JPS59150019A - Production of seamless steel pipe having high toughness - Google Patents

Abstract

PURPOSE:To obtain a seamless steel pipe having high toughness and good weldability by making a steel contg. a decreased amt. of C and Mn and a specified amt. of Ni, Cu and carbide forming element into a pipe by a seamless method and subjecting the pipe to reheating, cooling, heating and tempering treatments under specific conditions. CONSTITUTION:A steel consisting of 0.02-0.10wt% C, 0.02-0.50% Si, 0.70- 1.80% Mn, <=2.0% Mn + 10C, 0.10-1.00% Cu, 0.05-0.70% Ni, 0.15<=Ni+Cu<= 1.00% and further >=1 kind of 0.010-0.050% Ti and 0.020-0.070% Nb and the balance Fe with inevitable impurities is melted. Such steel is made into a pipe at a finishing temp. of >=700 deg.C by a Mannesmann plug mill hot seamless steel pipe making method. The pipe is heated to the Ac3 point or above after cooling and is cooled at a cooling rate of 20-70 deg.C/sec down to a temp. region of <=300 deg.C. The pipe is further heated to a temp. region of 550- Ac1 transformation point and is subjected to a tempering treatment. The seamless steel pipe having high toughness and good weldability is thus obtd. with less alloy components.

Description

DETAILED DESCRIPTION OF THE INVENTION This invention is based on 52 to 707 laths of AP Engineering Standard 5LX.
The present invention relates to a method for manufacturing seamless steel pipes having relatively low strength, high toughness, and good weldability at low cost.

Conventionally, measures to improve the strength and toughness of steel sheets include rolling a material containing appropriate alloying elements and then reheating it to quench and temper it, or directly quenching it after normal hot rolling, and then tempering it. Generally, a so-called controlled rolling or controlled cooling method is employed, which involves quality treatment, microstructural refinement by low-temperature rolling, and further accelerated cooling after rolling to obtain a steel sheet with higher strength and toughness.

However, in the Man-i-Suman Pragmill seamless steel tube manufacturing method, it is difficult to ensure the toughness of non-tempered copper steel tubes because low-temperature rolling is difficult due to the characteristics of the manufacturing equipment. Improvements were common.

However, in order to obtain sufficient toughness through tempering treatment, it is necessary to ensure a martensitic rate of 90% or more in the quenched structure. Especially for seamless steel pipes, quenching treatment from the outside is usually required. In order to obtain a stable martensitic structure, especially on the inner surface, it was necessary to contain a large amount of quenching elements.

In particular, when manufacturing line pipes of API standard (American petroleum standard) 5LX-, 52-70 class seamless steel pipes that are used in cold regions and require relatively low strength and high toughness, heat treatment is necessary. If we do this, the toughness will be ensured, but the strength will be too high, and even if we reduce the strength by tempering at a relatively high temperature, welding of circumferential joints on-site will cause welding cracks during welding work and thermal effects. In order to reduce the structural change in the part, preheating and postheating steps are required, and there are problems in that the manufacturing cost increases due to the addition of a large amount of alloying elements, which is an economical problem.

An object of the present invention is to provide a method for manufacturing a high-toughness seamless steel pipe that does not cause such problems.

As a result of intensive research in order to achieve this purpose, the inventors of the present invention discovered that the C1 and 1 buttons below are reinforced with ferrite.
One or two of Cu and Ni are contained, and carbide-forming elements T1 and Nb are added to this to refine the initial austenite grains, delay the precipitation of pro-eutectoid cementite during cooling, and make the carbide a core. By attempting to generate fine ferrite, we discovered that by applying quenching and tempering to a non-tempered seamless steel pipe with a fine ferrite pearlite structure, a high-toughness seamless steel pipe with a fine ferrite structure could be obtained. It is.

According to the method of the present invention, not only can a seamless steel pipe with higher toughness than before be obtained, but also a system with less hardening of the circumferential joint weld and good weldability because it requires less alloying components. , it became possible to reduce manufacturing costs.

The gist of the present invention is that C: 0.02 to 010 (hereinafter, the ratio representing the component ratio is the weight ratio), 'Si
: Contains 0.02 to 0.50%, Mn: 07o to 180%, and the sum CC (%) + of 10 times the amount of C and Mll
1OMn (%)] is 20 or less, and Cu:
0.10-1. O0%, Ni: 0.05-0.70
0.15 <Ni (%) + Cu [%
)<1. Contains Ti at a value that satisfies OO, and further contains Ti:
0.010~○o50 Section, Nbi O, 02~007
Preferably, it also contains one or more of 'ri/
C and Nb/C of 0.10% or more), the balance being iron and unavoidable impurity elements.
Finish pipe making at 700°C or higher, cool down and then reheat to 3 AC points or higher, cooling at a cooling rate of 20 to 70°C to 300°C or lower.
Forced cooling in the range of / sec, and then 500℃~A
C. The method is characterized in that a high-toughness seamless steel pipe having a fine ferrite structure is manufactured by heating to a transformation point and performing a tempering treatment.

The reasons for limiting the amount of each additive element and manufacturing conditions of the steel according to the method of the present invention as described above will be described below.

C increases the strength of steel pipes, but if it is contained in an amount exceeding 0.10%, bainite will precipitate, so the content should not exceed 0.10%.

This is one of the greatest features of this invention.

In order to occupy a large amount of ferrite structure, the lower the C content, the better, but in consideration of economic efficiency, the lower limit was set to 02% iJ, which does not require holding refining (decarburization treatment).

Mn is a necessary component not only for deoxidizing but also for maintaining strength. However, if it exceeds 180%, it is unfavorable for toughness and weldability, and in particular, bainite and martensite are generated by quenching, so the upper limit was set at 1.80%.

The limit of Mn was set to 0.70% to ensure deoxidation, S fixation strength, and toughness.

Also, the sum of C and 10 times 1vin [C (%) + 10Mn
(%)] was set at 20% or less because 20% is the limit at which C can be dissolved as a solid solution while maintaining the ferrite state. This is because it precipitates and deteriorates toughness.

Sl is added to deoxidize and obtain the desired strength, but if it is less than 0.02%, sufficient deoxidation will not occur and
If it exceeds 50%, the toughness will deteriorate, so the content should be reduced to 0.
It was set as 02 to 050 cracks.

CU and Ni have the effect of strengthening the ferrite matrix and ensuring the desired strength of the steel pipe.

In particular, in the steel pipe of the present invention, in order to obtain a fine ferrite structure after heat treatment, the C and 1vln contents are lowered and the formation of bainite and martensite is suppressed, thereby compensating for the decrease in strength due to this.1. It also has the effect of improving toughness, so it is an element that needs to be added in one or more types.

However, the content of CU is 0% and the content of Ni is 0%.
70%, and when the sum of both exceeds 1.00, precipitation of bainite structure is observed during quenching, leading to deterioration of the toughness of the steel pipe.On the other hand, when the Cu content is 0 and 10%, N1
The content of ○05 is 0.1, and the total of both is 0.1.
If it is less than 5%, the desired effect cannot be obtained, so
0.10 to 1.00% Cu.

N1 is 005-070% and the sum of both is 0,
'15 to 1.00.

T1 and Nb are carbide-forming elements that play an important role in forming fine ferrite precipitation sites during quenching and imparting strength and toughness to steel. 0.01%Nb( is ineffective below 0.02%, i.e. Ti(%)/C(%) and Nb(
%)/C(%) must each be 0.10 or more.9 On the other hand, if the T1 content exceeds 005% and the NI) content exceeds O and O'7%, In this case, the strength improvement effect and ferrite refinement effect become saturated, and bainite precipitates and toughness deteriorates immediately.
Each was limited to 7%.

In addition, with this invention, a seamless steel pipe of 7-0. 'Finishing pipe making at 0°C or above' means that the temperature at the outlet of the outer diameter straightening machine (sizer mill), which is the final process of pipe making, is 700°C or above; Ferrite precipitates before the completion of the heat treatment, distortion occurs due to corrective pressing, and a fine grain structure cannot be obtained during heat treatment, resulting in a significant local decrease in strength and toughness.

Cooling after pipe production can be done by either rapid cooling or cooling, but if a bainite structure appears after cooling, the austenite crystal grains will become larger during reheating and the toughness of the product will deteriorate slightly. , it is desirable to have a pearlite structure.

Reheating after rolling may be performed at an AC point of 3 or above to form austenite, but if it is too high, the austenite will become grain-sized, so it is desirable to keep the temperature at about AC3+'l OO°C□.

Furthermore, if the cooling rate exceeds 70° C./sec, bainite will be generated, whereas if the cooling rate is less than 20° C./sec, the crystal grains will become coarse, making it impossible to obtain the desired strength and toughness.

When the cooling stop temperature range exceeds 300°C, bainite transformation occurs from retained austenite.

Tempering causes transformation when the temperature exceeds Ac1,
Furthermore, since no improvement in toughness can be expected below 550°C, AC1
The temperature range was 550°C.

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

Steel having the composition shown in Table 1 is made into billets using a conventional method. After heating these billets to 1200°C, they are passed through each pipe-making process using the Mannesmann plug mill system to
Pipe manufacturing was completed at 0°C, and the steel pipe was allowed to cool down to 200°C or less, and then reheated and heat treated.

Seamless steel pipes having various compositions of A and P shown in Table 1 were heat treated under the conditions shown in Table 2.

The results of the first tensile test of each steel pipe obtained in this way.

The results of the Nyarupee impact test and the lightning test are also shown in Table 2.

From the results shown in Table 2, it is clear that the steel pipes manufactured by the method of the present invention have excellent toughness and high strength. In actual welding tests, even though no preheating or postheating is performed during welding, there is little structural change in the heat-affected zone, and the fracture surface transition temperature in the pipe circumferential direction is low. The brittle propagation characteristics have been improved, and results have been obtained that fully satisfy low-temperature toughness.

On the other hand, it is clear that the bainite structure produced by the comparative method, in which the manufacturing conditions are outside the scope of the present invention, is inferior to the method of the present invention in terms of fracture surface transition temperature and toughness.

In addition, No. The figure shows a microscopic structure diagram of a seamless steel pipe 10 produced by the method of the present invention, and Figure 2 shows a microscopic structure diagram of a similar state of comparative method 14, but the method 10 of the present invention has fine ferrite. Comparative method] 4 is bainite,
It can be seen that it is martensite.

As mentioned above, this invention improves strength by subjecting a seamless steel pipe having a ferrite and pearlite structure formed with a pearlite occupation area ratio of 10% or less as-rolled to a quenching arm-returning heat treatment to change the structure to fine ferrite. This technology is extremely effective in simplifying the welding process, improving weldability, and reducing costs.

[Brief explanation of drawings]

FIG. 1 is an example of a microscopic structure diagram of a seamless steel pipe produced by the method of the present invention, and FIG. 2 is a microscopic structure diagram of a comparative seamless steel pipe. Applicant: Sumitomo Metal Industries, Ltd.

Claims (1)

[Claims] Contains C: 0.02-010%, Si: 0.02-050 So, Mn: 070-180 So, and satisfies the formula: Mn (%) + 100 (%) < 20. , and Cu: 0.10~1.00%, Ni: 0.05~O,'70%, and one or two of them are represented by the formula: 0,15 <Ni (%) + Cu (%) <1. o. Component composition ( % by weight) was processed using the Mannesmann plug mill hot seamless steel pipe manufacturing method at a finishing temperature of '7.
After making the pipe at 00'C or higher and cooling it, heat it again to AC3 or higher, then cool it down to a temperature range below 300°C, at a cooling rate of 20~B, then further to 550~Ac, in the transformation point temperature range. A method for manufacturing a high-toughness seamless steel pipe having a fine ferrite structure, which is characterized by subjecting it to heating and tempering.