1500 MPa grade press hardening steel by thin slab casting and direct rolling and method for producing the same

Abstract

A press hardening steel by a thin slab casting and direct rolling has a tensile strength of 1500 MPa or more. The press hardening steel has a components by weight percent: C: 0.21-0.25%, Si: 0.26-0.30%, Mn: 1.0-1.3%, P≤0.01%, S≤0.005%, Als: 0.015-0.060%, Cr: 0.25-0.30%, Ti: 0.026-0.030% or Nb: 0.026-0.030% or V: 0.026-0.030%, or a mixture of two or more of the above in any proportion; B: 0.003-0.004%, and N≤0.005%. A method for producing the press hardening steel includes following steps: hot metal desulphurization; electric-furnace or converter smelting and refining; continuous casting; descaling, then entering a soaking furnace; heating and soaking; high-pressure water descaling, then entering a rolling mill; hot rolling; cooling; coiling; austenitizing; die deforming and quenching.

Claims

1. A method for producing a press hardening steel, wherein the press hardening steel is produced by directly rolling and casting a slab and has a tensile strength of 1500 MPa or more, and the press hardening steel comprises components by weight percent of C: 0.21-0.25%, Si: 0.26-0.30%, Mn: 0-1.3%, P≤0.01%, S≤0.005%, Als: 0.015-0.060%, Cr: 0.25-0.30%, Ti: 0.026-0.030% or Nb: 0.026-0.030% or V: 0.026-0.030%, or a mixture of two or more of the above in any proportion; B: 0.003-0.004%, N≤0.005%, and a balance of Fe and inevitable impurities, the method comprising following steps: step 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%; step 2: performing conventional electric furnace or converter smelting, and conventional refining; step 3: performing continuous casting, and controlling a degree of superheat of tundish molten steel to be 15° C. to 30° C., a thickness of slab to be 52 mm to 55 mm, and a casting speed to be 3.7 m/min to 7.0 m/min; step 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; step 5: performing conventional soaking on the slab, and controlling inside the soaking surface in a weak oxidizing atmosphere, i.e. a residual oxygen content in the furnace being 0.5% to 5.0%; step 6: heating the slab, and controlling a temperature of the slab entering the furnace to be 820° C. to 1050° C. and a temperature of the slab leaving the furnace to be 1190° C. to 1210° C.; step 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; step 8: hot rolling, controlling a first pass reduction rate to be 52% to 63%, a second pass reduction rate to be 50% to 60% and a final pass reduction rate to be 10% to 16%, controlling a rolling speed to be 8 m/s to 12 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 850° C. to 890° C.; step 9: cooling to a coiling temperature in a manner of laminar cooling, water curtain cooling or intensified cooling; step 10: performing coiling, and controlling the coiling temperature to be 655° C. to 675° C.; step 11: performing austenitizing after uncoiling and blanking, controlling an austenitizing temperature to be 905° C. to 920° C., and holding for 3 minutes to 4 minutes; step 12: die punching and deforming, and keeping a pressure for 10 seconds to 20 seconds in a die; and step 13: performing quenching, controlling a quenching cooling speed to be 20° C./s to 40° C./s, and then naturally cooling to a room temperature.

2. The method for producing the press hardening steel according to claim 1, wherein the rolling process of the slab is carried out in rolling mill arrangement forms such as a 6 finishing mills production line or a 1 roughing mill+6 finishing mills production line, or a 2 roughing mills+6 finishing mills production line, or a 7 finishing mills production line, or a 3 roughing mills+4 finishing mills production line, or 2 roughing mills+5 finishing mills production line, or a 1 roughing mill+5 finishing mills production line.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a microstructure of a product according to the present invention.

DESCRIPTION OF THE EMBODIMENTS

(2) The present invention is described in detail below.

(3) Table 1 is a list of chemical component values of various embodiments and comparative examples of the present invention.

(4) Table 2 is a list of main process parameter of various embodiments and comparative examples of the present invention.

(5) Table 3 is a list of property detection cases of various embodiments and comparative examples of the present invention.

(6) In various embodiments of the present invention, production is performed according to following process:

(7) 1) Hot melt desulphurize, and control S≤0.002%, an exposed surface of the molten iron after slagging off being not lower than 96%.

(8) 2) Perform conventional electric furnace or converter smelting, and conventional refining.

(9) 3) Perform continuous casting, and control a degree of superheat of tundish molten steel to be in the temperature of 15-30° C., a thickness of a slab to be 52-55 mm, and the casting speed to be 3.7-7.0 m/min.

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

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

(12) 6) Heat the slab, and control a temperature of the slab entering the furnace to be 820-1050° C. and a temperature of the slab leaving the furnace to be 1190-1210° C.

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

(14) 8) Perform hot rolling, control a first pass reduction rate to be 52-63%, a second pass reduction rate to be 50-60% and a final pass reduction rate to be 10-16%, control a rolling speed to be 8-12 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 850-890° C.

(15) 9) Cool to a coiling temperature in a manner of laminar cooling, water curtain cooling or intensified cooling.

(16) 10) Perform coiling, and control the coiling temperature to be 655-675° C.

(17) 11) Perform austenitizing after uncoiling and blanking, control an austenitizing temperature to be 850-920° C., and hold for 3-5 min.

(18) 12) Perform die punching and deforming, and keep a pressure for 10-20 s in a die.

(19) 13) Perform quenching, control a quenching cooling speed to be 20-40° C./s, and then naturally cool to a room temperature.

(20) 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 B N 1 0.24 0.27 1.02 0.005 0.005 0.024 0.26 0.030 — — 0.0032 0.003 2 0.225 0.30 1.10 0.008 0.002 0.036 0.30 0.026 0.027 — 0.0036 0.002 3 0.21 0.29 1.30 0.004 0.003 0.022  0.295 — 0.030 — 0.0040 0.004 4 0.25 0.26 1.00 0.004 0.005 0.060 0.25 — 0.026 0.026 0.0035 0.005 5 0.23 0.28 1.20 0.010 0.001 0.015 0.27 0.028 — — 0.0030 0.004 6 0.22 0.285 1.22 0.003 0.003 0.055 0.28 — — 0.030 0.0034 0.002 7 0.246 0.265 1.26 0.006 0.002 0.045 0.29 0.024 — 0.025 0.0038 0.003 Comparative 0.20 0.08 1.50 0.010 0.006 0.040 — 0.10  — — — 0.006 example 1 Comparative 0.13 0.45 1.3 0.025 0.005 0.04 0.50 0.02  0.02  — — 0.004 example 2

(21) TABLE-US-00002 TABLE 2 List of main process parameter values of various embodiments and comparative examples of the present invention Temperature Finish Temperature Quenching Pressure of slab Tapping rolling Coiling Austenitizing holding cooling keeping into furnace temperature temperature temperature temperature time speed time in Embodiment ° C. ° C. ° C. ° C. ° C. min ° C./s dies 1 897-910 1197-1210 878-890 655-664 910 4 30 12 2 820-833 1195-1207 850-862 657-672 920 3 27 19 3 1034-1048 1200-1210 872-884 663-675 905 3 38 17 4 975-987 1190-1205 863-874 658-671 870 5 20 20 5 850-865 1197-1209 865-877 656-669 880 4 26 15 6  998-1013 1196-1208 868-880 661-671 870 4 22 13 7 929-942 1192-1206 881-890 659-674 890 5 40 10 Comparative — 1232-1245 890-905 602-617 — — — — example 1 Comparative — — 895-915 647-658 — — — — example 2

(22) 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 0.8 1120 1620 6.4 2 1.5 1080 1560 7.2 3 1.2 1100 1600 6.8 4 2.0 1050 1510 7.5 5 1.8 1070 1545 7.3 6 1.0 1090 1550 6.7 7 0.9 1060 1530 6.5 Comparative 1.2 705 755 22 example 1 Comparative 1.5 570 650 20 example 2

(23) As can be seen from Table 3, a short process for directly rolling from thin slabs makes the strength of the inventive steel up to 1500 MPa, which can achieve the purpose of replacing cold forming with thermoforming and meanwhile have the strength much higher than that of existing short-process products, which is of great significance for promoting the development of lightweight automobiles.

(24) The present specific implementation is merely exemplary and does not limit the implementation of the technical solutions of the present invention.