JP6282577B2 - High strength high ductility steel sheet - Google Patents

Description

  The present invention relates to a high-strength, high-ductility steel plate useful as an automatic thin steel plate and the like, and more particularly to a technique for improving the strength / ductility balance of a steel plate.

  For example, steel sheets used in automobile frame parts and the like are required to have higher strength for the purpose of collision safety and fuel efficiency reduction by reducing the weight of the car body, and excellent molding for processing into complex frame parts In addition to workability, weldability is also required when parts are joined together and assembled into an assembly. For this reason, as the mechanical characteristics specifically required (hereinafter, also simply referred to as “characteristics”), the yield ratio (YR) is 0.7 or more while suppressing the carbon amount to 0.3 mass% or less. Further, there is a demand for the development of a steel sheet that can secure a tensile strength (TS) × elongation (EL) × stretch flangeability (λ) of 1000000 MPa ·% ·% or more.

  In a high-strength steel sheet of 980 MPa or higher, it is effective to use TRIP steel, TBF steel, or the like utilizing the TRIP effect of retained austenite in order to achieve both high strength and high ductility. In order to further improve the strength-ductility balance of these steels, various studies have been made on the amount of retained austenite, the average carbon concentration, and the form, and steel plates having better properties have been proposed (for example, Patent Documents 1 to 3). reference).

  For example, in Patent Document 1, when the form of retained austenite in a steel structure is classified into a lath shape and an island shape, the ratio of the island-like retained austenite is controlled within a certain range, so that elongation and press forming stability are controlled. A high-strength thin steel sheet excellent in resistance has been proposed. In this technology, in addition to good elongation at room temperature, good elongation at a temperature of 100 to 200 ° C. has been realized, but YR, which is an effective material factor, and strength-ductility balance are sufficient as impact characteristics. It is assumed that it has not been secured, and it is not considered to satisfy the above demand level.

  Patent Document 2 proposes a high-strength cold-rolled steel sheet that significantly improves the uniform elongation in the 45 ° direction relative to the rolling direction by increasing the degree of accumulation of the crystal orientation of the austenite phase in the steel structure. Yes. However, the characteristics in the rolling direction and the direction perpendicular to the rolling, which are general ductility evaluation directions, are not specially mentioned and are not considered to satisfy the above-mentioned desired level.

  Further, Patent Document 3 proposes a high-strength thin steel sheet having improved paint bake hardenability and strength-ductility balance by imparting a C concentration difference between the surface and the inside of residual austenite grains in the steel structure. Yes. However, in this technique, the C concentration difference is imparted to the residual austenite grains only for the purpose of improving the baking bake hardenability, and as in the present invention, the stability of the residual austenite is improved. It is not intended to give the required carbon concentration distribution to the retained austenite in order to increase ductility, and is completely different from the technical idea of the present invention.

JP 2012-41573 A JP2012-21225A JP 2012-31505 A

  Therefore, an object of the present invention is to obtain a strength-ductility that can ensure a yield ratio (YR) of 0.7 or more and a tensile strength (TS) × elongation (EL) × stretch flangeability (λ) of 1000000 MPa ·% ·% or more. An object of the present invention is to provide a high-strength, high-ductility steel sheet having an excellent balance.

The high strength and high ductility steel sheet according to the first invention of the present invention is
Ingredient composition is mass%,
C: 0.10% or more, less than 0.35%,
Si + Al: 0.5 to 2.0%,
Mn: 1.0-4.0%,
P: 0 to 0.05%,
S: 0 to 0.01%
The balance consists of iron and inevitable impurities,
Steel structure
The residual austenite is 8% or more in terms of the area ratio with respect to the entire structure, and the balance is composed of one or more of bainite, martensite, tempered bainite, and tempered martensite,
Regarding the carbon concentration in the retained austenite,
The average carbon concentration is 0.9 to 1.2% by mass,
The standard deviation of the carbon concentration distribution is 0.35% by mass or more,
A region having a carbon concentration of 1.5% by mass or more is characterized by having an area ratio of 1.0% or more with respect to the entire tissue.

The high strength and high ductility steel sheet according to the second invention of the present invention is
In the first invention,
Ingredient composition is mass%, and
One or two or more of Cu, Ni, Mo, Cr and B are contained in total of 1.0% or less.

The high strength and high ductility steel sheet according to the third aspect of the present invention is
In the first or second invention,
Ingredient composition is mass%, and
One or more of V, Nb, Ti, Zr and Hf are contained in a total of 0.2% or less.

The high strength and high ductility steel sheet according to the fourth aspect of the present invention is
In any one of the first to third inventions,
Ingredient composition is mass%, and
One or two or more of Ca, Mg, and REM are included in a total of 0.01% or less.

  According to the present invention, not only by defining the amount (area ratio) of retained austenite and average carbon concentration, but also by controlling the distribution of carbon concentration, the TRIP phenomenon can be developed from the early stage to the late stage of deformation. By realizing a high work hardening rate, the yield ratio (YR) is 0.7 or more, and the tensile strength (TS) × elongation (EL) × stretch flangeability (λ) can be ensured to be 1000000 MPa ·% ·% or more. It has become possible to provide a high-strength, high-ductility steel sheet having an excellent balance between strength and ductility.

It is a figure which shows typically the diffraction peak of residual (gamma) measured by the X ray diffraction method. It is a figure which shows typically the heat processing conditions for manufacturing the high intensity | strength highly ductile steel plate which concerns on this invention.

  In order to solve the above problems, the present inventors have obtained a steel sheet made of TBF steel as its mechanical properties with a yield ratio (YR) of 0.7 or more, tensile strength (TS) × elongation (EL) × elongation. Various studies have been made on measures that can secure a flangeability (λ) of 1000000 MPa ·% ·% or more. As a result, the inventors have thought that the desired characteristics can be secured by the following thought research.

  That is, in order to further improve the strength-ductility balance as compared with the prior art, it is necessary to more effectively utilize retained austenite (hereinafter also referred to as “residual γ”) that promotes the TRIP phenomenon. However, since there is an upper limit on the C content in the steel sheet from the viewpoint of ensuring the weldability of the steel sheet, there is a limit in increasing the residual γ amount and the average carbon concentration in the residual γ.

  Therefore, the present inventors paid attention to the carbon concentration distribution in the residual γ. That is, in order to achieve high strength and high ductility by the TRIP phenomenon, it is important to achieve a high work hardening rate from the initial stage to the middle stage of deformation, and for this purpose, unstable residual γ with a low carbon concentration. There must be some. On the other hand, in order to maintain a high work hardening rate even when the amount of deformation increases, that is, in the later stage of deformation, it is necessary to create a stable residual γ having a high carbon concentration.

  That is, it is important that the residual γ is not only high stability or low stability, and a wide range of stability, that is, carbon concentration distribution exists.

  As a result of further investigation based on the above findings, the present inventors have completed the present invention.

  Hereinafter, a steel structure (hereinafter sometimes simply referred to as “structure”) that characterizes the high-strength and highly ductile steel sheet (hereinafter also referred to as “the steel sheet of the present invention”) according to the present invention will be described.

[Steel structure of the steel sheet of the present invention]
As described above, the steel sheet of the present invention is based on the structure of TBF steel. In particular, after containing a predetermined amount of residual γ having a predetermined carbon concentration, the carbon concentration distribution in the residual γ is controlled. This is different from the prior art.

<Residual austenite: 8% or more in area ratio with respect to the whole structure>
Residual γ is useful for improving ductility, and in order to exert such an effect effectively, the area ratio to the whole tissue should be 8% or more, preferably 9% or more, more preferably 10% or more. is necessary.

<Balance: one or more of bainite, martensite, tempered bainite, and tempered martensite>
By preventing the formation of ferrite and building the parent phase with bainite and martensite, which are fine and uniform structures, and / or their tempered structures, it is possible to prevent deformation at low loads by refining the matrix structure. The yield strength YS can be increased.

<Average carbon concentration (% Cγ _R ) in residual γ: 0.9 to 1.2% by mass>
% C gamma _R is the residual γ is an indicator that affects the stability of transforms to martensite during deformation. %, The C gamma _R is too low, since the residual γ is unstable, after stressing, since the deformation-induced martensitic transformation occurs before the plastic deformation, the required elongation can not be obtained. On the other hand,% the C gamma _R is too high, too residual γ is stable, be added processing for deformation-induced martensitic transformation does not occur, not again obtain the required stretch flangeability. To obtain the required elongation,% C gamma _R is required to be 0.9 to 1.2 mass%. Preferably it is 1.0-1.2 mass%.

<Standard deviation of carbon concentration distribution in residual γ: 0.35 mass% or more>
This is because, in order to increase and maintain the work hardening rate from the initial stage to the late stage of the deformation, the residual γ having different stability is created by widening the carbon concentration distribution in the residual γ. In order to effectively exert such an action, the standard deviation of the carbon concentration distribution in the residual γ is 0.35% by mass or more, preferably 0.40% by mass or more, more preferably 0.45% by mass or more. There is a need to.

<A region where the carbon concentration in the residual γ is 1.5 mass% or more: 1.0% or more in terms of the area ratio with respect to the entire structure>
In order to increase the elongation, it is important that the stability of the residual γ is high when the amount of strain is increased. To that end, it is not sufficient that the carbon concentration is high on average, but the stability is high. That is, it is necessary that a certain amount or more of residual γ having a high carbon concentration exists. Specifically, the area where the carbon concentration in the residual γ is 1.5% by mass or more is 1.0% or more, preferably 1.5% or more, more preferably 2.0% or more in terms of the area ratio to the whole structure. It is necessary to let

[Measurement method of area ratio of residual γ, average carbon concentration in residual γ (% Cγ _R ), and carbon concentration distribution thereof]
Here, each measuring method of the area ratio of residual γ, the average carbon concentration (% Cγ _R ), and the carbon concentration distribution will be described.

The area ratio (Vγ _R ) and average carbon concentration (% Cγ _R ) of residual γ were measured by X-ray diffraction after grinding to a thickness of ¼ of the steel plate and then chemical polishing (ISIJ Int. Vol. 33, (1933), No. 7, p. 776). In the present invention, a two-dimensional micro part X-ray diffractometer (RINT-RAPIDII) manufactured by Rigaku Corporation was used as the X-ray diffractometer, and a Co-Kα ray was used as the X-ray.

  Regarding the structures other than residual γ, the steel sheet was corroded with nital and observed with an optical microscope (magnification 400 times) to identify a structure other than residual γ.

Next, regarding the distribution of carbon concentration in the residual γ, the three diffraction peaks (200) _γ , (220) _γ and (311) _γ measured with the X-ray diffractometer were used as follows. Asked.

First, as shown in the schematic diagram of FIG. 1, with respect to the three diffraction peaks of (200) _γ , (220) _γ and (311) _γ , 2θ (2θ _avg (hkl)) and diffraction intensity are maximized, respectively. Its half width Δ2θ (hkl) was determined. Here, (hkl) means (200), (220) or (311) (the same shall apply hereinafter).

Next, from the above 2θ _avg (hkl), d (hkl) was obtained from the following formula (1) using the Bragg condition: λ = 2dsin θ (d: diffraction grating constant, λ: wavelength of Co-Kα ray). .

d (hkl) = λ / {2 sin (2θ _avg (hkl) / 2)} (1)

Then, the following equation (3), the crystal lattice constant _a 0 seek (hkl), and these three crystal lattice constant _a 0 a (hkl) and arithmetic mean determined crystal lattice constant _{a 0.}

a ₀ (hkl) = d (hkl) √ (h ² + k ² + l ² ) (2)

And Dyson's equation (Dyson DJ, Holmes B. (1970), “Effect of alloying additions on the lattice parameter austenite” shown in the following equation (3), J. Iron Steel 9-4, 46: 46. .) _Was used to determine the carbon concentration% C _avg (unit: mass%). (This carbon concentration% C _avg is used only as an index for defining the carbon concentration distribution, and strictly speaking, the average carbon concentration% Cγ _R measured separately is not necessarily exactly the same. Note that.)

% C _avg = (1 / 0.033) · (a0−0.0012 ·% Mn + 0.00157 ·% Si−0.0056 ·% Al) Formula (3)
Here,% Mn,% Si, and% Al are the contents (mass%) of Mn, Si, and Al, respectively, in the steel sheet.

  Next, the half-value width Δ% C of the carbon concentration distribution in the 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 Formula (4)
2θ _H (hkl) = 2θ _avg (hkl) + Δ2θ (hkl) / 2 Equation (5)

Therefore, by using the above 2θ _L (hkl) and 2θ _H (hkl), respectively, by using the Bragg condition and the above equations (1) to (3) in the same procedure as described above, the half width of the carbon concentration distribution is Upper and lower limit values% _CL and% _CH 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 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)} · 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) (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 (9) is obtained from equations (7) and (8). The relationship is obtained.

{1 / √ (2πσ _{% C} )} · exp {− (Δ% C / 2) ² / (2σ _{% C} ² )} = 1 / {2√ (2πσ _{% C} )} Equation (9)

By deforming the equation (9), the following equation (10) is derived as an expression for obtaining the standard deviation _{sigma% C} from the half width delta% C, the half width delta% C in the equation (10) The standard deviation σ _{% C} was calculated by substituting.

σ _{% C} = √ {(Δ% C / 2) ² / (2ln2)} (10)

Then, using the average value% C _avg and σ _{% C} of the carbon concentration distribution in the residual γ determined as described above, the carbon concentration is 1 by the cumulative density function g (x) shown in the following formula (11). The following expression (12) is derived as an expression for obtaining the area ratio Vγ _R (C ≧ 1.5%) of the entire tissue in the region of 0.5 mass% or more, and Vγ _R (C ≧ 1.5%) was calculated.

g (x) = (1/2) · [1 + erf {(x−u) / √ (2σ ² )}] (11)

Vγ _R (C ≧ 1.5%) = Vγ _R {1-g (1.5)}
= Vγ _R [0.5-erf {(1.5-% C _avg ) / √ (2σ _{% C} ² )}] Equation (12)
Here, Vγ _R is the area ratio of the total residual γ.

  Next, the component composition which comprises this invention steel plate is demonstrated. Hereinafter, all the units of chemical components are mass%. In addition, “content” of each component may be simply referred to as “amount”.

[Component composition of the steel sheet of the present invention]
C: 0.10% or more and less than 0.35% C is an essential element for ensuring strength and ductility by contributing to securing the amount (area ratio) of retained austenite. In order to effectively exhibit such an action, it is necessary to contain C by 0.10% or more, preferably 0.12% or more, more preferably 0.14% or more. However, since the weldability is deteriorated when the C amount becomes excessive, the C amount is less than 0.35%, preferably 0.32% or less, more preferably 0.30% or less, and further preferably 0.28% or less. And

Si + Al: 0.5 to 2.0%
Si and Al are elements that effectively suppress the decomposition of residual austenite and the formation of carbides. In order to effectively exhibit such an action, it is necessary to contain Si and Al in total of 0.5% or more, preferably 0.7% or more, and more preferably 0.9% or more. However, even if Si and Al are contained excessively, the above effect is saturated and not only is economically wasteful but also causes hot brittleness, so the total amount of Sl and Al is 2.0% or less. 1.9% or less, more preferably 1.8% or less.

Mn: 1.0-4.0%
Mn is an element necessary for stabilizing austenite and obtaining desired retained austenite. In order to effectively exhibit such an action, it is necessary to contain Mn at 1.0% or more, preferably 1.3% or more, more preferably 1.6% or more. However, if the amount of Mn is excessive, adverse effects such as occurrence of cracking of the cast slab are observed, so the amount of Mn is 4.0% or less, preferably 3.5% or less, more preferably 3.0% or less.

P: 0 to 0.05%
P inevitably exists as an impurity element, but is an element that may be contained in order to ensure a desired residual γ. However, if P is excessively contained, secondary workability deteriorates, so the amount of P is 0.05% or less, preferably 0.03% or less, and more preferably 0.02% or less.

S: 0 to 0.01%
Since S is also unavoidably present as an impurity element and forms sulfide-based inclusions such as MnS and becomes a starting point of cracking and deteriorates workability, the amount of S is 0.01% or less, preferably 0.005% or less, more preferably 0.003% or less.

  The steel of the present invention contains the above-described elements as essential components, and the balance is iron and inevitable impurities. In addition, the following allowable components can be contained within a range not impairing the function of the present invention.

One or more of Cu, Ni, Mo, Cr, and B: 1.0% or less in total These elements are useful as steel strengthening elements, as well as for stabilizing residual γ and securing a predetermined amount. It is an effective element. In order to effectively exhibit such an action, it is recommended that these elements are contained in a total amount of 0.001% or more, and further 0.01% or more. However, even if these elements are contained excessively, the above effect is saturated and is economically wasteful. Therefore, these elements are added in a total amount of 1.0% or less, and further 0.5% or less. Is preferred.

One or more of V, Nb, Ti, Zr, and Hf: 0.2% or less in total These elements have the effect of precipitation strengthening and refinement of the structure, and are useful elements for increasing the strength. In order to effectively exhibit such an action, it is recommended that these elements are contained in a total amount of 0.01% or more, and further 0.02% or more. However, even if these elements are contained excessively, the above effects are saturated and economically useless. Therefore, these elements are 0.2% or less in total amount, and further 0.1% or less. It is preferable to do this.

One or more of Ca, Mg and REM: 0.01% or less in total These elements are elements that control the form of sulfides in steel and are effective in improving workability. Here, examples of the REM (rare earth element) used in the present invention include Sc, Y, and lanthanoid. In order to effectively exhibit the above action, it is recommended to contain these elements in a total amount of 0.001% or more, and further 0.002% or more. However, even if these elements are contained excessively, the above effects are saturated and economically useless, so these elements are 0.01% or less in total amount, further 0.005% or less. It is preferable to do this.

  Next, preferable production conditions for obtaining the steel sheet of the present invention will be described below.

[Preferred production method of the steel sheet of the present invention]
The steel sheet of the present invention can be manufactured by hot-rolling a steel material satisfying the above component composition, followed by cold rolling, and then performing a heat treatment, for example, under the conditions of the following steps (1) to (4). (See FIG. 2).

[Heat treatment conditions]
(1) The cold-rolled sheet is heated to the second heating temperature T2: [0.3 × Ac1 + 0.7 × Ac3] to [0.8 × Ac1 + 0.2 × Ac3], and the second holding time t2: 5 s at that temperature. After maintaining the above or heating the same temperature range at an average heating rate of 4 ° C./s or less,
(2) Further, after heating to the third heating temperature T3: [Ac3 + 10 ° C.] to 950 ° C. and holding at that temperature for the third holding time t3: 180 s or less,
(3) After cooling the third heating temperature T3 to 500 ° C. at an average cooling rate CR1: 20 ° C./s or more,
(4) Austempering temperature T4: At 350 to 480 ° C. Austempering holding time t4: After holding for 10 s or longer, cool to room temperature.

  Hereinafter, the reason why the heat treatment conditions are recommended will be described.

<(1) Second heating temperature T2: [0.3 × Ac1 + 0.7 × Ac3] to [0.8 × Ac1 + 0.2 × Ac3] for the second holding time t2: 5 s or more, or the same temperature Heating range at an average heating rate of 4 ° C./s or less>
The austempering treatment in the above step (4) is caused by causing Mn concentration distribution during reverse transformation in the two-phase region temperature range by holding or gradually heating in the two-phase region temperature range of ferrite / austenite. This is because the local rate difference of the bainite transformation at the time is increased and the carbon concentration distribution in the residual γ is widened.
The holding time t2 in the temperature range is more preferably 10 s or more, and further preferably 20 s or more, but it is recommended that the holding time t2 be 200 s or less from the viewpoint of productivity.
Ac1 and Ac3 are derived from the chemical composition of the steel sheet, by Lesley, “Iron & Steel Materials Science”, translated by Koda Narumi, Maruzen Co., Ltd., 1985, p. 273 can be obtained using the equation described in H.273.

<(2) Third holding temperature T3: [Ac3 + 10 ° C.] to 950 ° C. Third holding time t3: 180 s or less>
This is to prevent the ferrite from remaining until the subsequent cooling by making the structure into an austenite single phase structure by holding in the austenite single phase region temperature region.
If the third heating temperature T3 is less than [Ac3 + 10 ° C.], ferrite remains, and ferrite growth cannot be suppressed in the subsequent cooling process of the above step (3), and ferrite is excessively formed. On the other hand, when the third heating temperature T3 exceeds 950 ° C. or the third holding time t3 exceeds 180 s, Mn distributed during the two-phase heating in the step (1) becomes uniform, and the residual γ The carbon concentration distribution cannot be widened.

<(3) Cooling from the third heating temperature T3 to 500 ° C. at an average cooling rate HR1: 20 ° C./s or higher>
This is to prevent the formation of ferrite and to make a bainite-based structure.
The average cooling rate HR3 in this temperature range is more preferably 25 ° C./s or more, and further preferably 30 ° C./s or more.

<(4) Austempering temperature T4: 350 to 480 ° C., austempering holding time t4: Hold for 10 s or more, then cool to room temperature>
This is because bainite transformation is promoted and carbon is concentrated to untransformed austenite to obtain stable residual γ.

[Modification of heat treatment conditions]
In addition, you may comprise the said process (1) like the following process (1a).

(1a) The cold-rolled sheet is heated to the first heating temperature T1: [Ac1-100 ° C] to [Ac1-30 ° C] and held at that temperature for the first holding time: 10 s or more, or the same temperature range. After heating at an average heating rate of 2 ° C./s or less, the second heating temperature T2: [0.3 × Ac1 + 0.7 × Ac3] to [0.8 × Ac1 + 0.2 × Ac3] and the second holding time t2: Hold for 5 s or longer.

  In this way, Mn is concentrated in cementite by holding or gradually heating in the ferrite / cementite two-phase region temperature range for a predetermined time in advance, and between ferrite / austenite during the subsequent ferrite / austenite two-phase region heating. By promoting the distribution of the Mn concentration, the local speed difference of the bainite transformation during the austempering treatment in the step (4) can be increased, and the carbon concentration distribution in the residual γ can be broadened.

  Moreover, you may comprise the said process (4) like the following process (4a).

(4a) Austempering temperature T4: After holding austempering holding temperature t4: 10 s or more at 350 to 480 ° C., after reheating to reheating temperature T5: 500 to 600 ° C. and holding at that temperature for reheating holding time t5: 30 s or less Cool to room temperature.

  Thus, this invention steel plate can also be made into a plated steel plate by reheating to the temperature range which does not decompose | disassemble residual (gamma), and alloying a plating layer.

  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, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  The steels having the components shown in Table 1 below were manufactured by vacuum melting, and after hot forging into steel plates with a thickness of 30 mm, hot rolling was performed. The hot rolling conditions do not substantially affect the final structure and properties of the steel sheet of the present invention, but in this example, after heating to 1200 ° C, the end temperature of hot rolling is 880 ° C in multi-stage rolling. The plate thickness was 2.5 mm under the conditions. Then, it cooled to 500 degreeC with the cooling rate of 30 degrees C / s, and stopped cooling, and after inserting in the furnace heated to 500 degreeC, it hold | maintained for 30 minutes, Then, furnace-cooled and it was set as the hot-rolled sheet. The hot-rolled sheet was pickled to remove the surface scale, and then cold-rolled to 1.4 mm to obtain a cold-rolled sheet.

  And it heat-processed on the conditions shown in following Table 2 by using the said cold rolled sheet as a starting material. The average heating rate from room temperature to the first heating temperature (holding temperature) is constant 10 ° C./s, the average heating rate from the next heating temperature (holding temperature) to 20 ° C./s is constant, and the next heating temperature ( The average heating rate up to (holding temperature) was fixed at 10 ° C./s. The average heating rate from the austempering temperature T4 to the reheating temperature T5 was constant 10 ° C./s, and the average cooling rate from the austempering temperature T4 or the reheating temperature T5 to room temperature was constant 10 ° C./s.

For each steel plate after the heat treatment, the area ratio of residual γ, the average carbon concentration (Cγ _R ) in the residual γ, and the carbon concentration distribution thereof were measured by the measurement method described in the above-mentioned section. Was measured.
In addition, all the structures of the steel plates used in this example, the remainder other than retained austenite and ferrite were composed of one or more of bainite, martensite, tempered bainite, and tempered martensite. In Table 3, only the area ratios of retained austenite and ferrite are shown.

  Moreover, in order to evaluate the strength-ductility balance for each steel plate after the heat treatment, the yield strength YS, the tensile strength TS, and the elongation (total elongation) EL were measured by a tensile test. In addition, the tension test produced the JIS No. 5 test piece and implemented according to JISZ2241. Further, in order to evaluate the stretch flangeability λ of each steel plate, the hole expansion ratio was measured according to the Japan Iron and Steel Federation standard JFST1001.

  The measurement results are shown in Table 3 below. In the same table, the properties of the steel sheet after the heat treatment are those with a yield ratio (YR) of 0.7 or more and tensile strength (TS) × elongation (EL) × stretch flangeability (λ) of 1000000 MPa ·% ·% or more. Was determined to be acceptable (O), and the others were determined to be unacceptable (X).

  As shown in Table 3 above, the steel No., which is an inventive steel (evaluation is ○). 3, 4, 9-11, 14, 18-27 are steels that satisfy the requirements of the structure provision of the present invention as a result of heat treatment under the recommended conditions using steel types that satisfy the requirements of the composition provision of the present invention. In addition, it was confirmed that a high-strength and ductile steel sheet having mechanical properties satisfying the acceptance criteria and having an excellent strength-ductility balance was obtained.

  On the other hand, steel No. which is a comparative steel (evaluation of x). 1, 2, 5-8, 12, 13, 15-17 do not satisfy at least one of the requirements of the component prescription and the structure prescription of the present invention, and the characteristics do not satisfy the acceptance criteria.

  That is, Steel No. Although 1, 2, 5-8, and 12 use steel types that satisfy the requirements of the component provisions of the present invention, they are manufactured under conditions that partially deviate from the recommended production conditions, so the requirements of the organization provisions are satisfied. The characteristics are inferior.

  On the other hand, Steel No. 13 and 15 to 17 are manufactured under the recommended manufacturing conditions, but use a steel type that partially deviates from the requirements of the component provisions of the present invention, so the requirements of the organization regulations are not satisfied and the characteristics are inferior. .

  From the above, the applicability of the present invention was confirmed.

Claims (4)

Ingredient composition is mass%,
C: 0.10% or more, less than 0.35%,
Si + Al: 0.5 to 2.0%,
Mn: 1.0-4.0%,
P: 0 to 0.05%,
S: 0 to 0.01%
The balance consists of iron and inevitable impurities,
Steel structure
The residual austenite is 8% or more and 15.3% or less in terms of the area ratio with respect to the entire structure, and the balance is composed of one or more of bainite, martensite, tempered bainite, and tempered martensite,
Regarding the carbon concentration in the retained austenite,
The average carbon concentration is 0.9 to 1.2% by mass,
The standard deviation of the carbon concentration distribution is 0.40 % by mass or more,
A high-strength, high-ductility steel sheet characterized in that a region having a carbon concentration of 1.5 mass% or more is 1.0% or more in terms of the area ratio relative to the entire structure. Ingredient composition is mass%, and
The high-strength and high-ductility steel sheet according to claim 1, comprising 1.0% or less of one or more of Cu, Ni, Mo, Cr and B in total. Ingredient composition is mass%, and
The high-strength and high-ductility steel sheet according to claim 1 or 2, comprising one or more of V, Nb, Ti, Zr and Hf in a total of 0.2% or less. Ingredient composition is mass%, and
The high-strength and high-ductility steel sheet according to any one of claims 1 to 3, which contains one or more of Ca, Mg and REM in a total of 0.01% or less.