KR20170070230A - High-strength high-ductility steel sheet - Google Patents

Abstract

The steel structure contains a specific alloy component and the remaining austenite is 8% or more by area in total structure and the remainder is at least one of bainite, martensite, tempering bainite and tempering martensite , The average carbon concentration of the carbon concentration in the retained austenite, and the standard deviation of the carbon concentration distribution are within a specific range, respectively.

Description

HIGH-STRENGTH HIGH-DUCTILITY STEEL SHEET [0002]

The present invention relates to a high strength and high ductility steel sheet useful as an automotive thin steel plate or the like, and more particularly to a technique of improving the strength and ductility balance of a steel sheet.

For example, a steel plate used for skeleton parts of automobiles is required to have a higher strength for the purpose of collision safety and fuel consumption reduction by weight reduction of the vehicle, and in addition to excellent molding processability for machining into a complicated skeleton part, In addition, weldability is required when the parts are joined together and assembled into an assembly. For this reason, it is preferable to use a resin having a yield ratio (YR) of 0.7 or more, a tensile strength (TS) x elongation (EL) of less than 0.3 mass% ) X tensile strength (lambda) of 1000000 MPa ·% ·% or more.

In order to achieve both high strength and high ductility in a high strength steel sheet of 980 MPa or more in grade, it is effective to use TRIP steel or TBF steel utilizing the TRIP effect due to retained austenite. 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 shape, and steel sheets having better properties have been proposed (see, for example, 3).

For example, in Patent Document 1, when the shape of the retained austenite in the steel structure is classified into the lath shape and the island shape, by controlling the ratio of the retained austenite in the island shape to a certain range, A high strength thin steel sheet has been proposed. In this technique, in addition to good elongation at room temperature, good elongation at a warming of 100 to 200 DEG C is achieved. However, it is assumed that YR and a strength-ductility balance, which are effective material factors as collision characteristics, It is not considered to satisfy the demand level.

Patent Document 2 proposes a high strength cold rolled steel sheet in which the degree of uniformity of the orientation of the austenite phase in the steel structure is increased to remarkably improve the uniform elongation in the direction of 45 DEG with respect to the rolling direction. However, the characteristics in the rolling direction and the direction perpendicular to the rolling direction, which are general ductility evaluation directions, are not particularly mentioned, and it is not considered to satisfy the above-mentioned demand level.

Further, Patent Document 3 proposes a high-strength thin steel sheet improved in paint baking hardenability and strength-ductility balance by imparting a C concentration difference to the surface and the interior of the retained austenite grains in the steel structure. However, the application of the difference in C concentration to the retained austenite grains in this technique is merely aimed at improving the coating baking hardenability, and the stability of the retained austenite is improved as in the present invention, , It is not intended to give the carbon concentration distribution necessary for the retained austenite, so that it is completely different from the technical idea of the present invention.

Japanese Patent Application Laid-Open No. 2004-41573 Japanese Patent Application Laid-Open Publication No. 2012-21225 Japanese Patent Application Laid-Open No. 2001-31505

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a strength-ductile balance material which can secure a tensile strength (TS) × elongation (EL) × stretch flangeability (λ) of 1,000,000 MPa ·% Strength steel sheet having excellent strength and ductility.

In the high strength and high ductility steel sheet according to the first invention of the present invention,

The composition of matter, in% by mass,

C: not less than 0.10% and not more than 0.35%

Si + Al: 0.5 to 2.0%

Mn: 1.0 to 4.0%

P: 0 to 0.05%

S: 0 to 0.01%

, The balance being iron and inevitable impurities,

However,

The retained austenite has an area ratio of not less than 8%

And the remainder is composed of one or more of bainite, martensite, tempering bainite and tempering martensite,

With respect to the carbon concentration in the retained austenite,

Its average carbon concentration is 0.9 to 1.2% by mass,

The standard deviation of the carbon concentration distribution is 0.35 mass% or more,

And an area having a carbon concentration of 1.5% by mass or more is 1.0% or more in terms of an area ratio with respect to the whole texture.

In the high strength and high ductility steel sheet according to the second invention of the present invention,

In the first invention,

The composition of matter is, by mass%, further,

Cu, Ni, Mo, Cr and B in a total amount of not more than 1.0%.

In the high strength and high ductility steel sheet according to the third invention of the present invention,

In the first or second invention,

The composition of matter is, by mass%, further,

V, Nb, Ti, Zr and Hf in a total amount of not more than 0.2%.

In the high strength and high ductility steel sheet according to the fourth invention of the present invention,

In any one of the first to third inventions described above,

The composition of matter is, by mass%, further,

Ca, Mg, and REM in a total amount of 0.01% or less.

According to the present invention, not only the amount of the retained austenite (area ratio) and the average carbon concentration are specified, but also the distribution of the carbon concentration is controlled, thereby realizing the TRIP phenomenon from the initial stage to the late stage of deformation and realizing a high work hardening rate (YR) of 0.7 or more and a tensile strength (TS) x elongation (EL) x elongation flange formability (lambda) of 1000000 MPa ·% ·% or more. So that a ductile steel sheet can be provided.

1 is a diagram schematically showing a diffraction peak of residual? As measured by an X-ray diffraction method.
2 is a diagram schematically showing a heat treatment condition for manufacturing a high strength and high ductility steel sheet according to the present invention.

(YR) of 0.7 or more and a tensile strength (TS) x elongation (EL) x elongation flangeability (lambda) of at least 0.7 as a mechanical property of a steel sheet made of TBF steel, Has been repeatedly examined for measures that can secure 1000000 MPa ·% ·% or more. As a result, by the following accident study, it has been found that the desired characteristics can be secured.

That is, in order to further improve the strength-ductility balance as compared with the prior art, it is necessary to more effectively utilize the retained austenite (hereinafter also referred to as "residual?") For promoting the TRIP phenomenon. However, from the viewpoint of securing the weldability of the steel sheet, there is a limitation on the C content in the steel sheet, so there is a limit to increase the residual? Amount and the average carbon concentration in the residual?.

Therefore, the present inventors paid attention to the carbon concentration distribution in the residual?. That is, in order to achieve high strength and high ductility by the TRIP phenomenon, it is important to realize a high work hardening rate from the early stage of deformation to the middle stage, and in order to do so, it is necessary that some unstable residual? . On the other hand, in order to maintain a high work hardening rate even when the amount of deformation increases, that is, even in the latter stage of deformation, it is necessary to prepare a stable residual gamma having a high carbon concentration.

That is, it is important that the residual? Has not only a high stability or a low stability, but also a stability, that is, a carbon concentration distribution widely exists.

The present inventors have further studied based on the above findings, and as a result, they have completed the present invention.

Hereinafter, a steel structure (hereinafter also referred to simply as " structure ") which characterizes a high-strength high-ductile steel sheet according to the present invention (hereinafter also referred to as " steel sheet according to the present invention "

[Steel structure of steel sheet of the present invention]

As described above, the steel sheet of the present invention is based on the structure of the TBF steel. In particular, since the carbon concentration distribution in the residual? Is controlled after containing a predetermined amount of residual? Having a predetermined carbon concentration, Which is different from the prior art.

≪ Residual austenite: 8% or more in terms of area ratio to the whole structure >

The residual? Is useful for improving the ductility. In order to effectively exhibit such action, it is necessary to make the area ratio to the total structure at least 8%, preferably at least 9%, more preferably at least 10% . On the other hand, the area ratio of the residual? Is preferably 20% or less, more preferably 18% or less, particularly preferably 16% or less.

≪ Balance: at least one of bainite, martensite, tempering bainite and tempering martensite >

It is possible to prevent the formation of ferrite and to prevent the generation of ferrite and to improve the yield strength due to the prevention of deformation at the time of deformation due to the fineness of the hosiery structure by constructing the parent phase with a fine and uniform structure of bainite or martensite and / YS can be increased.

≪ Average carbon concentration (% C _{? R} ) in residual?: 0.9 to 1.2 mass%

% C _γR is an index that affects the stability of residual γ to martensitic transformation during deformation. If the% C _{? R} is too low, the residual? Is unstable, so that after the stress is applied, the processed organic martensite transformation occurs before the plastic deformation, so that the required elongation can not be obtained. On the other hand, if the% C _{? R} is too high, the residual? _Becomes excessively stable, and the processed organic martensite transformation does not occur even if the processing is carried out, so that the necessary stretch flangeability can not be obtained. In order to obtain the necessary elongation, the% C _{? R} must be set to 0.9 to 1.2% by mass. The lower limit of the% C _{? R} is preferably 1.0% by mass.

<Standard deviation of carbon concentration distribution in residual y: 0.35 mass% or more>

In order to increase and maintain the work hardening rate from the initial stage to the later stage of deformation, the carbon concentration distribution in the residual? Is widened to produce the residual? Having different stability. In order to effectively exhibit such an effect, the standard deviation of the carbon concentration distribution in the residual? Should be 0.35% by mass or more, preferably 0.40% by mass or more, and more preferably 0.45% by mass or more. On the other hand, in the high-strength high-ductile steel sheet according to the present invention, the standard deviation of the carbon concentration distribution in the residual? Is practically an upper limit of about 0.70 mass%, preferably 0.65 mass% or less.

&Lt; Area where the carbon concentration in the residual? Is not less than 1.5% by mass: not less than 1.0%

In order to increase the elongation, it is important that the stability of the residual? When the amount of distortion increases is high. For this purpose, it is not sufficient if only the average carbon concentration is high and the residual? Is required to be present in a certain amount or more. Concretely, it is necessary that the area where the carbon concentration in the residual? Is not less than 1.5% by mass is not less than 1.0%, preferably not less than 1.5%, more preferably not less than 2.0% as the area ratio with respect to the whole texture. On the other hand, in the high-strength high-ductile steel sheet according to the present invention, the area ratio of the region having the carbon concentration in the residual? Of 1.5% by mass or more to the whole structure is about 1/2 of the total residual? Area, 5 or less, more preferably 1/3 or less.

[Method of measuring the area ratio of residual?, The average carbon concentration ( _? C _{? R} ) in residual? And the carbon concentration distribution thereof]

Here, the measurement method of the area ratio of the residual?, The average carbon concentration (% C _{? R} ) and the carbon concentration distribution will be described.

The area ratio V _{? R} and the average carbon concentration (% C _{? R} ) of the residual? Were measured by an X-ray diffraction method after grinding to a thickness of 1/4 of the steel sheet and chemical polishing (ISIJ Int. Vol 33, (1933), No. 7, p. 776). On the other hand, in the present invention, a two-dimensional micrographic X-ray diffraction apparatus (RINT-RAPIDII) manufactured by Rigaku Corporation was used as the X-ray diffraction apparatus, and Co-K?

On the other hand, for the tissues other than the residual?, The steel sheet was detached and corroded and observed under an optical microscope (magnification: 400x) to identify tissues other than residual?.

Subsequently, the distribution of the carbon concentration in the residual? Was determined as follows using three diffraction peaks of (200)?, (220)? And (311)? Measured by the above X-ray diffractometer.

First, as shown in the schematic diagram of FIG. 1, 2θ (2θ _avg (hkl)) at which the diffraction intensity becomes maximum is obtained for three diffraction peaks of (200) γ, (220) The half-value width? 2? (Hkl) was obtained. Here, (hkl) means (200), (220) or (311) (the same applies hereinafter).

Subsequently, from the 2θ _avg (hkl), the Bragg condition: λ = 2dsinθ saved d (hkl) from the following formula (1) using (d:: the diffraction grating constant, λ Co-Kα line wavelength).

d (hkl) =? / {2 sin (2? _avg (hkl) / 2)} ... Equation (1)

The crystal lattice constants a ₀ (hkl) were obtained by the following equation (3), and the three crystal lattice constants a ₀ (hkl) were arithmetically averaged to obtain a crystal lattice constant a ₀ .

_{a 0 (hkl) = d (} hkl) √ (h 2 + k 2 + l 2) ... Equation (2)

(Dyson DJ, Holmes B. (1970), "Effect of alloying additions on the lattice parameter austenite", J. Iron Steel Inst., 208: 469-474) shown in the following formula (3) To obtain a carbon concentration% C _avg (unit: mass%). (On the other hand, this carbon concentration% C _avg is used only as an index for defining the carbon concentration distribution, and it can not be said that it is strictly consistent with the above-mentioned average carbon concentration% C _γR . )

% C _avg = (1 / 0.033) (a ₀ -0.0012.% Mn + 0.00157.% Si-0.0056.% Al) Equation (3)

Here,% Mn,% Si and% Al are the contents (mass%) of Mn, Si and Al in the steel sheet, respectively.

Next, the half value width?% C of the carbon concentration distribution in the residual? Was obtained in the following order.

First, the diffraction angles at the upper and lower limits of the half-value width? 2 (hkl) of the diffraction angle 2? (Hkl) of each peak were determined by the following formulas (4) and (5) (see FIG.

_{2θ L (hkl) = 2θ avg} (hkl) -Δ2θ (hkl) / 2 ... Equation (4)

2? _H (hkl) = 2? _Avg (hkl) +? 2? (Hkl) / 2 Equation (5)

Thus, by using Bragg conditions and the above equations (1) to (3) in the same order as above using the 2? _L (hkl) and 2? _H (hkl), the upper and lower limits % C _L and% C _H were obtained. Then, the full width at half maximum of the carbon concentration distribution?% C was calculated by the following equation (6).

Δ% C =% C _H -% C _L ... Equation (6)

Then, assuming that the carbon concentration distribution is a normal distribution, the standard deviation? _{% C} was calculated from the half-value width?% C as follows.

That is, the probability density function f (x) of the normal distribution is expressed by the following equation (7) from the average value u and the standard deviation?.

f (x) = {1 /? (2??)} exp {- (xu) ² / ( ² ? ² )} ... Equation (7)

The probability f (u) in the average value is obtained by the following equation (8) by substituting x = u into the above equation (7).

f (u) = 1 /? (2?) ... Equation (8)

Then, the average value u =% a value half as much as move up and down in the half-height width from C _{avg Δ% C (% C avg} ± Δ% C / 2) the probability density at _{f (% C avg ± Δ%} C / 2 ) Is 1/2 of the probability density f (u) = f (% C _avg ) at the average value u =% C _avg , the relationship of the following equation (9) is obtained from the equations (7) and (8) .

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

The following equation (10) is derived from the equation (9) to obtain the standard deviation? _{% C} from the half-width width?% C and therefore the standard deviation? _{% C} was calculated.

? _{% C} =? {?% C / 2) ² / (2ln2)} ... Equation (10)

And, at least by the cumulative density function g (x) shown in the following formula (11) using the mean value% C _avg and _{σ% C} of the carbon concentration distribution of the obtained residue γ as described above, the carbon concentration is 1.5 mass% (12) is derived from the area ratio V _{? R} (C _? 1.5%) with respect to the entire tissue of the region, and V _{? R} (C _? 1.5%) is calculated using this formula (12).

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

V _{? R} (C _? 1.5%) = V _{? R} {1-g (1.5)}

= V _{? R} [0.5-erf {(1.5% C _avg ) / ( ^2? _{% C} ² )}] Equation (12)

Here, V _{? R} is the area ratio of the total residual _?.

Next, the composition of components constituting the inventive steel sheet will be described. Hereinafter, the units of the chemical components are all% by mass. Further, the &quot; content &quot; of each component may be simply described as &quot; amount &quot;.

[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 securing strength and ductility by contributing to securing the amount (area ratio) of retained austenite. In order to exhibit such an effect effectively, it is necessary to contain C of 0.10% or more, preferably 0.12% or more, and more preferably 0.14% or more. However, when the amount of C is excessive, the weldability is deteriorated. Therefore, the amount of C is less than 0.35%, preferably not more than 0.32%, more preferably not more than 0.30%, more preferably not more than 0.28%.

Si + Al: 0.5 to 2.0%

Si and Al are elements that effectively inhibit the formation of carbide by decomposition of the retained austenite. In order to exhibit such an effect effectively, it is necessary to contain Si and Al in a total amount of not less than 0.5%, preferably not less than 0.7%, more preferably not less than 0.9%. However, even if Si and Al are contained excessively, the above effect is saturated, which is economically useless and causes hot brittleness. Therefore, the total amount of Si and Al is 2.0% or less, preferably 1.9% or less And not more than 1.8%.

Mn: 1.0 to 4.0%

Mn is an element necessary for stabilizing austenite and obtaining a desired retained austenite. In order to exhibit such an effect effectively, it is necessary to contain 1.0% or more, preferably 1.3% or more, and more preferably 1.6% or more of Mn. However, when the Mn content is excessive, adverse effects such as cracking of the cast slab are observed. Therefore, the Mn content is 4.0% or less, preferably 3.5% or less, more preferably 3.0% or less.

P: 0 to 0.05%

P is inevitably present as an impurity element, but it is an element that can be contained in order to secure a desired residual?. However, if P is contained excessively, the secondary processability deteriorates. Therefore, the P content is 0.05% or less, preferably 0.03% or less, more preferably 0.02% or less.

S: 0 to 0.01%

S is inevitably present as an impurity element and forms a sulfide inclusion such as MnS to become a starting point of cracking and deteriorates workability. Therefore, the S content is 0.01% or less, preferably 0.005% or less, more preferably 0.005% Is 0.003% or less.

The steel of the present invention contains the above element as an essential component and the remainder is iron and inevitable impurities. However, the following allowable components may be added to the steel in the 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 strengthening elements for steel and are effective elements for stabilizing residual γ and securing a predetermined amount. In order to effectively exhibit such an action, it is recommended that these elements be contained in an amount of 0.001% or more, more preferably 0.01% or more. However, even if these elements are contained in excess, the above effects are saturated and economically unnecessary. Therefore, the total amount of these elements is preferably 1.0% or less, more preferably 0.5% or less.

V, Nb, Ti, Zr and Hf: not more than 0.2% in total

These elements have effects of precipitation strengthening and fine structure, and are useful elements for increasing the strength. In order to effectively exhibit such an action, it is recommended that these elements be contained in an amount of 0.01% or more, more preferably 0.02% or more. However, even if these elements are contained in excess, the above effects are saturated and economically unnecessary. Therefore, it is preferable that the total amount of these elements is 0.2% or less, more preferably 0.1% or less.

Ca, Mg and REM: 0.01% or less in total

These elements are effective elements for improving workability by controlling the form of sulfides in steel. Examples of REM (rare earth element) used in the present invention include Sc, Y, lanthanoid and the like. In order to exhibit the above-mentioned action effectively, it is recommended to contain these elements in a total amount of 0.001% or more, more preferably 0.002% or more. However, even if these elements are contained in excess, the above effects are saturated and economically unnecessary. Therefore, it is preferable that the total amount of these elements is 0.01% or less, more preferably 0.005% or less.

Next, preferable manufacturing conditions for obtaining the inventive steel sheet will be described below.

[Preferred Production Method of Steel Sheet of the Present Invention]

The steel sheet of the present invention can be produced by hot-rolling a steel material satisfying the above-mentioned composition, then cold-rolling the steel sheet, and then heat-treating the steel sheet under the following conditions (1) to (4) .

[Heat treatment conditions]

(1) The cold-rolled sheet is heated to the second heating temperature T2: [0.7 x Ac1 + 0.3 x Ac3] to [0.2 x Ac1 + 0.8 x Ac3] and maintained at the temperature for the second holding time t2: After heating the temperature range at an average heating rate of 4 DEG C / s or less,

(2) further heating at a third heating temperature T3: [Ac3 + 10 占 폚] to 950 占 폚, maintaining the third holding time t3 at 180 占 폚 for 180 seconds or less,

(3) cooling from the third heating temperature T3 to 500 DEG C at an average cooling rate CR1 of at least 20 DEG C / s,

(4) Austempering temperature T4: Maintain the maintenance time at t4: 10 s or more at 350 to 480 캜, and then cool to room temperature.

Hereinafter, the reason why the above-mentioned heat treatment condition is recommended will be described.

(1) The second holding time t2 is maintained at the second heating temperature T2: [0.7 x Ac1 + 0.3 x Ac3] to [0.2 x Ac1 + 0.8 x Ac3] Heating at the following average heating rate &gt;

The Mn concentration distribution is caused during the reverse transformation in the bimetallic temperature region by holding or heating the ferrite / austenite at a bimetallic temperature region for a predetermined time or longer, The local velocity difference of the knight transformation is increased to widen the carbon concentration distribution in the residual?.

The holding time t2 in the temperature range is more preferably 10 s or more, and even more preferably 20 s or more, but from the viewpoint of productivity, it is recommended that the holding time t2 is 200 s or less.

On the other hand, Ac1 and Ac3 are derived from the chemical composition of the steel sheet by Leslie, &quot; Steel Materials Science &quot;, Kodanariasu, Maruzen Co., Ltd., 1985, p. 273 &lt; / RTI &gt;

(2) The third heating and holding temperature T3: [Ac3 + 10 占 폚] to 950 占 폚 and the third holding time t3: 180 seconds or less>

The ferrite is retained in the austenite single phase inverse temperature region so as to prevent the ferrite from remaining in the austenite single phase structure until the subsequent cooling.

If the third heating temperature T3 is less than [Ac3 + 10 占 폚], the ferrite remains and the growth of ferrite can not be suppressed during the cooling process of the subsequent step (3). On the other hand, if the third heating temperature T3 is more than 950 DEG C or the third holding time t3 is more than 180s, the Mn distributed during heating in the two-phase heating in the step (1) becomes uniform, It can not be made wide.

(3) Cooling from the third heating temperature T3 to 500 占 폚 at an average cooling rate HR1 of 20 占 폚 / s or more>

To prevent the formation of ferrite and to make the structure of bainite-based body.

The average cooling rate HR3 in this temperature range is more preferably 25 DEG C / s or more, and still more preferably 30 DEG C / s or more.

(4) Austempering temperature T4: Holding time at t4 for at least 10 seconds, at austempering holding time at 350 to 480 DEG C,

To accelerate bainite transformation and to concentrate carbon with untransformed austenite to obtain stable residual?.

[Modified examples of heat treatment conditions]

On the other hand, the step (1) may be configured as the following step (1a).

(1a) The cold-rolled sheet is heated at a first heating temperature T1: [Ac1-100 占 폚] to [Ac1-30 占 폚] and maintained at that temperature for a first holding time of 10 seconds or longer, And the second holding time t2 is maintained at the second heating temperature T2: [0.7 x Ac1 + 0.3 x Ac3] to [0.2 x Ac1 + 0.8 x Ac3] for not less than the second holding time t2: 5 s.

As described above, Mn is concentrated in the cementite by holding or heating in advance in the ferrite / cementite 2-phase temperature region for a predetermined time, thereby accelerating the distribution of Mn concentration between the ferrite / austenite in the subsequent ferrite / austenite bimetallic heating , The local velocity difference of the bainite transformation during the oust tempering process in the step (4) can be made larger, and the carbon concentration distribution in the residual? Can be widened.

The step (4) may be configured as in the following step (4a).

(4a) Austempering temperature T4: 350 to 480 占 폚 maintains the maintenance time t4: 10 s or more, reheats the reheat temperature T5 to 500 to 600 占 폚, maintains the reheat holding time t5 to 30 seconds or less, Cool to room temperature.

As described above, the steel sheet of the present invention can be made of a plated steel sheet by re-heating to a temperature range where the residual? Is not decomposed to alloy the plated layer.

The present invention will be described in more detail with reference to the following examples. However, the present invention is of course not limited by the following examples, and it is also possible to carry out the modifications appropriately within a range that is suitable for the purpose Of course, all of which are included in the technical scope of the present invention.

Example

The steel having the components shown in the following Table 1 was made of a vacuum solvent, hot-forged to form a steel sheet having a thickness of 30 mm, and hot-rolled. The conditions of the hot rolling do not essentially affect the final structure and properties of the steel sheet of the present invention. However, in this embodiment, after heating to 1200 deg. C, multi-stage rolling is performed under the conditions of the end temperature of hot rolling of 880 deg. . Thereafter, the steel sheet was cooled to 500 deg. C at a cooling rate of 30 deg. C / s to stop the cooling, inserted into a furnace heated to 500 deg. C, held for 30 minutes and then cooled to obtain a hot rolled sheet. The hot-rolled sheet was pickled to remove the scale of the surface, and then cold-rolled to 1.4 mm to obtain a cold-rolled sheet.

Then, the cold-rolled sheet was used as a starting material and heat-treated under the conditions shown in Table 2 below. On the other hand, the average heating rate from the room temperature to the initial heating temperature (holding temperature) is fixed at 10 占 폚 / sec, the average heating rate to the next heating temperature (holding temperature) is 20 占 폚 / Holding temperature) was set at a constant 10 ° C / s. The average heating rate from the tempering temperature T4 to the reheating temperature T5 was fixed at 10 占 폚 / sec, and the average cooling rate from the tempering temperature T4 or the reheating temperature T5 to the room temperature was fixed at 10 占 폚 / sec.

For each of the steel sheets subjected to the heat treatment, the area ratio of the residual?, The average carbon concentration (? R) in the residual? And the carbon concentration distribution thereof were measured by the measuring method described in the section " .

On the other hand, in all of the structures of the steel sheet used in the present embodiment, the balance other than the retained austenite and ferrite was composed of one or more of bainite, martensite, tempering bainite and tempering martensite , And only area ratios of retained austenite and ferrite are shown in Table 3 below.

For each of the steel sheets subjected to the heat treatment, the yield strength YS, the tensile strength TS and the elongation (total elongation) EL were measured by a tensile test in order to evaluate the strength-ductility balance. On the other hand, the tensile test was carried out in accordance with JIS Z 2241 by preparing a JIS No. 5 test piece. Further, in order to evaluate the extension flange lambda of each steel sheet, the hole expanding rate was measured in accordance with JFST1001 of Japan Steel Federation.

The measurement results are shown in Table 3 below. (*) Of the steel sheet after the heat treatment is such that the yield ratio (YR) is 0.7 or more and the tensile strength (TS) x elongation (EL) x stretch flangeability (?) Is 1000000 MPa% ), And the other ones were rejected (x).

As shown in the above Table 3, the steel No. 1 having the inventive steel (evaluation is?). 3, 4, 9 to 11, 14 and 18 to 27 show the inventive steel which satisfies the requirements of the present invention as a result of the heat treatment under the recommended conditions by using the steel material satisfying the requirements of the present invention. , The mechanical properties satisfied the acceptance criteria, and it was confirmed that a steel sheet with high strength and excellent strength-ductility balance was obtained.

On the other hand, in the case of the steel No. 1 which is the comparative steel (evaluation x). 1, 2, 5 to 8, 12, 13, and 15 to 17 do not satisfy at least one of the requirements of the constitutional regulations and the organizational regulations of the present invention, and the characteristics do not satisfy the acceptance criteria.

That is, 1, 2, 5 to 8, and 12 use a steel grade that meets the requirements of the specification of the present invention. However, since the steel grade is manufactured under a condition partially deviating from the recommended manufacturing conditions, This is behind.

On the other hand, 13, and 15 to 17 are manufactured under the recommended manufacturing conditions, but because they use steel grades that deviate somewhat from the requirements of the present invention, they do not meet the requirements of the organization regulations and are inferior in properties.

The applicability of the present invention was confirmed from the above.

While the invention has been described in detail and with reference to specific embodiments thereof, it is evident to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.

The present application is based on Japanese Patent Application (Japanese Patent Application No. 2014-238710) filed on November 26, 2014, the content of which is incorporated herein by reference.

The steel sheet of the present invention is excellent in balance of strength and ductility, and is useful for automotive steel sheets and the like.

Claims (2)

The composition of matter, in% by mass,
C: not less than 0.10% and not more than 0.35%
Si + Al: 0.5 to 2.0%
Mn: 1.0 to 4.0%
P: 0 to 0.05%
S: 0 to 0.01%
, The balance being iron and inevitable impurities,
However,
The residual austenite is not less than 8% in area ratio to the entire structure and the remainder is composed of one or more of bainite, martensite, tempering bainite and tempering martensite,
With respect to the carbon concentration in the retained austenite,
Its average carbon concentration is 0.9 to 1.2% by mass,
The standard deviation of the carbon concentration distribution is 0.35 mass% or more,
The area where the carbon concentration is 1.5% by mass or more is 1.0% or more
Wherein the high strength steel sheet is a high strength steel sheet. The method according to claim 1,
Wherein the composition comprises, by mass%, any one of the following (a) to (c).
(a) one or more of Cu, Ni, Mo, Cr and B in a total amount of not more than 1.0%
(b) one or more of V, Nb, Ti, Zr and Hf in a total amount of not more than 0.2%
(c) 0.01% or less in total of one or more of Ca, Mg and REM

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