STEEL, STEEL BAR AND MANUFACTURING METHOD THEREOF

Abstract

Provided is a steel comprising the following chemical composition in percentage by mass: 0.150-0.250% of C, 0.10-0.50% of Si, 0.60-1.50% of Mn, 0.30-1.20% of Cr, 0.20-0.80% of Mo, 2.00-4.00% of Ni, 0-0.10% of Nb, 0.0010-0.0050% of B, 0-0.12% of V, 0.003-0.06% of Ti, 0.01-0.08% of Al, the balance being Fe and unavoidable impurities. Also provided is a steel bar and a manufacturing method thereof. The steel bar is made from the above steel. The manufacturing method comprises the steps of smelting and casting, heating, forging or rolling, quenching, and tempering.

Claims

1. A steel comprising the following chemical composition in percentage by mass: C: 0.15˜0.25%, Si: 0.10˜0.50%, Mn: 0.60˜1.50%, Cr: 0.30˜1.20%, Mo: 0.20˜0.80%, Ni: 2.00˜4.00%, Nb: 0˜0.10%, B: 0.0010˜0.0050%, V: 0˜0.12%, Ti: 0.003˜0.06%, Al: 0.01˜0.08%, and balance of Fe and inevitable impurities.

2. The steel according to claim 1, further comprising 0≤Cu≤0.30% and/or 0≤Ca≤0.005%.

3. The steel according to claim 1, wherein the inevitable impurities satisfy at least one of the following: P≤0.015%, S≤0.003%, H≤0.0002%, N≤0.0150%, and O≤0.0030%.

4. The steel according to claim 1, wherein the microstructure of the steel is tempered martensite and tempered bainite.

5. The steel according to claim 1, wherein the steel has a yield strength of ≥950 MPa, a tensile strength of ≥1150 MPa, a Charpy impact energy below −20° C. Akv of ≥75 J, an elongation rate of ≥15%, and a percentage reduction of area after fracture of ≥55%.

6. A steel bar made from the steel according to claim 1.

7. The steel bar according to claim 6, wherein the steel bar has a diameter of 180 mm or less.

8. A manufacturing method of the steel bar according to claim 6, comprising the steps of: smelting and casting; heating; forging or rolling; quenching: wherein the austenitizing temperature during the quenching step is 840-1050° C., then water quenching after austenitizing; and tempering: wherein the tempering temperature is 500-650° C., then air cooling or water cooling after the tempering.

9. The manufacturing method according to claim 8, wherein the heating temperature during the heating step is 1050-1250° C.

10. The manufacturing method according to claim 8, wherein the finishing rolling temperature or finishing forging temperature is ≥800° C.

11. The steel bar of claim 6, wherein the steel further comprises 0≤Cu≤0.30% and/or 0≤Ca≤0.005%.

12. The steel bar of claim 6, wherein the inevitable impurities satisfy at least one of the following: P≤0.015%, S≤0.003%, H≤0.0002%, N≤0.0150%, and O≤0.0030%.

13. The steel bar of claim 6, wherein the microstructure of the steel is tempered martensite and tempered bainite.

14. The steel bar of claim 6, wherein the steel has a yield strength of ≥950 MPa, a tensile strength of ≥1150 MPa, a Charpy impact energy below −20° C. Akv of ≥75 J, an elongation rate of ≥15%, and a percentage reduction of area after fracture of ≥55%.

15. The method of claim 8, wherein the steel bar has a diameter of 180 mm or less.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0054] FIG. 1 shows the metallographic structure of the low temperature ductile steel bar with ultra-high strength according to Example 4 under optical microscope.

[0055] FIG. 2 shows the metallographic structure of the low temperature ductile steel bar with ultra-high strength according to Example 4 under scanning electron microscope.

DETAILED DESCRIPTION

[0056] The embodiments of the present disclosure will be further described below in conjunction with the drawings and examples. However, the explanation and description are not intended to unduly limit the technical solutions of the present disclosure.

Examples 1-6

[0057] The low temperature ductile bars with ultra-high strength according to Examples 1-6 are produced by the following steps:

[0058] Smelting and casting based on the chemical compositions listed in Table 1, wherein the smelting step can adopt converter steelmaking or circuit steelmaking, and casting to continuous casting slab;

[0059] Heating the casing slab to 1050˜1250° C. and holding the temperature for ≥1.5 h;

[0060] Forging or rolling based on the parameters shown in Table 2;

[0061] Quenching: the austenitizing temperature of the quenching step is 840-1050° C., then water quenching after austenitizing;

[0062] Tempering: the tempering temperature is 500-650° C., then air cooling or water cooling after the tempering.

[0063] It should be noted that air cooling or slow cooling can be adopted after forging or rolling.

[0064] Table 1 lists the mass percentage of each chemical element in the low temperature ductile bars with ultra-high strength according to Examples 1-6.

TABLE-US-00001 TABLE 1 (wt %, and the balance of Fe and other inevitable impurities except P, S, H, N and O) Ex. C Si Mn P S Cr Mo Nb Ni Cu V Al Ti Ca H N O B 1 0.15  0.50 0.50 0.015 0.002 1.18 0.80 0.06 2.00 0.28 0.12 0.05 0.05  0.005 0.00015 0.0120 0.0030 0.0008 2 0.170 0.40 0.70 0.011 0.002 1.06 0.70 0.08 2.30 0.24 0.09 0.04 0.06  0.001 0.00020 0.0080 0.0013 0.0013 3 0.200 0.30 0.80 0.009 0.001 0.92 0.50 0.05 2.70 0.17 0.07 0.07 0.02  0.002 0.00012 0.0030 0.0020 0.0019 4 0.210 0.20 1.10 0.007 0.003 0.70 0.40 0.03 3.00 0.10 0.05 0.08 0.015 0.003 0.00010 0.0050 0.0025 0.0029 5 0.220 0.20 1.30 0.010 0.002 0.50 0.30 0.02 3.40 0.06 0.04 0.02 0.003 0.003 0.00012 0.0060 0.0015 0.0038 6 0.240 0.10 1.50 0.008 0.001 0.30 0.20 0   3.70 0   0.02 0.01 0.010 0    0.00007 0.0040 0.0012 0.0045

[0065] Table 2 lists the specific process parameters of the low temperature ductile bars with ultra-high strength according to Examples 1-6.

[0066] Those skilled in the art may determine the holding duration, heating duration and tempering duration according to features such as diameter of the steel bar or the like to ensure that the steel bar is evenly heated, so that the core portion would reach the same temperature as the surface of the steel bar.

TABLE-US-00002 TABLE 2 Round steel forging Bloom heating step or rolling step Finishing Finishing rolling forging Quenching Tempering Heating Holding temp. for Heating or rolling Austenitizing Heating Tempering Tempering temp. duration inter. temp. temp. quenching duration temp. duration No. (° C.) (min) slab (° C.) (° C.) (° C.) (° C.) (min) (° C.) (min) Ex. 1 1050 240 800 1070 800 840 30 500 60 Ex. 2 1080 270 880 1100 830 870 100 530 60 Ex. 3 1080 300 880 1120 940 900 150 560 100 Ex. 4 1200 360 950 1150 980 930 150 590 180 Ex. 5 1200 420 950 1200 1020 980 170 610 260 Ex. 6 1200 480 950 1250 1020 1040 300 650 300

[0067] The low temperature ductile bars with ultra-high strength of Examples 1-6 were then subjected to performance test. The test results of round steel having different diameters are listed in Table 3 below.

TABLE-US-00003 TABLE 3 Steel bar Yield Tensile Percentage Longitudinal diameter strength strength Elongation reduction of area impact energy at Example (mm) (MPa) (MPa) rate (%) after fracture (%) −20° C. (J) 1 20 1000 1170 17.0 65 129/135/139 2 50 1020 1180 17.0 63 126/132/127 3 70 1060 1200 16.5 62 113/120/124 4 100 1110 1220 16.0 61 111/107/119 5 130 1130 1230 16.0 62 105/109/107 6 180 1140 1250 15.5 60 97/101/95 Note: The three columns of the longitudinal impact energy at −20° C. represent the test results of three parallel samples, respectively.

[0068] According to Table 3, the Examples according to the present disclosure has a yield strength of ≥950 MPa, a tensile strength of ≥1150 MPa, a Charpy impact energy below −20° C. Akv of ≥75 J, an elongation rate of ≥15%, a percentage reduction of area after fracture of ≥55%. Thus, it can be concluded that the low temperature ductile bars with ultra-high strength have good strength, toughness at low temperature and pliability.

[0069] FIG. 1 shows the metallographic structure of the low temperature ductile steel bar with ultra-high strength according to Example 4 under optical microscope. FIG. 2 shows the metallographic structure of the low temperature ductile steel bar with ultra-high strength according to Example 4 under scanning electron microscope.

[0070] Combining FIG. 1 and FIG. 2, it can be seen that the microstructure of the low temperature ductile steel bar with ultra-high strength according to Example 4 is tempered martensite and tempered bainite.

[0071] To sum up, the chemical composition system adopted in the low temperature ductile steel with ultra-high strength according to the present disclosure fully utilizes the influence of various alloying elements on phase transformation and microstructure to ensure the strength, low temperature impact toughness and elongation rate of the steel, so as to obtain a high-strength steel with a tensile strength of 1150 MPa and balanced ultra-high toughness and strong pliability.

[0072] Further, the low temperature ductile steel with ultra-high strength according to the present disclosure has a tensile strength of 1150 MPa. The application of tempering process after the quenching step for the rolled or forged low temperature ductile steel bar with ultra-high strength can help form a matrix structure of tempered martensite and tempered bainite on which fine carbides are precipitated, which eliminates the internal stress of the steel and provide good uniformity of the structure.

[0073] In addition, the reasonable design and broad process window of the composition and manufacturing process of the low temperature ductile steel with ultra-high strength would achieve mass commercial production of the bars or steel plates.

[0074] It should be noted that the portion of prior art in the protection scope of the present disclosure is not limited to the embodiments given herein. All prior art that does not contradict the solutions of the present disclosure, including but not limited to the previous patent documents, prior publications, prior applications, etc., can all be included in the protection scope of the present disclosure.

[0075] In addition, the combination of the technical features in the present disclosure is not limited to the combination described in the claims or the combination described in the specific examples. All technical features described herein can be freely combined in any way, unless contradicts between each other.

[0076] It should also be noted that the above-listed embodiments are only specific examples of the present disclosure. Obviously, the present disclosure should not be unduly limited to such specific embodiments. Changes or modifications that can be directly or easily derived from the present disclosure by those skilled in the art are intended to be within the protection scope of the present disclosure.