500 MPA GRADE LOW YIELD RATIO WEATHER-RESISTANT BRIDGE STEEL AND MANUFACTURING METHOD THEREFOR

Abstract

Disclosed is 500-MPa low-yield-ratio weather-resistant bridge steel and a manufacturing method therefor; the weather-resistant bridge steel includes the following components in percentage by mass: C: 0.04%-0.09%, Si: 0.15%-0.30%, Mn: 1.40%-1.50%, P: 0.009%-0.015%, S: ≤0.002%, Nb: 0.020%-0.050%, Ti: 0.010%-0.020%, V: 0.010%-0.030%, Cu: 0.30%-0.40%, Ni: 0.30%-0.45%, Cr: 0.45%-0.60%, Mo: 0.08%-0.15%, Alt: 0.02%-0.04%, and the balance Fe and inevitable impurities; through scientific component designing and a matched manufacturing method combining controlled rolling and cooling and tempering, the weather-resistant bridge steel has a low yield ratio, high low-temperature toughness and high elongation.

Claims

1. 500 MPa low-yield-ratio weather-resistant bridge steel, comprising the following components in percentage by mass: C: 0.04%-0.09%, Si: 0.15%-0.30%, Mn: 1.40%-1.50%, P: 0.009%-0.015%, S: ≤0.002%, Nb: 0.020%-0.050%, Ti: 0.010%-0.020%, V: 0.010%-0.030%, Cu: 0.30%-0.40%, Ni: 0.30%-0.45%, Cr: 0.45%-0.60%, Mo: 0.08%-0.15%, Alt: 0.02%-0.04%, and the balance Fe and inevitable impurities.

2. The 500-MPa low-yield-ratio weather-resistant bridge steel according to claim 1, wherein a metallographic structure is tempered bainite.

3. The 500-MPa low-yield-ratio weather-resistant bridge steel according to claim 1, wherein a 8 mm-80 mm thick steel plate has a yield ratio ≤0.83.

4. The 500-MPa low-yield-ratio weather-resistant bridge steel according to claim 3, wherein an atmospheric corrosion resistance index I ≥6.5.

5. The 500-MPa low-yield-ratio weather-resistant bridge steel according to claim 1, wherein in the components in percentage by mass, C is 0.06%-0.09% and Mn is 1.40%-1.46%.

6. A manufacturing method for the 500-MPa low-yield-ratio weather-resistant bridge steel according to claim 1, comprising processes of smelting, continuous casting, soaking, rolling, relaxation, cooling and off-line tempering, wherein a continuous casting billet is heated in the soaking process until a center temperature reaches 1130° C.-1230° C.; the rolling process is to conduct recrystallization zone rolling and non-recrystallization zone rolling on a descaled continuous casting billet, and an accumulated deformation amount of the recrystallization zone rolling is 50% or more of a thickness of the continuous casting billet; an intermediate billet holds a temperature at 800° C.-990° C., a temperature-holding thickness is 2 times-4 times of a final-product thickness, the non-recrystallization zone rolling is conducted after a temperature is reached, and a finishing temperature is controlled to be 790° C.-830° C.; in the relaxation process, relaxation is conducted until an initial cooling temperature is 730° C.-760° C.; the cooling process is to conduct laminar cooling from the initial cooling temperature, control a self-tempering temperature to be 420° C.-600° C., and then conduct air-cooling to a room temperature; and in the off-line tempering process, a tempering temperature is 450° C.-550° C., heat preservation is conducted at the temperature for 20 min-40 min, heat preservation time being proportionate to the final-product thickness, and then natural cooling is conducted to the room temperature.

7. The manufacturing method according to claim 6, wherein a 150 mm-320 mm thick continuous casting billet is used in manufacturing of a 8 mm-80 mm thick final product.

8. The manufacturing method according to claim 7, wherein in the continuous casting process, stacking cooling is conducted on the continuous casting billet for 24 h or more, stacking cooling time increases with increasing of a thickness of the continuous casting billet, and for a 320 mm continuous casting billet, the stacking cooling time is 48 h or more.

9. The manufacturing method according to claim 8, wherein a uniform temperature of the continuous casting billet in the soaking process is less than 20° C.

10. The manufacturing method according to claim 9, wherein in the soaking process, heating time ≥ the thickness of the continuous casting billet * 1 min/mm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 is an optical metallographic structure image of a product after magnification of 500 times in Embodiment 2.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] The present disclosure is further described below with reference to specific embodiments.

[0030] Embodiment 1 included the components in percentage by mass: C: 0.04%, Si: 0.28%, Mn: 1.50%, P: 0.014%, S: 0.0010%, Nb: 0.020%, Ti: 0.015%, V: 0.010%, Ni: 0.30%, Cu: 0.40%, Cr: 0.45%, Mo: 0.08%, Alt: 0.02%, and the balance Fe and inevitable impurities. Smelting, refining, alloying and calcium treatment were conducted on raw materials, then molten steel was obtained, and the molten steel was subjected to slab continuous casting, a thickness of a casting billet being 150 mm and an atmospheric corrosion resistance index I being 6.51. Stacking cooling was conducted on a continuous casting billet 24 h or more, and the billet was soaked at 1230° C., a uniform temperature being less than 20° C. After heating was conducted for 150 min, dephosphorization was conducted, and then rolling was conducted at two stages. A temperature of recrystallization zone rolling was 1080° C., a total deformation amount was 79%, and a thickness of an intermediate billet was controlled to be 4 times of that of a final product. An initial rolling temperature of non-recrystallization zone rolling was 990° C., a thickness of the final product was 8 mm, and a finishing temperature was 830° C.

[0031] After rolling finishing, relaxation was conducted to an initial cooling temperature of 730° C., laminar cooling was conducted on a steel plate at the initial cooling temperature, a self-tempering temperature being 600° C., then air-cooling was conducted to a room temperature, then the steel plate was tempered at a tempering temperature of 550° C., and heat preservation was conducted at the temperature for 20 min.

[0032] Through observation on a metallographic structure of a sample after controlled rolling and cooling and tempering, it was found that a microstructure type was “tempered bainite”, and the material had yield strength of 575 MPa, tensile strength of 693 MPa, a yield ratio of finished steel of 0.83, -40° C. Akv of 180 J, and elongation A of 20%.

[0033] Embodiment 2 included the components in percentage by mass: C: 0.06%, Si: 0.30%, Mn: 1.46%, P: 0.010%, S: 0.0015%, Nb: 0.040%, Ti: 0.020%, V: 0.020%, Ni: 0.35%, Cu: 0.30%, Cr: 0.60%, Mo: 0.10%, Alt: 0.04%, and the balance Fe and inevitable impurities. Smelting, refining, alloying and calcium treatment were conducted on raw materials, then molten steel was obtained, and the molten steel was subjected to slab continuous casting, a thickness of a casting billet being 320 mm and an atmospheric corrosion resistance index I being 6.70. Stacking cooling was conducted on a continuous casting billet 48 h or more, and the billet was soaked at 1160° C., a uniform temperature being less than 20° C. After heating was conducted for 352 min, dephosphorization was conducted, and then rolling was conducted at two stages. A temperature of recrystallization zone rolling was 1070° C., a total deformation amount was 53%, and a thickness of an intermediate billet was controlled to be 2.5 times of that of a final product. An initial rolling temperature of non-recrystallization zone rolling was 850° C., a thickness of the final product was 60 mm, and a finishing temperature was 810° C.

[0034] After rolling finishing, relaxation was conducted to an initial cooling temperature of 750° C., laminar cooling was conducted on a steel plate at the initial cooling temperature, a self-tempering temperature being 480° C., then air-cooling was conducted to a room temperature, then the steel plate was tempered at a tempering temperature of 500° C., and heat preservation was conducted at the temperature for 35 min.

[0035] As shown in FIG. 1, through observation on a metallographic structure of a sample after controlled rolling and cooling and tempering, it was found that a microstructure type was “tempered bainite”, and the material had yield strength of 556 MPa, tensile strength of 682 MPa, a yield ratio of finished steel of 0.82, -40° C. Akv of 225 J, and elongation A of 21%.

[0036] Embodiment 3 included the components in percentage by mass: C: 0.09%, Si: 0.15%, Mn: 1.40%, P: 0.0090%, S: 0.0020%, Nb: 0.035%, Ti: 0.018%, V: 0.030%, Ni: 0.45%, Cu: 0.37%, Cr: 0.50%, Mo: 0.15%, Alt: 0.02%, and the balance Fe and inevitable impurities. Smelting, refining, alloying and calcium treatment were conducted on raw materials, then molten steel was obtained, and the molten steel was subjected to slab continuous casting, a thickness of a casting billet being 320 mm and an atmospheric corrosion resistance index I being 6.53. Stacking cooling was conducted on the casting billet 48 h or more, and the billet was soaked at 1130° C., a uniform temperature being less than 20° C. After heating was conducted for 320 min, dephosphorization was conducted, and then rolling was conducted at two stages. A temperature of recrystallization zone rolling was 1040° C., a total deformation amount of rough rolling was 50%, and a thickness of an intermediate billet was controlled to be 2.0 times of that of a final product. An initial rolling temperature of non-recrystallization zone rolling was 800° C., a thickness of the final product was 80 mm, and a finishing temperature was 790° C.

[0037] After rolling finishing, relaxation was conducted to an initial cooling temperature of 760° C., laminar cooling was conducted on a steel plate at the initial cooling temperature, a self-tempering temperature being 420° C., then air-cooling was conducted to a room temperature, then the steel plate was tempered at a tempering temperature of 450° C., and heat preservation was conducted at the temperature for 40 min.

[0038] Through observation on a metallographic structure of a sample after controlled rolling and cooling and tempering, it was found that a microstructure type at low magnification was “tempered bainite” with high uniformity, and the material had yield strength of 545 MPa, tensile strength of 673 MPa, a yield ratio of finished steel of 0.81, -40° C. Akv of 216 J, and elongation A of 22%.

[0039] Embodiment 4 included the components in percentage by mass: C: 0.05%, Si: 0.20%, Mn: 1.45%, P: 0.015%, S: 0.0012%, Nb: 0.050%, Ti: 0.010%, V: 0.018%, Ni: 0.40%, Cu: 0.38%, Cr: 0.48%, Mo: 0.12%, Alt: 0.025%, and the balance Fe and inevitable impurities. Smelting, refining, alloying and calcium treatment were conducted on raw materials, then molten steel was obtained, and the molten steel was subjected to slab continuous casting, a thickness of a casting billet being 260 mm and an atmospheric corrosion resistance index I being 6.59. Stacking cooling was conducted on the casting billet 36 h or more, and the billet was soaked at 1200° C., a uniform temperature being less than 20° C. After heating was conducted for 286 min, dephosphorization was conducted, and then rolling was conducted at two stages. A temperature of recrystallization zone finish rolling was 1100° C., a total deformation amount of rough rolling was 63%, and a thickness of an intermediate billet was controlled to be 3.0 times of that of a final product. An initial rolling temperature of non-recrystallization zone rolling was 870° C., a thickness of the final product was 32 mm, and a finishing temperature was 810° C.

[0040] After rolling finishing, relaxation was conducted to an initial cooling temperature of 740° C., laminar cooling was conducted on a steel plate at the initial cooling temperature, a self-tempering temperature being 550° C., then air-cooling was conducted to a room temperature, then the steel plate was tempered at a tempering temperature of 480° C., and heat preservation was conducted at the temperature for 30 min.

[0041] Through observation on a metallographic structure of a sample after controlled rolling and cooling and tempering, it was found that a microstructure type was “tempered bainite”, and the material had yield strength of 571 MPa, tensile strength of 713 MPa, a yield ratio of finished steel of 0.80, -40° C. Akv of 332 J, and elongation A of 21%.

[0042] It may be seen from the above embodiments that for the 500 MPa low-yield-ratio weather-resistant bridge steel produced by a heavy and medium plate mill, the yield ratio of the weather-resistant bridge steel is effectively reduced through component designing and a matched manufacturing process including controlled rolling and cooling and off-line tempering, and a yield ratio of finished steel may be ensured to be ≤0.83.