JPH09143612A - High strength hot rolled steel plate member low in yield ratio - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hot rolled steel plate member for structural purpose having a low yield ratio of <=80% and >=540kgf/mm<2> high strength. SOLUTION: A steel having a chemical compsn. contg., by weight, 0.05 to 0.25% C, 0.01 to 1.0% Si, 0.50 to 2.0% Mn, 0.05% P, <=0.03% S, <=0.08% sol.Al and <=0.01% N, and the balance Fe with inevitable impurities is subjected to hot rolling and is coiled, and after that, the steel plate is subjected to cold working, is thereafter reheated to a ferrite-austenite coexistent temp. region and is subjected to hot forming to form its structure after cooling into a mixed one essentially consisting of ferrite and martensite.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention has a strength class of 54.
High-strength hot-rolled steel plate members with a low yield ratio of 0 N / mm ² class or more and 780 N / mm ² class or less, and are used as structural hot-working members such as automobile suspension materials such as wheels and construction materials. .

[0002]

2. Description of the Related Art Conventionally, hot-rolled steel sheet for the purpose of lowering the yield ratio has been developed for the purpose of improving formability (shape freezing) and impact resistance as shown in DP steel for automobile steel sheets, for example. However, the yield ratio of the member processed and formed from such a steel sheet is high, the impact property and the material homogeneity as the member are poor, and it cannot be said that the member has sufficient quality.

As the hot rolled steel sheet for construction, conventionally, rolled steel for general structure (JIS G 3101) and rolled steel for welded structure (JIS G 3106) have been widely used. In order to improve the property, especially in order to improve the seismic resistance, it is desired to lower the yield ratio, and a rolled steel material for building structures (JIS G 3136) has been established, and the yield ratio has been specified. However, as a hot-rolled steel plate member for construction, for example, a hot-rolled steel strip is formed into a tubular shape by cold working, an opening edge is electrically resistance welded to obtain a steel pipe, and then this steel pipe is subjected to cold working to be formed. The worked steel plate member has a higher yield point and a higher yield ratio due to work hardening during forming by cold working. Also, the impact value is significantly reduced. In particular, it is not uncommon for the yield ratio to be 90% or more because the material is greatly deteriorated in the cold-worked portion.

[0004]

For this reason, various measures have been taken to suppress the increase in the yield ratio, but there is a limit to the reduction in the yield ratio. For example, Japanese Patent Laid-Open No. 4-3
In Japanese Patent No. 23319, there is described a method of heating only a portion subjected to severe cold working to reduce the yield ratio of the entire member, but with this method, heating of the flat plate portion becomes insufficient,
The strain generated by cold working remains and thermal strain occurs due to the temperature difference with the heating part.Therefore, strength difference occurs between the severely worked part and the flat plate part, and the yield ratio reduction effect and impact. The effect of improving the value is small.

There is also a manufacturing method in which quenching and tempering treatment is carried out after cold working for increasing the strength, but in this method, since the structure of the member becomes tempered martensite, the yield ratio becomes high.

In the case of thick steel plates, steel plates having a mixed structure of ferrite and martensite (ferrite + martensite) have been used as raw material plates for lowering the yield ratio. Since the steel plate members are manufactured by welding the joints, etc., at the member corners including the welded parts, the ferrite +
Martensite structure cannot be obtained. For this reason, there are problems that the yield ratio is not lowered and welding softening occurs.

The present invention has been made in view of the above problems, and an object of the invention is to provide a structural hot-rolled steel plate member having a low yield ratio and high strength.

[0008]

The high-strength hot-rolled steel sheet member of the present invention has a chemical composition of wt%, C: 0.05 to 0.25%, S
i: 0.01-1.0%, Mn: 0.50-2.0%, P: 0.05% or less, S: 0.03% or less, sol.Al: 0.08% or less N: 0.01% or less, the balance consisting of Fe and unavoidable impurities The steel is hot-rolled and wound, and then the steel sheet is cold-worked, then reheated to the ferrite / austenite coexistence temperature range, hot-formed in this temperature range, and the microstructure after cooling is ferrite and martensite. The site is a mixed organization of the main body. Here, the mixed structure mainly composed of ferrite and martensite means 50% of the amount of martensite.
It means that less than bainite may be partially contained in the structure.

As the chemical composition of the steel plate member, in addition to the above-mentioned components, one or more of the following components may be contained, if desired. Cr: 0.05 to 0.80%, Ni: 0.50% or less, Mo: 0.40% or less, Nb: 0.08% or less, V: 0.30%
Below, Ti: 0.10% or less, B: 0.0005 to 0.0050%, C
a: 0.006% or less

According to the present invention, not only the chemical composition of the hot-rolled steel sheet to be used as the original sheet but also the production conditions are properly adjusted to the microstructure after hot working, so that the tensile strength after hot forming ( A hot-rolled steel sheet member having a high yield strength (TS) of 540 N / mm ² or more and a low yield ratio of 80% or less can be obtained.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION First, the reasons for limiting the steel composition of the hot-rolled steel sheet member according to the present invention will be described. Below, the unit is
wt%. C: 0.05 to 0.25% C is an element effective for increasing the strength of the steel sheet. 0.
If it is less than 05%, its action is too small, and it is 0.25%.
If it exceeds the range, the weldability and toughness will be significantly deteriorated. Therefore, the lower limit is 0.05% and the upper limit is 0.25%. It is preferably 0.08 to 0.20%.

Si: 0.01 to 1.00% Si has a deoxidizing action on molten steel, and also improves strength and ductility by a solid solution strengthening action in a cooling stage after hot working. 0.
If it is less than 01%, such an effect is too small, while on the other hand, 1.00
If it exceeds%, Si scale is likely to be generated in the hot rolling stage, and the steel sheet surface properties are deteriorated. Therefore, the lower limit is 0.01% and the upper limit is 1.00%.

Mn: 0.50 to 2.00% Mn is an element that enhances the strength like C and Si, and is an essential element for enhancing the strength by cooling after hot working. If it is less than 0.50%, the effect is too small,
If it exceeds 2.00%, a band-like structure is formed and ductility in the rolling direction is deteriorated. Therefore, the lower limit is 0.50% and the upper limit is 2.00%.

P: 0.05% or less S: 0.03% or less P and S deteriorate the toughness, so the smaller the better, the upper limits are 0.05% and 0.03%, respectively.

Sol.Al: 0.08% or less Al is added as a deoxidizing agent for steel, but if added in a large amount, the cleanliness of the steel deteriorates and the toughness also deteriorates.
The upper limit of sol. (Solid solution) Al is 0.08%.

N: 0.01% or less N decreases the ductility and weldability, so the less the better, the upper limit is made 0.01%. The steel used in the present invention comprises, in addition to the above components, the balance Fe and unavoidable impurities, but in order to further improve strength and toughness, one or more of the following components can be further contained, if necessary.

Cr: 0.05 to 0.80% Cr is effective in increasing the strength of steel by precipitation hardening and the like, and is particularly effective in improving the strength characteristics after reheating. To obtain this effect, 0.05% or more is necessary. On the other hand, if contained in a large amount, low temperature toughness and weldability are deteriorated, so the upper limit is made 0.80%.

Mo: 0.40% or less Mo, like Cr, is effective in increasing the strength of steel,
Particularly, it is effective for improving the strength characteristics after reheating and for improving the high temperature strength characteristics. However, if contained in a large amount, the weldability deteriorates and the cost increases, so the upper limit is 0.4.
0%.

Nb: 0.08% or less Nb is effective in increasing the strength of the steel by precipitation hardening and is also effective in refining the grain size, but if it is contained in a large amount, the weldability is deteriorated. The upper limit is 0.08%.

V: 0.30% or less V is effective for increasing the strength of steel by precipitation hardening and the like, and is particularly effective for improving the strength characteristics after reheating, but if added in a large amount, it deteriorates the weldability. Therefore, the upper limit is 0.30
%.

Ti: 0.10% or less Ti enhances the strength of steel and forms TiN, which is effective in refining austenite grains and is effective in improving toughness, but if added in large amounts, it deteriorates weldability. Therefore, the upper limit is made 0.10%.

B: 0.0005 to 0.0050% B is effective for increasing the strength by adding a trace amount, and
It has the effect of significantly increasing the hardenability. 0.0005%
If it is less than 1, the effect is too small, while if added in a large amount, a B compound is formed and the toughness is deteriorated.
0050%.

Ca: 0.006% or less Ca has the effect of reducing the mechanical anisotropy and improving the ductility and toughness through morphology control of changing the form of sulfide from expanded to spherical. However, if a large amount is added, the ductility and toughness rather decrease due to the increase of non-metallic inclusions in the steel, so the upper limit is made 0.006%.

The hot-rolled steel sheet member of the present invention is obtained by hot rolling a steel having the above-mentioned components, winding the steel, cold working the steel sheet, and then adding the steel in the ferrite-austenite coexistence temperature range of 700 to 700.
Reheating is performed at 950 ° C., hot forming is performed in this temperature range, and the structure after cooling is a mixed structure mainly composed of ferrite and martensite.

In the hot rolling, the slab may be rolled immediately after continuous casting (direct rolling), or the ingot may be heated and then rolled, but the heating temperature during hot rolling is 1100 to 1100.
1300 ° C. is preferred. If it is less than 1100 ° C, the effect of improving strength due to Nb or the like is insufficient, while if it exceeds 1300 ° C, austenite is coarsened and toughness deteriorates. However, since the desired effect of the present invention can be obtained even at a heating temperature outside the above range, the heating temperature is not limited to this range.

The finishing temperature of hot rolling may be in a range that allows cold forming and is not particularly limited, but preferably 800 ° C to 850 ° C just above the transformation point. The take-up temperature after hot rolling is also not particularly specified, but if the take-up temperature is extremely low, the low-temperature transformation forming structure increases, the strength remarkably increases, and the cold workability deteriorates significantly. . Further, when the winding temperature becomes extremely higher than usual, the surface properties (grain boundary oxidation) become remarkable. Therefore, the winding temperature range may be about 400 to 650 ° C.

The rolled steel sheet is rewound and subjected to an appropriate cold working, for example, forming into a predetermined shape by roll forming, and then a temperature range of Ac / _one ferrite / austenite coexistence, usually 700 to 950. After being reheated to a temperature range of ° C and formed into an appropriate member shape such as a square tube by hot working, the metal structure of the member is cooled to obtain a mixed structure mainly composed of ferrite and martensite. The mixed structure mainly composed of ferrite and martensite means that a small amount of bainite of less than 50%, especially 25% or less of the amount of martensite may be partially contained in the structure. With such a structure, the yield ratio of the member after hot forming can be lowered, and a homogeneous and excellent material can be obtained.

If the reheating temperature is lower than the ferrite / austenite coexisting temperature, austenite is not formed. Therefore, martensite is not formed even by the subsequent hot working, and if the structure of the member is mainly composed of ferrite + martensite. Therefore, a hot-rolled steel sheet member having a low yield strength cannot be obtained. Further, when the temperature exceeds the coexistence temperature of ferrite and austenite, the steel structure is almost austenized during reheating, so that C is not concentrated in austenite.
Martensite is not generated at a relatively slow cooling rate after hot working, and the target structure cannot be obtained in the hot rolled steel sheet member. In the present invention, since hot working is performed in the ferrite / austenite coexisting temperature range, concentration of C occurs in austenite, the martensite transformation point decreases, and after hot working, transformation occurs even at a relatively slow cooling rate, Usually, the desired mixed structure can be obtained by air cooling or air blow cooling.

[0029]

EXAMPLES Steel grades No. A to have the chemical composition shown in Table 1
G (inventive steel), HI (comparative steel) sample steels are melted,
It was hot rolled and wound to produce a hot rolled steel sheet having a plate thickness of 12 mm. After cold working (cold rolling with a rolling reduction of 7%) using this steel sheet, it was reheated to various temperatures, hot worked (hot rolling with a rolling reduction of 10%), and air-cooled. Table 2 shows the results of examining the metal structure, tensile properties, and impact properties of the steel sheet member after hot working.

[0030]

[Table 1]

[0031]

[Table 2]

From Table 2, sample Nos. 1 to 8 according to the embodiment
Then, the tensile strength after hot working is 590 to 780 N / mm ²
The yield ratio is 72 to 78%, and a member having high strength and low yield ratio is obtained. Also, the impact value is 34J or more,
It can be seen that it has excellent toughness. In No. 8, some bainite (about 25% of martensite in area ratio)
%) Was produced, but since it was a small amount, the characteristics did not deteriorate.

On the other hand, sample Nos. 9 to 1 which are comparative examples
In No. 1, the steel composition is within the composition range of the present invention, but since the reheating temperature is out of the range of the present invention, the structure becomes a mixed structure of ferrite + pearlite, and the yield ratio is 84 to 86%.
It is high. In addition, sample Nos. 12 to 15 of comparative examples
Because the composition is out of the steel composition range of the present invention,
Even if it is reheated within the range of the present invention, it becomes a ferrite + bainite structure, and the yield ratio is as high as 84 to 89%.

[0034]

EFFECT OF THE INVENTION According to the hot-rolled steel sheet member of the present invention, the strength after hot working is as high as 540 N / mm ² or more, and the yield ratio is 80% or less. It is suitable as a steel plate member.

Claims (2)

[Claims] 1. A chemical composition, by weight, C: 0.05 to 0.25%, Si: 0.01 to 1.0%, Mn: 0.50 to 2.0%, P: 0.05% or less, S: 0.03% or less, sol.Al: 0.08% or less N: 0.01% or less of N, 0.01% or less, the balance Fe and unavoidable impurities, hot-rolled and wound, after cold working the steel sheet, reheated to the ferrite-austenite coexistence temperature range A high-strength hot-rolled steel sheet member having a low yield ratio, which is obtained by performing hot forming in this temperature range, and the structure after cooling is a mixed structure mainly composed of ferrite and martensite. 2. The composition as set forth in claim 1, wherein the chemical composition is wt%, Cr: 0.05 to 0.80%, Ni: 0.50% or less, Mo: 0.40% or less, Nb: 0.08% or less, V: The high-strength hot-rolled steel sheet member having a low yield ratio according to claim 1, containing one or more of 0.30% or less, Ti: 0.10% or less, B: 0.0005 to 0.0050%, and Ca: 0.006% or less.