JPH11152544A - Hot rolled steel sheet for working having ultrafine grain, its production and production of cold rolled steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a hot rolled steel sheet which contains ultrafine grains and is small in the anisotropy of mechanical properties by regulating the average ferrite grain size and the aspect ratio of ferrite grains to < specified value and regulating the ratio of the average ferrite grain size to the average grain size of the secondary phase to a specified range. SOLUTION: The average ferrite grain size in regulated to <2 μm, the aspect ratio of ferrite grains is regulated to <1.5, and the average ferrite grain size dm (μm) and the average grain size ds (μm) of the secondary phase satisfy 0.3<dm/ds<3. The compsn. of the steel sheet preferably has a compsn. contg., by weight, 0.01 to 0.3% C, <=3.0% Si, <=3.0% Mn and <=0.5% P, contg. the other metals by 0 to 0.005% in total, and the balance substantial iron. The stock for a hot rolled steel sheet is melted is immediately or temporarily cooled and is heated to <=1200 deg.C, and at the time of subjecting it to hot rolling, the rolling reduction in the dynamic recrystallization region is executed by the passes of >=5 stands.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention is advantageous when applied to uses for automobiles, home appliances, mechanical structures, constructions, etc.
The present invention relates to a hot-rolled steel sheet which has ultrafine ferrite grains having an average grain size of less than 2 μm as hot-rolled, is excellent in ductility, toughness, fatigue strength and the like, and has a small anisotropy of these properties, and a method for producing the same. In addition, the present invention relates to a cold-rolled steel sheet having excellent workability using the hot-rolled steel sheet as a material.

[0002]

2. Description of the Related Art Steel materials used for automobile materials and structural materials are required to have excellent mechanical properties such as strength, workability and toughness. Since it is effective to refine the structure to comprehensively enhance these mechanical properties, many production methods aiming at the fine structure are being sought. In recent years, with regard to high-tensile steel sheets, there is a strong need for high-tensile steel sheets that can achieve both low cost and high-performance characteristics, and the target is shifting to the development of steel sheets that meet these needs. For the purpose of suppressing deterioration of ductility, toughness, durability ratio, and the like, miniaturization of the structure of high-strength steel is an important issue. Furthermore, in cold-rolled steel sheets also used for automobile materials, it is said that the grain refinement of the hot-rolled steel sheets used as the material is effective in improving workability, particularly r-value (Rankford value). Refining the structure of a hot-rolled steel sheet as a base material is also an important issue.

[0003] In general, means for refining the structure in the prior art include a large rolling reduction method, a controlled rolling method and a controlled cooling method. Among them, there is proposed a method of refining the structure by large rolling reduction, which is represented by, for example, JP-A-58-123823. The essential point of the refining mechanism in these methods is to promote the γ → α strain-induced transformation by applying a large pressure to the austenite grains, and a certain degree of refining is achieved by such a method. However, there is a manufacturing problem that it is difficult to realize with a general hot strip mill, such as making the rolling reduction per pass 40% or more. In addition, the refinement of the final structure obtained due to such difficult-to-realize manufacturing conditions has a limit, and the average crystal grain size is at most about 5 μm. In addition, since the crystal grains are flattened by the high rolling reduction, there is a problem that anisotropy occurs in the mechanical properties and the fracture absorption energy decreases due to separation.

On the other hand, as a steel sheet to which a crystal refinement method belonging to a controlled rolling method or a controlled cooling method is applied, there is a precipitation strengthened steel sheet containing Nb or Ti. These steel sheets use the precipitation strengthening action of Nb and Ti to increase the tensile strength,
Utilizing the recrystallization inhibiting action of austenite grains provided by the method, ferrite grains are refined by γ → α strain-induced transformation from unrecrystallized deformed austenite grains when subjected to low-temperature finish rolling. However, these steel sheets have a problem in that the anisotropy of mechanical properties is large, for example,
In the case of automotive steel sheets subjected to press forming, the forming limit is determined by the characteristic level in the direction in which ductility is the least ductile.Therefore, in such a highly anisotropic steel sheet, the effect of refining the structure may not appear as a characteristic at all. is there. The same applies when used for structural materials, etc.
Since the anisotropy such as fatigue strength increases, the effect of making the structure finer may not appear as a characteristic at all. Further, in any of these methods, the particle size obtained was at most about 2 μm. There is also known a means for suppressing grain growth by performing a quenching treatment immediately after hot rolling (for example, Japanese Patent Publication No. 4-1608), but this method also limits fine grains of about 4 μm. It is.

[0005]

As described above, in the prior art, the achievable final ferrite grain size is 2 μm.
m was the limit. Since the effect of improving mechanical properties by refining crystal grains is inversely proportional to the square root of the crystal grain size,
Although this improvement effect only shows a gradual improvement in the region where the grain size is 2 μm or more, a significant improvement in characteristics can be achieved if the crystal grain size of less than 2 μm is realized.

[0006] The present invention solves the problems of the prior art, and can be easily implemented with a general hot strip mill.
In addition, the base material for hot-rolled steel sheets and cold-rolled steel sheets, which have low anisotropy of mechanical properties and achieve ultra-fine grains having a final ferrite grain size of less than 2 μm, which could not be achieved by the prior art, is advantageous. The purpose is to propose with the manufacturing method.

[0007]

The present invention is a hot-rolled steel sheet containing ferrite as a main phase, characterized in that the average ferrite grain size is less than 2 μm and the aspect ratio of the ferrite grains is less than 1.5. , A hot-rolled steel sheet for processing having ultra-fine grains. The present invention also relates to a hot-rolled steel sheet containing ferrite as a main phase, wherein the average ferrite grain size is less than 2 μm, the aspect ratio of the ferrite grains is less than 1.5, and the average ferrite grain size is dm (μm). Average grain size of two phases
ds (μm) is a hot-rolled steel sheet for processing having ultrafine grains satisfying the following expression: 0.3 <dm / ds <3. Further, the present invention relates to a hot-rolled steel sheet containing ferrite as a main phase, wherein the average ferrite grain size is less than 2 μm, the aspect ratio of the ferrite grains is less than 1.5, and the average ferrite grain size is dm (μm). The average crystal grain size ds (μm) of the two phases satisfies the following equation: 0.3 <dm / ds <3, and the interval between the nearest second phase particles is less than twice the crystal grain radius of the second phase. This is a hot-rolled steel sheet for processing having ultrafine grains having a second phase in which the ratio is less than 10%. The preferred composition range of the hot-rolled steel sheet for processing according to the present invention is C: 0.01.
~ 0.3 wt%, Si: 3.0 wt% or less, Mn: 3.0 wt% or less,
P: 0.5 wt% or less, Ti: 0 to 1.0 wt%, N
b: 0 to 1.0 wt%, V: 0 to 1.0 wt%, Cr: 0 to 1.0 wt%
%, Cu: 0 to 3.0 wt%, Mo: 0 to 1.0 wt%, Ni: 0 to 1.
0 wt%, one or more of Ca, REM, B, and one or more of them in total from 0 to 0.005 wt%, with the balance being substantially iron. Further, among the above preferred component compositions,
When the Mn content is 0.5 wt% or more, the second phase has a structure containing one or more of martensite, bainite, retained austenite, pearlite, and acicular ferrite. Further, the present invention provides a method for producing a material for hot-rolled steel sheet, immediately or once cooled and heated to 1200 ° C. or less to perform hot rolling, wherein the reduction in the dynamic recrystallization region is reduced by 5 stands or more. This is a method for producing a hot-rolled steel sheet for processing having ultrafine grains, which is performed in a pass. Further, the hot-rolled steel sheet for processing of the present invention more preferably has a bake hardening amount of 100 MPa or more. In the method for producing a hot-rolled steel sheet for processing according to the present invention, the roll or the steel sheet can be heated by the heating means provided between the roll stands of the finish rolling equipment. Further, the hot-rolled steel sheet for processing of the present invention can be used as a base material for a cold-rolled steel sheet having ultra-fine grains. Cold rolling at 50-90% reduction rate, then 600 ° C
There is a method of performing annealing at a temperature not higher than the Ac ₃ transformation point.

[0008] In the present invention, the aspect ratio of ferrite grains refers to the ratio between the major axis and the minor axis of the ferrite grains. Practically, since the ferrite grains extend in the rolling direction, the ratio of the major axis to the minor axis on the cross section in the rolling direction is substituted. Further, in the present invention, the average particle size of the ferrite particles is defined as the average particle size in a cross section in the rolling direction according to a conventional method. Second
The average crystal grain size of a phase is obtained by measuring the area and the number of crystals of a structure other than ferrite, which is the main phase, from a crystal structure photograph, and converting it into a diameter (diameter) equivalent to a circle having the area. . Also in the case of obtaining the individual second phase particle size, it is to be converted into a circle equivalent. A steel sheet having ferrite as a main phase means that the ferrite phase has a volume fraction of 50% or more. Also, in the steel sheet of the present invention, Ti
The lower limit of the content such as the amount being 0% means that those components may not be added in some cases.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The inventors have conducted research and development to solve the above problems, and as a result, during hot rolling,
By repeatedly performing reduction in the dynamic recrystallization region,
It has been found that ferrite can be made into ultrafine grains. And it is not necessary to reduce the rolling in the dynamic recrystallization region to a large value, and therefore, a good structure in which the aspect ratio of the ferrite grains is less than 1.5 is obtained. It was also found out.

The average ferrite grain size as described above is 2 μm.
m, and the ferrite grains have an aspect ratio of less than 1.5, because the crystal grains are fine, not only are the mechanical properties such as strength, toughness, and ductility particularly excellent, but also their anisotropy is low. . Moreover, since the grain boundary area is larger than that of a steel sheet having a grain size of 2 μm or more, a large amount of solid solution C is trapped in the crystal grain boundaries. Therefore, solid solution C during baking coating
Is diffused into the grains to fix dislocations,
Excellent paint bake hardening ability of 100 MPa or more. Therefore, it can be easily processed at the time of forming processing, but can obtain high strength by heat treatment such as baking after painting, and is particularly suitable as a steel plate for automobiles.

Among the hot-rolled steel sheets whose main phase is ferrite, the average ferrite grain size is less than 2 μm, and the aspect ratio of the ferrite grains is less than 1.5, the average ferrite grain size dm (μm) and the second phase The average grain size ds (μm) of the steel sheet satisfying the following equation: 0.3 <dm / ds <3, particularly, has a small difference in crystal grain size, so that the steel sheet is uniformly deformed, necked, wrinkled, and has poor surface properties. Is less likely to occur. For this reason, the workability is good, and it is particularly suitable for a processing method for expanding a hole. Also, the fatigue properties and fracture toughness are extremely good.

The hot-rolled steel sheet according to the present invention having the above-mentioned characteristics is applicable to a wide range of fields and applications from mild steel sheets to steel sheets for automobile structures, steel sheets for high-strength automobiles for processing, steel sheets for home appliances, structural steel sheets and the like. The present invention can be applied to a steel sheet (hereinafter, the term “steel sheet for processing” in this specification is used in a meaning including all of these uses). Therefore, DP (Dua
l Phase) steel and TRIP (Transformation Induced Plasti
city) can be applied to a composite structure steel sheet containing one or more of martensite, bainite, retained austenite, pearlite, and acicular ferrite as a second phase such as steel. It is also possible to use a steel sheet having a structure containing a small amount of pearlite or cementite as the second phase. Furthermore, S
The amount can be reduced to 0.002 wt% or less to improve hole spreading properties and fatigue crack propagation stopping properties, and can be used as a steel plate for automobile wheels.

FIG. 1 shows the results of a study on the relationship between the average ferrite grain size and mechanical properties of a hot-rolled steel sheet. In this investigation, C: 0.03 wt%, Si: 0.1 wt%, Mn: 0.2 wt%, P:
A steel containing 0.01 wt%, S: 0.003 wt%, and Al: 0.04 wt% is heated to 1100 ° C., then subjected to rough rolling under ordinary conditions, and then subjected to various finishing rolling equipment including 7 stands. The test was performed on hot-rolled steel sheets having various ferrite crystal grain sizes obtained by applying rolling under finish rolling conditions. For the steel sheet having a grain size of less than 2 μm, the temperature difference between the steel sheet temperature at the entrance of the first stand and the steel sheet temperature at the exit side of the final (seventh) stand at the time of finish rolling is 60 ° C. or less. Was also obtained at a temperature difference of about 30 ° C. or less. Also,
Examination of the aspect ratio revealed that all of the steel sheets having a grain size of less than 2 μm obtained by the above-mentioned method were less than 1.5. Incidentally, the bake hardening amount (BH amount) in the figure was determined by heating at 170 ° C. for 20 minutes after a 2% prestrain, and then performing a tensile test again to determine the amount of increase in load.

From the figure, it can be seen that the average ferrite grain size is 2 μm.
m, compared to steel plates of 2 μm or more,
It can be seen that various characteristics are significantly improved. This tendency is
The same was true not only for the steel sheets of the component compositions tested, but also for steel sheets of other component systems. In addition, the average ferrite grain size is 1
Various characteristics were further improved by setting the thickness to μm or less. Therefore, in the present invention, the average ferrite grain size is 2 μm.
m, the aspect ratio of ferrite grains is limited to less than 1.5. When the average ferrite grain size was less than 2 μm, the grain size of the second phase was examined.
It was in the range of more than 0.5 to less than 2.

In the steel sheet having ferrite as a main phase according to the present invention, the average ferrite grain size dm (μm) and the average crystal grain size ds (μm) of the second phase satisfy the following equation: 0.3 <dm / ds <3. It is more preferable to satisfy. This is because if there is a large difference in crystal grain size between the main phase ferrite and the second phase crystal, mechanical properties may be degraded.
This is considered to be because when the difference in crystal grain size is large, deformation during processing becomes non-uniform. As a result of studying the preferred range of the ratio of the crystal grain size between the main phase and the second phase by the inventors, it was found that the ratio was 0.2
It was found that when the value was larger than 3 and smaller than 3, the mechanical properties were good and uniform deformation occurred. More preferably, it is in the range of 0.5 <dm / ds <2.

Further, in the steel sheet according to the present invention, with respect to the second phase, the rate at which the interval between the nearest second phase particles is less than twice the crystal grain radius of the second phase is less than 10%. It is preferable to have ultrafine particles having the following. As a result of various studies on the distribution state of the second phase, the inventors have found that if the second phase is distributed in a band shape or a row shape (layer shape), sufficient improvement in mechanical properties, particularly stretch flangeability, cannot be obtained. Therefore, it has been found that a so-called island-like distribution mode in which the second phases are relatively isolated without dense second phases is desirable. As an evaluation means showing such a form distributed in an island shape, as for the second phase, the ratio at which the interval between the nearest second phase particles is less than twice the crystal grain radius of the second phase is 10%.
If it is less than this, the characteristics are improved. The volume ratio of the second phase to the whole is preferably in the range of 3 to 30%.

The preferred composition range of the steel sheet of the present invention is as follows. (C: 0.01 to 0.3 wt%) C is a cheap reinforcing component,
The necessary amount is added according to the desired steel sheet strength. C content is 0.
If the content is less than 01 wt%, the crystal grains become coarse, and the ferrite average crystal grain size of 2 μm or less, which is the object of the present invention, cannot be achieved, and if added in a large amount exceeding 0.3 wt%, the workability deteriorates. And the weldability also deteriorates.
Preferably, the content is about 0.3 wt%. When the structure contains a small amount (10% or less) of cementite or pearlite as the ferrite single phase or the second phase, C
Is preferably about 0.01 to 0.1 wt%.

(Si: 3.0 wt% or less) Si effectively contributes to the increase in strength while improving the strength-elongation balance as a solid solution strengthening component, and suppresses ferrite transformation to obtain a desired second phase volume. Works effectively to obtain a texture with high elongation, but excessive addition limits the upper limit to deteriorate ductility and surface properties.
3.0 wt%. More preferably, 0.05 to 2.0 wt%
Range. When the ferrite single phase or the second phase has a structure containing a small amount (10% or less) of cementite or pearlite, the content of Si is preferably 1.0 wt% or less.

(Mn: 3.0 wt% or less) Mn contributes to the refinement of crystal grains through the action of lowering the Ar ₃ transformation point, and also promotes the formation of martensite and a retained austenite phase in the second phase. Through the strength-ductility balance,
It has the effect of increasing the strength-fatigue strength ductility balance. Furthermore, it has the effect of detoxifying harmful solid-solution S as MnS. However, if too much is added, the steel hardens and the strength is reduced.
Since the ductility balance is deteriorated, the upper limit is set to 3.0 wt%.
When the second phase has a structure containing one or more of martensite, bainite, pearlite, retained austenite and acicular ferrite, it is preferable to contain 0.5 wt% or more in order to obtain such a structure. . More preferably, it is in the range of 1.0 to 2.0 wt%. When the ferrite single phase or the second phase has a structure containing a small amount (10% or less) of cementite or pearlite, Mn is 2.0 wt% or less, more preferably 0.1 to 1.0 wt%.
% Is preferable.

(P: 0.5 wt% or less) Since P is also useful as a reinforcing component of steel, it can be added according to the desired steel sheet strength. However, excessive addition causes segregation at the grain boundaries and deterioration of brittleness. Therefore, the upper limit is 0.5 wt%. More preferably, it is in the range of 0.005 to 0.2 wt%.

Ti, Nb, V, and Mo are components useful in the present invention for forming a carbonitride and refining crystal grains to obtain an ultrafine structure of 2 μm or less. It also has the effect of improving strength. Therefore,
In the present invention, one or more of Ti, Nb, V and Mo are
Add as needed. Particularly, Ti forms a carbonitride even at a relatively low temperature and is stably present in the steel. Therefore, the above-mentioned effect is easily exerted even at a low slab heating temperature.
In the present invention, in order to exert these effects, it is preferable to contain 0.01 wt% or more, and if added in an excessively large amount, the effects are saturated and the cost is increased. %, More preferably 0.5 wt%
% Or less. In the case of a structure containing a small amount (10% or less) of cementite or pearlite as the ferrite single phase or the second phase, the content of these components is 0.3 wt%.
The content is preferably at most 0.1 wt% or less.

Like Cr, Cu and Ni, Mn can also be contained as a strengthening component, if necessary. However, if too much is added, the strength-ductility balance is rather deteriorated. Cr is set to about 1.0 wt%. In addition, in order to fully exhibit the effect, 0.01wt
% Is preferably contained.

Since Ca, REM and B have the effect of improving the workability by controlling the shape of the sulfide and increasing the grain boundary strength,
Although it can be contained as needed, excessive addition may adversely affect cleanliness and recrystallization, so it is preferably about 50 ppm or less. Note that B also has an effect of reducing aging when a cold-rolled steel sheet is obtained by continuous annealing.

In the steel sheet of the present invention, Mn is contained in an amount of 0.5 wt% or more in the above preferred composition range, and the second phase contains one or more of martensite, bainite, retained austenite, pearlite and acicular ferrite. Composite tissue. The present invention is not limited to this, and a steel sheet having a structure containing a small amount of pearlite or cementite as the ferrite single phase or the second phase can also be used.

Next, a method for manufacturing a steel sheet according to the present invention will be described. Molten steel adjusted to a predetermined component composition range, as a rolled material by continuous casting or ingot-bulking-rolling, and hot rolled to this rolled material, when subjected to rolling,
It may be cooled once and reheated to 1200 ° C or less, or
Direct rolling or hot charge rolling (HCR) may be used. Further, as in the thin slab continuous casting method, a slab cast by continuous casting may be directly hot-rolled. In the case of reheating, heating at a low temperature of 1200 ° C. or less is advantageous because the crystal grains do not become coarse. Also in the case of direct rolling, it is desirable to start rolling after cooling to 1200 ° C. or less from the viewpoint of suppressing grain growth during rolling. Average ferrite grain size dm
The slab heating temperature is desirably 1150 ° C. or lower so that (μm) and the average crystal grain size ds (μm) of the second phase satisfy the following expression: 0.3 <dm / ds <3. In order to disperse the second phase in an island shape, the slab heating temperature is preferably 1100 ° C. or lower. In any case, the lower limit is sufficient if the finish rolling temperature can be ensured, and is about 900 ° C. at present.

[0026] Hot rolling is the most important aspect of the present invention. That is, the hot rolling is performed in a dynamic recrystallization region by a rolling pass of 5 stands or more, and the ferrite average crystal grain size of less than 2 μm expected in the present invention and the aspect ratio are reduced.
In order to obtain a structure having ultra-fine grains in which the average ferrite grain size dm (μm) is less than 1.5 and the average crystal grain size ds (μm) of the second phase satisfies the following equation: 0.3 <dm / ds <3. It is important.

In order to apply the reduction in the dynamic recrystallization region, for example, it is effective to apply the reduction in five or more continuous stands while minimizing the temperature reduction of the rolling material during finish rolling. The temperature difference between the temperature of the steel plate on the first stand entrance side and the temperature of the steel sheet on the last stand exit side is preferably 60 ° C. or less, more preferably 30 ° C. or less. The five consecutive stands represent stands that actually perform rolling. For example, there is no problem even if a stand that is not lowered in an open state is sandwiched.

When rolling is performed in the dynamic recrystallization region in the finish rolling including the latter stage, it is preferable that the reduction in the dynamic recrystallization region includes the final stand in order to obtain a good aspect ratio. Further, in order to easily realize the reduction in the dynamic recrystallization region, it is desirable to apply the reduction immediately above the Ar ₃ transformation point. The rolling reduction of each stand rolling in the dynamic recrystallization zone is:
High pressure reduction is not necessary, but rather high pressure reduction is not preferable because the aspect ratio of crystal grains is deteriorated. At most 20% is good. The lower limit of the rolling reduction is not particularly limited as long as dynamic recrystallization occurs, but is preferably 4% or more. When the dynamic recrystallization region is a higher temperature region, the dynamic recrystallization region rolling may be performed from a stage after the rough rolling to a stage before the finish rolling. Preferable rolling conditions are the same as in the case of including the latter stage of finish rolling.

The finish rolling as described above can be carried out even in a normal finishing rolling equipment by extremely reducing the cooling of the steel sheet and the equipment during hot rolling. However, a heating means is provided between the finishing rolling stands. Thus, heating the material to be rolled or the roll can more easily prevent a temperature drop of the steel sheet during finish rolling. FIG. 2 shows an example of such a heating means. The example shown in FIG. 3A is a high-frequency heating apparatus, which applies an alternating magnetic field to a steel sheet to generate an induced current and heat the steel sheet. The heating means of the present invention is not limited to the high-frequency heating device shown in FIG. 1A, but may be heated by an electric heater as shown in FIG.
Alternatively, a direct current heating heater may be used. In addition, at the time of hot rolling, it goes without saying that the reduction may be performed while applying lubrication.

The steel sheet that has been subjected to the above finish rolling is wound up into a coil. The winding temperature and the cooling rate after winding are not particularly limited, and are appropriately determined according to the steel sheet to be manufactured. In the case of a composite structure steel sheet such as a DP steel or a TRIP steel, a desired composite structure can be obtained under conditions such as quenching the respective martens and bainite regions through the nose of the ferrite region on the cooling curve and winding up. However, a steel sheet having a structure containing a small amount of pearlite or cementite as a ferrite single phase or the second phase,
Rolling to avoid the cooling curve produced by the second phase structure,
Winding and cooling may be performed. Further, in order to obtain a structure having an island-like distributed second phase in which the distance between adjacent second phase particles is less than twice the crystal grain radius of the second phase is less than 10%, It is desirable that the slab heating temperature be 1100 ° C. or less, cooling is performed immediately after finish rolling, and cooling is performed at a cooling rate of 30 ° C./s or more. Immediately after the finish rolling, immediate quenching for cooling is more preferable in order to obtain the steel sheet of the present invention in which ultrafine grains are obtained, since crystal grains can be prevented from becoming coarse. Preferred quenching conditions are within 0.5 seconds after rolling.
Cooling at 30 ° C / s or more.

The steel sheet satisfying the ferrite grain size and the aspect ratio according to the present invention can be used as a hot-rolled steel sheet for various uses and also as a base material for a cold-rolled steel sheet. Since the crystal grains are fine and homogeneous, they are particularly suitable for use in cold-rolled steel sheets for processing and the like, and can provide steel sheets having excellent r-values.

In order to manufacture such a cold-rolled steel sheet for processing, cold rolling is performed at a rolling reduction of 50 to 90%, and annealing is performed at a transformation point of 600 to Ac ₃ . If the rolling reduction is less than 50%, good workability cannot be obtained,
Even if a reduction exceeding 90% is applied, the characteristics are saturated. In both cases where the annealing temperature is lower than 600 ° C. and where the temperature exceeds the Ac ₃ point transformation point, good workability cannot be obtained. After quenching after annealing, an overaging treatment may be performed. Further, not only continuous annealing but also a method of winding around a coil and performing box annealing may be used.

[0033]

EXAMPLES Example 1 A steel material having the composition shown in Table 1 was heated and hot-rolled under various conditions shown in Table 2 to obtain a hot-rolled steel sheet. Each steel sheet must be finished within 0.3 seconds after finish rolling.
Cooling was started at 50 ° C / s. Further, for the steel type B, lubrication rolling was performed. Table 3 shows the results of examining the mechanical properties of these steel sheets. Further, using these hot-rolled steel sheets as base materials, cold-rolling and annealing were performed at cold-rolling reduction rates and annealing temperatures shown in Table 4 to obtain cold-rolled steel sheets. Table 4 also shows the mechanical properties of these cold-rolled steel sheets. Each of the hot-rolled steel sheets of the present invention had a tensile strength of 40 kgf / mm ² or more.
As is evident from Table 3, the inventive steel having an average ferrite grain size of less than 2 μm according to the present invention has a lower strength than the comparative steel.
It has excellent elongation balance, durability ratio, toughness, small anisotropy and good BH content.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

[0037]

[Table 4]

(Example 2) C: 0.06% by weight, Si: 0.9% by weight
%, Mn: 1.3 wt%, P: 0.01 wt%, and 0.0008 to 0.00 S
Using samples varied in the range of 6 wt%, the average crystal grain size was 7 μm (6.0 to 8.0 μm) and less than 2 μm (0.7 μm).
1.01.0 μm). Incidentally, pearlite is formed as the second phase of the steel sheet, and the ratio of the average crystal grain size of ferrite and pearlite is 0.5 to 2 when the average crystal grain size is less than 2 μm, and the average crystal grain size is 7 mm. 0.1 in case
-4. A hot-rolled steel sheet having an average crystal grain size of less than 2 μm is manufactured by the method according to the present invention, and the distribution of the second phase grains is controlled by controlling the slab heating temperature and the like to make the distribution between the nearest second phase grains. A group in which the interval is less than 10% at a ratio of less than twice the crystal grain radius of the second phase, and 10 to 30%
And got a group. As shown in FIG. 3, the steel sheet was punched into a 20 mmφ diameter (d ₀ ) and expanded with a conical punch (vertical angle 60 °), and the hole expansion ratio (dd) until cracks occurred in the steel sheet as shown in FIG. ₀₎ / d ₀₎ was measured. FIG. 4 shows the results. Curve A in the figure shows an average ferrite crystal grain size of less than 2 μm, an aspect ratio of 1.3, and a dm / ds of 1.
8 shows a group in which the ratio of the distance between the nearest second phase particles being less than twice the crystal grain radius of the second phase is 10% or less (average 8%). Curve B shows a ferrite crystal grain size of less than 2 μm, an aspect ratio of 1.3, a dm / ds of 1.8,
The group in which the distance between the nearest second phase particles is less than twice the crystal grain radius of the second phase is 10 to 30% (average 23%). Curve C shows a group in which the average crystal grain size of ferrite is 7 μm and the aspect ratio is 2.5. The group indicated by the curves A and B is the hot-rolled steel sheet of the present invention, and the group indicated by the curve C is the comparative hot-rolled steel sheet. As shown in the figure, in the hot-rolled steel sheet according to the present invention, a good hole expansion ratio was obtained, and in particular, excellent characteristics were obtained when the S content was reduced to 0.002 wt% or less. Further, by distributing the second phase in an island shape, the hole expansion rate was further improved. Therefore, the hot-rolled steel sheet according to the present invention is also suitable for applications requiring hole expandability, such as automobile wheels.

Example 3 A steel material having the composition shown in Table 5 was heated and hot-rolled under various conditions shown in Table 6 to obtain a hot-rolled steel sheet. Here, the dynamic recrystallization zone rolling was performed from a stage after the rough rolling to a stage before the finish rolling. Each steel plate is
After finish rolling, cooling was started at 50 ° C / s within 0.3 seconds. Further, lubricating rolling was performed on steel type C (Nos. 6 and 7). Table 7 shows the results of examining the mechanical properties of these steel sheets. Using the obtained hot-rolled steel sheets of steel types B (Nos. 4, 5) and D (Nos. 8, 9) as base materials, cold rolling and annealing were performed at a cold rolling reduction of 75% and an annealing temperature of 750 ° C. Performed to obtain a cold-rolled steel sheet. Table 7 shows the mechanical properties of these cold rolled steel sheets.
It is described together. The number 8 (steel type D) is 1000 ° C.
At 800 ° C and a reduction of 80%, then 60
Once cooled to 0 ° C, the temperature is raised again to 850 ° C and
After a reduction of 90% at 0 ° C., the mixture was allowed to cool. In these steels, the volume fraction of the second phase was 3 to 30%. As is clear from Table 7, the inventive steels having an average ferrite grain size of less than 2 μm according to the present invention are superior in strength-elongation balance, durability ratio, and toughness as compared with the comparative steels. The steel in which the ratio dm / ds of the average particle diameter of the second phase to the average of the second phase is controlled to be more than 0.3 to less than 3 has further excellent durability ratio, toughness, small anisotropy, and good BH content. I have.

[0040]

[Table 5]

[0041]

[Table 6]

[0042]

[Table 7]

[0043]

According to the present invention, 40 kgf / mm ²
In the low-carbon hot-rolled steel sheet described above, good mechanical properties can be obtained without anisotropy.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between ferrite average grain size and mechanical properties of a hot-rolled steel sheet.

FIG. 2 is a diagram showing a steel sheet heating means in a finish rolling facility.

FIG. 3 is a diagram illustrating a method of measuring a hole expansion ratio.

FIG. 4 is a diagram showing the relationship between the S amount of a steel sheet and the hole expansion rate.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Roll stand 2 Roll-down roll 3 Backup roll 4 Rolled material 5 High frequency heating device 6 Heater heating device

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Osamu Furukun 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Technical Research Institute of Kawasaki Steel Co., Ltd. (72) Inventor Susumu Okada 2-3-2 Uchisaiwaicho, Chiyoda-ku, Tokyo No. Kawasaki Steel Corporation Tokyo Head Office

Claims (10)

[Claims] 1. A hot-rolled steel sheet containing ferrite as a main phase, characterized by having an average ferrite grain size of less than 2 μm and an aspect ratio of ferrite grains of less than 1.5, for processing having ultra-fine grains. Hot rolled steel sheet. 2. A hot-rolled steel sheet comprising ferrite as a main phase, wherein the average ferrite grain size is less than 2 μm, the aspect ratio of the ferrite grains is less than 1.5, and the average ferrite grain size is dm (μm). The average grain size ds (μm) of the phase is
A hot-rolled steel sheet for processing having ultra-fine grains that satisfies the following formula: 0.3 <dm / ds <3. 3. A hot-rolled steel sheet containing ferrite as a main phase, wherein the average ferrite grain size is less than 2 μm, the aspect ratio of the ferrite grains is less than 1.5, and the average ferrite grain size is dm (μm). The average grain size ds (μm) of the phase is
An ultrafine particle having a second phase that satisfies the following equation: 0.3 <dm / ds <3, and the ratio at which the distance between the nearest second phase particles is less than twice the crystal grain radius of the second phase is less than 10%. Hot rolled steel sheet for processing with grains. 4. C: 0.01 to 0.3 wt%, Si: 3.0 wt% or less, Mn: 3.0 wt% or less, P: 0.5 wt% or less, Ti: 0 to 1.0 wt%, Nb: 0 to 0 wt% 1.0 wt%, V: 0 to 1.
0 wt%, Cr: 0 to 1.0 wt%, Cu: 0 to 3.0 wt%, Mo: 0
~ 1.0 wt%, Ni: 0 ~ 1.0 wt%, one or more of
One, two or more of Ca, REM and B are 0 to 0.005 wt in total
The hot-rolled steel sheet for processing having ultra-fine grains according to claim 1, wherein the hot-rolled steel sheet comprises ultra-fine grains. 5. C: 0.01 to 0.3 wt%, Si: 3.0 wt% or less, Mn: 0.5 to 3.0 wt%, P: 0.5 wt% or less, Ti: 0 to 1.0 wt%, Nb: 0 ~ 1.0 wt%, V: 0 ~ 1.
0 wt%, Cr: 0 to 1.0 wt%, Cu: 0 to 3.0 wt%, Mo: 0
~ 1.0 wt%, Ni: 0 ~ 1.0 wt%, one or more of
One, two or more of Ca, REM and B are 0 to 0.005 wt in total
%, With the balance substantially consisting of iron and a structure containing one or more of martensite, bainite, retained austenite, pearlite and acicular ferrite as the second phase. 4. A hot-rolled steel sheet for processing having ultra-fine grains according to 2 or 3. 6. The hot-rolled steel sheet for processing having ultrafine grains according to claim 1, wherein the bake hardening amount is 100 MPa or more. 7. A hot rolling process in which a material for a hot-rolled steel sheet is melted and immediately or once cooled and heated to 1200 ° C. or less to perform hot rolling, wherein a reduction in a dynamic recrystallization zone is performed by 5 stands or more. A method for producing a hot-rolled steel sheet for processing having ultra-fine grains, wherein 8. A roll or a steel plate is heated by a heating means provided between roll stands of a finish rolling facility.
The method for producing a hot-rolled steel sheet for processing according to the above. 9. A base material for a cold rolled steel sheet having ultrafine grains having the structure and composition according to claim 1. Description: 10. A method for producing a cold-rolled steel sheet, wherein the base material for a cold-rolled steel sheet according to claim 9 is subjected to cold rolling at a reduction ratio of 50 to 90%, and then annealing at 600 ° C. to the Ac ₃ transformation point.