JPH07242947A - Production of high ductility hot rolled high tensile strength steel plate excellent in corrosion resistance - Google Patents

Abstract

PURPOSE:To produce a hot rolled high tensile strength steel plate provided with excellent corrosion resistance, ductility, bore expandability and weldability. CONSTITUTION:This is a method for producing a high ductility hot rolled high tensile strength steel plate excellent in corrosion resistance and bore expandability in which a steel having a compsn. contg. 0.05 to 0.25% C, <=2.5% Si, <=2.5% sol. Al, 0.8 to 2.5% Mn, 0.10 to 0.80% Cu, 0.020 to 0.15% P and 0.01 to 0.50% Ni and satisfying (So+Al) >=1.0 is treated in the following process to form its structure into a one essentially consisting of polygonal ferrite and contg. retained austenite by >=5% in a volume ratio. It is held under heating to 1000 to 1100 deg.C and is immediately subjected to hot rough rolling at >=50% total draft, and the rough rolling is finished at 880 to 940 deg.C. Successively, it is subjected to finish rolling at >=60% total draft, and the finish rolling is ended at 780 to 840 deg.C. Next, it is subjected to accelerated cooling to 300 to 450 deg.C at 10 to 50 deg.C/s cooling rate and is coiled. The same steel may be incorporated with specified amounts of one or more kinds among Ca, Zr, rare earth elements, Nb, Ti and V.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled high-tensile steel sheet for working, which is suitable as a steel sheet for high-strength members in automobiles, industrial machines, etc., which has excellent corrosion resistance, ductility and hole expandability, and also has weldability. It relates to a manufacturing method.

[0002]

2. Description of the Related Art Hot-rolled steel sheets produced by continuous hot rolling have been widely used as relatively inexpensive structural materials in automobiles and various industrial machines. Most of the members to be molded are often formed by pressing. Therefore, there is a strong demand for "combination of high strength and high workability".

Generally, when the strength of a steel sheet is increased, the workability is deteriorated, but as a steel sheet having both high strength and high workability, for example, a DP as disclosed in JP-A-55-44551.
Steel (Dual Phase steel) is known.

This DP steel is a [ferrite + martensite] dual-phase structure steel and has a "low yield ratio and high ductility".
It is characterized by that. However, the characteristics of this steel according to JIS No. 5 tensile test piece are 60 kg class high tensile strength [tensile strength (T
(S): 60kgf / mm ² ], elongation (EL) is about 30%, tensile strength / elongation balance (TS × EL) value is less than 2000, and 80kg class high tensile strength [TS: 80kgf / mm ² ] EL Is about 20%, and the value of (TS × EL) is
The limit is less than 1800, and DP steel cannot cope with higher ductility.

On the other hand, apart from this, as a means for significantly improving the ductility of high-strength steel sheets, "method of utilizing transformation-induced plasticity of retained austenite (hereinafter referred to as TRIP)"
Has been proposed (see, for example, JP-A-55-145121). According to this method, TS is 110 kgf / mm ² or more and EL is 22
%, And it is considered possible to manufacture high-ductile high-strength steel sheets with a (TS × EL) value of more than 2400. However, in this method, the C content is 0.40 to 0.85% by weight (hereinafter,
It is necessary to adjust the content of the steel sheet to a higher value (% representing the composition ratio is to be a weight%), so the resulting steel sheet is inferior in terms of weldability,
It is difficult to widely use it as a steel sheet for automobiles.

As a means to secure retained austenite and obtain high ductility of a steel sheet under a low C content, a steel containing about 0.2% C, about 1.5% Si, and about 1.5% Mn is hot-rolled. However, a method has been proposed in which finish rolling is performed in the vicinity of Ar ₃ point, accelerated cooling is performed at a cooling rate of 40 ° C./s or more, and then winding is performed in the vicinity of 400 ° C. (see Japanese Patent Laid-Open No. 63-4017). However, with this method, although the elongation can be greatly improved as compared with DP steel, since the crystal grain size is relatively coarse, hole expansibility, which is one of the important indicators of workability, is not improved, The hole expandability of the hot-rolled steel sheet obtained is only about 30% that of DP steel.

Therefore, the steel sheet produced by the above method cannot be used for parts requiring stretch-flangeability, and is a hot-rolled high-tensile steel sheet for working which is sufficiently satisfactory in hole expansibility. Appearance is strongly desired.

[0008] In recent years, with the tightening of regulations on fuel consumption of automobiles, thin steel sheets used for undercarriage parts of automobiles in order to promote weight reduction of vehicle bodies have tended to have high tension and thin wall. However, with the expansion of the use of road anti-icing materials, these steel sheets are required to have corrosion resistance at the same time, and the amount of rust-prevented steel sheets used is also increasing.

By the way, as a method of imparting corrosion resistance to a steel sheet, there is a method of plating a protective film on the surface of the steel sheet as represented by a zinc-based plated steel sheet. However, when arc-welding galvanized steel sheets, welding defects such as blowholes frequently occur in the lap fillet weld due to evaporation of zinc in the plating film, leading to reduced fatigue strength. And the problem that sufficient component strength cannot be obtained occurs.

Therefore, a small amount of elements such as Cu, P, and Cr are added to a steel plate base material represented by corrosion-resistant steel, which does not cause the problem of deterioration of weldability, to form a dense corrosion film (Japanese Patent Laid-Open No. 2000-242242). (See No. 54-9113). However, even in such a steel sheet, when the strength is further increased and the thickness is further reduced, there arises a problem that the corrosion resistance becomes insufficient, and further higher base metal corrosion resistance is desired.

[0011]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide welding properties in addition to corrosion resistance, ductility and hole expandability. It is to provide a method for manufacturing a hot-rolled high-tensile steel plate.

[0012]

The summary of the present invention is as follows.
The method for producing a high-ductility hot-rolled high-strength steel sheet according to (1) to (3).

(1) C: 0.05 to 0.25% by weight, Si: 2.
5% or less, sol.Al: 2.5% or less, Mn: 0.8 to 2.5%, Cu:
0.10-0.80%, P: 0.020-0.15% and Ni: 0.01-0.
Steel containing 50% and satisfying (Si + Al) ≥ 1.0%, the balance being Fe and unavoidable impurities, is processed by the following steps and conditions, and contains 5% or more by volume of retained austenite. A method for producing a high-ductility hot-rolled high-strength steel sheet excellent in corrosion resistance and hole expansibility, characterized by having a structure mainly composed of polygonal ferrite.

After heating and holding at 1000 to 1100 ° C., hot rough rolling with a total reduction of 50% or more is immediately performed, and 880 to 94
The rough rolling is completed at 0 ° C.

Subsequently, finish rolling with a total reduction of 60% or more is performed, and the finish rolling is completed at 780 to 840 ° C.

Then, at a cooling rate of 10 to 50 ° C./s, 300 to 45
Accelerated cooling to 0 ° C.

Then, it is wound up.

(2) In addition to the above component (1), in addition, one or two of Ca: 0.0002 to 0.010%, Zr: 0.01 to 0.10%, and rare earth element: 0.002 to 0.10% in weight%. Steel containing more than one kind is treated in the above step (1), and the volume ratio is 5%.
A method for producing a high-ductility hot-rolled high-strength steel sheet excellent in corrosion resistance and hole expansibility, characterized by having a structure mainly composed of polygonal ferrite containing retained austenite as described above.

(3) In addition to the above component (1) or (2), in addition, in% by weight, Nb: 0.005 to 0.10%, Ti: 0.005 to 0.
10% and V: 0.005 to 0.20% of steel containing one or more of them is treated in the above step (1) to obtain a volume ratio of 5
% Of retained austenite is used as a main structure of polygonal ferrite, and a method for producing a high-ductility hot-rolled high-strength steel sheet excellent in any of corrosion resistance and hole expandability.

The above-mentioned "structure mainly composed of polygonal ferrite" means that "a structure other than retained austenite (for example, bainite)" is "substantially composed of polygonal ferrite" to such an extent that the properties of the polygonal ferrite are not significantly affected. Means "organization".

In order to achieve the above object, the inventors of the present invention show a sufficiently high strength even for automobiles, etc., particularly in the range of C content where satisfactory weldability is obtained, and "excellent corrosion resistance and ductility. As a result of various investigations for feasibility of a hot-rolled high-strength steel sheet having an amount of austenite sufficient to utilize the TRIP effect leading to hole expandability, the following results were obtained.
The new findings shown in A) to C) were obtained.

A) Ferrite grains can be refined by low temperature heating and low temperature hot rolling.

In the conventional hot rolling process, since the slab heating temperature is high, the austenite grains are coarse, and the austenite grains after rough rolling performed at high temperature are also coarse. Therefore, the ferrite grains finally obtained by finish rolling could not be refined so much. The reason will be described in detail below.

The hot-rolled steel sheet is produced by hot rolling a continuously cast slab having a sheet thickness of 200 mm or more to a sheet thickness of about 3 mm in a hot strip mill line consisting of heating-rough rolling-finish rolling-winding steps. To be done. Of these, finish rolling is generally performed at a high speed in a tandem mill composed of 6 or 7 units. By the way, in finish rolling, heat removal mainly through a rolling roll is larger than heat generation due to heating, and the temperature of the steel sheet decreases. If the temperature of the steel sheet before the start of rolling is low, the finish temperature of finish rolling may be too low, or the deformation resistance of the steel sheet may be increased and the weight of rolling may be too large. Therefore, conventionally, large reduction finish rolling at low temperature was impossible.

As a result of various studies, the present inventors have found that by performing finish rolling while compensating the temperature of the steel sheet, it is possible to perform large reduction finish rolling at low temperature even for low alloy steel.

That is, the temperature of the steel sheet before finish rolling can be lowered by applying, for example, electric heating to the finish rolling machine to compensate the temperature of the steel sheet. Therefore, the slab heating temperature and the rough rolling temperature can be lowered. 1100 ° C
By heating the slab at a low temperature below, re-solution of carbonitride finely precipitated in the slab is suppressed, so that coarsening of austenite particles is suppressed and comparatively fine austenite particles are obtained.

Further, by carrying out rough rolling at a low temperature, processing-recrystallization of austenite is repeated and fine and uniform austenite grains are obtained at the end of rough rolling. Furthermore, by starting finish rolling at a low temperature, work hardening of austenite is promoted, substantially low temperature large reduction finish rolling is achieved, and very fine ferrite grains that could not be obtained by the conventional hot rolling process. can get.

B) The hole expandability can be improved by refining the ferrite crystal grains.

As a result of performing temperature compensation (for example, applying current heating) of the steel sheet during hot finish rolling and performing the above low temperature heating and low temperature hot rolling, extremely fine polygonal ferrite grains with a grain size number of 14 or more. And a retained austenite and bainite phase were uniformly dispersed. It was also found that, by obtaining such a metal structure, it is possible to significantly improve the hole expandability as well as the elongation.

C) By refining the ferrite crystal grains and containing Si and Al, the corrosion resistance is further improved by adding a trace amount of Cu, P and Ni.

The steel sheet to which trace amounts of Cu, P and Ni were added was subjected to the above-mentioned hot rolling, and the steel sheet obtained by refining the crystal grains was subjected to a wet-dry cycle test to investigate the maximum perforation depth. Then, it was found that the effect of improving the corrosion resistance by adding a small amount of Cu, P, and Ni is further increased by making the ferrite crystal grains fine and the inclusion of Si and Al.

It is to be noted that the ferrite crystal grains are refined on the steel sheet.
Although it is not clear why the addition of a small amount of Cu, P, and Ni further improves the corrosion resistance of the steel sheet by adjusting the conditions for the inclusion of Si and Al, the progress of pitting corrosion is suppressed by the refinement of ferrite crystal grains. It is considered that this is due to the fact that uniform corrosion progresses on the entire surface of the steel sheet and that the stability of the corrosion products is further increased by Si and Al which generate relatively stable oxides.

[0033]

Next, the reason why the chemical composition of the raw material steel, the structure of the steel sheet, and the manufacturing conditions thereof are limited as described above in the present invention will be described in detail together with the operation thereof. % Means% by weight.

A) Chemical composition of raw steel and microstructure of steel sheet C: 0.05 to 0.25% C stabilizes austenite in the untransformed austenite as the ferrite transformation progresses in the cooling process after hot rolling. Together, it is an element that contributes to strengthening the steel sheet. If the C content is less than 0.05%, the effects of securing strength and stabilizing austenite are not sufficient. On the other hand, 0.25
%, The weldability remarkably deteriorates, and the amount of polygonal ferrite decreases and the amount of bainite increases too much, so that the hole expandability deteriorates and the corrosion resistance also decreases. Therefore, C
The content range was set to 0.05 to 0.25%.

Si: 2.5% or less Si has the effect of promoting the formation of polygonal ferrite, helping C to concentrate in untransformed austenite, and delaying the precipitation of cementite. Therefore, it is an extremely effective element for remaining austenite, and is also an effective element for solid-solution strengthening the polygonal ferrite and increasing the strength of the steel sheet. Furthermore, it has the effect of further improving the corrosion resistance by Cu, P, and Ni together with the refinement of crystal grains. However, when the content exceeds 2.5%, these effects are saturated, and hard martensite is likely to be formed, resulting in deterioration of hole expansibility. Therefore, the Si content is 2.5%
The following was set. However, the content is controlled in relation to sol.Al as described later.

Sol.Al: 2.5% or less Al is a ferrite stabilizing element like Si, and promotes the formation of polygonal ferrite to promote the concentration of C in untransformed austenite and delays the precipitation of cementite. , Has the effect of promoting the retention of austenite.
Further, it has the effect of further improving the corrosion resistance of Cu, P and Ni together with the refinement of crystal grains. Moreover, the effect is more pronounced than the addition of the same weight% of Si. However, if the Al content is so
The effect is saturated when l.Al exceeds 2.5%, and
It promotes the formation of inclusions and deteriorates ductility and hole expandability. Therefore, the content of sol.Al was set to 2.5% or less.

Si + Al: 1.0% or more Si and Al have the function of promoting the retention of austenite as described above, and also have the effect of further improving the corrosion resistance of Cu, P and Ni together with the refinement of crystal grains. In order to obtain these effects, the total content of Si and Al must be 1.0% or more.

Therefore, the total content of Si + Al is set to 1.0% or more.

Mn: 0.8 to 2.5% Mn is an austenite stabilizing element, which lowers the Ms point of untransformed austenite and improves the hardenability, and suppresses the transformation of untransformed austenite into pearlite. However, this effect cannot be obtained when the Mn content is less than 0.8%. On the other hand, if it exceeds 2.5%, it becomes difficult to form a sufficient amount of polygonal ferrite in the cooling process after hot rolling, so that the concentration of C in the untransformed austenite becomes insufficient and the austenite is stabilized. I can't make it happen. Therefore, the range of Mn content is 0.8-
It was set at 2.5%.

Cu: 0.10 to 0.80% Cu is a main component for improving the corrosion resistance and has the effect of improving the corrosion resistance through the formation of a dense corrosion film. However, if the content is less than 0.10%, this effect cannot be obtained, while if it exceeds 0.80%, this effect is saturated and the economic efficiency is impaired. Therefore, the range of Cu content is 0.
It was set at 10 to 0.80%.

P: 0.020 to 0.15% P also has the effect of enhancing corrosion resistance through the formation of a dense corrosion film. However, if the content is less than 0.020%, this effect cannot be obtained, while if it exceeds 0.15%, the toughness and hole expandability deteriorate. Therefore, the range of P content is
It was set to 0.020 to 0.15%.

Ni: 0.01 to 0.50% Ni has the function of preventing the deterioration of hot workability associated with the addition of Cu, and the function of improving corrosion resistance. However, if the content is less than 0.01%, these effects cannot be obtained, while if it exceeds 0.50%, the economical efficiency is impaired. Therefore,
The range of Ni content was set to 0.01 to 0.50%.

The raw material steel to be subjected to the method of the present invention may contain at least one selected from the following element groups.

Ca, Zr, and rare earth elements: Each of these elements has the effect of adjusting the form of inclusions to improve cold workability, so at least one element is contained if necessary.

In the case of Ca, its content is less than 0.0002%,
If the content of Zr is less than 0.01% and the content of rare earth element is less than 0.002%, the above effect cannot be obtained. On the other hand, if the Ca content exceeds 0.010%, the Zr content exceeds 0.10%, and the rare earth element content exceeds 0.10%, conversely, the amount of inclusions in the steel becomes too large and the workability deteriorates. Therefore, the range of Ca content is 0.0002 to 0.010%, Zr
The content range is 0.01 to 0.10%, and the rare earth element content range is 0.01 to 0.10%.

Nb, Ti and V: Nb, Ti and V are all elements which are precipitated as carbonitrides on the ferrite material and effectively act to increase the strength of the steel sheet, and if necessary, one or more of them may be contained.

In any case, if the content is less than 0.005%, the desired effect cannot be obtained, while if Nb and Ti exceed 0.10% and V exceeds 0.20%, the effect is saturated. Because it is not economical. Therefore, the ranges of Nb and Ti contents were set to 0.005 to 0.10%, and the range of V content was set to 0.005 to 0.20%.

Further, it is possible to select one or more kinds from the group of Ca, Zr and rare earth elements and the group of Nb, Ti and V, and to carry out composite addition within the above content range.

The impurities that are inevitably mixed in the steel include S, Cr and Mo. For example, it is desirable to suppress the content of S as follows.

S: S is an impurity element which forms sulfide inclusions and deteriorates workability, so its content is 0.05.
It is desirable to keep the percentage below. However, it is more preferably 0.003% or less from the viewpoint of ensuring further excellent workability.

The raw steel having the above-mentioned chemical composition is, for example, a converter,
It is melted in an electric furnace or open furnace. The steel type may also be rimmed steel, capped steel, semi-killed steel or killed steel. The production of the raw steel billet may be performed by any means of "ingot-bulk rolling" or "continuous casting".

The reason why the structure of the steel sheet according to the method of the present invention is mainly composed of polygonal ferrite containing 5% or more of retained austenite by volume is as follows.

That is, the ductility is mainly secured by polygonal ferrite, but in order to further improve the ductility, the residual austenite needs to be 5% or more by volume.

B) Manufacturing Conditions for Hot Rolled Steel Sheet In the present invention, 10 raw steel slabs to be subjected to hot rolling are used.
After heating and holding at 00 ~ 1100 degrees, the total reduction rate is immediately 50
% Hot rolling at a temperature of 880 to 940 ℃, then finish rolling at a total reduction of 60% or more, finish rolling at 780 to 840 ℃, and finish rolling at 10 to 10 50 ° C / s
By accelerating cooling to 300-450 ℃ at the cooling rate and winding up, 5% or more of austenite by volume ratio remains, and the fine polygonal ferrite-based main component is sufficient to improve ductility and hole expansibility. It is possible to get an organization.

By heating the raw steel slab to 1000 to 1100 ° C., the alloying elements can be solid-solved in austenite, and heating at low temperature can prevent coarsening of austenite grains. If the heating temperature is lower than 1000 ° C., the above lower limit temperature during rough rolling cannot be satisfied due to the temperature decrease during rough rolling. On the other hand, when the temperature exceeds 1100 ° C, the austenite grains become coarse, and the polygonal ferrite grains generated from the austenite grains after finish rolling cannot be sufficiently refined.

Immediately after the above heating, hot rough rolling with a total rolling reduction of 50% or more is performed, and by rolling at 880 to 940 ° C., uniform and fine austenite grains can be obtained. If the total rolling reduction is less than 50% or the rough rolling end temperature exceeds 940 ° C, the polygonal ferrite grains generated from the austenite grains after finish rolling cannot be sufficiently refined. On the other hand, if the rough rolling finish temperature is lower than 880 ° C, the deformation resistance of the steel sheet increases due to the temperature decrease during finish rolling, and the rolling load becomes too large.Therefore, the lower limit of the total reduction rate during finish rolling in the next process is set. Not only can we not meet,
The lower limit of the finish rolling finish temperature cannot be satisfied.

After completion of the rough rolling, the total rolling reduction is
By finishing rolling at 60% or more, and finishing rolling at 780 to 840 ° C, the austenite is refined and the austenite is work hardened to promote the formation of polygonal ferrite. Therefore, after finish rolling, 10 to 50 ℃
A sufficient amount of polygonal ferrite can be generated during accelerated cooling at a cooling rate of / s.

At this time, it is desirable to perform temperature compensation of the steel sheet in order to secure the above-mentioned total rolling reduction and finishing temperature. Any means may be used, but for example, a direct electric heating method as disclosed in JP-A-4-356314 is suitable.

The total rolling reduction is less than 60% or the finishing temperature is
If the temperature exceeds 840 ° C, the work hardening of austenite becomes insufficient and the polygonal ferrite grains do not become sufficiently fine,
Hole expandability deteriorates, and corrosion resistance also decreases. Finishing temperature 780
If the temperature is lower than 0 ° C., ferrite is generated during hot rolling and becomes work ferrite, which deteriorates workability.

When the cooling rate after finish rolling is less than 10 ° C./s,
Perlite is formed during cooling and no austenite remains. On the other hand, when the cooling rate exceeds 50 ° C./s, a sufficient amount of ferrite is not formed and austenite does not remain.

In the above accelerated cooling, the temperature of the steel sheet is 300 to 45.
It is necessary to carry out the treatment until it reaches 0 ° C and wind it in this temperature range. When the winding temperature exceeds 450 ℃, pearlite is generated,
Not enough austenite remains. On the other hand, when the coiling is carried out in a temperature range below 300 ° C, the formation of martensite is promoted, and the ductility and hole expansibility are deteriorated.

The hot-rolled steel sheet according to the present invention described above has a C content in the range of 0.05 to 0.25%, and therefore has the weldability of the level required for a steel sheet for high strength members.
Have a sufficient amount of residual austenite to utilize.
Moreover, by making the crystal grain size finer, the high ductility steel sheet has improved hole expandability, which was a weak point of the conventional retained austenite steel sheet. Further, it is a steel sheet excellent in corrosion resistance without impairing weldability.

Therefore, according to the present invention, the weldability is good, the tensile strength and elongation balance (TS × EL) are good by forming retained austenite, and the heat expandability and corrosion resistance are excellent. A rolled steel plate can be obtained.

When the steel sheet produced by the method of the present invention is subjected to surface treatment such as hot dip galvanizing, hot dip galvanizing, and electroplating, in addition to excellent ductility and hole expansibility, It is possible to obtain a surface-treated steel sheet having excellent corrosion resistance.

[0065]

Example A raw steel slab having the chemical composition shown in Tables 1 and 2 was rolled under the conditions shown in Tables 3 and 4 to produce a steel plate having a thickness of 2.3 mm. Steel types A to T are steels satisfying the composition defined in the present invention, and steels U to Z, α and β are comparative example steels.

From the obtained steel sheet, JIS No. 5 tensile test pieces were sampled and their mechanical properties were investigated. Also, thickness 2.3 mm x width
A φ14 mm hole was punched out from a 120 mm × 120 mm long steel plate test piece with a clearance of 15%, and the hole expandability 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.

Regarding corrosion resistance, from the steel plate, width 30 mm x length
A 70 mm test piece was cut out, and "wet keeping (humidity: 95% or more, temperature: holding at room temperature for 16 hours) → salt spray (5% saline, holding at 35 ° C x 6 hours) → drying (holding at 50 ° C x 2 hours) ) "
The corrosion test was carried out with 1 cycle as a cycle, and the change in the maximum corrosion depth was investigated. The results are also shown in Tables 3 and 4.

As is clear from Tables 3 and 4, TS was 70 kgf / mm ² in the hot-rolled steel sheet manufactured under the conditions specified in the present invention.
While achieving the above high strength and high ductility with EL of 30% or more, it exhibits excellent punching workability with a hole expansion ratio of 80% or more. Furthermore, it has excellent corrosion resistance compared to conventional corrosion resistant steel. Therefore, it is understood that high corrosion resistance, high strength and high workability are satisfied at the same time.

On the other hand, in the test No. 28 using the steel sheet of the steel type U whose C content exceeds the range defined in the present invention, although the elongation is relatively high, the hole expandability and the corrosion resistance are deteriorated.
In Test No. 30 using the steel sheet of the steel type W having the C content below the range of the present invention, the strength is significantly reduced.

In Test No. 29 using a steel sheet of steel type V in which the total content of Si and sol.Al is less than the range defined in the present invention, strength, elongation, hole expansibility and corrosion resistance are all deteriorated.

In the test No. 31 using the steel sheet according to the steel type X whose Mn content is below the range defined by the present invention, both strength and elongation are lowered.

In Test No. 32 using the steel sheet of the steel type Y having the Si content exceeding the range specified in the present invention, the hole expansibility is deteriorated.

In the test No. 33 and the test No. 34 using the steel sheet of the steel grade Z or the steel grade α, respectively, in which the P or Cu content is less than the range defined by the present invention, the corrosion resistance is lowered.

In Test No. 35 using the steel sheet of the steel type β in which the P content exceeds the range defined in the present invention, the hole expansibility is deteriorated.

In Test No. 22 in which the finishing rolling finishing temperature exceeds the range defined by the present invention, the polygonal ferrite grains were not sufficiently refined, and the hole expandability and the corrosion resistance were deteriorated.

In Test No. 21, which was manufactured under the condition that the coiling temperature exceeded the range defined by the present invention, retained austenite was not sufficiently generated and the elongation was reduced. In Test No. 23, which was manufactured under the condition that the winding temperature was lower than the range defined by the present invention, retained austenite was not sufficiently generated, elongation was reduced, and hole expandability was not good.

In Test No. 24, which was manufactured under conditions in which the finish rolling finish temperature, the cooling rate, etc. were out of the ranges defined in the present invention, retained austenite was not sufficiently generated and the elongation was reduced.

Total reduction of hot rough rolling, its end temperature,
In the test Nos. 25, 26 and 27, which were manufactured under the conditions in which the total rolling reduction of finish rolling was out of the range defined in the present invention, the polygonal ferrite grains were not sufficiently refined, and the hole expandability and corrosion resistance were not sufficient. It is falling.

[0079]

[Table 1]

[0080]

[Table 2]

[0081]

[Table 3]

[0082]

[Table 4]

[0083]

According to the present invention, corrosion resistance, strength, ductility,
A highly workable hot-rolled high-strength steel sheet having excellent hole expandability and weldability can be stably manufactured. When this steel sheet is applied to industrial equipment members such as undercarriage parts of automobiles, it is possible to further improve the performance and life of those products, which brings about extremely useful effects in industry.

Continuation of front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display area C22C 38/16

Claims (3)

[Claims] 1. C: 0.05 to 0.25%, Si: 2.5% by weight
Below, sol.Al: 2.5% or less, Mn: 0.8 to 2.5%, Cu: 0.10.
~ 0.80%, P: 0.020 ~ 0.15% and Ni: 0.01 ~ 0.50%
And satisfies (Si + Al) ≥ 1.0% with the balance being Fe
And steel containing unavoidable impurities are heated and held at 1000 to 1100 ° C, and then hot rough rolling with a total reduction of 50% or more is immediately performed, and after rough rolling at 880 to 940 ° C is completed, the total rolling is continued. Finish rolling with a rolling reduction of 60% or more, 780 ~ 84
Finish rolling at 0 ℃, 3 at the cooling rate of 10 ~ 50 ℃ / s
After accelerating cooling to 00-450 ℃, wind it up, 5% by volume
A method for producing a high-ductility hot-rolled high-strength steel sheet excellent in corrosion resistance and hole expansibility, characterized by having a structure mainly composed of polygonal ferrite containing retained austenite as described above. 2. C: 0.05 to 0.25%, Si: 2.5% by weight
Below, sol.Al: 2.5% or less, Mn: 0.8 to 2.5%, Cu: 0.10.
~ 0.80%, P: 0.020 ~ 0.15% and Ni: 0.01 ~ 0.50%
And (Si + Al) ≥ 1.0%, and Ca:
Steel containing at least one of 0.0002 to 0.0100%, Zr: 0.01 to 0.10%, and rare earth element: 0.002 to 0.10%, the balance being Fe and inevitable impurities, 1000
Immediately after heating and holding at ~ 1100 ℃, the total rolling reduction is 50%.
After performing the above hot rough rolling and finishing the rough rolling at 880 ~ 940 ℃, finish rolling with a total reduction of 60% or more, finish the finish rolling at 780 ~ 840 ℃, 10 ~ 50 ℃ After accelerated cooling to 300-450 ℃ at a cooling rate of / s, winding,
A method for producing a high-ductility hot-rolled high-strength steel sheet having excellent corrosion resistance and hole expandability, which has a structure mainly composed of polygonal ferrite containing 5% or more by volume of retained austenite. 3. In addition to the component of claim 1 or 2, further
Nb: 0.005-0.10%, Ti: 0.005-0.10% and V:
Steel containing one or more of 0.005 to 0.20% is heated and held at 1000 to 1100 ° C, and then immediately hot rough rolled with a total reduction of 50% or more at 880 to 940 ° C. After finishing the rough rolling, finish rolling with a total rolling reduction of 60% or more is continued, finish rolling at 780 to 840 ℃, and finish rolling to 10 to 50
One of the corrosion resistance characteristics is characterized by accelerating cooling to 300 to 450 ° C at a cooling rate of ℃ / s, and then winding it to obtain a structure mainly composed of polygonal ferrite containing 5% or more of retained austenite by volume. A method for producing a high-ductility hot-rolled high-strength steel sheet having excellent hole expandability.