HIGH-STRENGTH STEEL ALLOWING LOW-TEMPERATURE WELDING AND HIGH-HEAT INPUT WELDING AND PRODUCTION METHOD THEREOF

Abstract

A high-strength steel allowing low-temperature welding and high-heat input welding and a production method thereof are provided, which belongs to the technical field of steel production. The high-strength steel includes the following chemical components by mass fraction: 0.03-0.16% of C, 0.05-0.5% of Si, 1.0-1.9% of Mn, 0.002-0.02% of P, 0.001-0.01% of S, 0.005-0.07% of A1, 0.005-0.04% of Ti, 0.1-0.5% of Cr, 0.0005-0.005% of B, 0.002-0.01% of Mg+Zr, 0.001-0.008% of O, 0.004-0.01% of N, and the balance of Fe and residual elements. Magnesium and zirconium are added to form magnesium/zirconium oxide, titanium and boron are added to form titanium/boron nitride, and the two types of precipitates work synergistically to improve the microstructure of a heat-affected zone. The method optimizes the chemical composition and production process of existing high-strength steel.

Claims

1. A high-strength steel allowing low-temperature welding and high-heat input welding, comprising the following chemical components by mass fraction: 0.03-0.16% of C, 0.05-0.5% of Si, 1.0-1.9% of Mn, 0.002-0.02% of P, 0.001-0.01% of S, 0.005-0.07% of Al, 0.005-0.04% of Ti, 0.1-0.5% of Cr, 0.0005-0.005% of B, 0.002-0.01 % of Mg+Zr, 0.001-0.008% of O, 0.004-0.01 % of N, and the balance of Fe and residual elements; and a compound containing titanium nitride or boron nitride in the steel is recorded as titanium/boron nitride, and a compound containing magnesium oxide or zirconium oxide is recorded as magnesium/zirconium oxide; an area number density of the titanium/boron nitride particles with a size of 0.02-0.2 .Math.m in the steel is recorded as a/mm.sup.2, and an area number density of the magnesium/zirconium oxide particles with a size of 0.2-2 .Math.m is recorded as b/mm.sup.2; in a steel matrix, a and b satisfy relational expressions 10<a/b<100 and 10.sup.4<a+10b<10.sup.5; and in a heat-affected zone during steel welding, a and b satisfy relational expressions 5<a/b<50 and 10.sup.4<2a+10b<10.sup.5.

2. The high-strength steel allowing low-temperature welding and high-heat input welding according to claim 1, further comprising the following chemical components by mass fraction: one or more selected from the group consisting of 0.1-0.5% of Mo, 0.1-0.5% of Ni, 0.1-0.5% of Cu, 0.01-0.06% of Nb, and 0.01-0.06% of V.

3. High-strength steel allowing low-temperature welding and high-heat input welding according to claim 1, wherein in the steel matrix, by a number of particles, 3-30% of the magnesium/zirconium oxide with a size of 0.2-2 .Math.m is attached with the titanium/boron nitride.

4. The high-strength steel allowing low-temperature welding and high-heat input welding according to claim 1, wherein under conditions that a preheating temperature of the steel is lower than 100° C. and a welding heat input is 5-50 kJ/cm, and under conditions that the preheating temperature is lower than 50° C. and the welding heat input is 50-500 kJ/cm, the heat-affected zone during welding has impact toughness greater than or equal to 47 J at -40° C.

5. The high-strength steel allowing low-temperature welding and high-heat input welding according to claim 2, wherein under conditions that a preheating temperature of the steel is lower than 100° C. and a welding heat input is 5-50 kJ/cm, and under conditions that the preheating temperature is lower than 50° C. and the welding heat input is 50-500 kJ/cm, the heat-affected zone during welding has impact toughness greater than or equal to 47 J at -40° C.

6. The high-strength steel allowing low-temperature welding and high-heat input welding according to claim 3, wherein under conditions that a preheating temperature of the steel is lower than 100° C. and a welding heat input is 5-50 kJ/cm, and under conditions that the preheating temperature is lower than 50° C. and the welding heat input is 50-500 kJ/cm, the heat-affected zone during welding has impact toughness greater than or equal to 47 J at -40° C.

7. A production method of the high-strength steel allowing low-temperature welding and high-heat input welding according to claim 1, comprising the following processing steps: step 1, wire making: conducting nitrogenation and oxygenation treatment on alloys of titanium, boron, magnesium, and zirconium to obtain oxynitride alloys, crushing the oxynitride alloys into powder with a particle size less than 3 mm, and wrapping the powder with a steel strip to make alloy cored wires, wherein a core material of the alloy cored wires has a weight of 200-500 g/m and an outer diameter of 9-16 mm,x and the steel strip has a thickness of 0.3-0.6 mm; and the core material of the alloy cored wires comprises the following chemical components by mass fraction: 20-45% of Ti, 5-30% of Mg+Zr, 1-10% of B, 1-15% of N, 1-10% of O, 1-50% of Fe, 1-50% of Si, 1-50% of Mn, and residual elements; and installing the alloy cored wires on a wire feeder of a refining station; step 2, steel smelting: smelting molten iron and/or scrap steel into molten steel by a converter or electric furnace, tapping the steel to a ladle, and blowing an argon gas at a bottom during smelting and tapping; transporting the ladle to the refining station for refining, and adjusting components and temperature of the molten steel, wherein a gas used during refining is an argon gas, and the molten steel is prevented from absorbing nitrogen in the air; after dissolved oxygen of the molten steel is less than 0.001 wt.% and dissolved nitrogen is less than 0.004 wt.%, feeding the alloy cored wires; and adjusting alloy components of the molten steel to meet chemical component requirements of the high-strength steel allowing low-temperature welding and high-heat input welding, and leaving the station; and casting the molten steel with full protection to obtain a billet; and step 3, rolling: sending the billet directly to a rolling mill for rolling, or hot charging or cold charging the billet into a heating furnace for heating at 1,150-1,300° C. for 30-300 min; and sending the heated billet the rolling mill for rolling, wherein the rolling mill conducts primary rolling at 1,100-1,250° C. and final rolling at 750-1,100° C.; and cooling the rolled steel to a room temperature, so as to obtain the high-strength steel allowing low-temperature welding and high-heat input welding.

8. The production method of the high-strength steel allowing low-temperature welding and high-heat input welding according to claim 7, further comprising the following chemical components by mass fraction: one or more selected from the group consisting of 0.1-0.5% of Mo, 0.1-0.5% of Ni, 0.1-0.5% of Cu, 0.01-0.06% of Nb, and 0.01-0.06% of V.

9. The production method of the high-strength steel allowing low-temperature welding and high-heat input welding according to claim 7, wherein in step 1, an alloy of titanium and boron is subjected to nitrogenation treatment, an alloy of magnesium and zirconium is subjected to oxygenation treatment, and the two alloys are mixed to form cored wires or each made into cored wires for use in step 2.

10. The production method of the high-strength steel allowing low-temperature welding and high-heat input welding according to claim 7, wherein in step 2, refining methods for the molten steel is one or more selected from the group consisting of ladle furnace (LF) refining, Ruhrstahl-Heraeus (RH) refining, and vacuum degassing (VD) refining.

11. The production method of the high-strength steel allowing low-temperature welding and high-heat input welding according to claim 7, wherein in step 2, the alloy cored wires are fed at 100-200 m/min.

12. The production method of the high-strength steel allowing low-temperature welding and high-heat input welding according to claim 7, wherein in step 2, after the alloy cored wires are fed, the molten steel is bottom-blown with the argon gas for at least 3 min.

13. The production method of the high-strength steel allowing low-temperature welding and high-heat input welding according to claim 7, wherein in step 3, the rolled steel is subjected to on-line controlled cooling or off-line heat treatment.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] FIG. 1 is a photo of an optical microstructure of a heat-affected zone during welding of a high-strength H-beam allowing low-temperature welding and high-heat input welding in Example 2 of the present disclosure without preheating and with a welding heat input of 30 kJ/cm. It can be seen from the figure that a microstructure of the heat-affected zone during welding is significantly refined, and the welding properties are significantly improved.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0043] The specific implementation of the solution of the present disclosure will be described in detail below through the examples, but the protection scope of the present disclosure is not limited to the examples.

Example 1

[0044] In the present example, a high-strength steel plate allowing low-temperature welding and high-heat input welding included the following chemical components by mass fraction: 0.03% of C, 0.2% of Si, 1.9% of Mn, 0.002% of P, 0.0015% of S, 0.02% of Al, 0.02% of Ti, 0.005% of Mg+Zr, 0.004% of O, 0.004% of N, and the balance of Fe and residual elements. In a steel matrix, a number of titanium/boron nitrides with a size of 0.02-0.2 .Math.m was a=32,000/mm.sup.2, a number of magnesium/zirconium oxides with a size of 0.2-2 .Math.m was b=2,100/mm.sup.2, and a and b satisfied relational expressions 10<a/b<100 and 10.sup.4<a+10b<10.sup.5.

[0045] A preparation method for the above steel included the following processing steps: nitrogenation and oxygenation treatment were conducted on alloys of titanium, boron, magnesium, and zirconium to obtain oxynitride alloys. The oxynitride alloys were crushed into powder with a particle size less than 3 mm. The powder was wrapped with a steel strip to make alloy cored wires. A core material of the alloy cored wires had a weight of 300 g/m and an outer diameter of 10 mm, and the steel strip had a thickness of 0.3 mm. The core material of the alloy cored wires include the following chemical components by mass fraction: 20% of Ti, 20% of Mg+Zr, 5% of B, 3% of N, 6% of O, 15% of Fe, 15% of Si, 12% of Mn, and residual elements. The alloy cored wires were installed on a wire feeder of a refining station.

[0046] Molten iron was smelted into molten steel by a converter, the steel was tapped to a ladle, and an argon gas was blown at a bottom during smelting and tapping. The ladle was transported to the refining station for LF refining, and components and temperature of the molten steel were adjusted. A gas used during refining was an argon gas, and the molten steel was prevented from absorbing nitrogen in the air. After dissolved oxygen of the molten steel reached 0.0003 wt.% and dissolved nitrogen reached 0.0025 wt.%, the alloy cored wires were fed at 100 m/min, and the molten steel was bottom-blown with the argon gas for 3 min. Alloy components of the molten steel were adjusted to meet chemical component requirements of the high-strength steel allowing low-temperature welding and high-heat input welding, and the steel left the station. The molten steel was cast with full protection to obtain a billet.

[0047] The billet was hot charged into a heating furnace for heating at 1,150° C. for 60 min. The heated billet was sent to a steel plate mill for rolling, and subjected to primary rolling at 1,100° C. and final rolling at 780° C. The rolled steel was cooled to a room temperature, so as to obtain the high-strength steel plate allowing low-temperature welding and high-heat input welding.

[0048] Under conditions that a preheating temperature of the steel plate was 50° C. and a welding heat input was 15 kJ/cm, the heat-affected zone during welding had impact toughness of 240 J at -40° C. Under conditions of no preheating and a welding heat input of 300 kJ/cm, the heat-affected zone during welding had impact toughness of 270 J at -40° C. In the 15 kJ/cm heat-affected zone during welding of the steel plate, a number of titanium/boron nitrides with a size of 0.02-0.2 .Math.m was a=24,000/mm.sup.2, a number of magnesium/zirconium oxides with a size of 0.2-2 .Math.m was b=1,300/mm.sup.2, and a and b satisfied relational expressions 5<a/b<50 and 10.sup.4<2a+10b<10.sup.5.

Example 2

[0049] In the present example, a high-strength steel allowing low-temperature welding and high-heat input welding included the following chemical components by mass fraction: 0.16% of C, 0.2% of Si, 1.6% of Mn, 0.004% of P, 0.002% of S, 0.02% of Al, 0.015% of Ti, 0.2% of Cr, 0.0015% of B, 0.008% of Mg+Zr, 0.003% of O, 0.006% of N, 0.1% of Mo, and the balance of Fe and residual elements. In a steel matrix, a number of titanium/boron nitrides with a size of 0.02-0.2 .Math.m was a=34,500/mm.sup.2, a number of magnesium/zirconium oxides with a size of 0.2-2 .Math.m was b=2,600/mm.sup.2, and a and b satisfied relational expressions 10<a/b<100 and 10.sup.4<a+10b<10.sup.5. By a number of particles, 15% of the magnesium/zirconium oxide with a size of 0.2-2 .Math.m was attached with the titanium/boron nitride.

[0050] A preparation method for the above steel included the following processing steps: nitrogenation and oxygenation treatment were conducted on alloys of vanadium, titanium, magnesium, and calcium to obtain oxynitride alloys. The oxynitride alloys were crushed into powder with a particle size less than 3 mm. The powder was wrapped with a steel strip to make alloy cored wires. A core material of the alloy cored wires had a weight of 400 g/m and an outer diameter of 12 mm, and the steel strip had a thickness of 0.5 mm. The core material of the alloy cored wires include the following chemical components by mass fraction: 25% of V, 15% of Ti, 10% of Mg+Ca, 16% of N, 3% of O, 15% of Fe, 8% of Si, 5% of Mn, and residual elements. The alloy cored wires were installed on a wire feeder of a refining station.

[0051] Molten iron and scrap steel were smelted into molten steel by an electric furnace, the steel was tapped to a ladle, and an argon gas was blown at a bottom during smelting and tapping. The ladle was transported to the refining station for RH refining, and components and temperature of the molten steel were adjusted. A gas used during refining was an argon gas, and the molten steel was prevented from absorbing nitrogen in the air. After dissolved oxygen of the molten steel reached 0.0002 wt.% and dissolved nitrogen reached 0.003 wt.%, the alloy cored wires were fed at 120 m/min, and the molten steel was bottom-blown with the argon gas for 4 min. Alloy components of the molten steel were adjusted to meet chemical component requirements of the high-strength steel allowing low-temperature welding and high-heat input welding, and the steel left the station. The molten steel was cast with full protection to obtain a billet.

[0052] The billet was cold charged into a heating furnace for heating at 1,300° C. for 90 min. The heated billet was sent to a H-beam mill for rolling, and subjected to primary rolling at 1,250° C. and final rolling at 1,100° C. The rolled steel was cooled to a room temperature, so as to obtain the high-strength H-beam allowing low-temperature welding and high-heat input welding.

[0053] Under conditions of no preheating of the H-beam and a welding heat input of 30 kJ/cm, the heat-affected zone during welding had impact toughness of 160 J at -40° C. Under conditions that a preheating temperature was 45° C. and a welding heat input was 120 kJ/cm, the heat-affected zone during welding had impact toughness of 230 J at -40° C. In the 120 kJ/cm heat-affected zone during welding of the steel plate, a number of titanium/boron nitrides with a size of 0.02-0.2 .Math.m was a=19,500/mm.sup.2, a number of magnesium/zirconium oxides with a size of 0.2-2 .Math.m was b=1,600/mm.sup.2, and a and b satisfied relational expressions 5<a/b<50 and 10.sup.4<2a+10b<10.sup.5.

Example 3

[0054] In the present example, a high-strength steel allowing low-temperature welding and high-heat input welding included the following chemical components by mass fraction: 0.07% of C, 0.4% of Si, 1.3% of Mn, 0.002% of P, 0.004% of S, 0.03% of Al, 0.02% of Ti, 0.3% of Cr, 0.0035% of B, 0.006% of Mg+Zr, 0.006% of O, 0.008% of N, 0.2% of Cu, 0.1% of Mo, and the balance of Fe and residual elements. In the steel matrix, a number of titanium/boron nitrides with a size of 0.02-0.2 .Math.m was a=42,500/mm.sup.2, a number of magnesium/zirconium oxides with a size of 0.2-2 .Math.m was b=3,500/mm.sup.2, and a and b satisfied relational expressions 10<a/b<100 and 10.sup.4<a+10b<10.sup.5. By a number of particles, 25% of the magnesium/zirconium oxide with a size of 0.2-2 .Math.m was attached with the titanium/boron nitride.

[0055] A preparation method for the above steel included the following processing steps: nitrogenation and oxygenation treatment were conducted on alloys of vanadium, titanium, magnesium, and calcium to obtain oxynitride alloys. The oxynitride alloys were crushed into powder with a particle size less than 3 mm. The powder was wrapped with a steel strip to make alloy cored wires. A core material of the alloy cored wires had a weight of 500 g/m and an outer diameter of 16 mm, and the steel strip had a thickness of 0.4 mm. The core material of the alloy cored wires include the following chemical components by mass fraction: 45% of Ti, 12% of Mg+Zr, 8% of B, 3% of N, 6% of O, 20% of Fe, 1% of Si, 1% of Mn, and residual elements. The alloy cored wires were installed on a wire feeder of a refining station.

[0056] Molten iron and scrap steel were smelted into molten steel by a converter, the steel was tapped to a ladle, and an argon gas was blown at a bottom during smelting and tapping. The ladle was transported to the refining station for LF-RH refining, and components and temperature of the molten steel were adjusted. A gas used during refining was an argon gas, and the molten steel was prevented from absorbing nitrogen in the air. After dissolved oxygen of the molten steel reached 0.0005 wt.% and dissolved nitrogen reached 0.0025 wt.%, the alloy cored wires were fed at 150 m/min, and the molten steel was bottom-blown with the argon gas for 14 min. Alloy components of the molten steel were adjusted to meet chemical component requirements of the high-strength steel allowing low-temperature welding and high-heat input welding, and the steel left the station. The molten steel was cast with full protection to obtain a billet.

[0057] The billet was hot charged into a heating furnace for heating at 1,250° C. for 120 min. The heated billet was sent to a heavy and medium plate mill for rolling, and subjected to primary rolling at 1,200° C. and final rolling at 1,050° C. The steel plate had a thickness of 52 mm. The rolled steel was cooled to a room temperature and normalized at 900° C., so as to obtain the high-strength steel plate allowing low-temperature welding and high-heat input welding.

[0058] Under conditions that a preheating temperature of the steel plate was as low as 80° C. and a welding heat input was 5 kJ/cm, the heat-affected zone during welding had impact toughness of 220 J at -40° C. Under conditions of no preheating and a welding heat input of 500 kJcm, the heat-affected zone during welding had impact toughness of 180 J at -40° C. In the 500 kJ/cm heat-affected zone during welding of the steel plate, a number of titanium/boron nitrides with a size of 0.02-0.2 .Math.m was a=27,500/mm.sup.2, a number of magnesium/zirconium oxides with a size of 0.2-2 .Math.m was b=2,600/mm.sup.2, and a and b satisfied relational expressions 5<a/b<50 and 10.sup.4<2a+10b<10.sup.5.

Example 4

[0059] In the present example, a high-strength steel plate allowing low-temperature welding and high-heat input welding included the following chemical components by mass fraction: 0.05% of C, 0.5% of Si, 1.9% of Mn, 0.006% of P, 0.003% of S, 0.01% of Al, 0.04% of Ti, 0.3% of Cr, 0.003% of B, 0.01% of Mg+Zr, 0.008% of O, 0.008% of N, 0.1% of V, 0.1% of Nb, and the balance of Fe and residual elements. In the steel, a number of titanium/boron nitrides with a size of 0.02-0.2 .Math.m was 46,500/mm.sup.2, and a number of magnesium/zirconium oxides with a size of 0.2-2 .Math.m was 3,400/mm.sup.2. By a number of particles, 8% of the magnesium/zirconium oxide with a size of 0.2-2 .Math.m was attached with the titanium/boron nitride.

[0060] A preparation method for the above steel included the following processing steps: nitrogenation and oxygenation treatment were conducted on alloys of vanadium, titanium, magnesium, and calcium to obtain oxynitride alloys. The oxynitride alloys were crushed into powder with a particle size less than 3 mm. The powder was wrapped with a steel strip to make alloy cored wires. A core material of the alloy cored wires had a weight of 250 g/m and an outer diameter of 14 mm, and the steel strip had a thickness of 0.6 mm. The core material of the alloy cored wires include the following chemical components by mass fraction: 30% of Ti, 25% of Mg+Zr, 8% of B, 5% of N, 6% of O, 10% of Fe, 8% of Si, 13% of Mn, and residual elements. The alloy cored wires were installed on a wire feeder of a refining station.

[0061] Molten iron and scrap steel were smelted into molten steel by an electric furnace, the steel was tapped to a ladle, and an argon gas was blown at a bottom during smelting and tapping. The ladle was transported to the refining station for LF-VD refining, and components and temperature of the molten steel were adjusted. A gas used during refining was an argon gas, and the molten steel was prevented from absorbing nitrogen in the air. After dissolved oxygen of the molten steel reached 0.0006 wt.% and dissolved nitrogen reached 0.0035 wt.%, the alloy cored wires were fed at 200 m/min, and the molten steel was bottom-blown with the argon gas for 4 min. Alloy components of the molten steel were adjusted to meet chemical component requirements of the high-strength steel allowing low-temperature welding and high-heat input welding, and the steel left the station. The molten steel was cast with full protection to obtain a billet.

[0062] The billet was directly sent to a rolling mill for rolling, and subjected to final rolling at 800° C. The rolled steel was cooled on-line to 630° C., and air-cooled to a room temperature, so as to obtain the high-strength steel plate allowing low-temperature welding and high-heat input welding.

[0063] Under conditions that a preheating temperature of the steel plate was 90° C. and a welding heat input was 30 kJ/cm, the heat-affected zone during welding had impact toughness of 200 J at -40° C. Under conditions that a preheating temperature was 40° C. and a welding heat input was 350 kJ/cm, the heat-affected zone during welding had impact toughness of 175 J at -40° C. In the 350 kJ/cm heat-affected zone during welding of the steel, a number of titanium/boron nitrides with a size of 0.02-0.2 .Math.m was a=27,500/mm.sup.2, a number of magnesium/zirconium oxides with a size of 0.2-2 .Math.m was b=2,400/mm.sup.2, and a and b satisfied relational expressions 5<a/b<50 and 10.sup.4<2a+10b<10.sup.5.

Example 5

[0064] In the present example, a high-strength steel allowing low-temperature welding and high-heat input welding included the following chemical components by mass fraction: 0.08% of C, 0.2% of Si, 1.5% of Mn, 0.003% of P, 0.003% of S, 0.02% of Al, 0.02% of Ti, 0.1% of Cr, 0.0025% of B, 0.06% of Mg+Zr, 0.005% of O, 0.005% of N, 0.1% of Ni, 0.1% of Cu, and the balance of Fe and residual elements. In the steel, a number of titanium/boron nitrides with a size of 0.02-0.2 .Math.m was 29,500/mm.sup.2, and a number of magnesium/zirconium oxides with a size of 0.2-2 .Math.m was 1,600/mm.sup.2. By a number of particles, 20% of the magnesium/zirconium oxide with a size of 0.2-2 .Math.m was attached with the titanium/boron nitride.

[0065] A preparation method for the above steel included the following processing steps: nitrogenation and oxygenation treatment were conducted on alloys of vanadium, titanium, magnesium, and calcium to obtain oxynitride alloys. The oxynitride alloys were crushed into powder with a particle size less than 3 mm. The powder was wrapped with a steel strip to make alloy cored wires. A core material of the alloy cored wires had a weight of 350 g/m and an outer diameter of 14 mm, and the steel strip had a thickness of 0.6 mm. The core material of the alloy cored wires include the following chemical components by mass fraction: 35% of Ti, 20% of Mg+Zr, 5% of B, 8% of N, 6% of O, 5% of Fe, 10% of Si, 7% of Mn, and residual elements. The alloy cored wires were installed on a wire feeder of a refining station.

[0066] Molten iron and scrap steel were smelted into molten steel by an electric furnace, the steel was tapped to a ladle, and an argon gas was blown at a bottom during smelting and tapping. The ladle was transported to the refining station for RH refining, and components and temperature of the molten steel were adjusted. A gas used during refining was an argon gas, and the molten steel was prevented from absorbing nitrogen in the air. After dissolved oxygen of the molten steel reached 0.0008 wt.% and dissolved nitrogen reached 0.0035 wt.%, the alloy cored wires were fed at 180 m/min, and the molten steel was bottom-blown with the argon gas for 10 min. Alloy components of the molten steel were adjusted to meet chemical component requirements of the high-strength steel allowing low-temperature welding and high-heat input welding, and the steel left the station. The molten steel was cast with full protection to obtain a billet.

[0067] The billet was cold charged into a heating furnace for heating at 1,200° C. for 150 min. The heated billet was sent to a rolling mill for rolling, and subjected to primary rolling at 1,150° C. and final rolling at 1,100° C. The rolled steel was cooled to a room temperature. The steel plate was subjected to off-line quenching and tempering heat treatment, the quenching was conducted at 900° C., and the heating was conducted for 30 min, and the tempering was conducted at 580° C. for 60 min, so as to obtain the high-strength steel plate allowing low-temperature welding and high-heat input welding.

[0068] Under conditions of no preheating of the steel plate and a welding heat input of 45 kJ/cm, the heat-affected zone during welding had impact toughness of 240 J at -40° C. Under conditions of no preheating and a welding heat input of 250 kJ/cm, the heat-affected zone during welding had impact toughness of 200 J at -40° C. In the 250 kJ/cm heat-affected zone during welding of the steel, a number of titanium/boron nitrides with a size of 0.02-0.2 .Math.m was a=16,000/mm.sup.2, a number of magnesium/zirconium oxides with a size of 0.2-2 .Math.m was b=1,800/mm.sup.2, and a and b satisfied relational expressions 5<a/b<50 and 10.sup.4<2a+10b<10.sup.5.

Comparative Example 1

[0069] In the comparative example, a high-strength steel plate included the following chemical components by mass fraction: 0.04% of C, 0.25% of Si, 1.85% of Mn, 0.002% of P, 0.002% of S, 0.025% of Al, 0.02% of Ti, 0.005% of Mg+Zr, 0.004% of O, 0.004% of N, 0.1% of Cr, 0.1% of Mo, 0.2% of Ni, and the balance of Fe and residual elements. In steel matrix, a number of titanium/boron nitrides with a size of 0.02-0.2 .Math.m was a=63,500/mm.sup.2, a number of magnesium/zirconium oxides with a size of 0.2-2 .Math.m was b=500/mm.sup.2, and a and b did not satisfy relational expressions 10<a/b<100 and 10.sup.4<a+10b<10.sup.5.

[0070] A preparation method for the above steel included the following processing steps: molten iron was smelted into molten steel by a converter, the steel was tapped to a ladle, and an argon gas was blown at a bottom during smelting and tapping. The ladle was transported to the refining station for LF refining, and components and temperature of the molten steel were adjusted. A gas used during refining was an argon gas. Alloys with various elements were added separately, the components of the molten steel were adjusted to meet the requirements of the components of the steel plate, and the steel left the station. The molten steel was cast with full protection to obtain a billet.

[0071] The billet was hot charged into a heating furnace for heating at 1,150° C. for 60 min. The heated billet was sent to a rolling mill for rolling, and subjected to primary rolling at 1,100° C. and final rolling at 780° C. The rolled steel was cooled to a room temperature, so as to obtain the high-strength steel plate.

[0072] Under conditions that a preheating temperature of the steel plate was 80° C. and a welding heat input was 15 kJ/cm, the heat-affected zone during welding had impact toughness of 35 J at -40° C. Under conditions of no preheating and a welding heat input of 100 kJ/cm, the heat-affected zone during welding had impact toughness of 18 J at -40° C. In the 100 kJ/cm heat-affected zone during welding of the steel plate, a number of titanium/boron nitrides with a size of 0.02-0.2 .Math.m was a=55,000/mm.sup.2, a number of magnesium/zirconium oxides with a size of 0.2-2 .Math.m was b=400/mm.sup.2, and a and b did not satisfy relational expressions 5<a/b<50 and 10.sup.4<2a+10b<10.sup.5.

[0073] However, the comparative example does not optimally control the distribution of oxides and nitrides, and the welding properties are difficult to meet the requirements.