JP3168665B2 - Hot-rolled high-strength steel sheet with excellent workability and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a high-strength hot-rolled steel sheet having excellent workability and having a tensile strength of 50 kgf / mm ² or more, which is suitable as a structural member formed into various shapes by press working, stretch flange working and the like. The present invention relates to the manufacturing method.

[0002]

2. Description of the Related Art A so-called "hot rolled steel sheet" manufactured by continuous hot rolling has been widely applied to various industrial equipment such as automobiles as a relatively inexpensive structural material. However, there are many members which are used by being formed by press working, and therefore, there is a strong demand for "combining high strength and high ductility".

As a steel material which is considered to be excellent in both strength and ductility, for example, DP steel (Dual Phase steel: ferrite + martensitic two-phase structure steel) as described in JP-A-55-44551 is disclosed. It has been known. This D
Features of P steels is that it "ductile yield ratio is low," but still the tensile strength: a current situation is that the elongation of about 30% of a material of 60 kgf / mm ^2, the surface of the ductile Therefore, a further improved material has been desired.

As a means for improving the ductility of a high-strength steel sheet, a method utilizing TRIP (transformation-induced plasticity) of retained austenite has been developed (for example, Japanese Unexamined Patent Publication No. Sho 5).
No. 5-145121). According to this method, a tensile strength (Ts) of 110 kgf / mm ² or more and an elongation (EL) of 22% or more, and a high ductile high-strength steel sheet exceeding 2400 as “Ts × EL” can be manufactured. It is. However, in this method, the C content needs to be adjusted to a high value of 0.35 to 0.85% (hereinafter, “%” representing the component ratio is referred to as “weight%”). The range of application as a steel sheet was narrow.

[0005] As a means for obtaining high ductility in steel by securing retained austenite under a low C content, there has been proposed a method in which a high Si content steel is greatly reduced in a low temperature austenite region ( JP-A-63-4017). In this method, the structure of the steel sheet is substantially changed to a structure of ferrite, bainite and retained austenite.
Is concentrated in untransformed austenite and stabilized to obtain residual austenite.

However, although the steel sheet obtained by the above method has high ductility, in addition to "bainite containing no carbide" which is important for formation of retained austenite, "hard bainite containing carbide" is formed in a coarse and band-like manner. Therefore, there was a problem that the hole-expandability was poor.
In addition, there is a problem that manufacturing conditions for retaining austenite have a very narrow allowable range, and it is difficult to stably manufacture a desired steel sheet. In particular, when pearlite was formed during the manufacturing process of the steel sheet, the amount of retained austenite was significantly reduced, and the realization of the structure containing retained austenite became extremely unstable.

As described above, in order to obtain retained austenite from a low C steel having good weldability, it is necessary to generate "a sufficient amount of ferrite to enrich C in untransformed austenite". For this reason, the means for securing low austenitic retained austenite of low-C steel, which has been conventionally adopted, is to "reduce the finishing temperature of hot rolling", and therefore the load on the hot rolling mill is large. In addition, since austenite does not remain at a low cooling rate at which pearlite is generated, strict control of cooling after hot rolling is required to avoid generation of pearlite. In addition, at a cooling rate at which pearlite is not generated, a large amount of hard bainite containing fine carbides is easily generated inevitably, so that it is very difficult to obtain high hole expandability.

[0008] In view of the above, the object of the present invention is to provide 50 kg of structural material for automobiles and industrial equipment.
The objective was to establish an industrial means capable of stably providing a hot-rolled high-tensile steel sheet having both f / mm ² or higher tensile strength and excellent ductility, hole expandability and weldability.

[0009]

In order to achieve the above-mentioned object, the present inventors have developed a high strength, especially for automobiles, having a low C content range in which satisfactory weldability can be obtained. And "T leads to excellent ductility and hole expandability.
As a result of various studies in search of the possibility of realizing a high-ductility hot-rolled high-tensile steel sheet containing a sufficient amount of austenite to utilize the RIP effect, the following findings can be obtained. Was.

A) In order to ensure good weldability and high ductility in a hot-rolled steel sheet, it is necessary to reduce its C content and leave 5% or more of retained austenite in particular by volume. B) However, reducing the C content inevitably decreases the strength of the steel sheet.
50 kgf / mm by forming a structure composed of polygonal ferrite containing stenite and pearlite and bainite
It is possible to stably secure a tensile strength of ² or more. C) In addition, as described above, an appropriate amount (3% by volume)
(20%) reduces the amount of hard bainite, so the hole expandability of the steel sheet is significantly improved. D) In addition, the addition of a small amount of Nb into the steel sheet leads to a reduction in load. Even if hot rolling at a high finishing temperature is used, and even if the cooling rate after hot rolling is set to 0.5 to 20 ° C./s (thus producing pearlite), the ferrite is sufficiently formed and the control is easy. 5% or more of retained austenite is stably secured.

The present invention has been completed by conducting further research based on the above findings and the like. "The hot-rolled high-strength steel sheet is made of C: 0.05 to 0.25%, Si: 2.5% or less, Mn: 0.8
~ 2.5%, Nb: 0.003 ~ 0.05%, Ti: 0.04% or less, Al: 2.0
% Or less, N: 0.0100% or less, or further contains at least one of Ca: 0.0002 to 0.01%, Zr: 0.01 to 0.10%, rare earth element: 0.002 to 0.10%, and Si (%) + Al (%) ≧ 1.0 and the balance is a component composition composed of Fe and unavoidable impurities, and a residual Au content of 5% or more by volume.
By having a structure that contains stainless steel and 3-20% pearlite and the balance is polygonal ferrite and bainite, it has a tensile strength of 50 kgf / mm ² or more and excellent ductility and hole expandability. "C: 0.05-0.25%, Si: 2.5% or less, Mn:
0.8 to 2.5%, Nb: 0.003 to 0.05%, Ti: 0.04% or less, Al: 2.0
% Or less, N: 0.0100% or less, or further contains at least one of Ca: 0.0002 to 0.01%, Zr: 0.01 to 0.10%, rare earth element: 0.002 to 0.10%, and Si (%) + Al (%) Satisfies ≧ 1.0 and the remainder is composed of a slab consisting of Fe and unavoidable impurities, re-heated to 1100 ° C or higher, hot-rolled, and finish-rolled in a temperature range of Ar ₃ points or higher And 0.
By cooling to a temperature range of 550 ° C. or less at a cooling rate of 5 to 20 ° C./s, the above “containing 5% or more of residual austenite by volume and 3 to 20% of pearlite and the remainder being polygonal High-ductility hot-rolled high-strength steel sheet having a structure composed of ferrite and bainite, with a tensile strength of 50 kgf / mm ² or more, and excellent ductility and hole expandability. Has a great feature.

[0012]

Next, the reason why the composition and structure of the steel plate (slab) and the manufacturing conditions thereof are limited as described above in the present invention will be described in detail together with the operation. A) Chemical composition CC of steel sheet (slab) In the cooling process after hot rolling, C is concentrated in untransformed austenite with the progress of ferrite transformation to stabilize austenite, thereby making C in the hot rolled steel sheet. Has an effect of securing sufficient residual austenite to obtain the TRIP effect, but if its content is less than 0.05%, sufficient residual austenite is obtained.
Unable to secure the stain. On the other hand, if the content of C exceeds 0.25%, the weldability becomes a problem, and the second phase (bainite phase) becomes too large to deteriorate the hole expandability. Therefore, the C content is determined to be 0.05 to 0.25%, but is preferably adjusted to 0.10 to 0.25% if possible.

Mn Mn has an important effect of suppressing the transformation of untransformed austenite into pearlite or martensite, but if its content is less than 0.8%, the desired effect due to the above-mentioned effect is not obtained. I can't secure it. However, if Mn is contained in an amount exceeding 2.5%, sufficient polygonal ferrite cannot be obtained in the cooling process after hot rolling, and the concentration of C into untransformed austenite is also insufficient. Cannot achieve the desired high ductility or TRIP effect. Therefore, the Mn content was limited to 0.8 to 2.5%.

Si Si promotes the formation of polygonal ferrite, assists the concentration of C into untransformed austenite, and has a function of delaying the precipitation of cementite, so that it is easy to obtain retained austenite. Preferred components above. It also has the effect of significantly strengthening ferrite by solid solution strengthening. However, if the content of Si exceeds 2.5%, the surface quality and weldability of the steel sheet deteriorate, so the Si content is
It is determined to be 2.5% or less.

Similar to Si, Al has the same effect as "promoting the formation of polygonal ferrite to assist the concentration of C in untransformed austenite and delay the precipitation of cementite".
Therefore, the lower limit of the Si content is determined in relation to the Al content. That is, if “Si (%) + Al (%) <1.0”, a desired effect cannot be obtained in the above-described operation, so that the content of Si and Al is “Si (%) + Al (%) ≧ 1.0”. It is determined that the relationship is satisfied.

Al Al, as described above, Al has an effect of “promoting the formation of polygonal ferrite to assist the enrichment of C into untransformed austenite and delaying the precipitation of cementite”, similar to Si. Is a component that makes it easy to obtain retained austenite. Moreover, its addition does not lead to deterioration of the surface properties of the steel sheet. Further, the effect of the above action is more remarkable than the addition of Si at the same weight ratio, and the ferrite to be formed is fine and does not promote the formation of coarse bainite which deteriorates hole expandability. However, even if Al is contained in excess of 2.0%, the effect is saturated, and the amount of inclusions becomes too large and the hole expandability deteriorates.
2.0%, preferably the content of Al is 0.1 to 2.0
It is better to adjust to the range of%. As described above, the Al content is adjusted in the range of “Si (%) + Al (%) ≧ 1.0” in relation to the Si content, and FIG. 1 shows the relationship between the Al content and the Si content in the present invention. 5 is a graph showing a region of quantity.

Nb Nb has an effect of appropriately suppressing pearlite transformation of austenite and creating a condition in which austenite remains even at a cooling rate at which pearlite is formed. If it is less than 0.003%, the desired effects of the above-mentioned effects cannot be obtained. On the other hand, if the content of Nb exceeds 0.05%, the effect is saturated, which is economically disadvantageous. Therefore,
The Nb content was determined to be 0.003 to 0.05%.
It is preferable to adjust within the range of 0.03%.

FIG. 2 is a graph illustrating the relationship between the amount of Nb added to a hot-rolled steel sheet and the volume ratio of retained austenite and pearlite. In addition, the steel plate subjected to this investigation changed the Nb content of the steel slab whose chemical composition substantially corresponds to “Steel A” in Table 1 described later, and changed the Nb content to 125 ° C.
A hot-rolled steel sheet having a thickness of 2 mm under hot-rolling conditions of 0 ° C. and a finishing temperature of 900 ° C. The cooling rate from the end of hot-rolling to winding was 4 ° C./s. FIG. 2 also shows that when the Nb content is 0.003% or more, excessive pearlite transformation is suppressed, and a retained austenite with a volume ratio of 5% or more can be obtained.

Ti Ti is a component that is added because it has an action of preventing cracking of the slab, but if it is contained in excess of 0.04%, the effect of the action is saturated. Therefore, the Ti content is
0.04% or less.

[0020] N N but is undesirable impurity element so generates a Nb nitride wasting Nb in the steel, defined since it acceptable as long as containing up to 0.0100%, the content of 0.0100% or less Was.

Since Ca, Zr, and rare earth elements all have the effect of adjusting the shape of inclusions and improving the cold workability of a hot-rolled steel sheet, one or two of them may be used as necessary. The above additions are made. However, if the contents are respectively less than 0.0002% of Ca, less than 0.01% of Zr and less than 0.002% of rare earth element, the desired effects by the above-mentioned effects cannot be obtained. On the other hand, 0.01% of Ca and 0.10% of Zr are not obtained. %
, And when the rare earth elements are contained in amounts exceeding 0.10%, the inclusions in the steel become too large, and conversely, the workability deteriorates. Therefore, Ca content is 0.0002-0.01%, Zr content is
0.01-0.10%, and rare earth element content is 0.002-0.10
%.

The "unavoidable impurities" other than N inevitably mixed into steel include O, P, S, Cu, Ni, Cr, and Mo.
However, for example, it is desirable to regulate the contents of P and S as follows, if possible. Since PP is an impurity element that adversely affects weldability, its content is preferably as low as possible, but it can be said that it is desirable to suppress the P content to 0.05% or less in order to secure desired weldability. Since S is an impurity element that forms MnS-based inclusions and lowers workability, its content is preferably as low as possible. However, in order to secure desired workability, the P content is suppressed to 0.05% or less. It can be said that it is desirable.

Incidentally, the steel having the above-described composition is melted by, for example, a converter, an electric furnace, or an open hearth furnace. The steel type may be any of rimed steel, capped steel, semi-killed steel and killed steel. Also, the production of steel slabs may be performed by any means of "ingot-bulk rolling" or "continuous casting".

B) Volume fraction of microstructure retained austenite in hot-rolled steel sheet If the volume fraction of retained austenite in the hot-rolled steel sheet is less than 5%, desired high ductility cannot be ensured, so -Stenite ratio was determined to be 5% or more by volume.

When the volume ratio of pearlite in the hot-rolled steel sheet is less than 3%, relatively high ductility is ensured if a residual austenite with a volume ratio of 5% or more is obtained. Although you can,
Good hole expandability cannot be achieved because hard bainite is generated. In addition, since the amount of ferrite produced is reduced, the ductility also tends to be slightly lower. On the other hand, if the volume ratio of pearlite exceeds 20%, the pearlite transformation of untransformed austenite proceeds excessively, so that the amount of retained austenite decreases, and sufficient elongation cannot be secured. . Therefore, the percentage of pearlite is 3% by volume.
Although it has been determined to be ２０20%, it is preferable to adjust it to 5 ％ 15% if possible.

FIG. 3 shows that a steel slab having a chemical composition corresponding to “Steel A” in Table 1 described below was heated to a heating temperature of 12
Hot-rolling to a thickness of 2 mm under hot-rolling conditions of 50 ° C, finishing temperature: 900 ° C, and changing the cooling rate after hot-rolling to 0.01 to 50 ° C / s,
After cooling to 00 ° C., the furnace is cooled from this temperature.
The results of the experiments in which the pearlite volume ratio was changed by performing the “winding process corresponding to ° C” were arranged. The relationship between the pearlite volume ratio and the amount of retained austenite, and the pearlite volume ratio and the original thickness J
It is a graph which shows the relationship with the tensile strength, elongation, and punching hole expansion rate in IS5 tensile test. Also from FIG.
When the light volume ratio is in the range of 3 to 20%, it can be confirmed that both ductility and hole expanding properties are high.

The "expandability of punched hole" in FIG.
Was investigated by a method in which a 14φ hole punched out on a test piece having a size of 95 mm × 95 mm with a 5% clearance was expanded using a 50φ cylindrical punch until a through-thickness crack occurred. The hole expansion rate was determined by the following equation (1). Hole expansion rate = [(hole diameter at occurrence of through-thickness crack in sheet thickness)-(original hole diameter)] / original hole diameter ... (1) Also, the "amount of retained austenite" is X from the center of the steel sheet.
A test piece for a line test was taken and measured.

C) Manufacturing conditions of hot-rolled steel sheet Now, having the above-mentioned component composition, containing 5% or more by volume of retained austenite and 3 to 20% of pearlite, the balance being polygonal ferrite. The hot-rolled steel sheet of the present invention having a structure composed of bainite has a steel slab having the above-described composition.
Hot rolling is started after heating and holding at 100 ° C. or more.
r After finishing the continuous hot finish rolling at ₃ or more, 0.5 to 20
It can be manufactured by cooling to a temperature range of 550 ° C. or lower at a cooling rate of ° C./s.

Heating / Hot Rolling Conditions Prior to hot rolling, the steel slab having the predetermined composition was 1100
If the heating / holding temperature is lower than 1100 ° C., Nb cannot be sufficiently dissolved in austenite to obtain a hot-rolled steel sheet having desired characteristics. . Here, the steel slab to be inserted into the heating furnace may be a slab kept at a high temperature after casting or a slab left at room temperature.

In the method of the present invention, the finish rolling is terminated at Ar ₃ or higher. However, when the finishing temperature is set to less than Ar ₃ , strain due to hot rolling is introduced into ferrite grains formed by ferrite transformation during hot rolling. In addition, the workability of the manufactured hot-rolled steel sheet deteriorates. In order to suppress the precipitation of Nb in the austenite region and reduce the load on the hot rolling mill, the finishing temperature is preferably 850 ° C. or higher, more preferably 900 ° C. or higher.

Cooling Conditions after Hot Rolling In the method of the present invention, cooling conditions after hot rolling are also extremely important. Polygonal ferrite effective for ductility is sufficiently formed, and tensile strength: 50 kgf / mm
"Pullite which does not degrade hole expandability" is produced in the amount necessary to achieve ² or more, while the remainder is carbide-free bainite and retained austenite.

When the cooling rate is less than 0.5 ° C./s,
Since the pearlite transformation of untransformed austenite proceeds excessively during cooling, residual austenite cannot be obtained. In addition, since cementite precipitates at the ferrite grain boundaries and the grain boundaries become brittle, the hole expandability deteriorates. On the other hand, the cooling rate is 20 ° C / s
Is exceeded, the formation of polygonal ferrite is insufficient and the concentration of C in the untransformed austenite is insufficient, so that the austenite is not stabilized and is hard to contain fine carbides instead of pearlite. Bainite is generated and the hole spreading property is deteriorated. Therefore, the cooling rate after hot rolling is
Although 0.5 to 20 ° C./s is specified, it is preferable to adjust the range to 1 to 10 ° C./s if possible.

In the method of the present invention, cooling at the above-mentioned cooling rate is 5
The process is continued until the temperature reaches 50 ° C. or lower. However, if the temperature at the end of cooling exceeds 550 ° C., pearlite transformation proceeds excessively and residual austenite cannot be obtained. Then, after the completion of the cooling, it may be allowed to cool down without being wound up or rolled up, but in the range of 300 to 550 ° C. from the viewpoint of promoting the formation of bainite containing no carbide. It can be said that winding is preferable.

The hot-rolled steel sheet according to the present invention, which is manufactured according to the above conditions, has a tensile strength exceeding 50 kgf / mm ² or more and is excellent in all of ductility, hole expandability and weldability. Although workability is exhibited, it goes without saying that a surface-treated steel sheet having excellent ductility and hole-expanding properties can be obtained by subjecting the steel sheet of the present invention to surface treatment such as hot-dip galvanizing, galvannealing, and electroplating.

Next, the effects of the present invention will be described more specifically with reference to examples.

EXAMPLE A steel having the chemical composition shown in Table 1 was melted in a 50 kg vacuum melting furnace, and then a slab having a thickness of 60 mm was formed by hot forging.
To obtain a hot-rolled steel sheet having a thickness of 2 mm. In addition,
Winding was simulated by furnace cooling, and the furnace cooling starting temperature was taken as the winding temperature.

Next, JIS No. 5 tensile test pieces were sampled from the obtained steel sheets, and their mechanical properties were examined. In addition, a hole expansion test was also conducted for each steel plate. This investigation was conducted until a 14 mm punched hole was punched out of a 95 mm x 95 mm test piece with 5% clearance until a thickness through crack occurred with a 50 mm cylindrical punch. I went in a way to spread. In addition, the hole expansion rate was obtained by the above-mentioned equation (1). In addition, X
Specimens for the line test were collected, and the amount of retained austenite was investigated. These results are also shown in Table 2.

[0037]

[Table 1]

[0038]

[Table 2]

As is clear from the results shown in Table 2,
The hot rolled steel sheet produced according to the present invention has more than 5% retained austenite and 3-20% pearlite, so that the strength-ductility balance (tensile strength x elongation) is more than 2500 high ductility Further, the strength-hole expansion balance (tensile strength × hole expansion ratio) also showed good hole expansion properties exceeding 3000.

On the other hand, the hot-rolled steel sheets obtained in Test No. 25 in which Nb was not added and in Test No. 11 in which the hot-rolling heating temperature was lower than 1100 ° C. did not produce solid-dissolved Nb. Since the transformation progressed excessively, retained austenite was not secured, and the ductility was low.

The hot-rolled steel sheet obtained in Test No. 12 having a low hot-rolling finishing temperature has a reduced ductility because strain is applied to the ferrite during hot rolling. The hot rolled steel sheets obtained in Test Nos. 13 and 14 in which the cooling rate after hot rolling was higher than the range specified in the present invention had poor hole expandability because no pearlite was generated. In particular, in Test No. 13, in which the finishing temperature was high, the resulting hot-rolled steel sheet had insufficient generation of polygonal ferrite, a small amount of retained austenite, and low ductility. In Test No. 15 in which the cooling rate after hot rolling was lower than the range specified in the present invention, the pearlite transformation of the obtained hot rolled steel sheet progressed excessively, and the residual
Stenite was not formed, and thus the ductility was low.

Further, in Test No. 24 in which the total content of [Si + Al] is smaller than the range specified in the present invention, the ductility of the hot-rolled steel sheet obtained due to excessive progression of the pearlite transformation is low. .

[0043]

As described above, according to the present invention, it is possible to stably provide a high-tensile hot-rolled steel sheet for processing having excellent strength, ductility, hole expandability, and weldability, and can be used around automobiles. Industrially extremely useful effects are obtained, for example, it is possible to further improve the quality by applying to industrial equipment members such as parts.

[Brief description of the drawings]

FIG. 1 is a graph showing regions of Al content and Si content in a hot-rolled steel sheet of the present invention.

FIG. 2 is a graph illustrating the relationship between the amount of Nb added to a hot-rolled steel sheet and the volume ratio of retained austenite and the volume ratio of pearlite.

FIG. 3 is a graph illustrating the relationship between the pearlite volume ratio of a hot-rolled steel sheet and the amount of retained austenite, tensile strength, elongation, and hole expansion ratio.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 9/46-9/48

Claims (4)

(57) [Claims] 1. The content of C, Si, Mn, Nb, Ti, Al and N is 0.05 to 0.25% by weight, Si: 2.5% or less, and Mn: 0.8 by weight.
~ 2.5%, Nb: 0.003 ~ 0.05%, Ti: 0.04% or less, Al: 2.0
%, N: 0.0100% or less, and Si (%) + Al (%) ≧ 1.0, with the balance being Fe and unavoidable impurities. −
A hot-rolled high-strength steel sheet excellent in workability, characterized by having a structure composed of polygonal ferrite and bainite and containing a balance of 3-20% of pearlite. 2. The content of C, Si, Mn, Nb, Ti, Al and N is 0.05 to 0.25% by weight, Si: 2.5% or less, and Mn: 0.8 by weight.
~ 2.5%, Nb: 0.003 ~ 0.05%, Ti: 0.04% or less, Al: 2.0
% Or less, N: 0.0100% or less, Ca: 0.0002 to 0.01%, Zr: 0.01 to 0.10%, Rare earth element: At least one of 0.002 to 0.10%, and Si (%) + Al (%) ≧ 1.0 and a balance of Fe and inevitable impurities, and a residual volume of at least 5% by volume.
A hot-rolled high-strength steel sheet excellent in workability, characterized by having a structure composed of polygonal ferrite and bainite and containing a balance of 3-20% of pearlite. 3. The content of C, Si, Mn, Nb, Ti, Al and N in terms of weight ratio is as follows: C: 0.05 to 0.25%, Si: 2.5% or less, Mn: 0.8
~ 2.5%, Nb: 0.003 ~ 0.05%, Ti: 0.04% or less, Al: 2.0
%, N: 0.0100% or less, and a steel slab having a composition of Si (%) + Al (%) ≧ 1.0 and the balance of Fe and unavoidable impurities is reheated to 1100 ° C or higher. Exit finish rolling at a temperature range of not lower than Ar ₃ point while rolling, 0.5 to
Characterized by cooling at a cooling rate of 20 ° C./s to a temperature range of 550 ° C. or less, characterized by containing at least 5% by volume of residual austenite and 3 to 20% of pearlite and the balance being polygonal ferrite A method for producing a hot-rolled high-strength steel sheet having a structure composed of steel and bainite and having excellent workability. 4. The content of C, Si, Mn, Nb, Ti, Al and N in terms of weight ratio is as follows: C: 0.05 to 0.25%, Si: 2.5% or less, Mn: 0.8
~ 2.5%, Nb: 0.003 ~ 0.05%, Ti: 0.04% or less, Al: 2.0
% Or less, N: 0.0100% or less, Ca: 0.0002 to 0.01%, Zr: 0.01 to 0.10%, Rare earth element: At least one of 0.002 to 0.10%, and Si (%) + Al (%) ≧ A steel slab satisfying 1.0 and having the balance of Fe and inevitable impurities is reheated to 1100 ° C. or higher, hot rolled, and finish rolling is completed in a temperature range of Ar ₃ points or more.
Characterized by cooling at a cooling rate of 20 ° C./s to a temperature range of 550 ° C. or less, characterized by containing at least 5% by volume of residual austenite and 3 to 20% of pearlite and the balance being polygonal ferrite A method for producing a hot-rolled high-strength steel sheet having a structure composed of steel and bainite and having excellent workability.