1900 MPa GRADE PRESS HARDENING STEEL BY MEDIUM THIN SLAB CASTING AND DIRECT ROLLING AND METHOD FOR PRODUCING THE SAME

Abstract

A press hardening steel by a medium and thin slab casting and direct rolling has a tensile strength 1900 MPa or more. The press hardening steel includes following chemical components by weight percent: C: 0.31-0.40%, Si: 0.36-0.44%, Mn: 1.6-2.0%, P0.006%, S0.004%, Als: 0.015-0.060%, Cr: 0.36-0.49%, Ti: 0.036-0.045% or Nb: 0.036-0.045% or V: 0.036-0.045%, or a mixture of the above two or more in any ratio, B: 0.004-0.005%, Mo: 0.26-0.35%, Ni: 0.11-0.20%, and N0.005%. A method for producing the press hardening steel includes following steps: desulphurizing molten iron; electric furnace or converter smelting and refining; continuous casting; descaling treatment before entering a soaking furnace; heating and soaking; high-pressure water descaling before entering a rolling mill; hot rolling; cooling; coiling; austenitizing; die deforming and quenching.

Claims

1. A press hardening steel, produced by a medium and thin slab casting and direct rolling and having a tensile strength 1900 MPa or more, the press hardening steel comprising following components by weight percent: C: 0.31%-0.40%, Si: 0.36%-0.44%, Mn: 1.6%-2.0%, P0.006%, S0.004%, Als: 0.015%-0.060%, Cr: 0.36%-0.49%, Ti: 0.036%-0.045% or Nb: 0.036%-0.045% or V: 0.036%-0.045%, or a mixture of the above two or more in any ratio, B: 0.004%-0.005%, Mo: 0.26%-0.35%, Ni: 0.11%-0.20%, N0.005%, and a balance of Fe and inevitable impurities, wherein a quenched microstructure is a full lath martensite; and mechanical properties are that a yield strength is larger than or equal to 1300 MPa, a tensile strength is larger than or equal to 1900 MPa, and elongation A.sub.80 mm is larger than or equal to 5%.

2. A method for producing the press hardening steel according to claim 1, wherein the method comprising following steps: 1) desulphurizing molten iron, and controlling S to be smaller or equal to 0.002%, an exposed surface of the molten iron after slagging off being not lower than 96%; 2) performing conventional electric furnace or converter smelting, and conventional refining; 3) performing continuous casting, and controlling a degree of superheat of tundish molten steel to be 15 C. to 30 C., a thickness of a slab to be 61 mm to 150 mm, and a casting speed to be 3.0 m/min to 5.0 m/min; 4) performing descaling treatment before the slab enters a soaking furnace, and controlling a pressure of descaling water to be 300 bar to 400 bar; 5) performing conventional soaking on the slab, and controlling inside of the soaking furnace in a weak oxidizing atmosphere even if a residual oxygen content in the furnace is 0.5% to 5.0%; 6) heating the slab, and controlling a temperature of the slab entering the furnace to be 800 C. to 1000 C. and a temperature of the slab leaving the furnace to be 1185 C. to 1215 C.; 7) performing high-pressure water descaling before entering a rolling mill, and controlling the pressure of the descaling water to be 280 bar to 420 bar; 8) hot rolling, controlling a first pass reduction rate to be 40% to 50%, a second pass reduction rate to be 40% to 50% and a final pass reduction rate to be 10% to 16%, controlling a rolling speed to be 3 m/s to 8 m/s, performing medium-pressure water descaling between a first pass and a second pass under the pressure of the descaling water of 200 bar to 280 bar, and controlling a finishing rolling temperature to be 860 C. to 900 C.; 9) cooling to a coiling temperature in a manner of laminar cooling, water curtain cooling or intensified cooling; 10) performing coiling, and controlling the coiling temperature to be 565 C. to 595 C.; 11) performing austenitizing after uncoiling and blanking, controlling an austenitizing temperature to be 930 C. to 980 C., and holding for 6 min to 15 min; 12) die punching and deforming, and keeping a pressure for 6 seconds to 9 seconds in a die; and 13) performing quenching, controlling a quenching cooling speed to be 50 C./s to 100 C./s, and then naturally cooling to a room temperature.

3. The method for producing the press hardening steel according to claim 2, wherein a rolling process of the medium and thin slab is carried out in a short-process production line in any one of rolling mill arrangement forms such as a 6F production line or a 1R+6F production line, or a 2R+6F production line, or a 7F production line, or a 3R+4F production line, or 2R+5F production line, or a 1R+5F production line.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] FIG. 1 is a microstructure of a product according to the present invention.

DESCRIPTION OF THE EMBODIMENTS

[0041] The present invention is described in detail below.

[0042] Table 1 is a list of chemical component values of various embodiments and comparative examples of the present invention.

[0043] Table 2 is a list of main process parameter of various embodiments and comparative examples of the present invention.

[0044] Table 3 is a list of property detection cases of various embodiments and comparative examples of the present invention.

[0045] In various embodiments of the present invention, production is performed according to following process:

[0046] 1) Hot melt desulphurize, and control S0.002%, an exposed surface of the molten iron after slagging off being not lower than 96%.

[0047] 2) Perform conventional electric furnace or converter smelting, and conventional refining.

[0048] 3) Perform continuous casting, and control a degree of superheat of tundish molten steel to be 15-30 C., a thickness of a slab to be 61-150 mm, and the casting speed to be 3.0-5.0 m/min.

[0049] 4) Perform descaling treatment before the slab enters a soaking furnace, and control a pressure of descaling water to be 300-400 bar.

[0050] 5) Perform conventional soaking on the slab, and control inside of the soaking furnace in a weak oxidizing atmosphere, i.e. a residual oxygen content in the furnace being 0.5-5.0%.

[0051] 6) Heat the slab, and control a temperature of the slab entering the furnace to be 800-1000 C. and a temperature of the slab leaving the furnace to be 1185-1215 C.

[0052] 7) Perform high-pressure water descaling before entering a rolling mill, and control the pressure of the descaling water to be 280-420 bar.

[0053] 8) Perform hot rolling, control a first pass reduction rate to be 40-50%, a second pass reduction rate to be 40-50% and a final pass reduction rate to be 10-16%, control a rolling speed to be 3-8 m/s, perform medium-pressure water descaling between a first pass and a second pass under the pressure of the descaling water of 200-280 bar, and control a finishing rolling temperature to be 860-900 C.

[0054] 9) Cool to a coiling temperature in a manner of laminar cooling, water curtain cooling or intensified cooling.

[0055] 10) Perform coiling, and control the coiling temperature to be 565-595 C.

[0056] 11) Perform austenitizing after uncoiling and blanking, control an austenitizing temperature to be 930-980 C., and hold for 6-15 min.

[0057] 12) Perform die punching and deforming, and keep a pressure for 6-9 s in a die.

[0058] 13) Perform quenching, control a quenching cooling speed to be 50-100 C./s, and then naturally cool to a room temperature.

[0059] The rolling process of the medium and thin slab is carried out in a short-process production line in any one of rolling mill arrangement forms such as a 6F production line or a 1R+6F production line, or a 2R+6F production line, or a 7F production line, or a 3R+4F production line, or 2R+5F production line, or a 1R+5F production line.

TABLE-US-00001 TABLE 1 Chemical component (wt. %) of various embodiments and comparative examples of the present invention Embodiment C Si Mn P S Als Cr Ti Nb V Mo Ni B N 1 0.37 0.39 1.91 0.004 0.003 0.015 0.38 0.045 0.27 0.16 0.0043 0.003 2 0.38 0.43 1.72 0.003 0.002 0.036 0.48 0.042 0.036 0.26 0.0046 0.002 3 0.39 0.36 1.60 0.005 0.003 0.029 0.47 0.045 0.30 0.0040 0.004 4 0.31 0.44 1.86 0.006 0.004 0.060 0.49 0.044 0.041 0.29 0.0049 0.005 5 0.35 0.40 1.95 0.004 0.001 0.035 0.36 0.036 0.35 0.11 0.0050 0.004 6 0.32 0.42 2.00 0.003 0.002 0.057 0.46 0.045 0.34 0.20 0.0048 0.002 7 0.40 0.38 1.75 0.002 0.002 0.043 0.42 0.038 0.036 0.32 0.0041 0.003

TABLE-US-00002 TABLE 2 List of main process parameter values of various embodiments and comparative examples of the present invention Temperature Finish of slab Tapping rolling Coiling Austenitizing Temperature Quenching Pressure into furnace temperature temperature temperature temperature holding time cooling speed keeping time Embodiment C. C. C. C. C. min C./s in dies 1 926-940 1202-1215 886-900 576-590 950 12 70 7.5 2 861-955 1197-1210 860-873 571-585 980 10 80 6.6 3 845-858 1199-1214 882-898 584-595 930 15 60 6.5 4 972-988 1185-1198 874-888 567-586 960 6 50 8 5 800-811 1190-1204 867-885 573-593 950 7 100 9 6 989-1000 1186-1201 884-897 565-587 970 13 90 6 7 825-838 1200-1213 863-877 570-581 980 9 95 7

TABLE-US-00003 TABLE 3 List of mechanical property cases of various embodiments and comparative examples of the present invention Yield Tensile Thickness strength R.sub.p0.2 strength R.sub.m Elongation Component mm MPa MPa A.sub.80 mm % 1 5.0 1360 1960 5.6 2 7.0 1380 1985 5.1 3 2.1 1430 2055 5.4 4 3.5 1340 1930 6.3 5 4.5 1345 1920 5.8 6 10.0 1400 2010 5.4 7 9.0 1425 2020 5.7

[0060] As can be seen from Table 3, the present application successfully makes the strength of the inventive steel up to 1900 MPa through a short process, which is of great significance for promoting the development of lightweight automobiles.

[0061] The present specific implementation is merely exemplary and does not limit the implementation of the technical solutions of the present invention.