GB2101014A

GB2101014A - Manufacture of seamless steel pipe

Info

Publication number: GB2101014A
Application number: GB08219839A
Authority: GB
Inventors: Manfred Hartl; Dieter Vespermann; Friedrich Huls; Fritz Kegel
Original assignee: Mannesmann AG
Current assignee: Vodafone GmbH
Priority date: 1981-07-09
Filing date: 1982-07-08
Publication date: 1983-01-12
Also published as: AR230652A1; DE3127373C2; ATA166682A; IT1151206B; FR2509325A1; GB2101014B; DE3127373A1; AT389320B; JPS589921A; IT8221425A0; FR2509325B1

Abstract

In the manufacture of seamless steel pipes, in particular for the petroleum industry, a solid in got is pierced, and is extended in a first heating; the hollow body is re-heated; and is then longitudinally rolled and cooled in air. According to the present invention, improved metal properties are achieved by lowering the hollow body temperature, after extending, but before re-heating; by storage in air, normally for 2.5 to 6 minutes, to a temperature below the Ar1 point, but not less than 500 DEG C. After the longitudinal rolling, the formed pipe is cooled with liquid, normally with water or a water-air-mixture by approximately 100 DEG to below the Ar1 point, before final cooling in air.

Description

SPECIFICATION High quality seamless steel pipes The invention relates to the manufacture of seamless steel pipes, particularly high quality pipes for the petroleum industry. More specifically, it relates to a manufacturing method in which a solid ingot is pierced and extended in a first heating into a hollow body with approximately the wall thickness of the finished pipe; re-heated to a temperature of for example 50 to 130 C overthe AC3 point; and longitudinally rolled to the finished pipe diameter.

A known method of the above type is described as a continuous pipe rolling mill method in the journal 3R International, February, 1977, and has been used in the manufacture of high quality pipes. After stretch reducing to finished pipe diameter and cooling to room temperature, pipes for the petroleum industry are re-heated up to austenitation temperature and, depending on the steel composition, quenched in oil or water and tempered to the desired strength. To fulfil special requirements it has aiso been proposed to normalise pipes prior to hardening by heat treatment and then temper them.

It is known in the hot-rolling of steel, to quench harden from the rolling heat, thereby saving the expense of a separate heating step for quench hardening. While adequate strength can be achieved in this method, the toughness and extension properties are frequently unsatisfactory. This is generally attributed to the coarsening of the grain structure which takes place.

The present invention seeks to avoid the need for multiple heating in rolling and tempering from room temperature, particularly in the manufacture of high quality pipes, while achieving high strengths with sufficient toughness and expansion properties. To this end, a method according to the invention for the manufacture of a seamless steel pipe comprises piercing and extending a solid ingot in a first heating to a hollow body; cooling the body by storage in air to a temperature below the Ar1 point, but no less than 500"C; re-heating the hollow body and longitudinal rolling to the finished pipe diameter; and cooling the formed pipe first with liquid by substantially 100,0 to a temperature below the Ar, point and then in air.Normally, the cooling of the body prior to re-heating is accomplished in 2.5 to 6 minutes; and the re-heat temperature is 50 to 1 300C over the Ac3 point.

The longitudinal rolling usually takes place with at least approximately 1.1 to approximately 2.5 times extension, and preferably within this range. The liquid cooling ofthe formed pipe is typically accomplished using water or a water/air mixture.

Although in discussing the known method, a report is indeed given of a temperature drop to approximately 700"C, and hence below Ar1 in the static iron-carbon diagram, this is in practice only a quick surface cooling and neither an intentional nor actual structural transformation ofthe pipe body.

Hence the grain refinement that can be achieved in the method of the invention is not brought about, neither through the re-heating to 50-130" over Ac3.

However, here the important point is an improvement in thetoughness and expansion properties of the pipe. 1;7 addition, the result in the same sense is that the hollow body quickly cools to a temperature by approximately 100" below Ar1 after the longitudinal rolling to the finished pipe diameter, and only then is slowly cooled in the air.

The method of the present invention is particularly suited to the manufacture of pipes of which the steel has the following composition: 0.25 - 0.45 % C 0.7 -1.7%Mn 0.3 - 0.5 % Si 0.1 -0.3%Cr 0.07-0.14%V 0 -0.014%N the balance being iron and unavoidable impurities.

The resulting pipe can have a yield point of 480 to 650 N/mm2; and a notch toughness of at least 30J, measured at room temperature on an ISO-Vtestpiece. Generally, the yield point is related to the carbon, manganese and chromium contents, the lower and higher thresholds for the yield point or responding to those of carbon, manganese and/or chromium. For example a yield point of 480 to 540 N/mm2 corresponds to 0.25 to 0.3% C; and at least one of 0.7 to 0.9% Mn and 0.1 to 0.15% Cr. At the other extreme, a yield point of 600 to 650 N/mm2 corresponds to 0.4 to 0.45% C; and at least one of 1.5 to 1.7% Mn and 0.25 to 0.3% Cr.We have found that a yield point of 520 N/mm2 and the quality grade N 80 of the specification of the American Petroleum Institute (API) can be achieved with the above steel composition, but modified as follows:- 0.38 to 0.42% C 1.45 to 1.65% Mn 0.3 to 0.5% Si 0.015to 0.2% Cr 0.08 to 0.1% V 0.02 to 0.04% AC up to 0.012% N up to 0.02% S up to 0.025% P In particular, the invention enables the strength range required for oil field pipes from quality grade N 80 to be completely covered, without tempering with special heating up to quench-hardening and subsequent annealing heating being necessary.

Compared with the quick cooling from the rolling heat to martensite structure and the subsequent annealing, the method ofthe invention can achieve the same tensile strength, while the yield point, toughness and extension properties are consider ably improved.

1. Amethodforthe manufacture of a seamless steel pipe comprising piercing and extending a solid ingot in a first heating to a hollow body; cooling the body by storage in air to a temperature below the Ar point, but no less than 500"C; re-heating the hollow body and longitudinal rolling to the finished pipe diameter; and cooling the formed pipe first with liquid by substantially 10000 to a temperature below the Ar1 point and then in air.

2. A method according to Claim 1 wherein said

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION High quality seamless steel pipes The invention relates to the manufacture of seamless steel pipes, particularly high quality pipes for the petroleum industry. More specifically, it relates to a manufacturing method in which a solid ingot is pierced and extended in a first heating into a hollow body with approximately the wall thickness of the finished pipe; re-heated to a temperature of for example 50 to 130 C overthe AC3 point; and longitudinally rolled to the finished pipe diameter. A known method of the above type is described as a continuous pipe rolling mill method in the journal 3R International, February, 1977, and has been used in the manufacture of high quality pipes. After stretch reducing to finished pipe diameter and cooling to room temperature, pipes for the petroleum industry are re-heated up to austenitation temperature and, depending on the steel composition, quenched in oil or water and tempered to the desired strength. To fulfil special requirements it has aiso been proposed to normalise pipes prior to hardening by heat treatment and then temper them. It is known in the hot-rolling of steel, to quench harden from the rolling heat, thereby saving the expense of a separate heating step for quench hardening. While adequate strength can be achieved in this method, the toughness and extension properties are frequently unsatisfactory. This is generally attributed to the coarsening of the grain structure which takes place. The present invention seeks to avoid the need for multiple heating in rolling and tempering from room temperature, particularly in the manufacture of high quality pipes, while achieving high strengths with sufficient toughness and expansion properties. To this end, a method according to the invention for the manufacture of a seamless steel pipe comprises piercing and extending a solid ingot in a first heating to a hollow body; cooling the body by storage in air to a temperature below the Ar1 point, but no less than 500"C; re-heating the hollow body and longitudinal rolling to the finished pipe diameter; and cooling the formed pipe first with liquid by substantially 100,0 to a temperature below the Ar, point and then in air.Normally, the cooling of the body prior to re-heating is accomplished in 2.5 to 6 minutes; and the re-heat temperature is 50 to 1 300C over the Ac3 point. The longitudinal rolling usually takes place with at least approximately 1.1 to approximately 2.5 times extension, and preferably within this range. The liquid cooling ofthe formed pipe is typically accomplished using water or a water/air mixture. Although in discussing the known method, a report is indeed given of a temperature drop to approximately 700"C, and hence below Ar1 in the static iron-carbon diagram, this is in practice only a quick surface cooling and neither an intentional nor actual structural transformation ofthe pipe body. Hence the grain refinement that can be achieved in the method of the invention is not brought about, neither through the re-heating to 50-130" over Ac3. However, here the important point is an improvement in thetoughness and expansion properties of the pipe. 1;7 addition, the result in the same sense is that the hollow body quickly cools to a temperature by approximately 100" below Ar1 after the longitudinal rolling to the finished pipe diameter, and only then is slowly cooled in the air. The method of the present invention is particularly suited to the manufacture of pipes of which the steel has the following composition: 0.25 - 0.45 % C 0.7 -1.7%Mn 0.3 - 0.5 % Si 0.1 -0.3%Cr 0.07-0.14%V 0 -0.014%N the balance being iron and unavoidable impurities. The resulting pipe can have a yield point of 480 to 650 N/mm2; and a notch toughness of at least 30J, measured at room temperature on an ISO-Vtestpiece. Generally, the yield point is related to the carbon, manganese and chromium contents, the lower and higher thresholds for the yield point or responding to those of carbon, manganese and/or chromium. For example a yield point of 480 to 540 N/mm2 corresponds to 0.25 to 0.3% C; and at least one of 0.7 to 0.9% Mn and 0.1 to 0.15% Cr. At the other extreme, a yield point of 600 to 650 N/mm2 corresponds to 0.4 to 0.45% C; and at least one of 1.5 to 1.7% Mn and 0.25 to 0.3% Cr.We have found that a yield point of 520 N/mm2 and the quality grade N 80 of the specification of the American Petroleum Institute (API) can be achieved with the above steel composition, but modified as follows:- 0.38 to 0.42% C 1.45 to 1.65% Mn 0.3 to 0.5% Si 0.015to 0.2% Cr 0.08 to 0.1% V 0.02 to 0.04% AC up to 0.012% N up to 0.02% S up to 0.025% P In particular, the invention enables the strength range required for oil field pipes from quality grade N 80 to be completely covered, without tempering with special heating up to quench-hardening and subsequent annealing heating being necessary. Compared with the quick cooling from the rolling heat to martensite structure and the subsequent annealing, the method ofthe invention can achieve the same tensile strength, while the yield point, toughness and extension properties are consider ably improved. CLAIMS

2. A method according to Claim 1 wherein said cooling of the body prior to re-heating is accomp lished in 2.5 to 6 minutes.

3. A method according to Claim 1 or Cialm 2 wherein the re-heat temperature is 50 to 130"C cver the Ac3 point.

4. A method according to any preceding Claim wherein the longitudinal rolling accomplishes an extension ofthe body in the range 1.1 to 2.5.

5. A method according to any preceding Claim wherein the formed pipe is first cooled with water or a water/air mixture.

6. A method forthe manufacture of seamless steel pipes according to Claim 1 and substantially as herein described.

7. A seamless steel pipe manufactured by a method according to any preceding Claim, having a yield point of 480 to 650 N/mm2, a notch toughness of at least 30J, measured at room temperature on an ISO-V-testpiece, and comprising by weight: 0.25 - 0.45 % C 0.7 -1.7 %Mn 0.3 - 0.5 % Si 0.1 -0.3%Cr 0.07-0.14%V 0 -0.014% N the balance being iron and unavoidable impurities.

8. A pipe according to Claim 7 having a yield point of 480 to 540 N/mm2 and comprising by weight 0.025 to 0.3% C; and at least one of 0.7 to 0.9% Mn and 0.1 to 0.15% Cr.

9. A pipe according to Claim 7 having a yield point of 600 to 650 N/mm2 and comprising by weight 0.4 to 0.45% C; and at least one of 1.5 to 1.7% Mn and 0.25 to 0.3% Cr.

10. A pipe according to Claim 8 having a yield point of 520 N/mm2 and comprising by weight: 0.38 - 0.42 % C 1.45- 1.65 % Mn 0.3 -0.5 % Si 01.5 - 0.20 % Cr 0.08-0.1 %V 0.02 - 0.04 % AC up to 0.012 % N upto0.02 %S up to 0.025 % P.

11. A seamless steel pipe manufactured by a method according to any of Claims 1 to 6 and substantially as herein described.