JP2003193194A - High strength steel sheet having excellent weldability and hole expansibility and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength, high ductility steel sheet in which the weldablity and hole expansibility of a high strength steel sheet having a tensile strength of ≥800 MPa are simultaneously improved, and to provide a production method therefor. <P>SOLUTION: The high strength steel sheet having excellent weldability and hole expansibility has a composition containing, by mass, 0.01 to 0.20% C, 0.01 to 2.5% Si, 0.01 to 3% Mn, 0.0010 to 0.1% P, 0.0010 to 0.05% S,, and 0.005 to 2% Al, and further containing one or two kinds selected from 0.01 to 5.0% Mo and 0.001 to 1.0% Nb in the ranges so as to saatisfy the following inequality (A), and the balance Fe with inevitable impurities, and has a microstructure containing bainite or bainitic ferrite by ≥70% in an area ratio, and has a tensile strength of ≥800 MPa, and the production method uses the same high strength steel sheet: (3.0Nb+2.5Mo+1/10Si+Mn)-(2C<SP>0.5</SP>+2)>0 (A). <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to building materials, home electric appliances,
The present invention relates to a high-strength steel sheet suitable for automobiles and the like, which has excellent weldability and hole expandability, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for high-strength steel sheets with good workability, especially for the purpose of improving fuel efficiency and durability in automobile bodies. In addition, the tensile strength is 800 MPa because of the needs of collision safety and expansion of cabin space.
Steel sheets of higher grade or higher are being used in parts such as reinforcements. When assembling members using such high-strength materials, ductility, bendability, hole expandability, weldability, etc. are more serious problems than high-strength steel sheets with a tensile strength of up to about 590 MPa. Is required. The following measures are taken for each characteristic.

For example, regarding hole expandability, CAMP-ISI
As described in J vol.13 (2000) p.395, bainite is used as the main phase to improve the hole expansibility, and as for the overhang formability, the current phase is formed by generating retained austenite in the second phase. It is disclosed that it exhibits the same bulging property as that of retained austenitic steel. Furthermore, it is also shown that when austenite treatment with Ms temperature or less is performed to generate retained austenite with a volume ratio of 2 to 3%, the tensile strength × hole expansion ratio becomes maximum. However, 800 MPa
The weldability and the softening behavior in the heat-affected zone, which are actualized beyond the range, are not considered. Regarding weldability, softening behavior in HAZ (HAZ softening behavior)
Is often regarded as a problem. On the other hand, for example, as disclosed in JP-A-2000-87175, Nb and Mo carbides (Nb,
It has been shown that the precipitation of Mo) C suppresses the HAZ softening behavior. However, although this technique considers fatigue strength, it does not sufficiently consider workability such as hole expandability. Further, the effect of suppressing the HAZ softening behavior is also low in strength level, and it cannot be said that the weldability and workability of an extremely high strength material of 800 MPa or more are sufficient. In particular,
When the tensile strength is 800 MPa or more, welding itself becomes difficult, and it becomes more remarkable at 980 MPa or more. Therefore, in some cases, in addition to conventional welding methods such as spot welding, laser welding and the like are partially applied. However, because of the high strength, the base material has a significantly more remarkable material variation in the welded portion and the heat affected zone than the high strength material of 590 MPa class or higher.

In order to increase the ductility of high strength materials,
It is common to actively utilize complex organization. However, when martensite or retained austenite is used for the second phase, there is a problem that the hole expandability is significantly reduced (for example, CAMP-ISIJ, vol.13 (2000), p.3).
91). Further, this document discloses that the main phase is ferrite and the second phase is martensite, and the hole expansion ratio is improved by reducing the hardness difference between the two, but the hole expansion ratio is 70%. If it is less than%, it is not a significant improvement.

[0005]

SUMMARY OF THE INVENTION The present invention solves the problems of the prior art as described above, and has a tensile strength of 800 MPa.
An object of the present invention is to provide a high-strength steel sheet in which the weldability and hole expandability of the above high-strength steel sheet are simultaneously improved, and a manufacturing method thereof.

[0006]

Means for Solving the Problems As a result of various studies, the present inventors have found that the tensile strength is 800 MPa or more.
As a method of simultaneously improving weldability and hole expandability, by limiting the microstructure and the composition range and formula (A), softening behavior of the weld heat affected zone is suppressed while maintaining high strength of 800 MPa or more. Furthermore, it was found that the hole expansion ratio: (hole inner diameter before hole expansion test / hole diameter before hole expansion test-1) × 100 can secure a hole expandability of 70% or more. The present invention has been completed based on the above findings, and the gist thereof is as follows.

(1)% by mass, C: 0.01 to 0.2
0%, Si: 0.01 to 2.5%, Mn: 0.01 to 3
%, P: 0.0010 to 0.1%, S: 0.0010
0.05%, Al: 0.005 to 2%, Mo: 0.01 to 5.0%, Nb: 0.01 to 1.
1% or 2% of 0% is contained in a range satisfying the following formula (A), the balance is Fe and inevitable impurities, and the microstructure contains 70% or more in area ratio as bainite or bainitic ferrite. A high-strength steel plate with excellent weldability and hole expandability, which has a tensile strength of 800 MPa or more. (3.0Nb + 2.5Mo + 1 / 10Si + Mn) - (2C 0.5 +2)> 0 ··· (A) (2) In addition, in mass%, Cr: 0.01~5%, N
i: 0.01 to 5%, Cu: 0.01 to 5%, Co:
0.01 to 5%, W: 0.01 to 5% of 1 type or 2
A high-strength steel sheet excellent in weldability and hole expandability according to (1), which contains at least one kind.

(3) Further, in% by mass, Zr, Hf, Ta, T
0.001% to 1% or more of i and V in total
A high-strength steel sheet having excellent weldability and hole expandability according to (1) or (2), which is characterized by containing. (4) Further, in mass%, B: 0.0001 to 0.1%
A high-strength steel sheet having excellent weldability and hole expandability according to any one of (1) to (3), which contains

(5) Further, in% by mass, Ca, Y, Rem
1 or 2 or more of 0.001 to 0.5% in total is contained, The high-strength steel sheet excellent in weldability and hole expandability according to (1) to (4). (6) A casting slab composed of the components described in (1) to (5) is directly or once cooled and then reheated. After hot rolling, the rolled hot rolled steel sheet is pickled, cold rolled, and then annealed. maximum temperature _{0.8 × (Ac 3 -Ac 1)} + Ac 1 (℃) or A
After annealing at c ₃ +30 (° C) or less, 1 to 150 ° C /
A method for producing a high-strength steel sheet excellent in weldability and hole expansibility, characterized by cooling to a temperature range of 200 to 500 ° C. at a cooling rate of 2 seconds and subsequently maintaining the temperature range for 1 to 3000 seconds.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below.
The inventors have found that, in mass%, C: 0.01 to 0.2%, S
i: 0.01 to 2.5%, Mn: 0.01 to 3%, P:
0.0010 to 0.1%, S: 0.0010 to 0.05
%, Al: 0.005 to 2%, based on a steel sheet consisting of the balance Fe and unavoidable impurities, added with each alloy, melted, and heated again as cast or once cooled,
The hot rolled steel sheet rolled after hot rolling was pickled, cold rolled, and then annealed to prepare a cold rolled annealed sheet. For the steel sheet, microstructure observation, hole expansion test prescribed by the Iron and Steel Federation, tensile test according to JIS, laser welding with steel sheets,
After that, a ball head overhanging test was performed to compare and evaluate each characteristic.

As a result, it was found that by controlling the finally obtained microstructure, it is possible to obtain a high-strength steel sheet having a tensile strength of 800 MPa or more and excellent weldability and hole expandability. Next, a preferable microstructure of the base steel sheet will be described. In order to secure sufficient hole expandability, it is effective to use bainite or bainitic ferrite, and the area ratio is determined to be 70% or more. The bainite referred to here includes both upper bainite in which carbide is formed at the lath boundary and lower bainite in which fine carbide is formed in lath. In addition, bainitic ferrite means bainite without carbide, and for example, acicular ferrite is one example. In order to improve the hole expandability, lower bainite in which carbide is finely dispersed or bainitic ferrite without carbide is the main phase, and the area ratio is 97.
% Is desirable.

On the other hand, prevention of softening in the heat affected zone of welding becomes a problem. On the other hand, the weldability of a high-strength material having a tensile strength of 800 MPa or more is ensured by satisfying the formula (A) defining the components as described later. From the viewpoint of ensuring ductility and increasing strength, ferrite may be included in an area ratio of less than 30%. On the other hand, inclusion of austenite and / or martensite is not desirable from the viewpoint of hole expanding workability and softening behavior of the weld heat affected zone, but if the area ratio is less than 3%, no remarkable deterioration of properties is observed. Therefore, the area ratio may be less than 3%. Further, inclusions such as oxides and sulfides may be inevitably included.

When the formula (A) is not satisfied, the tensile strength cannot be secured at 800 MPa or more, the softening of the weld heat affected zone cannot be suppressed, and the hole expandability becomes difficult to secure. (3.0Nb + 2.5Mo + 1 / 10Si + Mn) - (2C 0.5 +2)> 0 ··· (A) addition, as the remaining structure of the microstructure in addition to the above, carbides, nitrides, sulfides, oxide Area ratio 1 for 1 or 2 or more objects
Even when the content is less than or equal to%, it can be used in the present invention, and these are included in the area ratio of the main phase. The phases of the above microstructure, ferrite (bainitic ferrite), bainite, austenite, martensite, the interfacial oxide phase and the remaining structure are identified, the existing positions are observed, and the area ratio is measured by using Nital reagent and JP 59-21947
Corrosion of a steel plate rolling direction cross section or a rolling right-angle direction cross section by the reagent disclosed in Japanese Patent Publication No. 3) and quantification by observation with an optical microscope at a magnification of 500 to 1000 times and an electron microscope at a magnification of 1000 to 100000 times (scanning and transmission types) It is possible. It is possible to obtain the area ratio of each tissue by observing each of 20 or more visual fields and by the point counting method or image analysis.

Next, the reasons for limiting the preferred ranges of the steel sheet components in the present invention will be described. C is an element added for the purpose of controlling the ratios of the main phase and the second phase for ensuring a good strength-hole expandability balance. It also affects the fine homogenization of the substrate. In order to secure the strength and the area ratio of each second phase, the lower limit was set to 0.001% by mass (hereinafter, the same), and the upper limit capable of maintaining weldability and hole expandability was set to 0.20%. Preferably, C: 0.03 to 0.
When it is 10%, a good balance between strength and hole expandability can be obtained. Si is an element added for the purpose of suppressing the formation of a relatively coarse carbide that deteriorates the strength-ductility balance, and its lower limit is 0.01% by mass. Further, since excessive addition adversely affects weldability and the upper limit is 2.
It was set to 5% by mass. Preferably Si: 0.05 to 0.2
By setting it as%, a more remarkable effect can be obtained.

Mn is added for the purpose of increasing the strength. It is also effective in suppressing ferrite transformation and converting the main phase into bainite or bainitic ferrite. Further, it is added for the purpose of suppressing carbide precipitation, which is one of the causes of strength reduction and deterioration of hole expandability, and pearlite formation.
From these things, it was 0.01 mass% or more. on the other hand,
Excessive addition promotes martensite formation and causes a remarkable decrease in ductility, so the upper limit was 3% by mass. Preferably, by setting Mn: 0.5 to 3.0%, a good balance between strength and hole expandability can be obtained. P is a strengthening element. Further, lowering P improves hole expandability, but extremely lowering is economically disadvantageous, so the lower limit was made 0.0010 mass%. Further, addition of a large amount adversely affects weldability and manufacturability during casting and hot rolling, so 0.1% was made the upper limit.

As for S, lowering S is effective for improving hole expandability. On the other hand, since extremely low cost is economically disadvantageous,
The lower limit of 0010 mass% and the upper limit of 0.05 mass% are that addition of an amount exceeding this adversely affects weldability and manufacturability during casting and hot rolling. A
l is added as a deoxidizing element. Therefore, 0.005
The addition amount was at least mass%. On the other hand, excessive addition impairs weldability and plating wettability, so the upper limit was made 2%.

Mo suppresses the formation of carbides and pearlite that deteriorate the strength-hole expandability balance. In addition, it is effective in suppressing ferrite transformation and making the main phase bainite or bainitic ferrite, and is an important additive element for obtaining a very good balance of good strength-hole expandability-weldability. Therefore, the lower limit is 0.01
It was defined as mass%. Further, excessive addition causes ductility deterioration, so the upper limit was made 5.0%. Nb forms fine carbides, nitrides or carbonitrides and is extremely effective in strengthening steel sheets. It also delays ferrite transformation and promotes the formation of bainite and bainitic ferrite.
Furthermore, since it is also effective in suppressing softening of the heat affected zone of welding, the addition amount is set to 0.01% by mass or more. On the other hand, excessive addition deteriorates ductility and hot workability, so the upper limit was made 1.0 mass%.

Further, in order to control the weldability and the hole expandability in a well-balanced manner at a strength level of 800 MPa or higher, the formula (A) is satisfied. (3.0Nb + 2.5Mo + 1 / 10Si + Mn) - (2C 0.5 +2)> 0 ··· (A) addition, the steel to which the present invention is directed is the purpose of further improving the strength, Cr, Ni , Cu, Co, W may be contained alone or in combination. Cr is an element added for the purpose of strengthening and suppressing the formation of carbides and the purpose of forming bainite and bainitic ferrite, and is 0.01% or more, and addition of an amount exceeding 5% adversely affects workability. This was set as the upper limit. Ni is 0.01% by mass or more for the purpose of strengthening by improving the hardenability, and if added in an amount of more than 5% by mass, workability, in particular, the hardness increase of martensite contributes adversely and exerts an adverse effect. And

Cu is added in an amount of 0.01% by mass or more for the purpose of strengthening, and if it is added in an amount exceeding 5% by mass, workability and manufacturability are adversely affected. Since Co has a good balance of strength-hole expansibility by controlling bainite transformation, Co.
The amount added was 01% by mass or more. On the other hand, the upper limit of addition is not particularly set, but it is desirable to set it to 5% by mass or less because it is an expensive element and addition in a large amount impairs economic efficiency. W
Indicates that the strengthening effect appears at 0.01% by mass or more, and the upper limit of 5% by mass is that addition of an amount exceeding this adversely affects the workability.

Further, the steel targeted by the present invention is Z which is a strong carbide forming element for the purpose of further improving the strength.
It may contain one or more of r, Hf, Ta, Ti and V. These elements form fine carbides, nitrides or carbonitrides and are extremely effective in strengthening the steel sheet,
If necessary, one kind or two or more kinds are added in a total amount of 0.001% by mass or more. On the other hand, since ductility and hot workability are deteriorated, the upper limit of the total addition amount of one or more kinds is set to 1% by mass. B can also be added if desired. B is effective in strengthening grain boundaries and strengthening steel materials when added in an amount of 0.0001% by mass or more, but when the addition amount exceeds 0.1% by mass, the effect is not only saturated, but also processed. Therefore, the upper limit was set to 0.1% by mass because of the decrease in the property.

When Ca, Y and Rem are added in appropriate amounts, they contribute to the control of inclusions, particularly to fine dispersion, so 0.001
% Or more, while excessive addition lowers manufacturability such as castability and hot workability and ductility of steel sheet products, so 0.5
Mass% was made the upper limit. As the inevitable impurities, for example, N
, Sn, etc., but the effect of the present invention is not impaired even if these elements are contained in the range of 0.02 mass% or less.
A method for manufacturing a high-strength steel sheet having such a structure and excellent in weldability and hole expandability will be described below.

When the steel sheet of the present invention is manufactured by hot rolling and cold rolling / annealing, the slab adjusted to have predetermined components is directly or once cooled and then reheated to perform hot rolling. The reheating temperature at this time is preferably 1100 ° C. or higher and 1300 ° C. or lower. When the reheating temperature becomes high, coarse grains and thick oxide scale are formed, while at low temperature heating, rolling resistance becomes high. Further, after hot rolling, if the surface scale is removed by a high pressure descaling device or pickling, the surface of the product can be cleaned well, which is advantageous when plating is applied. After that, the product is finished by cold rolling and annealing. In addition, electroplating, hot dip galvanizing, and hot dip galvanizing do not hinder the present invention.
The hot rolling completion temperature is determined by the chemical composition of the steel, Ar.
Generally, it is performed at a temperature of ₃ transformation temperature or higher, but if the temperature is from Ar ₃ to a low temperature of about 10 ° C., the properties of the final steel sheet will not be deteriorated. In addition, the coiling temperature after cooling is set to the bainite transformation start temperature or higher determined by the chemical composition of steel,
It is possible to avoid increasing the load during cold rolling more than necessary,
This is not limited to the case where the total reduction ratio of cold rolling is small, and the properties of the final steel sheet are not deteriorated even when wound at a temperature below the bainite transformation temperature of steel. The total reduction ratio of cold rolling is set from the relationship between the final plate thickness and the cold rolling load, but if it is 40% or more, it is sufficient for recrystallization and does not deteriorate the properties of the final steel plate.

In annealing after cold rolling, the annealing temperature is expressed by the temperatures Ac1 and Ac3 which are determined by the chemical composition of the steel (for example, "Steel Material Science": W. C. Leslie, translated by Shigeyasu Koda, Maruzen P273). 0.8 x (Ac ₃
If it is less than −Ac ₁ ) + Ac ₁ (° C.), the amount of austenite obtained at the annealing temperature is small, so that bainite or bainitic ferrite cannot be mainly produced in the final steel sheet. In addition, the higher the annealing temperature is, the larger the grain size and surface oxidation are promoted, and the upper limit of the annealing temperature is set to Ac in order to increase the manufacturing cost.
_{It was set to 3} + 30 (° C). The annealing time in this temperature range needs to be 10 seconds or more to make the temperature of the steel plate uniform and to secure austenite. However, if it exceeds 30 minutes, the generation of the grain boundary oxidized phase is promoted and the cost is increased. The subsequent primary cooling is important for producing bainite or bainitic ferrite while suppressing the transformation from the austenite phase to the ferrite phase to some extent. This cooling rate is 1 ℃
If it is less than / sec, there is a concern that the generation of ferrite or pearlite may be promoted and the strength may be reduced, so the lower limit of the cooling rate was set to 1 ° C / sec. On the other hand, the cooling rate is 150 ℃
If it exceeds / sec, the amount of hard phase such as martensite phase in the final steel sheet will increase and it will be difficult in operation.

If this primary cooling is performed to less than 200 ° C., a large amount of martensite is generated during cooling, which promotes hole expandability and delayed fracture, so the cooling stop temperature is 200 to
It was set to 500 ° C. Further, when the cooling stop temperature exceeds 500 ° C., carbide is generated in a short time during the subsequent holding, which causes a decrease in strength, so this was made the upper limit. Further, in order to promote the progress of bainite transformation, the temperature is maintained in this temperature range. If the dwell time is long, it is not preferable in terms of productivity and carbides are generated, so 300
It is desirable to set it within 0 seconds. Further, in order to allow the bainite transformation to proceed, it is desirable to hold for 1 second or more, preferably 15 seconds to 20 minutes. If it is less than 200 ° C, bainite transformation is unlikely to occur, and if it exceeds 500 ° C, carbides are generated and it becomes difficult to leave a sufficient retained austenite phase. Regarding the welding method, it is within the scope of the present application even if a commonly used welding method such as arc, TIG, MIG, mash and laser welding is performed.

[0025]

EXAMPLES The present invention will be described in more detail below with reference to examples.
explain. A steel plate having a composition as shown in Table 1 is 1200 ° C.
Heated to Ar₃ After hot rolling is completed at the transformation temperature or higher and after cooling
Above the bainite transformation start temperature determined by the chemical composition of each steel
After pickling, the rolled steel strip is cold rolled to a thickness of 1.2 mm.
It was After that, the following formula was calculated from the composition (mass%) of each steel.
What Ac₁ And Ac₃ The transformation temperature was calculated. Ac₁ = 723-10.7 × Mn% + 29.1 × Si
%, Ac₃ = 910-203 × (C%)^1/2-15.2 x N
i% + 44.7 × Si% + 104 × V% + 31.5 × M
o% -30 × Mn% -11 × Cr% + 400 × Al%, These Ac₁ And Ac₃ Yaki calculated from transformation temperature
10% H at blunt temperature₂ -N ₂ The temperature was raised and held in the atmosphere
Then, at a cooling rate of 3 to 150 ° C / sec, up to 200 to 450 ° C.
And hold for 1-3000 seconds, then cool
did.

JIS No. 5 tensile test pieces were sampled from these steel sheets and their mechanical properties were measured. Further, a hole expansion test was conducted in accordance with the Iron and Steel Federation standards to obtain the hole expansion ratio. Regarding the weldability, laser welding was performed with steel plates butted together, and a ball head overhanging test was performed with resin sheet lubrication to measure the overhanging height and the fracture position with respect to the base material. Table 2
Table 3 shows the microstructure and each material, and Table 3 shows each manufacturing condition and material. It can be seen that the invention steel satisfying the outline of the present invention is excellent in weldability, ductility, strength (tensile strength of 800 MPa or more), and hole expandability. On the other hand, in the comparative examples that deviate from the conditions of the present invention, the ball head protrusion height, the tensile strength, and the hole expandability of the welded portion are inferior. Fig. 1 shows the ball head overhang height ratio of the welded portion: R, tensile strength: TS / MPa, and hole expansion ratio: λ = 1-d / d ₀
(D: hole diameter after hole expansion test / mm, d ₀ : base hole diameter / m
The relationship between the value multiplied by m) and the value of the formula (A) is shown. It can be seen that the invention steel satisfying all of the structure, the manufacturing method, and the formula (A) has all three favorable properties of workability, tensile strength, and hole expandability.

[0027]

According to the present invention, the tensile strength is 800 MPa.
It is possible to obtain a high-strength and high-ductility steel sheet in which the weldability and hole expansibility of the above high-strength steel sheet are improved at the same time, and a method for producing the same.

[Table 1]

[Table 2]

[Table 3]

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between R × TS × λ and a value on a left side of Expression (A).

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Toshiki Nonaka             5-3 Tokai-cho, Tokai-shi, Aichi Nippon Steel Corporation             Ceremony Company Nagoya Steel Works F term (reference) 4K037 EA01 EA02 EA05 EA06 EA09                       EA10 EA11 EA13 EA15 EA16                       EA17 EA19 EA20 EA23 EA25                       EA27 EA28 EA29 EA31 EA32                       EA33 EA35 EA36 EB06 EB07                       EB08 EB09 EB11 FJ00 FK02                       FK03

Claims (6)

[Claims] 1. In mass%, C: 0.01 to 0.20%, Si: 0.01 to 2.5%, Mn: 0.01 to 3%, P: 0.0010 to 0.1% , S: 0.0010 to 0.05%, Al: 0.005 to 2%, and Mo: 0.01 to 5.0%, Nb: 0.001 to 1.0%, one kind Alternatively, two kinds are contained in a range satisfying the following formula (A), the balance is Fe and inevitable impurities, and the microstructure contains bainite or bainitic ferrite in an area ratio of 70% or more.
A high-strength steel plate with excellent weldability and hole expandability, which has a tensile strength of 800 MPa or more. (3.0Nb + 2.5Mo + 1 / 10Si + Mn)-(2C ^0.5 +2)> 0 ・ ・ ・ (A) 2. Further, in mass%, Cr: 0.01 to 5%, Ni: 0.01 to 5%, Cu: 0.01 to 5%, Co: 0.01 to 5%, W: 0 0.01 to 5% of 1 type (s) or 2 or more types are contained, The high strength steel plate excellent in the weldability and hole expandability of Claim 1 characterized by the above-mentioned. 3. Further, 0.001 to 1% of Zr, Hf, Ta, Ti, and V in total of 0.001 to 1% is contained in mass%. High-strength steel sheet with excellent weldability and hole expandability described in. 4. Further, B: 0.0001 to% by mass.
The high-strength steel sheet having excellent weldability and hole expandability according to claim 1, wherein the high-strength steel sheet contains 0.1%. 5. Further, in mass%, 1 of Ca, Y, and Rem is used.
A high-strength steel sheet having excellent weldability and hole expandability according to any one of claims 1 to 4, which contains 0.001 to 0.5% in total of one kind or two or more kinds. 6. A cast slab comprising the components of claims 1 to 5 is directly or once cooled and then reheated, the hot rolled steel sheet after hot rolling is pickled, cold rolled, and then annealed. Maximum temperature is 0.8 × (Ac ₃ −Ac ₁ ) + Ac ₁
After annealing above (° C) and below Ac ₃ +30 (° C), 1
A high-strength steel sheet excellent in weldability and hole expansibility, characterized by being cooled to a temperature range of 200 to 500 ° C at a cooling rate of ~ 150 ° C / sec, and subsequently kept in the same temperature range for 1 to 3000 seconds. Manufacturing method.