JPH07233416A - Production of building low yield ratio ts (tensile strength) 590n/mm2 class steel tube by cold forming - Google Patents

Abstract

PURPOSE:To secure a low yield ratio and to improve the productivity and economical efficiency by limiting working conditions at the time of tube making using a hot rolled steel tube with a specified compsn. subjected to hardening and tempering treatment. CONSTITUTION:A steel having a compsn. contg., by weight, 0.04 to 0.11% C, <=0.5% Si, 0.6 to 1.6% Mn, <=0.03% P, <=0.1% S, 0.7 to 1.2% Cu, 0.30 to 1.00% Ni, 0.05 to 0.50% Cr, 0.05 to 0.30% Mo, 0.005 to 0.25% Ti, <=0.06% Al and 0.001 to 0.006% N, furthermore satisfying the conditions in Di shown by the formula, and the balance Fe with inevitable impurities is subjected to hot rolling and is thereafter subjected to air cooling or forced cooling. After that, it is reheated to a temp. of the Ac3 or above and is thereafter subjected to hardening or hardening and tempering. By the use of this steel plate, it is formed into a steel tube by cold forming in the range of t/D (t: plate thickness and D: the outer diameter of the steel tube) <=10%, which is thereafter reheated to the temp. range of 650 to 750 deg.C and is annealed.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a low yield ratio TS59 by cold forming used for various structures in the fields of construction and civil engineering.
The present invention relates to a method for manufacturing a 0 N / mm ² class steel pipe.

[0002]

2. Description of the Related Art Generally, when cold working is applied to a steel material, YP (yield point) and TS rise due to work hardening,
At that time, since the increase in YP is larger than that in TS, the yield ratio (hereinafter referred to as YR) also increases. Therefore, the cold-formed steel pipe has a small plastic deformation ability after yielding, and is difficult to apply to a building structure.

On the other hand, as a method of manufacturing a low YR steel pipe, there are a centrifugal casting method, a method of quenching and tempering a steel tube, and the like.
The centrifugal casting method is low in productivity and economical efficiency, and the method of quenching and tempering a steel tube is economical and dimensional accuracy of the steel tube. It was inferior in comparison.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for manufacturing a steel pipe having a low YR by cold forming. The steel pipe manufactured according to the present invention has a low YR.
And has high productivity, economy and dimensional accuracy.

[0005]

Means for Solving the Problems The present invention solves the above-mentioned problems and achieves the object, and the gist thereof is as follows. (1) C: 0.04 to 0.11% by weight, Si: 0.
5% or less, Mn: 0.6 to 1.6%, P: 0.03% or less, S: 0.01% or less, Cu: 0.7 to 1.2%, N
i: 0.30 to 1.00%, Cr: 0.05 to 0.50
%, Mo: 0.05 to 0.30%, Ti: 0.005 to
0.025%, Al: 0.06% or less, N: 0.001
˜0.006%, and further Di≡0.367√
C (1 + 0.7Si) (1 + 3.33Mn) (1 + 0.
35Cu) (1 + 0.36Ni) (1 + 2.16Cr)
(1 + 3Mo) (1 + 1.7V) (1 + 1.77Al)
= 1.5 to 2.5, and hot rolling the steel with the balance being iron and unavoidable impurities, followed by air cooling or controlled cooling and reheating to a temperature of Ac ₃ or higher for quenching or quenching, After tempering, the obtained steel sheet was t / D (t:
Cold forming characterized by cold forming in the range of plate thickness, D: steel pipe outer diameter) ≦ 10% to produce a steel pipe, and then reheating to a temperature range of 650 to 750 ° C. to normalize Manufacturing method of low yield ratio TS (tensile strength) 590 N / mm ² class steel pipe for building by molding.

(2) C: 0.04 to 0.11 by weight ratio
%, Si: 0.5% or less, Mn: 0.6 to 1.6%,
P: 0.03% or less, S: 0.01% or less, Cu: 0.
7-1.2%, Ni: 0.30-1.00%, Cr:
0.05 to 0.50%, Mo: 0.05 to 0.30%,
Ti: 0.005 to 0.025%, Al: 0.06% or less, N: 0.001 to 0.006%, and
Nb: 0.005-0.03%, V: 0.01-0.0
5%, Ca: 0.001 to 0.006% of 1 type or 2
Containing more than one species, and further, Di≡0.367√C (1+
0.7Si) (1 + 3.33Mn) (1 + 0.35C
u) (1 + 0.36Ni) (1 + 2.16Cr) (1+
3 Mo) (1 + 1.7 V) (1 + 1.77 Al) = 1.
Steel satisfying 5 to 2.5 and the balance being iron and unavoidable impurities is hot-rolled, then air-cooled or controlled-cooled, and reheated to a temperature of Ac ₃ or higher for quenching or quenching, tempering, T / D (t: plate thickness,
D: Steel pipe outer diameter) cold-formed within a range of ≦ 10% to produce a steel pipe, and then reheat to a temperature range of 650 to 750 ° C. to normalize and construct by cold forming Low yield ratio TS (tensile strength) 590N / mm ² class steel pipe manufacturing method.

[0007]

The present invention will be described in detail below. As a result of various studies, the inventors of the present invention need to combine the optimization of the components of the steel sheet with an appropriate heat treatment (normalizing) after cold working in order to reduce YR after cold working. I found that there is.

The main points of the present invention are (1) the limitation of the components and manufacturing method of the steel sheet to be subjected to cold working, and (2) the material control technology by heat treatment after cold working of the steel sheet. First, the reason for limiting the component range of the starting steel of the present invention will be described. C is necessary to secure the strength of the base material, but if it is contained in a large amount, it is a heat treatment (normalizing) performed after cold forming.
Causes significant deterioration in toughness. From such a viewpoint, the C content is set to a range of 0.04 to 0.11%.

Since Si is an element unfavorable in terms of weldability and HAZ (heat affected zone) toughness, its upper limit was made 0.5%. Further, Al and Ti alone are sufficient for deoxidizing steel,
Si does not necessarily have to be added. Mn is an essential element for securing strength and toughness, and its lower limit is 0.6.
%. However, if the Mn content is too large, the weldability and the toughness of the base material and HAZ deteriorate, so the upper limit was made 1.6%.

In the starting steel of the present invention, the reason why the impurities P and S are 0.03% or less and 0.01% or less, respectively, is to further improve the low temperature toughness of the base material and the welded portion. . A decrease in the amount of P prevents grain boundary fracture, and a decrease in the amount of S prevents deterioration of toughness due to MnS. The preferable amounts of P and S are 0.01% or less and 0.005% or less, respectively.

Cu is required to be at least 0.7% in order to obtain characteristics as a TS590N / mm ² class steel pipe. However, addition of Cu in excess of 1.2% makes it difficult to sufficiently reduce YR, so the upper limit was made 1.2%. Ni improves the strength and toughness of the base metal without adversely affecting the weldability and HAZ toughness, and is also effective in preventing Cu-cracks. However, the Ni content is 0.30
If it is less than 0.1%, the effect is not obtained, and if it exceeds 1.00%, since Ni is an extremely expensive element and the economic efficiency is impaired, the upper limit was made 1.00%.

[0012] Cr is an element that enhances the strength of the base material and the welded portion, and at least 0.05% is necessary. However, if the amount of Cr is too large, the weldability and HAZ toughness are significantly deteriorated, so the upper limit was made 0.50%. Mo is an element that improves both strength and toughness, and it is necessary to add 0.05% or more to the TS590N / mm ² class steel pipe. But,
If the amount of Mo is too large, the weldability and HAZ toughness are unfavorable, so the upper limit was made 0.30%.

Ti forms carbonitrides and improves HAZ toughness. When the amount of Al is small, an oxide of Ti is formed to improve the HAZ toughness, but when the amount of Ti is less than 0.005%, there is no effect, and when it exceeds 0.025%, the HAZ is increased.
Since it has an unfavorable effect on toughness, its addition range is limited to 0.005 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 is not limited in the present invention. 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 0.06%.

N is generally contained in steel as an unavoidable impurity, but it is combined with Nb and V to form a carbonitride to increase strength, and TiN is formed to form the above-mentioned material. Enhances the properties of HAZ. Therefore, the N content must be at least 0.001%. However, if the amount of N increases, it deteriorates the HAZ toughness and the occurrence of surface flaws in the continuously cast slab, so the upper limit was made 0.006%.

The basic components of the starting steel of the present invention are as described above, and the objects can be sufficiently achieved by this, but the elements described below, that is, Nb, V and Ca.
By selectively adding, more preferable results can be obtained with respect to the improvement of strength and toughness characteristics. The added amount of the selective addition element will be described below.

Nb forms fine carbonitrides to increase strength and improve HAZ toughness. However, when the amount of Nb is less than 0.005%, the effect is small, and 0.03%
When it exceeds, it becomes difficult to sufficiently reduce YR.
V is an element having almost the same effect as Nb, but its precipitation hardening ability is slightly inferior to that of Nb. If the V content is less than 0.01%, the effect is small, and if it exceeds 0.05%, it becomes difficult to sufficiently reduce YR.

Ca controls the morphology of sulfide (MnS),
It has the effects of increasing Charpy absorbed energy and improving low temperature toughness. However, the amount of Ca is 0.001%
If it is less than 0.006%, there is no practical effect, and if it exceeds 0.006%, a large amount of CaO and CaS is formed to become a large inclusion,
Not only the toughness of steel but also the cleanliness is impaired, and the weldability and lamella tear resistance are adversely affected. Therefore, the range of the amount of Ca added is set to 0.001 to 0.006%.

Also, the YR of the steel pipe is set to a predetermined value (80%).
In order to achieve the following, it is necessary to adjust the hardenability of steel to an appropriate range, and therefore Di≡0.367√
C (1 + 0.7Si) (1 + 3.33Mn) (1 + 0.
35Cu) (1 + 0.36Ni) (1 + 2.16Cr)
(1 + 3Mo) (1 + 1.7V) (1 + 1.77Al)
It is necessary to adjust within the range of 1.5 to 2.5 with the index indicated by. The reason is that if this value is less than 1.5, the YR will not be sufficiently reduced by the normalizing heat treatment described below, and if it exceeds 2.5, the TS590N / mm ² class steel pipe will have excessive strength and a significant deterioration in toughness. This is because.

The method for producing a steel sheet is as follows. After hot-rolling the steel limited to the above components, air cooling or controlled cooling is performed to obtain an Ac
Reheat to a temperature of ₃ or higher and quench or quench and temper. In this case, as for the cooling after hot rolling, the required characteristics can be obtained by either air cooling or controlled cooling, but controlled cooling is preferable from the viewpoint of improving the toughness due to grain refinement of the structure. Next, the heat treatment (normalizing) temperature after cold forming (t / D ≦ 10%) has a lower limit temperature of 650 in order to sufficiently release the strain in cold working and lower YR. ℃. Further, normalizing at a temperature that is too high causes not only release of cold strain but also insufficient strength, particularly significant decrease in YP due to decrease in precipitation hardening of Cu, so the upper limit temperature is set to 75.
Set to 0 ° C.

[0020]

EXAMPLE A steel plate was manufactured by a known converter, continuous casting, and thick plate process, and its strength and toughness were investigated. 1 of Table 1
7 shows the chemical composition of the present invention, and 8-15 shows the chemical composition of the comparative example. Table 2 shows the steel pipe manufacturing conditions of the present invention example and the comparative example and their mechanical properties.

[0021]

[Table 1]

[0022]

[Table 2]

Inventive Examples 1 to 7 in Table 2 are the strength of the base material, Y
A good balance of R and toughness is achieved. On the other hand, in Comparative Example 8, since the Di value is low, YR is high. In Comparative Example 9, the C content is high, and the toughness of the steel pipe is deteriorated. In Comparative Example 10, the amount of Cu is low, and the strength of the steel pipe is insufficient. Further, in Comparative Example 11, the amount of Cu is high and the YR of the steel pipe is high. In Comparative Example 12, Di
Since the value is too high, the strength of the steel pipe exceeds the specified strength and the toughness is deteriorated. In Comparative Example 13, the normalizing temperature was 63.
Since it is as low as 0 ° C, the YR of the steel pipe is high. In Comparative Example 14, since the normalizing temperature is as high as 770 ° C., YS is significantly low and is below the standard strength. In Comparative Example 15, since t / D is as high as 12%, the YR of the steel pipe is high.

[0024]

INDUSTRIAL APPLICABILITY The steel pipe manufactured according to the present invention has a low YR and an excellent plastic deformation capacity after yielding. As a result, the safety of structures such as buildings and bridges can be greatly improved.

Claims (2)

[Claims] 1. By weight ratio, C: 0.04 to 0.11%, Si: 0.5% or less, Mn: 0.6 to 1.6%, P: 0.03% or less, S: 0.0. 01% or less, Cu: 0.7 to 1.2%, Ni: 0.30 to 1.00%, Cr: 0.05 to 0.50%, Mo: 0.05 to 0.30%, Ti: 0.005 to 0.025%, Al: 0.06% or less, N: 0.001 to 0.006%, and Di ≡ 0.367√C (1 + 0.7Si) (1 + 3.3)
3Mn) (1 + 0.35Cu) (1 + 0.36Ni)
(1 + 2.16Cr) (1 + 3Mo) (1 + 1.7V)
(1 + 1.77Al) = 1.5 to 2.5 is satisfied, and the balance is steel and iron and inevitable impurities are hot-rolled, followed by air cooling or controlled cooling to obtain Ac ₃
Reheating to the above temperature and quenching or quenching and tempering, the obtained steel sheet is subjected to cold forming within the range of t / D (t: sheet thickness, D: steel pipe outer diameter) ≤ 10% to form a steel pipe. Made,
Then, a method for manufacturing a low yield ratio TS (tensile strength) 590 N / mm ² class steel pipe for construction by cold forming, which comprises reheating to a temperature range of 650 to 750 ° C. and normalizing. 2. A weight ratio of C: 0.04 to 0.11%, Si: 0.5% or less, Mn: 0.6 to 1.6%, P: 0.03% or less, S: 0.0. 01% or less, Cu: 0.7 to 1.2%, Ni: 0.30 to 1.00%, Cr: 0.05 to 0.30%, Mo: 0.05 to 0.30%, Ti: 0.005 to 0.025%, Al: 0.06% or less, N: 0.001 to 0.006%, and Nb: 0.005 to 0.03%, V: 0.01 to 0 0.05%, Ca: 0.001 to 0.006%, one or more kinds are contained, and further, Di≡0.367√C (1 + 0.7Si) (1 + 3.3
3Mn) (1 + 0.35Cu) (1 + 0.36Ni)
(1 + 2.16Cr) (1 + 3Mo) (1 + 1.7V)
(1 + 1.77Al) = 1.5 to 2.5 is satisfied, and the balance is steel and iron and inevitable impurities are hot-rolled, followed by air cooling or controlled cooling to obtain Ac ₃
Reheating to the above temperature and quenching or quenching and tempering, the obtained steel sheet is subjected to cold forming within the range of t / D (t: sheet thickness, D: steel pipe outer diameter) ≤ 10% to form a steel pipe. Made,
Then, a method for manufacturing a low yield ratio TS (tensile strength) 590 N / mm ² class steel pipe for construction by cold forming, which comprises reheating to a temperature range of 650 to 750 ° C. and normalizing.