JP2009127089A - High-strength cold rolled steel sheet with excellent isotropy, elongation and stretch-flangeability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength cold rolled steel sheet which is improved in elongation and stretch-flangeability and reduced in anisotropy of elongation and has more excellent formability. <P>SOLUTION: The cold rolled steel sheet has a composition containing, by mass, 0.03 to 0.30% C, 0.1 to 3.0% Si, 0.1 to 5.0% Mn, ≤0.1% P, ≤0.005% S, ≤0.01% N, 0.01 to 1.00% Al and the balance iron with inevitable impurities, and has a structure containing tempered martensite with Hv hardness of ≤380 by ≥50% (including 100%) in area ratio and the balance ferrite, wherein as the state of distribution of cementite particles in the tempered martensite, the number of the cementite particles having ≥0.1 μm circle-equivalent diameter is ≤3 pieces per μm<SP>2</SP>of the tempered martensite, and the maximum degree of integration of (110) crystal plane in the ferrite is ≤1.7. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a high-strength steel sheet excellent in workability, and in particular, relates to a high-strength steel sheet having low mechanical property anisotropy and enhanced 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 a high-strength steel sheet having a tensile strength of 780 MPa or more, and both elongation (total elongation; El) and stretch flangeability (hole expansion ratio; λ) are enhanced, and mechanical characteristics, particularly elongation anisotropy. For example, a steel sheet with a tensile strength of 780 MPa class has a total elongation of 15% or more, a hole expansion ratio of 100% or more, and a steel sheet with a tensile strength of 980 MPa class has a total elongation of 10%. % And a hole expansion ratio of 100% or more, and an elongation anisotropy (difference in elongation between a rolling direction and a direction perpendicular to the rolling direction) is as small as possible (for example, less than 1%).

  In response to the above needs, many high-strength steel sheets with improved balance between stretch and stretch flangeability have been proposed based on various structural control concepts, but stretch and stretch flangeability satisfy the above required level. As a result, there has not yet been a product that has been made compatible, and there is still no improvement in the anisotropy of elongation.

  For example, Patent Document 1 discloses a high-tensile cold-rolled steel sheet that contains at least one of Mn, Cr, and Mo in a total amount of 1.6 to 2.5% by mass and is substantially composed of a single-phase structure of martensite. Although the hole expansion ratio (stretch flangeability) is 100% or more, the elongation does not reach 10% (see the invention example in Table 6 of the same document), and the anisotropic elongation There is no mention of sex.

  Patent Document 2 discloses a high-tensile steel sheet having a two-phase structure of ferrite with an area ratio of 65 to 85% and the balance being tempered martensite.

  Patent Document 3 discloses a high-tensile steel plate having a two-phase structure in which the average crystal grain sizes of ferrite and martensite are both 2 μm or less and the volume ratio of martensite is 20% or more and less than 60%. .

Although all the high-tensile steel sheets disclosed in Patent Documents 2 and 3 have an elongation of 10% or more, the hole expansion ratio (stretch flangeability) does not reach 100% (see Patent Document 2). The invention examples in Table 2 and the examples in Table 2 of Patent Document 3) are not mentioned.
JP 2002-161336 A JP 2004-256872 A JP 2004-232022 A

  Accordingly, an object of the present invention is to provide a high-strength cold-rolled steel sheet with improved formability, in which both elongation and stretch flangeability are enhanced and elongation anisotropy is reduced.

The invention described in claim 1
% By mass (hereinafter the same for chemical components)
C: 0.03 to 0.30%,
Si: 0.1 to 3.0%,
Mn: 0.1 to 5.0%,
P: 0.1% or less,
S: 0.005% or less,
N: 0.01% or less,
Al: 0.01 to 1.00%
And the balance has a component composition consisting of iron and inevitable impurities,
Tempered martensite having a hardness of 380 Hv or less contains 50% or more (including 100%) in area ratio, and the balance has a structure made of ferrite,
As a distribution state of the cementite particles in the tempered martensite, there are 3 or less cementite particles having an equivalent circle diameter of 0.1 μm or more per 1 μm ^{2 of the} tempered martensite, and
A high-strength cold-rolled steel sheet excellent in isotropy, elongation, and stretch flangeability, characterized in that the maximum degree of integration of the (110) crystal plane of the ferrite is 1.7 or less.

The invention described in claim 2
Ingredient composition further
Cr: 0.01-1.0% and / or Mo: 0.01-1.0%
The high strength cold-rolled steel sheet having excellent isotropy, elongation and stretch flangeability according to claim 1.

The invention according to claim 3
Ingredient composition further
Cu: 0.05-1.0% and / or Ni: 0.05-1.0%
The high-strength cold-rolled steel sheet having excellent isotropy, elongation, and stretch flangeability according to claim 1.

The invention according to claim 4
Furthermore,
Ca: 0.0005 to 0.01% and / or Mg: 0.0005 to 0.01%
The high-strength cold-rolled steel sheet having excellent isotropy, elongation and stretch flangeability according to any one of claims 1 to 3.

  According to the present invention, in a two-phase structure composed of ferrite and tempered martensite, the hardness and area ratio of tempered martensite, the distribution state of cementite particles in the tempered martensite, and the accumulation of (110) crystal planes of ferrite By properly controlling the degree, it is possible to improve stretch flangeability while ensuring stretch, and to reduce the anisotropy of stretch, so that a high-strength steel sheet with better formability can be provided. became.

  The present inventors pay attention to a high-strength steel sheet (see Patent Documents 2 and 3 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 anisotropy of the elongation is reduced and the stretch flangeability can be improved, it is considered that a high-strength steel sheet satisfying the above desired level can be obtained, and the influence of various factors on the stretch and stretch flangeability We have conducted intensive studies such as investigating. As a result, in addition to reducing the proportion of ferrite, reducing the hardness of the tempered martensite, and preventing the cementite particles precipitated in the martensite during tempering from becoming coarse, It was found that the difference in elongation between the rolling direction and the direction perpendicular to the rolling direction can be reduced by limiting the degree of integration of the (110) crystal plane of the ferrite to a predetermined value or less. It came to complete.

  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 2 and 3 above. In particular, the hardness of the tempered martensite is 380 Hv or less. It is different from the steel sheets of Patent Documents 2 and 3 in that it is controlled and the distribution state of cementite particles precipitated in the tempered martensite is controlled.

<Tempered martensite with a hardness of 380 Hv or less: 50% or more (including 100%) in area ratio>
By limiting the hardness of the tempered martensite and increasing the deformability of the tempered martensite, the stress concentration at the interface between the ferrite and the tempered martensite is suppressed, and cracking at the interface is prevented and the elongation is prevented. By ensuring the flangeability and making the structure mainly tempered martensite, high strength can be ensured even if the hardness of the tempered martensite is lowered.

  In order to effectively exhibit the above action, the tempered martensite has a hardness of 380 Hv or less (preferably 370 Hv or less, more preferably 350 Hv or less), and the tempered martensite has an area ratio of 50% or more, preferably 60%. Above, more preferably 70% or more (including 100%). The balance is ferrite.

<Cementite particles with an equivalent circle diameter of 0.1 μm or more: 3 or less per 1 μm ^{2 of} tempered martensite>
Stretch flangeability can be improved by controlling the size and number of cementite particles precipitated in martensite during tempering. That is, the stretch flangeability can be improved by reducing the number of coarse cementite particles that are the starting points of fracture when the stretch flange is deformed. In addition, in this manner, cementite particles having an appropriate size (for example, 0.02 μm or more and less than 0.1 μm) are dispersed in martensite along with preventing coarsening of the cementite particles. Working as a source increases the work hardening index and contributes to improved elongation.

In order to effectively exert the above action, coarse cementite particles having an equivalent circle diameter of 0.1 μm or more are limited to 3 or less, preferably 2.5 or less, more preferably 2 or less per 1 μm ^{2 of} tempered martensite. To do.

<Maximum degree of integration of (110) crystal plane of ferrite is 1.7 or less>
When the (110) crystal plane of ferrite (hereinafter sometimes referred to as “(110) α”) is excessively accumulated in a specific direction, the specific direction and the direction in which the (110) crystal plane is not accumulated so much are obtained. Since the slip system acting when stress is applied changes, the elongation varies depending on the direction of the tensile load. Therefore, by controlling the degree of integration of the (110) crystal plane of ferrite, it is possible to reduce mechanical properties, particularly the anisotropy of elongation (El).

  In order to effectively exhibit the anisotropy suppressing effect, the maximum degree of integration of the (110) crystal plane of ferrite is 1.7 or less, preferably 1.6 or less, and more preferably 1.5 or less.

  Hereinafter, a method for measuring the hardness and area ratio of tempered martensite, the size and number of cementite particles, and the degree of integration of the (110) plane will be described.

  First, regarding the area ratio of martensite, each test steel sheet was mirror-polished, corroded with a 3% nital solution to reveal the metal structure, and then a scanning type with a magnification of 20000 times for approximately 4 μm × 3 μm region 5 fields of view. An electron microscope (SEM) image was observed, and the area ratio of martensite was calculated from the area ratio of each area, with the area not containing cementite being ferrite and the remaining area being martensite by image analysis.

  Next, regarding the hardness of martensite, the Vickers hardness (98.07N) Hv of the surface of each test steel sheet is measured according to the test method of JIS Z 2244, and the hardness of martensite is expressed using the following formula (1). Conversion to HvM was performed.

HvM = (100 × Hv−VF × HvF) / VM (1)
However, HvF = 102 + 209 [% P] +27 [% Si] +10 [% Mn] +4 [% Mo] −10 [% Cr] +12 [% Cu] (Toshio Fujita et al .: “Design and Theory of Steel Materials” ( Maruzen Co., Ltd., published on September 30, 1981, p.10, Fig. 2.1, reads the degree of influence of each alloy element amount on the change in yield stress of low C ferritic steel (straight line) (Note that other elements such as Al and N do not affect the hardness of the ferrite.)
Here, HvF: hardness of ferrite, VF: area ratio (%) of ferrite, VM: area ratio (%) of martensite, [% X]: content (mass%) of component element X.

Regarding the size and the number of the cementite particles, each sample steel plate was mirror-polished and corroded with 3% nital to reveal the metal structure, and then the region inside the martensite was analyzed in a 100 μm ² region. A scanning electron microscope (SEM) image with a magnification of 10,000 times is observed for the field of view, and a white portion is marked as a cementite particle from the contrast of the image and marked, and the area of each marked cementite particle is circled by image analysis software. The equivalent diameter was calculated, and the number of cementite particles of a predetermined size existing per unit area was determined.

For the degree of integration of the (110) crystal plane of ferrite, see Japan Iron and Steel Institute, 3rd edition, Iron and Steel Handbook I Basics [Maruzen], p. The positive pole figure of the (110) crystal face of ferrite was created by the FM method described in 465, and the maximum value of the pole density was taken as the degree of integration.

  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: 0.03-0.30%
C is an important element that affects the area ratio of martensite and the amount of cementite precipitated in the martensite and affects the strength and stretch flangeability. If it is less than 0.03%, the strength cannot be ensured. On the other hand, if it exceeds 0.30%, the martensite hardness becomes too high, and stretch flangeability cannot be ensured. The range of C content is preferably 0.05 to 0.25%, more preferably 0.07 to 0.20%.

Si: 0.1-3.0%
Si has an effect of suppressing the coarsening of cementite particles during tempering, and is a useful element that contributes to both stretch flangeability by preventing the formation of coarse cementite particles. If it is less than 0.10%, the cementite particles are coarsened during tempering, so that stretch flangeability cannot be ensured. On the other hand, if it exceeds 3.0%, the formation of austenite during heating is inhibited, so the martensite area ratio is reduced. It cannot be ensured, and stretch flangeability cannot be ensured. The range of Si content becomes like this. Preferably it is 0.30-2.5%, More preferably, it is 0.50-2.0%.

Mn: 0.1 to 5.0%
Mn, like Si, has the effect of suppressing the cementite coarsening during tempering, and prevents the formation of coarse cementite particles, thereby contributing to improvement of stretch flangeability and ensuring hardenability. It is a useful element. If it is less than 0.1%, the cementite particles become coarse during tempering, so that stretch flangeability cannot be secured. On the other hand, if it exceeds 5.0%, austenite remains at the time of quenching (at the time of cooling after annealing), Reduces stretch flangeability. The range of Mn content is preferably 0.30 to 2.5%, more preferably 0.50 to 2.0%.

P: 0.1% or less P is inevitably present as an impurity element and contributes to an increase in strength by solid solution strengthening, but segregates at the prior austenite grain boundaries and embrittles the grain boundaries to increase stretch flangeability. Since it deteriorates, it is made 0.1% or less. 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: 0.01 to 1.00%
Al combines with N to form AlN and reduces the solid solution N that contributes to the occurrence of strain aging, thereby preventing the stretch flangeability from deteriorating and contributing to the strength improvement by solid solution strengthening. If it is less than 0.01%, 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: 0.01-1.0% and / or Mo: 0.01-1.0%
These elements are useful elements for increasing the precipitation strengthening amount while suppressing deterioration of stretch flangeability by precipitating as fine carbides instead of cementite. Addition of less than 0.01% of each element cannot effectively exert the above-described effect, while addition of more than 1.0% of each element results in excessive precipitation strengthening and high martensite hardness. It will become too long and flangeability will fall.

Cu: 0.05 to 1.0% and / or Ni: 0.05 to 1.0%
These elements are elements useful for improving the balance between elongation and stretch flangeability because it becomes easy to obtain moderately fine cementite by suppressing the growth of cementite. When Cu and Ni are added at less than 0.05%, and B is added at less than 0.0002%, the above effects cannot be exhibited effectively. On the other hand, if each element exceeds 1.0%, austenite remains at the time of quenching and the stretch flangeability is deteriorated.

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.

  And after the said cold rolling, annealing, re-annealing, and further tempering are performed.

[Annealing conditions]
As annealing conditions, heating to Ac3 point or higher (may be repeated to more than Ac3 point if necessary), sufficiently austenite single phase is formed, and then cooled to 200 ° C. or lower. The cooling method is arbitrary. This suppresses the accumulation of the (110) crystal plane of the ferrite in a specific direction.

[Re-annealing conditions]
As re-annealing conditions, re-annealing heating temperature: [(Ac1 + Ac3) / 2] to 1000 ° C., re-annealing holding time: 3600 s or less, and then from re-annealing heating temperature to directly below Ms point 50 ° C. The cooling rate is 1 ° C./s or more from the re-annealing heating temperature to the temperature of 600 ° C. or more (first cooling end temperature) below the re-annealing heating temperature (first cooling). After cooling slowly at a rate), it is preferable to rapidly cool to a temperature below the Ms point (second cooling end temperature) at a cooling rate (second cooling rate) of 50 ° C./s or less.

<Re-annealing heating temperature: [(Ac1 + Ac3) / 2] to 1000 ° C., re-annealing holding time: 3600 s or less>
This is because the area ratio of martensite that is sufficiently transformed into austenite during re-annealing heating and transformed from austenite during subsequent cooling is secured by 50% or more.

  If the re-annealing heating temperature is less than [(Ac1 + Ac3) / 2] ° C., the amount of transformation to austenite is insufficient during re-annealing heating, so the amount of martensite that is transformed from austenite during subsequent cooling is reduced, resulting in an area ratio. On the other hand, if the temperature exceeds 1000 ° C., the austenite structure becomes coarse and the bendability and toughness of the steel sheet deteriorate, and the annealing equipment deteriorates.

  Further, if the re-annealing holding time exceeds 3600 s, productivity is extremely deteriorated, which is not preferable.

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

  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.

<Slow cooling at a cooling rate of 1 ° C./s or higher to a temperature of 600 ° C. or higher below the re-annealing heating temperature>
This is because by forming a ferrite structure having an area ratio of less than 50%, the elongation can be improved while the stretch flangeability is secured.

  When the temperature is less than 600 ° C. or the cooling rate is less than 1 ° C./s, ferrite is not formed, and the strength and stretch flangeability cannot be secured.

[Tempering conditions]
As the tempering conditions, the temperature after re-annealing cooling to the first tempering heating temperature: 325 to 375 ° C., between 100 to 325 ° C. is heated at an average heating rate of 5 ° C./s or more, and the first step. Tempering holding time: after holding for 50 s or more, the second tempering heating temperature T: heated to 400 ° C. or more, and the second tempering holding time t (s) is Pg = exp [−9649 / ( T + 273)] × t <1.2 × 10 ⁻³ , and Pt = (T + 273) [log (t) +17]. When the temperature T is changed during the second stage holding, the following formula (2) may be used as Pg.

  Cementite particles are uniformly precipitated in the martensite structure by holding at around 350 ° C, which is the temperature range where the precipitation of cementite from martensite is the fastest, and then heated and held at a higher temperature range, so that cementite This is because the particles can be grown to an appropriate size.

<First tempering heating temperature: Heating between 325 and 375 ° C., between 100 and 325 ° C. at an average heating rate of 5 ° C./s or more>
When the first-stage tempering heating temperature is less than 325 ° C or more than 375 ° C, or the average heating rate between 100 to 325 ° C is less than 5 ° C / s, the precipitation of cementite particles in the martensite is uneven. As a result, the ratio of coarse cementite particles increases due to subsequent growth during heating and holding in the second stage, and stretch flangeability cannot be obtained.

<Second-stage tempering heating temperature T: Heated to 400 ° C. or higher, and second-stage tempering holding time t (s) is Pg = exp [−9649 / (T + 273)] × t <1.2 × 10 ^{− 3} and hold under the condition of Pt = (T + 273) [log (t) +17]>
Here, Pg = exp [−9649 / (T + 273)] × t is a precipitate grain growth model described in the formula (4.18) of Koichi Sugimoto et al .: Material Histology [Asakura Shoten Publishing], p106. It is a parameter that defines the size of cementite particles as precipitates, with variables originally set and simplified.

  In addition, Pt = (T + 273) [log (t) +17] is obtained from the Japan Institute of Metals: Iron and Steel Materials Course, Modern Metallurgy Materials 4, p. 50 is a parameter that defines the hardness of tempered martensite.

  When the second stage tempering heating temperature T is less than 400 ° C., the holding time t required for growing the cementite particles to an appropriate size becomes too long.

When Pg = exp [−9649 / (T + 273)] × t ≧ 1.2 × 10 ⁻³ , the cementite particles are coarsened, and the number of cementite particles of 0.1 μm or more becomes too large, so that stretch flangeability can be secured. Disappear.

Further, when Pt = (T + 273) [log (t) +17] <13600, the hardness of martensite is not sufficiently lowered, and stretch flangeability cannot be secured.

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

  For each steel plate after heat treatment, the area ratio of martensite and its hardness, as well as the size and number of cementite particles were measured by the measurement method described in the above [Best Mode for Carrying Out the Invention]. did.

Furthermore, for each steel sheet, tensile strength TS, measured elongation El _C, and stretch flangeability λ of L direction elongation El _L and C in (rolling direction) (rolling direction and perpendicular direction). Note that the tensile strength TS and the elongation El _{C in the C} direction have a major axis in the direction perpendicular to the rolling direction, and the elongation El _{L in the} L direction has a major axis along the rolling direction, and are described in JIS Z 2201. No. test piece was prepared and measured according to JIS Z 2241. Then, the difference ΔEl = El _L −E1 _C between the elongation in the L direction and the C direction _was calculated, and a case where ΔEl was less than 1% was regarded as passing with a small elongation anisotropy. 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.

  Table 3 shows the measurement results.

  As shown in the table, Steel No. 1 to 3, 5, 7, 10, 11, 13 to 17, 25 all have an elongation El of 15% or more and a stretch flangeability (hole expansion ratio) λ of 100 when the tensile strength TS is 780 MPa or more. When the tensile strength TS is 980 MPa or more, the elongation El is 10% or more, the stretch flangeability (hole expansion ratio) λ satisfies 100% or more, and the elongation anisotropy is small. ] A high-strength cold-rolled steel sheet having the isotropy and elongation and stretch flangeability that satisfies the demand level described in the section] was obtained.

  On the other hand, steel No. which is a comparative example. 4, 6, 8, 9, 12, 19 to 24, 26, and 27 are inferior in any of the characteristics.

  For example, steel no. No. 4 has a martensite area ratio of less than 50% and is excellent in elongation. However, the tensile strength and stretch flangeability are inferior, and the maximum integration degree of (110) α exceeds 1.7. Isotropic.

  Steel No. No. 6 has an area ratio of martensite of 50% or more because the C content is too high, but in addition to its hardness being too high, there are too many coarsened cementite particles, so it has excellent tensile strength. Although the elongation anisotropy is small, both the absolute value of elongation and stretch flangeability are inferior.

  Steel No. No. 8, because the Si content is too high, the area ratio of martensite is less than 50%, and its hardness is too high, so it has excellent tensile strength and elongation, and has low elongation anisotropy, but stretch flangeability Is inferior.

  Steel No. In No. 9, the cementite particles are coarsened due to the Mn content being too low, and the tensile strength and elongation are excellent, and the elongation anisotropy is small, but the stretch flangeability is inferior.

  Steel No. No. 12, since the austenite remains at the time of quenching (during cooling after annealing) because the Mn content is too high, the tensile strength and elongation are excellent, and the elongation anisotropy is small, but the stretch flangeability is low. Inferior.

  Steel No. Nos. 18 to 24 do not satisfy the requirements for defining the hardness of martensite or the dispersion state of cementite particles because the re-annealing conditions or tempering conditions are out of the recommended range, and excellent tensile strength and anisotropy of elongation. Is small, but at least the stretch flangeability is inferior.

  Steel No. Nos. 26 and 27 do not satisfy the requirement to define the degree of accumulation of (110) α because the annealing conditions are out of the recommended range, and the tensile strength, the absolute value of elongation and the stretch flangeability are excellent. The anisotropy of is increased.

  Incidentally, the distribution state of cementite particles in the martensite structure of the invention example (steel No. 2) and the comparative example (steel No. 19) is illustrated in FIG. In the inventive example, the cementite particles of an appropriate size are uniformly dispersed, whereas in the comparative example, it is recognized that a large number of coarse cementite particles are present.

Further, FIG. 2 illustrates a positive electrode diagram of (110) α by the FM method of the invention example (steel No. 2) and the comparative example (steel No. 26). It can be seen that the inventive examples are clearly less anisotropic than the comparative examples.

It is a figure which shows the distribution state of the cementite particle | grains in a martensitic structure. It is a positive electrode dot diagram of the (110) crystal plane of ferrite.

Claims (4)

% By mass (hereinafter the same for chemical components)
C: 0.03 to 0.30%,
Si: 0.1 to 3.0%,
Mn: 0.1 to 5.0%,
P: 0.1% or less,
S: 0.005% or less,
N: 0.01% or less,
Al: 0.01 to 1.00%
And the remainder has a component composition consisting of iron and inevitable impurities,
Tempered martensite having a hardness of 380 Hv or less contains 50% or more (including 100%) in area ratio, and the balance has a structure made of ferrite,
As a distribution state of the cementite particles in the tempered martensite, there are 3 or less cementite particles having an equivalent circle diameter of 0.1 μm or more per 1 μm ^{2 of the} tempered martensite, and
The maximum integration degree of the (110) crystal face of the ferrite is 1.7 or less. A high-strength cold-rolled steel sheet excellent in isotropy, elongation and stretch flangeability. Ingredient composition further
Cr: 0.01-1.0% and / or Mo: 0.01-1.0%
The high-strength cold-rolled steel sheet having excellent isotropy, elongation and stretch flangeability according to claim 1. Ingredient composition further
Cu: 0.05-1.0% and / or Ni: 0.05-1.0%
The high-strength cold-rolled steel sheet having excellent isotropy, elongation, and stretch flangeability according to claim 1 or 2. Furthermore,
Ca: 0.0005 to 0.01% and / or Mg: 0.0005 to 0.01%
The high-strength cold-rolled steel sheet having excellent isotropy, elongation and stretch flangeability according to any one of claims 1 to 3.

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