JPS5819438A - Production of steel pipe having high strength and high toughness - Google Patents

Abstract

PURPOSE:To produce a steel pipe having high strength and high toughness by subjecting a welded steel pipe contg. specific ratios of C, Si, Mn, P, S, solAl to hardening and tempering treatments under specific conditions. CONSTITUTION:The entire part of a welded steel pipe or seamless steel pipe consisting of one or both of base material parts and weld zones contg., by wt%, 0.05-0.30% C, 0.05-0.80% Si, 0.5-2.0% Mn, <=0.025% P, 0.02-0.10% solAl, further contg. >=1 kind among <=0.5% Cu, <=2.5% Ni, <=1% Cr, <=1% Mo, <=0.15% Nb, <=0.15% V according to need, and the balance Fe and unavoidable impurities, is heated within 2min from the Ac3 transformation point up to the temp. region of the equationI, and before the holding time (t) of the equation II is elapsed, the pipe is hardened. The pipe is heated up to the temp. region of 450 deg.C-Ac1 transformation point within 8min heating time from ordinary temp. and before the holding time exceeds 5min, the pipe is subjected to a tempering treatment by water or air cooling.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of manufacturing a steel pipe having high strength and high toughness.

In recent years, the increase in global energy demand has led to a wave of large-scale oil and natural gas field development in harsh areas such as Alaska, and the demand for steel pipes that can withstand such harsh environments is rapidly increasing. Furthermore, due to the increasing scale of development and the trend toward efficient transportation, these steel pipes are increasingly required to have higher strength and toughness.

Conventionally, one method for obtaining steel pipes that has both high strength and high toughness at the same time is to put the steel pipes directly into a heating furnace and heat them for a long time to quench and temper them (pipe QT treatment). was known.

However, in such conventional long-time heating and long-time heat treatment methods using furnace heating, it is necessary to add large amounts of various alloying elements such as Ni, Cr, and Mo to the steel pipe material in order to obtain high strength performance. This was extremely disadvantageous from the viewpoint of weldability and material cost.

In addition, a method for manufacturing steel pipes by induction heating has been developed, which makes it easy to obtain high strength with short heating times, but steel pipes manufactured by this method have drawbacks such as large variations in toughness.

From the above-mentioned viewpoints, the present inventors conducted research to manufacture steel pipes with high strength and high toughness using rapid heating by induction heating and quenching and tempering methods that involve short-time heat treatment. (b) A range in which austenite grains do not grow abnormally during quenching, by heating at a high temperature that does not cause austenite crystal grains to become coarse and allows carbides to be dissolved as much as possible to make the precipitation effect effective. (C) If the strength is prevented from decreasing by heating for a short time during tempering,
They discovered that heat-treated steel pipes with sufficient strength and toughness can be manufactured at low cost without adding special alloying elements as much as possible.

In other words, low-cost steel pipes with high strength and high toughness are
This invention was made based on the knowledge that steel can be manufactured by adjusting the composition of the steel itself and defining heat treatment conditions, and has a C: 0.05 to 0.3.
0 (hereinafter referred to as weight %), Si: 0.0 5
~0.80%.

Mn: 0.5-2.0%, p: 0.025 or less, S: 0. O15% or less, sot, bamboo: 0.02 to 0.10%, or further contains: Cu: 0.5% or less, Ni: 2.5% or less, Cr: 1% or less, Mo: 1- Containing one or more of the following: Nb: 0.15% or less, V: 015% or less, Fe and unavoidable impurities: residue, having either or both of the base metal part and the welded part of the composition The entire welded steel pipe or seamless steel pipe with the above composition is heated from the Ac3 transformation point to the temperature range T shown by
The temperature should be raised within 2 minutes, and the holding time t (minutes) should be quenched before it exceeds t, = 6.58 After raising the temperature to the temperature range of 450℃ to ACI transformation point with a heating time of 8 minutes or less, water cooling or air cooling is performed before the holding time exceeds 5 minutes, resulting in high strength and high toughness. This method is characterized by a low-cost manufacturing method for steel pipes that have both of the above features.

In other words, in this invention, the heating during quenching is rapidly heated and held for a short time to make the steel structure fine-grained, and the heating during tempering is heated rapidly and held for a short time to recover toughness and reduce strength. The present invention is characterized in that a steel pipe with a high yield strength of 7 ok/- or more and a Charpy impact value (vTs) of -80° C. or less can be obtained.

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 the content is less than 0.05%, the desired effect cannot be obtained, whereas if the content exceeds 0.30%, the desired effect cannot be obtained. Since the toughness will deteriorate, the content should be reduced to 0.05 to 0.
It was limited to 30%.

(b) 5i The Si component is added as a deoxidizing agent for steel, but if its content is less than 0.05%, the deoxidizing effect will not be sufficient, while if it is contained in excess of 0.80%, Since the low-temperature toughness deteriorates, the content should be increased from 0.05 to 0.
limited to sop.

(c) Mn The Mn component has the effect of improving strength and toughness, but if the content is less than 0.5%, the desired effect cannot be obtained; If the content exceeds 0, the weldability will be greatly reduced, so the content should be reduced to 0.
．． 5-2. Limited to O.J.

(d) sot, A1 5oL, M is added as a deoxidizing agent for steel and is effective in suppressing grain growth during quenching, but if its content is 0.
If it is less than 0.02%, the above effect is not sufficient; lO
If the content exceeds 0.02 to 0.0%, inclusions will increase and the toughness will deteriorate. It was limited to 100%.

Its content is o, 02 s’! JE'H was established.

Established.

(g) Cu, Ni, Cr, Mo, Nb
, VCu, Ni, Or, Mo, Nb, and V components are all effective for increasing strength and improving toughness, but in addition, the Cu component improves corrosion resistance, and the Ni component improves low temperature resistance. The Mo component also has the effect of improving toughness and preventing temper brittleness. However, if the Cu component content exceeds 0.5%, surface defects due to hot brittleness will occur9, and if the Nl component exceeds 2.6%, the cost will be higher than the allowable limit. If the Cr or Mo component exceeds 1% and the Nb or V component exceeds 0.15%, low temperature toughness and weldability will deteriorate, so the content of the Cu component is reduced to 0.
．． 5% or less, Ni content 2.5% or less, cr
The content of the acid component was limited to 1% or less, the content of the Mo component was limited to 1% or less, the content of the Nb component was limited to 15% or less, and the content of the component (2) was limited to 15% or less.

In order to make the steel quality uniform and to make effective use of precipitates, it is desirable that the heating temperature during quenching be as high as possible to obtain a fine grain structure. There is an optimal heating temperature range to obtain a fine-grained structure in steel pipes, and as is clear from Figure 1, which shows the relationship between the heating temperature during rapid heating and the austenite grain size, S contained in steel
It is determined from the amounts of oL, A1, and N, and the temperature is 50° C. lower than the N and N complete solid solution temperature. This A
tN complete solid solution temperature 'ro is the sol contained in the steel,
A11. It is determined from the amount (5) and the amount of N (a) using the following equation.

Furthermore, in the case of austenitization by rapid heating, the coarsening temperature of austenite and nite grains is 50°C as compared to the TO mentioned above.
corresponds to a lower temperature. Therefore, the upper limit value of the rapid heating temperature to obtain a fine-grained structure is Since it is desirable to make , the rapid heating temperature T(6) is
The range of

(II) Temperature rising time and holding time in quenching process The temperature rising time from Ac3 transformation point to T'C was set within 2 minutes because 2
As shown in Figure 2, if the temperature exceeds 10 minutes, crystal grains will grow and it will be difficult to obtain good toughness. (10
00 to T) The reason why the range was set not to exceed +1 was because, as shown in Figure 3, if the time exceeds t, abnormally grown grains of 100μ or more will partially occur, resulting in significant mixed grains, leading to deterioration of toughness. This is because it becomes

(Reason for setting the 119 tempering temperature to 450°C to the ACI transformation point and limiting the heating time and holding time.

If the tempering temperature is outside the range of 450°C to AC1 transformation point, it will not be possible to obtain the desired excellent strength and toughness, and if the heating time exceeds 8 minutes or the holding time exceeds 5 minutes. Once this is done, there is no difference from ordinary hardened and tempered materials.

Note that N, which is an unavoidable impurity contained in the steel pipe in the method of the present invention, is usually suppressed to 1100 ppm or less, preferably 50 ppm or less.

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

Next, the present invention will be explained with reference to examples and in comparison with the pQv example.

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) under the conditions also shown in Table 1. Inventive steel pipes 1 to 13 and comparative steel pipes 1 to 6 were manufactured by cooling with water.

Note that Comparative Steel Pipes 1 to 6 were all manufactured under heat treatment conditions that were outside the scope of the present invention (indicated by a pot 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, the present invention has industrially useful effects such as the ability to obtain steel pipes with high strength and high toughness that can withstand harsh usage conditions at low cost using a relatively simple method. is brought about.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between heating temperature during rapid heating and austenite grain size, Figure 2 is a diagram showing the relationship between heating time during quenching and Charpy fracture surface transition temperature, and Figure 3 is a diagram showing the relationship between heating temperature during quenching and Charpy fracture surface transition temperature. FIG. 3 is a relationship diagram between heating temperature and holding time showing a region where abnormal growth of austenite grains occurs during heating. v1 Figure 5eaJ ('c) Figure 2 Figure 3

Claims (2)

[Claims] (1) C: 0.05-0.30chi, Si: 0.0
5-0.80chi, Mn: 0.5-2.0chi, P: 0.025- or less, S: O, O 15- or less, soL, ldl: 0.02-0.10%, Fe and unavoidable Impurities: Remaining: A welded steel pipe having either or both of a base metal part and a welded part having a composition (more than 1% by weight) consisting of the following, or the entire seamless steel pipe having the above composition, from an AC11 transformation point to 1- (more than 1% by weight) The temperature is raised within 2 minutes to the temperature range T shown in weight%), and the holding time t (minutes) is t, -a, 58xlo (1000~T) -1-1
The temperature is then heated from room temperature to the temperature range of 450°C to AC1 transformation point within 8 minutes, and tempered by water cooling or air cooling as soon as the holding time exceeds 5 minutes. A method of manufacturing a steel pipe with high strength and high toughness, which is characterized by subjecting it to a treatment. (2) C: 0.05-0.30%, S 〇. 0
Contains 5-0.80%, Mn: 0.5-2.0%, P: 0.025% or less, S: O, O1B or less, sot, AL2 0.02-0.10%. With and further. Cu: 0.5% or less, Ni: 2.5% or less, Cr: 1% or less, Mo: 1- or less, Nb: O, 15% or less, V: 0.15% or less, one or more of the following. A welded steel pipe having either or both of a base metal part and a welded part, or a seamless steel pipe having the above composition, containing Fe and unavoidable impurities: the remainder, (in weight %), is subjected to AC3 transformation. From the point, -'32≦TccJ≦1.95-IJAj'J 3
(N% -3”'1.95-1oIrcA1%:) (N
%) (weight % or more) within 2 minutes, and the holding time t (minutes) is t=6.58X10""(1000~T)"56F-
1, then heated from room temperature to a temperature range of 450°C to AC1 transformation point with a heating time of no more than 8 minutes, and tempered with water or air cooling while a holding time does not exceed 5 minutes. A method of manufacturing a steel pipe with high strength and high toughness, which is characterized by subjecting it to a treatment.