1900 MPa grade press hardening steel by thin slab casting and directly rolling and method for producing the same

Abstract

A press hardening steel by a thin slab and having a tensile strength of 1900 MPa or more includes following components by weight: C: 0.31-0.40%, Si: 0.36-0.44%, Mn: 1.6-2.0%, P≤0.006%, S≤0.004%, 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 any two or more of the above in any proportion, B: 0.004-0.005%, Mo: 0.26-0.35%, and N≤0.005%. A method for producing the press hardening steel includes following steps: molten iron desulphurization; smelting and refining by an electric furnace or converter; 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 method for producing a press hardening steel, the press hardening steel having a tensile strength of 1900 MPa or more, and comprising following components by weight percent: C: 0.31-0.40%, Si: 0.36-0.44%, Mn: 1.6-2.0%, P≤0.006%, S≤0.004%, 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 any two or more of the above in any proportion, B: 0.004-0.005%, Mo: 0.26-0.35%, and N≤0.005%, and a balance of Fe and inevitable impurities, 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 electric furnace or converter smelting, and 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 48 mm to 52 mm, and a casting speed to be 4.0 m/min to 7.0 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 soaking on the slab, and controlling inside of a soaking furnace in a weak oxidizing atmosphere in which the soaking furnace has an atmosphere in which the oxygen content is 0.5% to 5.0%; 6) heating the slab, and controlling a temperature of the slab entering the furnace to be 850° C. to 1050° C. and a tapping temperature of the slab leaving the furnace to be 1210° C. to 1230° 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 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 870° C. to 910° C.; 9) cooling to a coiling temperature in a manner of laminar cooling, water curtain cooling or intensified cooling; 10) performing coiling, and controlling a coiling temperature to be 605° C. to 635° C.; 11) performing austenitizing after uncoiling and blanking, controlling an austenitizing temperature to be 910° C. to 920° C., and holding for 3 minutes to 4 minutes; 12) performing die stamping forming, and keeping a pressure for 10 seconds to 20 seconds in a die; and 13) performing quenching, controlling a quenching cooling speed to be 20° C./s to 24° C./s, and then naturally cooling to room temperature.

2. The method for producing the press hardening steel according to claim 1, wherein: the hot rolling of the slab is carried out in a short-process production line in any one of rolling mill arrangement forms 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 molten iron, 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 48-52 mm, and the casting speed to be 4.0-7.0 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 of 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 850-1050° C. and a tapping temperature of the slab leaving the furnace to be 1210-1230° 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 870-910° 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 a coiling temperature to be 605-635° 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 the room temperature.

(20) The rolling process of the 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.

(21) 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 B N 1 0.38 0.42 1.9 0.004 0.004 0.027 0.38 0.045 — — 0.27 0.0042 0.003 2 0.36 0.43 1.7 0.005 0.002 0.036 0.49 0.042 0.036 — 0.26 0.0045 0.002 3 0.40 0.36 1.6 0.005 0.003 0.029 0.47 — 0.045 — 0.30 0.0040 0.004 4 0.32 0.39 1.8 0.004 0.004 0.060 0.48 — 0.044 0.041 0.29 0.0048 0.005 5 0.35 0.40 1.95 0.006 0.001 0.015 0.36 0.036 — — 0.35 0.0050 0.004 6 0.31 0.44 2.0 0.003 0.002 0.055 0.45 — — 0.045 0.34 0.0049 0.002 7 0.39 0.38 1.75 0.005 0.002 0.043 0.42 0.038 — 0.036 0.32 0.0041 0.003

(22) TABLE-US-00002 TABLE 2 List of main process parameter values of various embodiments and comparative examples of the present invention Temperature of slab Finish Temperature Quenching Pressure into Tapping rolling Coiling Austenitizing holding cooling keeping furnace temperature temperature temperature temperature time speed time Embodiment ° C. ° C. ° C. ° C. ° C. min ° C./s in dies 1 1036-1050 1210-1222 880-900 616-630 920 3 33 14 2 954-969 1220-1230 896-910 623-635 900 4 29 17 3 880-891 1212-1225 870-889 605-617 910 3 38 18 4  995-1008 1216-1229 882-895 608-623 880 4 20 10 5 850-863 1211-1223 889-902 622-634 850 5 24 16 6 1019-1030 1219-1230 883-898 612-627 890 3 22 12 7 875-887 1215-1227 875-887 615-632 860 5 40 20

(23) TABLE-US-00003 TABLE 3 List of mechanical property cases of various embodiments and comparative examples of the present invention Yield Tensile Elongation Thickness strength R.sub.p0.2 strength R.sub.m A.sub.80 mm Component mm MPa MPa % 1 0.8 1420 2050 5.2 2 1.5 1390 1970 5.8 3 1.2 1410 1985 5.4 4 2.0 1330 1920 6.2 5 1.8 1350 1950 5.9 6 1.0 1400 2010 5.4 7 0.9 1395 2000 5.5

(24) As can be seen from Table 3, a short process for direct rolling from a thin slab makes the strength of the inventive steel up to 2100 MPa, which can achieve the purpose of replacing cold forming with press hardening and is of great significance for promoting the development of lightweight automobiles.

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