WO2017150117A1 - High strength steel sheet and manufacturing method therefor - Google Patents

Abstract

An aspect of the present invention is a high strength steel sheet with a specific component composition, wherein: the metal structure of the steel sheet comprises polygonal ferrite, bainite, tempered martensite, and retained martensite; when the metal structure is observed with a scanning electron microscope, the metal structure satisfies polygonal ferrite: 10-50 area%, bainite:10-50 area%, and tempered martensite: 10-80 area% with respect to the metal structure overall; and when the metal structure is measured by X-ray diffractometry, the metal structure satisfies retained austenite: at least 5 volume%, retained austenite with a carbon concentration of 1.0 mass% or less: at least 3.5 volume%, and retained austenite with a carbon concentration of 0.8 mass% or less: 2.4 volume% or less with respect to the metal structure overall.

Description

High strength steel plate and manufacturing method thereof

The present invention relates to a high-strength steel plate and a manufacturing method thereof. Specifically, it has excellent workability at room temperature, and the tensile strength when processing at a temperature of 100 to 350 ° C. is much lower than the molding load when processing at room temperature is 980 MPa or more. The present invention relates to a high-strength steel sheet and a manufacturing method thereof.

Steel sheets used for automobile structural parts and the like are required to have a high strength of 980 MPa or more in order to realize collision safety for passengers and fuel efficiency improvement. On the other hand, a steel plate is usually formed into a part shape at room temperature, and this forming is subjected to press working. Therefore, the steel sheet is required to have good press workability (hereinafter sometimes simply referred to as workability).

A TRIP (Transformation Induced Plasticity) steel sheet is known as a steel sheet having both strength and workability (for example, Patent Document 1). A TRIP steel sheet is a steel sheet containing metastable austenite (hereinafter sometimes referred to as residual austenite, sometimes referred to as residual γ), and transforms into martensite when the steel sheet deforms under stress. This has the effect of promoting the hardening of the deformed portion and preventing the concentration of strain, thereby improving the uniform deformability and exhibiting good elongation. Thus, although the elongation of the TRIP steel sheet is good, the strength of the steel sheet itself is high, so that the forming load during press working is large and the load on the press machine is excessive. Therefore, the TRIP steel sheet may not be applied depending on the part shape. Therefore, it is desired to reduce the load on the press machine and reduce the molding load at the time of pressing. That is, it is recommended that the strength is low during press working and high strength when used after press working.

As a method of reducing the forming load during the press working, a method of press forming by heating the steel plate to, for example, about 100 ° C. to A ₁ point is conceivable. In general, since the deformation resistance is reduced by warming the steel plate, the forming load during press working can be reduced.

Techniques described in Patent Documents 2 and 3 are known as techniques for reducing the forming load during press working by warming and forming a steel plate.

Patent Document 2 describes a high-tensile steel sheet having a tensile strength of 450 MPa or more and excellent in warm formability and shape freezing property, which is suitable for a processing method in which heating is performed in a temperature range of 350 ° C. to A ₁ point and press forming. Has been. This high-tensile steel sheet satisfies a predetermined component composition, and the ratio of the tensile strength at 450 ° C. to the tensile strength at room temperature is 0.7 or less, and the crystal structure of the steel is that of the martensite phase. The volume ratio is 10% or more and 80% or less, the average diameter of each dispersed martensite phase is 8 μm or less, and the volume ratio of the ferrite phase is the largest in the structure other than martensite. The high-strength thin steel sheet described in Patent Document 2 has a small decrease in tensile strength at 150 ° C., and in order to obtain a sufficient effect of reducing the load at the time of forming, after all, it is in a temperature range of 350 ° C. to A ₁ point. It is necessary to heat and press mold. However, when heated to such a high temperature, the surface state of the steel sheet is impaired by oxidation, and the energy for heating the steel sheet increases.

Patent Document 3 describes a high-strength steel sheet that sufficiently reduces strength when warm-formed at 150 to 250 ° C., but can ensure high strength of 980 MPa or more when used at room temperature after forming. This high-strength steel sheet has an area ratio of 5 to 20% of retained austenite with respect to the entire structure, and the C concentration (Cγ _R ) of the retained austenite is controlled to 0.5 to 1.0% by mass.

By the way, depending on the shape of the structural component, etc., not only warm pressing but also warm pressing may be performed at room temperature. Therefore, the steel plates used for the above parts are also required to have excellent workability at room temperature.

Since the heating temperature of the high-strength steel sheet described in Patent Document 3 is 150 to 250 ° C., the problem that occurs in the high-tensile steel sheet described in Patent Document 2 does not occur. The processability at room temperature is not considered.

Japanese Patent No. 5728115 Japanese Patent Laid-Open No. 2003-113442 JP 2013-181184 A

The present invention has been made paying attention to the above-mentioned circumstances, and the purpose thereof is excellent in workability at room temperature, in particular, elongation and hole expansibility, for a high-strength steel sheet having a tensile strength of 980 MPa or more. Another object of the present invention is to provide a high-strength steel sheet having a reduced forming load when processed at a temperature of 100 to 350 ° C., and a method for producing the same.

One aspect of the present invention is, in mass%, C: 0.10 to 0.5%, Si: 1.0 to 3%, Mn: 1.5 to 3%, P: more than 0%, 0.1% Hereinafter, S: more than 0%, 0.05% or less, Al: 0.005 to 1%, and N: more than 0%, 0.01% or less, with the balance being iron and inevitable impurities. (1) The metal structure of the steel sheet includes polygonal ferrite, bainite, tempered martensite, and retained austenite. (2) When the metal structure is observed with a scanning electron microscope, Polygonal ferrite: 10-50 area%, bainite: 10-50 area%, tempered martensite: 10-80 area%, (3) When the metal structure was measured by X-ray diffraction, Residual austenite: 5.0% by volume or less with respect to the entire structure Residual austenite having a carbon concentration of 1.0% by mass or less: 3.5% by volume or more, and retained austenite having a carbon concentration of 0.8% by mass or less: 2.4% by volume or less. It is a steel plate.

The above and other objects, features and advantages of the present invention will become apparent from the following detailed description and the accompanying drawings.

FIG. 1 is a diagram schematically showing a diffraction peak of residual γ measured by an X-ray diffraction method. FIG. 2 is a schematic diagram for explaining a form in which at least one selected from the group consisting of retained austenite and carbide is connected. FIG. 3A is a schematic diagram for explaining a distribution state of bainite and tempered martensite. FIG. 3B is a schematic diagram for explaining a distribution state of bainite and tempered martensite. FIG. 4 is a schematic view showing an annealing pattern when manufacturing a high-strength steel sheet according to the present invention.

Hereinafter, embodiments according to the present invention will be described, but the present invention is not limited thereto.

First, the metal structure of the high-strength steel sheet according to the embodiment of the present invention will be described.

The metal structure of the high-strength steel sheet is a mixed structure containing polygonal ferrite, bainite, tempered martensite, and retained austenite.

And the metal structure of the high-strength steel sheet is
(A) When the metal structure is observed with a scanning electron microscope,
Polygonal ferrite: 10 to 50 area%,
Bainite: 10-50 area%,
Tempered martensite: 10 to 80% by area satisfied,
(B) When the metal structure is measured by the X-ray diffraction method,
Residual austenite: 5.0% by volume or more,
Residual austenite having a carbon concentration of 1.0% by mass or less: 3.5% by volume or more,
It is important to satisfy the residual austenite having a carbon concentration of 0.8% by mass or less: 2.4% by volume or less.

First, after describing the requirement (B) that characterizes the present invention, the requirement (A) will be described.

[Residual γ]
Residual γ is a structure necessary for improving uniform deformability and ensuring good elongation by the TRIP effect. Further, the residual γ is a structure necessary for securing strength.

In the high-strength steel sheet, in order to exert such effects, the volume ratio of residual γ (hereinafter sometimes referred to as Vγ _R ) is 5.0% or more, preferably 8%, with respect to the entire metal structure. Above, more preferably 10% or more. However, when the amount of residual γ produced becomes excessive, the hole expansion rate λ decreases, and the processability at room temperature cannot be improved. Thus, in the high strength steel sheet, the volume fraction V.gamma _R of residual γ, relative to the entire metal structure, preferably 30% or less, more preferably 25% or less.

The residual γ may be generated between laths, or an aggregate of lath-like structures, for example, blocks, packets, or old γ grain boundaries on the MA mixed phase in which fresh martensite and residual γ are combined. It may exist in a lump as a part. MA is an abbreviation for Martensite-Authentite Constituent.

The volume ratio Vγ _R of the residual γ is a value measured by an X-ray diffraction method.

The high-strength steel sheet has a volume fraction of retained austenite having a carbon concentration of 1.0 mass% or less [hereinafter referred to as Vγ _R (C), particularly when the metal structure is measured by an X-ray diffraction method. ≦ 1.0%). ] Is a volume fraction of retained austenite having a carbon concentration of 0.8% by mass or less [hereinafter, Vγ _R (C ≦ 0.8%). ] Of 2.4% or less is important. That is, as described below, it is important to appropriately generate residual γ having a carbon concentration of more than 0.8 mass% and 1.0 mass% or less.

First, it will be explained that the load during warming can be sufficiently reduced by setting the volume ratio of residual γ having a carbon concentration of 1.0% by mass or less to a certain level or more.

As an index of load reduction at the time of warm forming, as shown in Patent Document 3 above, ΔTS, that is, tensile strength at room temperature (hereinafter sometimes referred to as room temperature TS), it is warm. The value (room temperature TS−warm TS) obtained by subtracting the tensile strength (hereinafter sometimes referred to as “warm TS”) can be used. It can be said that the larger the ΔTS, the more the load during warming is sufficiently reduced.

To increase the value of ΔTS,
(1) Decrease in deformation resistance due to high processing temperature; and (2) Residual γ is unstable at room temperature and improves tensile strength TS, but is stable at warm temperature and does not improve tensile strength TS;
A method of using can be considered.

Since (1) does not depend on the material, it is effective to have the residual γ shown in (2) above in order to obtain a steel plate having a larger ΔTS, and for that purpose, carbon The residual γ having a low concentration, specifically, the volume fraction Vγ _R (C ≦ 1.0%) of the residual γ having a carbon concentration of 1.0% by mass or less is 3.5% by mass with respect to the entire metal structure. It has been found that the generation is positive. Vγ _R (C ≦ 1.0%) is preferably 4.0% by volume or more, and more preferably 4.5% by volume or more. The upper limit of Vγ _R (C ≦ 1.0%) is not particularly limited, and the maximum value of Vγ _R (C ≦ 1.0%) is equal to the volume ratio of residual γ contained in the steel sheet. Vγ _R (C ≦ 1.0%) is preferably 10% by volume or less, more preferably 8% by volume or less.

As described above, it was found that the molding load during warming can be reduced by positively generating residual γ having a carbon concentration of 1.0% by mass or less. However, if a large amount of residual γ with a too low carbon concentration is generated, the residual γ is transformed into hard fresh martensite (FM) at the initial stage of hole expansion at room temperature, and becomes a strain concentrated portion, and the hole expansion ratio λ As a result, it was found that the workability at room temperature deteriorates. Therefore, in order to increase the hole expansion ratio λ and improve the processability at room temperature, it is necessary to suppress the formation of residual γ with a carbon concentration that is too low.

Based on these viewpoints, the relationship between the carbon concentration of residual γ and the workability at room temperature was examined. The volume fraction Vγ _R (C ≦ 0.8%) of residual γ with a carbon concentration of 0.8% by mass or less was calculated. The present inventors have found that the content may be 2.4% by volume or less with respect to the entire metal structure. Vγ _R (C ≦ 0.8%) is preferably 2.3% by volume or less, more preferably 2.2% by volume or less, and still more preferably 2.1% by volume or less. Vγ _R (C ≦ 0.8%) is preferably as small as possible, and most preferably 0% by volume.

As described above, by setting Vγ _R (C ≦ 1.0%) to 3.5% by volume or more, ΔTS can be increased, and the molding load at warm temperature can be reduced more than the molding load at room temperature. By suppressing Vγ _R (C ≦ 0.8%) to 2.4% by volume or less, it is possible to increase the hole expansion ratio λ when the hole expansion processing is performed at room temperature, and to improve the workability at room temperature. .

The present inventors confirmed that residual γ having a carbon concentration exceeding 1.0 mass% is stable at both room temperature and warm, so that the influence on ΔTS and the hole expansion rate λ at room temperature is small. ing.

Here, the relationship between the prior art and the present invention will be described.
The concept of controlling the stability of the residual γ with the carbon concentration of the residual γ has been conventionally known. For example, as in Patent Document 3, a technique for reducing the strength at the time of molding at 150 to 250 ° C. by controlling the average value of the carbon concentration of residual γ within a predetermined range is already known. .

On the other hand, in the present invention, focusing on the carbon concentration of individual residual γ, not the average carbon concentration of residual γ, the residual γ having a carbon concentration of more than 0.8 mass% and 1.0 mass% or less is positive. It is greatly different from the conventional technology in that it is generated in the above. That is, even if the amount of residual γ is the same and the average value of the carbon concentration of the residual γ is the same, the amount of residual γ generated and the carbon concentration of 1.0% by mass or less The present inventors have found that the characteristics obtained are greatly changed if the amount of γ produced is different.

Further adding to the above Patent Document 3, since the average value of the carbon concentration in the residual γ contained in the steel sheet disclosed in the above Patent Document 3 is very low, the residual γ having a carbon concentration of 0.8% by mass or less. The amount produced is expected to be quite large and the composition is also different.

Next, each measurement method of the volume ratio of residual γ (Vγ _R ), the average carbon concentration of residual γ (hereinafter sometimes referred to as% C _avg ), and the carbon concentration distribution of residual γ with respect to the entire metal structure. Will be described.

The volume fraction of residual γ and the average carbon concentration of residual γ are measured by X-ray diffraction after grinding to a thickness of ¼ of the steel plate and then chemical polishing. The measurement principle is ISIJ Int. Vol. 33, 1933, no. 7, p. 776 can be referred to. In this X-ray diffraction method, an X-ray diffractometer (RINT-1500) manufactured by Rigaku Corporation was used as the X-ray diffractometer, and Co-Kα rays were used as the X-ray.

The distribution of carbon concentration of residual γ was determined as follows using three diffraction peaks of (200) _γ , (220) _γ , and (311) _γ measured with the above X-ray diffractometer.

First, as shown in the schematic diagram of FIG. 1, 2θ (2θ _avg (hkl)) at which the diffraction intensity is maximum for each of the three diffraction peaks of (200) _γ , (220) _γ , and (311) _γ. And its half-value width Δ2θ (hkl). Here, (hkl) means (200), (220) or (311) (the same shall apply hereinafter). FIG. 1 is a diagram schematically showing a diffraction peak of residual γ measured by an X-ray diffraction method.

Next, from the above 2θ _avg (hkl), using the Bragg condition: λ _B = 2 d sin θ (d: lattice spacing, λ _B : wavelength of Co—Kα ray), d (hkl) is obtained from the following equation (1). Asked.
d (hkl) = λ _B / {2 sin (2θ _avg (hkl) / 2)} (1)

Then, the following equation (2), lattice constant _a 0 seek (hkl), and their three lattice constants _a 0 (hkl) and arithmetic mean of the determined lattice constants _{a 0.}
a ₀ (hkl) = d (hkl) √ (h ² + k ² + l ² ) (2)

And carbon concentration% _Cavg (unit: mass%) was calculated _| required using following formula (3).
% C _avg = (1 / 0.033) × (a ₀ −3.572) (3)

Next, the half-value width Δ% C of the carbon concentration distribution of residual γ was determined by the following procedure.

First, the diffraction angles at the upper and lower limits of the half-value width Δ2θ (hkl) of the diffraction angle 2θ (hkl) of each peak were determined by the following formulas (4) and (5) (see FIG. 1).
2θ _L (hkl) = 2θ _avg (hkl) −Δ2θ (hkl) / 2 (4)
2θ _H (hkl) = 2θ _avg (hkl) + Δ2θ (hkl) / 2 (5)

Therefore, by using the above 2θ _L (hkl) and 2θ _H (hkl) respectively, and using the Bragg condition and the above formulas (1) to (3) in the same procedure as described above, the half width of the carbon concentration distribution is obtained. Upper and lower limit values% C _L and% C _H were determined. And the half value width (DELTA)% C of carbon concentration distribution was calculated | required by following formula (6).
Δ% C =% C _H- % C _L (6)

Then, assuming that the carbon concentration distribution is a normal distribution, the standard deviation σ% C was calculated from the half-value width Δ% C as follows. That is, the probability density function f (x) of the normal distribution is expressed by the following formula (7) from the average value u and the standard deviation σ.
f (x) = {1 / √ (2πσ ² )} × exp {− (x−u) ² / (2σ ² )} (7)

The probability f (u) in the average value is obtained by the following formula (8) by substituting x = u into the above formula (7).
f (u) = 1 / √ (2πσ ² ) (8)

Then, the probability density f (% C _avg ± Δ% C) at a value (% C _avg ± Δ% C / 2) shifted up and down from the average value u =% C _avg by ½ of the half-value width Δ% C / 2) is ½ of the probability density f (u) = f (% C _avg ) at the average value u =% C _avg , the following equation is obtained from the above equations (7) and (8): The relationship (9) is obtained.
{1 / √ (2πσ% C ² )} × exp {− (Δ% C / 2) ² / (2σ% C ² )} = 1 / {2√ (2πσ% C ² )} (9)

By modifying the above equation (9), the following equation (10) is derived as an equation for obtaining the standard deviation σ% C from the half-value width Δ% C. Therefore, the half-value width Δ% C is derived from this equation (10). Was substituted to calculate the standard deviation σ% C.
σ% C = √ {(Δ% C / 2) ² / (2ln2)} (10)

Then, using the average value% C _avg and the standard deviation σ% C of the carbon concentration distribution in the residual γ determined as described above, the cumulative distribution function g (x) shown in the following formula (11) The following equation (12) is derived as an equation for obtaining the volume fraction Vγ _R (C ≦ 1.0%) of residual γ with a carbon concentration of 1.0 mass% or less with respect to the entire tissue, and this equation (12) is used. Vγ _R (C ≦ 1.0%) was calculated.
g (x) = (1/2) × [1 + erf {(x−u) / √ (2σ ² )}] (11)
Vγ _R (C ≦ 1.0%) = Vγ _R × g (1.0) = Vγ _R × (1/2) × [1 + erf {(1.0−% C _avg ) / √ (2σ% C ² ) }] (12)

Further, using the average value% C _avg and the standard deviation σ% C of the carbon concentration distribution in the residual γ determined as described above, the cumulative distribution function g (x) shown in the above equation (11) is used to calculate the metal The following equation (13) is derived as an equation for obtaining the volume fraction Vγ _R (C ≦ 0.8%) of residual γ having a carbon concentration of 0.8 mass% or less with respect to the entire tissue, and using this equation (13) Vγ _R (C ≦ 0.8%) was calculated.
_{Vγ R (C ≦ 0.8%)} = Vγ R × g (0.8) = Vγ R × (1/2) × [1 + erf {(0.8-% C avg) / √ (2σ% C 2) }] (13)

In the above formula (12) and Formula (13), V.gamma _R is the total volume fraction of retained γ to the entire metal structure.
Next, the requirement (A) will be described.

[Polygonal ferrite]
Polygonal ferrite is softer than bainite and is a structure that acts to increase the elongation of the steel sheet and improve the workability at room temperature. In order to exert such an effect, the area ratio of polygonal ferrite is 10% or more, preferably 20% or more, more preferably 25% or more with respect to the entire metal structure. However, since the strength decreases when the amount of polygonal ferrite produced becomes excessive, the area ratio of polygonal ferrite is 50% or less, preferably 45% or less, more preferably 40% or less, based on the entire metal structure. To do.

The area ratio of the polygonal ferrite can be measured with a scanning electron microscope.

[Bainite]
Bainite generated by the bainite transformation is a structure that effectively acts to concentrate C into austenite and obtain residual γ. Moreover, since bainite has the intensity | strength between polygonal ferrite and tempered martensite, it is a structure | tissue which raises both intensity | strength and elongation with good balance. In order to exhibit such an effect, the area ratio of bainite is 10% or more, preferably 15% or more, more preferably 20% or more with respect to the entire metal structure. However, when the amount of bainite produced becomes excessive, the strength decreases, so that the area ratio of bainite is 50% or less, preferably 40% or less, more preferably 30% or less with respect to the entire metal structure.

The bainite is a structure in which the average of the interval between at least one selected from the group consisting of residual γ and carbide is 1 μm or more when observed with a scanning electron microscope after the section of the steel sheet is corroded with nital. In addition to bainitic ferrite with no carbide precipitation, it includes those with partial precipitation of carbide.

[Tempered martensite]
Tempered martensite is a structure that acts to increase both the strength and the hole expansion ratio λ in a well-balanced manner. In order to exhibit such an effect, the area ratio of tempered martensite is 10% or more, preferably 15% or more, more preferably 20% or more with respect to the entire metal structure. However, if the amount of tempered martensite produced becomes excessive, the amount of residual γ produced decreases significantly, and the elongation decreases. Therefore, the area ratio of tempered martensite is preferably 80% or less of the entire metal structure, preferably Is 70% or less, more preferably 60% or less.

The tempered martensite is an average of the interval between at least one selected from the group consisting of residual γ and carbide when less than 1 μm is selected from the group consisting of residual γ and carbide when the steel sheet is subjected to nital corrosion and then observed with a scanning electron microscope. Organization.

Here, the “average of at least one interval selected from the group consisting of residual γ and carbide” will be described. The average is the distance between the center positions of the adjacent residual γ, the distance between the center positions of the adjacent carbides, or the residual γ when the cross section of the steel sheet is subjected to Nital corrosion and observed with a scanning electron microscope. It is a value obtained by averaging the results of measuring the distance between the center positions of the carbides adjacent to the residual γ. The distance between the center positions means the distance between the center positions obtained by obtaining the center positions of each residual γ or each carbide and measuring the most adjacent residual γ, the carbides, or the residual γ and the carbide. The center position determines the major axis and minor axis of residual γ or carbide, and is the position where the major axis and minor axis intersect.

However, when residual γ or carbide precipitates on the boundary of the lath, a plurality of residual γ and carbide are connected to form a needle shape or a plate shape. In this case, the distance between the center positions is not a distance between at least one selected from the group consisting of residual γ and carbide, but at least one type selected from the group consisting of residual γ and carbide as shown in FIG. The distance between the lines 11 formed continuously in the direction, that is, the distance between the laths, is the distance 12 between the center positions. FIG. 2 is a schematic diagram for explaining a form in which at least one selected from the group consisting of retained austenite and carbide is connected.

The distribution state of bainite and tempered martensite is not particularly limited, and both bainite and tempered martensite may be generated in the old austenite grains, and bainite and tempered martensite are generated for each old austenite grain. It may be.

The distribution state of bainite and tempered martensite is schematically shown in FIGS. 3A and 3B. FIG. 3A shows a state in which both bainite 21 and tempered martensite 22 are mixed and formed in the prior austenite grains 23, and FIG. 3B shows bainite 21 and tempered martensite 22 for each prior austenite grain 23. Shows how each is generated. The black circle 24 shown in each figure shows the MA mixed phase. 3A and 3B are schematic diagrams for explaining the distribution state of bainite and tempered martensite.

[Others]
The metal structure of the high-strength steel sheet may be composed of polygonal ferrite, bainite, tempered martensite, and residual γ. However, as long as the effects of the present invention are not impaired, other structures include MA mixed phase, pearlite. Or you may have remainder structures, such as fresh martensite. Any remaining structure becomes a starting point of cracking and deteriorates the workability at room temperature, so it is preferable that the remaining structure be as small as possible. The remaining structure is preferably 25 area% or less in total when the cross section of the steel plate is subjected to nital corrosion and then observed with a scanning electron microscope.

The area ratio of polygonal ferrite, bainite, and tempered martensite is measured with a scanning electron microscope, whereas the volume fraction of residual γ is measured with an X-ray diffraction method, and the measurement method is different. The total area ratio and volume ratio of these tissues may exceed 100%.

Next, the component composition of the high-strength steel sheet according to this embodiment will be described. Hereinafter,% in the component composition means mass%.

The high-strength steel plate has C: 0.10 to 0.5%, Si: 1.0 to 3%, Mn: 1.5 to 3%, P: more than 0%, 0.1% or less, S: 0 %, 0.05% or less, Al: 0.005 to 1%, and N: more than 0%, 0.01% or less.

C is an element that increases the strength of the steel sheet, and is also an element that is necessary for stabilizing austenite and securing residual γ. In order to exert such an effect, the C amount is 0.10% or more. The amount of C is preferably 0.13% or more, more preferably 0.15% or more. However, if C is contained excessively, weldability deteriorates, so the C content is 0.5% or less. The amount of C is preferably 0.30% or less, more preferably 0.25% or less.

Si is a solid solution strengthening element and is an element that contributes to increasing the strength of the steel sheet. Si is an important element for suppressing the precipitation of carbides and condensing and stabilizing C in austenite to secure residual γ. In order to exert such effects, the Si amount is set to 1.0% or more. The Si amount is preferably 1.2% or more, more preferably 1.3% or more. However, if Si is excessively contained, reverse transformation of polygonal ferrite to austenite does not occur during annealing and soaking, and polygonal ferrite remains excessively, resulting in insufficient strength. Moreover, a scale is remarkably formed at the time of hot rolling, and a scale trace is attached to the steel sheet surface, which deteriorates the surface properties. For these reasons, the Si content is 3% or less. The amount of Si is preferably 2.5% or less, more preferably 2.0% or less.

Mn is an element that acts as a hardenability improving element, suppresses excessive formation of polygonal ferrite during cooling, and increases the strength of the steel sheet. Mn also contributes to stabilizing the residual γ. In order to exhibit such an effect, the amount of Mn is 1.5% or more. The amount of Mn is preferably 1.8% or more, more preferably 2.0% or more. However, when Mn is contained excessively, the formation of bainite is remarkably suppressed, the desired amount of bainite cannot be secured, and the balance between strength and elongation is deteriorated. In addition, adverse effects such as slab cracking occur. Therefore, the Mn content is 3% or less. The amount of Mn is preferably 2.8% or less, more preferably 2.7% or less.

P is an inevitable impurity, and if contained excessively, it promotes grain boundary embrittlement due to grain boundary segregation and deteriorates workability at room temperature, so the P content is 0.1% or less. The amount of P is preferably 0.08% or less, more preferably 0.05% or less. The amount of P is preferably as small as possible, but is usually about 0.001%.

S is an unavoidable impurity, and if it is excessively contained, sulfide inclusions such as MnS are formed and the workability at room temperature deteriorates as a starting point of cracking. Therefore, the S amount is 0.05% or less. . The amount of S is preferably 0.01% or less, more preferably 0.005% or less. The amount of S is preferably as small as possible, but is usually about 0.0001%.

Al, like Si, is an important element for securing the residual γ by suppressing the precipitation of carbides. Al is an element that also acts as a deoxidizer. In order to exert such an effect, the Al amount is set to 0.005% or more. The amount of Al is preferably 0.010% or more, more preferably 0.03% or more. However, if Al is contained excessively, a lot of inclusions are precipitated in the steel sheet, and the workability at room temperature deteriorates. Therefore, the Al content is set to 1% or less. The amount of Al is preferably 0.8% or less, more preferably 0.5% or less.

N is an inevitable impurity, and if it is contained in excess, a large amount of nitride precipitates and becomes the starting point of cracking, and the workability at room temperature deteriorates, so the N amount is 0.01% or less. The N amount is preferably 0.008% or less, more preferably 0.005% or less. The amount of N is preferably as small as possible, but is usually about 0.001%.

The basic components of the high-strength steel plate are as described above, and the balance is iron and inevitable impurities. As an inevitable impurity, mixing of elements brought in depending on the situation of raw materials, materials, manufacturing facilities, etc. is allowed within a range that does not impair the effects of the present invention.

The high-strength steel plate may further contain at least one element belonging to the following (a) to (e) as another element. Further, the elements belonging to the following (a) to (e) may be contained alone, or a plurality of elements belonging to the following (a) to (e) may be contained in combination.
(A) At least one selected from the group consisting of Cr: more than 0%, 1% or less, and Mo: more than 0%, 1% or less.
(B) At least one selected from the group consisting of Ti: more than 0%, 0.15% or less, Nb: more than 0%, 0.15% or less, and V: more than 0%, 0.15% or less.
(C) At least one selected from the group consisting of Cu: more than 0%, 1% or less, and Ni: more than 0%, 1% or less.
(D) B: more than 0% and 0.005% or less.
(E) At least one selected from the group consisting of Ca: more than 0%, 0.01% or less, Mg: more than 0%, 0.01% or less, and rare earth elements: more than 0%, 0.01% or less.

(A) Cr and Mo are elements that suppress the formation of excessive polygonal ferrite during cooling and prevent a decrease in strength. In order to effectively exert such effects, the Cr content is preferably 0.02% or more, more preferably 0.1% or more, and further preferably 0.2% or more. The amount of Mo is preferably 0.02% or more, more preferably 0.1% or more, and further preferably 0.2% or more. However, if Cr and Mo are contained excessively, the generation of bainite is remarkably suppressed as in Mn, the desired amount of bainite cannot be secured, and the balance between strength and elongation may be deteriorated. The content is preferably 1% or less, more preferably 0.8% or less, and still more preferably 0.5% or less. The amount of Mo is preferably 1% or less, more preferably 0.8% or less, and still more preferably 0.5% or less. Cr and Mo may contain either one or both.

(B) Ti, Nb, and V are all elements that act to refine the metal structure and improve the strength and toughness of the steel sheet. In order to effectively exhibit such effects, the Ti content is preferably 0.01% or more, more preferably 0.015% or more, and further preferably 0.020% or more. The Nb content is preferably 0.01% or more, more preferably 0.015% or more, and further preferably 0.020% or more. The amount of V is preferably 0.01% or more, more preferably 0.015% or more, and further preferably 0.020% or more. However, the effect is saturated even if Ti, Nb, and V are contained excessively. In addition, carbides may precipitate at the grain boundaries and workability at room temperature may deteriorate. For these reasons, the Ti content is preferably 0.15% or less, more preferably 0.12% or less, and still more preferably 0.10% or less. The Nb content is preferably 0.15% or less, more preferably 0.12% or less, and still more preferably 0.10% or less. The V amount is preferably 0.15% or less, more preferably 0.12% or less, and still more preferably 0.10% or less. Ti, Nb, and V may contain any 1 type, and may contain 2 or more types chosen arbitrarily.

(C) Cu and Ni are elements that act to improve the corrosion resistance of the steel sheet. In order to exhibit such an effect effectively, the amount of Cu is preferably 0.01% or more, more preferably 0.05% or more, and further preferably 0.10% or more. The amount of Ni is preferably 0.01% or more, more preferably 0.05% or more, and further preferably 0.10% or more. However, the effect is saturated even if Cu and Ni are contained excessively. Moreover, hot workability may deteriorate. For these reasons, the Cu content is preferably 1% or less, more preferably 0.8% or less, and still more preferably 0.5% or less. The amount of Ni is preferably 1% or less, more preferably 0.8% or less, and still more preferably 0.5% or less. Cu and Ni may contain either one or both.

(D) B, like Cr and Mn, is an element that suppresses excessive formation of polygonal ferrite during cooling and prevents a decrease in strength. In order to effectively exert such effects, the B content is preferably 0.0001% or more, more preferably 0.0005% or more, and further preferably 0.0010% or more. However, when B is contained excessively, the formation of bainite is remarkably suppressed like Cr and Mn, the desired amount of bainite cannot be secured, and the balance between strength and elongation may be deteriorated. For these reasons, the B content is preferably 0.005% or less, more preferably 0.004% or less, and still more preferably 0.003% or less.

(E) Ca, Mg, and rare earth elements (Rare Earth Metal; REM) are all elements that have the effect of finely dispersing inclusions in the steel sheet. In order to effectively exert such effects, the Ca content is preferably 0.0001% or more, more preferably 0.0005% or more, and further preferably 0.0010% or more. The amount of Mg is preferably 0.0001% or more, more preferably 0.0005% or more, and still more preferably 0.0010% or more. The amount of rare earth elements is preferably 0.0001% or more, more preferably 0.0005% or more, and still more preferably 0.0010% or more. However, when Ca, Mg, and rare earth elements are contained excessively, forgeability and hot workability may deteriorate. Therefore, the Ca content is preferably 0.01% or less, more preferably 0.005% or less, and still more preferably 0.003% or less. The Mg content is preferably 0.01% or less, more preferably 0.005% or less, and still more preferably 0.003% or less. The rare earth element content is preferably 0.01% or less, more preferably 0.005% or less, and still more preferably 0.003% or less. Ca, Mg, and rare earth elements may contain any 1 type, and may contain 2 or more types chosen arbitrarily.

In the high-strength steel sheet, the rare earth element means a lanthanoid element (15 elements from La to Lu), Sc (scandium), and Y (yttrium).

The surface of the high-strength steel sheet has an electrogalvanized (EG) layer, a hot dip galvanized (GI) layer, or an alloyed hot dip galvanized (GA) layer. It may be. That is, the present invention includes high-strength electrogalvanized steel sheets, high-strength hot-dip galvanized steel sheets, and high-strength galvannealed steel sheets.

Next, a method for manufacturing a high-strength steel sheet according to this embodiment will be described with reference to FIG. FIG. 4 is a schematic diagram showing an annealing pattern when manufacturing the high-strength steel sheet, where the horizontal axis represents time (seconds) and the vertical axis represents temperature (° C.).

[Soaking process]
First, a steel sheet satisfying the above component composition is heated to a T1 temperature range of 800 ° C. or more and Ac ₃ points or less, and held in the T1 temperature range for 40 seconds or more and soaked (soaking step). The steel plate may be a hot-rolled steel plate or a cold-rolled steel plate. In addition, the soaking temperature in the T1 temperature region is hereinafter referred to as “T1”, and the soaking time in the T1 temperature region is hereinafter referred to as “t1”. The holding includes not only constant temperature holding but also a mode in which the temperature fluctuates within the T1 temperature range.

By soaking in the two-phase temperature range of polygonal ferrite and austenite, a predetermined amount of polygonal ferrite can be generated. If the soaking temperature T1 in the T1 temperature range is too low, polygonal ferrite is excessively generated and the strength is lowered. On the other hand, if the soaking temperature T1 is too low, the stretched structure generated during cold rolling remains and the elongation decreases, so that the workability at room temperature cannot be improved. Therefore, the soaking temperature T1 is set to 800 ° C. or higher. The soaking temperature T1 is preferably 810 ° C. or higher, more preferably 820 ° C. or higher. However, if the soaking temperature T1 is too high, it becomes an austenite single phase region, and the amount of polygonal ferrite produced is insufficient, so that the elongation decreases and the workability at room temperature cannot be improved. Therefore, the soaking temperature T1 is set to Ac ₃ points or less. The soaking temperature T1 is preferably Ac ₃ point −10 ° C. or lower, more preferably Ac ₃ point −20 ° C. or lower.

If the soaking time t1 in the T1 temperature range is too short, the steel sheet cannot be heated uniformly, so that the carbide remains undissolved and the formation of residual γ is suppressed. As a result, the elongation decreases and the processability at room temperature cannot be improved. Therefore, the soaking time t1 is set to 40 seconds or more. The soaking time t1 is preferably 50 seconds or longer, more preferably 80 seconds or longer. Although the upper limit of soaking time t1 is not specifically limited, If soaking time t1 is too long, productivity will worsen. Therefore, the soaking time t1 is preferably 500 seconds or less, and more preferably 450 seconds or less.

The temperature of Ac ₃ point of the steel sheet is represented by the following formula (II) described in “Leslie Steel Material Science” (published by Maruzen Co., Ltd., William C. Leslie, published May 31, 1985, p.273). It can be calculated from In the following formula (II), [] indicates the content (% by mass) of each element, and the content of elements not included in the steel sheet may be calculated as 0% by mass.
Ac ₃ points (° C.) = 910−203 × [C] ^1/2 + 44.7 × [Si] −30 × [Mn] −11 × [Cr] + 31.5 × [Mo] −20 × [Cu] − 15.2 × [Ni] + 400 × [Ti] + 104 × [V] + 700 × [P] + 400 × [Al] (II)

[First cooling step]
After the soaking, when the Ms point represented by the following formula (I) is 350 ° C. or higher, up to an arbitrary cooling stop temperature T2 satisfying 350 ° C. or lower and 100 ° C. or higher, or by the following formula (I) When the represented Ms point is lower than 350 ° C., it is cooled to an arbitrary cooling stop temperature T2 that satisfies the Ms point or lower and 100 ° C. or higher (first cooling step). In the first cooling step, cooling is performed from 700 ° C. to 300 ° C. or the higher one of the cooling stop temperatures T2 at an average cooling rate of 5 ° C./second or more.

By controlling the average cooling rate in the section from 700 ° C. to 300 ° C. or the higher one of the cooling stop temperatures T2 after soaking (hereinafter sometimes referred to as CR1), A predetermined amount of polygonal ferrite can be generated. That is, when the average cooling rate CR1 in the section is less than 5 ° C./second, polygonal ferrite is excessively generated and the strength is lowered. Therefore, the average cooling rate CR1 in the above section needs to be controlled to 5 ° C./second or more, preferably 10 ° C./second or more, more preferably 15 ° C./second or more. The upper limit of the average cooling rate CR1 in the above section is not particularly limited, but if the average cooling rate CR1 becomes too large, temperature control becomes difficult. Therefore, the average cooling rate CR1 in the above section is preferably 80 ° C./second or less, more preferably 60 ° C./second or less.

The cooling stop temperature T2 is 100 to 350 ° C. However, when the Ms point calculated by the following formula (I) is less than 350 ° C., the cooling stop temperature T2 is set to 100 ° C. to Ms point.

If the cooling stop temperature T2 is too low, tempered martensite is excessively generated and the amount of residual γ is reduced, so that elongation is lowered and workability at room temperature cannot be improved. On the other hand, if the cooling stop temperature T2 is too low, a large amount of residual γ with a carbon concentration exceeding 1.0% by mass is generated, the amount of residual γ with a carbon concentration of 1.0% by mass or less is relatively small, and ΔTS is The warm forming load cannot be sufficiently reduced compared with the forming load at room temperature. The reason why a large amount of residual γ with a carbon concentration exceeding 1.0% by mass is generated is that film-like residual γ remains between the laths in the tempered martensite, and the carbon concentration of the residual γ is high. Therefore, the cooling stop temperature T2 is set to 100 ° C. or higher. The cooling stop temperature T2 is preferably 110 ° C. or higher, more preferably 120 ° C. or higher. However, if the cooling stop temperature T2 is too high, the amount of tempered martensite generated is reduced, so that the subsequent bainite transformation is difficult to proceed and the C enrichment to austenite is difficult to proceed. As a result, the residual γ with a carbon concentration of 0.8% by mass or less increases, the hole expansion rate λ decreases, and the processability at room temperature cannot be improved. Therefore, when the Ms point is 350 ° C. or higher, the cooling stop temperature T2 is set to 350 ° C. or lower. The cooling stop temperature T2 is preferably 330 ° C. or lower, more preferably 300 ° C. or lower. On the other hand, when the Ms point is lower than 350 ° C., the cooling stop temperature T2 is set to be equal to or lower than the Ms point. The cooling stop temperature T2 is preferably Ms point −20 ° C. or lower, more preferably Ms point −50 ° C. or lower.

The temperature of the Ms point can be calculated from the following formula (I) in consideration of the polygonal ferrite fraction (Vf) in the formula described in the “Leslie Steel Material Science” (p. 231). In the following formula (I), [] indicates the content (mass%) of each element, and the content of elements not included in the steel sheet may be calculated as 0 mass%. Vf represents the polygonal ferrite fraction (area%), but since it is difficult to directly measure the polygonal ferrite fraction during production, the soaking is performed separately under the same conditions as the production conditions for the high-strength steel sheet. After performing the step, the polygonal ferrite fraction in the sample obtained by cooling to room temperature at the same average cooling rate as the production conditions of the high-strength steel plate in the first cooling step may be Vf.
Ms point (° C.) = 561−474 × [C] / (1−Vf / 100) −33 × [Mn] −17 × [Ni] −17 × [Cr] −21 × [Mo] (I )

[Reheating process]
After cooling to the above cooling stop temperature T2, it is reheated to a T3 temperature range of more than 350 ° C. and 540 ° C. or less, and held at the T3 temperature range for 50 seconds or longer (reheating step). In addition, the reheating temperature in the T3 temperature range is hereinafter referred to as “T3”, and the holding time in the T3 temperature range is hereinafter referred to as “t3”. The holding includes not only constant temperature holding but also a mode in which the temperature fluctuates within the T3 temperature range.

By holding for 50 seconds or more in the T3 temperature range, it is possible to generate residual γ having a carbon concentration of more than 0.8% by mass and 1.0% by mass or less, so that the workability at room temperature remains good. A high-strength steel sheet with reduced warm load can be realized.

If the reheating temperature T3 is too low, the amount of residual γ having a carbon concentration of 1.0% by mass or less is reduced and ΔTS is lowered, so that the molding load during warming cannot be reduced. Therefore, the reheating temperature T3 is over 350 ° C. The reheating temperature T3 is preferably 360 ° C. or higher, more preferably 370 ° C. or higher. However, if the reheating temperature T3 is too high, the bainite transformation does not proceed sufficiently, the amount of residual γ decreases, and the elongation EL decreases. Further, among the residual γ, the amount of residual γ having a carbon concentration of 0.8% by mass or less increases, and the hole expansion rate λ decreases. As a result, the processability at room temperature cannot be improved. Therefore, the reheating temperature T3 is set to 540 ° C. or lower. The reheating temperature T3 is preferably 520 ° C. or lower, more preferably 500 ° C. or lower.

Also, if the holding time t3 is too short, the bainite transformation does not proceed sufficiently, so that C concentration to austenite does not proceed sufficiently and the amount of residual γ produced decreases. As a result, the elongation EL decreases. In addition, the concentration of C in each austenite varies, the amount of residual γ with a carbon concentration of 0.8% by mass or less increases, and the hole expansion ratio λ decreases. As a result, processability at room temperature deteriorates. Accordingly, the holding time t3 is set to 50 seconds or longer. The holding time t3 is preferably 80 seconds or longer, more preferably 100 seconds or longer. The upper limit of the holding time t3 is not particularly limited, but in consideration of productivity, for example, 20 minutes or less is preferable.

[Second cooling step]
After holding in the reheating step, cooling is performed at an average cooling rate of 10 ° C./second or more from the T3 temperature range to 300 ° C., and from 300 ° C. to 150 ° C., an average cooling rate of more than 0 ° C./second, 10 ° C. Cool in less than 1 second (second cooling step). After the above holding, when cooling from the T3 temperature range to 150 ° C., it is important to perform two-stage cooling with 300 ° C. as the boundary, and the high temperature side up to 300 ° C. can increase ΔTS by rapid cooling, The molding load during warming can be reduced. On the low temperature side from 300 ° C., the hole expansion ratio λ can be increased by slow cooling, so that the workability at room temperature can be improved.

That is, if the average cooling rate up to 300 ° C. (hereinafter, sometimes referred to as CR2) is too small after reheating, the concentration of C into bainite transformed and untransformed austenite proceeds during cooling, and carbon While the amount of residual γ exceeding 1.0% by mass increases, the amount of residual γ having a carbon concentration of 1.0% by mass or less decreases. As a result, ΔTS decreases, and the molding load during warming cannot be reduced. Therefore, it is necessary to control the average cooling rate CR2 to 10 ° C./second or more, preferably 15 ° C./second or more, more preferably 20 ° C./second or more. The upper limit of the average cooling rate CR2 is not particularly limited, but if the average cooling rate CR2 becomes too large, temperature control becomes difficult. Therefore, the average cooling rate CR2 is preferably 80 ° C./second or less, more preferably 60 ° C./second or less.

On the other hand, bainite transformation does not proceed during cooling from 300 ° C. to 150 ° C., but C diffusion proceeds. Thus, by reducing the average cooling rate in this temperature range (hereinafter sometimes referred to as CR3), the amount of residual γ with a carbon concentration of 0.8 mass% or less can be reduced. The reason for this is unknown, but among the variations in the C concentration that occurs in each residual γ, C diffuses during cooling to the residual γ that is lower than the original C concentration. It can be reduced. The source of C is considered to be tempered martensite. In addition, fresh martensite (FM) inevitably transformed from γ undergoes self-tempering during cooling. As a result, the hole expansion rate λ is increased and the processability at room temperature is improved. Therefore, the average cooling rate CR3 in the above section needs to be controlled to less than 10 ° C./second, preferably 5 ° C./second or less, more preferably 2 ° C./second or less.

After cooling to 150 ° C., it may be cooled to room temperature according to a conventional method.

[Plating]
An electrogalvanized layer, a hot-dip galvanized layer, or an alloyed hot-dip galvanized layer may be formed on the surface of the high-strength steel plate.

The conditions for forming the electrogalvanized layer, hot dip galvanized layer, or alloyed hot dip galvanized layer are not particularly limited, and conventional electrogalvanized (EG) treatment, hot dip galvanized (GI) treatment, galvannealed alloyed zinc Plating (GA) treatment can be employed. Accordingly, an electrogalvanized steel sheet (hereinafter sometimes referred to as “EG steel sheet”), a hot dip galvanized steel sheet (hereinafter sometimes referred to as “GI steel sheet”), and an alloyed hot dip galvanized steel sheet (hereinafter referred to as “GA steel sheet”). Is sometimes obtained).

As a method for producing an EG steel sheet, after the second cooling step, the steel sheet is energized while being immersed in a zinc solution at 55 ° C., for example, and electrogalvanizing is performed.

As a method for producing a GI steel sheet, the above reheating process may serve as a hot dip galvanizing process. That is, after reheating to the T3 temperature range, the hot dip galvanization may be performed by immersing in a plating bath adjusted to the temperature of the T3 temperature range to serve as both hot dip galvanization and holding in the T3 temperature range. . At this time, the residence time in the T3 temperature region may satisfy the requirement for the holding time t3.

As a method for producing a GA steel sheet, after galvanizing at a temperature in the T3 temperature range, an alloying treatment may be subsequently performed in the T3 temperature range. At this time, the residence time in the T3 temperature region may satisfy the requirement for the holding time t3.

The amount of galvanized adhesion is not particularly limited, and for example, it may be about 10 to 100 g / m ² per side.

The plate thickness of the high-strength steel plate is not particularly limited, but may be a thin steel plate having a plate thickness of 3 mm or less, for example.

The tensile strength (TS) of the high-strength steel plate is 980 MPa or more, and preferably 1100 MPa or more.

Also, the high-strength steel plate is excellent in workability at room temperature (TS × EL, λ). Specifically, the high-strength steel plate preferably has a TS × elongation (EL) of 16000 MPa ·% or more, and more preferably 18000 MPa ·% or more. The high-strength steel sheet preferably has a hole expansion rate λ of 20% or more, and more preferably 25% or more.

Further, as described above, the high-strength steel sheet is excellent in workability at room temperature (TS × EL, λ), and the forming load during warming is sufficiently reduced. Specifically, the high-strength steel sheet preferably has a ΔTS of 150 MPa or more, and more preferably 180 MPa or more. This warm processing means forming at a temperature of about 100 to 350 ° C.

The above-mentioned high-strength steel sheet is suitably used as a material for structural parts of automobiles. Structural parts of automobiles include, for example, front and rear side members and crashing parts such as crash boxes, pillars and other reinforcements (for example, bears, center pillar reinforcements, etc.), roof rail reinforcements, side sills , Body components such as floor members and kick sections, shock-absorbing parts such as bumper reinforcements and door impact beams, and seat parts.

This specification discloses various modes of technology as described above, and the main technologies are summarized below.

One aspect of the present invention is, in mass%, C: 0.10 to 0.5%, Si: 1.0 to 3%, Mn: 1.5 to 3%, P: more than 0%, 0.1% Hereinafter, S: more than 0%, 0.05% or less, Al: 0.005 to 1%, and N: more than 0%, 0.01% or less, with the balance being iron and inevitable impurities. (1) The metal structure of the steel sheet includes polygonal ferrite, bainite, tempered martensite, and retained austenite. (2) When the metal structure is observed with a scanning electron microscope, Polygonal ferrite: 10 to 50 area%, bainite glaze: 10 to 50 area%, tempered martensite: 10 to 80 area%, (3) When the metal structure was measured by X-ray diffraction, Residual austenite: 5.0% by volume with respect to the entire structure Furthermore, the residual austenite having a carbon concentration of 1.0% by mass or less: 3.5% by volume or more and the residual austenite having a carbon concentration of 0.8% by mass or less: 2.4% by volume or less are satisfied. It is a strength steel plate.

According to such a configuration, the elongation and hole expandability at room temperature are good, and the molding load when processing at a temperature of 100 to 350 ° C. is markedly reduced from the molding load when processing at room temperature. A high-strength steel sheet having a tensile strength of 980 MPa or more can be provided.

The high-strength steel sheet further contains at least one element selected from the group consisting of Cr: more than 0%, 1% or less, and Mo: more than 0%, 1% or less as other elements, as other elements. Also good.

Further, the high-strength steel plate, as other elements, in mass%, Ti: more than 0%, 0.15% or less, Nb: more than 0%, 0.15% or less, and V: more than 0%, You may contain at least 1 sort (s) chosen from the group which consists of 0.15% or less.

The high-strength steel sheet further contains at least one element selected from the group consisting of Cu: more than 0%, less than 1%, and Ni: more than 0%, less than 1% as other elements. May be.

The high-strength steel sheet may further contain B: more than 0% and 0.005% or less in terms of mass% as other elements.

In addition, the high-strength steel sheet further includes, as other elements, mass%, Ca: more than 0%, 0.01% or less, Mg: more than 0%, 0.01% or less, and rare earth elements: more than 0%. , At least one selected from the group consisting of 0.01% or less.

The high-strength steel sheet includes a high-strength steel sheet having an electrogalvanized layer, a hot-dip galvanized layer, or an alloyed hot-dip galvanized layer on the surface.

In another aspect of the present invention, a steel sheet satisfying the above component composition is heated to a T1 temperature range of 800 ° C. or higher and Ac ₃ points or lower, and held in the T1 temperature range for 40 seconds or more to soak. When the Ms point represented by the following formula (I) is 350 ° C. or higher after the heating step and soaking, up to any cooling stop temperature T2 that satisfies 350 ° C. or lower and 100 ° C. or higher, or the following formula (I) In the case where the Ms point represented by is less than 350 ° C., the cooling stop temperature T2 is less than the Ms point and is 100 ° C. or higher, and from 700 ° C. to 300 ° C. or higher of the cooling stop temperature T2 A first cooling step in which the average cooling rate is 5 ° C./second or higher, and a reheating step in which the temperature is reheated to a T3 temperature range of more than 350 ° C. and 540 ° C. and held for 50 seconds or more in the T3 temperature range And after holding, from the T3 temperature range to 300 ℃ And a second cooling step of cooling at an average cooling rate of 10 ° C./second or more and cooling from 300 ° C. to 150 ° C. at an average cooling rate of more than 0 ° C./second and less than 10 ° C./second. It is a manufacturing method of a steel plate.
Ms point (° C.) = 561−474 × [C] / (1−Vf / 100) −33 × [Mn] −17 × [Ni] −17 × [Cr] −21 × [Mo] (I )

In the above formula (I), Vf is the same average cooling rate as the production conditions for the high-strength steel plate in the first cooling step after separately performing the soaking step under the same conditions as the production conditions for the high-strength steel plate. Means the fraction of polygonal ferrite (area%) in the sample obtained by cooling to room temperature. Moreover, in the said formula (I), [] has shown content (mass%) of each element, and content of the element which is not contained in a steel plate is calculated as 0 mass%.

In the method for producing the high-strength steel plate, electrogalvanization may be performed after the second cooling step.

In the method for producing the high-strength steel sheet, hot dip galvanization or alloyed hot dip galvanization may be performed in the reheating step.

According to the present invention, the component composition and metal structure of the steel sheet are appropriately controlled, and in particular, the carbon concentration of residual γ is strictly controlled, so that the elongation and hole expansibility at room temperature are good, It is possible to provide a high-strength steel sheet having a tensile strength of 980 MPa or more and a method for producing the same, in which a forming load when processing at a temperature of 100 to 350 ° C. is significantly reduced as compared with a forming load when processing at room temperature.

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 may be implemented with modifications within a range that can meet the above and the gist described below. Of course, these are all possible and are included in the technical scope of the present invention.

A steel material was manufactured by melting steel containing the components shown in Table 1 below, the balance being iron and inevitable impurities. The obtained steel material was heated and held at 1250 ° C. for 30 minutes, then hot-rolled so that the reduction rate was about 90% and the finish rolling temperature was 920 ° C. From this temperature, the steel was wound at an average cooling rate of 30 ° C./second. It was cooled to a take-up temperature of 600 ° C. and wound up. After winding, it was cooled to room temperature to produce a hot-rolled steel plate having a thickness of 2.6 mm.

The obtained hot-rolled steel sheet was pickled to remove the surface scale, and then cold-rolled at a cold rolling rate of 46% to produce a cold-rolled steel sheet having a thickness of 1.4 mm.

The obtained cold-rolled steel sheet was continuously annealed to produce a test material. That is, the obtained cold-rolled steel sheet was heated to a soaking temperature T1 (° C.) shown in the following Table 2-1 or Table 2-2, and a soaking time t1 (shown in the following Table 2-1 or Table 2-2). Second) and soaking, and then cooled to the cooling stop temperature T2 (° C.) shown in Table 2-1 or Table 2-2 below. The average cooling rate CR1 (° C./second) from 700 ° C. to 300 ° C. or higher of the cooling stop temperature T2 is shown in the following Table 2-1 or Table 2-2.

Tables 2-1 and 2-2 below also show the composition of the components shown in Table 1 below and the Ms point (° C.) calculated based on the above formula (I).

Next, heating is performed from the cooling stop temperature T2 (° C.) to the reheating temperature T3 (° C.) shown in Table 2-1 or Table 2-2 below. The indicated holding time t3 (second) was held.

After holding, it was cooled to room temperature. At this time, the reheating temperature T3 (° C.) to 300 ° C. is cooled at an average cooling rate CR2 (° C./second) shown in Table 2-1 or Table 2-2 below, and from 300 ° C. to 150 ° C., Cooling was performed at an average cooling rate CR3 (° C./second) shown in Table 2-1 or Table 2-2 below.

Some of the test materials obtained by continuous annealing were subjected to the following plating treatment to produce EG steel plates, GI steel plates, and GA steel plates.

[Electrogalvanizing (EG) treatment]
After continuous annealing, it is cooled to room temperature, and then the specimen is immersed in a galvanizing bath at 55 ° C., subjected to electrogalvanizing treatment at a current density of 30 to 50 A / dm ² , washed with water and dried to provide an EG steel sheet. Manufactured. The amount of electrogalvanized adhesion was 10 to 100 g / m ² per side.

[Hot galvanizing (GI) treatment]
After heating from the cooling stop temperature T2 (° C.) to the reheating temperature T3 (° C.) shown in Table 2-1 or Table 2-2 below, the plate is immersed in a hot dip galvanizing bath at a temperature of 460 ° C. The GI steel sheet was manufactured by cooling to room temperature. The residence time in the T3 temperature range is shown in the column of holding time t3 (seconds) shown in Table 2-1 or Table 2-2 below. The adhesion amount of hot dip galvanizing was 10 to 100 g / m ² per side.

[Alloyed hot dip galvanizing (GA) treatment]
After immersion in the hot dip galvanizing bath, alloying treatment was further performed at the temperature shown in Table 2-1 or Table 2-2 below, and then cooled to room temperature to produce a GA steel sheet. The residence time in the T3 temperature range is shown in the column of holding time t3 (seconds) shown in Table 2-1 or Table 2-2 below. The adhesion amount of the galvannealed alloy was 10 to 100 g / m ² per side.

The categories of the obtained specimens are shown in Table 2-1 or Table 2-2 below. In the table, cold rolling indicates a cold rolled steel sheet, EG indicates an EG steel sheet, GI indicates a GI steel sheet, and GA indicates a GA steel sheet.

For the obtained specimens (meaning including cold-rolled steel sheet, EG steel sheet, GI steel sheet, GA steel sheet, the same applies hereinafter), the observation of the metal structure and the evaluation of the mechanical properties were performed according to the following procedure.

≪Observation of metal structure≫
Among the metal structures, the area ratios of polygonal ferrite, bainite, and tempered martensite were calculated based on the results observed with a scanning electron microscope, and the volume ratio of residual γ was measured by the X-ray diffraction method.

[Polygonal ferrite, bainite, tempered martensite]
After polishing the cross section parallel to the rolling direction of the test material, it was subjected to Nital corrosion, and the 1/4 position of the plate thickness was observed with a scanning electron microscope at a magnification of 3000 times for 5 fields of view. The observation visual field was about 40 μm × about 30 μm. The area ratio of polygonal ferrite can be measured by observation with this scanning electron microscope. Further, the area ratio between bainite and tempered martensite is the average (average interval) of the interval between at least one selected from the group consisting of residual γ and carbide observed as white or light gray in the observation field. Measured based on the measured method, bainite and tempered martensite were distinguished by the measured average interval, and measured by the point calculation method.

The measurement results are shown in Table 3-1 or Table 3-2 below. In Table 3-1 or Table 3-2 below, F represents the area ratio of polygonal ferrite, B represents the area ratio of bainite, and TM represents the area ratio of tempered martensite. The balance is residual γ, MA mixed phase in which fresh martensite and residual γ are combined, pearlite, and fresh martensite.

[Residual γ]
After grinding to 1/4 position of the thickness of the specimen, the ground surface was chemically polished, and then the volume ratio of residual γ with respect to the entire metal structure was measured by X-ray diffraction (ISIJ Int. Vol. 33, 1933). Year, No. 7, p. 776).

Further, the carbon concentration in the residual γ is measured by the procedure described above, and the volume ratio of the residual γ having a carbon concentration of 1.0 mass% or less [Vγ _R (C ≦ 1.0%)] with respect to the entire metal structure, and The volume fraction [Vγ _R (C ≦ 0.8%)] of residual γ having a carbon concentration of 0.8% by mass or less was calculated.

As reference data, the average value% C _avg of the carbon concentration distribution in the residual γ and the standard deviation σ% C are also shown in Table 3-1 or Table 3-2 below.

≪Evaluation of mechanical properties≫
[Tensile strength at room temperature (TS), elongation (EL)]
Tensile strength (TS) and elongation (EL) at room temperature (25 ° C.) were measured by performing a tensile test based on JIS Z2241. As the test piece, a test piece obtained by cutting out a No. 5 test piece defined in JIS Z2201 so that the direction perpendicular to the rolling direction of the test material is the longitudinal direction was used. The tensile speed of the tensile test was 10 mm / min.

The results of TS and EL measured at room temperature are shown in Table 3-1 or Table 3-2 below.

Also, the product of TS and EL measured at room temperature (TS × EL) was calculated, and the results are shown in Table 3-1 or Table 3-2 below.

[ΔTS]
In order to evaluate the degree of load reduction during warm forming, a value obtained by subtracting the tensile strength (warm TS) at warm (200 ° C) from the tensile strength (room temperature TS) at room temperature (25 ° C) (room temperature TS). -Warm TS = ΔTS) was calculated. The tensile test was performed under the same conditions as described above except that the tensile speed was 1000 mm / min in order to simulate the speed at the time of press working, and the test was performed at two levels of room temperature and 200 ° C. The calculated ΔTS is shown in Table 3-1 or Table 3-2 below.

[Hole expandability]
The hole expandability was evaluated by the hole expansion rate (λ) measured by performing a hole expansion test based on JIS Z2256. The measurement results are shown in the column of “λ (%)” in Table 3-1 or Table 3-2 below.

In the present invention, the case where TS was 980 MPa or more was evaluated as high strength. Further, when TS × EL was 16000 MPa ·% or more and λ was 20% or more, it was evaluated that the processability at room temperature was excellent. Moreover, when ΔTS was 150 MPa or more, it was evaluated that the molding load during warming was reduced. And the case where all of TS, TSxEL, (lambda), and (DELTA) TS satisfy a reference value was set as the pass. On the other hand, the case where any of TS, TS × EL, λ, or ΔTS did not satisfy the reference value was regarded as unacceptable.

The following can be considered from Table 1, Table 2-1, Table 2-2, Table 3-1, and Table 3-2.

No. 1, 7, 8, 10, 13, 15, 17, 20, 22, 23, 26 to 31, 33, 37, 38, 40, and 42 are examples that satisfy the requirements defined in the present invention. The TS measured at room temperature is 980 MPa or higher and has high strength. Further, TS × EL and λ satisfy the acceptance criteria of the present invention, and the processability at room temperature is good. Furthermore, since ΔTS satisfied the acceptance criteria of the present invention, the molding load during warming could be reduced.

On the other hand, No. 2 to 6, 9, 11, 12, 14, 16, 18, 19, 21, 24, 25, 32, 34 to 36, 39, and 41 are examples that do not satisfy any of the requirements defined in the present invention. In addition, at least one of the properties of strength, workability at room temperature, and reduction of warm forming load is deteriorated.

The details will be described below.

No. 2, 6, 12, and 41 are examples in which the average cooling rate CR3 from 300 ° C. to 150 ° C. after holding in the reheating step is too large, and a large amount of residual γ having a carbon concentration of 0.8 mass% or less is generated. Λ was reduced and the processability at room temperature could not be improved.

No. 3, 5, and 39, the average cooling rate CR2 from the reheating temperature (the holding temperature in the reheating process) to 300 ° C. is too small, and the residual γ amount with a carbon concentration of 1.0 mass% or less cannot be secured. In this example, ΔTS was small, and the molding load during warming could not be reduced. In addition, No. 3 is an example simulating the above-mentioned Patent Document 1. As described in paragraph [0128] of the above-mentioned Patent Document 1, the average cooling rate to room temperature after holding was set to 5 ° C./second.

No. 4, 19, and 32 are examples in which the cooling stop temperature T2 after soaking is too high. No. In 4, 19, and 32, since tempered martensite was not generated or the amount of generation was small, TS at room temperature was low, and strength could not be secured. No. In 4, 19, and 32, since a large amount of residual γ having a carbon concentration of 0.8 mass% or less was generated, λ was small, and workability at room temperature could not be improved. In addition, No. In No. 19, the retention time t3 was relatively long, and the amount of polygonal ferrite produced was relatively small, so it is considered that bainite was excessively produced. That is, the smaller the amount of polygonal ferrite produced, the less likely it is that C is concentrated in the surrounding austenite, so the bainite transformation is considered to occur faster.

No. No. 9 is an example in which polygonal ferrite was excessively generated because the average cooling rate CR1 in the section from 700 ° C. to 300 ° C. was too small after soaking. As a result, no. No. 9 could not secure the desired TS.

No. 11 is an example in which polygonal ferrite was hardly generated because the soaking temperature T1 was too high. As a result, no. No. 11, TS × EL was low, and the processability at room temperature could not be improved.

No. 14 is an example in which the cooling stop temperature T2 after soaking is too low. No. In No. 14, bainite was not generated, and tempered martensite was excessively generated, so that the amount of residual γ generated could not be secured. As a result, no. No. 14, TS × EL was low, and the processability at room temperature could not be improved. No. No. 14 could not secure the amount of residual γ having a carbon concentration of 1.0% by mass or less, so ΔTS was small, and the molding load during warming could not be reduced.

No. No. 16 is an example in which polygonal ferrite is excessively generated because the soaking temperature T1 is too low. As a result, no. No. 16 could not secure the desired TS. No. No. 16, TS × EL decreased, and the processability at room temperature could not be improved. This is presumably because the work structure introduced during cold rolling remained.

No. 18 is an example in which the reheating temperature T3 is too high. No. In No. 18, bainite hardly formed and the amount of residual γ could not be secured, so TS × EL was lowered and workability at room temperature could not be improved. No. In No. 18, since residual γ having a carbon concentration of 0.8% by mass or less was excessively generated, λ became small, and workability at room temperature could not be improved.

No. No. 21 is an example in which the amount of residual γ having a carbon concentration of 1.0% by mass or less could not be secured because the reheating temperature T3 was too low. As a result, no. In No. 21, ΔTS was small, and the molding load during warming could not be reduced.

No. No. 24 is an example in which a desired residual γ amount could not be ensured because the soaking time t1 was too short. As a result, no. In No. 24, TS × EL decreased, and the processability at room temperature could not be improved.

No. 25 is an example in which the holding time t3 is too short. No. 25, because the production amount of bainite and residual γ could not be ensured, TS × EL decreased, and since the residual γ with a carbon concentration of 0.8% by mass or less was excessively generated, λ became small, at room temperature. It was not possible to improve the workability.

No. 34 to 36 are examples in which the component composition does not satisfy the requirements defined in the present invention.

No. In 34, since the amount of Si was too small, TS decreased and the strength could not be secured. No. No. 34 could not secure the yield of residual γ, so TS × EL decreased, and the processability at room temperature could not be improved. No. No. 34 was an example in which the amount of residual γ with a carbon concentration of 1.0% by mass or less could not be ensured, ΔTS became small, and the molding load during warming could not be reduced.

No. In 35, since the amount of C was too small, TS decreased and the strength could not be secured. No. For 35, the amount of residual γ produced could not be secured, so TS × EL decreased, and the processability at room temperature could not be improved. No. No. 35 was an example in which the amount of residual γ with a carbon concentration of 1.0% by mass or less could not be ensured, ΔTS became small, and the molding load during warm could not be reduced.

No. In No. 36, since the amount of Mn was too small, polygonal ferrite was excessively generated, TS was lowered, and the strength could not be secured. No. For 36, the amount of residual γ produced could not be secured, so TS × EL decreased, and the processability at room temperature could not be improved. No. No. 36 was an example in which the amount of residual γ with a carbon concentration of 1.0% by mass or less could not be ensured, ΔTS became small, and the molding load during warm could not be reduced.

This application is based on Japanese Patent Application No. 2016-0383304 filed on February 29, 2016 and Japanese Patent Application No. 2016-182966 filed on September 20, 2016. The contents thereof are included in the present application.

In order to express the present invention, the present invention has been properly and fully described through the embodiments with reference to the drawings. However, those skilled in the art can easily change and / or improve the above-described embodiments. It should be recognized that this is possible. Therefore, unless the modifications or improvements implemented by those skilled in the art are at a level that departs from the scope of the claims recited in the claims, the modifications or improvements are not limited to the scope of the claims. To be construed as inclusive.

According to the present invention, the component composition and metal structure of the steel sheet are appropriately controlled, and in particular, the carbon concentration of residual γ is strictly controlled, so that the elongation and hole expansibility at room temperature are good, Provided are a high-strength steel sheet having a tensile strength of 980 MPa or more and a method for producing the same, in which a forming load when processing at a temperature of 100 to 350 ° C. is significantly reduced as compared with a forming load when processing at room temperature.

Claims (6)

% By mass
C: 0.10 to 0.5%,
Si: 1.0-3%,
Mn: 1.5 to 3%,
P: more than 0%, 0.1% or less,
S: more than 0%, 0.05% or less,
Al: 0.005 to 1%, and N: more than 0% and 0.01% or less,
The balance is a steel plate made of iron and inevitable impurities,
(1) The metal structure of the steel sheet includes polygonal ferrite, bainite, tempered martensite, and retained austenite,
(2) When the metal structure is observed with a scanning electron microscope,
Polygonal ferrite: 10 to 50 area%,
Bainite: 10-50 area%,
Tempered martensite: 10 to 80% by area satisfied,
(3) When the metal structure is measured by the X-ray diffraction method,
Residual austenite: 5.0% by volume or more,
Residual austenite having a carbon concentration of 1.0% by mass or less: 3.5% by volume or more,
Residual austenite having a carbon concentration of 0.8% by mass or less: A high-strength steel sheet satisfying 2.4% by volume or less. Furthermore, as other elements,
Cr: more than 0%, 1% or less,
Mo: more than 0%, 1% or less,
Ti: more than 0%, 0.15% or less,
Nb: more than 0%, 0.15% or less,
V: more than 0%, 0.15% or less,
Cu: more than 0%, 1% or less,
Ni: more than 0%, 1% or less,
B: more than 0%, 0.005% or less,
Ca: more than 0%, 0.01% or less,
The high-strength steel sheet according to claim 1, comprising at least one selected from the group consisting of Mg: more than 0% and 0.01% or less, and rare earth elements: more than 0% and 0.01% or less. The high-strength steel sheet according to claim 1, which has an electrogalvanized layer, a hot-dip galvanized layer, or an alloyed hot-dip galvanized layer on the surface of the steel sheet. A method for producing the high-strength steel sheet according to claim 1 or 2,
A soaking step of heating the steel sheet satisfying the above component composition to a T1 temperature range of 800 ° C. or more and Ac ₃ points or less, and holding and soaking in the T1 temperature range for 40 seconds or more
After soaking,
When the Ms point represented by the following formula (I) is 350 ° C. or higher, up to an arbitrary cooling stop temperature T2 that satisfies 350 ° C. or lower, 100 ° C. or higher, or the Ms point represented by the following formula (I) is 350 In the case of less than ℃, when cooling to an arbitrary cooling stop temperature T2 that satisfies the Ms point or lower and 100 ° C or higher, the average cooling rate from 700 ° C to 300 ° C or the higher one of the cooling stop temperatures T2 A first cooling step of 5 ° C./second or more;
A reheating step of reheating to a T3 temperature range of more than 350 ° C. and 540 ° C. or less, and holding for 50 seconds or more in the T3 temperature range;
After holding, the temperature is cooled from the T3 temperature range to 300 ° C. at an average cooling rate of 10 ° C./second or more, and from 300 ° C. to 150 ° C. is cooled at an average cooling rate of more than 0 ° C./second and less than 10 ° C./second. A method for producing a high-strength steel sheet, comprising a cooling step.
Ms point (° C.) = 561−474 × [C] / (1−Vf / 100) −33 × [Mn] −17 × [Ni] −17 × [Cr] −21 × [Mo] (I )
In the above formula (I), Vf is the same average cooling rate as the production conditions for the high-strength steel plate in the first cooling step after separately performing the soaking step under the same conditions as the production conditions for the high-strength steel plate. Means the fraction of polygonal ferrite (area%) in the sample obtained by cooling to room temperature. Moreover, in said formula (I), [] has shown content (mass%) of each element, and content of the element which is not contained in a steel plate is calculated as 0 mass%. The method for producing a high-strength steel sheet according to claim 4, wherein electrogalvanizing is performed after the second cooling step. The method for producing a high-strength steel sheet according to claim 4, wherein hot-dip galvanizing or alloying hot-dip galvanizing is performed in the reheating step.

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