JP2000199034A - High tensile strength hot rolled steel plate excellent in workability and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To impart an excellent balance of strength-elongation to the steel plate by specifying the compsn. contg. C, Si, Mn, P, Ti and Fe and forming a structure composed of the main phase of ferrite having specified average grain size and a 2nd phase of martensite and austenite having specified volume ratios. SOLUTION: This steel plate has a compsn. contg., by weight, 0.01 to 0.3% C, <=1.0% Si, <=3.0% Mn, <=0.5% P and 0.03 to 0.3% Ti, if required, contg. one or more kinds of <=0.3% Nb and <=0.3% V as well, moreover contg. one or more kinds among <=1.0% Cu, <=1.0% Ni, <=1.0% Cr and <=1.0% Mo, furthermore contg. one or more kinds among Ca, rare earth metals and B by <=0.005% in total, and the balance substantial Fe. Moreover, it has a structure consisting of the main phase of ferite and a 2nd phase, in which the average grain size of ferrite is <=3.5 μm, the average grain size of the 2nd phase is <=3.5 μm, and also, the 2nd phase has martensite of >=70% volume ratio and austenite of >=20%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet which is advantageous for use in automobiles, home appliances, mechanical structures, constructions, etc., and in particular, has ultra-fine grains as hot-rolled. The present invention relates to a hot-rolled steel sheet excellent in balance, ductility, toughness and strength-elongation.

[0002]

2. Description of the Related Art Steel materials used for automobiles, home appliances, mechanical structures, and construction are required to have excellent mechanical properties such as strength, workability, and toughness. this house,
As for the strength, conventionally, high-strength steel sheets whose strength has been increased by various methods have been proposed. For example, in a ferrite single phase structure, a solid solution strengthened steel sheet added with a solid solution strengthening element such as Si, Mn, or P, or a precipitation strengthened steel sheet added with a carbonitride forming element such as Nb or Ti, or a ferrite phase Known are a dual-structure (DP) steel sheet strengthened by a second phase such as martensite and bainite, and a steel sheet strengthened by refining crystal grains. However, in the solid solution strengthened steel sheet, the amount of the added alloy element is large, so that the cost is increased, the workability such as ductility is reduced, and the strength obtained is limited. Further, DP steel sheet has a good strength-ductility balance, but is inferior in hole expandability, and some problems remain, such as strict cooling control after rolling is required for adjustment of the second phase structure. Was. Further, in the case of high-strength steel by grain refinement, the yield strength is high, so that the yield ratio is high and the press formability remains low.

[0003] In recent years, with regard to high-strength steel sheets, the target is shifting to the development of high-strength steel sheets that can achieve both low cost and high-performance characteristics. In addition, in steel sheets for automobiles,
In order to protect the occupant in the event of a collision, it is required to have not only high strength but also excellent impact resistance. For these reasons, high-strength steel sheets require strength, other toughness,
It is necessary to comprehensively improve the mechanical properties, such as workability, etc.In order to suppress the deterioration of ductility, toughness, durability ratio, etc. accompanying the increase in tensile strength, it is important to refine the structure of high-tensile steel. Has become.

Recently, there has been proposed a method in which austenite grains before hot rolling are extremely refined and rolled, and the structure is refined by utilizing dynamic recrystallization and further controlled cooling.
87798, JP-A-9-143570, JP-A-10-8138
No., published in Japanese Unexamined Patent Publication No. JP-A-9-87798 discloses that
Mn: 1.0 to 2.5 wt%, Ti: 0.05 to 0.30 wt%, or T
i: A slab containing 0.05 to 0.30 wt% and Nb: 0.30 wt% or less is heated to a temperature of 950 to 1100 ° C., and rolling is performed at least twice at a rolling reduction of 20% or more per pass. After performing hot rolling at a rolling temperature of not less than the Ar ₃ transformation point, it is cooled at a cooling rate of 20 ° C./s or more,
A method for producing a high-tensile hot-rolled steel sheet which is wound at a temperature of 75 ° C. and has a structure of 75% by volume or more of polygonal ferrite having an average crystal grain size of less than 10 μm and 5 to 20% by volume of retained austenite is disclosed.

[0005] JP-A-9-143570 discloses that Ti: 0.05-0.
A steel containing one or two of 3 wt% and Nb: 0.10 wt% or less is heated to a temperature of 950 to 1100 ° C., and rolling is performed at least twice so that a rolling reduction per pass is 20% or more. above is performed, the finish rolling temperature and hot rolled so that the Ar ₃ transformation point or higher, Ar ₃ transformation point to 750 ° C. and cooled at 20 ° C. / s or more cooling rate, the temperature range of 750 ° C. below to 600 ° C. For 5-20 seconds at 550 ° C again at a cooling rate of 20 ° C / s or more.
Cool to below temperature and wind at below 550 ° C,
A method for producing a high-tensile hot-rolled steel sheet having an ultrafine structure with a ferrite content of 80% by volume or more and an average ferrite grain size of less than 10 μm is disclosed.

[0006] JP-A-10-8138 discloses that Mn: 1.0 wt%
Hereinafter, Ti: 0.05 to 0.30 wt%, or all or 1 of Ti
Slab containing twice the amount of Nb instead of 950-11
Heating to a temperature of 00 ° C, rolling at a rolling reduction of 20% or more per pass at least twice or more, hot rolling at a finish rolling temperature of the Ar ₃ transformation point or more, and then 20 ° C / s or more A method for producing a high-tensile hot-rolled steel sheet having an ultrafine grain structure composed of ferrite and retained austenite, cooled at a cooling rate of 350 to 550 ° C., is disclosed.

[0007] Also, Japanese Patent Application Laid-Open No.
C: 0.02 to 0.2%, Si: 0.1 to 1.5%, Mn: 0.5 to
3.0%, S: Including 0.010% or less, P: 0.03-0.15%,
Cr: 0.1 to 2.0%, Mo: 0.1 to 1.0%, one or more selected from the group consisting of Fe and unavoidable impurities, the balance being 80 to 80% of a ferrite phase having an average grain size of 10 μm or less.
Occupies 97%, and the remainder has an average diameter of 0.
A hot-rolled high-strength steel sheet having excellent formability and impact resistance comprising a second phase mainly composed of martensite, which is 2 to 1.5 times, is disclosed.

[0008]

SUMMARY OF THE INVENTION
JP-A-9-87798, JP-A-9-143570, JP-A-10-813
Although the technique described in Japanese Patent Publication No. 8 focuses on the refinement of crystal grains, the particle size obtained was at most about 3.6 μm. Further, although the strength and ductility are improved in the steel sheets manufactured using these techniques, the anisotropy of the mechanical properties is large and the strength-elongation balance is still high, particularly from the viewpoint of the workability of the steel sheet for automobiles. Not enough.

A high-strength steel sheet manufactured by the technique described in Japanese Patent Application Laid-Open No. 10-195588 has improved formability and impact resistance as compared with the conventional steel sheet, but has a high demand for steel sheets for automobiles which are currently required. From the viewpoint of workability, the strength-hole expanding workability balance and the strength-elongation balance were still insufficient.
The present invention advantageously solves the above-mentioned problems of the prior art, has ultra-fine grains, has a low yield ratio, is excellent in strength-elongation balance, strength-hole expanding workability balance, and is excellent in press formability. An object is to provide a high-tensile hot-rolled steel sheet.

[0010]

Means for Solving the Problems The present inventors have conducted intensive studies in order to achieve the above-mentioned object, and as a result, during hot rolling, the rolling was repeatedly performed in a dynamic recrystallization temperature region of an austenite region, and was relatively low. By quenching after light pressure reduction,
It has been found that the ferrite grains as the main phase can be made into ultrafine grains having a size of 3.5 μm or less, and the second phase can be made finer than the main phase and dispersed in an island shape. In addition, A
Heating to the temperature range of the α-γ2 phase range of c ₁ transformation point or more (Ac ₁ transformation point + 80 ° C) or less and then performing cooling annealing significantly lowers the yield ratio and further remarkably improves the strength-elongation balance. Has been found to be able to produce a high-strength steel sheet.

Regarding the experimental results on which the present invention was based,
explain. C: 0.12%, Si: 0.3%, Mn: 0.8%, Ti: 0.16%,
P: contains 0.005% and has an average ferrite grain size of 1.5
A hot-rolled steel sheet (Ac ₁ transformation point: 740 ° C.) containing ferrite having a main phase of μm or 4.5 μm was subjected to continuous annealing by changing the heating temperature in the range of 650 ° C. to 880 ° C. The holding time at the soaking temperature was constant at 40 sec. The cooling rate after soaking is
At 30 ° C / s, it was rapidly cooled to 300 ° C. After continuous annealing, a tensile test was performed, yield strength YS, tensile strength TS, elongation El
Was calculated, and the yield ratio YR and the strength-elongation balance TS × El were calculated. In addition, the tensile test was implemented also about the steel plate as hot rolled. The results are shown in FIGS.

1 and 2, the hot-rolled ultra-fine grained steel sheet having a ferrite grain size (initial grain size) of 1.5 μm was converted to an α-transformation point not less than the Ac ₁ transformation point (Ac ₁ transformation point + 80 ° C.) or less. It can be seen that heating to the γ2 phase region increases TS, decreases YS, and significantly improves YR and TS × El. Initial particle size
At 4.5 μm, no such significant improvement is seen.

According to further studies by the present inventors, a fine-grained steel sheet having an initial grain size of 3.5 μm or less is heated to a temperature higher than the Ac ₁ transformation point to cause an α → γ reverse transformation, thereby causing a microstructure in the cooled state. The two phases have a structure mainly composed of a martensite phase having an average crystal grain size of 3.5 μm or less and containing austenite.
This allows low YS without adding a large amount of alloying elements.
Thus, it was found that a steel sheet having an extremely good strength-elongation balance was obtained. Note that the steel sheet having such a structure had a good balance between strength and hole expandability. On the other hand, when heating is performed at a temperature exceeding the Ac ₁ transformation point + 80 ° C., crystal grains grow, the strength is reduced, and the material properties are deteriorated. On the other hand, in a steel sheet having an initial grain size of more than 3.5 μm, sufficient reverse transformation and enrichment of alloying elements in the second phase hardly occur by short-time annealing, so that martensite or the like hardly occurs after cooling.

The present inventors have further studied based on the above findings and completed the present invention. That is, in the present invention, C: 0.01 to 0.3%, Si: 1.0% by weight.
%, Mn: 3.0% or less, P: 0.5% or less, Ti: 0.03 ~
Containing 0.3% or further Al: 0.10% or less, the balance being substantially composed of iron, having ferrite as a main phase, and having a structure of a main phase and a second phase, The average particle size of the ferrite is 3.5 μm or less, the average particle size of the second phase is 3.5 μm or less, and the second phase is
A high-tensile hot-rolled steel sheet having excellent workability, characterized by having martensite of 70% or more and austenite of 2% or more by volume, and in the present invention, in addition to the above-mentioned composition, further comprises a weight %, It is preferable that the composition contains one or two of Nb: 0.3% or less and V: 0.3% or less. In the present invention, in addition to the above-mentioned compositions, the composition further contains , Cu: 1.0% or less, Ni: 1.0% or less, Cr:
It is preferable that the composition contains one or more of 1.0% or less and Mo: 1.0% or less. In the present invention, in addition to each of the above-described compositions, Ca, REM
, B is preferably a composition containing 0.005% or less in total.

Further, the present invention is a steel sheet having an average particle size has the following fine particles 3.5 [mu] m, and heated to a temperature range of Ac ₁ transformation point or above (Ac ₁ transformation point + 80 ° C.), then the annealing cooling This is a method for producing a high-tensile hot-rolled steel sheet excellent in workability, characterized by being applied. In addition, the present invention relates to the following:
01-0.3%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.
When the rolling steel material containing 5% or less and Ti: 0.03 to 0.3% is reheated to 1100 ° C or less, or subjected to hot rolling after reaching 1100 ° C or less, the hot rolling is performed by:
Rolling is performed at least 5 passes or more in the dynamic recrystallization temperature range, and hot rolling is performed so that the finish rolling temperature is equal to or higher than the Ar ₃ transformation point. After the completion of hot rolling, a cooling rate of 30 ° C./s or more is required within 0.5 sec. After the hot-rolled steel sheet in cooling and, Ac ₁ to the heat-rolled steel sheet
Heating to a temperature range not lower than the transformation point (Ac ₁ transformation point + 80 ° C.) and lower, and then annealing at a cooling rate preferably in a range of 10 to 100 ° C./s is performed. This is a method for producing a tension hot-rolled steel sheet. In the present invention,
The annealing may include an overaging treatment of maintaining or slowly cooling at a temperature in the range of 200 to 450 ° C. during the cooling after heating.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the reasons for limiting the chemical components of the high-tensile hot-rolled steel sheet of the present invention will be described. C: 0.01 to 0.3% C is an inexpensive reinforcing component, and contains a necessary amount according to the desired steel sheet strength. If the C content is less than 0.01%, the crystal grains become coarse, and the average particle size of the ferrite intended in the present invention.
It is not possible to achieve less than 3.5 μm. Also, the C content is 0.
If it exceeds 3%, workability is deteriorated and weldability is also deteriorated. Therefore, C is set in the range of 0.01 to 0.3%. More preferably, it is in the range of 0.05 to 0.2%.

Si: 1.0% or less Si effectively contributes to an increase in strength while improving strength-elongation balance as a solid solution strengthening component. In addition, it effectively acts to suppress the formation of ferrite and obtain a structure having a desired second phase volume fraction, but excessive addition deteriorates ductility and surface properties. Therefore, the content of Si is set to 1.0% or less. In addition,
Preferably it is 0.01-0.7%.

Mn: 3.0% or less Mn contributes to the refinement of crystal grains through the action of lowering the Ar ₃ transformation point. In addition, through the action of promoting the formation of martensite and retained austenite in the second phase, the strength-
Has the effect of increasing the elongation balance. Further, it has an effect of rendering harmful solid solution S harmless as MnS. However, a large amount of addition hardens the steel and rather degrades the strength-elongation balance. Therefore, Mn is set to 3.0% or less. Incidentally, preferably at least 0.05%, more preferably 0.5%
~ 2.0%.

P: 0.5% or less P is useful as a strengthening component and can be added according to the desired strength of the steel sheet. However, excessive addition segregates at grain boundaries and causes embrittlement. Therefore, P is set to 0.5% or less. Incidentally, excessive reduction may increase the cost, and is preferably 0.001 to 0.2%, more preferably 0.005 to 0.2%.
%.

Ti: 0.03-0.3% Ti is present as TiC and is used to refine the initial austenite grains during the hot rolling heating stage and induce dynamic recrystallization during the subsequent hot rolling process. Works effectively. In order to exert such an effect, at least 0.03%
The above content is necessary, but if the content exceeds 0.3%, the effect is saturated and an effect corresponding to the content cannot be expected.
Therefore, Ti is set in the range of 0.03 to 0.3%. In addition, Preferably, it is 0.05 to 0.20%.

Al: 0.10% or less Al acts not only as a deoxidizing agent but also as an AlN to refine crystal grains, and may be contained in an appropriate amount. However, when the content exceeds 0.10%, oxide-based inclusions increase, and the cleanliness decreases. For this reason, Al is 0.10%
It is preferable to limit to the following. Incidentally, preferably 0.005
~ 0.07%.

One or two of Nb: 0.3% or less and V: 0.3% or less Nb and V both form carbonitrides and refine initial austenite grains in a hot rolling and heating stage. It has an action, and if necessary, by overlapping with Ti,
Further, it effectively acts on the occurrence of dynamic recrystallization. But 0.
Even if it is contained in a large amount exceeding 3%, the effect is saturated and an effect commensurate with the content cannot be expected. Therefore, both Nb and V are 0.3%
It is desirable to do the following.

Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.
0% or less, Mo: 1.0% or less One or more of them. Cu, Mo, Ni, and Cr can be contained as reinforcing components, if necessary, but a large amount of them may be used instead. -Deteriorating ductility balance; Therefore, Cu, Mo, N
It is desirable that both i and Cr be 1.0% or less. In order to sufficiently exhibit the above-mentioned effects, it is preferable to contain at least 0.01% or more.

0.005% or less in total of one or more of Ca, REM, and B Ca, REM, and B all have the effect of improving workability by controlling sulfide shape and increasing grain boundary strength. And can be contained as needed. However, excessive content may adversely affect cleanliness and recrystallization, so that the total content is desirably 0.005% or less.

The hot-rolled steel sheet of the present invention is substantially composed of Fe, except for the above-mentioned composition. The hot-rolled steel sheet of the present invention has a structure composed of a main phase composed of ferrite having an average grain size of 3.5 μm or less and a second phase having an average grain size of 3.5 μm or less. The main phase preferably has a volume ratio of 80% or more. 80
%, The ductility decreases. It is more preferably at most 97%.

When the ferrite grains are refined, the desired strength can be secured with a smaller amount of alloying elements compared to the conventional high-strength steel, and the characteristics other than the strength are less deteriorated, and the subsequent plating property is also improved. It will be good. However, when the average grain size of ferrite exceeds 3.5 μm, the increase in strength due to the refinement of crystal grains is small, the amount of alloy addition increases, and the ductility further deteriorates. For this reason, the average ferrite grain size was 3.5 μm.
m. The average particle size of the second phase is 3.5 μm
When the average particle size of the second phase exceeds the limit, the improvement in toughness and ductility is reduced. Therefore, the average particle size of the second phase is limited to 3.5 μm or less.

The volume ratio of the second phase is desirably 3 to 20%. If the volume fraction of the second phase is less than 3%, the strength-ductility balance is poor, and if it exceeds 20%, the ductility is deteriorated. The second phase has at least 70% by volume martensite and at least 2% by volume austenite with respect to the entire second phase. If the volume fraction of martensite in the second phase is less than 70%, a low yield ratio cannot be obtained, a high yield ratio which is a defect of a steel sheet having fine grains, and a strength-elongation balance is low. If the volume fraction of austenite in the second phase is less than 2%, only low strength-elongation balance and strength-hole expanding workability balance can be obtained. Only when the volume fraction of austenite is 2% or more, TS × El becomes 22000 MPa ·% or more.

In the present invention, the average grain size of the ferrite and the second phase grains is the average grain size in the cross section in the rolling direction according to a conventional method. Next, a method for producing a hot-rolled steel sheet according to the present invention will be described. In the present invention, the average particle size (initial particle size)
There the steel sheet having the fine particle 3.5 [mu] m, and heated to a temperature range of Ac ₁ transformation point or above (Ac ₁ transformation point + 80 ° C.), then subjected to annealing for cooling. The following steps (between molten steel and hot rolling / coil winding) up to before annealing are preferable examples for obtaining a steel sheet having fine grains having an average grain size of 3.5 μm or less. However, the present invention is not limited to this manufacturing method. In addition, the steel sheet having the above fine grains contains ferrite as a main phase (by volume ratio of 50% or more, preferably 70% or more).
It is preferable to obtain a predetermined ferrite volume ratio after annealing.

The molten steel adjusted to the above component composition range is
A rolled material is obtained by continuous casting or ingot-bulking rolling, and the rolled material is subjected to hot rolling to obtain a hot-rolled steel sheet. The hot rolling may be reheating rolling in which the rolled material is cooled and then reheated, or may be direct rolling or hot charge rolling. Further, a continuously cast slab such as a thin slab continuous casting method may be directly hot-rolled. When reheating, to refine the initial austenite grains,
It is desirable to heat to 1100 ° C or lower. Also in the case of direct rolling, it is preferable to start rolling after cooling to 1100 ° C. or less in order to promote dynamic recrystallization. In order to set the finishing rolling temperature in the austenite range, it is preferable that the reheating temperature or the direct rolling start temperature is 900 ° C. or higher.

In the present invention, when hot rolling is performed on a rolled material having the above-mentioned temperature, it is preferable that the rolling is repeatedly performed at least 5 times or more in the dynamic recrystallization temperature range. The austenite grains are refined by repeatedly applying the reduction in the dynamic recrystallization temperature range. Because the finer austenite grains progress as the number of times of causing dynamic recrystallization increases,
It is preferable to reduce the pressure in at least 5 passes or more, and in 5 or more successive passes. With less than 5 passes, the degree of refinement of the austenite grains is small and the average ferrite grain size is 3.5.
It is difficult to achieve fine particles of μm or less.

The rolling reduction in the dynamic recrystallization temperature range is not particularly limited as long as dynamic recrystallization occurs, but it is 4 to 20% per pass, preferably less than 20%. It is desirable to do. If the rolling reduction per pass is less than 4%, dynamic recrystallization does not occur. On the other hand, if the rolling reduction per pass exceeds 20%, the anisotropy of mechanical properties, for example, Δ
El, becomes big. In addition, the final rolling pass in the dynamic recrystallization temperature range requires a reduction of 13 to 30% in order to reduce the size of the second phase.
It is desirable that

It should be noted that, although the annealing is performed at the Ac ₁ transformation point or more, it is not preferable that the second phase exists in an aggregated state before annealing, and the island shape (the island shape in the present invention is defined as a particle of the second phase). (It means a state in which the ratio of other second phases present at intervals of less than the diameter is 20% or less). According to the above rolling conditions, an island-like second phase can be obtained. The dynamic recrystallization temperature referred to in the present invention is a strain-stress obtained by simulating rolling conditions by a measuring device (for example, “Processing For Master” manufactured by Fuji Denki Koki Co., Ltd.) capable of controlling temperature and strain independently. A value measured in advance from the relationship is used. Dynamic recrystallization temperature depends on steel composition, heating temperature,
Although it varies depending on the rolling reduction, rolling reduction, etc., it is said that it exists in the temperature range of 850-1100 ° C, usually in the range of 250-100 ° C. Incidentally, the temperature range of the dynamic recrystallization temperature range, the higher the rolling reduction per pass, or the higher the Ti content,
Expanding.

From the viewpoint of refining the structure, it is necessary to perform rolling at a temperature as low as possible within the dynamic recrystallization temperature range.
The transformation site of the α transformation is advantageously increased. Therefore, in order to promote the refinement of austenite grains, the temperature from (lower limit temperature of dynamic recrystallization) + 80 ° C., preferably (lower limit temperature of dynamic recrystallization) + 60 ° C. to the lower limit temperature of dynamic recrystallization Applying the pressure of 3 passes or more in the range will reduce the average crystal grain to 3.
It is preferable for the thickness to be 5 μm or less.

In order to obtain the number of times of rolling in the low temperature range of the dynamic recrystallization, a heating means may be provided between the rolling stands to heat the material to be rolled or the roll. In particular, it is effective to install the heating means at a position where the temperature is significantly reduced. As the heating means, the steel plate may be heated by a high-frequency heating device, the roll may be heated using an electric heater, or the heating may be performed by direct current heating.

It is needless to say that, during the hot rolling, the reduction may be performed while lubricating. In the present invention, rolling conditions other than rolling in the dynamic recrystallization temperature range are not particularly limited, but the rolling finish temperature is set to the Ar ₃ transformation point or higher. If the rolling finish temperature is lower than the Ar ₃ transformation point, the ductility and toughness of the steel sheet deteriorate.

In the hot-rolled steel sheet which has been hot-rolled under the above-described conditions, the austenite grains at this point are almost equiaxed crystal grains. , Γ → α transformation, the growth of ferrite grains is suppressed, and the structure is refined. Therefore, it is preferable to start cooling within 0.5 sec, preferably within 0.3 sec after the end of rolling. Cooling started after rolling
If it exceeds 0.5 sec, grain growth becomes remarkable.

The cooling rate is 30 ° C./s or more. If the cooling rate is less than 30 ° C./s, ferrite grains grow, making it impossible to achieve finer grains and making it difficult to distribute the second phase finely and in islands. The hot rolled steel sheet cooled at a cooling rate of 30 ° C./s or more, preferably to a temperature range of 350 to 650 ° C., is immediately wound around a coil. The winding temperature and the cooling rate after winding are not particularly limited. It is determined appropriately according to the steel sheet to be manufactured. However, when the winding temperature is high, the second phase has a structure mainly composed of pearlite, and the ferrite grains tend to grow.
On the other hand, if the winding temperature is too low, winding becomes difficult. For this reason, the winding temperature is 350-650 ° C
It is desirable to be within the range.

Next, the hot-rolled steel sheet is annealed. The annealing method need not be particularly limited, but is preferably continuous annealing from the viewpoint of production efficiency. The soaking temperature is in a temperature range from the Ac ₁ transformation point to the (Ac ₁ transformation point + 80 ° C.) or less.
By heating to this temperature range, a part is transformed into the γ phase. The soaking time is 1 to 300 sec, preferably 20 to 1 sec.
Desirably, it is set to 00 sec.

Next, the steel sheet is preferably 10 to 100 ° C. /
At a cooling rate of s, it is cooled to 200-600 ° C. If the cooling rate after soaking is less than 10 ° C./s, diffusion of C occurs, and it becomes difficult to form a second phase mainly composed of martensite and containing austenite. On the other hand, the cooling rate is 100 ° C / s
Even if it is faster than the above, there is no change in the tissue fraction of the second phase,
In addition, since it is very difficult to increase the cooling rate above the facility, it is desirable to set the upper limit to 100 ° C / s. The quenching stop temperature after soaking is preferably in the temperature range of 200 to 600 ° C. If the quenching stop temperature is less than 200 ° C., shape defects are likely to occur. When the temperature is lower than 400 ° C., the diffusion of C is slow.
Since the effect of the cooling rate is small, rapid cooling is preferably stopped in a temperature range of 400 ° C. or more in terms of cost. On the other hand, 600 ℃
Is exceeded, the diffusion of C occurs and the martensite fraction of the second phase decreases.

After the cooling is stopped, an overaging treatment can be performed. The overaging condition is 20 to 1 in the temperature range of 200 to 450 ° C.
It is preferable to keep the temperature for 80 sec or cool slowly.

[0041]

EXAMPLE A molten steel having the composition shown in Table 1 was made into a slab (rolled material) by a continuous casting method. Table 2 shows these slabs.
Heating, hot rolling, and cooling after rolling were performed under various conditions shown in (1) to obtain a hot-rolled steel sheet (sheet thickness 2 to 4 mm). The manufacturing conditions N
o.3 and No.5 performed lubrication rolling. Then, these hot-rolled steel sheets were subjected to continuous annealing for heating and cooling under the conditions shown in Table 2. For some steel sheets, during cooling,
Overage treatment was applied.

The obtained steel sheets were examined for structure, tensile properties and hole expandability, and the results are shown in Table 3. The organization
For the section in the rolling direction of the steel sheet, the volume fraction of ferrite, the grain size and the
The structure, volume fraction, and particle size of the phase were measured. Regarding the tensile properties, the tensile properties (yield point YS, tensile strength TS, elongation El) were measured using a JIS No. 5 test piece in the rolling direction of the steel sheet.

The hole expandability is 10 mmφ (D
After the punched hole of ( ₀ ) is machined, it is expanded by a conical punch with a vertex angle of 60 °, and the hole diameter D immediately after the crack penetrates the plate thickness is obtained. Λ = ｛(D−D ₀ ) / D _The evaluation was made based on the λ value obtained from ₀ ｝ × 100%. Table 3 shows the results.

[0044]

[Table 1]

[0045]

[Table 2]

[0046]

[Table 3]

In each of the steel sheets of the present invention, the average grain size of ferrite is 3.5 μm or less, and the average grain size of the second phase is 3.
Less than 5 μm, the volume of martensite in the second phase is 70% by volume
As described above, it has a structure with an austenite amount of 2% by volume or more, a low yield ratio, a TS × El value of as high as 22000 MPa ·% or more, and a λ value of at least 90%. It has a high tensile strength hot rolled steel sheet with excellent workability. The presence or absence of the overaging treatment does not significantly affect the processability.

On the other hand, the steel sheet No. 7 having a high slab heating temperature, no occurrence of dynamic recrystallization, a large average ferrite grain size, and a small amount of martensite in the second phase is out of the range of the present invention. Indicates that the TS × El value is low. Further, the steel sheet No. 8 out of the range of the present invention has a low annealing temperature, a small amount of the second phase martensite, an elongation, and a low TS × El value. The steel sheet No. 9 has a high annealing temperature, a large average ferrite grain size, a small amount of martensite and an austenite in the second phase, a low elongation, and a low TS × El value.
Steel sheets No. 23 and No. 24 have a low Ti content, a large ferrite average grain size, elongation, and a low TS × El value. Steel sheet No. 25 has a low C content, a large average ferrite grain size, and low values such as elongation.

[0049]

According to the present invention, there is provided a fine powder having excellent fineness, excellent mechanical properties, excellent balance of strength-elongation, excellent balance of strength-hole expandability, and excellent press formability. High-strength hot-rolled steel sheet can be manufactured at low cost, and it has a remarkable industrial effect.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of annealing temperature on YS and TS.

FIG. 2 is a graph showing the effect of annealing temperature on YR and TS × El.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Osamu Furukun 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba F-term in Technical Research Laboratory, Kawasaki Steel Co., Ltd. 4K032 AA01 AA02 AA04 AA05 AA08 AA11 AA14 AA16 AA17 AA19 AA22 AA23 AA27 AA29 AA31 AA35 AA36 AA40 BA01 CA02 CB02 CC03 CC04 CD03 CE01 CE02 CF02 4K037 EA01 EA02 EA05 EA06 EA09 EA11 EA13 EA15 EA16 EA17 EA19 EA20 EA23 EA25 EA27 04

Claims (7)

[Claims] 1. The composition contains, by weight, C: 0.01 to 0.3%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.5% or less, and Ti: 0.03 to 0.3%, with the balance substantially consisting of Fe. The ferrite has a main phase and a structure composed of a main phase and a second phase, and has an average grain size of the ferrite of 3.5 μm or less and an average grain size of the second phase of 3.5 μm or less. A high-strength hot-rolled steel sheet excellent in workability, wherein the second phase has martensite having a volume ratio of 70% or more and austenite having a volume ratio of 2% or more. 2. In addition to the composition, further in weight%
Nb: 0.3% or less, V: one or two of 0.3% or less
The high-tensile hot-rolled steel sheet having excellent workability according to claim 1, wherein the composition includes a seed. 3. In addition to the composition, further in weight%
Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, M
o: The hot-rolled steel sheet with excellent workability according to claim 1 or 2, wherein the composition contains one or more of 1.0% or less. 4. In addition to the composition, further in weight%
One, two or more of Ca, REM and B are added in a total of 0.00
4. The hot-rolled steel sheet with excellent workability according to claim 1, wherein the steel sheet has a composition containing 5% or less. 5. A steel sheet having fine grains having an average grain size of 3.5 μm or less is heated to a temperature in a range of Ac ₁ transformation point or more (Ac ₁ transformation point + 80 ° C.) or less, and then subjected to annealing for cooling. A method for producing a high-strength hot-rolled steel sheet with excellent workability. 6. A rolling steel material containing, by weight, C: 0.01 to 0.3%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.5% or less, Ti: 0.03 to 0.3%, ° C or below, or when performing hot rolling after reaching 1100 ° C or below, the hot rolling is reduced by at least 5 passes or more in the dynamic recrystallization temperature range, and the finish rolling temperature is adjusted to Ar. ₃ and hot rolled to transformation point or higher, after the end of hot rolling, after a hot-rolled steel sheet was cooled at 30 ° C. / s or more cooling rate within 0.5 sec, Ac ₁ transformation point or above to the heat-rolled steel sheet (Ac ₁ transformation point + 80 ℃)
A method for producing a high-tensile hot-rolled steel sheet excellent in workability, characterized by performing annealing in the following temperature range and then cooling. 7. The cooling after heating in the annealing is 10 to
The method for producing a high-tensile hot-rolled steel sheet excellent in workability according to claim 5, wherein the steel sheet is cooled at a cooling rate in a range of 100 ° C./s.