JPH0387318A - Production of steel tube or square steel tube having low yield ratio - Google Patents

Abstract

PURPOSE:To produce a steel tube having low yield ratio by subjecting a steel tube made of low carbon steel to heating up to a temp. in a specific region and then to water cooling at specific cooling velocity. CONSTITUTION:A steel tube (or square steel tube) composed of low carbon steel or low carbon low alloy is heated up to a temp. in the region between (Ac3-250 deg.C) and (Ac3-20 deg.C) and water-cooled at >=30 deg.C/sec cooling rate. Moreover if the improvement of toughness is required, the tube is further tempered at 200-600 deg.C. By this method, the steel tube (or square steel tube) reduced in yield ratio and having strength as high as >= about 50kgf/mm<2> can be obtained at a low cost.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method of manufacturing a steel pipe or square pipe with a low yield ratio.

(Prior Art) In recent years, various demands have been increasing across various fields that handle steel materials, such as improving usage characteristics to improve competitiveness and reducing manufacturing costs.

Among these, in the field of construction, it is desired to reduce the yield ratio in order to improve the safety of structures, especially in order to improve their seismic resistance. Until now, this requirement has been strong mainly in the thick plate field, but recently this requirement has become stronger in the steel pipe field. Various methods have been studied for manufacturing thick steel plates with a low yield ratio, but unfortunately, in the field of steel pipes, there are currently almost no examples of such methods being studied, at least for construction purposes. For example, ERW steel pipes are manufactured by forming hot coils, but the yield ratio increases due to work hardening during forming.
It is considered to be a disadvantageous manufacturing method for manufacturing steel pipes with a low yield ratio. For example, there is a method for manufacturing ERW steel pipes for oil wells with a low yield ratio in JP-A-57-16118, but in this method a considerable amount of C is added to achieve a low yield ratio (C amount: 0. 26~0.48%), Ceq from the viewpoint of weldability
It cannot be applied to architectural structures where upper limits are specified. Similarly, there is Japanese Patent Application Laid-Open No. 57-16119 as a manufacturing method for low yield ratio, high tensile resistance welded steel pipes, but this method involves manufacturing extremely low YRm at the hot coil stage, and In order to suppress work hardening, the amount of strain is considerably limited.
In actual operation, it is quite difficult.

(Problem to be solved by the invention) As a low yield ratio steel pipe for construction or a square pipe, the tensile strength is 4.
There is a requirement for a yield ratio of 75% or less at the 0-60 kg level, but this is impossible with current manufacturing methods. In other words, in the non-tempered type, the so-called azurol type, which is manufactured by simply forming a hot coil into a round shape, the structure is due to work hardening during forming, and in the tempered type, so-called QT type, the structure becomes tempered martensite. , a yield ratio of 75% or less has not been achieved.

(Means for Solving the Problems) Therefore, the present inventors conducted numerous experiments and detailed studies in order to reduce the yield ratio. The necessity of creating a two-phase structure consisting of ferrite and a second phase of carbide was confirmed. Furthermore, it was confirmed that in order to lower the yield ratio, it is important to lower the yield point and increase the tensile strength.

Based on this knowledge, the present invention has made it possible to manufacture steel pipes or square pipes with a low yield ratio.
3-250~Ac, heated to -20℃, then heated to 30℃
A method for manufacturing a steel pipe (or square pipe) with a low yield ratio, characterized by water cooling at a cooling rate of at least C/sec, and then tempering at a temperature range of 200 to 600 C, if necessary.

(Function) In the present invention, by setting the heating temperature to a high level between Ac and Aca transformation points and then cooling with water, the effect of work hardening on the vibrator bottom shape and subsequent square tube forming is removed, and
We have succeeded in achieving dual phase steel.

Furthermore, by lowering the tempering temperature, the synergistic effect of not softening the second phase part more than necessary,
This makes it possible to manufacture steel pipes (or square pipes) with a low yield ratio.

Next, the conditions for steel pipe forming, square pipe forming, heating, cooling, and tempering of the present invention will be described.

First, regarding steel pipe forming and subsequent square tube forming,
There are no particular regulations, and any molding is acceptable. For example, steel pipes are manufactured using different manufacturing methods, such as seamless steel pipes, electric resistance welded steel pipes,
It can be classified into UO steel pipes, spiral steel pipes, forge welded pipes, etc.
The present invention is acceptable with any of these manufacturing methods. Of course, it is also acceptable to directly form a rectangular tube from a plate such as a hot coil and weld it by electric resistance welding. This is to set the heating temperature in the subsequent heat treatment to a temperature at which processing strain is removed.

Next, the heating temperature after molding is set to Ac5-250~Ac5-20t.
: By heating to this temperature range,
This is because the aim was to simultaneously achieve dual-phase steel after cooling and eliminate forming distortion. That is, if the steel is heated directly above it and then cooled with water, it becomes a two-phase steel, but the strength of the ferrite is high because of residual processing strain in the ferrite, and as a result, a low yield ratio cannot be achieved. This temperature was set as the lower limit because it is possible to achieve both duplex steel formation and strain removal by heating the steel to a higher temperature than the intermediate temperature between Ac and Ac3, or even Ac, to a higher temperature than -250 t. As the heating temperature is increased, the yield ratio passes through the lowest limit and the yield ratio increases. This is because the area ratio of ferrite decreases, and as it approaches Ac, the yield ratio increases rapidly. This is because the area ratio of ferrite approaches zero. From this, Ac3-20°C was set as the upper limit of the heating temperature.

Ac3-250 ~ Ac, water cooling after heating to -20°C changes the part into austenite during reheating and hardens the C-enriched part into a quenched structure, increasing the tensile strength and obtaining a low yield ratio. It's for a reason. If the water cooling is insufficient, the quenched structure will not harden sufficiently and, as a result, a low yield ratio will not be obtained. Therefore, the cooling rate was specified to be 30° C./sec or more.

By the way, depending on the type of steel, there are some steels whose toughness is not good only by water cooling after heating, and it may be necessary to perform tempering treatment after water cooling in order to improve the toughness. At that time, the tempering temperature should be determined as follows: Regarding the two-phase structure of ferrite and second phase carbide, if the second phase part, which has been sufficiently hardened by previous water cooling, is tempered at too high a temperature, it will become too soft, resulting in a decrease in tensile strength. Since this causes an increase in yield ratio, the upper limit was set at 600°C. However, if the tempering temperature is too low to be less than 200°C, the tempering effect will be almost gone and the toughness may not be improved, so the lower limit was set at 200°C.

The method of the present invention can be applied to low carbon steel or low carbon low alloy steel to which special elements are added with good results. Preferred component compositions include: C: 0.05-0.30% Si: 0.02-0.50% Mn: 0.5 (1-2.00% A): 0.001-0.100% N : Depending on the required characteristics of low carbon steel whose basic component is 0.0005 to 0.0100%, or strength steel in addition to the above basic components, Cu: 2.0% or less Ni: 9.5% or less Cr: 5. 5% or less Mo: 2.0% or less Nb: 0.15% or less V: 0.3% or less Ti: 0.15% or less B: 0.0QO3 to 0.0030% Ca: 0.
1f below 0080%! Alternatively, two or more kinds may be added.

Cu is added because it is useful for increasing strength and improving corrosion resistance, but 2
．． Even if it is added in excess of 0%, there is almost no increase in strength, so the upper limit of the content is set at 2.0%.

Ni is added because it is useful for improving low-temperature toughness, but since it is an expensive element, the upper limit of the content is 9.5%.

Or is added because it is useful for increasing strength and improving corrosion resistance, but if too much, it impedes low temperature toughness and weldability, so the content is 5.
The upper limit is 5%.

MO is useful for increasing strength, but if too much MO impedes weldability, so the upper limit of the content is 2.0%.

Nb is added because it is useful for refining austenite grains and increasing strength, but if too much, it impedes weldability, so the upper limit of the content is set to 0.15%.

(2) is useful for precipitation strengthening, but if too large, it impedes weldability, so the upper limit of content is 0.3%.

Ti is added because it is useful for refining austenite grains, but if it increases, it inhibits weldability, so the content is limited to 0.3%.
is the upper limit.

B has the effect of significantly increasing the hardenability of steel when added in a small amount. In order to effectively obtain this effect, it is necessary to add at least 0.0003%. However, if added in excess, B compounds are generated and the toughness is deteriorated, so the upper limit is set to 0.0030%.

Ca is added because it is useful for controlling the morphology of sulfide-based inclusions, but if the amount is too large, it will shape the inclusions in the steel and deteriorate the properties of the steel, so the upper limit of the content is set at o, ooa%.

(Example) Table 1 shows the chemical composition of the test materials, and Table 2 shows the size of the steel pipe or square pipe, heat treatment conditions, and mechanical properties of the obtained steel pipe.

Steel pipe No., AI, Bl, C1゜Dl, E shown in Table 2
l, Fl, Gl, Hl, It, Jl.

Kl, Ll, Ml, Nl, 01. Pi, QlR1, St
, TI, 01. Vl is a steel according to the present invention, and is distantly related to the low yield ratio (yield ratio of 70% or less) targeted by the present invention.

On the other hand, A2 has a high yield ratio because the heating temperature is too high. A3 has a high yield ratio because the heating temperature is too low. A4 has a high yield ratio because the cooling rate after heating is insufficient. A5 has a high yield ratio because the tempering temperature is too high.

Further, B2 has a high yield ratio because the tempering temperature is too high. C2 has a high yield ratio due to insufficient cooling rate. B2 has a high yield ratio because the heating temperature is too low.

(Effects of the Invention) As explained in detail above, the present invention enables low-yield ratio steel pipes or square pipes with high strength of 50 Jf/mm2 or more to be manufactured at low cost without using particularly expensive alloying elements. As a result, the industrial effects are significant.

4 others

Claims (1)

[Claims] 1. A yield ratio characterized by heating a low carbon steel or a low carbon low alloy steel pipe to Ac_3-250-Ac_3-20°C and subsequently cooling it with water at a cooling rate of 30°C/sec or more. A method of manufacturing low-cost steel pipes. 2 Low carbon steel or low carbon low alloy steel pipe is heated to Ac_3-250 to Ac_3-20°C, then water-cooled at a cooling rate of 30°C/sec or more, and then heated to 20°C.
A method for manufacturing a steel pipe with a low yield ratio, characterized by tempering at a temperature range of 0 to 600°C. 3 Low carbon steel or low carbon low alloy square tube is heated to Ac_3-250 to Ac_3-20°C and then water-cooled at a cooling rate of 30°C/sec or more. Production method. 4. Low carbon steel or low carbon low alloy steel pipe is heated to Ac_3-250 to Ac_3-20°C, then water-cooled at a cooling rate of 30°C/sec or more, and then heated to 20°C.
A method for manufacturing a square tube with a low yield ratio, characterized by tempering at a temperature range of 0 to 600°C.