JP3578435B2 - Hot-rolled steel sheet for structural use excellent in press formability and surface properties and method for producing the same - Google Patents

Description

【００１９】
【表２】

【００２０】
表３には表１に示した材料の曲げと突き曲げ試験にて加工性の評価を行った結果である。比較例５は密着あるいは０．５ｔＲ，１．０ｔＲ曲げのいずれの試験でも割れが発生しているに対し、 Ｃａ，Ｃａ／Ｓを規制して鋼板中の介在物の形態制御した本発明例１〜４は、いずれの曲げ試験条件においても割れが発生しておらず加工性に優れていることがわかる。
【００２１】
【表３】

【００２２】
【発明の効果】
本発明では、Ｃａ，Ｃａ／Ｓを規制して鋼板中の介在物の形態制御し、熱間圧延条件の制御により板厚中央部に縞状（バンド）組織のないフェライトとパーライトの均質な混合組織を得てプレス成形性や靱性を得ることができる。また、鋼板の表面性状を向上させるためにＣｕとＮｉ添加量を規制して酸化スケール剥離性を改善しているので製造過程で問題となるスケール疵が防止できる。
これまで熱延ままでは比較的軽度な成形加工部品に限られていた構造用熱延鋼板のプレス成形加工性を向上させた高強度で高靱性の材料が提供できるので、成形加工中に発生する割れやクラックに起因した生産性の低下を改善できる。
【図面の簡単な説明】
【図１】（Ｃａ／Ｓ）と硫化物系介在物量との関係を示す図である。
【図２】（Ｃａ／Ｓ）と衝撃異方性との関係を示す図である。[0001]
TECHNICAL FIELD OF THE INVENTION
TECHNICAL FIELD The present invention relates to a hot-rolled steel sheet for structural use having a tensile strength of 490 N / mm ² or more, which is excellent in press formability and surface properties, and a method for producing the same.
[0002]
[Prior art]
Steel materials used as materials for gear parts, chain parts, frame parts, etc. for automobiles and industrial machines are carbon steel for mechanical structures specified in JIS G4051 and structural materials such as SCR420 and SCM420 specified in JISG4102-4106. Alloy steel is used. These steel materials are stamped into a predetermined shape or press-molded, and after heat treatment such as carburizing quenching and tempering, the materials are improved, and some of them are subjected to shot peening or the like, and then painted to be commercialized. These steel materials are required to be soft and excellent in workability before forming, and to be excellent in strength, impact resistance, wear resistance and the like after heat treatment.
[0003]
On the other hand, when carbon steel for mechanical structure or structural alloy steel such as SCR420 and SCM420 is used at a high strength level as hot rolled, workability is 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]
[Problems to be solved by the invention]
The present invention improves the press-formability of high-strength hot-rolled steel sheets for structural use, which was previously limited to relatively light-formed parts when hot-rolled as it is, and has been a concern at the time of press-forming. An object of the present invention is to improve the productivity by lowering the productivity caused by cracks and cracks generated therein and to prevent scale flaws of the structural steel sheet to improve the surface properties.
[0005]
[Means for Solving the Problems]
According to the present invention, in order to achieve the above object, C: 0.10 to 0.50%, 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 to 1.50%, Mo: 0.10 to 0.50%, Acid-soluble Al: 0.05% or less, Ca: 0.003% or more, Ca / S: 0.60 to 1.70, the balance substantially consisting of Fe, and the metal structure is homogeneous between ferrite and pearlite. Provided is a structural hot-rolled steel sheet having a mixed structure. This hot-rolled steel sheet is subjected to hot rolling at a total draft of 50% or more and a final draft of 10% or more after heating the steel slab to 1150 to 1300 ° C, and the finishing temperature is changed from Ar ₃ transformation point to Ar ₃ + 50 ° C. It is manufactured by controlling the winding temperature to 400 to 600 ° C. and the average cooling rate from hot-rolling finishing to winding to 30 to 60 ° C./second, and is subjected to temper rolling and pickling by ordinary methods.
[0006]
BEST MODE FOR CARRYING OUT THE INVENTION
In the present invention, the morphology of inclusions in a steel sheet is controlled by regulating Ca, Ca / S, and the uniform mixing of ferrite and pearlite without a banded (band) structure at the center of the sheet thickness is controlled by controlling the hot rolling conditions. The press formability and toughness are improved by the structure. 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 added are regulated to improve the scale surface.
[0007]
Hereinafter, the reasons for limiting the components and contents of the hot rolled steel sheet for a structure in the present invention will be described.
C: 0.10 to 0.50%
C is an important element in determining the properties of steel. Strength and hardness increase with the addition amount, and wear resistance and the like improve. However, workability and toughness decrease, so the upper limit is set to 0.50%. The lower limit was set to 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 processed, 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 may adversely affect 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.
[0008]
Mn: 0.60-1.00%
Although the strength increases with the addition amount of Mn, the anisotropy of the material becomes strong due to the increase of Mn-based inclusions which cause the deterioration of press workability and the manifestation of a striped (band) structure. From the viewpoint of high strength and press workability, the Mn content was set to 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. acceptable.
S: 0.006% or less S content is preferably as low as P, but if it is 0.006% or less, sulfide-based inclusions are reduced and workability is improved.
[0009]
Cu: 0.30% or less Addition of Cu concentrates at the boundary between the oxide scale generated during hot rolling and the steel sheet surface, and improves the scale releasability. However, when added in an amount of 0.20% or more, fine cracks are likely to be generated on the surface of the steel sheet due to embrittlement of molten metal. For this reason, a desirable addition range of Cu is 0.10 to 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 is 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 of molten metal embrittlement, so there is no need to add Ni.
[0010]
Cr: 0.90 to 1.50%
Cr is an additive element that improves the quenching properties and mechanical properties of steel, but the strength and toughness are improved by increasing the amount of added Cr, but when the content exceeds 1.50%, the proof stress decreases. Therefore, 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 or 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%.
[0011]
Acid-soluble Al: 0.05% or less Al is added for deoxidation, fixation of nitrogen, and adjustment of 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.
[0012]
Ca: 0.003% or more Ca is an element that exerts an effect on deoxidation and desulfurization, and adjusts the form and distribution of sulfide-based nonmetallic inclusions in the deoxidized steel to improve workability. In order to obtain this effect, a value of 0. It requires an addition amount of 003% or more.
Ca / S: 0.60 to 1.70
According to FIG. 1 showing the relationship between Ca / S and sulfide-based inclusions, if the Ca / S ratio is 0.60 or more, a steel material having a small amount of sulfide-based inclusions is obtained, so that the workability is good. Become. From FIG. 2 showing the relationship between the anisotropy due to the impact value and Ca / S, if the ratio of Ca / S is 0.60 or more, the value of the anisotropy due to the impact value becomes 0.5 or more, A steel material having a small toughness and a 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.
[0013]
After smelting the steel of the above component in a converter, it is degassed, continuously cast into slabs, 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.
[0014]
Heating temperature 1150-1300 ° C:
If the temperature is 1150 ° C. or less, there is a problem that slab cracks occur during rough rolling and surface quality is reduced due to reduced 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 obtaining a structure in which carbides are dispersed with a total reduction ratio of 50% or more and a final reduction ratio of 10% or more. If the total draft is less than 50% and the final draft 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 tends to be formed, so that improvement in workability and toughness cannot be expected.
[0015]
Finishing temperature Ar ₃ transformation point to Ar ₃ + 50 ° C .:
Mechanical properties such as the finish rolling Ar ₃ transformation point to Ar 3 + By ending with ₅₀ low temperature range of ℃ fine ferrite grains are obtained strength and toughness is improved. If the finishing temperature does not reach the Ar ₃ transformation point, the deformation resistance increases, which impairs the sheet passing property in the hot rolling process. Further, since the two-phase 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, thereby deteriorating the workability. On the other hand, when the finishing temperature exceeds Ar ₃ + 50 ° C., the hot-rolled structure becomes coarse and 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 the mechanical properties in the coil is impaired.
[0016]
Average cooling rate from hot rolling finish to winding at a winding temperature of 400 to 600 ° C 30 to 60 ° C / sec:
The steel strip after hot rolling is wound in a temperature range of 400 to 600 ° C. in order to prevent the formation of a striped (band) structure that affects workability and toughness and to obtain a fine grained structure of ferrite and pearlite. Can be If the temperature is lower than 400 ° C., the strength rises remarkably, and the workability is impaired. Conversely, if the winding temperature exceeds 600 ° C., the strength of 490 N / mm 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 the temperature is lower than 30 ° C./sec, the desired structure cannot be obtained, and the ratio of banded (band) structure increases, and 490 N / mm ² or more And good workability cannot be obtained. When the average cooling rate increases, 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 then a hot-rolled steel sheet having a thickness of 4.5 mm 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 gauge 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. The hole diameter (d) at the time of occurrence of was measured, and the hole expansion ratio (λ) defined by the following equation was obtained.
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 more, and have improved Rankford values and hole expansion ratios and are excellent in workability as compared with Comparative Examples. It can also be seen that the scale flaws on the surface of the steel sheet were slight and the surface properties were excellent. In addition, it can be seen that Inventive Example 4 has a larger scale flaw than Inventive Examples 1 to 3 to which Cu is added, but has an 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 adding Ca, so that the Rankford value and the hole expansion ratio were low, and the degree of scale flaws was large.
[0018]
[Table 1]

[0019]
[Table 2]

[0020]
Table 3 shows the results of evaluating the workability of the materials shown in Table 1 by bending and butt bending tests. Comparative Example 5 shows cracks in both the adhesion test and the 0.5tR and 1.0tR bending tests, but the present invention example 1 in which the form of inclusions in the steel sheet was controlled by regulating Ca and Ca / S. Nos. 4 to 4 show that no cracking occurred under any of the bending test conditions and the workability was excellent.
[0021]
[Table 3]

[0022]
【The invention's effect】
In the present invention, the morphology of inclusions in a steel sheet is controlled by regulating Ca and Ca / S, and the uniform mixing of ferrite and pearlite without a banded (band) structure in the center of the sheet thickness is controlled by controlling the hot rolling conditions. By obtaining a structure, press formability and toughness can be obtained. In addition, since the addition of Cu and Ni is regulated in order to improve the surface properties of the steel sheet and the oxide scale releasability is improved, scale flaws, which are problematic in the production process, can be prevented.
Until now, hot-rolled as-is was limited to relatively light-formed parts. 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 the decrease in productivity due to cracks and cracks.
[Brief description of the drawings]
FIG. 1 is a graph 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)

In weight percent,
C: 0.10 to 0.50%,
Si: 0.35% or less,
Mn: 0.60-1.00%,
P: 0.03% or less,
S: 0.006% or less Cu: 0.30% or less,
Ni: 0.25% or less,
Cr: 0.90 to 1.50%,
Mo: 0.10 to 0.50%,
Acid-soluble Al: 0.05% or less,
Ca: 0.003% or more,
Ca / S: 0.60 to 1.70,
A hot-rolled steel sheet for structural use excellent in press formability and surface properties, the balance being substantially composed of Fe, and the metal structure being a homogeneous mixed structure of ferrite and pearlite. In weight percent,
C: 0.10 to 0.50%,
Si: 0.35% or less,
Mn: 0.60-1.00%,
P: 0.03% or less,
S: 0.006% or less Cu: 0.30% or less,
Ni: 0.25% or less,
Cr: 0.90 to 1.50%,
Mo: 0.10 to 0.50%,
Acid-soluble Al: 0.05% or less,
Ca: 0.003% or more,
Ca / S: 0.60 to 1.70,
A steel slab whose balance is substantially made of Fe,
After heating to 1150 to 1300 ° C, hot rolling is performed at a total draft of 50% or more and a final draft of 10% or more, the finishing temperature is Ar ₃ transformation point to Ar ₃ + 50 ° C, and the winding temperature is 400 to 600 ° C. A method for producing a hot-rolled steel sheet for structural use having excellent press formability and surface properties in which the average cooling rate from hot-rolling finishing to winding is 30 to 60 ° C / sec.

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