JPH10251794A - Hot rolled steel plate for structural purpose, excellent in press formability and surface characteristic, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a hot rolled steel plate for structural use, having >=490N/mm<2> tensile strength and excellent in press formability and surface characteristic. SOLUTION: The hot rolled steel plate gas a composition which consists of, by weight, 0.10-0.50% C, <=0.35% Si, 0.60-1.00% Mn, <=0.03% P, <=0.006% S, <=0.030% Cu, <=0.25% Ni, 0.90-01.50% Cr, 0.10-0.50% Mo, <=0.05% acid-soluble Al, >=0.003% Ca, and the balance essentially Fe and in which Ca/S is regulated to 0.60-1.70 and also has a metallic structure composed of homogeneous mixed structure of ferrite and pearlite. This steel plate can be produced by controlling finishing temp. at hot rolling to a temp. between the Ar3 transformation point and (Ar3 +50 deg.C) and also controlling coiling temp. and average cooling rate from finishing to coiling to 400-600 deg.C and (30 to 60 deg.C)/sec, respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet for structural use having excellent press formability and surface properties and a tensile strength of 490 N / mm ² or more, and a method for producing the same.

[0002]

2. Description of the Related Art Steel materials used as materials for gear parts, chain parts, frame parts and the like for automobiles and industrial machines include carbon steel for mechanical structures specified in JIS G4051 and SCR42 specified in JIS G4102 to 4106.
0, SCM420 and other structural alloy steels are used.
These steel materials are stamped into a predetermined shape or press-molded, and after heat treatment such as carburizing quenching and tempering, the quality of the materials is improved. These steel materials are soft and have excellent workability before forming, and have strength and strength after heat treatment.
It is required to have excellent impact resistance, wear resistance, and the like.

On the other hand, carbon steel for machine structure, SCR420,
When a structural alloy steel such as SCM420 is used at a high strength level as hot rolled, workability has been improved by performing softening treatment such as annealing. In addition, structural alloy steels suffer from deterioration in the surface quality of steel sheets and products due to scale flaws caused by oxide scale generated during hot rolling in the manufacturing process, and reduced production yield due to excessive peeling of thick oxide scale. Was a problem.

[0004]

SUMMARY OF THE INVENTION The present invention is intended to improve the press-formability of a high-strength hot-rolled steel sheet for a structure, which was previously limited to relatively light-formed parts when hot-rolled as it is, and to provide a press-forming process. It is an object of the present invention to improve the productivity, which is caused by cracks and cracks generated during the forming process, which has been concerned at the time of forming, and to prevent scale flaws of the structural steel sheet to improve the surface properties.

[0005]

According to the present invention, in order to achieve the above-mentioned object, C: 0.10 to 0.50% by weight,
Si: 0.35% or less, Mn: 0.60 to 1.00%,
P: 0.03% or less, S: 0.006% or less, Cu:
0.30% or less, Ni: 0.25% or less, Cr: 0.9
0 to 1.50%, Mo: 0.10 to 0.50%, acid-soluble Al: 0.05% or less, Ca: 0.003% or more, C
a / S: 0.60 to 1.70, the balance being substantially Fe, and providing a structural hot-rolled steel sheet whose metal structure is a homogeneous mixed structure of ferrite and pearlite. This hot-rolled steel sheet is subjected to hot rolling at a total reduction rate of 50% or more and a final reduction rate of 10% or more after heating the steel slab to 1150 to 1300 ° C., and the finishing temperature is changed from the Ar ₃ transformation point to Ar ₃ + 50 ° C. It is manufactured by controlling the winding temperature at 400 to 600 ° C. and the average cooling rate from hot rolling finishing to winding at 30 to 60 ° C./second, and is subjected to temper rolling and pickling by ordinary methods.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, the form of inclusions in a steel sheet is controlled by regulating Ca and Ca / S, and no striped (band) structure is formed at the center of the sheet thickness by controlling hot rolling conditions. Press formability and toughness are improved by a homogeneous mixed structure of ferrite and pearlite. Further, in order to prevent scale flaws generated during hot rolling, which is a problem in the production process of steel materials, and to improve the surface properties of the steel sheet, the amounts of Cu and Ni are regulated to improve the scale surface.

Hereinafter, the reasons for limiting the components and contents of the hot-rolled structural steel sheet according to the present invention will be described. C: 0.10 to 0.50% C is an important element in determining the properties of steel. The strength and hardness increase with the amount of addition, and wear resistance and the like are improved, but workability and toughness are reduced. As it decreases, the upper limit is 0.50%
And the lower limit was 0.10% from the viewpoint of strength. Si: 0.35% or less Si is added as a deoxidizing agent and a quenching promoting element. However, in a steel material required to be severely worked, if the content exceeds 0.35%, the steel becomes hard and causes embrittlement. There is a possibility that the addition of a large amount of Si adversely affects the scale surface on the surface of the steel sheet. Therefore, the upper limit of the Si content is set to 0.35% or less.

Mn: 0.60-1.00% Although the strength increases with the amount of Mn added, the increase in Mn-based inclusions and the stripes (bands) which cause the deterioration of press workability.
The anisotropy of the material becomes stronger due to the manifestation of the tissue. From the viewpoint of high strength and press workability, the Mn content is 0.60 to 1.
00%. P: 0.03% or less P and S not only lower the toughness of the steel material, but also segregate at the crystal grain boundaries of the steel to cause grain boundary embrittlement, so that it is preferably as low as possible. acceptable. S: 0.006% or less Although the S content is preferably as low as P, it is preferably 0.006% or less.
% Or less, sulfide-based inclusions are reduced and workability is improved.

Cu: 0.30% or less With the addition of Cu, it is concentrated at the boundary between the oxide scale generated during hot rolling and the steel sheet surface, and scale releasability is improved. However, if added in an amount of 0.20% or more, fine cracks are likely to be generated on the steel sheet surface due to the embrittlement of the molten metal. Therefore, the desirable addition range of Cu is 0.10 to 0.10.
0.15%. Ni: 0.25% or less Addition of approximately the same amount as Cu is required to suppress fine cracks generated on the steel sheet surface due to molten metal embrittlement which becomes a problem when adding 0.20% or more of Cu. . However, when the addition of Cu is less than 0.20%, there is no problem with the embrittlement of molten metal, so there is no need to add Ni.

Cr: 0.90 to 1.50% Cr is an additive element for improving the quenching properties and mechanical properties of steel, but the strength and toughness are improved by increasing the amount of added Cr. When the content exceeds 1.50%, the proof stress is reduced, so the Cr content is set to 0.90 to 1.50%. Mo: 0.10 to 0.50% When Mo is added to Cr-added steel and Cr-Ni-containing steel in combination, the mechanical properties are further improved, and the hardenability and the temper softening resistance are increased. When the amount of Mo added increases, the mechanical properties are further improved, but the production cost becomes expensive. Therefore, the amount of Mo added is set to 0.10 to 0.50%.

Acid-soluble Al: 0.05% or less Al is added for deoxidizing, fixing nitrogen, and adjusting crystal grain size. In the present invention, the content of the acid-soluble Al is set to 0.05% or less in order to improve the workability at the time of forming by suppressing the growth of austenite crystal grains.

Ca: 0.003% or more Ca is an element exhibiting an effect on deoxidation and desulfurization, and adjusts the form and distribution of sulfide-based nonmetallic inclusions in deoxidized steel to improve workability. . In order to obtain this effect, use 0. 003
% Or more is required. Ca / S: 0.60 to 1.70 From FIG. 1 showing the relationship between Ca / S and sulfide-based inclusions,
When the ratio of a / S is 0.60 or more, a steel material having few sulfide-based inclusions can be obtained, so that the workability is good. From FIG. 2 showing the relationship between the anisotropy by the impact value and Ca / S,
When the Ca / S ratio is 0.60 or more, the anisotropy value according to the impact value becomes 0.5 or more, and a steel material with small anisotropy and high toughness can be obtained. However, if Ca / S exceeds 1.70, the number of oxide-based composite inclusions increases and the workability deteriorates, so the upper limit was set to 1.70.

After melting the steel of the above component in a converter, it is degassed to form a slab by continuous casting, and then charged into a heating furnace and hot rolled to obtain a strip. In the production of a structural hot rolled steel sheet having a high strength level, a high strength steel material having good workability can be obtained by controlling hot rolling conditions and cooling after finish rolling.

Heating temperature 1150-1300 ° C .: 11
If the temperature is lower than 50 ° C., there is a problem that the surface quality is deteriorated due to the slab crack during the rough rolling and the scale releasability. If the temperature is 1300 ° C. or higher, problems such as high-temperature brittleness, quality deterioration due to large scale peeling, yield reduction due to scale loss, and the like become problems. Total reduction 50% or more, final reduction 10% or more: Press workability and toughness are improved by setting the total reduction 50% or more and the final reduction 10% or more to obtain a structure in which carbides are dispersed. If the total reduction is less than 50% and the final reduction is less than 10%, it is difficult to obtain a uniform carbide-dispersed structure.
On the other hand, if the rolling reduction of the hot rolling is small, recrystallization is delayed and a banded structure of ferrite and pearlite is easily formed, so that improvement in workability and toughness cannot be expected.

Finishing temperature Ar ₃ transformation point to Ar ₃ +50
° C.: finish rolling to improve the mechanical properties such as Ar ₃ transformation point to Ar ₃ + 50 ° C. of that ends at low temperature region fine ferrite grains are obtained strength and toughness. Finish temperature is Ar
_{If the} transformation point is not reached, the deformation resistance increases, which impairs the threadability in the hot rolling process. Further, since the two-phase zone rolling of austenite and ferrite is performed, processed ferrite is easily generated, so that coarse crystal grains are generated on the surface layer of the steel sheet after rolling, and the workability is deteriorated. On the other hand, when the finishing temperature is Ar ₃ +
If the temperature exceeds 50 ° C., the hot-rolled structure becomes coarse and the workability is deteriorated, and the cooling strain in the cooling process after rolling is increased to deteriorate the shape of the steel sheet. In addition, since cooling unevenness easily occurs, the stability of mechanical properties in the coil is impaired.

Average cooling rate from hot rolling finishing to winding at a winding temperature of 400 to 600 ° C. to 30 to 60 ° C./sec: The strip after hot rolling has strips (bands) which affect workability and toughness. It is wound in a temperature range of 400 to 600 ° C. in order to prevent the formation of a structure and obtain a fine-grained structure of fine ferrite and pearlite.
If the temperature is lower than 400 ° C., the strength is remarkably increased and the workability is impaired.
A strength of m ² or more cannot be obtained. On the other hand, the average cooling rate until winding is also necessary to obtain the desired structure,
If it is less than / sec, the desired structure cannot be obtained and the ratio of the banded (band) structure increases, so that a strength of 490 N / mm ² or more and good workability cannot be obtained. If the average cooling rate is increased, the steel sheet becomes harder and good workability cannot be obtained, so the upper limit was set to 60 ° C./sec.

[0017]

EXAMPLE A slab obtained by melting steels 1 to 5 having the components shown in Table 1 was roughly rolled to a thickness of 30 mm, and a sheet thickness of 4.5.
mm hot rolled steel sheet was manufactured. The finishing temperature was 850 ° C., the winding temperature was 500 to 600 ° C., and the average cooling rate was 30 to 50 ° C./sec. A sample was cut out from the hot rolled sheet, and a hardness measurement, a tensile test, and a hole expansion test were performed. The surface properties of the steel sheet were visually observed to determine the scale flaws of the steel sheet. The tensile test was performed using a JIS No. 5 test piece with the distance between the reference points of the parallel portions being 50 mm. The hole expansion test is performed by punching a hole of 10 mm (do) with a clearance of 10% in the center of a 150 mm square steel plate, and pushing up the hole with a punch of a ball head. Diameter at the time of occurrence (d)
Was measured to determine a hole expansion ratio (λ) defined by the following equation. λ = (d−do) / do × 100 As shown in Table 2, Examples 1 to 3 of the present invention have a tensile strength of 490 N
/ Mm2 or higher, and the Rankford value and the hole expansion ratio are improved and the workability is excellent as compared with the comparative example, and the scale flaws on the steel sheet surface are small and the surface properties are excellent. Recognize. In addition, it can be seen that Example 4 of the present invention has a larger scale flaw than Examples 1 to 3 of the present invention in which Cu is added, but has improved Rankford value and hole expansion ratio, and is excellent in workability. In Comparative Example 5, the amount of S added was larger than the range of the present invention, and the inclusion control was not performed by the addition of Ca, so that the Rankford value and the hole expansion rate were low, and the degree of scale flaws was large.

[0018]

[Table 1]

[0019]

[Table 2]

Table 3 shows the results of evaluating the workability of the materials shown in Table 1 by bending and thrust tests. Comparative Example 5
Cracks occur in both the adhesion and the 0.5tR and 1.0tR bending tests, whereas Ca, Ca / S
Examples 1 to 4 of the present invention in which the shape of inclusions in the steel sheet was controlled by regulating
No. 4 shows that cracking did not occur under any of the bending test conditions and the workability was excellent.

[0021]

[Table 3]

[0022]

According to the present invention, the form of inclusions in a steel sheet is controlled by regulating Ca and Ca / S, and by controlling the hot rolling conditions, ferrite having no banded (band) structure in the center of the sheet thickness can be obtained. Press formability and toughness can be obtained by obtaining a homogeneous mixed structure of pearlite. Further, in order to improve the surface properties of the steel sheet, the amounts of Cu and Ni added are regulated to improve the oxide scale releasability, so that scale flaws which are problematic in the production process can be prevented. Until now, hot-rolled parts were limited to relatively light forming parts, so high-strength, high-toughness materials with improved press-formability of hot-rolled steel sheets for structural use can be provided. It is possible to improve a decrease in productivity caused by cracks and cracks.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between (Ca / S) and the amount of sulfide-based inclusions.

FIG. 2 is a diagram showing a relationship between (Ca / S) and impact anisotropy.

Claims (2)

[Claims] C. 0.10 to 0.50% by weight,
Si: 0.35% or less, Mn: 0.60 to 1.00%,
P: 0.03% or less, S: 0.006% or less Cu: 0.
30% or less, Ni: 0.25% or less, Cr: 0.90 or less
1.50%, Mo: 0.10 to 0.50%, acid soluble A
l: 0.05% or less, Ca: 0.003% or more, Ca /
S: 0.60 to 1.70, the balance being substantially Fe, and a structural hot-rolled steel sheet excellent in press formability and surface properties having a homogeneous microstructure of ferrite and pearlite. 2. C: 0.10 to 0.50% by weight,
Si: 0.35% or less, Mn: 0.60 to 1.00%,
P: 0.03% or less, S: 0.006% or less Cu: 0.
30% or less, Ni: 0.25% or less, Cr: 0.90 or less
1.50%, Mo: 0.10 to 0.50%, acid soluble A
l: 0.05% or less, Ca: 0.003% or more, Ca /
S: 0.60 to 1.70, a steel slab consisting essentially of Fe, with the balance being heated to 1150 to 1300 ° C, subjected to hot rolling at a total draft of 50% or more and a final draft of 10% or more,
Excellent finishing temperature Ar ₃ transformation point to Ar ₃ + 50 ° C., the coiling temperature 400 to 600 ° C., an average cooling rate from the hot rolling finish to the take-up press formability and surface properties to 30 to 60 ° C. / sec Of hot rolled steel sheets for structural use.