JP2013181183A - High strength cold rolled steel sheet having low in-plane anisotropy of yield strength, and method of producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength cold rolled steel sheet having low in-plane anisotropy of yield strength and excellent press formability.SOLUTION: A high strength cold rolled steel sheet has a composition comprising by mass%, C: 0.06 to 0.12%, Si: not more than 0.7%, Mn: 1.2 to 2.6%, P: not more than 0.020%, S: not more than 0.03%, sol. Al: 0.01 to 0.5%, and N: not more than 0.005%, with the balance being Fe and inevitable impurities, and has a structure comprising a ferrite phase in a volume ratio of 60% or more as a main phase and a martensite phase in a volume ratio of 5% or more and not more than 20%, relative to the whole steel sheet structure, with a three dimensional crystal orientation distribution function at {φ1, Φ, φ2}={0°, 35°, 45°} made to be not higher than 2.5.

Description

  The present invention relates to a high-strength cold-rolled steel sheet having a low in-plane anisotropy of yield strength, which is useful for the use of steel sheets for automobiles, and a method for producing the same.

In recent years, in order to suppress CO ₂ emissions from the viewpoint of global environmental conservation, there has been a demand for improved fuel efficiency of automobiles. In addition, in order to ensure the safety of passengers in the event of a collision, it is also required to improve safety centering on the collision characteristics of the automobile body. For this reason, the weight reduction and reinforcement of the automobile body are being actively promoted.
In order to satisfy the weight reduction and strengthening of automobile bodies at the same time, it is effective to reduce the plate thickness within the range where rigidity does not become a problem by increasing the strength of component materials. Actively used for parts.

The weight reduction effect increases as the strength of the steel plate increases. In the automobile industry, for example, as a skeletal structure material for absorbing shock, there is a trend to use a steel plate having a tensile strength (TS) of 500 MPa or more, or even 590 MPa or more. .
On the other hand, since many automobile parts made of steel plates are formed by press working, the automobile steel plates are required to have excellent press formability. However, high strength steel sheets are required to be improved because press formability, ductility, and deep drawability are greatly deteriorated as compared with ordinary mild steel sheets.

For example, up to 440 MPa class, high strength steel sheets are based on steel that has been converted to IF (Interstitial free) by adding Ti and Nb in solid solution C and solid solution N to a very low carbon steel sheet with excellent formability. There are steel sheets to which solid solution strengthening elements such as Si, Mn, and P are added.
Further, at 500 MPa or higher, or even 590 MPa or higher, a composite structure steel plate is put into practical use, and there are a DP steel plate having a two-phase structure of ferrite and martensite and a TRIP steel plate utilizing residual γ. The former is characterized by low yield strength and high work hardening ability due to residual strain around martensite. The latter has a feature that uniform elongation is increased by plasticity-induced martensitic transformation.

In general, the mechanical properties of a high-strength steel sheet may be evaluated by tensile properties in a specific direction such as a direction perpendicular to rolling, and the in-plane anisotropy may be evaluated by an in-plane anisotropy Δr of Rankford value (r value). Here, Δr is obtained by the following formula using Rankford values r _L , r _D , r _{C in} 0 ° (L direction), 45 ° (D direction) and 90 ° direction (C direction) with respect to the rolling direction. It is done.
Δr = (r _L + r _C −2r _D ) / 2

  However, analysis of actual press forming has revealed that the shape freezing property and surface distortion after forming a part are greatly influenced by the in-plane anisotropy of the yield strength. Therefore, improvement in press formability can be expected by reducing the in-plane anisotropy of yield strength.

  Regarding a steel sheet having a small in-plane anisotropy, for example, Patent Document 1 discloses a cold-rolled steel sheet for automobile outer panel components having excellent bake hardenability and a small in-plane anisotropy, and a method for manufacturing the same. . In this technique, Δr is defined by the amount of C and the rolling reduction during cold rolling, and both in-plane anisotropy and dent resistance can be achieved. Moreover, it is necessary to start cooling within 2 seconds after hot rolling and to cool at a cooling rate of 70 ° C./s or more over a temperature range of 100 ° C. or more. However, the in-plane anisotropy here is Δr and does not necessarily match the in-plane anisotropy of the yield strength.

As for steel sheets relating to ductile in-plane anisotropy, for example, Patent Document 2 discloses a steel sheet having excellent shape freezing property and a method for manufacturing the same. Such a steel sheet is a composite structure steel having a volume fraction of ferrite or bainite as a maximum and containing martensite in a volume fraction of 1% or more and 25% or less, and is a sheet having a thickness of at least 1/2 to 1/4. Surface,
(1) The average value (A) of the X-ray random intensity ratio of {100} <011> to {223} <110> orientation group is 4.0 or more,
(2) The average value (B) of the X-ray random intensity ratio of the three crystal orientations of {554} <225>, {111} <112> and {111} <110> is 5.5 or less,
(3) (A) / (B) ≧ 1.5
(4) {100} <011> X-ray reflection random intensity ratio is equal to or greater than {211} <011> X-ray random intensity ratio,
And at least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction is 0.7 or less, and the anisotropy ΔuE1 of uniform elongation is 4% or less, and the local elongation is The anisotropy ΔLE1 is 2% or more and ΔuEl is ΔLE1 or less.
However,
ΔuEl = {| uEl (L) −uEl (45 °) | + | uEl (C)
-UEl (45 °) |} / 2
ΔLEl = {| LEl (L) −LEl (45 °) | + | LEl (C)
-LEl (45 °) |} / 2
The uniform elongation in the direction parallel to the rolling direction (L direction), vertical (C direction) and 45 ° is defined as uEl (L), uEl (C) and uEl (45 °), respectively, and parallel to the rolling direction (L Direction), vertical (C direction), and local elongation in the 45 ° direction are denoted by LEl (L), LEl (C), and LEl (45 °), respectively. In addition, it is necessary to optimize the hot rolling finishing conditions and to perform winding at a critical temperature or lower according to the Mn equivalent.

  However, there is a problem in that the development of the {100} <011> texture decreases the drawability, and the relationship between the yield strength and the in-plane anisotropy is not clear.

JP 2004-197155 A JP 2005-256020 A

  As described above, conventional steel sheets for automobiles have focused on improving the r value and elongation to improve the formability. However, as disclosed in Patent Document 1, in order to obtain a steel sheet having a small r-value in-plane anisotropy, the technique of rapid cooling after hot rolling to obtain a bainite structure has a problem that the strength level is limited. Further, in Patent Document 2, there is a problem that the phase ratio of the structure is likely to change as the manufacturing conditions change, and that the in-plane anisotropy, particularly the in-plane anisotropy of the yield strength, cannot be reduced when the structure changes. It was.

  The present invention advantageously solves the above problems, particularly focusing on the yield strength, improving the press formability by reducing the in-plane anisotropy, the tensile strength (TS) is 500 MPa or more, Alternatively, another object is to provide a high-strength cold-rolled steel sheet having a very low in-plane anisotropy of yield strength even with a high strength of 590 MPa or more together with its manufacturing method.

Generally, in the texture of cold-rolled steel sheets, it is said that an α fiber whose <100> direction is parallel to the RD direction and a γ fiber whose <111> direction is parallel to the ND direction are developed. The value becomes higher.
Here, the orientation group of α fibers appearing in the texture of the cold-rolled steel sheet is {001} <110> to {111} <110>, and is represented by a three-dimensional orientation space in which three variables of Euler angles are taken as a rectangular coordinate axis. Φ1 = 0 °, φ2 = 45 °, and φ = 0-55 °.

The inventors of the present invention have made extensive studies to solve the above-mentioned problems. As a result, the mechanical characteristics, particularly the in-plane anisotropy of the yield strength, have a specific orientation ({φ1, Φ, φ2} = { 0 °, 35 °, 45 °}) and strongly correlated with the three-dimensional crystal orientation distribution function, and not related to the accumulation of γ fibers related to other orientations, for example, the r value, which is an index of deep drawability. I found.
As a result of further research, the in-plane anisotropy of yield strength is affected by the microstructure, and the three-dimensional crystal orientation distribution function at {φ1, Φ, φ2} = {0 °, 35 °, 45 °} It was found that the in-plane anisotropy of the yield strength can be stably reduced by controlling the volume fraction of the martensite phase with respect to the entire steel sheet structure.

The present invention has been developed based on the above findings, and the gist thereof is as follows.
(1) In mass%,
C: 0.06 to 0.12%,
Si: 0.7% or less,
Mn: 1.2 to 2.6%
P: 0.020% or less,
S: 0.03% or less,
sol.Al: 0.01 to 0.5% and N: 0.005% or less, the balance is made of Fe and inevitable impurities, and is a volume fraction relative to the whole steel sheet structure, with the ferrite phase as the main phase. %, And the martensite phase is 5% to 20%, and the three-dimensional crystal orientation distribution function at {φ1, Φ, φ2} = {0 °, 35 °, 45 °} is 2.5 or less. A high-strength cold-rolled steel sheet with low in-plane anisotropy of yield strength.

(2) The steel sheet further contains at least one selected from Cr: 0.5% or less and Mo: 0.5% or less in terms of mass%, as described in (1) above High strength cold-rolled steel sheet with low in-plane anisotropy of yield strength.

(3) A steel slab having the component composition according to any one of (1) to (2) above is hot-rolled at a finishing temperature of 840 ° C. or higher and 950 ° C. or lower, and then a reduction rate of 30% or higher and 70% or lower. After cold rolling, annealing is performed at a temperature of 800 ° C. or more and A ₃ points or less, and the temperature range from the annealing temperature to 400 ° C. is cooled at a rate equal to or higher than the critical cooling rate CR (° C./s) represented by the following formula. A method for producing a high-strength cold-rolled steel sheet having low in-plane anisotropy of yield strength.
Record
logCR = −3.50 [% Mo] −1.20 [% Mn] −2.0 [% Cr] −0.32 [% P] +3.50
However, [% M] is the content of element M in steel (mass%).

  According to the present invention, a high-strength cold-rolled steel sheet having low in-plane anisotropy of yield strength and excellent press formability can be obtained. Therefore, the high-strength cold-rolled steel sheet of the present invention is extremely useful when applied to automobile parts.

Hereinafter, the present invention will be specifically described.
First, the reason why the component composition is limited to the above range will be described. The unit of the content of each element is mass% unless otherwise specified.
C: 0.06 to 0.12%
C is an element necessary for ensuring a predetermined amount of the second phase fraction and increasing the strength and controlling the in-plane anisotropy of the yield strength. If the amount of C is less than 0.06%, it is difficult to ensure 5% or more of the martensite phase, which is not preferable. On the other hand, if the amount of C exceeds 0.12%, the proportion of the second phase, which is a structure other than the ferrite phase, increases, and the volume fraction of the ferrite phase cannot be increased to 60% or more, resulting in reduced ductility. . In addition, since the second phase such as the martensite phase forms a network and surrounds the ferrite, the influence of the texture structure of the ferrite phase is difficult to be exhibited, and it is difficult to control the in-plane anisotropy of the yield strength. . Accordingly, the C content is in the range of 0.06 to 0.12%. Preferably it is 0.06 to 0.10% of range.

Si: 0.7% or less
In addition to the effect of improving the surface quality by delaying the scale formation in hot rolling, Si has an effect of moderately delaying the alloying reaction between the iron and zinc in the plating bath or alloying treatment, This has the effect of increasing the work hardening ability of the ferrite phase. From this viewpoint, it is preferable to contain approximately 0.01% or more. However, when the Si content exceeds 0.7%, the appearance quality deteriorates. Therefore, the Si content is 0.7% or less. Preferably it is 0.3% or less.

Mn: 1.2 to 2.6%
Mn is added to increase hardenability and increase the ratio of martensite in the second phase. From the viewpoint of such complex organization, the lower limit of the Mn content is 1.2%. On the other hand, if the amount of Mn is too large, the α → γ transformation temperature in the annealing process is lowered, and γ grains are formed at the fine ferrite grain boundaries immediately after recrystallization or at the interface between recovered grains in the middle of recrystallization. For this reason, not only does the second phase become fine and ductility is lowered, but also the in-plane anisotropy of the yield strength cannot be controlled. From such a viewpoint, the upper limit of the Mn content is 2.6%. Preferably it is 1.2 to 2.1% of range.
Since the amount of martensite produced varies depending on the cooling rate after annealing, it is necessary to control the cooling rate in accordance with the amounts of Mn, Cr, and Mo as will be described later.

P: 0.020% or less When the amount of P exceeds 0.020%, surface defects due to deterioration of weldability or segregation occur. Therefore, the P content is 0.020% or less.

S: 0.03% or less S has an effect of improving the primary scale peelability of the steel sheet and improving the plating appearance quality. However, when the amount of S increases, the amount of MnS precipitated in the steel increases too much, reducing the ductility such as the elongation and stretch flangeability of the steel sheet, and the press formability. Moreover, the hot ductility at the time of hot-rolling a slab is reduced, and surface defects are easily generated. Furthermore, the corrosion resistance is slightly reduced. From such a viewpoint, the S amount is set to 0.03% or less. Preferably it is 0.01% or less, More preferably, it is 0.005% or less, More preferably, it is 0.002% or less.

sol.Al: 0.01 to 0.5%
In addition to being useful as a deoxidizing element for steel, sol.Al has the effect of improving the normal temperature aging resistance by fixing solute N present as an impurity, so the amount of sol.Al is 0.01% or more. To do. On the other hand, if the amount of sol.Al exceeds 0.5%, it leads to an increase in cost and further induces surface defects. Therefore, the amount of sol.Al is 0.01 to 0.5%.

N: 0.005% or less When N is too much, the aging resistance at room temperature is deteriorated, and a large amount of Al or Ti is required to fix solute N, so it is preferable to reduce it as much as possible. From such a viewpoint, the N amount is set to 0.005% or less.

Although the basic components have been described above, in the present invention, the following elements can be appropriately added as needed.
Cr: 0.5% or less
Cr, like Mn, is an element that contributes to high strength through complex organization. In order to obtain this effect, the Cr content is preferably 0.1% or more. However, excessive addition of Cr not only saturates this effect, but also increases costs. Therefore, the Cr content is 0.5% or less.

Mo: 0.5% or less
Mo is an element that improves hardenability, suppresses the formation of pearlite, and contributes to high strength. In order to obtain this effect, the Mo amount is preferably 0.1% or more. However, Mo is an extremely expensive element, and a large amount of addition leads to a significant cost increase. From such a viewpoint, the Mo amount is set to 0.5% or less.
Since the amount of martensite produced varies depending on the cooling rate after annealing, it is necessary to control the cooling rate in accordance with the amounts of Mn, Cr, and Mo as will be described later.

  In the steel sheet of the present invention, components other than those described above are Fe and inevitable impurities. However, as long as the effects of the present invention are not impaired, the inclusion of components other than those described above is not rejected.

Next, the reason why the steel structure ratio in the steel sheet of the present invention is limited to the above range will be described.
Ferrite phase: 60% or more in volume fraction In the present invention, the ferrite texture is controlled, and if the ferrite phase becomes too small, that is, if the second phase, which is a phase other than ferrite, increases, the texture is controlled. It becomes difficult to control the in-plane anisotropy of the yield strength. Furthermore, the second phase such as the martensite phase surrounds the periphery of the ferrite on the network, so that the macro plastic behavior of the steel sheet does not depend on the crystal orientation of the ferrite. From such a viewpoint, the ferrite phase is included in a volume fraction of 60% or more with respect to the entire structure of the steel sheet. Preferably it is 75% or more.

Martensite phase: 5% or more and 20% or less in volume fraction The martensite phase is a useful structure that not only contributes to increasing the strength, but also reduces the yield ratio and improves the shape freezing property. From such a viewpoint, the martensite phase is included in a volume fraction of 5% or more with respect to the entire structure of the steel sheet. On the other hand, from the viewpoint of controlling the in-plane anisotropy of the yield strength, when the martensite phase exceeds 20%, the martensite surrounds the periphery of the ferrite on the network, thereby controlling the texture of the ferrite. The meaning is lost and it is not preferable. Accordingly, the martensite phase is in the range of 5% or more and 20% or less in terms of volume fraction relative to the entire steel sheet structure.
In the steel sheet of the present invention, it is preferable that the above-described ferrite phase is a main phase and the second phase is mainly composed of a martensite phase. The other phases other than the above-described ferrite phase and martensite phase are preferably 5% or less, more preferably 3% or less, in terms of volume fraction relative to the entire structure of the steel sheet.

Here, the volume fraction of each phase was measured by the point count method (based on ASTM E562-83 (1988)), and the area fraction was defined as the volume fraction. The area ratio of each phase was obtained by collecting a test piece from each of the obtained cold-rolled annealed plates, and corroding the vertical section (L section) parallel to the rolling direction with nital after polishing, and using a scanning electron microscope (SEM). In addition, the type of phase was identified by observation at 4000 times, and the area ratio of the ferrite phase as the main phase and the area ratio of the martensite phase were determined.
In the structure photograph, ferrite is a slightly black contrast region, and the region where carbides are generated in a lamellar or dot array is pearlite and bainite, and particles with white contrast are martensite.

{φ1, Φ, φ2} = 3-dimensional crystal orientation distribution function at {0 °, 35 °, 45 °}: 2.5 or less Further, the texture in the steel sheet of the present invention was evaluated by a three-dimensional crystal orientation distribution function. . Conventionally, pole figures by X-ray diffraction (XRD) have been used for texture analysis. The pole figure represents a statistical crystal orientation distribution regarding a large number of crystal grains, and is therefore a method suitable for determining the preferred orientation. However, the texture of polycrystalline materials often exhibits a number of preferred orientations as well as a single preferred orientation. For example, in a fiber texture that is an orientation group rotated around a certain crystal axis, it is difficult to accurately evaluate the existence ratio of each orientation from the pole figure. Therefore, a three-dimensional crystal orientation distribution function was created based on the information of the pole figure, and the existence ratio of each orientation was evaluated.

When evaluating the three-dimensional crystal orientation distribution function, it was determined by the series expansion method from the incomplete pole figures of (200), (211), and (110) obtained by the reflection method.
As a result, in the steel structure having the volume fraction of the ferrite phase and martensite phase as described above, the α fiber has a specific orientation ({φ1, Φ, φ2} = {0 °, 35 °, 45 °}). It was investigated that the in-plane anisotropy of the yield strength is reduced when the three-dimensional crystal orientation distribution function is 2.5 or less. However, it is important that the volume fraction of the ferrite phase and martensite phase is controlled as described above. For example, in the case of a ferrite single phase, the optimum ferrite that can reduce the in-plane anisotropy of the yield strength The texture is different from the above.

  The reason why the in-plane anisotropy of the yield strength is small when the three-dimensional crystal orientation distribution function at {φ1, Φ, φ2} = {0 °, 35 °, 45 °} is 2.5 or less is not necessarily Although not clear, the inventors think as follows. In general, the crystal orientation of {φ1, Φ, φ2} = {0 °, 35 °, 45 °} is likely to occur in ferrite transformed from cold rolling or processed austenite, and its three-dimensional crystal orientation distribution function If it is high, the in-plane anisotropy of mechanical properties tends to be increased. Therefore, in order to reduce the in-plane anisotropy, it is necessary to control the three-dimensional crystal orientation distribution function within a certain range. However, the optimum value varies depending on the steel type. In particular, in a composite steel sheet containing 60% or more of the ferrite phase as the main phase of the present application and 5 to 20% of the martensite phase, the three-dimensional crystal orientation distribution function may be 2.5 or less. Is optimal.

Next, the manufacturing method of this invention is demonstrated.
First, the steel slab to be used is desirably produced by a continuous casting method in order to prevent macro segregation of components, but may be produced by an ingot-making method or a thin slab casting method. In addition to the conventional method in which the slab is manufactured and then cooled to room temperature and then reheated, it is charged directly into the heating furnace without being cooled and placed in a heating furnace and hot rolled, or a little heat retention After carrying out, an energy saving process such as direct rolling which is immediately hot rolled can be applied without any problem.

  The slab heating temperature is preferably low because the precipitates are coarsened to develop a {111} recrystallized texture and improve deep drawability. However, when the slab heating temperature is less than 1000 ° C., the rolling load increases, and the risk of trouble occurring during hot rolling increases. For this reason, it is preferable that slab heating temperature shall be 1000 degreeC or more. Further, from the viewpoint of increase in scale loss accompanying an increase in oxidized weight, the upper limit of the slab heating temperature is preferably 1300 ° C.

  The steel slab heated under the above conditions is subjected to hot rolling consisting of rough rolling and finish rolling. Here, the steel slab is made into a sheet bar by rough rolling. The conditions for rough rolling need not be specified, and may be performed according to a conventional method. From the viewpoint of lowering the slab heating temperature and preventing troubles during hot rolling, it is effective to utilize a so-called sheet bar heater that heats the sheet bar.

Finishing temperature: 840 ° C. or more and 950 ° C. or less Next, the sheet bar is finish-rolled to obtain a hot-rolled steel sheet. At this time, the finishing temperature, that is, the finish rolling outlet temperature (FT) is set to 840 ° C. or more and 950 ° C. or less. This is to obtain a texture preferable for in-plane anisotropy of yield strength after cold rolling and recrystallization annealing. When FT is less than 840 ° C., not only the load during hot rolling becomes high, but also in some component systems, ferrite zone rolling occurs and the texture changes greatly. On the other hand, if the FT exceeds 950 ° C., not only the structure becomes coarse, but also the rolling cannot be sufficiently performed in the austenite non-recrystallized state, so that the in-plane anisotropy of the yield strength increases after cold rolling annealing.

  Moreover, in order to reduce the rolling load at the time of hot rolling, it is good also as lubrication rolling between some or all passes of finishing rolling. Performing the lubrication rolling is effective from the viewpoint of uniforming the shape of the steel sheet and homogenizing the material. The coefficient of friction during lubrication rolling is preferably in the range of 0.10 to 0.25. Furthermore, it is preferable from the viewpoint of the operational stability of hot rolling to join successive sheet bars together and to perform continuous rolling.

  The coil winding temperature (CT) is not particularly specified, but is preferably 400 ° C. or higher and 720 ° C. or lower. In particular, when CT exceeds the upper limit, the crystal grains become coarse and the strength is reduced.

Reduction ratio in cold rolling: 30% or more and 70% or less The hot-rolled steel sheet obtained as described above is cold-rolled. The hot-rolled steel sheet before cold rolling is preferably pickled before cold rolling in order to remove scale. Pickling may be performed under normal conditions. If the rolling reduction during cold rolling is less than 30%, the recrystallization speed changes and it becomes difficult to control the anisotropy of the yield strength. On the other hand, if it exceeds 70%, strain is locally introduced around the carbide precipitated during hot rolling, and the texture of the ferrite after annealing begins to change greatly, making it difficult to obtain the desired texture. It becomes. Therefore, the rolling reduction in cold rolling is 30% or more and 70% or less.

Annealing temperature: 800 ° C. or more and A ₃ points or less The cold-rolled steel sheet obtained as described above is heated to a range of 800 ° C. or more and A ₃ points or less and annealed in the same temperature range. If the annealing temperature is less than 800 ° C., the γ fraction during soaking cannot be secured, and a sufficient martensite phase cannot be formed after cooling. On the other hand, the annealing temperature is more than _three points A, the higher the γ fraction, the texture after the inverse transformation greatly changes, it is difficult to obtain the desired texture. Accordingly, the annealing temperature is set to 800 ° C. or more and A ₃ points or less.

Cooling rate in a temperature range from the annealing temperature to at least 400 ° C .: critical cooling rate CR (° C./s) or more In order to form a martensite phase having a predetermined volume fraction, the cold-rolled steel plate annealed as described above is used. The temperature range from the annealing temperature to at least 400 ° C. is cooled at a rate equal to or higher than the critical cooling rate CR (° C./s) represented by the following formula.
logCR = −3.50 [% Mo] −1.20 [% Mn] −2.0 [% Cr] −0.32 [% P] +3.50
However, [% M] is the content of element M in steel (mass%).

  When the average cooling rate in the temperature range is less than the critical cooling rate, martensite is hardly formed and a ferrite single-phase structure is formed. For this reason, not only is the structure strengthening insufficient, but the in-plane anisotropy of the yield strength cannot be controlled. On the other hand, if the cooling rate exceeds 100 ° C./s, the self-tempering of martensite that occurs during continuous cooling becomes insufficient, and the martensite becomes excessively hardened, resulting in an increase in yield strength and a decrease in ductility. Therefore, the cooling rate is preferably 100 ° C./s or less. In order to control the cooling rate, it is preferable to perform annealing in a continuous annealing line.

As mentioned above, although the basic process of the manufacturing method of this invention was demonstrated, you may add the following process.
A plating layer may be formed on the surface of the steel sheet by adding a surface treatment such as electroplating or hot dipping after the cold-rolled steel sheet annealing step. The plating layer is not limited to pure zinc plating or zinc-based alloy plating, but may be various plating layers conventionally applied to the steel sheet surface such as Al plating or Al-based alloy plating.

  Further, temper rolling or leveler processing may be applied to the cold-rolled annealed plate or plated steel plate produced as described above for the purpose of adjusting the shape correction, surface roughness, and the like. The total elongation of temper rolling or leveler processing is preferably in the range of 0.2 to 15%. If it is less than 0.2%, the intended purpose of shape correction and roughness adjustment cannot be achieved. On the other hand, if it exceeds 15%, it tends to cause a significant decrease in ductility, which is not preferable.

Molten steel having various compositions shown in Table 1 was melted in a converter and made into a steel slab by a continuous casting method. These steel slabs were heated to 1250 ° C. and made into sheet bars by rough rolling, and then subjected to finish rolling under the conditions shown in Table 2 to obtain hot rolled steel sheets. These hot-rolled steel plates were pickled and then made into cold-rolled steel plates by cold rolling at the rolling reduction shown in Table 2. Subsequently, these cold-rolled steel sheets were subjected to continuous annealing in the continuous annealing line under the conditions shown in Table 2. Furthermore, the obtained cold-rolled annealed steel sheet was subjected to temper rolling with an elongation of 0.5%. The following values were determined by A ₃ point in Table 1 is a thermodynamic calculation software Thermo-Calc (registered trademark).

The cold rolled annealed sheet thus obtained was examined for tensile properties and steel structure.
(1) Tensile properties JIS No. 5 tensile specimens were sampled from 0 ° (L direction), 45 ° (D direction) and 90 ° direction (C direction) with respect to the rolling direction of each cold-rolled annealed sheet obtained, A tensile test was performed at a crosshead speed of 10 mm / min in accordance with the provisions of JIS Z 2241 to determine yield strength (YS), tensile strength (TS), and uniform elongation (UEl). Here, the representative values of tensile strength (TS) and uniform elongation (UEl) were the tensile strength TS _L and uniform elongation UEl _{L of the} specimens taken from the 0 ° direction.
Further, ΔYP _L was used as an index of in-plane anisotropy of yield strength. This ΔYP _L indicates the in-plane anisotropy of the yield strength normalized by YP _L , and was calculated from the following equation.
ΔYP _L = {(YP _L / YP _L ) + (YP _C / YP _L ) −2 (YP _D / YP _L )} / 2
= (YP _L + YP _C -2YP _D ) / (2YP _L )
However,
YP _L = YS _L / YS _L
YP _D = YS _D / YS _L
YP _C = YS _C / YS _L
, And _the shows the yield strength of YS _L, YS _D, YS _C is 0 ° (L direction), 45 ° (D direction) and the direction of 90 ° (C direction) were taken from the test piece.
If the absolute value of ΔYP _L is 0.03 or less, it can be said that the in-plane anisotropy of the yield strength is excellent.

(2) Steel structure
(a) Three-dimensional crystal orientation distribution function (200), (211), (110) obtained by the reflection method by performing X-ray diffraction of the plate surface at the 1/4 thickness of each cold-rolled annealed plate. ) To obtain the three-dimensional crystal orientation distribution function by the series expansion method, and evaluated the three-dimensional crystal orientation distribution function at {φ1, Φ, φ2} = {0 °, 35 °, 45 °}. .

(b) Volume fraction of phase For the volume fraction of each phase, the area ratio of each phase was measured by the point counting method described above, and the area ratio was defined as the volume fraction.

  The survey results are shown in Table 2.

  As is clear from Table 2, in the steel plates of the present invention, the three-dimensional crystal orientation distribution function at {φ1, Φ, φ2} = {0 °, 35 °, 45 °} is 2.5 or less, It can be seen that the in-plane anisotropy of the yield strength is reduced even when the strength (TS) is 500 MPa or more.

  The high-strength cold-rolled steel sheet having low in-plane anisotropy of yield strength obtained according to the present invention is suitably used in various fields such as automobile steel sheets, household appliances and beverage cans.

Claims (3)

% By mass
C: 0.06 to 0.12%,
Si: 0.7% or less,
Mn: 1.2 to 2.6%
P: 0.020% or less,
S: 0.03% or less,
sol.Al: 0.01 to 0.5% and N: 0.005% or less, the balance is made of Fe and inevitable impurities, and is a volume fraction relative to the whole steel sheet structure, with the ferrite phase as the main phase. %, And the martensite phase is 5% to 20%, and the three-dimensional crystal orientation distribution function at {φ1, Φ, φ2} = {0 °, 35 °, 45 °} is 2.5 or less. A high-strength cold-rolled steel sheet with low in-plane anisotropy of yield strength.   2. The surface of yield strength according to claim 1, wherein the steel sheet further contains at least one selected from Cr: 0.5% or less and Mo: 0.5% or less by mass%. High strength cold-rolled steel sheet with small internal anisotropy. A steel slab having the composition according to claim 1 is hot-rolled at a finishing temperature of 840 ° C. or higher and 950 ° C. or lower and then cold-rolled at a rolling reduction of 30% or higher and 70% or lower. And annealing at a temperature of 800 ° C. or more and A ₃ points or less, and cooling the temperature range from the annealing temperature to 400 ° C. at a rate equal to or higher than the critical cooling rate CR (° C./s) represented by the following formula. A method for producing a high-strength cold-rolled steel sheet having low in-plane anisotropy of yield strength.
Record
logCR = −3.50 [% Mo] −1.20 [% Mn] −2.0 [% Cr] −0.32 [% P] +3.50
However, [% M] is the content of element M in steel (mass%).