Manufacturing method of high strength, high toughness seamless steel pipe for linepipe

Abstract

A manufacturing method of high-strength, high-toughness seamless steel pipe for linepipe comprises: (1) hot piercing a billet, in which the carbon equivalent value Ceq defined by expression (a): Ceq=C+Mn / 6+(Cu+Ni) / 5+(Cr+Mo+V) / 5[%] . . . (a) is not greater than 0.60 wt %, and then subjecting the pierced billet to hot rolling at a temperature of 950° C. or higher to obtain a seamless steel pipe; (2) putting the seamless steel pipe immediately, without cooling to Ar3 point or lower, into a furnace kept at Ar3 point+50° C. to 1100° C. to carry out heating; (3) cooling the seamless steel pipe at a cooling rate of 5° C. / sec or higher; and (4) tempering the seamless steel pipe at temperature of 550° C. to Ac1 point. A high-strength and high-toughness seamless steel pipe with little strength unevenness can be manufactured, which can be effectively employed for linepipes in cold areas.

Description

TECHNICAL FIELD [0001] The present invention relates to a manufacturing method of seamless steel pipe for linepipe, and more particularly, the invention relates to a manufacturing method of high-strength and good-toughness seamless steel pipe for linepipe. BACKGROUND ART [0002] In prior art, in order to obtain high-strength and good-toughness steel, structure-grain-refinement is usually applied by using the process comprising in sequence of hot rolling, cooling to Ar1 or lower, reheating, quenching, and tempering (herein after referred to as “reheating-quenching process”), or by controlling the rolling step. [0003] When using the reheating-quenching process to refine structures and thus manufacturing linepipe having high strength of API, X60 or greater, precipitation-enhancing elements such as Ti, Nb, and / or hardness-improving elements such as Cr, Mo, Cu, Ni need to be added. [0004] However, excessive addition of these elements would cause the deterioration of toughness of welded jo...

AI Technical Summary

Benefits of technology

[0020] Limiting the carbon equivalent value defined by expression (a) to 0.06 or lower is for ensuring the weldability of steel pipe. Generally, seamless steel pipes for linepipe are peripherally welded and jointed at laying location. For the steel pipe having a great carbon equivalent value, cracks occur during the welding and thus pre-heating is required for preventing the cracking. When the Ceq is controlled to be 0.60 or lower, the peripheral welding can be carried out without excessive operations such as pre-heating.

[0032] Ti is not an indispensably added element. But, its suitable addition and the control of finish rolling temperature (that will be described hereinafter) would suppress the unevenness of strength and simultaneously improve strength owing to precipitate enhancement. However, if the addition of Ti exceeds 0.10%, the excessive precipitate enhancement would degrade toughness. Therefore, the Ti content is defined within a range of 0% to 0.10%.

[0034] Similar to Ti, Nb is not an indispensably added element. But, its suitable addition and the control of finish rolling temperature (that will be described hereinafter) would suppress the unevenness of strength while improving strength owing to precipitate enhancement and simultaneously improving the toughness due to the refinement of crystal grains. However, if the addition of Nb exceeds 0.05%, toughness would be degraded. Therefore, the Nb content is defined within a range of 0% to 0.05%.

Problems solved by technology

In order to realize this object, the inventor has carried out repeated researches and found that the precipitating behaviors of Ti, Nb and V vary along with the alteration of the finish rolling temperature, and thus resulting in great strength unevenness between each steel pipe.

Therefore, the unevenness of the strength of steel occurs due to the variation of addition amounts of Ti and Nb and the finish rolling temperature as well.