JP3245224B2 - Manufacturing method of low yield ratio steel pipe for building by cold forming. - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a low yield ratio steel pipe by cold forming for use in various structures in the fields of construction and civil engineering.

[0002]

2. Description of the Related Art Generally, when a steel material is subjected to cold working, YP and TS rise due to work hardening, and YP and TS increase in comparison with TS.
The yield ratio (hereinafter, referred to as YR) also increases due to the large rise of the steel pipe, and the steel pipe formed by cold forming has a drawback that it is difficult to apply to a building structure because the plastic deformation capacity after yielding is small.

On the other hand, there are centrifugal casting, quenching and tempering in steel pipes as methods for producing low YR steel pipes, and centrifugal casting is quenching steel pipes in terms of low productivity and economy. However, tempering was inferior to steel pipes manufactured by cold forming of steel sheets in terms of economy and dimensional accuracy of the steel pipes.

[0004]

SUMMARY OF THE INVENTION The present invention provides a technique for manufacturing a steel pipe having a low YR by cold forming a steel sheet. The steel pipe manufactured according to the method of the present invention has low YR and high productivity, economy and dimensional accuracy.

[0005]

The present invention overcomes the above-mentioned problems and achieves the object, and specific means are shown in the following (1) and (2).

(1) C 0.01 to 0.12 in weight ratio
%, Si 0.5% or less, Mn 0.9-1.6%, P
0.03% or less, S 0.01% or less, Nb 0.0
05-0.05%, Ti 0.005-0.025%,
Al 0.1% or less, N 0.006% or less,
A steel sheet whose remaining portion is made of iron and unavoidable impurities is hot-rolled, then air-cooled or water-cooled, and re-heated to a temperature of Ac ₃ or higher to perform a normalizing steel sheet, t / D (t: sheet thickness,
D: steel pipe outer diameter) ≦ 10%, a steel pipe is manufactured by cold forming, and then reheated to a temperature range of 700 to 850 ° C. to perform normalizing, characterized by a sheet thickness of 100 mm or less and a YR of A method for producing a low yield ratio steel pipe for a building having 80% or less.

(2) C 0.01 to 0.12 in weight ratio
%, Si 0.5% or less, Mn 0.9-1.6%, P
0.03% or less, S 0.01% or less, Nb 0.0
05-0.05%, Ti 0.005-0.025%,
Al 0.1% or less, N 0.006% or less, and Cu
0.05-1.5%, Ni 0.05-2.0%, C
r 0.05-1.0%, Mo 0.05-1.0%, V
0.005 to 0.10%, Ca 0.001 to 0.0
A steel containing one or more of 0.6% and the remainder consisting of iron and unavoidable impurities is hot-rolled, then air-cooled or water-cooled, and reheated to a temperature of Ac ₃ or more for normalizing. T / D (t: plate thickness, D: outer diameter of steel pipe)
A steel pipe is manufactured by cold forming in a range of ≦ 10%, and then reheated to a temperature range of 700 to 850 ° C. to perform normalizing.
% Low-yield-ratio steel pipe for architectural use.

[0008]

The present invention will be described below.

According to the study of the inventors, Y after cold working
In order to lower R, it has been found that it is necessary to combine steel sheet components with appropriate heat treatment (normalizing) after cold working.

Therefore, the gist of the present invention lies in (1) the limitation of the composition of the steel sheet to be subjected to cold working and the manufacturing method, and (2) the material control technique by heat treatment after cold working the steel sheet.

First, the reasons for limiting the component ranges will be described.

C is necessary to ensure the strength of the base material. However, if C is contained in a large amount, heat treatment (2) performed after cold forming.
(Temperature normalization) causes significant deterioration in toughness. From such a viewpoint, C is set to 0.01 to 0.12%.

[0013] Si is an element inevitably contained in steel for deoxidation. However, since Si is an element that is not preferable in terms of HAZ toughness and weldability, its upper limit is set to 0.5%.

Mn is a very important element that simultaneously improves the strength and toughness, and is required to be 0.9% or more.
If added in a large amount, the weldability, the base material and the toughness of the HAZ are deteriorated, so the upper limit is set to 1.6%.

The reason why the impurities P and S are set to 0.03% and 0.01% respectively in the steel of the present invention is to further improve the low-temperature toughness of the base material and the welded portion. The reduction of P prevents grain boundary destruction, and the reduction of S content is MnS
To prevent toughness degradation. Preferred P and S contents are 0.01% and 0.005% respectively.

Nb forms fine carbonitrides, increases strength, refines the structure during hot rolling, and improves HAZ toughness. However, if the content is 0.005% or less, there is no effect, and if it exceeds 0.05%, toughness is deteriorated by heat treatment after cold forming.

[0017] Ti forms a carbonitride to improve HAZ toughness. When the amount of Al is small, an oxide of Ti is formed to improve the HAZ toughness. When the amount of Al is small, Ti
To improve the HAZ toughness,
Less than 5% has no effect, and more than 0.025% HA
It has an unfavorable effect on Z toughness.
Limited to 0.025%.

Al is generally an element contained in the deoxidized upper steel, but the lower limit is not limited for the steel of the present invention because deoxidation is also performed by Si and Ti. However, when the amount of Al increases, the cleanliness of the steel deteriorates and the toughness of the welded portion deteriorates. Therefore, the upper limit was set to 0.1%. Although N is generally contained in steel as an unavoidable impurity, it combines with Nb and V to form a carbonitride to increase the strength, and forms TiN to form the HAZ as described above. Enhance. Therefore N
A minimum amount of 0.001% is required. However, an increase in the N content promotes deterioration of the HAZ toughness and generation of surface flaws in the continuously cast slab.
6%.

The basic components of the steel of the present invention are as described above, and the objectives can be sufficiently achieved. However, in order to further enhance the properties for the objectives, the following elements, namely Cu, Ni, C
When r, Mo, V, and Ca are selectively added, more favorable results can be obtained with respect to improvement in strength and toughness.

Next, the above-mentioned additional elements and the amounts thereof will be described.

Ni improves the strength and toughness of the base material without adversely affecting weldability and HAZ toughness.
If it is 5% or less, the effect is weak, and if it is 2.0% or more, it becomes extremely expensive and loses economy, so the upper limit was made 2.0%.

Cu has almost the same effect as Ni,
It is also effective in increasing the strength due to Cu precipitates and in improving corrosion resistance and weather resistance. In this case, if the Cu content exceeds 1.5%, the precipitation effect is saturated, and if it is 0.05% or less, there is no effect, so the Cu content is limited to 0.05 to 0.5%.

Mo improves both the strength and toughness of the base material,
In particular, it is an element effective for lowering the YR after the heat treatment in the two-phase region.
If the content is less than 0.05%, the effect is small.
Limited to 05-1.0%. Cr is an element that enhances the strength of the base material and the welded portion. When the Cr content is 0.5% or more, the weather resistance is also improved. However, when the Cr content exceeds 1.0%, the weldability and HAZ toughness are deteriorated. At less than 05%, the effect is weak. Therefore, the Cr content is set to 0.05 to 1.0%.

V is an element having almost the same effect as Nb, but has a slightly lower precipitation hardening ability than Nb. 0.005
% Or less, hardening is small, and if it exceeds 0.10%, toughness is deteriorated by heat treatment after cold forming.

Ca controls the form of sulfide (MnS),
This has the effect of increasing Charpy absorbed energy and improving low temperature toughness. However, if the Ca content is less than 0.001%, there is no practical effect, and if it exceeds 0.006%, CaO, C
aS is generated in large amounts to become large inclusions, which impair not only the toughness but also the cleanliness of the steel and adversely affect weldability and lamella tear resistance.
06%.

In the method for producing a steel sheet, the above-defined steel is hot-rolled, air-cooled or water-cooled, and reheated to a temperature of Ac ₃ or higher to normalize. In this case, the required properties can be obtained by cooling after hot rolling by air cooling or water cooling, but water cooling is preferable in terms of improvement in toughness due to grain refinement of the structure.

Next, the heat treatment (normalizing) temperature after the cold forming (t / D ≦ 10%) is used to sufficiently release the strain in the cold working, lower the YR, and increase the strength. The lower limit temperature is 700 ° C. Normalization at an excessively high temperature not only releases cold strain but also causes insufficient strength and an increase in YR. Therefore, the upper limit temperature is set to 850 ° C.

[0028]

EXAMPLE A steel plate was manufactured by a well-known converter, continuous casting, and thick plate process, and then a steel pipe was manufactured by cold forming, subjected to normalizing heat treatment, and its strength and toughness were investigated.

Tables 1 to 8 show the chemical compositions of the steels of the present invention and 9 to 16 show the chemical compositions of the comparative steels. In Table 1, steels 1-4 are TS60
0N / mm ² class, is obtained by the steel 5~8TS800N / mm ² class goal.

Table 2 shows the steel sheet production conditions of the steel of the present invention and the comparative steel and their mechanical properties.

The steels 1 to 8 of the present invention shown in Table 2 have strengths in steel pipes,
Toughness can be achieved in a well-balanced manner, and YR is also 80% or less.

On the other hand, in Comparative Steel 9, since C is high, the toughness of the steel pipe is deteriorated. Comparative steel 10 has low Mn,
Low strength in steel pipes. The comparative steel 11 has a high Mn and has a deteriorated toughness. Since the comparative steel 12 does not contain Nb, the crystal grains are not sufficiently refined during rolling and the toughness is deteriorated. The comparative steel 13 has a high Nb, and the toughness of the steel pipe is deteriorated. The comparative steel 14 has a high YR because the degree of cold work (t / D) is too large at 12%. The comparative steel 15 has a low annealing temperature, and thus has insufficient strength and has a low YR.
Is also higher. Since the comparative steel 16 has a high annealing temperature, the strength is insufficient and the YR is also high.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

The steel pipe produced by the chemical composition and production method of the present invention is a steel pipe having a low YR and excellent plastic deformation ability after yielding. As a result, the safety of structures such as buildings and bridges can be greatly improved.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 8/00-8/10 C21D 9/08 C22C 38/00-38/60

Claims (2)

(57) [Claims] C: 0.01 to 0.12%, Si: 0.5% or less, Mn: 0.9 to 1.6%, P: 0.03% or less, S: 0. 01% or less, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.025%, Al: 0.1% or less, N: 0.006% or less, with the balance being iron and inevitable Hot rolled steel containing natural impurities, air-cooled or water-cooled, and reheated to a temperature of Ac ₃ or higher to normalize the steel sheet, t / D
(T: plate thickness, D: outer diameter of steel pipe) A steel pipe is manufactured by cold forming within a range of ≦ 10%, and then reheated to a temperature range of 700 to 850 ° C. to normalize. Method for producing low yield ratio steel pipes. 2. C: 0.01 to 0.12%, Si: 0.5% or less, Mn: 0.9 to 1.6%, P: 0.03% or less, S: 0. 01% or less, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.025%, Al: 0.1% or less, N: 0.006% or less, Cu: 0.05 to 1. 5%, Ni: 0.05 to 2.0%, Cr: 0.05 to 1.0%, Mo: 0.05 to 1.0%, V: 0.005 to 0.10%, Ca: 0 0.001% to 0.006%, the balance consisting of iron and unavoidable impurities is hot rolled, then air-cooled or water-cooled, and reheated to a temperature of Ac ₃ or more. The normalized steel sheet is subjected to t / D (t: thickness, D: outer diameter of steel pipe) ≦ 10.
% Of the steel pipe by cold forming, and then 700
A method for producing a low yield ratio steel pipe for building, characterized by normalizing in a temperature range of from 850 ° C to 850 ° C.