RU2534703C2 - High-strength cold-rolled steel sheet with low in-plane anisotropy of yield point and method of its production - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: high-strength cold-rolled sheet steel with low in-plane anisotropy ΔYP_L making 0.03 or less. Sheet is made of steel containing in wt %: C: 0.06-0.12%, Si: 0.7% or less, Mn: 1.2-2.6%, P: 0.020% or less; S: 0.03% or less; sol. Al: 0.01-0.5%; N: 0.005% or less, at least one of Cr: 0.5 or less, and Mo: 0.5 or less, Fe and unavoidable impurities making the rest. Sheet microstructure consists, in terms of volume fraction relative to the entire microstructure, of 60% or more of ferrite phase as the main phase and 5-20% of martensite phase. Besides, it features the function of distribution of crystal 3D orientation of 2.5 or less in compliance with {φ1, Φ, φ2}={0°, 35°, 45°}.

EFFECT: breaking point of at least 500 MPa and better mouldability.

2 cl, 2 tbl, 1 ex

Description

FIELD OF THE INVENTION

The present invention relates to a high-strength cold-rolled steel sheet with low planar yield stress anisotropy, which is suitable for use as an automobile steel sheet, etc. and also to a manufacturing method.

State of the art

In recent years, in order to reduce the amount of CO ₂ emissions from the point of view of global environmental protection, there is an increasing need to improve the fuel economy of cars. In addition, increased vehicle safety is focused on the vehicle body characteristics in a collision, which are necessary to ensure the safety of passengers in a collision. Thus, certain measures are being taken to implement a lighter and more durable car body.

In order to simultaneously achieve weight reduction and increase the strength of the car body, it is effective to use more durable materials in order to reduce sheet thickness until there is no negative effect on stiffness. Accordingly, high-strength cold-rolled steel sheet is actively used today for car parts.

The stronger the steel sheet, the greater the effect of weight reduction. For example, there is a tendency in the automotive industry to use steel sheet with a tensile strength (TS) of at least 500 MPa, or even at least 590 MPa.

On the other hand, it is necessary that the automotive steel sheet has excellent stamping ability, since most automotive parts from steel sheet are formed by stamping. However, high-strength steel sheet is much inferior to conventional soft steel sheets in the ability to stamping, ductility and deep drawing, and therefore requires improvement in this regard.

As an example of a high strength steel sheet, for example up to grade 440 MPa, there is a steel sheet which is made by adding Ti and Nb in an amount suitable for fixing dissolved C and N in a particularly low carbon steel sheet with excellent formability, so as to obtain steel with a small the amount of metal inclusions as the main material, to which is then added a solid solution of reinforcing elements, such as Si, Mn, P, etc.

In addition, in the range of 500 MPa or more, or in the range of 590 MPa or more, a complex phase steel sheet, for example a DP steel sheet, with two phases of ferrite and martensite, and a TRIP steel sheet using the residual phase are practically used. The former is characterized by a low yield strength and high ability to strain hardening due to residual deformation near martensite, while the latter has the peculiarity that uniform elongation is enhanced due to ductility due to martensitic transformation.

As a rule, the mechanical characteristics of a high-strength steel sheet can be estimated by the tensile strength characteristic in a given direction, for example, the direction perpendicular to the rolling direction, and its planar anisotropy can be estimated by the planar anisotropy ∆r of the Lankford coefficient (r-value). Here ∆r can be calculated by the Lankford coefficient r _L , r _D and r _C in the directions 0 ° (L direction), 45 ° (D direction) and 90 ° (C direction) with respect to the rolling direction in accordance with the following formula:

Δr = (r _L + r _D -2r _C ) / 2.

However, based on an analysis of practical stamping, it was found that planar anisotropy of the yield strength significantly affects shape retention after forming parts and surface deformation. Thus, it is expected that the stamping ability can be improved by reducing the planar anisotropy of the yield strength.

Regarding a steel sheet with low planar anisotropy, for example, Patent Document 1 (JP 2004-197155) discloses a cold rolled steel sheet with excellent hardenability in a furnace and low planar anisotropy, which is suitably used for an external panel of a car body, as well as a method of manufacturing such a steel sheet . This technology consists in determining ∆r using the amount of C and the degree of compression during cold rolling, assuming that the required planar anisotropy and notch resistance can be realized simultaneously. In addition, in accordance with this technology, cooling must be started within 2 seconds after hot rolling and carried out in a temperature range of 100 ° C or more at a cooling rate of 70 ° C / s or more. However, the planar anisotropy here is determined by ∆r, which does not always coincide with the planar anisotropy of the yield strength.

Regarding a steel sheet related to planar anisotropy of plasticity, for example, Patent Document 2 (JP-2005-256020) discloses an excellent shape retention steel sheet, as well as a method for manufacturing such a steel sheet. The steel sheet is characterized as complex phase steel containing a maximum volume fraction of ferrite or bainite and martensite in the range of 1-25%. In this case, at least in the region of the sheet surface from 1/2 thickness to 1/4 thickness, all of the following conditions must be met:

(1) the average value (A) of the sample ratio of the X-ray intensities with the orientation of the crystal groups {100} <011> ~ {223} <110> is 4.0 or more;

(2) the average value (B) of the sample ratio of the X-ray intensities over the three orientations of the crystals {554} <225>, {111} <112> and {111} <110> is 5.5 or less;

1,5;(3) (A) / (B)

1.5;

(4) {100} <011> a sample ratio of X-ray intensities of at least {211} <011> a sample ratio of X-ray intensities.

∆LE1, гдеIn addition, the following conditions must be met: at least one of the r-magnitude in the rolling direction and the r-magnitude in the direction perpendicular to the rolling direction is 0.7 or less; the planar anisotropy of the uniform elongation ΔuE1 is 4% or less; the planar anisotropy of the local elongation ΔLE1 is 2% or more, and ΔuE1

ΔLE1, where

ΔuE1 = {| uE1 (L) -uE1 (45 °) | + | uE1 (C) -uE1 (45 °) |} / 2, and

∆LE1 = {| LE1 (L) -LE1 (45 °) | + | LE1 (C) -LE1 (45 °) |} / 2,

uniform elongation in the parallel direction (L direction), in the perpendicular direction (C direction) and in the direction of 45 ° to the rolling direction is defined as uE1 (L), uE1 (C) and uE1 (45 °), respectively, and local elongation in parallel direction (L direction), perpendicular (C direction) and 45 ° to the rolling direction is defined as LE1 (L), LE1 (C) and LE1 (45 °), respectively. In addition, it is necessary to optimize the conditions of the final hot rolling and winding at a critical temperature or less in accordance with an equivalent amount of Mn.

However, there is a problem that the development of texture {100} <011> reduces the ability to stretch, in addition, the effect on the planar anisotropy of the yield strength has not been clarified.

SUMMARY OF THE INVENTION

Problems Solved by the Invention

As described above, to improve the formability of conventional steel sheet for automobiles, efforts have been focused on increasing r-magnitude or elongation. However, since the technology disclosed in Patent Document 1 (JP 2004-197155 A) is to obtain a bainite phase by rapidly cooling after hot rolling to obtain a steel sheet with low planar anisotropy of r-magnitude, the problem remains that there may be only a limited level of strength is achieved. The technology disclosed in Patent Document 2 (JP-2005-256020) involves yet another problem, namely that the phase relation of the microstructure tends to change with changing manufacturing conditions, such that planar anisotropy, in particular planar anisotropy of yield strength, cannot always be reduced when the microstructure changes.

The aim of the present invention is the preferred solution to the above problems and the creation of high-strength cold-rolled steel sheet with a tensile strength (TS) of 500 MPa or more, or even higher than 590 MPa or more, and extremely low planar anisotropy of the yield strength at the same time, focusing on the yield strength and reducing it planar anisotropy, thereby improving the ability to stamping, and also creating a method for its manufacture.

Ways to solve the problem

In cold rolled steel sheet textures, it is generally believed that α fibers are formed in the <100> direction parallel to the RD direction, and γ fibers in the <111> direction parallel to the ND direction. When, in particular, γ fibers are formed, the r-value increases.

Here, the orientation of the α group of the fiber formed in the cold rolled steel sheet textures is {001} <110> ~ {111} <110>, and φ1 = 0 °, φ2 = 45 °, Φ = 0 ° ~ 55 °, which are shown in 3D orientations in space with rectangular coordinate axes corresponding to three variable Euler angles.

The inventors conducted extensive studies to solve the above problems and found that the mechanical characteristics, in particular the plane anisotropy of the yield strength, very closely correlate with the distribution function of the 3D orientation of the crystals of the indicated orientation of the α fiber along ({φ1, Φ, φ2} = {0 °, 35 °, 45 °}), and are not related to other orientations, such as the accumulation of γ fiber associated with the r-value, which is an indicator of deep drawing.

In further studies, it was found that the planar anisotropy of the yield strength also depends on the microstructure and, therefore, the planar anisotropy of the yield strength can be stably reduced by controlling the distribution function of the 3D crystal orientation along {φ1, Φ, φ2} = {0 °, 35 ° , 45 °}, as well as the volume fraction of the martensitic phase in the entire microstructure of the steel sheet.

The present invention has been completed based on knowledge and conclusions, as described above, and can be summarized as follows.

The first aspect of the present invention is a high-strength cold-rolled steel sheet with low planar anisotropy of the yield strength, including the following composition components,% mass:

C: 0.06-0.12%;

Si: 0.7% or less;

Mn: 1.2-2.6%;

P: 0.020% or less;

S: 0.03% or less;

Sol.Al: 0.01-0.5%;

N: 0.005% or less, and

the rest is Fe and unavoidable impurities, the steel sheet including, in terms of the volume fraction with respect to the entire microstructure of the steel sheet, 60% or more of the ferritic phase as the main phase, and 5-20% of the martensitic phase, and the distribution function of the 3D crystal orientation is 2.5 or less in {φ1, Φ, φ2} = {0 °, 35 °, 45 °}.

The second aspect of the present invention relates to a high-strength cold-rolled steel sheet with low planar yield stress anisotropy in accordance with the first aspect, in which the steel sheet includes, in wt.%, At least one of Cr: 0.5% or less, and Mo: 0.5% or less.

A third aspect of the present invention relates to a method for manufacturing a high-strength cold-rolled steel sheet with low planar yield stress anisotropy, comprising producing a steel slab with the composition components specified in the first or second aspects, hot rolling the steel slab at a final temperature of 840-950 ° C, and then cold rolling with a reduction ratio of 30-70%, then annealing at a temperature of 800 ° C or more and to point A ₃ or less, and subsequent cooling with a critical cooling rate CR (° · C / s ) or more, which is expressed by the following formula in the temperature range from the annealing temperature to 400 ° C:

log CR = -3.50 [% Mo] -1.20 [% Mn] -2.0 [% Cr] -0.32 [% P] +3.50,

where [% M] is the amount of element M contained in steel,% of the mass.

Effect of the invention

In accordance with the present invention, it is possible to obtain a high-strength cold-rolled steel sheet with low planar yield stress anisotropy and excellent stamping ability. The high-strength cold-rolled steel sheet obtained in the present invention is particularly suitable for use as automobile parts.

Preferred Embodiment

The present invention will be described in detail below.

First of all, the components of the composition of the high-strength cold-rolled steel sheet of the present invention are limited, as described above, for reasons that will be described in detail below, where the unit content of each element is "% mass.", Unless otherwise specified.

C: 0.06-0.12%

Carbon (C) is an element necessary to provide a given fraction of the ^2nd phase, increase strength and adjust yield strength with low planar anisotropy. A carbon content of less than 0.06% makes it difficult to ensure a martensitic phase content of at least 5%, which is not preferred. On the other hand, when the carbon content exceeds 0.12%, 2 ^nd phase besides ferrite phase is large percentage thereby becomes difficult to maintain the volume fraction of the ferrite phase is not less than 60%, thereby deteriorating ductility. In addition, 2 ^nd phase such as a martensite phase forms a network structure and tends to encircle the ferrite, so that the effect of texture is difficult ferrite phase and thereby control the planar anisotropy of the yield strength. Accordingly, the carbon content should be in the range of 0.06-0.12%, preferably in the range of 0.06-0.10%.

Si: 0.7% or less

Silicon (Si) slows down the formation of scale during hot rolling, and an insignificant amount is required to improve the surface quality, somewhat inhibits the alloying reaction between the iron base and zinc in a plating bath or during galvanizing with annealing, and improves hardening during high deformation, etc. In view of the foregoing, the Si content is preferably not less than about 0.01%. However, a Si content in excess of 0.7% impairs the appearance, thus, the Si content is set to 0.7% or less, preferably 0.3% or less.

Mn: 1.2-2.6%

Manganese (Mn) is added to improve dispersion hardening and increase the percentage of martensitic phases in the ^2nd phase. In terms of achieving such complex phases, the lower limit of the Mn content should be 1.2%. When the Mn content becomes too high, the temperature of transformation of α into γ during annealing becomes low, thus, a fine-grained ferrite grain is formed in the grain immediately after recrystallization or at the restored grain boundaries during recrystallization. Thus, the ^2nd phase becomes thinner and, as a result, ductility worsens, and it is also impossible to control the planar anisotropy of the yield strength. In light of this, the upper limit of the Mn content should be 2.6%, preferably the Mn content should be in the range of 1.2-2.1%. Since the amount of martensite formed varies depending on the cooling rate after annealing, the cooling rate should be controlled based on the amounts of Mn, Cr, and Mo, which will be explained later.

P: 0.020% or less

When the phosphorus (P) content is more than 0.020%, surface defects appear due to the deterioration of weldability and segregation, thus, the content of P is determined to be 0.020% or less.

S: 0.03% or less

Sulfur (S) has the effect of the ^1st improvement of the descaling properties of steel sheets and the quality of the appearance of the coating. However, when the S content is increased, too much MnS is released in the steel, impairing ductility, for example elongation and flanging ability during sheet steel deformation and stamping ability. In addition, ductility decreases during hot rolling of the slab, which tends to cause surface defects. In addition, corrosion resistance also deteriorates slightly. From this point of view, the content of S is determined to be 0.03% or less, preferably 0.01% or less, more preferably 0.005% or less, and more preferably 0.002% or less.

sol.Al: 0.01-0.5%

Soluble aluminum (sol.Al) can be used as an element for deoxidation of steel, and also affects the stabilization of dissolved N present as an impurity to improve aging resistance at room temperature. Thus, the content of sol.Al should be 0.01% or more. On the other hand, a sol.Al content exceeding 0.5% leads to an increase in cost, and a surface defect is induced. Thus, the content of sol.Al is in the range of 0.01-0.5%.

N: 0.005% or less

When the nitrogen (N) content is excessive, the aging resistance deteriorates at normal temperature, and a large amount of Al and Ti must be added to stabilize the dissolved N. Thus, the nitrogen content is preferably reduced as low as possible. From this point of view, the N content is determined to be 0.005% or less.

Although the composition of the main components has been explained above, the following elements can also be added as necessary, in accordance with the present invention.

Cr: 0.5% or less

Chromium (Cr) is an indispensable element that contributes to the reliable achievement of high strength steel sheet by the formation of complex phases, as well as Mn. To achieve the effect, the Cr content is preferably 0.1% or more. However, its excessive addition causes not only saturation of the effect, but also increases the cost, so the Cr content should be 0.5% or less.

Mo: 0.5% or less

Molybdenum (Mo) is an element that inhibits the formation of perlite by improving dispersion hardening to facilitate the production of high strength steel sheet. To achieve the effect, the Mo content is preferably 0.1% or more. However, molybdenum is so expensive that an excessive amount of the additive significantly increases its cost. From this point of view, the required Mo content is 0.5% or less. In addition, since the amount of martensite formed varies with the cooling rate after annealing, the cooling rate must be controlled based on the amounts of Mn, Cr, and Mo, which will be explained later.

The rest, i.e. components other than those described above comprise Fe and unavoidable impurities in the high-strength cold-rolled steel sheet of the present invention. However, the rest may additionally contain other components than those described above if the presence of such components does not adversely affect the functioning and effects of the present invention.

Next, the reasons why the microstructure of the steel should be limited to such a percentage will be explained.

Volume fraction of the ferrite phase: 60% or more

In accordance with the present invention, the texture of the ferrite is adjusted so that it tends to decrease too much. Namely, when the content of the ^2nd phase increases, in addition to the ferrite phase, it becomes difficult to control the planar anisotropy of the yield strength of the controlled textures. In addition, a ^2nd phase, such as a martensite phase, begins to surround the ferrite to form a network structure, so the macroscopic plastic behavior of the steel sheet will no longer depend on the orientation of the ferrite crystals. From this point of view, the volume fraction of the ferritic phase in the entire microstructure of the steel sheet should be 60% or more, preferably 75% or more.

Volume fraction of the martensite phase: 5% -20%

The martensitic phase is a useful phase, which contributes to an increase in the strength of the steel sheet, as well as a decrease in the ratio of yield strength to tensile strength to improve shape retention. Based on this, the volume fraction of the martensitic phase with respect to the entire microstructure of the steel sheet should be 5% or more. On the other hand, when the content of the martensitic phase exceeds 20%, martensite begins to surround the ferrite with the formation of a network structure and makes the control of ferrite texture absurd, which is not preferable in terms of controlling the plane anisotropy of the yield strength. Thus, the volume fraction of the martensitic phase with respect to the entire microstructure of the steel sheet should be in the range of 5-20%. In addition, the steel sheet in accordance with the present invention preferably consists of a ferrite phase as the main phase and a martensite phase as the ^2nd phase. The volume fractions of phases other than the above ferrite phase and martensite phase are preferably 5% or less, more preferably 3% or less with respect to the entire microstructure of the steel sheet.

The volume fraction of each phase is defined as the fraction of the area of each phase measured using the point counting method in accordance with ASTM E562-83 (1988). The fraction of the area of each phase is measured as follows, namely, a sample is taken of each cold-rolled annealed sheet, its cross section parallel to the direction of rolling (L section) is polished and etched with nital for studying at 4000x magnification using an SEM scanning electron microscope to determine the distribution and obtaining the area fraction of the ferritic phase as the main phase and the martensitic phase. In the microstructure photograph, areas slightly colored in black are considered as ferrite, areas where carbide is formed as a laminar structure or in the form of dots and lines are considered as perlite or bainite, and grains painted in white are martensite.

Distribution functions of the 3D crystal orientation along {φ1, Φ, φ2} = {0 °, 35 °, 45 °}: 2.5 or less.

In addition, the texture of the steel sheet in accordance with the present invention is evaluated by the distribution function of the 3D crystal orientation. Traditionally, the X-ray diffraction pattern (XRD) has been used to analyze the texture of steel sheet. Since the pole figure shows the statistical distribution of the orientation of the crystal in the mass of crystalline grain, it is a suitable way to determine the preferred orientation. However, the textures of polycrystalline materials tend to represent not only one preferred orientation, but also several preferred orientations. For example, in a fibrous structure, which is the orientation of a group rotating around a certain axis of the crystal, it is difficult to accurately assess the existing fraction of each orientation by measuring the pole figure. Thus, the distribution function of the 3D orientation of the crystals is obtained based on information from the pole figure to estimate the existing fraction of each orientation.

When evaluating the above distribution function of the 3D crystal orientation, the distribution function of the 3D crystal orientation is determined using the incomplete pole figures (200), (211) and (110) obtained by the reflection method in accordance with the series expansion method. Thus, it was found that in the microstructures of steel, including the volume fractions of the above phases of ferrite and martensite, when the distribution function of the 3D orientation of the crystals is 2.5 or less in a certain orientation of the α fiber: ({φ1, Φ, φ2} = {0 °, 35 °, 45 °}), the plane anisotropy of the yield strength becomes small. However, it is important to control the volume fractions of the phases of ferrite and martensite, as described above. For example, in the case of a single ferrite phase, the optimal ferrite texture, which reduces the planar anisotropy of the yield strength, differs from the above.

It is not always clear why the planar anisotropy of the yield strength decreases, provided that the distribution function of the SV orientation of the crystals over {φ1, Φ, φ2} = {0 °, 35 °, 45 °} is 2.5 or less. However, the inventors believe the following. Namely, in general, the crystal orientation in {φ1, Φ, φ2} = {0 °, 35 °, 45 °} usually appears in ferrite after cold rolling or transformation from deformed austenite. When the distribution function of the 3D crystal orientation is large, the planar anisotropy of the mechanical characteristics tends to increase, and it is necessary to control the distribution function of the 3D crystal orientation within an acceptable range to reduce planar anisotropy. However, the optimal value varies depending on the type of steel. In particular, in the complex phase of the steel sheet having a mixed structure of the ferrite phase of 60% or more as the main phase and the martensite phase of 5-20%, to which the present invention relates, the optimal distribution function of the 3D crystal orientation should be 2.5 or less.

Next will be described a method of manufacturing a high-strength cold-rolled steel sheet in accordance with the present invention.

The steel slab used is preferably fabricated by continuous casting to prevent macrosegregation of the components. However, casting and casting thin slabs are also acceptable. In addition to the traditional method, in which the manufactured slab is once cooled to room temperature and then reheated, energy-saving processes, such as hot loading during rolling, for introducing the hot slab into the hot rolling heating furnace without cooling, and direct hot rolling, can be applied without problems. to start hot rolling after quickly reaching the required temperature.

The slab heating temperature is preferably low for the development of {111} recrystallization texture by coarsening of precipitates to improve deep drawing. However, when the slab heating temperature is less than 1000 ° C, the rolling pressure increases, thus increasing the risk of problems during hot rolling. Thus, it is preferable that the heating temperature of the slab was 1000 ° C or more. In addition, from the point of view of increasing the loss of scale and the subsequent increase in the weight of the oxidation products, it is preferable that the upper limit of the optimum heating temperature of the slab was 1300 ° C.

Hot rolling is carried out on a steel slab heated under the above conditions, including rough rolling and finishing rolling. Here, a steel slab by rough rolling turns into a sheet blank. It is not necessary to indicate the conditions for rough rolling and can be performed in accordance with the prior art. In addition, the so-called sheet blank heater can be effectively used to heat the sheet blank so as to keep the slab heating temperature low and to prevent malfunctions during hot rolling.

End temperature: 840-950 ° C

Then finish rolling of the sheet billet is carried out to form a hot-rolled steel sheet. In this case, the end temperature, i.e. The finishing stand feed temperature (FT) should be in the range of 840-950 ° C to achieve textures that are preferable for planar anisotropy of the yield strength after cold rolling and recrystallization annealing. When the FT is less than 840 ° C, along with the high pressure during hot rolling, hot rolling in the ferrite region is carried out in part of the component systems, thereby substantially changing the texture. When the FT exceeds 950 ° C, the microstructures coarsen and rolling also cannot be performed completely under conditions of non-recrystallization of austenite, so the planar anisotropy of the yield strength will be increased after annealing during cold rolling.

In order to reduce the rolling pressure during hot rolling, lubrication can be applied during rolling in the part of the finish rolling or between full passages, which is effective in aligning the shape of the steel sheet and homogenizing the materials. The friction coefficient for rolling lubrication is preferably in the range of 0.10-0.25. In addition, it is preferable that adjacent sheet blanks be mutually connected so as to provide a continuous rolling process for continuous finish rolling from the point of view of stability during hot rolling.

Although it is not necessary to set the winding temperature (CT), the CT is preferably in the range 400-720 ° C. In particular, if CT exceeds the upper limit, the crystalline grain, as a rule, coarsens, thus, the strength decreases.

The degree of compression during cold rolling: in the range of 30-70%

Cold rolling of a hot-rolled steel sheet made under the above conditions is carried out. Preferably, the hot-rolled steel sheet is decapitated to remove scale before cold rolling. Duplication can be carried out under normal conditions. When the degree of compression during cold rolling is less than 30%, the recrystallization rate changes, so it is difficult to control the planar anisotropy of the yield strength. The degree of compression during cold rolling of more than 70% makes it difficult to achieve the desired textures, because the areas surrounding the carbide involved in hot rolling are partially deformed and the texture of ferrite after annealing begins to change significantly. Thus, the degree of compression during cold rolling should be 30-70%.

Annealing temperature: 800 ° C - point A ₃

The cold-rolled steel sheet manufactured under the above conditions is heated to 800 ° C — point A ₃ , and annealed in the same range. Annealing at temperatures below 800 ° C cannot provide the γ fraction during aging, and thus, a sufficient number of martensitic phases cannot be formed after cooling. While the annealing temperature above point A ₃ makes the content of the fraction too high and the textures change significantly after the reverse transformation, it is therefore difficult to achieve the desired textures. Thus, the annealing temperature should be in the range of 800 ° C - point A ₃ .

Cooling rate in the temperature range from annealing temperature to at least 400 ° C: critical cooling rate CR (° C / s) or more

To form the martensitic phase at a given volume fraction, the cold-rolled steel sheet annealed under the above conditions is cooled in the temperature range from the annealing temperature to at least 400 ° C with a critical cooling rate of CR (° C / s) or more, represented by the following formula :

logCR = -3.50 [% Mo] -1.20 [% Mn] -2.0 [% Cr] -0.32 [% P] +3.50,

where [% M] represents the amount of element M contained in steel (% mass.).

When the average cooling rate in the indicated temperature range is less than the critical cooling rate, it is difficult to form martensite, so that a microstructure of single-phase ferrite is formed. Thus, the steel sheet is not strong enough and planar anisotropy of the yield strength cannot be controlled either. A cooling rate above 100 ° / s causes insufficient self-tempering of the martensite formed during continuous cooling. Thus, martensite is excessively hardened and the yield strength is increased, and ductility is reduced. Thus, the cooling rate is preferably 100 ° / s or less. In addition, to control such a cooling rate, it is preferable to use a continuous annealing line for annealing.

The main flow chart of the present invention has been presented above, however, the following processes can be added:

After the above annealing process of the cold rolled steel sheet, surface treatment processes, for example galvanizing or hot dip galvanizing, can be added to form a coating layer on the surface of the steel sheet. Coating layers can be provided not only with a coating of pure zinc and zinc-based alloys, but also with an A1 coating and a coating based on the A1 alloy. Namely, various coating layers previously applied to the surface of the steel sheet may be acceptable.

To correct the shape and adjust the degree of surface roughness of the sheet, a cold-rolled annealed sheet or coated steel sheet made as described above can undergo a training or leveling process. The elongation coefficient during training or during the alignment process is preferably in the range of 0.2-15%. In the case of an elongation coefficient of less than 0.2%, a correction of the initially given shape and degree of roughness cannot be achieved. If the elongation ratio is more than 15%, ductility, as a rule, is significantly reduced, which is not preferred.

Examples

Molten steel having a wide range of compositional components, as shown in Table 1, is prepared by casting into ingots and continuous casting to obtain the corresponding samples of a steel slab. With each of the samples of the steel slab spend: heating to 1250 ° C; transformation into a sheet blank by rough rolling; fine rolling under the conditions presented in table 2, to obtain a hot-rolled steel sheet of steel; etching and cold rolling of a hot-rolled steel sheet with a reduction ratio shown in Table 2 to obtain a cold-rolled steel sheet; continuous annealing of cold-rolled steel sheet on a continuous annealing line under the conditions shown in table 2; and training annealed cold rolled steel sheet with an elongation coefficient of 0.5%. Point A ₃ in table 1 is calculated using the Thermo-Calc (registered trademark) software.

Steel type Chemical composition (% wt.) Point A ₃ (° C) Note FROM Si Mn P S sol.Al N Cr Mo A 0.0015 0.01 0.18 0.006 0.006 0,025 0.0020 - - 907 Steel comparison B 0,083 0,07 1.65 0.005 0.008 0,035 0.0018 - - 824 Matching steel C 0,064 0.20 1.78 0.012 0.004 0,037 0.0029 - - 832 Matching steel D 0.126 1.29 1.93 0.023 0.002 0,045 0.0034 - - 855 Steel comparison E 0,081 0.45 1.65 0.017 0.002 0,031 0.0039 - - 843 Matching steel F 0,064 0.48 1.45 0.012 0.004 0,037 0.0029 0.20 - 853 Matching steel G 0,085 0.02 1.85 0.012 0.004 0,037 0.0029 - 0.15 820 Matching steel

Explore the characteristics of tensile strength, texture and microstructure of samples of cold-rolled annealed sheet, obtained accordingly:

(1) Tensile Strength Characteristics

JIS No. 5 tensile test specimens are taken from each annealed cold rolled steel sheet in the 0 ° (L direction), 45 ° (D direction) and 90 ° (C direction) directions with respect to the rolling direction and the tensile test is carried out at a traverse speed 10 mm / min in accordance with JIS Z 2241 for yield strength (YS), tensile strength (TS) and uniform elongation (EUI). The characteristic values of ultimate strength (TS) and uniform elongation (EUI) are the tensile strength TS _L and uniform elongation UE1 _{L of} each specimen obtained for the 0 ° direction. In addition, ΔYP _{L is} used as an index representing the planar anisotropy of the yield strength. ∆YP _L represents the plane anisotropy of the yield strength normalized by YP _L and can be calculated by the following formula:

∆YP _L = {(YP _L / YP _L ) + (YP _C / YP _L ) -2 (YP _D / YP _L )} / 2

= (YP _L + YP _C -2YP _D ) / (2YP _L ),

where YP _L = YS _L / YS _L , YP _D = YS _D / YS _L , YP _C = YS _C / YS _L and YS _L , YS _D , YS _C represent the yield strength of each sample obtained at 0 ° (L direction) , 45 ° (D direction) and 90 ° (C direction), respectively. When the absolute value of ΔYP _L is 0.03 or less, planar anisotropy can be considered excellent.

(2) Steel texture and microstructure

(a) Distribution function of 3D crystal orientation

An x-ray phase analysis of each annealed cold-rolled steel sheet is carried out at 1/4 of the thickness from the sheet surface and the distribution function of the 3D crystal orientation is obtained from incomplete pole figures (200), (211) and (110) obtained by reflection. Then, the distribution function of the 3D crystal orientation is estimated by {φ1, Φ, φ2} = {0 °, 35 °, 45 °}.

(b) Volume fraction of each phase

The volume fraction of each phase is the fraction of the area measured by the point counting method, as described above.

The obtained test results are shown in table 2.

No. Steel type Hot Rolling Conditions Cold Rolling Conditions Annealing conditions for cold rolled steel sheet ODF ^{* 5} (Volume fraction) Yield strength Tensile strength Uniform elongation Note FT ^{* 2} (° C) The degree of compression (%) Pace. annealing (° C) Cooling rate ^{* 3} (° C / s) CR ^{* 4} (° C / s) Ferrite Phase (%) Martensite phase (%) YS _L (MPa) Yp _l Yp _d Yp _c ΔYP _L TS _L (MPa) UE1 _L (%) one A 905 70 880 twenty 1915 4.0 one hundred 0 167 1.00 1,04 1,07 -0.01 306 48.0 Compares. example 2 B 880 60 850 25 33 2.7 93 0 345 1.00 1,03 1.00 -0.03 462 33.6 Compares. example 3 C 880 60 830 25 23 1.8 87 12 421 1.00 0.99 0.93 -0.03 641 27.7 An example of the invention four D 880 60 830 25 fifteen 2.2 68 thirty 655 1.00 1.10 1.10 -0.05 986 16.8 Compares. example 5 E 880 55 840 25 33 5.2 90 four 417 1.00 0.99 1.05 0.04 626 28.1 Compares. example 6 F 890 60 840 25 23 1.9 90 9 395 1.00 0.98 0.96 0.00 635 29.5 An example of the invention 7 G 880 60 815 25 6 1.7 89 10 380 1.00 0.99 0.95 -0.02 640 28.5 An example of the invention ^{* 1} Underline means "out of range". ^{* 2} FT represents the feed temperature to the finishing stand. ^{* 3} Cooling rate is the average cooling rate in the range from the annealing temperature to 400 ° C. ^{* 4} CR is the critical cooling rate calculated by the formula: logCR = -3.50 [% Mo] -1.20 [% Mn] -2.0 [% Cr] -0.32 [% P] +3.50 where [% M] represents the amount of element M contained in steel (% mass.). ^{* 5} Distribution function of 3D crystal orientation along {φ1, Φ, φ2} = {0 °, 35 °, 45 °}.

You should take into account that from table 2 it is seen that the distribution function of the 3D orientation of the crystals of each steel sheet in accordance with the present invention is 2.5 or less according to {φ1, Φ, φ2} = {0 °, 35 °, 45 ° }. Thus, it can be recognized that the planar anisotropy of the yield strength decreases, although the tensile strength (TS) is 500 MPa or more.

Industrial applicability

The high-strength cold-rolled steel with low planar yield stress anisotropy, which can be obtained in accordance with the present invention, is suitable for use as a steel sheet for automobiles, as well as in a variety of areas, such as parts of household electrical appliances, beverage cans, etc. .

Claims (2)

1. High strength cold rolled steel sheet with an absolute value ΔYP _L of 0.03 or less, having a composition that includes, wt.%:
C: 0.06-0.12
Si: 0.7 or less
Mn: 1.2-2.6
P: 0.020 or less
S: 0.03 or less
sol.Al: 0.01-0.5
N: 0.005 or less
at least one of Cr: 0.5 or less, and Mo: 0.5 or less, and
Fe and unavoidable impurities - the rest,
moreover, the steel sheet includes, in terms of the volume fraction with respect to the entire microstructure of the steel sheet, of 60% or more of the ferritic phase as the main phase, 5-20% of the martensitic phase, and has a distribution function of the 3D crystal orientation of 2.5 or less over {φ1, Ф, φ2} = {0 °, 35 °, 45 °}. 2. A method of manufacturing a high-strength cold-rolled steel sheet with an absolute value ΔYP _L of 0.03 or less, comprising obtaining a steel slab having the composition specified in claim 1, hot rolling the steel slab at an end temperature of 840-950 ° C, followed by cold rolling with a reduction ratio of 30-70%, then annealing at a temperature of from 800 ° C or more to a point A ₃ or less, and subsequent cooling with a critical cooling rate of CR (° C / s) or higher, which is expressed by the following formula in the range temperature ry annealing to 400 ° C:
logCR = -3.50 [% Mo] -1.20 [% Mn] -2.0 [% Cr] -0.32 [% P] +3.50.