JPH0559485A - High ductility hot rolled high tensile strength steel sheet and its manufacture - Google Patents

Abstract

(57) [Summary] [Purpose] Stable provision of high-strength steel sheets for working that have sufficiently excellent strength, ductility, weldability, and hole expandability, as well as good surface properties. [Constitution] C: 0.05-0.25%, Si: 0.05% or less, Mn: 0.8
~ 2.5%, sol.Al: 1.0 ~ 2.5%, or further containing a specific amount of one or more of Ca, Zr, rare earth elements, Ti, V is heated and held at Ac ₃ point or more. 10 ~ 5 after finishing rolling at 780 ~ 840 ℃
Accelerated cooling to 300 to 450 ° C at a cooling rate of 0 ° C / s and winding, or finishing rolling at 780 to 940 ° C and a cooling rate of 10 ° C / s or more to a temperature range of 600 to 700 ° C. Accelerated cooling to 2-10 seconds, and then 300-450 ℃ at a cooling rate of 20 ℃ / s or more.
By accelerating cooling to, and winding, a hot rolled steel sheet having a structure mainly composed of polygonal ferrite containing 5% or more by volume of retained austenite is 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, 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 that are molded by pressing for their applications, and therefore there is a strong demand for "combining high strength and high ductility".

As a steel material excellent in both strength and ductility, for example, DP steel (Dual Phase steel: ferrite + martensite dual phase steel) as described in JP-A-55-44551 is known. .. It is said that the characteristic of this DP steel is "the yield ratio is low and the ductility is high", but the tensile strength is 60 kgf / mm ² and the elongation is about 30% at the present time. In view of the above, there has been a demand for an improved material.

By the way, a method utilizing TRIP (transformation-induced plasticity) of retained austenite has been developed as a means for improving the ductility of high-strength steel sheets (for example, JP-A-55-1).
45121). And according to this method
It is possible to manufacture a high-ductile high-strength steel sheet having a tensile strength of 110 kgf / mm ² or more and an elongation of 22% or more, and having a “value of [tensile strength] × [elongation]” exceeding 2400. However, with this method, it is necessary to adjust the C content to a high value of 0.35 to 0.85% (hereinafter,% representing the component ratio shall be% by weight), so the steel sheet obtained is inferior in weldability and is not suitable for automobiles. The scope of application as a steel sheet was narrow.

Incidentally, as a means for securing retained austenite under a low C content to obtain high ductility in the steel, a method of subjecting a high Si content steel to a large reduction in a low temperature austenite region has been proposed ( JP-A-63-4017). As described above, the addition of Si is effective for obtaining retained austenite, and therefore it is possible to secure high ductility in the steel material by adding an appropriate amount of Si, but on the other hand, coarse bainite is easily generated, and
If the amount of Si added is high, hard martensite is also likely to be formed, so hole expansibility, which is one of the important workability for steel sheets for automobiles, was low. In particular, when a large reduction is applied in the low temperature austenite region as described above, coarse bainite is likely to be formed into a band shape, which significantly deteriorates the hole expandability. Further, in the case of high Si-added steel, poor surface quality of hot-rolled steel sheet called island scale also became a big problem.

Therefore, the object of the present invention is to stabilize a high-strength steel plate for working which has sufficiently excellent strength, ductility, weldability and hole expandability, and has good surface properties. Was to establish a means that can be provided by.

[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 valuable findings can be obtained. It was

A) 1.0% or more even in the low Si content steel in which the "Si" effective for securing the retained austenite in the low C range is reduced and the deterioration of the surface property due to the island scale is suppressed. If austenite is contained by adding Al in a proportion of, it is possible to obtain an excellent balance of tensile strength and elongation.

That is, in Table 1, steel pieces (2.0 mm thick) containing Al in various proportions were heated to 950 ° C. to be austenitized, and then kept at 780 ° C. for 20 minutes, and subsequently, 450
The results of examining the amount of retained austenite and the tensile properties when rapidly cooled to ℃, held at the temperature of 450 ℃ for 10 minutes, and then air-cooled to room temperature are shown.
It can be seen that even if the content is reduced to 0.05% or less, if the Al content is increased to 1.0% or more, 5% or more of austenite can remain and excellent tensile strength / elongation balance can be imparted. ..

[0010]

[Table 1]

B) In the conventional Si-added retained austenite type high-strength steel, although a high elongation is obtained in the tensile test, only a hole expansion rate similar to that of DP steel is obtained in the hole expansion test, whereas Al addition is performed. In the case of residual austenitic high-strength steel, unexpectedly excellent elongation and high hole expansion rate can be satisfied at the same time.

That is, Table 2 shows that Si-added retained austenite type and Al-added retained austenite type high-strength steels (2.6 mm
The results of an examination of tensile properties and punched hole expandability with respect to (thickness) are shown. Here, the tensile properties were measured using JIS No. 5 test pieces, and the hole expansion ratio was obtained by punching a 14 mmφ hole in a steel piece of 2.6 mm × 120 mm × 120 mm with a clearance of 15% and then using a conical punch. .. From Table 2 as well, it can be seen that the Al-added retained austenitic steel exhibits a tensile strength / elongation balance equivalent to that of the conventional steel, and also exhibits a remarkably excellent tensile strength / hole expandability balance.

[0013]

[Table 2]

C) Further, since Al is a ferrite stabilizing element, it was considered that addition of Al was disadvantageous for leaving austenite remaining. However, addition of an appropriate amount of Al promotes 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.

D) Therefore, the weldability C does not significantly deteriorate.
If a structure mainly composed of polygonal ferrite containing 5% or more by volume of retained austenite is realized by adding 1% or more of Al to a low Si steel having a content range, it is required for automobile steel sheets and the like. A hot-rolled high-tensile steel sheet that sufficiently satisfies various properties such as strength, ductility, and hole expandability should be obtained.

E) 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-mentioned findings and the like. "The hot-rolled high-strength steel sheet C: 0.05 to 0.25%, Si: 0.05% or less, Mn: 0.8
~ 2.5%, sol.Al: 1.0-2.5% or more Ca: 0.0002-0.01%, Zr: 0.01-0.10%, rare earth element: 0.002-0.10%, Nb: 0.005-0.10%, Ti: 0.005
~ 0.10%, V: 0.005 ~ 0.20% of 1 type or more, the balance of which is Fe and unavoidable impurities, and is mainly composed of polygonal ferrite containing 5% or more by volume of retained austenite. It is characterized by having excellent ductility, hole expandability, and surface texture by having a structure, and further, "C: 0.05 to 0.25%, Si: 0.05% or less, Mn: 0.8
~ 2.5%, sol.Al: 1.0-2.5% or more Ca: 0.0002-0.01%, Zr: 0.01-0.10%, rare earth element: 0.002-0.10%, Nb: 0.005-0.10%, Ti: 0.005
~ 0.10%, V: 0.005 ~ 0.20% of one or more, and the balance of the steel consisting of Fe and unavoidable impurities, the steel is heated and held at Ac ₃ points or more, and then hot rolled, and After finishing rolling at 780 to 840 ° C., 10
Accelerated cooling to 300-450 ° C at a cooling rate of -50 ° C / s and winding, or finish rolling at 780-940 ° C and 600-700 ° C at a cooling rate of 10 ° C / s or more.
Accelerated cooling to the temperature range of 2 to 10 seconds, and then 300 to 45 at a cooling rate of 20 ° C / s or more.
By accelerating cooling to 0 ° C. and winding it, the high ductility hot rolling excellent in ductility, hole expansibility, and surface property having the above-mentioned “structure mainly composed of polygonal ferrite containing 5% or more by volume of retained austenite” Another feature is that it enables stable production of high-strength steel sheets. "

Here, the term "polygonal ferrite-based structure" means that a structure other than retained austenite (for example, bainite) does not affect the properties of the polygonal ferrite to a significant extent. It goes without saying that it means a "structure consisting of null ferrite".

[0019]

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.
It is said to be an extremely effective element for leaving austenite, since it has the effect of assisting the concentration of untransformed austenite and delaying the precipitation of cementite. About 1% is contained.
However, this Si causes "deterioration in appearance" of the steel sheet surface called island scale, and is also an element that deteriorates GA processability (hot dip galvanizing processability), and also coarse bainite and hard bainite. The formation of martensite is promoted to easily cause deterioration of hole expansibility. And, since these disadvantages become remarkable when the Si content exceeds 0.05%, in the present invention, the Si content is 0.05% or more.
Limited to less than or equal to%.

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 action cannot be obtained, while
%, 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 effect of promoting the uniform and fine formation of polygonal ferrite and suppressing the formation of coarse bainite that deteriorates the hole expandability, and the addition of Al causes the formation of island scales. Since it does not exist, good steel plate surface properties can be obtained. However, if the Al content is less than 1.0% as sol.Al, the desired effect due to the above action cannot be obtained, while
Even if added in excess of 2.5%, the effect is saturated, and the formation of inclusions is promoted to deteriorate ductility and hole expandability. Therefore, in the present invention, the sol.Al content is set to 1.0 to 2.5%. It was

Ca, Zr, and rare earth elements All of these components have the effect of adjusting the shape of inclusions to improve 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 0.002%. Not obtained, while Ca
0.01%, Zr 0.10%, and rare earth elements 0.10%
If the content is exceeded, the amount of inclusions in the steel becomes too large, and conversely the workability deteriorates. Therefore, the Ca content is 0.
0002-0.01%, Zr content was 0.01-0.10%, and rare earth element content was 0.002-0.10%.

Nb, Ti, and V Nb, Ti, and V are all elements that are precipitated 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 workability, it is desirable to keep its content to 0.05% or less. However, 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 of the present invention having the composition as described above 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 elements 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 the 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.

Furthermore, 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 subjected to hot rolling after heating and holding a steel slab having the predetermined component composition at an Ac ₃ point or higher,
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.degree. C., and thereafter, the austenite is accelerated and cooled to a temperature range of 600 to 700.degree. C. for 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.

[0036]

The hot-rolled steel sheet according to the present invention has a C content of 0.05 to 0.25% and therefore has a weldability of a level required as a steel sheet for high strength members, and further utilizes TRIP. A sufficient amount of austenite. Moreover, since the Si content is 0.05% or less, it has an excellent surface quality without island scale. Therefore, according to the present invention, despite the good weldability, residual austenite is present and the tensile strength / elongation balance is good,
Moreover, it is possible to realize a hot-rolled steel sheet having excellent surface properties and hole expandability. When the steel sheet of the present invention is subjected to surface treatment such as hot dip galvanizing, galvannealing, electroplating, etc., a surface-treated steel sheet having further 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 3 was heated and hot-rolled under the conditions shown in Tables 4 to 5.
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 4 to 5.

[0038]

[Table 3]

[0039]

[Table 4]

[0040]

[Table 5]

As is clear from the results shown in Tables 4 to 5, the hot-rolled steel sheet produced according to the present invention has a high tensile strength of 60 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 100% or more, and it is understood that both high strength and high workability are satisfied at the same time. Furthermore, it can be confirmed that the hot-rolled steel sheet of the present invention does not form island scales and exhibits a good surface condition.

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,
The strength, elongation, and hole expansion ratio are all inferior. In the test number 46 using the steel type Q in which the C content is below the specified range of the present invention, the strength is significantly inferior. In Test Nos. 22 and 47 using the steel type R in which the Si content exceeds the specified range of the present invention, both the strength and the expandability of the stretch hole are relatively good, but the surface properties of the steel sheet are inferior. In the test number 23 and the trial number 48 using the steel type S in which the Si content and the sol.Al content are out of the specified ranges of the present invention, the hole expandability and the steel sheet surface property are inferior.

Test numbers 16, 39 and 40 for conditions in which the winding temperature is out of the range specified by the present invention, and test numbers in which the finishing temperature, cooling rate, air cooling temperature, air cooling time, etc. are outside the ranges specified by the present invention
In all of the steel sheets obtained with Nos. 18, 19, 41 to 44, retained austenite was not sufficiently generated, elongation was reduced, and hole expandability was not good.

[0043]

[Summary of Effects] As described above, according to the present invention, it is possible to stably provide a high-ductility high-strength hot-rolled steel sheet having excellent strength, ductility, hole expandability, weldability, and surface properties. 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.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazutoshi Kunishige 4-53-3 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd.

Claims (12)

[Claims] 1. A weight ratio of C: 0.05 to 0.25%, Si: 0.05% or less, Mn: 0.8
~ 2.5%, sol.Al: 1.0-2.5%, the balance is Fe and unavoidable impurities, and 5% by volume
A high-ductility hot-rolled high-tensile steel sheet having excellent surface properties, ductility, and hole expandability, characterized by having a structure mainly composed of polygonal ferrite containing the above retained austenite. 2. By weight ratio, C: 0.05 to 0.25%, Si: 0.05% or less, Mn: 0.8
~ 2.5%, sol.Al: 1.0 ~ 2.5%, Ca: 0.0002 ~ 0.01%, Zr: 0.01 ~ 0.10%, Rare earth element: 0.002 ~ 0.10%, and the balance is Fe. And unavoidable impurities and volume ratio of 5%
A high-ductility hot-rolled high-tensile steel sheet having excellent surface properties, ductility, and hole expandability, characterized by having a structure mainly composed of polygonal ferrite containing the above retained austenite. 3. By weight ratio, C: 0.05 to 0.25%, Si: 0.05% or less, Mn: 0.8
~ 2.5%, sol.Al: 1.0 to 2.5%, Nb: 0.005 to 0.10%, Ti: 0.005 to 0.10%, V:
It has a structure mainly composed of polygonal ferrite containing 0.005 to 0.20% of one or more and the balance of Fe and unavoidable impurities, and containing residual austenite in a volume ratio of 5% or more. A high-ductility hot-rolled high-strength steel sheet with excellent surface properties, ductility, and hole expandability. 4. By weight ratio, C: 0.05 to 0.25%, Si: 0.05% or less, Mn: 0.8
~ 2.5%, sol.Al: 1.0 to 2.5%, Ca: 0.0002 to 0.01%, Zr: 0.01 to 0.10%, rare earth element: 0.002 to 0.10%, and Nb: 0.005 to 0.10%, Ti: 0.005 to 0.10%, V:
It has a structure mainly composed of polygonal ferrite containing 0.005 to 0.20% of one or more and the balance of Fe and unavoidable impurities, and containing residual austenite in a volume ratio of 5% or more. A high-ductility hot-rolled high-strength steel sheet with excellent surface properties, ductility, and hole expandability. 5. A weight ratio of C: 0.05 to 0.25%, Si: 0.05% or less, Mn: 0.8
~ 2.5%, sol.Al: 1.0-2.5% and the balance of steel consisting of Fe and unavoidable impurities is heated and held at Ac ₃ points or more, then hot-rolled and finish-rolled at 780-840 ° C. After finishing, 30 at a cooling rate of 10 to 50 ° C / s
Highly excellent in surface properties, ductility, and hole expansibility having a structure mainly composed of polygonal ferrite containing 5% or more of retained austenite by volume, which is characterized by accelerated cooling to 0 to 450 ° C and winding. Manufacturing method of ductile hot rolled high strength steel sheet. 6. A weight ratio of C: 0.05 to 0.25%, Si: 0.05% or less, Mn: 0.8
~ 2.5%, sol.Al: 1.0 ~ 2.5%, Ca: 0.0002 ~ 0.01%, Zr: 0.01 ~ 0.10%, Rare earth element: 0.002 ~ 0.10%, and the balance Fe and Steel consisting of unavoidable impurities is heated and held at Ac ₃ or higher and then hot-rolled. After finishing rolling at 780 to 840 ° C, 30 at a cooling rate of 10 to 50 ° C / s.
Highly excellent in surface properties, ductility, and hole expansibility having a structure mainly composed of polygonal ferrite containing 5% or more of retained austenite by volume, which is characterized by accelerated cooling to 0 to 450 ° C and winding. Manufacturing method of ductile hot rolled high strength steel sheet. 7. A weight ratio of C: 0.05 to 0.25%, Si: 0.05% or less, Mn: 0.8
~ 2.5%, sol.Al: 1.0 to 2.5%, Nb: 0.005 to 0.10%, Ti: 0.005 to 0.10%, V:
Contains at least one of 0.005 to 0.20% and the balance Fe
And steel consisting of unavoidable impurities is heated and held at an Ac ₃ point or higher and then hot-rolled. After finishing rolling at 780 to 840 ° C., 300 to 300 ° C. at a cooling rate of 10 to 50 ° C./s.
Highly ductile heat with excellent surface properties, 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 accelerated cooling to 450 ° C and winding. Manufacturing method of rolled high strength steel sheet. 8. A weight ratio of C: 0.05 to 0.25%, Si: 0.05% or less, Mn: 0.8
~ 2.5%, sol.Al: 1.0 to 2.5%, Ca: 0.0002 to 0.01%, Zr: 0.01 to 0.10%, rare earth element: 0.002 to 0.10%, and Nb: 0.005 to 0.10%, Ti: 0.005 to 0.10%, V:
Contains at least one of 0.005 to 0.20% and the balance Fe
And steel consisting of unavoidable impurities is heated and held at an Ac ₃ point or higher and then hot-rolled. After finishing rolling at 780 to 840 ° C., 300 to 300 ° C. at a cooling rate of 10 to 50 ° C./s.
Highly ductile heat with excellent surface properties, 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 accelerated cooling to 450 ° C and winding. Manufacturing method of rolled high strength steel sheet. 9. By weight ratio, C: 0.05 to 0.25%, Si: 0.05% or less, Mn: 0.8
~ 2.5%, sol.Al: 1.0-2.5% and the balance of steel consisting of Fe and unavoidable impurities is heated and held at Ac ₃ points or more, then hot-rolled and finish-rolled at 780-940 ° C. After finishing, 600 at a cooling rate of 10 ° C / s or more
Accelerated cooling to a temperature range of ~ 700 ° C
Polygonal ferrite containing residual austenite in a volume ratio of 5% or more, characterized by being cooled by air for 10 seconds and then further accelerated and cooled to 300 to 450 ° C at a cooling rate of 20 ° C / s or more. A method for producing a high-ductility hot-rolled high-strength steel sheet having a main structure and excellent in surface properties, ductility, and hole expandability. 10. A weight ratio of C: 0.05 to 0.25%, Si: 0.05% or less, Mn: 0.8.
~ 2.5%, sol.Al: 1.0 ~ 2.5%, Ca: 0.0002 ~ 0.01%, Zr: 0.01 ~ 0.10%, Rare earth element: 0.002 ~ 0.10%, and the balance Fe and Steel consisting of unavoidable impurities is heated and held at Ac ₃ or higher and then hot-rolled, and finish rolling is completed at 780 to 940 ° C, and then 600 at a cooling rate of 10 ° C / s or more.
Accelerated cooling to a temperature range of ~ 700 ° C
Polygonal ferrite containing residual austenite in a volume ratio of 5% or more, characterized by being cooled by air for 10 seconds and then further accelerated and cooled to 300 to 450 ° C at a cooling rate of 20 ° C / s or more. A method for producing a high-ductility hot-rolled high-strength steel sheet having a main structure and excellent in surface properties, ductility, and hole expandability. 11. A weight ratio of C: 0.05 to 0.25%, Si: 0.05% or less, Mn: 0.8.
~ 2.5%, sol.Al: 1.0 to 2.5%, Nb: 0.005 to 0.10%, Ti: 0.005 to 0.10%, V:
Contains at least one of 0.005 to 0.20% and the balance Fe
And steel consisting of unavoidable impurities is heated and held at an Ac ₃ point or higher and then hot-rolled, and finish rolling is completed at 780 to 940 ° C, and then 600 to 7 at a cooling rate of 10 ° C / s or more.
Accelerated cooling to a temperature range of 00 ° C and 2-10 in that temperature range
30 seconds at a cooling rate of 20 ° C / s or more after air cooling for 2 seconds
Highly excellent in surface properties, ductility, and hole expansibility having a structure mainly composed of polygonal ferrite containing 5% or more of retained austenite by volume, which is characterized by accelerated cooling to 0 to 450 ° C and winding. Manufacturing method of ductile hot rolled high strength steel sheet. 12. A weight ratio of C: 0.05 to 0.25%, Si: 0.05% or less, Mn: 0.8.
~ 2.5%, sol.Al: 1.0 to 2.5%, Ca: 0.0002 to 0.01%, Zr: 0.01 to 0.10%, rare earth element: 0.002 to 0.10%, and Nb: 0.005 to 0.10%, Ti: 0.005 to 0.10%, V:
Contains at least one of 0.005 to 0.20% and the balance Fe
And steel consisting of unavoidable impurities is heated and held at an Ac ₃ point or higher and then hot-rolled, and finish rolling is completed at 780 to 940 ° C, and then 600 to 7 at a cooling rate of 10 ° C / s or more.
Accelerated cooling to a temperature range of 00 ° C and 2-10 in that temperature range
30 seconds at a cooling rate of 20 ° C / s or more after air cooling for 2 seconds
Highly excellent in surface properties, ductility, and hole expansibility having a structure mainly composed of polygonal ferrite containing 5% or more of retained austenite by volume, which is characterized by accelerated cooling to 0 to 450 ° C and winding. Manufacturing method of ductile hot rolled high strength steel sheet.