JPH05105986A - Hot rolled high tensile strength steel sheet with high workability and its production - Google Patents

Abstract

PURPOSE:To stably provide a high tensile strength steel sheet for working combining sufficiently superior strength, ductility, weldability, and enlargement characteristic. CONSTITUTION:A steel which has a composition containing 0.05-0.25% C, 1.0-2.5% Si, 0.8-2.5% Mn, and 0.10-1.0% sol.Al or further containing one or more kinds among specific amounts of Ca, Zr, rare earth elements, Ti, and V is heated and held to and at a temp. not lower than the Ac3 point and hot- rolled. The resulting plate is subjected, after finish rolling is completed at 780-840 deg.C, to accelerated cooling down to 300-450 deg.C at 10-50 deg.C/s cooling rate and to cooling, or, the above plate is subjected, after finish rolling is completed at 780-940 deg.C, to accelerated cooling down to 600-700 deg.C at >=10 deg.C/s cooling rate, to air cooling at a temp. in the above temp. region for 2-10sec, further to accelerated cooling down to 300-450 deg.C at >=20 deg.C/s cooling rate. By this method, the hot rolled steel sheet having a structure composed essentially of polygonal ferrite and containing retained austenite by >=5% volume percentage can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength hot-rolled steel sheet for processing, which is suitable as a steel sheet for high-strength members in automobiles, industrial machines, etc. and has excellent ductility and hole expandability, and a method for producing the same.

[0002]

2. Description of the Related Art Today, "hot rolled steel sheet" produced by continuous hot rolling has been widely applied to various industrial equipments such as automobiles as a relatively inexpensive structural material. However, there are many members that are formed by press working for their applications, and therefore there is a strong demand for "combination of high strength and high workability".

However, it is generally unavoidable that the workability deteriorates when the strength of a steel sheet is increased, but as a steel material excellent in both strength and ductility, for example, JP-A-55-44551.
DP steel (Dual Phase steel) as described in Japanese Patent Publication
It has been known. This DP steel is a [ferrite + martensite] dual-phase steel, and is characterized by "a low yield ratio and high ductility". However, according to the characteristics of JIS No. 5 tensile test piece, it is a 60 kg class high tensile steel. {Tensile strength (TS): 60kgf / mm ² } Elongation (El) ≈ 30%, TS x El <2
000, 80kg class high tensile strength {tensile strength (TS): 80kg
Elongation (El) ≈ 20% at f / mm ² } and TS x El <1800 are the limits, and a material with further improved ductility has been desired.

On the other hand, apart from this, as a means for significantly improving the ductility of high strength steel sheet, TR of residual austenite is used.
A method utilizing IP (transformation-induced plasticity) has been proposed (see, for example, JP-A-55-145121). According to this method, the tensile strength (TS) is 110 kgf / mm ² or more, the elongation (El) is 22% or more, and it is possible to manufacture a high ductility and high strength steel plate having a “TS × El value” of more than 2400. Is considered to be. However, this method
The steel plate obtained from this is inferior in weldability because it is necessary to adjust it to a high value of 0.40 to 0.85% (% representing the composition ratio is hereinafter referred to as weight%), and it is difficult to widely use it as a steel plate for automobiles. there were.

As a means for securing retained austenite under a low C content to obtain high ductility in steel, a steel containing about 0.2% C, about 1.5% Si, and about 1.5% Mn. Is hot-rolled and finish-rolled near Ar ₃ and then 40
A method has been proposed in which accelerated cooling is performed at a cooling rate of ℃ / s or more, and then wound around 400 ℃ (Japanese Patent Laid-Open No. 63-4017).
issue). However, with this method, although the elongation can be greatly improved as compared with DP steel, hole expansibility, which is one of the important indicators of workability, is not improved, and the hole expansibility of the hot-rolled steel sheet obtained is not improved. Was only about 30% of DP steel plate. Therefore, there has been a strong demand for the appearance of a high-strength hot-rolled steel sheet for working which cannot be used for parts that require stretch-flangeability and is sufficiently satisfactory in hole expandability.

Therefore, the object of the present invention is to establish means for stably providing a high-strength steel plate for working which has sufficiently excellent strength, ductility, weldability and hole expandability. Was that.

[0007]

In order to achieve the above object, the inventors of the present invention, in particular, have a sufficiently high strength for use in automobiles and the like with a C content range in which weldability is satisfactory. Shown and "TR that leads to excellent ductility and hole expandability
As a result of various investigations for the feasibility of a high ductility hot rolled high strength steel sheet containing a sufficient amount of austenite to utilize the IP effect, the following findings were obtained. ..

A) "Si" is a very preferable component for securing retained austenite effective for improving ductility in a relatively low C content range preferable from the viewpoint of weldability, but Al is
Also has the same effect as Si, and conventional Si-added residual
When the appropriate amount of Al is added to the high-strength steel of the stenite type, the above effect is effectively exerted and its elongation and hole expansibility are significantly improved.

That is, in Table 1, the tensile properties and punch hole expandability of 2.6 mm thick "Si-added retained austenite type high-strength steel sheet" and "Si + Al-added retained austenite type high-strength steel sheet" are investigated. The results are shown. Here, JIS No. 5 test piece is used for tensile properties, and the hole expansion ratio is 2.6 mm.
A steel piece of × 120 mm × 120 mm was punched out with a hole of 14 mmφ with a clearance of 15% and then examined using a conical punch. From Table 1, it can be seen that the retained austenitic steel in which Si and Al are added together has the same tensile strength / elongation balance as the conventional Si-added retained austenitic high-strength steel, and has a significantly superior tensile strength. It can be seen that it shows a balance of the hole expandability.

[0010]

[Table 1]

B) Further, since Al is a ferrite stabilizing element, it was considered that the addition of Al was disadvantageous for remaining austenite, but the addition of an appropriate amount of Al promotes the formation of polygonal ferrite. In addition to improving the elongation of the product, the second phase is evenly dispersed and is effective for improving the hole expandability.

C) Therefore, 5% or more of retained austenite by volume ratio is contained by including a proper amount of Si and Al in a steel having a C content range in which weldability is not significantly deteriorated. If a structure mainly containing polygonal ferrite is realized, a hot-rolled high-strength steel sheet that sufficiently satisfies various properties such as strength, ductility, and hole expandability required for steel sheets for automobiles can be obtained.

D) Furthermore, the high ductility hot-rolled high-strength steel sheet excellent in hole expandability can be stably manufactured with good reproducibility by hot rolling a steel slab having a predetermined composition according to specific conditions. To be.

The present invention has been completed by further research based on the above findings and the like. "The hot-rolled high-strength steel sheet C: 0.05 to 0.25%, Si: 1.0 to 2.5%, M
n: 0.8 to 2.5%, sol.Al: 0.10 to 1.0%, or Ca: 0.0002 to 0.010%, Zr: 0.01 to 0.10%,
Rare earth element: 0.01 to 0.10%, Nb: 0.005 to 0.10%,
Polygonal containing at least one of Ti: 0.005 to 0.10%, V: 0.005 to 0.20%, the balance being Fe and unavoidable impurities, and containing 5% or more of retained austenite by volume. It is characterized by having excellent ductility and hole expandability by having a structure mainly composed of ferrite, and further, "C: 0.05-0.25%,
Si: 1.0 to 2.5%, Mn: 0.8 to 2.5%, sol.Al: 0.
Contains 10 to 1.0% or more Ca: 0.0002 to 0.010
%, Zr: 0.01 to 0.10%, rare earth element: 0.01 to 0.10.
%, Nb: 0.005 to 0.10%, Ti: 0.005 to 0.10%,
V: Steel containing 0.005 to 0.20% of 1 type or more and the balance of Fe and unavoidable impurities is heated and held at Ac ₃ point or more and then hot rolled, and 780 to 780
After finishing rolling at 840 ° C., accelerated cooling to 300 to 450 ° C. at a cooling rate of 10 to 50 ° C. and winding, or finishing rolling at 780 to 940 ° C.
Accelerated cooling to a temperature range of 600 to 700 ° C at a cooling rate of ℃ / s or more, air cooling in that temperature range for 2 to 10 seconds, and then accelerating to 300 to 450 ° C at a cooling rate of 20 ° C / s or more. By cooling and winding up, the high ductility hot rolled high strength steel sheet having the above-mentioned “structure mainly composed of polygonal ferrite containing 5% or more by volume of retained austenite” and excellent in ductility and hole expansibility is stably obtained. It also has a major feature in that it is manufactured.

The term "polygonal ferrite-based structure" as used herein means that "a structure other than retained austenite (for example, bainite)" is "substantially polygonal ferrite" to such an extent that the properties of the polygonal ferrite are not significantly affected. It goes without saying that the term "structure consisting of null ferrite" is used broadly.

[0016]

Next, the reason why the composition of the steel sheet (steel slab) and the manufacturing conditions thereof are limited as described above in the present invention will be described in detail together with its operation. A) The chemical composition C C of the steel sheet (steel piece) is concentrated in untransformed austenite with the progress of ferrite transformation in the cooling process after hot rolling to stabilize austenite and strengthen the steel sheet. However, if the content is less than 0.05%, the strength is not secured and the austenite stabilizing effect is not sufficient.
When C is contained in excess of 0.25%, the weldability is significantly deteriorated, and the amount of polygonal ferrite decreases and the amount of bainite increases too much, so that the hole expandability also deteriorates. Therefore, C
The content was set to 0.05 to 0.25%.

Si In general, Si promotes the formation of polygonal ferrite to form C.
Is a very effective element for remaining austenite because it has the effect of assisting the concentration of untransformed austenite and delaying the precipitation of cementite, and also solid-solution strengthens polygonal ferrite, It is also an effective component for increasing the strength of steel sheets. However, its content is 1.0%
If it is less than 2.5%, the above effect is not sufficiently secured, while if Si exceeds 2.5%, hard martensite is likely to be formed, resulting in deterioration of hole expandability. Therefore, the Si content is set to 1.0 to 2.5%.

Mn Mn is an austenite stabilizing element, and acts to lower the Ms point of untransformed austenite, improve hardenability, and suppress pearlite transformation of untransformed austenite. Exert. However, if the Mn content is less than 0.8%, the desired effect due to the above-mentioned action cannot be obtained.
%, It becomes difficult to form sufficient polygonal ferrite in the cooling process after hot rolling, so that the concentration of C in untransformed austenite becomes insufficient and the austenite is stabilized. I can't make it happen. Therefore, the Mn content is set to 0.8 to 2.5%.

Sol.Al Al is a particularly important component in the present invention. That is, Al
Like Si, it is a ferrite stabilizing element, promotes the formation of polygonal ferrite to promote the concentration of C in untransformed austenite, and promotes the retention of austenite through the action of delaying the precipitation of cementite. Moreover, the effect is more remarkable than when Si is added at the same weight ratio. In addition, it also has the function of promoting the uniform and fine formation of polygonal ferrite, suppressing the formation of coarse bainite that deteriorates the hole expandability, and improving the hole expandability of Si-added retained austenitic steel. There is. However, if the Al content is so
If the content of l.Al is less than 0.10%, the desired effect due to the above-mentioned action cannot be obtained, while if it exceeds 1.0%, the effect is saturated. Therefore, the sol.Al content is set to 0.10 to 1.0%.

Ca, Zr, and rare earth elements All of these components have the effect of adjusting the shape of the inclusions to improve the cold workability, and therefore, if necessary, one or more of them may be added. However, in the case of Ca, the content is less than 0.0002%, in the case of Zr, the content is less than 0.01%, and in the case of rare earth elements, the content is less than 0.01%, the desired effect due to the above action is obtained. Not obtained, while Ca
If 0.01%, Zr exceeds 0.10%, and rare earth elements exceed 0.10%, the amount of inclusions in the steel becomes excessive and conversely the workability deteriorates. Therefore, the Ca content is 0.00
02-0.01%, Zr content 0.01-0.10%, and rare earth element content 0.01-0.10%.

Nb, Ti, and V Nb, Ti, and V are all elements that precipitate as carbonitrides on the ferrite ground and are effective for further strengthening the steel sheet. Therefore, if necessary, one or more of them may be used. In any case, if the content is less than 0.005%, the desired effect is not obtained, while in Nb and Ti, the content exceeds 0.10%, and in the case of V, the content exceeds 0.20%. However, that effect is saturated and not economical. Therefore, the Nb content is 0.0
05-0.10%, Ti content was 0.005-0.10%, and V content was 0.005-0.20%.

The "unavoidable impurities" that are unavoidably mixed in the steel include P, S, Cu, Ni, Cr, Mo, etc. For example, the contents of P and S are as follows. It is desirable to regulate it. Since P P is an impurity element that adversely affects weldability, it is desirable to keep its content to 0.05% or less, but from the viewpoint of evenly dispersing polygonal ferrite.
It is more preferably 0.010% or less. Since S S is an impurity element that forms sulfide-based inclusions and deteriorates the workability, it is desirable to keep its content to 0.05% or less, but from the viewpoint of securing more excellent workability, It is more preferably 0.003% or less.

By the way, the steel having the composition as described above is melted by, for example, a converter, an electric furnace, an open furnace or the like. The steel type may be any of rimmed steel, capped steel, semi-killed steel or killed steel. Further, the production of the steel slab may be carried out by any means of "ingot-segmenting rolling" or "continuous casting".

B) Manufacturing Conditions for Hot Rolled Steel Sheet Now, the hot rolled steel sheet according to the present invention having the above-described composition and having a structure mainly composed of polygonal ferrite containing 5% or more by volume of retained austenite. The steel plate was heated and held at a temperature of Ac ₃ or higher for a steel slab having the above-mentioned composition, and then hot-rolled.
After finishing rolling at 0 ~ 840 ℃, 10 ~ 50 ℃ / s
It can be manufactured by accelerating cooling to 300 to 450 ° C. at a cooling rate of, and winding.

That is, the alloy element can be completely dissolved in austenite by heating the steel piece to the Ac ₃ point or higher. The steel piece to be inserted into the heating furnace may be a slab that remains at a high temperature after casting or a slab that has been left at room temperature.

Further, by completing the finish rolling at 780 to 840 ° C., the austenite can be made finer and work hardening of the austenite can be caused to accelerate the formation of polygonal ferrite. It is possible to generate a sufficient amount of polygonal ferrite during accelerated cooling at a cooling rate of 10 to 50 ° C./s after rolling. If the finishing temperature is lower than 780 ° C., ferrite is generated during hot rolling to form work ferrite, which deteriorates workability of the hot rolled steel sheet. On the other hand, the finishing temperature is 840
If the temperature exceeds ℃, the work hardening of austenite will be insufficient and polygonal ferrite will not be sufficiently generated, resulting in residual austenite.
Stenite is reduced.

The cooling rate after finish rolling is 10 ° C./s.
When it is less than pearlite, pearlite is formed during cooling and austenite does not remain. On the other hand, when the cooling rate exceeds 50 ° C./s, a sufficient amount of polygonal ferrite is not formed and austenite does not remain.

Further, if the winding temperature exceeds 450 ° C.
Light is produced and not enough austenite remains.
On the other hand, if the coiling is carried out in a temperature range below 300 ° C., the production of martensite is promoted and the ductility and hole expansibility deteriorate.

By the way, in addition to the above method, the high ductility high tensile strength hot rolled steel sheet according to the present invention is obtained by heating and holding a steel slab having the above-mentioned predetermined composition at an Ac ₃ point or higher, and then hot rolling it.
After finishing rolling at 0 to 940 ° C., accelerated cooling to a temperature range of 600 to 700 ° C. at a cooling rate of 10 ° C./s or more, and air cooling for 2 to 10 seconds in the temperature range, and then 20 ° C.
It can also be manufactured by accelerating cooling to 300 to 450 ° C. at a cooling rate of / s or more and winding.

That is, the austenite can be refined by finishing the finish rolling at 780 to 940 ° C., but thereafter, the austenite can be accelerated and cooled to the temperature range of 600 to 700 ° C. and the temperature range of 2 to 10 seconds. When air-cooled, the formation of polygonal ferrite is promoted and C in untransformed austenite is increased.
Is promoted, and a sufficient amount of austenite can remain. In this case, if the air cooling temperature range is less than 600 ° C. or if the air cooling time is less than 2 seconds, the production of polygonal ferrite is insufficient, while the air cooling temperature range is 700
If the temperature exceeds ℃ or the air cooling time exceeds 10 seconds, it will
Light is generated and austenite does not remain.

The reason why accelerated cooling is performed at a cooling rate of 10 ° C./s or more after finish rolling is to secure an air cooling time of 2 to 10 seconds on a limited hot run table. Also,
The reason why accelerated cooling is performed at 20 ° C./s or more after the air cooling is necessary to suppress the generation of pearlite.

Regarding the winding temperature, as in the case of the above-mentioned alternative method, when the temperature exceeds 450 ° C., pearlite is formed and austenite does not remain sufficiently, while in the temperature range below 300 ° C. When wound, martensite is promoted to be produced, and ductility and hole expandability deteriorate.

[0033]

The hot-rolled steel sheet according to the present invention has a C content of 0.05 to 0.25% and thus has a weldability of a level required for a steel sheet for high-strength members, and further utilizes TRIP. A sufficient amount of austenite. Moreover, the hole expandability, which was a weak point of the Si-added retained austenitic steel sheet, was sufficiently improved. Therefore,
According to the present invention, it is possible to realize a hot-rolled steel sheet having good weldability, residual austenite, good balance of tensile strength and elongation, and excellent hole expandability. .. In addition, the present invention steel sheet, hot dip galvanization,
When a surface treatment such as galvannealing or electroplating is performed, a surface-treated steel sheet having more excellent surface properties, ductility and hole expandability can be obtained.

Next, the effects of the present invention will be described more specifically by way of examples.

EXAMPLE A slab made of steels A to S having the chemical composition shown in Table 2 was heated and hot-rolled under the conditions shown in Tables 3 to 4.
Controlled cooling and winding were performed to obtain a hot-rolled steel sheet having a plate thickness of 2.3 mm.
Next, a JIS No. 5 tensile test piece was sampled from the obtained steel sheet, and the mechanical properties were investigated. Also, the size is 2.3mm thick x 1
A 14 mmφ hole was punched out in a steel plate test piece of 20 mm width × 120 mm length with a clearance of 15%, and the hole expansibility was investigated using a conical punch. Further, a test piece for X-ray test was sampled from the central portion of the steel sheet, and the amount of retained austenite was also investigated. The results are also shown in Tables 3 to 4.

[0035]

[Table 2]

[0036]

[Table 3]

[0037]

[Table 4]

As is clear from the results shown in Tables 3 to 4, the hot-rolled steel sheet manufactured according to the present invention has a high tensile strength of 70 kgf / mm ² or more and an excellent ductility of 30% or more. At the same time, it has excellent punching workability with a hole expansion ratio of 80% or more, and it is understood that both high strength and high workability are satisfied at the same time.

On the other hand, in Test Nos. 20 and 45 using steel type O having a C content exceeding the specified range of the present invention, the elongation is relatively high but the hole expansibility is poor. In the test number 21 using the steel type P whose sol.Al content is below the specified range of the present invention,
Similarly, the elongation is relatively high but the hole expansibility is poor. In the test number 46 using the steel type Q in which the C content and the Si content are below the specified range of the present invention, the strength is significantly inferior. In the test numbers 22 and 47 using the steel type R in which the Si content and the Mn content exceed the specified ranges of the present invention, the elongation and the hole expansibility are inferior. In the test numbers 23 and 48 using the steel type S whose Si content and sol.Al content are out of the specified ranges of the present invention, the elongation is relatively high but the hole expansibility is poor.

Test Nos. 16 and 44 under conditions in which the winding temperature is out of the range specified by the present invention, and Test No. 1 whose finishing temperature, cooling rate, air cooling temperature, air cooling time, etc. are outside the ranges specified by the present invention
The steel sheets obtained with Nos. 7, 18, 19, 39 to 44 all do not generate sufficient retained austenite, the elongation decreases, and the hole expandability is not good.

[0041]

[Summary of Effects] As described above, according to the present invention, it is possible to stably provide a high workability high strength hot rolled steel sheet which is excellent in strength, ductility, hole expandability and weldability.
By being applied to industrial equipment members such as automobile undercarriage parts, the performance and life of those products can be further improved, resulting in extremely useful effects in industry.

Claims (12)

[Claims] 1. A weight ratio of C: 0.05 to 0.25%, S
i: 1.0 to 2.5%, Mn: 0.8 to 2.5%, sol.Al: 0.1
It is characterized by containing 0 to 1.0% and the balance of Fe and inevitable impurities, and having a structure mainly composed of polygonal ferrite containing 5% or more by volume of retained austenite. , High workability hot rolled high tensile steel sheet with excellent ductility and hole expandability. 2. A weight ratio of C: 0.05 to 0.25%, S
i: 1.0 to 2.5%, Mn: 0.8 to 2.5%, sol.Al: 0.1
Including 0 to 1.0%, Ca: 0.0002 to 0.010%, Zr:
Polygonal ferrite containing 0.01 to 0.10%, rare earth element: 0.01 to 0.10%, at least one of which is Fe and unavoidable impurities, and 5% or more by volume of retained austenite. A highly workable hot-rolled high-tensile steel sheet having excellent ductility and hole expandability, which is characterized by having a main structure. 3. A weight ratio of C: 0.05 to 0.25%, S
i: 1.0 to 2.5%, Mn: 0.8 to 2.5%, sol.Al: 0.1
Including 0 to 1.0%, Nb: 0.005 to 0.10%, Ti: 0.
005 to 0.10%, V: 0.005 to 0.20%, and the remaining components are Fe and inevitable impurities.
A high workability hot-rolled high-tensile steel sheet having excellent ductility and hole expandability, which has a structure mainly composed of polygonal ferrite containing 5% or more by volume of retained austenite. 4. A weight ratio of C: 0.05 to 0.25%, S
i: 1.0 to 2.5%, Mn: 0.8 to 2.5%, sol.Al: 0.1
Including 0 to 1.0%, Ca: 0.0002 to 0.010%, Zr:
0.01 to 0.10%, one or more rare earth elements: 0.01 to 0.10%, and Nb: 0.005 to 0.10%, Ti: 0.005 to 0.10
%, V: 0.005 to 0.20% of at least one kind, and the balance of Fe and unavoidable impurities as the remaining components, and a structure mainly composed of polygonal ferrite containing 5% or more by volume of retained austenite. A high workability hot-rolled high-tensile steel sheet having excellent ductility and hole expandability, which is characterized by comprising. 5. A weight ratio of C: 0.05 to 0.25%, S
i: 1.0 to 2.5%, Mn: 0.8 to 2.5%, sol.Al: 0.1
Steel containing 0 to 1.0% and the balance consisting of Fe and unavoidable impurities is heated and held at an Ac ₃ point or higher and then hot rolled, and after finishing rolling at 780 to 840 ° C, 1
It has a structure mainly composed of polygonal ferrite containing 5% or more by volume of retained austenite, which is characterized by accelerated cooling to 300 to 450 ° C at a cooling rate of 0 to 50 ° C / s and winding. A method for producing a high workability hot rolled high strength steel sheet having excellent ductility and hole expandability. 6. A weight ratio of C: 0.05 to 0.25%, S
i: 1.0 to 2.5%, Mn: 0.8 to 2.5%, sol.Al: 0.1
Including 0 to 1.0%, Ca: 0.0002 to 0.010%, Zr:
Steel containing 0.01 to 0.10%, rare earth element: 0.01 to 0.10%, and balance of Fe and inevitable impurities is heated and held at Ac ₃ points or more, and hot-rolled.
After finishing rolling at 780 to 840 ° C, 10 to 50
It is characterized by accelerated cooling to 300 to 450 ° C at a cooling rate of ℃ / s and winding, and a residual o of 5% or more in volume ratio.
A method for producing a high workability hot-rolled high-strength steel sheet having a structure mainly composed of polygonal ferrite containing steenite and having excellent ductility and hole expandability. 7. A weight ratio of C: 0.05 to 0.25%, S
i: 1.0 to 2.5%, Mn: 0.8 to 2.5%, sol.Al: 0.1
Including 0 to 1.0%, Nb: 0.005 to 0.10%, Ti: 0.
Steel containing 005 to 0.10%, V: 0.005 to 0.20% and the balance of Fe and inevitable impurities is heated and held at an Ac of ₃ points or more, and hot-rolled.
After finishing rolling at ˜840 ° C., accelerated cooling to 300 to 450 ° C. at a cooling rate of 10 to 50 ° C./s and winding is performed, and residual austenite of 5% or more in volume ratio is retained. A method for producing a high workability hot-rolled high-strength steel sheet having a structure mainly containing polygonal ferrite and having excellent ductility and hole expandability. 8. A weight ratio of C: 0.05 to 0.25%, S
i: 1.0 to 2.5%, Mn: 0.8 to 2.5%, sol.Al: 0.1
Including 0 to 1.0%, Ca: 0.0002 to 0.010%, Zr:
0.01 to 0.10%, one or more rare earth elements: 0.01 to 0.10%, and Nb: 0.005 to 0.10%, Ti: 0.005 to 0.10
%, V: 0.005 to 0.20% of at least one steel, and the balance consisting of Fe and inevitable impurities, the steel is heated and held at Ac ₃ points or more, and hot-rolled at 780 to 840 ° C.
After finishing rolling is completed, accelerated cooling to 300 to 450 ° C. at a cooling rate of 10 to 50 ° C./s and winding up is performed, and a polygo containing 5% or more by volume of retained austenite. A method for producing a high workability hot-rolled high-strength steel sheet having a structure mainly composed of null ferrite and excellent in ductility and hole expandability. 9. A weight ratio of C: 0.05 to 0.25%, S
i: 1.0 to 2.5%, Mn: 0.8 to 2.5%, sol.Al: 0.1
A steel containing 0 to 1.0% and the balance of Fe and inevitable impurities is heated and held at an Ac ₃ point or higher and then hot rolled, and after finishing rolling at 780 to 940 ° C, 1
Accelerated cooling to a temperature range of 600 to 700 ° C at a cooling rate of 0 ° C / s or more, air cooling in that temperature range for 2 to 10 seconds, and then to 300 to 450 ° C at a cooling rate of 20 ° C / s or more. Highly workable hot-rolled high-strength steel sheet having a structure mainly composed of polygonal ferrite containing 5% or more by volume of retained austenite and having excellent ductility and hole expandability, characterized by accelerated cooling and winding. Manufacturing method. 10. A weight ratio of C: 0.05 to 0.25%,
Si: 1.0 to 2.5%, Mn: 0.8 to 2.5%, sol.Al: 0.
Including 10 to 1.0%, Ca: 0.0002 to 0.010%, Zr:
Steel containing 0.01% to 0.10%, rare earth element: 0.01% to 0.10%, and the balance being Fe and inevitable impurities is heated and held at Ac ₃ points or more, and then hot rolled,
After finishing rolling at 780 to 940 ° C., accelerated cooling to a temperature range of 600 to 700 ° C. at a cooling rate of 10 ° C./s or more, and air cooling in the temperature range for 2 to 10 seconds, and then 20
A ductility having a structure mainly composed of polygonal ferrite containing 5% or more of retained austenite by volume ratio, which is characterized by accelerating cooling to 300 to 450 ° C at a cooling rate of ℃ / s or more and winding. A method of manufacturing a hot-rolled, high-strength steel sheet with high workability and excellent hole expandability. 11. A weight ratio of C: 0.05 to 0.25%,
Si: 1.0 to 2.5%, Mn: 0.8 to 2.5%, sol.Al: 0.
Including 10 to 1.0%, Nb: 0.005 to 0.10%, Ti:
Steel containing 0.005 to 0.10%, V: 0.005 to 0.20%, and balance of Fe and inevitable impurities is heated and held at Ac ₃ points or more and hot-rolled.
After finishing rolling at 0 to 940 ° C, accelerated cooling to a temperature range of 600 to 700 ° C at a cooling rate of 10 ° C / s or more, and air cooling for 2 to 10 seconds in the temperature range, and then 20 ° C.
Ductility and holes having a structure mainly composed of polygonal ferrite containing 5% or more of retained austenite by volume ratio, characterized by being acceleratedly cooled to 300 to 450 ° C. at a cooling rate of / s or more and wound up. A method of manufacturing a high workability hot rolled high strength steel sheet having excellent expandability. 12. A weight ratio of C: 0.05 to 0.25%,
Si: 1.0 to 2.5%, Mn: 0.8 to 2.5%, sol.Al: 0.
Including 10 to 1.0%, Ca: 0.0002 to 0.010%, Zr:
0.01 to 0.10%, one or more rare earth elements: 0.01 to 0.10%, and Nb: 0.005 to 0.10%, Ti: 0.005 to 0.10
%, V: 0.005 to 0.20% of at least one steel and the balance of Fe and unavoidable impurities in the balance are heated and held at Ac ₃ points or more and then hot-rolled at 780 to 940 ° C.
After finishing rolling in, the product is accelerated cooled to a temperature range of 600 to 700 ° C at a cooling rate of 10 ° C / s or more, air-cooled in the temperature range for 2 to 10 seconds, and further cooled to 20 ° C / s or more. Highly excellent in ductility and hole expansibility having a structure mainly composed of polygonal ferrite containing 5% or more of retained austenite by volume ratio, which is characterized by accelerating cooling to 300 to 450 ° C. and winding. Processable hot rolled high strength steel sheet manufacturing method.