JPS5819439A - Production of high strength steel pipe having excellent low temperature toughness - Google Patents

Abstract

PURPOSE:To produce a high strength steel pipe having excellent low temp. toughness by subjecting a welded steel pipe contg. specific ratios of C, Si, Mn, solAl, P, S to hardening and tempering treatments under specific conditions. CONSTITUTION:The entire part of a welded steel pipe or seamless steel pipe having either or both of base material parts and weld zones contg., by wt%, <=0.30% C, <=0.80% Si, 0.5-2.0% Mn, <=0.10% solAl, <=0.025% P, <=0.015% S, and consisting of the balance Fe and unavoidable impurities is heated from the Ac3 transformation point up to the heating temp. within a 1,050 deg.C temp. range within 2min above the Ac3 transformation point and before the holding time (t) exceeds the value of the equation, the pipe is hardened. After the pipe is heated to 150-500 deg.C in <8min heating time from ordinary temp., the pipe is subjected to tempering treatment by water or air cooling before the holding time exceeds 5min.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a steel pipe that has high strength and excellent low-temperature toughness.

1. In recent years, due to the increase in global energy demand, Alaska,
Large-scale oil and natural gas fields have been discovered in places with harsh weather conditions such as Canada, the Arctic Circle, Siberia, and the North Sea.
The demand for steel pipes for oil wells and steel pipes for transportation to develop these pipes is rapidly increasing, but as the scale of development tends to increase and the environment in which they are used becomes more severe, there is a demand for steel pipes that have both high strength and high toughness. is becoming stronger and stronger.

Conventionally, in order to obtain steel pipes with high strength and high toughness, especially excellent low-temperature toughness, large amounts of various alloying elements such as Ni, Cr, and MO, which increase strength and toughness, are added to the steel material. However, these additive elements are expensive and inevitably increase the cost of the steel pipe itself.

From the above-mentioned viewpoints, the present inventors have achieved a high yield strength, such as API5LU80 to 1.00 of the American Petroleum Institute standard, and a fracture surface transition temperature without adding large amounts of expensive alloying elements. In order to manufacture quenched and tempered steel pipes at low cost, which have high strength (VTs) of -45°C or less and excellent low-temperature toughness, and which can be applied to the far northern part of Alaska, we are particularly interested in producing steel pipes that can be heated in a very short period of time. As a result of our research, we focused on quenching and tempering, which is a rapid heating and short-time heat treatment using induction heating, which has short holding times and is easy to obtain high strength even with steel of the same carbon equivalent. When quenching is performed by rapid heating and holding for a short time using induction heating, which is different from heating, the austenite crystal grains are made finer, and when tempering is rapidly heated and held at a low temperature for a short time, steel pipes are They found that there was little decrease in strength and that toughness was greatly improved.

In particular, as a result of examining the behavior of strength and toughness during low-temperature, short-time tempering, it was found that the toughness was sufficiently recovered even during short-time tempering with one tolerance, including the temperature increase time, and the degree of toughness was largely determined by the temperature reached and the composition. For example, even in 51-Mn steel, which is not affected by the effects of heat and does not contain any special elements, low-temperature tempering embrittlement hardly occurs, and it can be tempered for a short time including heating time.
We obtained the knowledge that high-strength, high-toughness steel pipes can be manufactured; this phenomenon occurs because the excessive lattice defects caused by quenching are eliminated by short-time low-temperature tempering, and time is not secured for P to diffuse into grain boundaries. It is thought that.

In other words, based on the knowledge that low-cost steel pipes with high strength and excellent low-temperature toughness can be produced by adjusting the chemical composition of the steel itself and by specifying the heat treatment conditions. This invention has been made, and therefore, C: 0.30% or less (hereinafter, "ch" is weight %), Si: 0.80% or less, Mn: 0.5 to 2.0. %, 806 M: o, 15 or less, P: 0.025 (or less, S: O, O, 15 or less, or in addition, Cu: o, 5 or less, Cr: l, Ni : 1 or less, Mo: 0.5% or less, Nl): 0.196 or less, v: O,lls or less, Ti: 0.296 or less, Contains one or more of the following, Fe and inevitable impurities: A welded steel pipe having either or both of the base metal part and the welded part with the composition consisting of , the temperature is raised above the Ac3 transformation point within 2 minutes, and the holding time t (minutes) is t, = 3.96X (1
,000-,-T) -) Quench before the temperature exceeds -1 (water-cooling, oil-cooling, or air-cooling may be used),
Furthermore, from room temperature, increase the temperature to within 8 minutes to 150~5
This method is characterized by the low-cost manufacturing method of steel pipes that have both high strength and good low-temperature toughness by performing water-cooling or air-cooling tempering treatment after the temperature is raised to 00°C and the holding time does not exceed 5 minutes. It has the following.

In other words, this invention uses rapid heating of steel pipes to refine the austenite grains, prevents local crystal grain growth by holding the pipe for a short time, and creates uniform and fine austenite grains. A high-strength, high-toughness steel pipe is manufactured by quenching and then low-temperature, short-time tempering. Furthermore, in order to impart high strength and high toughness, Cu is added to the steel pipe.

Cr, Ni, Mo, Nb, V, and T1
It is possible to contain one or more of these in combination.

Next, in the method for manufacturing a heat-treated steel pipe of the present invention, the reason why the composition range of the steel pipe and the heat treatment conditions are limited as described above will be explained.

a) C The C component has the effect of increasing the strength of steel pipes, but if its content exceeds 0.30%, the toughness deteriorates, so its content is limited to 0.30% or less. did.

b) Si Si component is necessary as a deoxidizing component of steel,
If the content exceeds 0.80%, the low temperature toughness tends to deteriorate, so the content is limited to 0.80% or less.

c) Mn The Mn component has the effect of improving the strength and toughness of steel pipes, but if its content is less than 0.5%, the desired effect cannot be obtained; on the other hand, if its content exceeds 2.0%, If it is contained, weldability will be extremely deteriorated, so its content should be reduced to 0.
．． It was limited to 5-2.0%.

d) sol, A11 soL, Al components are necessary as deoxidizing agents, but even if their content exceeds 0.10%, the deoxidizing agent/binding effect will be saturated, and the intervening As the toughness deteriorates due to the increase in the content, the content was reduced to 0.10%.
Limited to the following.

In order to significantly reduce the low-temperature brittleness of steel, its content was limited to 0.025% or less.

f) If the S content exceeds 0.015%, the impact absorption energy in the circumferential direction of the steel pipe will be significantly reduced, so the content was limited to 0.015% or less.

g) Cu, Or, Ni, Mo, Nb, V
, and TlCu, Cr, Ni, Mo, Nb,
Both V and T1 components are effective for increasing the strength and toughness of steel pipes, but when the CU acid component content exceeds 0.5%, surface defects due to hot embrittlement occur. If the Cr component is contained in excess of 1 inch, or M.

If the component is contained in excess of 0.5%, or
If the content of b, Y, and T1 exceeds 0.1%, it will have a negative effect on toughness, and even if the content exceeds 1% of the l"Ii acid component, the strength will not increase enough to justify the increase in cost. Since no improvement in toughness is observed, Cu
The component content is 0.5% or less, the Cr component and N1 component content is 1% or less, the MO component content is 0.5 inch or less, the Nb and V component content is 0.1 inch or less, and Ti The component content was limited to 0.2% or less.

h) Heat treatment conditions The reason why the quenching heating temperature T was set in the temperature range from AC3 point to 1050°C is that a fine-grained quenched structure can be obtained by quenching from this temperature range - especially when the temperature exceeds 1050°C. As is clear from the diagram in FIG. 1 showing the relationship between heating temperature and austenite grain size, hardening at a higher temperature is not preferable because it causes coarsening of crystal grains and deterioration of toughness.

In this quenching process, the heating time from room temperature is within 2 minutes, and the holding time t (minutes) after heating is 3.96X (100
The reason for setting the time not to exceed 0 to T) +1 is because if the heating time exceeds 2 minutes, grain growth will occur as shown in Figure 2, and toughness will deteriorate. ) exceeds 3.96 This is because it would cause deterioration.

The reason why the tempering temperature is set to 150 to 500°C is that tempering at a temperature in this range can recover toughness while suppressing the decrease in strength from as-quenched. As is clear from the diagram shown in Figure 3, it is difficult to recover the toughness, and on the other hand, when the temperature exceeds 500°C, as is clear from Figure 4, the strength decreases significantly. It is.

In this tempering process, the heating time from room temperature was set to within 8 minutes, and the holding time after heating was set to within 5 minutes. This is because the decrease becomes large and it becomes difficult to obtain the desired high strength.

Note that NK, which is an unavoidable impurity contained in the steel pipe in the method of the present invention, is usually suppressed to 100 p, pm or less, preferably 50 p, p, m or less.

As described above, by selecting the chemical composition and heat treatment conditions, it is possible to manufacture a steel pipe with high strength and toughness at low cost.

Next, the present invention will be explained using examples and comparing with comparative examples.

In each of the examples, seamless steel pipes having the chemical composition shown in Table 1 were manufactured by a conventional method, and then these steel pipes were heat-treated (quenching and tempering treatment) under the conditions also shown in Table 1. Inventive steel pipes 1 to 14 and comparative steel pipes 1 to 6 were manufactured by cooling the pipes by water cooling. Note that Comparative Steel Pipes 1 to 6 were all manufactured under heat treatment conditions outside the scope of the present invention (indicated by * in Table 1). Table 1 also shows the mechanical properties of the various steel pipes obtained as a result.

It is clear from the results shown in Table 1 that steel pipes with high strength and excellent low-temperature toughness can be obtained by the method of the present invention.

As described above, according to the present invention, it is possible to obtain a steel pipe with high strength that can withstand harsh usage environments and excellent low-temperature toughness by a relatively simple method at a low cost, which is useful for industrial purposes. This brings about a great effect.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between heating temperature during quenching and austenite grain size, Figure 2 is a diagram showing the relationship between heating time and austenite grain size, and Figure 3 is a diagram showing the relationship between tempering temperature and Charpy fracture. A diagram showing the relationship between surface transition temperature and FIG. 4 is a diagram showing the relationship between tempering temperature and yield strength. Applicant Sumitomo Metal Industries Co., Ltd. Agent Tomi 1) Kazuo Figure 1 The final temperature during quenching is p Shion Jl ('C)

Claims (2)

[Claims] (1) C: 0.30 or less, Si: 0.80' (76 or less, Mn: 0.5 to 2.0%, soL, A1: 0.10% or less, P: 0.025 or less. 9: 0.015% or less, Fe and unavoidable impurities: residue, (more than % by weight) Welded steel pipes having either or both of the base metal part and welded parts, or seamless steel pipes with the above composition. The whole is 1050 from the Act metamorphosis point.
At a heating temperature TcQi within the temperature range of °C, the temperature was raised above the AC3 transformation point within 2 minutes, and the holding time t (minutes) was 3.9.
6X (1000-T) -) 11-, then heated from room temperature to 150-500℃ within 8 minutes, and water cooled before holding time exceeds 5 minutes. Alternatively, a method for producing a high-strength steel pipe with excellent low-temperature toughness, characterized by subjecting it to air-cooled tempering treatment. (2) C: 0130% or less, 81: 0.80- or less, Mn: 0.5-2.0%, soL, Al: O, l O% or less, P: 0.02
5% or less, f3: O, O15% or less, Cu: 0.5% or less, Cr: 1% or less, Ni: 1% or less, Mo: 0.5- or less, Nb: 0 Contains one or more of the following: .1 inch or less, V: 0.1 inch or less, Ti: o, 2% or less. A welded steel pipe having either or both of a base metal part and a welded part having a composition consisting of Fe and unavoidable impurities (at least % by weight), or a seamless steel pipe having the above composition, is heated to 1050°C from the AC3 transformation point.
The temperature is raised above the AO3 transformation point within 2 minutes to the heating temperature T(6) within the temperature range of ℃, and the holding time t (minutes) exceeds t-3,96X(1000~T)-)-1. It is characterized by quenching the material for a while, then raising the temperature from room temperature to 150-500°C within 8 minutes, and then subjecting it to water-cooling or air-cooling tempering treatment before the holding time exceeds 5 minutes. A method for manufacturing high-strength steel pipes with excellent low-temperature toughness.