JPH09118952A - Member made of high-strength hot rolled steel sheet having lower yield ratio - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the member made of a hot rolled steel sheet for structure purpose having lower yield ratio and high strength. SOLUTION: After steel consisting of, by weight, 0.05-0.20% C, 0.01-1.0% Si, 0.50-2.0% Mn, <=0.05% P, <=0.03% S, <=0.08% sol. Al, <=0.014% N and the balance Fe with inevitable impurities is hot-rolled at a finishing temp. not lower than the point Ar3 , and it is coiled at 500-650 deg.C. Then, after cold-working the steel sheet, the steel sheet is reheated to a temp. of not lower than the point Ac3 and, after hot forming work is executed at a temp. of not lower than the point Ar3 , the structure is made into the mixed structure of ferrite and pearlite by cooling.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structural high-strength hot-rolled steel plate member having a low yield ratio, which is used for automobile suspension members such as wheels and construction members.

[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 seismic property, it is desired to lower the yield ratio in order to improve the seismic resistance, and rolled steel for building structures (JIS G 3136) has been enacted and the yield ratio has been specified. As a steel plate member, for example, a hot-rolled steel strip is formed into a tubular shape by cold working, an opening edge is subjected to electric resistance welding to obtain a steel pipe, and then a steel plate member formed by cold working of this steel pipe is The yield point rises due to work hardening during forming by hot working,
Higher yield ratio. 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 higher strength, but in this method, since the structure of the member becomes tempered martensite, the yield ratio becomes high.

Further, in the thick steel plate, the structures are ferrite and martensite (ferrite + ferrite) in order to lower the yield ratio.
Steel sheets with a mixed structure of martensite) are also used as member original plates, but since steel plate members are manufactured by welding the original plates together and welding the joints, the members including the welded parts The ferrite + martensite structure of the original plate cannot be obtained at the corners. 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 is, by weight%, C: 0.05 to 0.20% Si: 0.01 to 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, with the balance being Fe and inevitable impurities. After hot rolling at a finishing temperature of ₃ points or higher, coiling at 500 to 650 ° C, cold working of the steel sheet, reheating to a temperature of Ac ₃ points or higher and hot forming at a temperature of Ar ₃ points or higher. After that, the structure is cooled to ferrite and pearlite.

In order to further improve the strength and toughness of the member,
In addition to the above components, steel containing one or more of the following components may be used. Cr: 0.05 to 0.80% Ni: 0.50% or less Mo: 0.40% or less Nb: 0.08% or less V: 0.30% or less Ti: 0.10% or less B: 0.0005 ~ 0.0050% Ca: 0.006% or less

In the present invention, a solid solution strengthening element and a precipitation strengthening element are added, and the finishing temperature of hot rolling, winding temperature, heating temperature after cold forming and hot working temperature are set to predetermined values. By setting the structure after cooling to be a ferrite and pearlite structure, the yield point is lowered and the tensile strength is improved, whereby a hot rolled steel sheet member having a low yield ratio and high strength can be obtained.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION First, the reasons for limiting the components of the steel used in the present invention will be explained. All units are% by weight. C: 0.05 to 0.20% C is an element effective for increasing the strength of the steel sheet. 0.
If it is less than 05%, its effect is too small, while 0,20%
If it exceeds, the weldability and toughness will deteriorate. Therefore, the lower limit is 0.05% and the upper limit is 0.20%.

Si: 0.01 to 1.0% Si has a deoxidizing effect on molten steel, and improves strength and ductility by a solid solution strengthening effect in a cooling stage after hot working. 0.
If it is less than 01%, the effect is too small, while on the other hand 1.0%
If it exceeds the range, 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 1.0%.

Mn: 0.50 to 2.0% Mn, like Si, enhances the strength by solid solution strengthening by cooling after hot working. If it is less than 0.50%, such an effect is too small, while if it exceeds 2.0%, 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 2.0%.

P: 0.05% or less, S: 0.03% or less P and S deteriorate the toughness, so the upper limit of each is 0.0.
5% and 0.03%.

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 will deteriorate and the toughness will also decrease, so the upper limit is fixed. The amount of molten Al is 0.08%.

N: 0.01% or less N deteriorates ductility and weldability, so its upper limit is 0.0.
1%.

The steel used in the present invention is formed by the balance Fe and unavoidable impurities in addition to the above-mentioned components, but in order to further improve strength, toughness and the like, one or more of the following components may be further added if necessary. Can be included.

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

Mo: 0.40% or less Mo, like Cr, is effective in increasing the strength of steel,
In particular, it is effective in improving the strength characteristics after reheating and the high temperature strength characteristics. However, if added in a large amount, the weldability deteriorates,
In addition, the cost increases, so the upper limit is set to 0.40%.

Nb: 0.08% or less Nb is effective in increasing the strength of steel by precipitation hardening and is also effective in reducing the grain size. However, if a large amount is added, the weldability deteriorates, so the upper limit is 0.08%.
And

V: 0.30% or less V 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.
Addition of a large amount deteriorates weldability, so the upper limit is 0.3.
0%.

Ti: 0.10% or less Ti is effective in increasing the strength of steel, and Ti
Forming N is effective in making austenite grains finer, and is effective in improving toughness. However, if a large amount is added, the weldability is deteriorated, so the upper limit is made 0.10%.

B: 0.0005 to 0.0050% B is effective in 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 the above range, such an effect is too small, and if it is added in a large amount, a B compound is formed and the toughness is deteriorated, so the upper limit is made 0.0050%.

Ca: 0.006% or less Ca is effective in reducing mechanical anisotropy and improving ductility and toughness by controlling the morphology of sulfides from expanded to spherical. However, if a large amount is added, non-metallic inclusions increase in the steel and ductility and toughness deteriorate, so the upper limit is made 0.006%.

The hot-rolled steel sheet member of the present invention is 500-65 after hot-rolling steel having the above components at a finishing temperature of Ar ₃ or more.
After winding at 0 ° C, cold working the steel sheet, reheating to a temperature of Ac ₃ points or higher and hot forming at a temperature of Ar ₃ points or higher, then cooling to make the structure ferrite and pearlite. It is the one.

In hot rolling, the slab may be directly rolled immediately after continuous casting, or the ingot may be heated and then rolled. The heating temperature is preferably 1100 to 1300 ° C. 1100 ° C
If it is less than the above range, the solid solution of Nb becomes insufficient and the effect of Nb for improving the strength is insufficient. On the other hand, if the temperature exceeds 1300 ° C., austenite coarsens and the toughness decreases. However, even if the heating temperature is outside the above range, the intended effect of the present invention can be obtained. The finishing temperature for hot rolling is Ar ₃
Must be above the point. If the Ar is less than ₃ points, the processed structure remains, mixed grains are formed, and the anisotropy becomes remarkable.

The winding temperature must be 500 to 650 ° C. If it is less than 500 ° C., it will be overcooled and will have a two-phase structure of ferrite and bainite, the strength will remarkably increase, the ductility and toughness will deteriorate, and eventually the cold workability will deteriorate. On the other hand, if the winding temperature exceeds 650 ° C., the strength decreases and the grain boundary oxidation causes the surface properties to deteriorate remarkably.

The rolled steel sheet is unwound and formed into a predetermined shape by an appropriate cold working method. In the case of a square tube, cold pipe forming or welding is applied.

The cold worked steel sheet is then reheated to soften it. At this time, the heating temperature must be Ac ₃ or higher. By completely austenitizing, the strain remaining after cold working, that is, the effect of recovering dislocations is exerted, and further the structure is refined and the toughness needs to be raised to a heating temperature of Ac ₃ or higher. . On the other hand, if the Ac is less than ₃ points, the structure becomes a mixed grain, and improvement in toughness cannot be expected.

Then, after reheating, the desired shape is formed by hot working. At this time, the hot working temperature must be Ar ₃ or higher. If the Ar is less than ₃ points, the austenite will be excessively stretched, the structure will be mixed grains, and improvement in toughness cannot be expected.

Cooling after hot working is carried out by air cooling or blast cooling, and the structure is a mixed structure of ferrite and pearlite (including pseudo pearlite). With such a structure, the yield ratio can be lowered.

[0032]

EXAMPLES Various steels having the components shown in Table 1 were melted, and Table 2
After hot-rolling under the hot-rolling conditions shown in (1) and winding to produce a hot-rolled steel sheet having a plate thickness of 12 mm, and performing cold rolling with a rolling reduction of 7%,
After heating at 900 ° C for 10 min, 10% hot working at the same temperature (specifically, hot rolling with a reduction ratio of 10% by an actual machine).
Was given. The Ar ₃ points of the steels in Table 1 are about 740 to 810 ° C., and the Ac ₃ points are about 820 to 890 ° C., both the finishing temperature of hot working of the billet and the heating temperature after cold working are Ar. ₃ points and Ac ₃ points or more. The maximum amount of strain when processing a pipe having a circular cross section into a rectangular shape is equal to or less than the amount of strain generated by the hot working.

[0033]

[Table 1]

[0034]

[Table 2]

Tensile test pieces and impact test pieces were sampled from the steel sheet member after hot working, and the tensile properties and impact properties were investigated.
The results are also shown in Table 2. From Table 2, in Examples (Sample Nos. 1 to 3 and 7 to 13) using the steel of the present invention,
Tensile strength after hot working is 490 N / mm ² or more, YR
It can be seen that the (yield ratio) is 70% or less, the strength is high, the yield ratio is low, and the toughness is excellent.

On the other hand, the comparative examples (Sample Nos. 4 to 6) using steel grades outside the composition range of the steel of the present invention have low tensile strength after hot working. Although the comparative examples of Sample Nos. 14 and 15 use the steel of the present invention, since the winding temperature exceeds the lower limit and the upper limit of the range of the present invention, the strength of the hot-rolled steel sheet is remarkably low in Sample No. 14. Rises, making cold work difficult. Further, in Sample No. 15, the tensile strength after hot working is low.

As described above, according to the hot-rolled steel sheet member of the present invention, it has a high strength of 490 N / mm ² or more after hot working and a low yield ratio of 70% or less and impact characteristics. Is also good, and the present invention is suitable as a structural high strength steel plate member.

Claims (2)

[Claims] 1. By weight%, C: 0.05 to 0.20% Si: 0.01 to 1.0% Mn: 0.50 to 2.0% P: 0.05% or less S: 0.0. 03% or less sol.Al: 0.08% or less N: 0.01% or less, with the balance being Fe and inevitable impurities, after hot rolling at a finishing temperature of Ar ₃ points or more, 500 to 650 ° C. And then cold working the steel sheet, reheating to a temperature of Ac ₃ points or higher and hot forming at a temperature of Ar ₃ points or higher, then cooling to make the structure ferrite and pearlite. A high-strength hot-rolled steel sheet member having a low yield ratio. 2. The composition according to claim 1, further comprising, by weight%, Cr: 0.05 to 0.80% Ni: 0.50% or less Mo: 0.40% or less Nb: 0.08% or less. V: 0.30% or less Ti: 0.10% or less B: 0.0005 to 0.0050% Ca: 0.006% or less One or more of the following is contained, and the low yield ratio is high. High-strength hot-rolled steel plate member.