500 MPA grade longitudinally-welded steel pipe with low yield ratio and manufacturing method therefor

Abstract

A 500 MPa grade longitudinally welded steel pipe with a low yield ratio and a manufacturing method therefor, the components thereof by weight percentages being as follows: C 0.11%-0.16%, Si 0.15%-0.35%, Mn 0.8%-1.5%, V 0.06%-0.15%, Al 0.002%-0.04%, Ti 0%-0.05%, Nb 0%-0.05%, the balance being Fe and inevitable impurities, and the carbon equivalent Ceq being not more than 0.4%. According to the above component design, molten steel is subjected to smelting in a converter or electric furnace, and is cast into a slab; the slab is heated at 1,200 C.-1,300 C. and rolled into a plate strip, with the finished rolling temperature of the plate strip being between 840 C.-940 C.; the rolled plate strip is subjected to laminar flow cooling with a water curtain, the laminar flow cooling adopting the manner of rear-section cooling, and is coiled into a plate roll when being cooled to the temperature interval of 500 C.-560 C.; and after welding formation, a heat treatment on the welding seams is carried out at a heating temperature of 950 C.50 C. The steel pipe of the present invention has the characteristics of an excellent welding property, a high strength and toughness, a low yield ratio and the like, and the yield strength thereof is greater than 500 MPa and the yield ratio thereof is less than 0.85.

Claims

1. A method of manufacturing a longitudinally welded steel pipe comprising a) 0.11 wt %-0.16 wt % carbon (C); b) 0.15 wt %-0.35 wt/o silicon (Si); c) 0.8 wt %-1.5 wt % manganese (Mn); d) 0.06 wt %-0.15 wt % vanadium (V); e) 0.002 wt %-0.04 wt % aluminum (Al); f) 0 wt %-0.05 wt % titanium (Ti); g) 0 wt %-0.05 wt % niobium (Nb); and h) a balance of iron (Fe) and inevitable impurities; wherein the steel pipe has a carbon equivalent (Ceq) of <0.4%, and the steel pipe exhibits a yield strength of greater than 500 MPa, a yield ratio of less than 0.85 and a 0 C. full-scale impact energy of greater than 100 J; the method comprising: a) smelting molten steel; b) casting the smelted steel into a slab; c) heating the slab to a temperature of 1200 C.-1300 C.; d) rolling the heated slab into a plate strip at a temperature of about 840 C.-940 C.; e) laminar flow cooling the rolled plate strip with a water curtain, the laminar flow cooling adopting the manner of rear-section cooling, with the number of closed cooling water valves in the front section accounting for 15%-40% of the total number of cooling water valves, and cooling the strip to the temperature interval of 500 C.-560 C. within 30 seconds; f) coiling the cooled strip into a plate roll; g) welding the plate roll into a pipe; and h) heat treating welding seams of the welded pipe at a heating temperature of 950 C.50 C.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The present invention will be further described hereinafter in combination with embodiments.

(2) The components of the embodiments of the present invention can be found in Table 1, wherein Fe is the balance. The manufacturing methods and properties of the embodiments of the present invention can be found in Table 2.

(3) TABLE-US-00001 TABLE 1 unit: % by weight C Si Mn V Al Ti Nb Ceq Example 1 0.11 0.15 1.50 0.12 0.040 0.01 0.38 Example 2 0.13 0.30 1.11 0.10 0.020 0.02 0.34 Example 3 0.12 0.35 1.42 0.15 0.002 0.02 0.39 Example 4 0.16 0.25 0.80 0.06 0.010 0.05 0.31 Example 5 0.14 0.20 0.92 0.09 0.030 0.05 0.01 0.31 Comparative 0.13 0.30 1.20 0.02 0.02 0.34 Example 1 Comparative 0.19 0.35 1.20 0.12 0.02 0.41 Example 2

(4) TABLE-US-00002 TABLE 2 Proportion of the number of closed cooling water Finished Water 0 C. valves in the front rolling curtain Coiling full-scale section to the total temperature cooling temperature Rt Rtm Yield impact number of cooling C. times C. 0.5 MPa MPa ratio energy/J water valves Example 1 940 28 550 510 655 0.78 115 15% Example 2 900 15 560 545 660 0.83 120 24% Example 3 880 23 510 535 680 0.79 105 29% Example 4 850 12 540 540 645 0.84 110 37% Example 5 840 15 500 550 670 0.82 130 40% Comparative 870 22 500 430 500 0.86 120 Example 1 Comparative 900 25 550 580 710 0.82 40 Example 2

(5) As shown in Table 2, by adopting the chemical component design and production process system of the present invention, the yield strength of the material was greater than 500 MPa, the yield ratio was less than 0.85, the 0 C. full-scale impact energy was greater than 100 J, and the mechanical properties thereof were stable.

(6) After shearing and butt welding, plate roll forming and ERW pipe making, an ERW high-strength steel pipe was produced through the production processes of heat treatment on welding seams and the like, and the strengths of examples 1-5 all met the requirement of the yield strength of greater than 500 MPa. Comparative example 1 had simple components and did not contain the element V, thus being incapable of achieving the requirement of the yield strength of greater than 500 MPa at the coiling temperature of the present invention; comparative example 2 had a higher C content, and the impact toughness of the material decreased significantly, being incapable of meeting the strict requirements for impact load of the steel for a construction pile foundation. It can be seen that with the designed chemical components and process system of the present invention, ERW steel pipes meeting the high property requirement of the yield strength of greater than 500 MPa can be stably produced.