JPH0649541A - Production of low yield ratio building steel pipe by cold forming - Google Patents

Abstract

PURPOSE:To produce a low yield ratio steel pipe with high productivity by subjecting a hot rolled steel sheet having a specified compsn. as stock to normalizing treatment, thereafter working it into a steel pipe and executing normalization again at the time of producing a steel pipe from a steel sheet by a cold working method. CONSTITUTION:The hot rolled steel sheet having a compsn. contg., by weight, 0.01 to 0.12% C, <0.5% Si, 0.9 to 1.6% Mn, <0.03% P, <0.01% S, 0.005 to 0.05% Nb, 0.005 to 0.025% Ti, <0.1% Al and <0.006% or furthermore contg. one or >= two kinds among 0.05 to 1.5% Cu, 0.05 to 2.0% Ni, 0.05 to 1.0% Cr, 0.05 to 1.0% Mo, 0.005 to 0.10% V and 0.001 to 0.006% Ca is subjected to air cooling or water cooling after hot rolling. Next, this steel sheet is reheated at its Ac3 transformation point or above and is normalized to refine its compsn., and after that, it is subjected to cold working into the steel pipe. At this time, between the thickness (t) of the steel sheet and the outside diameter D of the steel pipe, the relationship of t/D <=10% is given, and the produced steel pipe is reheated to 700 to 850 deg.C and is subjected to normalizing treatment.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art Generally, when cold working is applied to steel, YP and TS rise due to work hardening, and YP and TS are higher than TS.
The yield ratio (hereinafter referred to as YR) also increases due to the large increase in the steel sheet, and the cold-formed steel pipe has a drawback that it is difficult to apply to a building structure because the plastic deformation ability after yielding is small.

On the other hand, methods for producing low YR steel pipes include centrifugal casting, quenching with steel pipes, tempering, etc. Centrifugal casting is a method of quenching steel pipes because of its low productivity and economical efficiency. However, tempering was inferior to steel pipes manufactured by cold forming of steel plates in terms of economic efficiency and dimensional accuracy of 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 plate. The steel pipe manufactured based on the method of the present invention has low YR and high productivity, economy and dimensional accuracy.

[0005]

Means for Solving the Problems The present invention overcomes the above-mentioned problems and achieves the object, and its concrete means are shown in (1) and (2) below.

(1) C 0.01 to 0.12 by weight
%, Si 0.5% or less, Mn 0.9 to 1.6%, P
0.03% or less, S 0.01% or less, Nb 0.0
05-0.05%, Ti 0.005-0.025%,
Containing Al 0.1% or less and N 0.006% or less,
A steel sheet, the balance of which is iron and inevitable impurities, is hot-rolled and then air-cooled or water-cooled, reheated to a temperature of Ac ₃ or higher and normalized to obtain 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 normalize and plate thickness 100 mm or less, YR A method of manufacturing a steel pipe with a low yield ratio of 80% or less.

(2) C 0.01 to 0.12 by weight ratio
%, Si 0.5% or less, Mn 0.9 to 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
0.005-0.10%, Ca 0.001-0.0
Steel containing 06% of one kind or two kinds or more, the balance of which is iron and unavoidable impurities, is hot-rolled, air-cooled or water-cooled, and then reheated to a temperature of Ac ₃ or higher for normalizing. T / D (t: plate thickness, D: steel pipe outer diameter)
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, and a plate thickness of 100 mm or less, YR is 80.
% Yield of low yield ratio steel pipe for construction.

[0008]

The present invention will be described below.

According to the research conducted by the inventors, Y after cold working
It has been found that it is necessary to combine the optimization of the components of the steel sheet and the appropriate heat treatment (normalization) after cold working in order to reduce R.

Therefore, the main points of the present invention are (1) the composition of the steel sheet to be subjected to cold working and the limitation of the manufacturing method, and (2) the material control technology by heat treatment after cold working of the steel sheet.

First, the reason for limiting the component range will be described.

C is necessary to secure the strength of the base material, but if it is contained in a large amount, heat treatment (2
Significant deterioration of toughness occurs during normalizing in the phase range). From such a viewpoint, C is set to 0.01 to 0.12%.

Si is an element that is inevitably contained in steel for deoxidation. However, since Si is an unfavorable element in HAZ toughness and weldability, its upper limit was set to 0.5%.

Mn is an extremely important element for simultaneously improving strength and toughness, and 0.9% or more is necessary, but
If a large amount is added, the weldability and the toughness of the base material and HAZ are deteriorated, so the upper limit was made 1.6%.

The reason why the impurities P and S in the steel of the present invention are set to 0.03% and 0.01% or less, respectively, is to further improve the low temperature toughness of the base material and the welded portion. Reduction of P prevents grain boundary destruction, and reduction of S amount reduces MnS
To prevent deterioration of toughness. The preferred P and S contents are 0.01% and 0.005% or less, respectively.

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

Ti forms carbonitrides and improves HAZ toughness. When the amount of Al is small, an oxide of Ti is formed to improve HAZ toughness. If the amount of Al is small, Ti
Form oxides and improve HAZ toughness, but 0.00
Less than 5% has no effect, and more than 0.025% HA
Since it has an unfavorable effect on Z toughness, it is 0.005
It is limited to 0.025%.

Al is generally an element contained in deoxidized upper steel, but since deoxidation is also performed by Si and Ti, the lower limit of the steel of the present invention is not limited. However, if the amount of Al increases, the cleanliness of the steel deteriorates and the toughness of the welded portion deteriorates, so the upper limit was made 0.1%. N is generally contained in steel as an unavoidable impurity, but it combines with Nb and V to form a carbonitride to increase the strength, and forms TiN to form HAZ as described above. Increase. Therefore N
A minimum amount of 0.001% is required. However, if the amount of N increases, the HAZ toughness deteriorates and the surface defects of the continuous casting slab are generated. Therefore, the upper limit is 0.00.
It was 6%.

The basic components of the steel of the present invention are as described above, and the object can be sufficiently achieved. However, in order to further improve the characteristics for the purpose, the elements described below, namely Cu, Ni, C
By selectively adding r, Mo, V, and Ca, more preferable results can be obtained with respect to improvement in strength and toughness.

Next, the above-mentioned additional element and its addition amount will be explained.

Ni improves the strength and toughness of the base metal without adversely affecting the weldability and HAZ toughness.
If it is 5% or less, the effect is small, and if it is 2.0% or more, it is extremely expensive and the economy is lost. Therefore, the upper limit was made 2.0%.

Cu has almost the same effect as Ni,
It also has the effect of increasing strength and improving corrosion resistance and weather resistance due to Cu precipitates. 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 strength and toughness of the base metal,
In particular, it is an element effective for lowering YR after heat treatment in the two-phase region.
If it is less than 0.05%, the effect is small, and if it exceeds 1.0%, the weld zone toughness and weldability are deteriorated, which is not preferable.
It is limited to 05 to 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, but when it exceeds 1.0%, the weldability and HAZ toughness are deteriorated, and If it is less than 05%, the effect is small. Therefore, the Cr content is 0.05 to 1.0%.

V is an element having almost the same effect as Nb, but its precipitation hardening ability is slightly inferior to that of Nb. 0.005
% Or less, the hardening is small, and if it exceeds 0.10%, the toughness deteriorates in the heat treatment after cold forming.

Ca controls the morphology of sulfide (MnS),
It has the effect of increasing Charpy absorbed energy and improving low temperature toughness. However, if the amount of Ca is less than 0.001%, there is no practical effect, and if it exceeds 0.006%, CaO, C
A large amount of aS is generated and becomes large inclusions, which not only impairs the toughness of steel but also the cleanliness and adversely affects the weldability and lamella tear resistance. Therefore, the range of Ca addition amount is 0.001 to 0.0
It is set to 06%.

In the method for producing a steel sheet, the steel having the above-mentioned limited components is hot-rolled, air-cooled or water-cooled, and then reheated to a temperature of Ac ₃ or higher for normalizing. In this case, the required characteristics can be obtained by cooling after hot rolling by air cooling or water cooling, but water cooling is preferable from the viewpoint of improving the toughness due to the fine graining of the structure.

Next, the heat treatment (normalizing) temperature after cold forming (t / D ≦ 10%) is used to sufficiently release the strain in cold working, to lower YR and increase strength. The lower limit temperature is 700 ° C. Further, normalizing at a temperature that is too high causes not only release of cold strain but also insufficient strength and increase in YR, so 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 and subjected to normalizing heat treatment, and its strength and toughness were investigated.

Tables 1 to 8 show the chemical compositions of the present invention steels, and 9 to 16 show the chemical compositions of the comparative steels. In Table 1, steels 1 to 4 are TS60
The target is 0 N / mm ² class, steel 5-8TS 800 N / mm ² class.

Table 2 shows the steel plate manufacturing conditions and the mechanical properties of the steel of the present invention and the comparative steel.

Steels 1 to 8 of the present invention shown in Table 2 are strengths in steel pipes,
The toughness is achieved in a well-balanced manner, and the YR is 80% or less.

On the other hand, Comparative Steel 9 has a high C content, so that the toughness of the steel pipe is deteriorated. Comparative steel 10 has a low Mn,
Low strength in steel pipe. The comparative steel 11 has a high Mn and deteriorates in toughness. Since the comparative steel 12 does not contain Nb, the grain size of the crystal grains is not sufficiently reduced during rolling, and the toughness is deteriorated. Comparative Steel 13 has a high Nb, and the toughness of the steel pipe is deteriorated. The comparative steel 14 has a cold working ratio (t / D) of 12%, which is too large, and thus has a high YR. Comparative steel 15 has a low annealing temperature, so its strength is insufficient and YR
Is also getting higher. Since the comparative steel 16 has a high annealing temperature, its strength is insufficient and YR is also high.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

INDUSTRIAL APPLICABILITY The steel pipe manufactured by the chemical composition and manufacturing method of the present invention has a low YR and an excellent plastic deformation ability after yielding. As a result, the safety of structures such as buildings and bridges can be greatly improved.

Claims (2)

[Claims] 1. A weight ratio of C: 0.01 to 0.12%, Si: 0.5% or less, Mn: 0.9 to 1.6%, P: 0.03% or less, S: 0.0. 01% or less, Nb: 0.005-0.05%, Ti: 0.005-0.025%, Al: 0.1% or less, N: 0.006% or less, and the balance is iron and unavoidable. Of steel made of mechanical impurities was hot-rolled, then air-cooled or water-cooled, and reheated to a temperature of Ac ₃ or higher to normalize the steel.
(T: plate 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 normalize it. For manufacturing low yield ratio steel pipes for automobiles. 2. By weight ratio, C: 0.01 to 0.12%, Si: 0.5% or less, Mn: 0.9 to 1.6%, P: 0.03% or less, S: 0.0. 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, and 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 Steel containing 0.001 to 0.006% of 1 or 2 or more, the balance of which is iron and inevitable impurities, is hot-rolled, air-cooled or water-cooled, and reheated to a temperature of Ac ₃ or higher. T / D (t: plate thickness, D: steel pipe outer diameter) ≦ 10
Steel pipe is manufactured by cold forming in the range of
To 850 ° C. normalizing the low yield ratio steel pipe for construction.