Steel plate resistant to zinc-induced crack and manufacturing method therefor

Abstract

The invention discloses a steel plate resistant to zinc-induced crack and a manufacturing method therefor. A low-alloy steel subjected to low C-ultra low Si-high Mn-low Al(Ti+Nb) microalloying treatment is taken as a basis; the Al content in the steel is appropriately reduced; the conditions are controlled so that Mn/C15, [(% Mn)+0.75(% Mo)](% C)0.16, Nb/Ti1.8 and Ti/N is between 1.50 and 3.40, CEZ0.44% and the B content is 2 ppm, Ni/Cu1.50; a Ca treatment is performed and the Ca/S ratio is controlled between 1.0 and 3.0, with (% Ca)(% S).sup.0.281.010.sup.3; and a TMCP process is optimized, so that a finished steel plate has a micro-structure of ferrite+bainite colonies which are tiny and dispersedly distributed, with an average grain size of not greater than 10 m, has homogeneous and excellent mechanical properties, excellent weldability and zinc-induced crack resistance, and is thus especially suitable as a zinc-spray coated corrosion-resistant steel plate for marine structures, a zinc-spray corrosion-resistant steel plate for extra-high voltage power transmission structures, a zinc-spray coated corrosion-resistant steel plate for coast bridge structures, and the like.

Claims

1. A steel plate consisting of in weight percentages: C: 0.05%-0.090%; Si: 0.20%; Mn: 1.35%-1.65%; P: 0.013%; S: 0.003%; Cu: 0.10%-0.30%; Ni: 0.20%-0.50%; Mo: 0.05%-0.20%; Nb: 0.015%-0.035%; Ti: 0.008%-0.018%; N: 0.0060%; Ca: 0.0010%-0.0040%; B: 0.0002%, and the balance being Fe and inevitable impurities; and at the same time the contents of the above-mentioned elements must satisfy the relationships as follows: Mn/C15; [(% Mn)+0.75(% Mo)](% C)0.16; CEZ0.44%, wherein, CEZ=C+Si/17+Mn/7.5+Cu/13+Ni/17+Cr/4.5+Mo/3+V/1.5+Nb/2+Ti/4.5+420B; Ni/Cu1.50; Nb/Ti1.8, and TUN is between 1.50 and 3.40; Ca/S is between 1.00 and 3.00, and (% Ca)(% S).sup.0.281.010.sup.3; wherein the finished steel plate has a yield strength of 460 MPa, a tensile strength of 550 MPa, and a single value of an impact energy at 60 C. of 47 J, the micro-structure of the finished steel plate is ferrite and bainite colonies which are tiny and dispersedly and homogeneously distributed, with an average grain size controlled at not greater than 10 m, and the micro-structure of a welding heat-affected zone is tiny and homogeneous ferrite and a small amount of pearlite; and wherein the S.sub.LM of the steel plate is 42%, wherein S.sub.LM=(the breaking strength of a galvanized tensile test bar containing periphery notches/the breaking strength of an un-galvanized tensile test bar containing periphery notches)100%.

2. A method for manufacturing the steel plate resistant to zinc-induced crack of claim 1, comprising the following steps: smelting and casting: a slab is formed by smelting and continuous casting according to the above-mentioned components and using a light reduction technique, the light reduction rate for continuous casting is controlled between 2% and 5%, the pouring temperature of a tundish is between 1530 C. and 1560 C., and the withdrawal speed is 0.6 m/min-1.0 m/min; heating: the heating temperature of the slab is 1050 C.1150 C., the slab is descaled with high pressure water after being removed from the furnace, and the descaling can be repeated if it is incomplete; rolling: a first stage is a normal rolling, wherein the maximum capacity of a rolling mill is used for an uninterrupted rolling, the pass reduction rate is 10%, the accumulated reduction rate is 45%, and the final rolling temperature is 980 C.; and a second stage adopts a controlled rolling in an austenite single phase region, wherein the initial rolling temperature of the controlled rolling is 800 C.850 C., the pass reduction rate of the rolling is 8%, the accumulated reduction rate is 50%, and the final rolling temperature is 760 C.800 C.; and cooling: after the controlled rolling is finished, the steel plate is immediately transported to accelerated cooling equipment to perform accelerated cooling on the steel plate, wherein the initial cooling temperature of the steel plate is 750 C.790 C., the cooling rate is 5 C./s, the stop-cooling temperature is 350 C.550 C., and thereafter the steel plate with a thickness of 25 mm is naturally air-cooled to not less than 300 C., and then slow-cooled and dehydrogenated, the slow cooling process consisting in maintaining the steel plate at not less than 300 C. for at least 36 hours; and the steel plate with a thickness of <25 mm is naturally air-cooled to room temperature.

3. The steel plate of claim 1, wherein the steel plate is a zinc-spray coated steel plate for marine structures, a zinc-spray steel plate for extra-high voltage power transmission structures, or a zinc-spray coated steel plate for coast bridge structures.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is the micro-structure of the steel in example 5 of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(2) The present invention is further illustrated below in conjunction with the embodiments and the drawings.

(3) See table 1 for the components of the steels in the embodiments of the present invention, and see tables 2 and 3 for the manufacturing process of the steels in the embodiments. Table 4 is the properties of the steels in the embodiments of the present invention.

(4) As shown in FIG. 1, the micro-structure of the finished steel plate of the present invention is ferrite+bainite colonies which are tiny and dispersedly and homogeneously distributed, with an average grain size controlled at not greater than 10 m, and the micro-structure of the welding heat-affected zone is tiny and homogeneous ferrite+a small amount of pearlite.

(5) In the present invention, through the combinational design of alloy elements and the strict control of residual B element in the steel, and the match with a suitable TMCP process, while ensuring the good mechanical properties and weldability of the steel plate as the base material, the welded joints, especially the welding heat-affected zone, of the steel plate has an excellent zinc-induced-crack-resistance, the organic unity of the high strength, good weldability and zinc-induced-crack-resistance is achieved, and the steel plate is particularly suitable as a zinc-spray coated corrosion-resistant steel plate for marine structures, a zinc-spray corrosion-resistant steel plate for extra-high voltage power transmission structures, a zinc-spray coated corrosion-resistant steel plate for coast bridge structures, and the like. Furthermore, the technique of the present invention is implemented through an on-line TMCP control process, the quenched-tempered heat treatment process is eliminated; not only the manufacturing cycle of the steel plate is shortened and the manufacturing costs of the steel plate is decreased, but also the production organization difficulty of the steel plate is reduced, and the production operating efficiency is improved; the relatively low noble alloy component design (especially the contents of Cu, Ni and Mo) greatly reduces the alloy costs of the steel plate; the ultra low C content, and low carbon equivalent and Pcm index greatly improve the weldability of the steel plate, especially high heat input weldability, thereby substantially enhancing the manufacturing efficiency of the on-site welding for users, saving the member-manufacturing costs for users, shortening the member-manufacturing time for users and creating great values for users; therefore such a steel plate is not only a high value-added and green and environmentally friendly product. The successful implementation of the technology in this patent marks that Baosteel makes a new breakthrough in the aspect of the key manufacturing technology of zinc-induced-crack-resistance steel plate, which improves the brand image and market competitiveness of the thick plate of Baosteel; it is not necessary to add any equipment during the production of a 550 MPa high-strength steel plate in the present invention, the manufacturing process is simple and the production process is easily controlled, and therefore, the manufacturing costs are low, and a very high cost performance and market competitiveness are achieved; and this technology has a strong adaptability, can be promoted to all the medium and heavy plate manufacturers having thermal treatment equipment, and has a very strong commercial popularization and a relatively high technology trade value.

(6) With the development of national economy in our country, the requirement of building an economical and harmonious society and the energy development have been put on the agenda, the ocean exploitation by humans is the most important; the steel plates for large-scale marine structures, offshore drilling platforms, drilling derricks and cross-sea bridges all need to spray zinc for anti-corrosion, the steel plate resistant to zinc-induced crack has a broad market prospect, and the 550 MPa-grade steel plate resistant to zinc-induced crack is still a bran-new steel type in our country; except for Baosteel, other iron and steel enterprises in our country never investigated and trial-manufactured. At present, this type of steel has been successfully trial-manufactured in Baosteel, and each mechanical performance index, weldability and zinc-induced-crack resistance thereof have reached an international advanced level.

(7) TABLE-US-00001 TABLE 1 Unit: weight percentage Steel sample C Si Mn P S Cu Ni Mo Nb Ti N Ca B Fe and impurities Example 1 0.05 0.17 1.38 0.013 0.0017 0.10 0.20 0.05 0.015 0.008 0.0043 0.0019 0.0002 the balance Example 2 0.07 0.11 1.35 0.010 0.0008 0.16 0.25 0.09 0.020 0.011 0.0038 0.0022 0.0001 the balance Example 3 0.06 0.20 1.50 0.011 0.0030 0.25 0.40 0.12 0.027 0.015 0.0046 0.0030 0.0001 the balance Example 4 0.09 0.10 1.60 0.007 0.0014 0.22 0.45 0.16 0.032 0.017 0.0053 0.0040 / the balance Example 5 0.07 0.09 1.65 0.008 0.0009 0.30 0.50 0.20 0.035 0.018 0.0060 0.0010 / the balance

(8) TABLE-US-00002 TABLE 2 1st stage rolling 2nd stage controlled rolling Accu- Final Controlled Final Accu- Light Pouring With- Heating Pass mulated rolling rolling rolling Pass mulated reduction temperature drawal temper- reduction reduction temper- temper- temper- reduction reduction rate of tundish speed ature rate rate ature ature ature rate rate Steel sample (%) ( C.) (m/min) ( C.) (%) (%) ( C.) ( C.) ( C.) (%) (%) Example 1 3 1560 1.0 1150 13 80 980 850 760 9 75 Example 2 2 1545 0.9 1130 10 75 995 830 775 8 75 Example 3 5 1530 0.7 1100 11 60 1000 820 800 8 60 Example 4 4 1550 0.8 1080 10 45 990 810 790 9 55 Example 5 3 1535 0.6 1050 12 50 1010 800 780 9 50

(9) TABLE-US-00003 TABLE 3 Controlled cooling process Slow cooling process Initial Stop- Slow Slow cooling Cooling cooling cooling cooling Steel temperature rate temperature temperature time sample ( C.) ( C./s) ( C.) ( C.) (hr.) Example 1 750 25 550 Natural air / cooling Example 2 765 15 500 311 36 Example 3 790 8 430 323 40 Example 4 780 6 400 335 40 Example 5 770 5 350 357 48

(10) TABLE-US-00004 TABLE 4 Product Welding plate preheating thickness YP TS Akv (40 C.) temperature S.sub.LM Steel sample (mm) MPa MPa % J ( C.) (%) Note Example 1 12 535 617 23 332, 367, 355; 351 0 63 no occurrence of zinc-induced cracks Example 2 25 527 623 25 363, 375, 344; 361 0 57 no occurrence of zinc-induced cracks Example 3 50 519 621 25 355, 349, 366; 357 0 60 no occurrence of zinc-induced cracks Example 4 65 530 636 26 324, 335, 348; 336 0 52 no occurrence of zinc-induced cracks Example 5 80 522 608 25 293, 303, 317; 304 0 50 no occurrence of zinc-induced cracks Note: S.sub.LM = (the breaking strength of a galvanized tensile test bar containing periphery notches/the breaking strength of an un-galvanized tensile test bar containing periphery notches) 100%, and S.sub.LM 42% indicates no occurrence of zinc-induced cracks.