: 0.100% or less, W: 0.100% or less. 500% or less, Cu: 2.00% or less, Ni: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Cr: 1.000% or less, Mo: 1.000% At least selected from the group consisting of B: 0.0050% or less, Mg: 0.0050% or less, Zr: 0.100% or less, REM: 0.0050% or less, and Ca: 0.0050% or less The high-strength steel sheet according to the above [3] or [4], containing 1 type.

According to the present invention, a high-strength steel sheet having a tensile strength of 1300 MPa or more and excellent workability can be obtained.
By using the high-strength steel sheet of the present invention, for example, as a vehicle body material, it is possible to improve fuel efficiency by reducing the weight of the vehicle body.

FIG. 4 is a schematic diagram showing a temperature pattern of heat treatment after cold rolling.

[High-strength steel plate]
The high-strength steel sheet of the present invention is a so-called cold-rolled steel sheet, and has a chemical composition and microstructure described below. Hereinafter, "high-strength steel sheet" or "cold-rolled steel sheet" may be simply referred to as "steel sheet".
The plate thickness of the steel plate is not particularly limited, and is, for example, 5.0 mm or less.
High strength means that the tensile strength (TS) is 1300 MPa or more.

<Component composition>
"%" in the component composition means "% by mass" unless otherwise specified.

<<C: 0.20% or more and 0.50% or less>>
C is an essential element for ensuring the amount of tempered martensite and martensite in the steel sheet and for increasing the strength of the steel sheet. In addition, C is an essential element for securing the amount of retained austenite in the steel sheet and increasing the ductility of the steel sheet.
From the viewpoint of ensuring the strength and workability of the steel sheet, the C content is 0.20% or more, preferably 0.21% or more, and more preferably 0.22% or more.
On the other hand, if the amount of C is too large, the steel sheet becomes embrittled. Therefore, the C content is 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less.

<<Si: 0.5% or more and 2.5% or less>>
Si is a useful element that contributes to improving the strength of steel sheets through solid-solution strengthening. Moreover, Si is a useful element that contributes to ensuring the amount of retained austenite in the steel sheet by suppressing the formation of carbides in the steel sheet.
From the viewpoint of obtaining the effect of adding Si, the amount of Si is 0.5% or more, preferably 0.6% or more, and more preferably 0.7% or more.
On the other hand, if the amount of Si is too large, the steel sheet becomes embrittled. Therefore, the Si content is 2.5% or less, preferably 2.2% or less, and more preferably 2.0% or less.

<<Mn: 2.5% or more and 5.0% or less>>
Mn is an element that stabilizes retained austenite, is effective in ensuring good ductility, and is an element that increases the strength of steel through solid-solution strengthening. Moreover, a large amount of retained austenite can be secured by concentrating Mn in retained austenite. From the viewpoint of obtaining such an effect of adding Mn, the Mn amount is 2.5% or more, preferably 2.6% or more, and more preferably 2.8% or more.
On the other hand, if an excessive amount of Mn is added, workability deteriorates due to segregation of Mn. Therefore, the Mn content is 5.0% or less, preferably 4.8% or less, and more preferably 4.5% or less.

<<P: 0.100% or less>>
P is an element useful for strengthening steel, but when the amount of P is too large, grain boundary segregation causes embrittlement. Therefore, the P content is 0.100% or less, preferably 0.050% or less, and more preferably 0.030% or less.
On the other hand, excessively reducing the amount of P leads to a significant increase in cost. Therefore, the P content is preferably 0.001% or more, more preferably 0.003% or more.

<<S: 0.0500% or less>>
Since S forms inclusions such as MnS and causes cracking when evaluating stretch flangeability, it is preferable to reduce the amount of S as much as possible. Therefore, the S content is 0.0500% or less, preferably 0.0300% or less, and more preferably 0.0100% or less.
On the other hand, excessively reducing the amount of S leads to a significant increase in cost. Therefore, the S content is preferably 0.0003% or more, more preferably 0.0005% or more, and even more preferably 0.0008% or more.

<<Al: 0.01% or more and 0.50% or less>>
Al is a useful element added as a deoxidizing agent in the steelmaking process. In order to obtain the effect of adding Al, the amount of Al is 0.01% or more, preferably 0.02% or more, and more preferably 0.03% or more.
On the other hand, if the amount of Al is too large, the risk of slab cracking during continuous casting increases. Therefore, the Al content is 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less.

<<N: 0.010% or less>>
N is an element that most significantly degrades the aging resistance of steel, and is preferably reduced as much as possible. Therefore, the N content is 0.010% or less, preferably 0.008% or less, and more preferably 0.007% or less.
On the other hand, excessively reducing the amount of N leads to a significant increase in cost. Therefore, the N content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more.

《Other Ingredients》
The component composition may further contain, in % by mass, at least one selected from the group consisting of the components described below.

(Ti: 0.100% or less)
Ti is useful for precipitation strengthening of steel. However, if the amount of Ti is too large, workability and shape fixability may become insufficient. Therefore, when Ti is contained, the amount of Ti is preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.060% or less, from the viewpoint of improving workability and the like.
On the other hand, when Ti is contained, the amount of Ti is preferably 0.005% or more, more preferably 0.008% or more, and still more preferably 0.010% or more, in order to obtain the effect of adding Ti.

(Nb: 0.100% or less)
Nb is useful for precipitation strengthening of steel. However, if the amount of Nb is too large, workability and shape fixability may become insufficient. Therefore, when Nb is contained, the Nb content is preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.060% or less, from the viewpoint of improving workability and the like.
On the other hand, when Nb is contained, the amount of Nb is preferably 0.005% or more, more preferably 0.008% or more, and still more preferably 0.010% or more, in order to obtain the effect of Nb addition.

(V: 0.100% or less)
V is useful for precipitation strengthening of steel. However, if the amount of V is too large, workability and shape fixability may become insufficient. Therefore, when V is contained, the amount of V is preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.060% or less, from the viewpoint of improving workability and the like.
On the other hand, when V is contained, the amount of V is preferably 0.005% or more, more preferably 0.008% or more, and still more preferably 0.010% or more in order to obtain the effect of adding V.

(Ta: 0.100% or less)
Ta, like Ti and Nb, contributes to increasing the strength of the steel sheet. In addition, Ta partially dissolves in Nb carbides and Nb carbonitrides and forms composite precipitates such as (Nb, Ta) (C, N). This is considered to have the effect of remarkably suppressing the coarsening of precipitates and stabilizing the contribution of precipitation strengthening to strength. Therefore, Ta may be contained as necessary.
However, even if Ta is contained excessively, the effect of stabilizing precipitates is saturated and the alloy cost increases. Therefore, the Ta content is preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.060% or less.
On the other hand, when Ta is contained, the amount of Ta is preferably 0.001% or more, more preferably 0.010% or more, and still more preferably 0.020% or more in order to obtain the effect of Ta addition.

(W: 0.500% or less)
W is effective for precipitation strengthening of steel and may be contained as necessary. However, if the amount of W is too large, the area ratio of hard martensite becomes excessive, and microvoids at grain boundaries of martensite increase during the hole expansion test, furthermore, crack propagation progresses, The stretch flangeability may deteriorate. Therefore, the W content is preferably 0.500% or less, more preferably 0.400% or less, and even more preferably 0.300% or less.
On the other hand, when W is contained, the amount of W is preferably 0.001% or more, more preferably 0.010% or more, and still more preferably 0.020% or more in order to obtain the effect of W addition.

(Cu: 2.00% or less)
Cu is an element that suppresses the formation of pearlite during cooling after heating at temperature T1, which will be described later. However, if the amount of Cu is too large, the amount of hard martensite becomes excessive, and it may be difficult to obtain the required workability. Therefore, when Cu is contained, the amount of Cu is preferably 2.00% or less, more preferably 1.00% or less, and even more preferably 0.80% or less, from the viewpoint of better workability.
On the other hand, when Cu is contained, the amount of Cu is preferably 0.01% or more, more preferably 0.08% or more, and still more preferably 0.10% or more, in order to obtain the effect of adding Cu.

(Ni: 1.00% or less)
Ni is an element that stabilizes retained austenite and is effective in securing better ductility. Furthermore, Ni is an element that further increases the strength of steel through solid-solution strengthening. Therefore, Ni may be contained as necessary.
However, if the amount of Ni is too large, the area ratio of hard martensite becomes excessive, and microvoids at grain boundaries of martensite increase during the hole expansion test, furthermore, crack propagation progresses, The hole expansibility may deteriorate. Therefore, the Ni content is preferably 1.00% or less, more preferably 0.70% or less, and even more preferably 0.40% or less.
On the other hand, when Ni is contained, the amount of Ni is preferably 0.005% or more, more preferably 0.01% or more, and still more preferably 0.10% or more, in order to obtain the effect of Ni addition.

(Sn: 0.200% or less, Sb: 0.200% or less)
Sn and Sb may be contained as necessary from the viewpoint of suppressing decarburization of a region of about several tens of μm in the surface layer of the steel sheet caused by nitridation or oxidation of the steel sheet surface. This prevents the area ratio of martensite from decreasing on the surface of the steel sheet, which is effective in ensuring strength and material stability.
Excessive addition of Sn and Sb, however, may lead to a decrease in toughness. Therefore, the Sn content and the Sb content are each preferably 0.200% or less, more preferably 0.100% or less, and still more preferably 0.050% or less.
On the other hand, when Sn and Sb are contained, the amount of Sn and the amount of Sb are each preferably 0.001% or more, more preferably 0.002% or more, in order to obtain the effect of adding Sn and Sb.

(Cr: 1.000% or less)
Cr is an element that has the effect of suppressing the formation of pearlite during cooling after heating at temperature T1, which will be described later. However, if the amount of Cr is too large, the amount of hard martensite becomes excessive, and it may be difficult to obtain the required workability. Therefore, when Cr is contained, the Cr content is preferably 1.000% or less, more preferably 0.800% or less, and even more preferably 0.500% or less, from the viewpoint of better workability.
On the other hand, when Cr is contained, the amount of Cr is preferably 0.005% or more, more preferably 0.010% or more, and still more preferably 0.015% or more, in order to obtain the effect of Cr addition.

(Mo: 1.000% or less)
Mo is an element that has the effect of suppressing the formation of pearlite during cooling after heating at temperature T1, which will be described later. However, if the amount of Mo is too large, the amount of hard martensite becomes excessive, and it may be difficult to obtain the required workability. Therefore, when Mo is contained, the Mo content is preferably 1.000% or less, more preferably 0.800% or less, and even more preferably 0.500% or less, from the viewpoint of better workability.
On the other hand, when Mo is contained, the amount of Mo is preferably 0.005% or more, more preferably 0.010% or more, and still more preferably 0.015% or more in order to obtain the effect of Mo addition.

(B: 0.0050% or less)
B is an element useful for suppressing the formation and growth of polygonal ferrite from austenite grain boundaries. However, if the amount of B is too large, workability may become insufficient. Therefore, when B is contained, the amount of B is preferably 0.0050% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less, from the viewpoint of improving workability.
On the other hand, when B is contained, the amount of B is preferably 0.0003% or more, more preferably 0.0004% or more, and still more preferably 0.0005% or more in order to obtain the effect of adding B.

(Mg: 0.0050% or less)
Mg is effective in ameliorating the adverse effects of sulfides on hole expansibility by spheroidizing the sulfides. Therefore, Mg may be contained as necessary.
However, if the amount of Mg is too large, inclusions and the like increase, which may cause defects on the surface and inside of the steel sheet. Therefore, the Mg content is preferably 0.0050% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less.
On the other hand, when Mg is contained, the amount of Mg is preferably 0.0005% or more, more preferably 0.0008% or more, in order to obtain the effect of adding Mg.

(Zr: 0.100% or less)
Zr is effective in ameliorating the adverse effects of sulfides on hole expandability by spheroidizing the sulfides. Therefore, Zr may be contained as necessary.
However, if the amount of Zr is too large, inclusions and the like increase, which may cause defects on the surface and inside of the steel sheet. Therefore, the Zr content is preferably 0.100% or less, preferably 0.080% or less, and more preferably 0.060% or less.
On the other hand, when Zr is contained, the amount of Zr is preferably 0.0005% or more, more preferably 0.010% or more, and still more preferably 0.020% or more, in order to obtain the effect of Zr addition.

(REM: 0.0050% or less)
REMs (rare earth metals) are effective in ameliorating the adverse effects of sulfides on hole expansibility by spheroidizing the sulfides. Therefore, REM may be contained as necessary.
However, if the amount of REM is too large, inclusions and the like increase, which may cause defects on the surface and inside of the steel sheet. Therefore, the REM amount is preferably 0.0050% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less.
On the other hand, when REM is contained, the amount of REM is preferably 0.0003% or more, more preferably 0.0005% or more, in order to obtain the effect of adding REM.

(Ca: 0.0050% or less)
Ca is an effective element for improving workability by controlling the morphology of sulfides. However, too much Ca may adversely affect the cleanliness of steel. Therefore, when Ca is contained, the amount of Ca is preferably 0.0050% or less, more preferably 0.0045% or less, and even more preferably 0.0040% or less.
On the other hand, when Ca is contained, the amount of Ca is preferably 0.0003% or more, more preferably 0.0005% or more, and still more preferably 0.0008% or more in order to obtain the effect of adding Ca.

《Remainder》
The balance of the component composition consists of Fe and unavoidable impurities.

<Microstructure>
Next, the microstructure (steel plate structure) of the steel plate will be described. Hereafter, the area ratio is the area ratio to the entire microstructure. The area ratio of each tissue is determined by the method described in Examples below.

<<Area ratio of ferrite: 0% or more and 10% or less>>
Ferrite is a soft tissue and contributes to ductility. However, too much ferrite makes it difficult to achieve the desired tensile strength. Furthermore, during working, strain is concentrated on soft ferrite mixed in the hard structure, which easily causes cracks, and as a result, desired workability cannot be obtained.
If the amount of ferrite is small, a small amount of ferrite will be isolated and dispersed in the hard structure, strain concentration can be suppressed, and deterioration of workability can be avoided. Therefore, the area ratio of ferrite is 10% or less, preferably 7% or less, and more preferably 5% or less. The area ratio of ferrite may be 0%, but may be, for example, 1% or more, preferably 2% or more, from the viewpoint of ensuring workability.

<<Area ratio of tempered martensite: 40% or more and less than 80%>>
Tempered martensite contributes to strength improvement. Therefore, the area ratio of tempered martensite is 40% or more, preferably 45% or more, and more preferably 50% or more.
On the other hand, when the tempered martensite is large, as will be described later, the desired amount of retained austenite cannot be secured, resulting in insufficient workability such as ductility. Therefore, the area ratio of tempered martensite is less than 80%, preferably less than 75%, and more preferably less than 70%.

Martensite can be determined by structural observation. Untempered martensite (as-quenched martensite) does not contain carbides in its structure. On the other hand, tempered martensite has carbides with random growth directions in the structure.

<<Area ratio of bainite: 5% or more and less than 20%>>
Sufficient promotion of bainite transformation is necessary to enrich C in untransformed austenite and obtain retained austenite. In order to obtain the desired tensile strength, the area ratio of bainite is 5% or more, preferably 6% or more, and more preferably 8% or more.
On the other hand, too much bainite results in a shortage of retained austenite. Therefore, the area ratio of bainite is less than 20%, preferably less than 18%, more preferably less than 15%.

<<Area ratio of martensite: 0% or more and 10% or less>>
Martensite is a hard structure and increases the strength of the steel sheet.
On the other hand, if the martensite content is too large, the bainite content is reduced and a stable amount of retained austenite cannot be ensured, resulting in a decrease in ductility. In addition, martensite becomes a starting point of voids and deteriorates stretch-flange formability.
Therefore, the area ratio of martensite is 10% or less, preferably 8% or less, and more preferably 6% or less. The area ratio of martensite may be 0%.

<<Area ratio of retained austenite: 10% or more and 20% or less>>
Retained austenite transforms into martensite due to the TRIP (Transformation Induced Plasticity) effect during working, and the hard martensite containing high C promotes high strength and at the same time improves ductility by increasing strain dispersion ability.
By coexisting such retained austenite with bainite and martensite, good workability can be obtained even in a high-strength region with a tensile strength (TS) of 1300 MPa or more. Specifically, the value of TS×El can be 18000 MPa·% or more, and the value of TS×λ can be 40000 MPa·%, and the balance between strength and workability is extremely excellent.
If the amount of retained austenite is too small, a sufficient TRIP effect cannot be obtained. Therefore, the area ratio of retained austenite is 10% or more, preferably 11% or more, and more preferably 12% or more.
On the other hand, if the amount of retained austenite is too much, the amount of hard martensite generated after the TRIP effect is exhibited becomes excessive, resulting in deterioration of toughness and stretch-flangeability. Therefore, the area ratio of retained austenite is 20% or less, preferably 19% or less, and more preferably 18% or less.

<<The ratio of the area ratio of martensite having an aspect ratio of less than 2.0 to the area ratio of all martensite: 0.5 or less>>
If the ratio (A/B) between the area ratio (A) of martensite having an aspect ratio of less than 2.0 and the area ratio (B) of all martensite is too large, sufficient ductility and stretch flangeability cannot be obtained. Sometimes. This is because if a large amount of martensite, which is extremely hard and has low deformability and poor toughness, has an aspect ratio of less than 2.0 (that is, massive), the material cannot be uniformly deformed when strain is applied, resulting in excellent This is because ductility and stretch flangeability may not be obtained.
Therefore, the ratio (A/B) is preferably 0.5 or less, more preferably 0.4 or less.

[Manufacturing method of high-strength steel plate]
Next, the method for manufacturing the high-strength steel sheet of the present invention (hereinafter also referred to as "the manufacturing method of the present invention") will be described. The manufacturing method of the present invention is also a method of manufacturing the above-described high-strength steel sheet of the present invention.

<Hot rolling>
A steel billet having the chemical composition described above is subjected to hot rolling to obtain a hot-rolled steel sheet. The conditions for hot rolling are not particularly limited, and a conventional method may be followed, but preferred conditions are as follows.
First, the billet is heated to a temperature range of 1100°C or higher and 1300°C or lower, and then hot rolling is completed in a temperature range of 870°C or higher and 950°C or lower. The obtained hot-rolled steel sheet is wound in a temperature range of 350°C or higher and 720°C or lower. The hot-rolled steel sheet is then pickled.

<Cold rolling>
The obtained hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet.
The draft of cold rolling is preferably 30% or more. As a result, during the heat treatment described later, fine austenite is generated, and fine retained austenite and martensite are finally obtained. may also improve.
On the other hand, although the upper limit of the rolling reduction in cold rolling is not particularly limited, it is preferably 85% or less, more preferably 75% or less, from the viewpoint of the load of cold rolling.

After cold rolling, the cold-rolled steel sheet is heat-treated.
The technical significance of the heat treatment in the present invention is roughly as follows.
Normally, high C and high Mn steels have high strength, but since the content of alloying elements is large, the bainite transformation speed is very slow, and the transformation start line during isothermal holding shifts to the long time side. . Therefore, the bainite transformation is delayed and it is difficult to utilize the retained austenite.
Therefore, when martensite and untransformed austenite coexist, the rate of bainite transformation is remarkably accelerated, and heating, cooling, and cooling stop are performed in a short time, and reheating is repeated twice.
That is, martensite nucleation due to cooling promotes bainite transformation during reheating due to the swing-back effect, and once bainite transformation has progressed, martensite nucleation occurs again due to cooling.
As a result, the bainite transformation can be fully utilized in a short period of time during the second reheating, and the retained austenite that contributes to the improvement of ductility is ensured, thereby achieving high ductility. At the same time, by preventing the formation of martensite, good stretch flangeability can also be achieved.
If reheating is not performed, or if reheating is performed only once, the proportion of martensite with a large aspect ratio increases, and ductility and stretch flangeability may become insufficient. Therefore, repeating reheating twice is important to obtain the desired texture and material.

FIG. 1 is a schematic diagram showing the temperature pattern of heat treatment after cold rolling. The heat treatment takes a temperature pattern (heat history) shown in FIG. A more detailed description will be given below.

<Heating for 15 seconds or more and 1000 seconds or less at the temperature T1 in the austenite single-phase region>
First, a cold-rolled steel sheet is heated at a temperature T1 in the austenite single phase region for 15 seconds or more and 1000 seconds or less.
The high-strength steel sheet of the present invention has tempered martensite and bainite obtained by transforming austenite as main structures. In order to secure the amount of tempered martensite and bainite in the steel sheet and increase the strength of the steel sheet, it is preferable that the amount of ferrite is as small as possible. Therefore, heating (annealing) at temperature T1 in the austenite single-phase region is required. The temperature T1 is preferably Ac ₃ (austenite transformation point) or higher, more preferably (Ac ₃ +15° C.) or higher.

Ac ₃ (unit: °C) is determined by the following formula.
Ac ₃ =937.2−436.5×[C%]+56×[Si%]−19.7×[Mn%]−4.9×[Cr%]−16.3×[Cu%]+38. 1 x [Mo%] + 124.8 x [V%] + 136.3 x [Ti%] - 19.1 x [Nb%] + 198.4 x [Al%] + 3315 x [B%]
In the above formula, [X%] is the content of component X (unit: % by mass) in the component composition described above. [X%] of component X not contained is set to 0.

The temperature T1 is not particularly limited as long as it is in the austenite single phase region, but if it is too high, the austenite grains grow significantly. As a result, the area ratio of tempered martensite increases, the ratio of martensite with a large aspect ratio increases, the area ratio of retained austenite decreases, and ductility and stretch flangeability may become insufficient. Therefore, the temperature T1 is preferably 1000° C. or lower, more preferably 950° C. or lower.

If the temperature T1 is less than the austenite single phase region, or if the heating time at the temperature T1 is too short, the reverse transformation to austenite does not proceed sufficiently during the heating at the temperature T1, and the area ratio of ferrite increases. , it may be difficult to secure the strength.
Therefore, the heating time at the temperature T1 is 15 seconds or longer, preferably 30 seconds or longer, and more preferably 60 seconds or longer.

On the other hand, if the heating time at the temperature T1 is too long, austenite coarsens during heating. As a result, the area ratio of martensite increases, the ratio of martensite with a large aspect ratio increases, the area ratio of retained austenite decreases, and ductility and stretch flangeability may become insufficient.
Therefore, the heating time at temperature T1 is 1000 seconds or less, preferably 800 seconds or less, and more preferably 600 seconds or less.

<Cooling to temperature T2 above Ms-50°C and below Ms°C>
Temperature T2 is important for achieving both strength and workability.
The cold-rolled steel sheet heated at temperature T1 is cooled to temperature T2, which is a cooling stop temperature of Ms-50°C or more and less than Ms°C. Cooling below Ms° C. transforms some of the austenite into martensite. The martensite generated during cooling to temperature T2 becomes nuclei for bainite transformation during holding at temperature T3, which will be described later. It is important for promoting bainite transformation during holding at temperature T3 and promoting carbon enrichment in the retained austenite.

When the temperature T2 is less than Ms−50° C., the amount of untransformed austenite converted to martensite at the temperature T2 becomes excessive, and finally tempered martensite is large and retained austenite is reduced, resulting in excellent strength and workability. cannot be compatible. Therefore, the temperature T2 is Ms-50° C. or higher.
On the other hand, when the temperature T2 is Ms° C. or higher, martensite cannot be secured at the time of stopping cooling. As a result, the bainite transformation during holding at the temperature T3 is delayed, and finally it becomes difficult to secure the desired amount of retained austenite. Therefore, temperature T2 is less than Ms.degree.

Ms (unit: °C) is determined by the following formula.
Ms=550−35×[Mn%]−13×[Si%]−10×[Cr%]−12×[Mo%]−600×{1−exp (−0.96×[C%])}
In the above formula, [X%] is the content of component X (unit: % by mass) in the component composition described above. [X%] of component X not contained is set to 0.

<The temperature is raised to a temperature T3 of 500°C or less and held for 15 seconds or more and 1000 seconds or less>
The cold-rolled steel sheet cooled to the temperature T2 is heated to a temperature T3 of 500° C. or less and held at the temperature T3 for 15 seconds or more and 1000 seconds or less.
In holding at temperature T3, martensite generated by cooling from temperature T1 to temperature T2 is tempered, untransformed austenite is transformed into bainite, and solute C is concentrated in austenite. Stabilization of austenite is advanced by these and the like.

If the temperature T3 is too high, the bainite transformation is suppressed, making it difficult to secure the desired amount of retained austenite. Therefore, the temperature T3 is 500° C. or lower, preferably 420° C. or lower.
On the other hand, if the temperature T3 is too low, the diffusion rate of dissolved C decreases and the amount of C enriched in austenite decreases, which may make it difficult to obtain the required C concentration in retained austenite. Therefore, the temperature T3 is preferably 250° C. or higher, more preferably 280° C. or higher, and even more preferably 300° C. or higher.

If the holding time at temperature T3 is too short, the bainite transformation will be insufficient and the desired microstructure will not be obtained. As a result, workability may become insufficient. Therefore, the holding time at the temperature T3 is 15 seconds or longer, preferably 50 seconds or longer, and more preferably 100 seconds or longer.

A holding time of 1000 seconds at the temperature T3 is sufficient due to the effect of accelerating the bainite transformation by the martensite generated at the temperature T2.
If the holding time at temperature T3 is too long, the final microstructure may not be stable retained austenite and, as a result, the desired ductility may not be obtained. Therefore, the holding time at temperature T3 is 1000 seconds or less, preferably 700 seconds or less, and more preferably 600 seconds or less.

<Cooling to temperature T4 which is Ms-200°C or more and less than temperature T2>
Martensite, which is the nucleus of bainite transformation, is generated again from austenite that has not undergone bainite transformation at the stage when the holding at temperature T3 is completed. Therefore, the cold-rolled steel sheet is cooled to temperature T4, which is a cooling stop temperature lower than temperature T2.
If the temperature T4 is equal to or higher than the temperature T2, the bainite transformation becomes insufficient after the temperature is raised to and held at the temperature T5 described later, and retained austenite that contributes to the improvement of ductility cannot be obtained. Therefore, the temperature T4 is less than the temperature T2.
On the other hand, if the temperature T4 is less than Ms−200° C., the amount of martensite generated becomes excessive when cooling is stopped at the temperature T4, the amount of tempered martensite after the temperature is raised to the temperature T5 becomes excessive, and the amount of retained austenite becomes excessive. decreases. This makes it impossible to achieve both excellent strength and workability. Therefore, the temperature T4 is Ms-200° C. or higher.

<The temperature is raised to a temperature T5 of 500°C or less and held for 15 seconds or more and 1000 seconds or less>
The cold-rolled steel sheet cooled to the temperature T4 is again heated to a temperature T5 of 500° C. or less, and held at the temperature T5 for 15 seconds or more and 1000 seconds or less.
In holding at temperature T5, martensite generated by cooling from temperature T3 to temperature T4 is tempered, and untransformed austenite is transformed into bainite. In this way, at the stage when the holding at temperature T5 is completed, a larger amount of solid solution C can be concentrated in austenite than after holding at temperature T3, the austenite is stabilized, and finally, it contributes to the improvement of ductility. Retained austenite can be secured.

If the temperature T5 is too high, bainite transformation is suppressed and the amount of retained austenite decreases. Therefore, the temperature T5 is 500° C. or lower, preferably 420° C. or lower.
On the other hand, if the temperature T5 is too low, the rate of diffusion of solid solution C decreases and the amount of C enriched in austenite decreases, which may make it difficult to obtain the required C concentration in retained austenite. Therefore, the temperature T5 is preferably 250° C. or higher, more preferably 280° C. or higher, and even more preferably 300° C. or higher.

If the holding time at temperature T5 is too short, the bainite transformation will be insufficient and the desired microstructure will not be obtained. As a result, workability may become insufficient. Therefore, the holding time at the temperature T5 is 15 seconds or longer, preferably 50 seconds or longer, and more preferably 100 seconds or longer.

A holding time of 1000 seconds at the temperature T5 is sufficient due to the effect of accelerating the bainite transformation by the martensite generated at the temperature T4.
If the holding time at temperature T5 is too long, the final microstructure may not have stable retained austenite, resulting in the desired strength and/or ductility. Therefore, the holding time at the temperature T5 is 1000 seconds or less, preferably 700 seconds or less, and more preferably 600 seconds or less.

As other conditions, the heating rate and cooling rate are as follows, for example.
The cold-rolled steel sheet after cold rolling is heated from room temperature to temperature T1, for example. The heating rate to temperature T1 is preferably 0.1° C./second or more, more preferably 0.5° C./second or more. On the other hand, 40° C./second or less is preferable, and 30° C./second or less is more preferable.

The cooling rate from temperature T1 to temperature T2 is preferably 5° C./second or more, more preferably 10° C./second or more. On the other hand, 50° C./second or less is preferable, and 40° C./second or less is more preferable.

The rate of temperature increase from temperature T2 to temperature T3 is preferably 5° C./second or more, more preferably 10° C./second or more. On the other hand, 100° C./second or less is preferable, and 50° C./second or less is more preferable.

The cooling rate from temperature T3 to temperature T4 is preferably 5° C./second or more, more preferably 10° C./second or more. On the other hand, 50° C./second or less is preferable, and 40° C./second or less is more preferable.

The rate of temperature increase from temperature T4 to temperature T5 is preferably 5° C./second or more, more preferably 10° C./second or more. On the other hand, 100° C./second or less is preferable, and 50° C./second or less is more preferable.

After being held at temperature T5, the cold-rolled steel sheet is cooled, for example, to room temperature. Suitable cooling methods include, for example, air cooling, gas cooling, furnace cooling and water cooling. At this time, the cooling rate from temperature T5 is preferably 5° C./second or more, more preferably 10° C./second or more. On the other hand, 1000° C./second or less is preferable, and 100° C./second or less is more preferable.

In the heat treatment described above, the holding temperature may not be constant as long as it is within the temperature range described above, and may vary within the temperature range described above.
As long as the heat history described above is satisfied, any equipment may be used for the heat treatment.
The surface of the steel sheet after the heat treatment may be subjected to temper rolling for shape correction.
A hot-dip galvanized steel sheet may be obtained by plating the steel sheet (the high-strength steel sheet of the present invention) after heat treatment. Furthermore, it is good also as an alloyed hot-dip galvanized steel sheet by alloying.

EXAMPLES The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the examples described below.

<Manufacturing of steel plate>
A steel slab obtained by melting a steel having the composition shown in Table 1 below (the balance consists of Fe and unavoidable impurities) is heated to 1250°C, and then finished hot rolled at 870°C, A hot-rolled steel sheet was obtained. The obtained hot-rolled steel sheet was coiled at 550° C. and then pickled. The pickled hot-rolled steel sheet was cold-rolled at a rolling reduction of 50% to obtain a cold-rolled steel sheet having a thickness of 1.2 mm.

Next, the obtained cold-rolled steel sheets were subjected to heat treatment in the temperature pattern shown in FIG. 1 under the conditions shown in Table 2 below to obtain steel sheets (high-strength steel sheets). The obtained steel sheet was subjected to temper rolling with a rolling reduction of 0.1%.

As another condition for the heat treatment of the temperature pattern shown in FIG. 1, the temperature rising rate from room temperature to temperature T1 was set to 2° C./sec. The cooling rate from temperature T1 to temperature T2 was set to 20° C./sec. The rate of temperature increase from temperature T2 to temperature T3 was set to 30° C./sec. The cooling rate from temperature T3 to temperature T4 was set to 15° C./sec. The rate of temperature increase from temperature T4 to temperature T5 was set at 20° C./sec. The cooling rate from temperature T5 to room temperature was 15° C./sec.

However, no. In 25, after holding at temperature T3, cooling was continued to room temperature without stopping cooling at temperature T4 to obtain a steel plate. That is, the second reheating was not performed.
Also, No. In 26, after heating at temperature T1, without cooling to temperature T2, it was cooled directly to temperature T3 and held at temperature T3. After holding at temperature T3, No. As in No. 25, the steel sheet was obtained by cooling to room temperature without stopping the cooling at temperature T4.

<Microstructure>
The area ratio of each structure in the obtained steel sheet was determined as follows. The results are shown in Table 3 below.

The area ratio of each structure in the entire microstructure of the steel plate was obtained by observing the cross section in the rolling direction and the 1/4 plate thickness surface of the steel plate with an optical microscope. The area occupied by each tissue within an arbitrarily set square area of 100 μm×100 μm was determined by image analysis using a cross-sectional photograph at a magnification of 1000 times. Observation was performed at N=5 (5 observation fields). For observation, etching was performed with a mixed solution of 3% by volume picral and 3% by volume sodium pyrosulfite.

The total area ratio of ferrite and bainite was determined assuming that the black region observed in this way was ferrite or bainite.
The total area ratio of tempered martensite, martensite and retained austenite was determined on the assumption that the remaining region other than the black region was tempered martensite, martensite and retained austenite.

Bainite is an aggregate of lath-like ferrite and has a structure containing Fe-based carbides.
Bainite and ferrite were distinguished using SEM (scanning electron microscope) and TEM (transmission electron microscope), and the area ratio of each was determined.

The area ratio of retained austenite was obtained as follows.
First, the amount of retained austenite was determined by the conventional X-ray diffraction method. The Kα line of Mo was used, and the ¼ surface of the steel plate was used as the measurement surface. Calculate the amount (volume ratio) of retained austenite from the ratio (peak intensity ratio) of the peak intensity of the (211) plane and (220) plane of austenite and the peak intensity of the (200) plane and (220) plane of ferrite. bottom. The calculated amount (volume ratio) of retained austenite was defined as the area ratio of retained austenite.

Tempered martensite and martensite were distinguished as follows.
Using a cross-sectional photograph (SEM photograph) in the rolling direction at a magnification of 1000 to 3000 times taken with a scanning electron microscope (SEM), image analysis was performed to determine the structure of each structure present in an arbitrarily set square area of 50 μm × 50 μm. The occupied area was obtained. Observation was performed at N=5 (5 observation fields). For observation, etching was performed with nital. In the SEM photograph, the area ratio was determined by defining martensite as having a smooth surface and tempering martensite as having carbides and the like observed on the surface.

Furthermore, the ratio (A/B) between the area ratio (A) of martensite having an aspect ratio of less than 2.0 and the area ratio (B) of all martensite was determined. The results are shown in Table 3 below.
The aspect ratio of martensite was calculated by drawing an ellipse circumscribing the martensite grain using Photoshop elements 13 and dividing the major axis length of the ellipse by the minor axis length.

<evaluation>
The obtained steel sheets were evaluated by the following methods. The results are shown in Table 3 below.
First, the obtained steel plate was subjected to a tensile test. The tensile test was performed according to JIS Z 2241 using a JIS No. 5 test piece (JIS Z 2201) with the width direction of the steel sheet as the longitudinal direction.
Tensile strength (TS) [MPa] and total elongation (El) [%] were measured to evaluate strength and ductility. The product of tensile strength and total elongation (TS x El) was calculated to evaluate the balance between strength and ductility. When TS×El≧18000 [MPa·%], the workability was evaluated to be excellent.
Furthermore, a 100 mm × 100 mm test piece was taken from the obtained steel plate, and a hole expansion test was performed three times in accordance with JFST 1001 (Iron Federation Standard) to obtain an average hole expansion ratio (λ) [%], and elongation Flangeability was evaluated. The product (TS×λ) of tensile strength and hole expansion rate was calculated to evaluate the balance between strength and stretch flangeability. When TS×λ≧40000 [MPa·%], the workability was evaluated to be excellent.

In Tables 1 to 3 below, underlines mean outside the range or preferred range of the present invention.

<Summary of evaluation results>
As shown in Table 3 above, No. 1 to No. 8 and no. 27 to No. All of the steel sheets No. 29 have a tensile strength of 1300 MPa or more, a TS x El value of 18000 MPa % or more, and a TS x λ value of 40000 MPa % or more, and have excellent strength and workability. It had both.

On the other hand, No. 9 to No. No. 26 steel sheets were insufficient in at least one of tensile strength, TS×El, and TS×λ. Specifically, it was as follows.

No. 9 (using steel type I with a small amount of C) had less tempered martensite, more bainite, less retained austenite, and insufficient strength and workability (ductility).
No. No. 10 (using steel type J with a low Mn content) had a large amount of ferrite, a small amount of retained austenite, and insufficient strength and workability (ductility).
No. No. 11 (using a steel type with a large amount of Mn) had insufficient workability (ductility, stretch flange formability).

No. Sample No. 12 (low temperature T1) had a large amount of ferrite, a small amount of retained austenite, and insufficient strength and workability (ductility).
No. Sample No. 13 (with a short heating time at temperature T1) contained a large amount of ferrite and had insufficient strength.
No. No. 14 (long heating time at temperature T1) has a large amount of martensite, a small amount of retained austenite, a large amount of martensite with an aspect ratio of less than 2.0, and insufficient workability (ductility, stretch flangeability). rice field.

No. Sample No. 15 (low temperature T2) had a large amount of tempered martensite, a small amount of retained austenite, and insufficient workability (ductility).
No. Sample No. 16 (high temperature T2) had less bainite, more martensite, less retained austenite, and insufficient workability (ductility and stretch flangeability).

No. Sample No. 17 (high temperature T3) had less bainite, more martensite, less retained austenite, and insufficient workability (ductility and stretch flangeability).
No. No. 18 (short holding time at temperature T3) had less bainite, more martensite, less retained austenite, and insufficient workability (ductility, stretch flangeability).
No. No. 19 (long holding time at temperature T3) had little retained austenite and insufficient workability (ductility).

No. Sample No. 20 (low temperature T4) had a large amount of tempered martensite, a small amount of retained austenite, and insufficient workability (ductility).
No. Sample No. 21 (high temperature T4) had less bainite, more martensite, less retained austenite, and insufficient workability (ductility and stretch flangeability).

No. Sample No. 22 (high temperature T5) had less bainite, more martensite, less retained austenite, and insufficient workability (ductility and stretch flangeability).
No. Sample No. 23 (short holding time at temperature T5) had less bainite, more martensite, less retained austenite, and insufficient workability (ductility and stretch flangeability).
No. No. 24 (long holding time at temperature T5) had little retained austenite and insufficient workability (ductility).

No. No. 25 (no second reheating) had less tempered martensite, less bainite, more martensite, less retained austenite, and insufficient workability (ductility, stretch flangeability).
No. No. 26 (no reheating, conventional continuous annealing) had less tempered martensite, more bainite, more martensite, less retained austenite, and poor workability (ductility, stretch flangeability).

Claims (2)

The area ratio of ferrite is 0% or more and 10% or less, the area ratio of tempered martensite is 40% or more and less than 80%, the area ratio of bainite is 5% or more and less than 20%, and the area ratio of martensite is A method for producing a high-strength steel sheet having a microstructure of 0% or more and 10% or less and an area ratio of retained austenite of 10% or more and 20% or less,
in % by mass,
C: 0.20% or more and 0.50% or less,
Si: 0.5% or more and 2.5% or less,
Mn: 2.5% or more and 5.0% or less,
P: 0.100% or less,
S: 0.0500% or less,
Al: 0.01% or more and 0.50% or less, and
N: contains 0.010% or less,
A steel slab having a chemical composition with the balance being Fe and unavoidable impurities is hot-rolled and then cold-rolled to obtain a cold-rolled steel sheet,
The cold-rolled steel sheet is heated at a temperature T1 in the austenite single phase region for 15 seconds or more and 1000 seconds or less, then cooled to a temperature T2 of Ms-50 ° C. or more and less than Ms ° C., and then raised to a temperature T3 of 500 ° C. or less. It is heated and held for 15 seconds or more and 1000 seconds or less, then cooled to a temperature T4 that is Ms-200° C. or more and less than the temperature T2, and then raised to a temperature T5 of 500° C. or less for 15 seconds or more. Hold for 1000 seconds or less,
The cooling rate from the temperature T1 to the temperature T2 is 5° C./second or more and 40° C./second or less,
The temperature increase rate from the temperature T2 to the temperature T3 is 5° C./second or more and 50° C./second or less,
The cooling rate from the temperature T3 to the temperature T4 is 5° C./second or more and 40° C./second or less,
A method for producing a high-strength steel sheet, wherein the temperature rising rate from the temperature T4 to the temperature T5 is 5° C./second or more and 50° C./second or less.
Here, when the content of the component X in the component composition is set to [X%] in units of mass %, Ms is obtained in units of °C by the following formula.
Ms=550−35×[Mn%]−13×[Si%]−10×[Cr%]−12×[Mo%]−600×{1−exp (−0.96×[C%])} The component composition is further, in mass %,
Ti: 0.100% or less,
Nb: 0.100% or less,
V: 0.100% or less,
Ta: 0.100% or less,
W: 0.500% or less,
Cu: 2.00% or less,
Ni: 1.00% or less,
Sn: 0.200% or less,
Sb: 0.200% or less,
Cr: 1.000% or less,
Mo: 1.000% or less,
B: 0.0050% or less,
Mg: 0.0050% or less,
Zr: 0.100% or less,
REM: 0.0050% or less, and
The method for producing a high-strength steel sheet according to claim 1, containing at least one selected from the group consisting of Ca: 0.0050% or less.

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