JP2002241897A - Steel sheet having small variation in yield strength and fracture elongation, high formability and low yield ratio, and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel sheet which has small vatiation in yield strength and fracture elongation, and has high formability and a low yield ratio. SOLUTION: The steel sheet has a composition containing C, Si, Mn and Al, that is selected from a group 1 (B), a group 2 (Ti, Nb, V and Zr), a group 3 (Cr, Mo, Cu and Ni) or a group 4 (Ca and rare earth elements), and has a crystal structure for which the average crystal grain size, volume ratio and aspect ratio of a ferritic phase, and further, the average crystal grain size of ferritic grain boundaries and its second phase are prescribed. In the method for producing the above steel sheet, hot rolling is conducted under the conditions where the difference in the temperature of a rough rolling stock at the start of finish rolling is <=100 deg.C. Provided that the variation in the finish rolling completion temperature is ΔFT, and the variation in the rough rolling completion temperature is ΔRT, the following inequality (a) is satisfied: ΔFT<=0.6×ΔRT...(a).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet used in steel structures, automobiles and home appliances, which has a small variation in characteristic values, has high formability and a low yield ratio, and a method for producing the same. . More specifically, the steel sheet of the present invention has a high yield strength so as to be optimally used for seismic / seismic / seismic isolation members of structures such as buildings and members requiring high formability for automobiles. And fluctuation of elongation at break is small,
It is a steel sheet having high formability and a low yield ratio, and can be used for hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, electroplated steel sheet, and organic coating treatment.

[0002]

2. Description of the Related Art When deformation or vibration due to an external force such as an earthquake is applied to a building structure, in order to prevent the building structure from being damaged or collapsed, it is necessary to quickly absorb the deformation and vibration energy due to the external force. . For this reason, a method has been proposed to prevent the building structure from being damaged or collapsed by using extremely low yield point steel or lead that plastically deforms before the main structural members such as columns and beams in part of the building structure. Have been.

[0003] For example, in Japanese Patent Application Laid-Open No. 9-2877051, as a method of using a steel sheet having a low yield point, a steel sheet having a C content of 0.01% or less and a microstructure having an average colony size of 80 µm or more is used. Proposed. Here, a colony is a group of crystal grains in which the orientations of adjacent crystal grains are substantially equal, that is, the orientation difference is within 20 degrees, and abrupt deterioration of ductility is caused by increasing the colony size. It is said that the building will be able to improve the damping performance of buildings when an earthquake occurs. However, in the proposed steel sheet, the yield strength is limited to 120 MPa or less, and it is not possible to obtain a higher yield strength. Moreover, there is no mention of the change in the yield strength or elongation at break of the steel sheet. On the other hand, steel sheets used for automobiles and home electric appliances are required to have excellent formability. Usually, to improve the formability of a steel sheet, there is a method of reducing impurities such as C, P, S and N. However, in this method, ferrite grains may be extremely coarsened. For this reason, not only high ductility cannot be obtained stably, but also the yield strength and the elongation at break increase greatly, and the predetermined formability as a steel sheet for automobiles and home electric appliances may not be ensured in some cases. Further, there is a problem that surface roughness is likely to occur during the forming of the steel sheet.

[0004] Further, when a steel sheet with reduced impurities is used as a vibration damping material during an earthquake, it is deformed preferentially over other structural members, making it difficult to stably absorb energy. Disadvantages. Further, when impurity elements in steel are reduced, austenite grains are coarsened during slab casting, and there is a problem that surface cracks exhibiting austenite grain boundary cracks are likely to occur during continuous casting or rough rolling after slab heating. Furthermore, in order to reduce impurity elements at the iron and steelmaking stages, measures such as lengthening the processing time of vacuum degassing and increasing the amount of desulfurization treatment agent are required, and the production cost of steel sheets will rise. There is also a problem.

[0005]

As described above, various improvements have been made to steel sheets that are suitable for members having a low yield ratio suitable for seismic resistance and vibration control of building structures or members that require high formability for automobiles. Attempts have been made, but there has been a problem that the effect of improving the properties is insufficient, and a steel sheet having the required properties cannot be stably obtained.

The present invention has been made in view of such a problem, and the strength-ductility balance of a steel sheet is selected by selecting the optimum conditions for the chemical composition, crystal structure, hot and cold rolling of steel. It is an object of the present invention to provide a steel sheet having excellent formability, a small variation in yield strength and elongation at break, high formability and a low yield ratio, and a method for producing the same.

[0007]

Means for Solving the Problems The present invention relates to the following (1) a steel sheet having a small change in yield strength and breaking elongation and having high formability and a low yield ratio, and (2) a method for producing the steel sheet. It is a gist. (1) In mass%, C: 0.0002 to 0.1%, Si: 0.003 to 2.0
%, Mn: 0.003 to 3.0% and Al: 0.002 to 2.0%, and one or two of the following first to fourth groups:
A steel plate containing substantially Fe and having P: 0.0002 to 0.15%, S: 0.0002 to 0.05% and N: 0.0005 to 0.015% as impurities, and the average grain size of ferrite phase The diameter is over 1μm ~ 50μm, and the volume ratio is
70% or more, the aspect ratio of the ferrite phase is 5 or less, 70% or more of the ferrite grain boundaries consist of large-angle grain boundaries, and the average crystal grain size of the second phase having the largest volume fraction among the remaining phases Is a steel sheet having a small variation in yield strength and elongation at break characterized by a high formability and a low yield ratio. Further, this steel sheet may be either a hot-rolled steel sheet or a cold-rolled steel sheet.

Group 1: B: contains 0.0002 to 0.01% Group 2: Contains one or more of Ti, Nb, V and Zr in total of 0.005 to 1.0% Group 3: Cr, Mo, Cu And one or two or more of Ni and 0.005% to 3.0% in total. Four groups: Ca: 0.005% or less and rare earth element: 0.20% or less (2) The component described in (1) above When hot rolling immediately after casting a slab using the steel contained, or 950-130
When hot rolling after reheating in a temperature range of 0 ° C., after rough rolling of the steel slab, the temperature difference in the coil in the coil longitudinal direction and width direction of the rough rolled material at the start of finish rolling is 100 ° C. or less. Characterized in that the rough-rolled material is re-heated or kept at the same time as hot rolling is performed under the conditions satisfying the following expression (a), the yield strength and the elongation at break are small, and the high formability and low yield ratio are small. And a method for producing a steel sheet having: ΔFT ≦ 0.6 × ΔRT (a) where ΔFT: fluctuation of finish rolling completion temperature (° C.), ΔR
T: Fluctuation of rough rolling completion temperature (° C.), ΔFT ≦ 30 ° C.
In the case of (1), the expression (a) is not applied.

[0009] The hot-rolled steel sheet obtained by the above-mentioned production method is further subjected to cold rolling at a reduction ratio of 50% or more, at 600 to 950 ° C.
It is preferable to perform the annealing treatment in the temperature range described above.

The characteristics of the steel sheet of the present invention, in which the fluctuation of the yield strength and the elongation at break and the high formability and the low yield ratio, are the characteristics of the steel sheet as shown in FIGS. Sometimes expressed as a strength-ductility balance.
When the properties of this steel sheet are quantitatively determined, the tensile strength TS ≧ 270 MPa, the variation in yield strength △ YS ≦ 50 MPa, the variation in elongation at break △ EL ≦ 5%, the yield ratio (yield strength) Is determined by satisfying all of the conditions of YR ≦ 0.75 and TS × EL ≧ 15000 MPa ·%. If all of these conditions are satisfied, it is determined that the material has characteristics. You.

The aspect ratio of the ferrite phase specified in the present invention is shown as a value obtained by dividing the maximum diameter by the minimum diameter.
Whether the ferrite grain boundary is a large-angle grain boundary is examined by examining the crystal orientation difference between adjacent ferrite grains, and is determined to be a large-angle grain boundary when the orientation difference between adjacent ferrite grains is 15 degrees or more. I have to do that.

Further, the crystal structure referred to as the second phase in the present invention includes various phases other than the ferrite phase such as pearlite, bainite, martensite, retained austenite, and cementite (hereinafter, these are simply referred to as “remainder phase”). Collectively, it refers to a phase having the largest volume ratio among precipitates (excluding carbides, nitrides, sulfides, oxides, phosphides, borides, and composite products thereof other than cementite).

The present inventors have conducted intensive experimental research to solve the above-mentioned problems, and have found the following, thereby completing the present invention.

In order to ascertain the characteristics of the steel sheets having the respective chemical compositions, steels containing the respective elements shown in Table 1 were cast into ingots in a vacuum melting furnace, and steel materials for rolling were prepared from these by ingot hot forging. After reheating the steel material, hot rolling is performed to make the equiaxed ferrite phase the main component, and as the remaining phase, cementite,
A hot rolled steel sheet having one or more of pearlite, bainite, martensite, and retained austenite was produced.

[0015]

[Table 1] For some steel types, an experiment was also conducted on direct-feed rolling in which hot rolling was started immediately after the ingot was formed in a vacuum melting furnace. The thickness of the obtained hot-rolled steel sheet was 2.6 mm. With respect to these steel sheets, the average crystal grain size of the equiaxed ferrite phase, the volume ratio thereof, the crystal orientation difference between adjacent ferrite grains, and the average crystal grain size of the remaining phase and the volume ratio thereof as hot rolled steel sheets were investigated. At the same time, tensile tests were performed on all steel types.
Some hot-rolled steel sheets were annealed at 820 ° C. × 50 sec after pickling.

Next, in order to grasp the characteristics of the cold-rolled steel sheet, a hot-rolled steel sheet having a thickness of 4.0 mm was manufactured from steel having the chemical composition shown in Table 1 above, and after pickling, cold-rolled at a rolling reduction of 80%. And 800 ℃ × 6
The same investigation was performed using a cold-rolled steel sheet annealed for 0 sec.

The average crystal grain size of the ferrite phase and the remaining phase is:
Light microscopic micrographs or scanning electron microscopic (SEM) micrographs were taken in five fields of view, and these were used to represent the average section length measured by the linear section method multiplied by 1.128.
The volume ratios of the ferrite phase and the residual phase were represented by an average value of the structural photographs of the five visual fields using an image analyzer. The crystal orientation difference between adjacent ferrite grains was measured by electron beam backscattering (EBSP).

[0018] The tensile test is performed in accordance with JIS Z 2201.
The test was performed using a tensile test piece. Average value V of each tensile test value
m _was evaluated at _{V m = (V 0 + 2V} 45 + V 90) / 4. Here, V ₀ , V ₄₅ and V ₉₀ indicate measured values obtained by performing a tensile test in the rolling direction, a direction at 45 ° to the rolling direction, and a direction at 90 ° to the rolling direction, respectively. The ΔV value, which is a fluctuation value of each tensile value, indicates the absolute value of the difference between the maximum value and the minimum value in the steel sheet.

FIG. 1 shows the relationship between the average crystal grain size of the ferrite phase and the second phase with respect to the strength-ductility balance of a hot-rolled steel sheet containing 70% or more by volume of the ferrite phase.
The symbol ● in the figure indicates that the properties of the steel are TS ≧ 270 MPa, TS × E
L ≧ 15000MPa ·%, ΔYS ≦ 50MPa, ΔEL ≦ 5%,
It satisfies all the conditions of YR ≦ 0.75. When this property is met, it indicates that the steel sheet has a small change in yield strength and elongation at break, and has high formability and a low yield ratio.

On the other hand, the symbols Δ and ▲ do not satisfy these characteristics. For example, TS ≧ 270 MPa, TS × EL <150
00 MPa ·%, ΔYS> 50 MPa, ΔEL> 5%, YR> 0.
It has one of the 75 characteristics.

As shown in FIG. 1, when the average crystal grain size of the ferrite phase is more than 1 μm and 50 μm or less and the average crystal grain size of the second phase is 50 μm or less, the strength-ductility balance is excellent. I have. However, in the figure, the average crystal grain size of the ferrite phase is 50 μm or less, and the average crystal grain size of the second phase is 50 μm.
m or less, if less than 70% of ferrite grain boundaries are large-angle grain boundaries, or if the aspect ratio of the ferrite phase exceeds 5, TS ≧ 270 MPa,
TS × EL ≧ 15000MPa ·%, YS ≦ 50MPa, ΔEL
≦ 5% and YR ≦ 0.75 cannot all be satisfied.

FIG. 2 shows that the average crystal grain size of the ferrite phase is 0.2.
It is a figure which shows the influence of the volume ratio of an equiaxed ferrite phase and a 2nd phase on the strength-ductility balance of the hot-rolled steel sheet whose 5-38 micrometers and the average crystal grain diameter of a 2nd phase are 0.3-30 micrometers. ●,
The symbols ■ and ▲ are the same as those in FIG.

As shown in FIG. 2, when the aspect ratio of the ferrite phase is 5 or less and the ratio of the ferrite phase is 70% or more by volume, the properties of the steel sheet are TS ≧ 27.
0MPa, TS × EL ≧ 15000MPa ·%, ΔYS ≦ 50MPa,
It satisfies all the conditions of ΔEL ≦ 5% and YR ≦ 0.75. However, in the figure, even if the volume fraction of the ferrite phase is 70% or more, the proportion of the large-angle grain boundaries among the ferrite grain boundaries is less than 70%, or the aspect ratio of the ferrite phase exceeds 5. In such a case, it can be seen that the above favorable characteristics cannot be satisfied.

Further, as a result of repeated tests on the cold-rolled steel sheets, the following findings (a) and (b) could be obtained regarding the effects of the properties of the hot-rolled steel sheets on the cold-rolled steel sheets.

A. 70 volume fraction of ferrite phase in hot rolled steel sheet
According to the investigation results on the properties of the cold-rolled steel sheet when the content is not less than 5%, the aspect ratio of the ferrite phase of the hot-rolled steel sheet is 5 or less and the average crystal grain size of the ferrite phase exceeds 1 μm.
0 μm or less, and when the average crystal grain size of the second phase is 50 μm or less, the properties of the cold-rolled steel sheet are also TS ≧ 270 MPa, TS ×
EL ≧ 15000MPa ·%, ΔYS ≦ 50MPa, ΔEL ≦ 5
%, YR ≦ 0.75. In other words, the properties of the hot-rolled steel sheet have small changes in the yield strength and elongation at break exhibited by the cold-rolled steel sheet, and directly affect the properties having high formability and a low yield ratio.

However, even if the average crystal grain size of the ferrite phase of the hot-rolled steel sheet is 50 μm or less and the average crystal grain size of the second phase is 50 μm or less, the proportion of the large-angle grain boundaries in the ferrite grain boundaries is small. If it is less than 70% or if the aspect ratio of the ferrite phase exceeds 5, the cold rolled steel sheet cannot satisfy all of the above characteristics.

B. Hot-rolled steel sheet with ferrite phase average grain size 0.5-36μm, second phase average grain size 0.3-30μm
According to the results of investigating the effect of the volume fraction of the equiaxed ferrite phase and the second phase of the hot-rolled steel sheet on the properties of the cold-rolled steel sheet, the aspect ratio of the ferrite phase of the hot-rolled steel sheet was 5 or less. When the ratio of the ferrite phase is 70% or more by volume, the characteristics of the cold-rolled steel are TS ≧ 270 MPa, TS × EL ≧ 1.
5000MPa ·%, △ YS ≦ 50MPa, △ EL ≦ 5%, YR ≦
Satisfies all conditions of 0.75.

However, when the volume ratio of the ferrite phase is 70
% Or more, if the proportion of the large-angle grain boundaries in the ferrite grain boundaries is less than 70% or if the aspect ratio of the ferrite phase exceeds 5, the properties of the cold-rolled steel sheet will be as described above. Cannot be all satisfied.

Next, the effect of the hot rolling conditions on the properties of the steel sheet was examined. Fluctuation of the finish rolling completion temperature in the longitudinal direction and width direction of the hot-rolled coil (difference between the highest temperature and the lowest temperature) △ FT is affected by the fluctuation of the rough rolling completion temperature △ RT. When the variation in ΔFT increases, the characteristic variation in the coil also increases. In order to suppress the characteristic fluctuation, stabilize the seismic energy absorption performance, and improve the formability during press working, it is necessary to suppress the fluctuation of ΔRT and ΔFT.

Specifically, the rough-rolled material is reheated or kept warm, and the temperature difference in the rolling longitudinal direction and the width direction of the rough-rolled material immediately before starting the finish rolling is set to 100 ° C. or less. The hot-rolled steel sheet and further hot-rolled steel sheet are cold-rolled by performing hot rolling under the condition that the relationship of RT satisfies the following expression (a), or more preferably by performing an annealing treatment at 600 ° C. to 950 ° C. after hot rolling. The characteristics of the cold-rolled coil that has been rolled and annealed can be made uniform in the rolling longitudinal direction and the width direction. However, if the condition of variation of the finish rolling completion temperature ΔFT ≦ 30 ° C. can be maintained,
Since there is no need to consider the effect of the variation ΔRT of the rough rolling completion temperature, it is not necessary to satisfy the condition of the following equation (a). ΔFT ≦ 0.6 × ΔRT (a) In addition, when directly hot-rolling the continuous cast slab without reheating, the slab temperature is set to 1300 ° C. or less, and When reheating, 1300 ° C or less 950
By heating at a temperature of not less than ° C., the solid re-solution of coarse precipitates present in the slab can be suppressed, and fine precipitation during hot rolling can be suppressed, so that the elongation at break can be improved.

[0031]

BEST MODE FOR CARRYING OUT THE INVENTION The reason why the steel sheet of the present invention and the method for producing the same are specified as described above is as follows. The chemical composition of the steel,
2. 2. crystal structure; 3. hot rolling and A description will be given separately for cold rolling. 1. Chemical composition of steel In the following description, the component composition indicates% by mass.

C: 0.0002-0.1% The higher the content, the lower the volume fraction of the ferrite phase and the higher the volume fraction of the hard residual phase, which adversely affects ductility and deep drawability. Therefore, it is better that the content of C is small, and it is set to 0.1% or less. Preferably 0.05% or less,
More preferably, it is 0.01% or less. On the other hand, the content is 0.00
In order to reduce the content to less than 02%, not only does the steelmaking cost increase, but also the ferrite grains become extremely coarse, so that high ductility cannot be obtained stably and the surface roughness tends to occur during the forming of the steel sheet. is there. Therefore, the C content is 0.0002
% Or more.

Si: 0.003 to 2.0% Since Si has the effect of improving the strength of steel without significantly impairing workability, it is contained in an amount of 2.0% or less. Further, it has the effect of promoting the formation of ferrite and increasing the amount of ferrite. If the Si content exceeds 2.0%, the workability of the steel deteriorates, so further addition is not preferred. Further, since Si degrades the surface properties of the steel in addition to the effect of hardening the steel, when the workability of the steel is emphasized, the smaller the Si content, the better. Therefore, in order to prevent a substantial adverse effect on workability and surface properties, 1.0
% Or less, more preferably 0.6% or less. On the other hand, in order to reduce the content to less than 0.003%, steelmaking costs increase, so the Si content is set to 0.003% or more.

Mn: 0.003 to 3.0% In general, an appropriate amount of Mn is contained for the purpose of preventing hot brittleness due to S. When Mn is used as a solid solution strengthening element, a desired effect cannot be obtained unless its content is 0.003% or more. However, when Mn is excessively contained, not only does formability deteriorate, but it becomes difficult to generate sufficient ferrite in a cooling process after hot rolling, and ductility and weldability are impaired. Become. In order to prevent this, the content of Mn is set to 3.0% or less, preferably 2.0% or less, more preferably 1.5% or less.

Al: 0.002 to 2.0% Al is used for deoxidization to obtain sound slabs, and promotes the formation of ferrite and increases the amount of ferrite, as well as Si, In order to secure the yield, the content is made 0.002% or more. On the other hand, the content
If the content exceeds 2.0%, the above effect is saturated, so the Al content is set to 2.0% or less. Preferably it is 1.2% or less, more preferably 0.1% or less.

The steel sheet of the present invention has, in addition to the above component elements,
The following component elements can be contained as needed.

B: 0.0002-0.01% (Group 1) B is not an essential element, but has an effect of improving the hardenability of steel, so that the crystal grain size and volume ratio of the ferrite phase and the remaining phase are controlled during the cooling process. Add if necessary. B is Ar
_Since it has the effect of lowering the _three points, it is effective to include it when it is difficult to complete finish rolling in the austenite temperature range. In particular, it is effective when manufacturing a hot rolled steel sheet having a thickness as thin as 2.0 mm or less. Furthermore, the ultra-low carbon steel sheet may be included because it has an effect of preventing “secondary cracking” that may occur during drawing.

When B is contained, it is contained in an amount of 0.0002% or more. However, when B is contained in more than 0.01%,
In some cases, the formation of ferrite is significantly suppressed, and the effect of preventing secondary working cracks saturates, thereby making the steel sheet brittle. Therefore, the B content is set to 0.01% or less, preferably 0.007% or less, and more preferably 0.005% or less.

One or more of Ti, Nb, V, and Zr: 0.005 to 1.0% (group 2) These elements are all contained in solid solution C, solid solution N, and solid solution S contained in steel. Has the effect of fixing as a precipitate and rendering it harmless, and is particularly effective in improving the deep drawability of cold-rolled annealed steel sheets. Further, it exerts an effect of increasing the strength of steel without significantly impairing ductility and deep drawability. Therefore, if necessary to ensure the deep drawability and strength of steel, one or more of these elements may be added in a total amount of 0.005 or more. However, when the total content exceeds 1.0%, the above effects are saturated, and conversely, ductility and deep drawability are reduced, so that the yield ratio is increased and the shape freezing property during press molding is deteriorated. Therefore, their content is set to 1.0% or less, preferably 0.5% or less, more preferably 0.2% or less.

One or more of Cr, Mo, Cu, and Ni: 0.005% to 3.0% (Group 3) These elements have an effect of improving hardenability similarly to Mn. By containing these elements in appropriate amounts, it becomes easy to control the crystal grain size and volume ratio of the ferrite phase and the remaining phase in the cooling process. Cu also has an effect of improving corrosion resistance. Therefore, if necessary, one or more of these elements are contained in a total of 0.005% or more. However, when these contents exceed 3.0%, the above effects are saturated, and when added in excess, the moldability is impaired. Therefore, the content of these should be 3.0% or less, preferably 1.0%
%, More preferably 0.5% or less.

Ca: 0.005% or less and rare earth element: 0.2
0% or less (Group 4) These elements are included as necessary because they adjust the shape of the inclusions and improve the cold workability. In the case where Ca is contained, if the content of Ca exceeds 0.005% and the content of the rare earth element exceeds 0.20%, the above-described effect is saturated. Therefore, these are set as the upper limits of the contents.

The steel sheet of the present invention suitably contains the above chemical composition, and the balance is composed of Fe and impurities. The contents of P, S and N among the impurities are controlled as follows.

P: 0.0002 to 0.15% P tends to make the steel hard and brittle, but can increase the strength of the steel without impairing the workability. However, if P is excessively contained, P segregates at the crystal grain boundaries and the steel becomes brittle. Therefore, the content is set to 0.15% or less. Preferably it is 0.12% or less, more preferably 0.10% or less. On the other hand, when the P content is reduced to less than 0.0002%, not only does the steelmaking cost increase, but also the ferrite grains become extremely coarse and high ductility cannot be obtained stably, and the surface roughness tends to occur during the formation of the steel sheet. For this reason, the lower limit of the content is set to 0.0002%.

S: 0.0002-0.05% Since S forms sulfide-based inclusions and reduces workability, its content is controlled to 0.05% or less. Preferably, in order to obtain more excellent workability, the content is preferably 0.03% or less, more preferably 0.015% or less. On the other hand, when the S content is 0.0
In order to reduce the content to less than 002%, not only does the steelmaking cost increase, but also the ferrite grains become extremely coarse and high ductility cannot be obtained stably, and the surface roughening tends to occur when the steel sheet is formed. For this reason, the lower limit of the content is set to 0.0002%.

N: 0.0005 to 0.015% N is preferably as small as possible, but if it is 0.015% or less, the effect is small in the present invention. However, its content is 0.015%
If it exceeds 300, it is necessary to add a large amount of Al or Ti, which is not economical. Therefore, its content is preferably set to 0.015% or less. Preferably it is 0.007% or less, more preferably 0.005% or less. However, reducing the N content to less than 0.0005% not only increases the steelmaking cost, but also causes the ferrite grains to become extremely coarse, so that high ductility cannot be stably obtained, and surface roughness occurs during the formation of the steel sheet. There is a problem that it is easy. Therefore, the N content is set to 0.0005% or more. 2. Crystal Structure The steel sheet of the present invention has an average ferrite phase crystal grain size of 1 μm.
Over 50 μm. Average grain size of ferrite phase is 50
When the size exceeds μm, deformation concentrates on a specific coarse crystal grain, and strain tends to be localized. For this reason, not only is it impossible to stably obtain high seismic energy absorption performance and good strength-characteristic balance, but also the characteristic fluctuations become large. Further, the surface of the steel sheet is roughened during press working, and the surface roughness tends to be large and the surface properties are likely to be unsatisfactory. Further, when the ferrite grain size of the hot-rolled steel sheet becomes large, it is formed from the vicinity of the old grain boundary during annealing after cold rolling.
Since the development of the 11 ° recrystallized texture is suppressed, high ductility and a high r value cannot be obtained stably, and the characteristic fluctuation also increases. For this reason, the average crystal grain size of the ferrite phase of the steel sheet is set to 50 μm or less, preferably 30 μm or less, and more preferably 10 μm or less.

On the other hand, when the average grain size is 1 μm or less, the ratio of the large-angle grain boundaries to the ferrite grain boundaries decreases,
It becomes difficult to make it 70% or more. Furthermore, the yield strength becomes extremely high, and it is not possible to obtain characteristics of a low yield ratio,
Press formability is greatly reduced. Therefore, the lower limit of the ferrite grain size is set to 1 μm.

Next, the volume ratio of the ferrite phase of the steel sheet was reduced to 70%.
It is necessary to do above. The ratio of ferrite phase is
If it is less than 70%, the strength of the steel sheet can be increased because the second phase having higher strength than the ferrite phase increases, but the ductility of the hot-rolled steel sheet and the cold-rolled steel sheet is significantly deteriorated, -Ductile balance cannot be properly maintained. For this reason, the ratio of the ferrite phase is 70% or more by volume, preferably 80% or more, more preferably 90% or more.

The aspect ratio of the ferrite phase is 5 or less. If the aspect ratio is more than 5, the microstructure will not have substantially isotropic properties as equiaxes, and the formability in the direction perpendicular to the direction of elongation will fluctuate.

It is necessary that 70% or more of the ferrite grain boundaries consist of large angle grain boundaries. When the ratio of the large-angle grain boundaries in the grain boundaries of the ferrite phase is less than 70%, relatively small-angle grain boundaries are relatively increased, and substantially coarse ferrite grains are increased. In this case, a high ductility, strength-ductility balance cannot be obtained stably, and the development of {111} recrystallized texture during annealing after cold rolling becomes difficult, and formability decreases.

As described above, when the ratio of the large-angle grain boundaries in the grain boundaries of the ferrite phase is less than 70%, the effect as a substantial ferrite grain boundary is reduced, and when the volume ratio of the ferrite phase is 70% or more, Good strength in a hot-rolled steel sheet or cold-rolled annealed steel sheet even if the average crystal grain size is 50 μm or less, and the aspect ratio is 5 or less, and at the same time, the average crystal grain size of the second phase described below is 50 μm or less. -Characteristic balance cannot be ensured, and characteristic fluctuations increase.

The phase occupying the maximum volume ratio among the remaining phases is defined as the second phase, and the average crystal grain size of the second phase is 50 μm or less.
Its volume fraction must be less than 30%. When the average crystal grain size of the second phase of the hot-rolled steel sheet exceeds 50 μm, the distribution of the hard second phase becomes uneven in the hot-rolled steel sheet, so that the yield strength changes {YS and elongation at break} EL. Becomes larger. Also,
Since cracks generated from the interface between the ferrite phase and the second phase during press molding are not easily prevented from propagating at the ferrite grain boundaries, ductility is reduced, and the strength-ductility balance is also reduced.

In the cold-rolled steel sheet, when the average crystal grain size of the second phase in the hot-rolled steel sheet is increased, the {111} recrystallization texture during cold rolling annealing is developed by randomizing the slip system in the vicinity of the second phase. Becomes difficult, and the moldability decreases. From such a point, the average crystal grain size of the second phase is 50 μm or less,
Preferably it is 30 μm or less, more preferably 10 μm or less.

The preferred lower limit of the average crystal grain size of the second phase is 0.1.
1 μm. When the average crystal grain size of the second phase is less than 0.1 μm, the effect of strengthening the ferrite phase is increased, the yield strength is increased, the yield ratio is increased, and the ferrite phase and the second phase are formed during press molding. This is because, rather than the dislocations proliferating from the interface, the second phase tends to hinder the movement of the dislocations, thereby deteriorating the formability. 3. Hot rolling The hot rolled steel sheet and the cold rolled steel sheet of the present invention are manufactured by the following method. 3-1 Slab heating When the temperature of the slab manufactured by the continuous casting method is 1300 ° C. or less and ₃ points or more of Ar, the slab can be directly fed as it is and hot-rolled. When the temperature of the slab is 1300 ° C. or higher, the slab is allowed to cool to a temperature of 1300 ° C. or lower, and then hot-rolled. If rolling after cooling, 95 slabs
After reheating to a temperature range of 0 to 1300 ° C., hot rolling is performed.

In rolling, the slab temperature is set to 1300 ° C. or lower because the slab temperature is lowered and precipitates such as MnS, AlN, TiS, and Ti ₄ C ₂ S ₂ which are coarsely precipitated during casting are re-dissolved. This is to suppress fine precipitation during hot rolling. Therefore, the slab temperature before hot rolling is
Preferably it is 1200 ° C. or lower, more preferably 1150 ° C. or lower. The heating time may be selected according to the size of the slab as long as the temperature becomes uniform throughout and the austenite crystal grains do not become coarse. 3-2 Rough rolling At least the final rolling pass of rough rolling is performed at _three points of Ar to 1150 ° C.
It is preferable to carry out in the temperature range described above. The total rolling reduction of the rough rolling is
It is preferably at least 30%. Thereby, the austenite crystal grains are refined, and the ferrite crystal grains after the γ / α transformation are easily refined. More preferably, the total draft is 40% or more. 3-3 Treatment of rough rolled material By reheating the rough rolled material or carrying out heat preservation before the start of finish rolling, it becomes easy to produce a hot rolled steel sheet and a cold rolled steel sheet having an excellent balance between strength and ductility. In other words, by this process, the temperature can be controlled such that the temperature difference in the coil in the coil longitudinal direction and the width direction of the rough-rolled material at the start of the finish rolling becomes 100 ° C. or less, and the condition of the above-mentioned equation (a) is satisfied. In addition, it becomes easy to control the fluctuation ΔFT of the finish rolling completion temperature in the coil.

When △ FT decreases, the temperature fluctuation in the cooling and winding process after finish rolling also decreases. Since the second phase formed in the cooling and winding process is a low-temperature transformation product, temperature fluctuations in the coil during cooling and winding directly change the type and volume ratio of the second phase, and furthermore, change characteristics in the coil. Connect directly. For this reason, when △ FT becomes small, the fluctuation of △ YS and △ EL becomes small.

The rough-rolled material is reheated or heat-treated before the start of finish rolling, and the finishing temperature is controlled so as to satisfy the condition of the above equation (a). The temperature is made uniform, and the temperature fluctuation in the coil in each of the steps of finish rolling, cooling, and winding can be suppressed. As a result, the microstructure can be made uniform, and characteristic fluctuations in the coil can be suppressed. The temperature difference in the coil of the rough rolled material at the start of the finish rolling is set to 100 ° C. or less, preferably 80 ° C. or less, more preferably 60 ° C. or less. The coefficient in the equation (a) is set to 0.6, but is preferably set to 0.5, and more preferably set to 0.4.

[0057] Further, by the above processing, when completing the finish rolling at austenite region is, △ since FT can be reduced, Ar ₃ over the entire coil even if the finish rolling completion temperature just above Ar ₃ point It can be rolled without falling below the point. As a result, the cumulative strain in the low-temperature austenite region can be increased, and the ferrite grain size after transformation can be efficiently reduced, and the grain boundary can be increased in angle.

When the finish rolling is completed in the ferrite region,
By performing the above treatment, the cumulative strain in the ferrite region can be increased, and the ferrite phase is recrystallized as hot rolled or heat treated after hot rolling (continuous annealing, box annealing, hot dip galvanizing, (For example, annealing before galvannealing). By annealing such a structure after cold rolling, the formability of the cold rolled steel sheet can be further improved.

Regardless of whether the finishing temperature is in the austenite region or the ferrite region, the above treatment facilitates obtaining the crystal structure characteristic of the steel sheet of the present invention, and has high formability and characteristic fluctuation. It is possible to obtain less hot-rolled steel sheets and cold-rolled steel sheets.

Furthermore, by reheating or preheating the rough rolled material before the finish rolling, even if the slab temperature before the rough rolling is lowered, the finish rolling can be performed without significantly lowering the finish rolling completion temperature. An increase in hot deformation resistance during rolling can be suppressed, and rolling can be performed without overloading the hot rolling mill. That is, by the above-described processing, the slab temperature before rough rolling can be lowered, so that it is possible to suppress fine precipitation during the above-described hot rolling, and to form a hot-rolled steel sheet or a cold-rolled steel sheet with high formability. It becomes easy to obtain. After roughly rolling the slab and winding it into a coil using a coil box, after rewinding and performing the finish rolling, and joining the leading end of the rough rolled material to the rear end of the preceding rough rolled material, The continuous finish rolling process of performing finish rolling is effective in making the characteristics in the coil uniform, but by combining these processes with the above-described processing, the characteristics can be made more uniform.

The surface of the rough rolled material is cooled, and the structure is transformed from the austenite phase to the ferrite phase.
By performing the reverse transformation to the austenite phase by the above treatment, the ferrite grain size in the surface layer portion of the hot-rolled steel sheet can be efficiently refined, and the grain boundary can be made large.

As a method of heating or keeping the heat of the rough-rolled material before the start of the finish rolling, a method of heating the rough-rolled material by high-frequency induction heating, a current heating method of directly applying a current to the rough-rolled material, and heating the combustion gas A gas heating method using a gas burner to be used or the like can be adopted. In particular, a rough bar heater of an induction heating method is effective, but the heating method is not limited to these methods.

In the thin slab casting process in which rough rolling can be simplified or omitted, there is no problem even if this process is used as a method of heating or keeping heat before the start of finish rolling. 3-4 Finish Rolling In order to obtain the hot-rolled steel sheet of the present invention, it is desirable that the above-described rough rolled material be processed and finish rolling be performed so as to satisfy the condition of the above-mentioned formula (a). For austenitic finish,
Finish rolling completion temperature is (Ar ₃ points + 100 ° C) or less, (Ar
₃ points -50 ° C) or more and the total draft in that temperature range is
Preferably, it is 50% or more. If the finishing temperature exceeds (Ar ₃ points + 100 ° C.) or the total draft is less than 50%, it becomes difficult to secure the microstructure specified in the present invention. This is because the degree of strain accumulation is small. The total draft in the specified temperature range is preferably 60% or more, more preferably 70% or more.

When the finish rolling completion temperature is (Ar₃(Point -50 ℃)
If it is lower than
You. In the case of ferrite finish rolling, the finishing rolling temperature is
(Ar ₃(Point -50 ° C), (Ar₃(-250 ° C)
Temperature range, and the total draft at this time should be 50% or more.
Is desirable. If these conditions cannot be satisfied,
It is difficult to secure the microstructure specified in the present invention.
You. That is, when the finishing temperature is (Ar₃Point -250 ° C)
If it is too low, it will not be possible to obtain a recrystallized
No, recrystallize the processed ferrite structure by annealing
However, the texture that improves the moldability was not obtained.
It is. The total reduction in this temperature range is preferably 60
%, More preferably 70% or more.

In the finish rolling, a friction coefficient μ between the rolling roll and the material to be rolled is set to 0.2 by using a hot lubricant.
The rolling is preferably performed as follows. Thereby, the work deformation in the thickness direction is made uniform, so that the ductility of the hot-rolled steel sheet or the cold-rolled steel sheet at the surface layer portion of the thickness can be improved. A commonly used hot lubricant may be used. For example, a mechanical oil having a reduced friction coefficient can be used. 3-5 Cooling and winding In order to make the ferrite crystal grains fine, it is preferable to cool at a cooling rate of 20 ° C./sec or more after the final rolling pass of finish rolling is completed. It is more preferably at least 30 ° C./sec. The time from the end of the final rolling pass to the start of cooling is preferably shorter than 3 seconds, more preferably shorter than 2 seconds.

If the winding temperature exceeds 800 ° C., increase in scale loss, deterioration of surface properties, coarsening of hot-rolled steel sheet ferrite crystal grains, etc. become problems.
It is preferable that the temperature is set to not more than ° C. However, in the case of recrystallization as hot rolling in the finish rolling in the ferrite region, the lower limit of the winding temperature is preferably 630 ° C. or higher. 4. Cold Rolling and Recrystallization Annealing A hot-rolled steel sheet is subjected to pickling by a usual method to remove oxides and dirt on the surface, then cold-rolled, and then recrystallization-annealed. In order to develop {111} recrystallization texture at the time of annealing after cold rolling, in order to develop {111} texture even in a hot-rolled steel sheet, 600
The hot-rolled steel sheet may be annealed under conditions of heating to a temperature of not less than ° C.

In cold rolling, a rolling texture is developed,
In order to develop {111} texture which is preferable for improving ductility and minimizing in-plane anisotropy by recrystallization annealing, the steel sheet is worked to a final thickness at a rolling reduction of 50% or more.

In the recrystallization annealing, in order to develop a texture favorable for deep drawability from the rolled texture introduced by cold rolling, the material is annealed by heating to a temperature range of 600 to 950 ° C. At a temperature lower than 600 ° C., recrystallization does not sufficiently proceed even after long-time annealing, while at a temperature higher than 950 ° C., the deep drawability decreases. Although the annealing method is not limited, a box annealing method, a continuous annealing method, or a continuous annealing usually performed in a hot-dip galvanizing treatment or an alloyed hot-dip galvanizing treatment may be used.

After cold rolling and recrystallization annealing, surface treatment such as temper rolling (skin pass) or hot dip galvanizing, galvannealing, electroplating, and organic coating may be applied. After being subjected to press working, these steel sheets are used for, for example, automobiles, home appliances, steel structures, and the like.

[0070]

EXAMPLES In order to confirm the effects of the steel sheet of the present invention, steel types A to J having the chemical compositions shown in Table 2 were cast using a vacuum melting furnace, and slabs having a thickness of 50 mm were produced by hot forging. A hot-rolled steel sheet was manufactured using the slab. Based on the conditions shown in Table 3, slab heating, rough rolling and finish rolling were performed on a laboratory scale to obtain a hot-rolled steel sheet having a thickness of 2.6 mm and a width of 250 mm. Heating of the rough rolled material was performed using a laboratory scale induction heating device.

The rolling conditions are as various as possible. For example, for steel type A of hot rolled Nos. 2 and ₃ , finish rolling is performed in a ferrite region in which the finish rolling completion temperature is (Ar ₃ points -150 ° C.), cooled to 700 ° C., and then gradually cooled (processing equivalent to winding).
When annealing is omitted, and after 800 ° C
A hot-rolled steel sheet was manufactured under two conditions when annealing at ℃. Rolling in the ferrite region was performed under high lubrication conditions with a friction coefficient between the steel sheet and the rolling roll of 0.12.

[0072]

[Table 2]

[Table 3] Table 3 shows the steel types B and D of hot rolled Nos. 4, 6, and 15.
Hot rolling was performed under the conditions shown in Table 1 to produce a 4.0 mm hot-rolled steel sheet. The resulting hot-rolled steel sheet having a thickness of 4.0 mm was pickled, cold-rolled based on the rolling conditions shown in Table 5, and further subjected to recrystallization annealing and temper rolling at a rolling reduction of 0.6%. A cold-rolled steel sheet having a thickness of 0.8 mm and a width of 250 mm was manufactured. Annealing for recrystallization was a continuous annealing method.

With respect to the obtained hot-rolled steel sheet and cold-rolled annealed steel sheet, a JIS Z 2201 No. 5 test piece was obtained from a total of three places, namely, both edges, a top part, a middle part, and a bottom part, and a half width. Were sampled, and the yield strength, tensile strength, elongation at break, and their variations were investigated. The survey results are shown in Tables 4 and 5.

According to the above-described method, the characteristic values of each tensile test were set to 0 °, 4 ° with respect to the rolling direction for the both edge portions from the direction parallel to the rolling direction, and to the half width.
The measured values of test specimens taken from 5 ° and 90 ° directions are averaged and used. The fluctuation value of the characteristic value was obtained as a value obtained by subtracting the minimum value from the maximum value of each characteristic. The yield ratio (YR) indicates the maximum value in the coil, and the TS × EL value indicates the minimum value in the coil.

As is evident from the results in Tables 4 and 5, the hot-rolled steel sheet and the cold-rolled steel sheet satisfying the chemical composition, crystal structure and rolling conditions specified in the present invention all have the yield strength in the coil. It can be seen that the change in elongation at break is small, the moldability is high, and the yield ratio is low. Specifically, the invention examples shown in Tables 4 and 5 show that TS ≧ 270 MPa,
TS × EL ≧ 15000MPa ·%, YS ≦ 50MPa, ΔEL
≦ 5%, and the conditions of YR ≦ 0.75 are all satisfied.

[0076]

[Table 4]

[Table 5]

According to the steel sheet of the present invention, the chemical composition of the steel,
By selecting the optimal conditions of crystal structure, hot and cold rolling, the steel sheet has excellent balance between strength and ductility, small variation in yield strength and elongation at break, high formability and low yield ratio. Can be achieved. Further, according to the production method of the present invention, a member for seismic, seismic control, and seismic isolation of a structure such as a building and a high formability for an automobile are required without coarsening crystal grains as in the related art. A steel sheet suitable for a member can be efficiently manufactured.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the average crystal grain size of a ferrite phase and a second phase with respect to the strength-ductility balance of a hot-rolled steel sheet containing 70% or more by volume of a ferrite phase.

FIG. 2 shows an average crystal grain size of a ferrite phase of 0.5 to 38 μm,
It is a figure which shows the influence of the volume ratio of an equiaxed ferrite phase and a 2nd phase on the intensity-ductility balance of the hot-rolled steel sheet whose average crystal grain diameter of a 2nd phase is 0.3-30 micrometers.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C21D 9/46 C21D 9/46 G C22C 38/58 C22C 38/58 (72) Inventor Koichi Hirano Osaka, Osaka, Japan Sumitomo Metal Industries Co., Ltd., 4-5-33, Kitahama, Chuo-ku, City (72) Inventor Etsuzo Kaneko 4-5-33, Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries, Ltd. (72) Inventor Akira Sakota People 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka F-term (reference) in Sumitomo Metal Industries, Ltd. 4K037 EA01 EA02 EA04 EA05 EA09 EA11 EA13 EA15 EA16 EA17 EA18 EA19 EA20 EA23 EA25 EA27 EA28 EA31 EA32 EB35 EA36 EB35 EB09 EB11 FA01 FA02 FA03 FB03 FC03 FC04 FE02 FE03 FF02 FF03 FJ04 FJ05 FJ06 JA01 JA06

Claims (5)

[Claims] (1) In mass%, C: 0.0002 to 0.1%, Si: 0.0
03-2.0%, Mn: 0.003-3.0% and Al: 0.002-2.
0%, and one or more of the following groups 1 to 4 are selected, and the balance is substantially composed of Fe: P: 0.0002 to 0.15%, S: 0.0002 to 0.05
% And N: 0.0005 to 0.015%, wherein the average crystal grain size of the ferrite phase is more than 1 μm to 50 μm, the volume ratio is 70% or more, and the aspect ratio of the ferrite phase is 5%.
Yield strength, wherein 70% or more of ferrite grain boundaries are composed of large-angle grain boundaries, and the average crystal grain size of the second phase having the largest volume fraction among the remaining phases is 50 μm or less. A steel sheet having high formability and a low yield ratio with small fluctuation in elongation at break. Group 1 ... B: contains 0.0002 to 0.01% Group 2 ... One or more of Ti, Nb, V and Zr contain 0.005 to 1.0% in total Group 3 ... Cr, Mo, Cu and Ni One or two or more of them contain 0.005% to 3.0% in total. Four groups: Ca: 0.005% or less and rare earth element: 0.20% or less 2. The steel sheet according to claim 1, wherein the steel sheet is a hot-rolled steel sheet or a cold-rolled steel sheet, and has a high formability and a low yield ratio. 3. Hot rolling immediately after casting a slab using the steel containing the component according to claim 1, or
When hot rolling after reheating in a temperature range of 950 to 1300 ° C., after rough rolling of the steel slab, the difference in coil temperature in the coil longitudinal direction and width direction of the rough rolled material at the start of finish rolling is 100
The method is characterized in that the rough-rolled material is reheated or kept at a temperature of not more than 0 ° C. and hot-rolled at the same time as satisfying the following equation (a). A method for producing a steel sheet having a property and a low yield ratio. ΔFT ≦ 0.6 × ΔRT (a) where ΔFT: fluctuation of finish rolling completion temperature (° C.), ΔR
T: Fluctuation of rough rolling completion temperature (° C.), ΔFT ≦ 30 ° C.
In the case of (1), the expression (a) is not applied. 4. The method according to claim 3, wherein after the hot rolling, annealing is carried out in a temperature range of 600 to 950 ° C., wherein the variation in yield strength and elongation at break is small and high formability and low yield ratio are obtained. A method for producing a steel sheet having: 5. The hot-rolled steel sheet produced according to claim 3 or 4 is further subjected to cold rolling at a rolling reduction of 50% or more to obtain 600%
A method for producing a steel sheet having a small change in yield strength and elongation at break, which has high formability and a low yield ratio, wherein the steel sheet is annealed in a temperature range of 950 ° C. to 950 ° C.