JP5329979B2 - High-strength cold-rolled steel sheet with an excellent balance between elongation and stretch flangeability - Google Patents

Description

  The present invention relates to a high-strength steel sheet excellent in workability, and in particular, to a high-strength steel sheet with improved elongation (total elongation) and stretch flangeability.

  For example, steel sheets used for automobile frame parts and the like are required to have high strength for the purpose of collision safety and fuel efficiency reduction by reducing the weight of the car body, and excellent forming process for processing into complex frame parts Sex is also required.

  For this reason, it is possible to provide a high-strength steel sheet having both improved elongation flangeability evaluated by elongation (total elongation; this refers to El) and hole expansion ratio (λ) while ensuring tensile strength. Is anxious.

  In response to the above needs, elongation and elongation of dual phase steel (DP steel) composed of ferrite and tempered martensite based on the concept of microstructure control that reduces the hardness difference between ferrite and martensite. Many high-strength steel sheets with improved flangeability balance have been proposed.

  For example, in Patent Document 1, regarding the hardness difference between the ferrite phase and the martensite phase, the martensite hardness is changed by changing the carbon concentration to martensite by changing the martensite fraction by cooling after annealing. In addition, a high-strength steel sheet is disclosed in which the hardness difference between the ferrite phase and the martensite phase is reduced by reducing the martensite hardness by tempering to ensure elongation and stretch flangeability.

  Patent Document 2 discloses a high-strength steel plate that secures stretch flangeability by adjusting the steel composition to increase the ferrite hardness and reducing the hardness difference between the ferrite phase and the martensite phase. .

  In these patent documents, the concept of component design and organization design in accordance with required mechanical characteristics is disclosed. On the other hand, with the recent increase in raw material costs, the formability (balance between stretch El and stretch flangeability λ) is improved without using an expensive alloy element and with the same component system as before and the structure as it is. There is a strong demand for it.

Japanese Patent Laying-Open No. 2005-256089 Japanese Patent Application Laid-Open No. 2004-2111119

  Accordingly, an object of the present invention is to provide a high-strength cold-rolled steel sheet that is excellent in the balance between elongation and stretch-flangeability, while targeting a dual-phase structure steel (DP steel) having the same component system as that of the prior art.

The invention described in claim 1
% By mass (hereinafter the same for chemical components)
C: more than 0.03% and 0.30% or less,
Si: less than 3.0% (including 0%)
Mn: 0.5 to 5.0%,
P: less than 0.1%,
S: 0.005% or less,
N: 0.01% or less,
Al: containing more than 0.01% and 1.00% or less, with the balance being composed of iron and inevitable impurities,
It has a two-phase structure composed of tempered martensite with an area ratio of 5% or more and 95% or less and the balance of ferrite,
Ri average Mn concentration _{C Mn · alpha} and the average Mn concentration ratio _{C Mn · α / C Mn ·} M than 0.95 der the _{C Mn · M} in tempered martensite in said ferrite,
A high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability, characterized in that the product EL × λ of stretch EL and stretch flangeability λ is 925% ·% or more .

The invention described in claim 2
Ingredient composition further
Cr: 0.01 to 1.0%,
Mo: 0.01 to 1.0%,
Cu: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
The high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to claim 1.

The invention according to claim 3
Ingredient composition further
Ca: 0.0005 to 0.01% and / or Mg: 0.0005 to 0.01%
The high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to claim 1.

  According to the present invention, in a two-phase structure composed of ferrite and tempered martensite, the ratio of the Mn concentration in ferrite and martensite is set to a certain value or more without changing the components and structure, so that elongation and elongation are increased. It has become possible to improve the balance of flangeability, and it has become possible to provide a high-strength steel plate with better formability.

It is a graph which illustrates typically the relationship between m ^* and V ( _alpha ) in Formula (3).

  The present inventors pay attention to a high-strength steel sheet (see Patent Documents 1 and 2 above) having a two-phase structure composed of ferrite and tempered martensite (hereinafter sometimes simply referred to as “martensite”) to ensure elongation. However, if the stretch flangeability can be improved, a high-strength steel sheet capable of improving the balance between stretch and stretch flangeability can be obtained, and intensive studies have been conducted such as investigating the effects of various factors on stretch flangeability.

  As a result of the examination, the following knowledge was obtained. That is, even in a limited component system, by increasing the ratio of the Mn concentration in ferrite and martensite to a certain value or more, the solid solution amount of Mn in the ferrite phase increases and the hardness of the ferrite increases. At the same time, the solid solution amount of Mn in the martensite phase decreases and the hardness of the martensite decreases. As a result, the difference in hardness between ferrite and martensite is reduced, stress concentration at the interface between ferrite and martensite is reduced, and stretch flangeability is improved. Or, since the strength of the ferrite itself has increased due to an increase in the amount of Mn solid solution, the strength of the steel sheet can be secured by securing a certain area ratio of ferrite (that is, even if the area ratio of martensite is reduced). However, the elongation can also be improved. Therefore, the ratio of the Mn concentration in ferrite and martensite is set to a certain value or more, and the ratio of the area ratio of ferrite and martensite is set within a predetermined range, so that the balance between elongation and stretch flangeability can be improved. I understood.

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

  Hereinafter, the structure characterizing the steel sheet of the present invention will be described first.

[Structure of the steel sheet of the present invention]
As described above, the steel sheet of the present invention is based on the same two-phase structure (ferrite + tempered martensite) as in Patent Documents 1 and 2, and in particular, the Mn concentration in ferrite and tempered martensite. It differs from the steel sheets of Patent Documents 1 and 2 in that the ratio is controlled to a certain value or more.

<Ratio of average Mn concentration C _{Mn · α in} ferrite to average Mn concentration C _{Mn · M in} tempered martensite C _{Mn · α} / C _{Mn · M} is 0.95 or more>
It is considered that the strength of the entire dual phase steel is determined by the addition rule of the volume ratio and strength of the ferrite phase that is the soft phase and the martensite phase that is the hard phase, and is expressed by the following formula (1).

_{TS total = V α × TS α} + V M × TS M ··· formula (1)
(Where TS _total : tensile strength of the entire duplex structure steel, TS _i : tensile strength of i phase, V _i : volume fraction of i phase, α: ferrite phase, M: martensite phase)

  Further, since the sum of the volume ratios of the respective phases is 1, the relationship of the following formula (2) is established.

V _α + V _M = 1 Equation (2)

  The following formula (3) is derived from the above formula (1) and formula (2).

V _α = 1− (A−1) / (m ^* −1) (3)
(Here, A = TS _total / TS _α , m ^* = TS _M / TS _α ; ratio of tensile strength of martensite and ferrite)

In the above formula (3), since the tensile strength TS _α of the ferrite phase is smaller than the tensile strength TS _M of the martensite phase and the tensile strength TS _{total of the} entire double-layer structure steel, A> 1, m ^* > 1, that is, , (A-1)> 0, (m ^* -1)> 0.

Here, it is known from experience that the ferrite volume fraction _Vα is substantially proportional to the elongation. In addition, since the tensile strength of each phase is almost proportional to the hardness, the ratio m ^* of the tensile strength of martensite and ferrite substantially corresponds to the ratio of the hardness of martensite and ferrite. It is empirically known that stretch flangeability improves as the ratio decreases.

As expressed by the above equation (3), V _α and m ^* are in a trade-off relationship via A, and this equation (3) is a graph in which the horizontal axis is m ^* and the vertical axis is V _α. In this case, since m ^* > 1, 0 ≦ V _α ≦ 1, a curve of y = −1 / x having asymptotic lines between V _α = 1 and m ^* = 1 is obtained (see FIG. 1). Here, when the concentration of Mn on the martensite side between the ferrite and martensite is large between the ferrite and martensite under the condition that the tensile strength TS _{total of the} whole duplex structure steel is set to the target strength (constant), Since the solid solution amount of Mn decreases, Ts _α decreases, the value of A in the intercept with the m ^* axis increases (A _{2 in} FIG. 1), and the entire curve shifts to the high m ^* side.

On the contrary, when the concentration of Mn on the martensite side between the ferrite and martensite is small, the solid solution amount of Mn in the ferrite increases, so that Ts _α increases, and the m ^* axis and The value of A in the intercept increases (A _{1 in} FIG. ₁ ), and the entire curve shifts to the low m * side.

Thus, as is clear from the above equation (3), towards the latter case, V _alpha is m ^* is reduced under certain conditions, or, if m ^* V _alpha is increased under certain conditions, The degree of freedom for selecting an appropriate combination of V _α and m ^* is widened, and the balance between elongation and stretch flangeability can be improved.

That the "V _alpha is m ^* is reduced in certain conditions", Mn of by devising a heat treatment process is also under the constraint of a limited component, to martensite side between the ferrite and martensite It was concentrated suppression, due to the concentration of the Mn, to suppress an excessive increase in excessive reduction and TS _M of TS _alpha, and ferrite by reducing the ratio m ^* of the hardness of the martensite and ferrite This means that stress concentration at the martensite interface is relaxed and stretch flangeability is improved.

Further, the above-mentioned “V _α increases under the condition that m ^* is constant” means that the smaller the Mn concentration on the martensite side between the ferrite and martensite, the more the M Since the strength increases, it means that the strength of the entire dual phase steel can be secured even if the ferrite fraction is increased, and elongation can be secured by increasing the ferrite fraction.

In order to effectively exhibit the above action, the ratio C _{Mn · α} / C _{Mn · M} of the average Mn concentration C _{Mn · α} in ferrite and the average Mn concentration C _{Mn · M in} tempered martensite is 0.95 or more. More preferably, it is 0.97 or more.

<A tempered martensite with an area ratio of 5% to 95% and the balance being ferrite>
While controlling the ratio of the Mn concentration in the ferrite and martensite and adjusting the ratio of the area ratio of ferrite and martensite, the strength is ensured while ensuring the balance between elongation and stretch flangeability.

  In order to effectively exhibit the above action, the tempered martensite has an area ratio of 5% or more (preferably 10% or more, more preferably 20% or more, particularly preferably 30% or more), 95% or less (preferably 90%). Hereinafter, it is more preferably 85% or less, particularly preferably 80% or less. The balance is ferrite.

  Hereinafter, a method for measuring the area ratio of tempered martensite and the average Mn concentration in ferrite and martensite will be described.

  First, the area ratio of tempered martensite was adjusted so that the rolling direction of each test steel sheet could be observed in the normal direction, then mirror-polished and corroded with nital liquid to reveal the metal structure. Then, three visual fields were observed with a scanning electron microscope at a magnification of 1000 times. About the martensite area ratio, the area | region where the white granular contrast in a scanning electron microscope image was contained was made into the martensite, and the ratio for which the area | region occupied to the whole was measured by image analysis, and it was set as the martensite area ratio.

  Next, the average Mn concentration in ferrite and martensite was determined as follows. That is, a line analysis was performed by EPMA at a distance of 100 μm along the thickness direction in the vicinity of t / 4 (t: thickness), under the measurement conditions of acceleration voltage: 15 kV, irradiation current: 0.3 μA, measurement interval: 0.2 μm. Went. Then, by using the results of this line analysis, the average Mn concentration is obtained for each crystal grain of ferrite and martensite identified from the structure photograph, and the average is further obtained for each phase, and the ferrite phase and martensite phase are obtained. The average Mn concentration was calculated.

  Next, the component composition which comprises this invention steel plate is demonstrated. Hereinafter, all the units of chemical components are mass%.

[Component composition of the steel sheet of the present invention]
C: more than 0.03% and 0.30% or less C is an important element that affects the martensite area ratio and affects the balance of tensile strength, elongation, and stretch flangeability. If it is 0.03% or less, the martensite area ratio is insufficient, so that the tensile strength and stretch flangeability cannot be secured. On the other hand, if it exceeds 0.30%, the ferrite area ratio is insufficient, so that elongation cannot be secured. The range of C content is preferably 0.05 to 0.25%, more preferably 0.07 to 0.20%.

Si: less than 3.0% (including 0%)
Si is a useful element that can increase tensile strength without decreasing elongation and stretch flangeability by solid solution strengthening. If it is 3.0% or more, the formation of austenite at the time of heating is inhibited, so the area ratio of martensite cannot be ensured and stretch flangeability cannot be ensured. The range of Si content becomes like this. Preferably it is 0.3-2.5%, More preferably, it is 0.5-2.0%.

Mn: 0.5 to 5.0%
Mn increases the tensile strength of the steel sheet by solid solution strengthening, has the effect of improving the hardenability of the steel sheet and promoting the generation of the low temperature transformation phase, and is a useful element for securing the martensite area ratio. is there. If it is less than 0.5%, sufficient hardenability cannot be secured, and a sufficient martensite area ratio cannot be secured during rapid cooling, so that tensile strength cannot be obtained. On the other hand, if it exceeds 5.0%, austenite remains and stretch flangeability is deteriorated. The range of the Mn content is preferably 0.7 to 4.0%, more preferably 1.0 to 3.0%.

P: Less than 0.1% P is unavoidably present as an impurity element, and contributes to an increase in tensile strength by solid solution strengthening, but segregates at the prior austenite grain boundaries and causes the grain boundaries to become brittle to stretch flangeability. Is less than 0.1%. Preferably it is 0.05% or less, More preferably, it is 0.03% or less.

S: 0.005% or less S is also unavoidably present as an impurity element, forms MnS inclusions, and becomes a starting point of cracks when expanding holes, thereby reducing stretch flangeability. . More preferably, it is 0.003% or less.

N: 0.01% or less N is also unavoidably present as an impurity element and lowers the elongation and stretch flangeability by strain aging, so the lower one is preferable, and the content is made 0.01% or less.

Al: more than 0.01% and 1.00% or less Al combines with N to form AlN, thereby reducing the solid solution N that contributes to the occurrence of strain aging and preventing deterioration of stretch flangeability. Contributes to improvement of tensile strength by melt strengthening. If it is 0.01% or less, solute N remains in the steel, so strain aging occurs and elongation and stretch flangeability cannot be secured. On the other hand, if it exceeds 1.00%, austenite formation during heating is inhibited. The area ratio of martensite cannot be secured, and stretch flangeability cannot be secured.

  The steel of the present invention basically contains the above components, and the balance is substantially iron and impurities. In addition, the following allowable components can be added as long as the effects of the present invention are not impaired.

Cr: 0.01 to 1.0%,
Mo: 0.01 to 1.0%,
Cu: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
These elements are useful elements that can increase tensile strength without lowering elongation and stretch flangeability by solid solution strengthening. When each element is added below the lower limit value, the above-mentioned effects cannot be exhibited effectively. On the other hand, when each element exceeds 1.0%, austenite remains at the time of quenching and the stretch flangeability is deteriorated. Let

Ca: 0.0005 to 0.01% and / or Mg: 0.0005 to 0.01%
These elements are useful elements for improving stretch flangeability by miniaturizing inclusions and reducing the starting point of fracture. If less than 0.0005% of each element is added, the above effect cannot be exhibited effectively. On the other hand, if more than 0.01% of each element is added, inclusions are coarsened and stretch flangeability is lowered. To do.

  Next, the preferable manufacturing method for obtaining this invention steel plate is demonstrated below.

[Preferred production method of the steel sheet of the present invention]
In order to manufacture the cold-rolled steel sheet as described above, first, steel having the above composition is melted and formed into a slab by ingot forming or continuous casting and then hot-rolled. As hot rolling conditions, the finishing temperature of finish rolling is set to Ar ₃ point or higher, and after appropriate cooling, winding is performed in a range of 450 to 700 ° C. After hot rolling is completed, pickling is performed and then cold rolling is performed. The cold rolling rate is preferably about 30% or more.

  Then, after the cold rolling, air cooling from annealing and further tempering are performed.

[Annealing conditions]
As an annealing condition, a temperature increase rate in a two-phase region (between Ac ₁ and Ac ₃ ) during heating is 3 ° C./s or more (more preferably 20 ° C./s or more, particularly preferably 40 ° C./s or more). And annealing heating temperature: Ac _{3 to} 1000 ° C. (more preferably Ac ₃ + 20 ° C. to Ac ₃ + 80 ° C.) and annealing holding time: 600 s (more preferably 300 s) or less, then annealing Air cooling at a cooling rate of 5 ° C./s or more (more preferably 10 ° C./s or more, particularly preferably 20 ° C./s or more) from a heating temperature to a temperature of 700 ° C. or lower and 450 ° C. or higher, followed by a temperature below the Ms point Quench at a cooling rate of 50 ° C./s or higher.

<Passing through the two-phase region at the time of heating at a temperature rising rate of 3 ° C./s or more (more preferably 20 ° C./s or more, particularly preferably 40 ° C./s or more)>
Since Mn is an austenite stabilizing element, Mn is more likely to be distributed to austenite than ferrite in the two-phase region. Such Mn concentration behavior to austenite occurs even when the temperature rises, and when passing through the two-phase region while heating to the austenite single-phase region, the diffusion of Mn to the austenite portion proceeds in the two-phase region. Then, even if Mn is concentrated and then heated to the austenite single phase region, the concentration of Mn in the two phase region is inherited. Therefore, rapid heating at a rate of temperature rise at the time of passing through the two-phase region, which was conventionally 1-2 ° C./s, is 3 ° C./s or more (more preferably 20 ° C./s or more, particularly preferably 40 ° C./s or more). As a result, the Mn concentration is reduced by shortening the passage time of the two-phase region and shortening the diffusion time of Mn into the austenite portion in the two-phase region.

<Annealing heating temperature: Ac ₃ or more and 1000 ° C. or less (more preferably Ac ₃ + 20 ° C. or more and Ac ₃ + 80 ° C. or less), annealing holding time: 600 s (more preferably 300 s) or less>
This is because it is made to be full austenite (austenite single-phase structure) during annealing and to reduce the concentration of Mn. When the annealing heating temperature is less than Ac ₃ ° C., a two-phase structure is formed, and the concentration of Mn into austenite is promoted between ferrite and austenite, and the ratio of the hardness of ferrite and tempered martensite in the final structure is increased and stretched. It becomes impossible to secure a balance between the stretch flangeability and the stretch. In order to make the full austenite more complete, the annealing holding temperature is more preferably set to Ac ₃ + 20 ° C. or higher. On the other hand, when the annealing holding temperature exceeds 1000 ° C., austenite grains become coarse and the grain size becomes non-uniform, so that stable mechanical properties cannot be secured. In consideration of the responsiveness of transformation to ferrite during air cooling after annealing, the annealing holding temperature is more preferably set to Ac ₃ + 80 ° C. or lower.

  When the annealing holding time exceeds 600 s, austenite grains become coarse and the grain size becomes non-uniform, so that stable mechanical properties cannot be secured. In consideration of the responsiveness of transformation to ferrite during air cooling after annealing, the annealing holding time is more preferably 300 s or less.

<Air cooling at a cooling rate of 5 ° C./s or more (more preferably 10 ° C./s or more, particularly preferably 20 ° C./s or more) from the annealing heating temperature to a temperature of 720 ° C. or less to 450 ° C. or more>
This is because ferrite is introduced during air cooling, and martensite that is transformed from austenite during the subsequent cooling is secured. If the air cooling temperature exceeds 720 ° C., ferrite is not introduced during air cooling, while if it is less than 450 ° C., martensite cannot be secured.

  In addition, when introducing ferrite from fluustenite by cooling, in order to suppress the concentration of Mn to the martensite side between ferrite and martensite while promoting ferrite transformation, 5 ° C./s or more, Air cooling is preferably performed at a cooling rate of 10 ° C./s or more, particularly preferably 20 ° C./s or more.

<Rapid cooling at a cooling rate of 50 ° C./s or higher to a temperature below the Ms point>
This is to suppress the formation of a ferrite or bainite structure from austenite during cooling and obtain a martensite structure.

  When the rapid cooling is finished at a temperature higher than the Ms point or when the cooling rate is less than 50 ° C./s, bainite is formed, and the strength of the steel sheet cannot be secured.

[Tempering conditions]
Tempering conditions are not particularly limited, but tempering (reheating treatment) is performed so as to maintain a temperature range of 150 to 550 ° C. for 60 seconds or more and 1200 seconds or less so as to obtain desired strength and formability. desirable.

  Steels having the components shown in Table 1 below were melted to produce 120 mm thick ingots.

Incidentally, Ac ₁ point of the steel, Ac ₃ point and Ms point are determined by the following equation, shown together with the steels of Ac ₃ point and Ms point in Table 1.

Ac ₁ (° C.) = 723 + 29.1 · [Si] −10.7 · [Mn] + 16.9 · [Cr] −16.9 [Ni] (4)

Ac ₃ (° C.) = 910−203 · √ [C] −15.2 · [Ni] + 44.7 · [Si] + 31.5 · [Mo] −330 · [Mn] + 11 · [Cr] + 20 · [ Cu] -720 · [P] -400 [Al] (5)

    Ms (° C.) = 550-361 · [C] −39 · [Mn] −20 · [Cr] −17 · [Ni] −10 · [Cu] −5 · [Mo] + 30 · [Al] 6)

  However, [C], [Ni], [Si], [Mo], [Mn], [Cr], [Cu], [P], and [Al] are C, Ni, Si, Mo, Mn, Content (mass%) of Cr, Cu, P, and Al is shown.

This was hot rolled to a thickness of 25 mm, and then hot rolled again to a thickness of 3.2 mm. After pickling this, it cold-rolled to 1.6 mm in thickness to make a test material, and heat-treated on the conditions shown in Table 2.

  About each steel plate after heat processing, the area ratio of martensite and the average Mn concentration in ferrite and martensite were measured by the measurement method described in the above section [Best Mode for Carrying Out the Invention].

  Moreover, about each said steel plate, tensile strength TS, elongation El, and stretch flangeability (lambda) were measured. The tensile strength TS and elongation El were measured in accordance with JIS Z 2241 by preparing a No. 5 test piece described in JIS Z 2201 with the long axis perpendicular to the rolling direction. Moreover, stretch flangeability (lambda) performed the hole expansion test according to the iron continuous standard JFST1001, and measured the hole expansion rate, and made this the stretch flangeability.

  The measurement results are shown in Tables 3 and 4. In addition, Δ (El · λ) in these tables is obtained by subtracting (El · λ) of the comparative steel from (El · λ) of each steel, and an improvement in the balance between the elongation El and the stretch flangeability λ. As an index indicating the effect, the larger the value, the better the balance between the elongation El and the stretch flangeability λ.

  As shown in these tables, Steel No. 2-6, 8, 10, 12, 14, 16, 34, 36, 38, 40, 42, and 44, all have a ratio of Mn concentration in ferrite and martensite of 0.95 or more and close to 1. On the other hand, the composition of the invention steel and steel is the same, but the heating rate in the two-phase region is out of the recommended range. 1, 7, 9, 11, 13, 15, 33, 35, 37, 39, 41 and 43 all have a ratio of the Mn concentration of 0.91 or less and less than 0.95.

  Each of the above invention steels has a tensile strength TS equal to or higher than that of the comparative example, and the balance (E · λ) between the elongation El and the stretch flangeability λ is improved by 100% ·% or more as compared with each of the comparative steels. You can see that

  Further, the steel No. 2 in which the heating rate or the air cooling rate in the two-phase region is more preferable or in a particularly preferable range. 3 to 6 are steel Nos. Which are comparative steels. Steel No. 1 in which (El × λ) is 300% ·% or more higher than that of No. 1 and both the heating rate and the air cooling rate in the two-phase region are simply in the preferred range. The balance between elongation El and stretch flangeability λ is further improved than 2.

  On the other hand, Steel No. In Nos. 18 and 22, the annealing heating temperature is too low or the air cooling rate is too slow, so that the concentration of Mn on the martensite side increases between ferrite and martensite, and steel No. 18 which is a comparative steel. Compared to 1, the tensile strength TS is almost equal, but the balance between the elongation El and the stretch flangeability λ is inferior.

  Steel No. In Nos. 17 and 19, the annealing temperature is too high or the annealing holding time is too long, so that the prior austenite grains become coarse, and steel No. 1 which is a comparative steel. Compared to 1, the tensile strength TS can be ensured to be almost equal, but the balance between the elongation El and the stretch flangeability λ is not improved.

  Steel No. Nos. 20 and 21 have a martensite area ratio outside the proper range because the air cooling temperature is too high or too low.

  Steel No. In No. 24, ferrite is not introduced into the steel structure because the C content is too high.

  On the other hand, Steel No. In 23 and 25, the martensite area ratio is not ensured because the C content is too low or the Si content is too high.

  Steel No. In No. 26, since the Mn content is too low, the hardenability cannot be secured, the martensite area ratio is not sufficiently secured, and bainite is included, so that the balance between elongation and stretch flangeability is inferior.

  On the other hand, Steel No. In No. 27, since the Mn content is too high, austenite remains at the time of quenching (during rapid cooling after annealing), so the balance between elongation El and stretch flangeability λ is inferior.

Steel No. 28-32, the content of P, S, Al or N is out of the scope of the present invention, does not meet at least one of the requirements for defining the structure of the present invention, or slightly different component system Steel No. which is a comparative steel. As compared with steel No. 1, the Mn concentration ratio between ferrite and martensite is less than that of steel No. 1. Even if it is equal to 1, the tensile strength TS is inferior, or when the tensile strength TS is substantially equal, the absolute values of elongation El and stretch flangeability λ are remarkably inferior.

Claims (3)

% By mass (hereinafter the same for chemical components)
C: more than 0.03% and 0.30% or less,
Si: less than 3.0% (including 0%)
Mn: 0.5 to 5.0%,
P: less than 0.1%,
S: 0.005% or less,
N: 0.01% or less,
Al: containing more than 0.01% and 1.00% or less, with the balance being composed of iron and inevitable impurities,
It has a two-phase structure composed of tempered martensite with an area ratio of 5% or more and 95% or less and the balance of ferrite,
Ri average Mn concentration _{C Mn · alpha} and the average Mn concentration ratio _{C Mn · α / C Mn ·} M than 0.95 der the _{C Mn · M} in tempered martensite in said ferrite,
A high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability, characterized in that the product EL × λ of stretch EL and stretch flangeability λ is 925% ·% or more . Ingredient composition further
Cr: 0.01 to 1.0%,
Mo: 0.01 to 1.0%,
Cu: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
The high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to claim 1, comprising one or more of the following. Ingredient composition further
Ca: 0.0005 to 0.01% and / or Mg: 0.0005 to 0.01%
The high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to claim 1 or 2.