1500 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 thin slab and having a tensile strength of 1500 MPa or more, includes following 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 any two or more of the above in any proportion, B: 0.003%-0.004%, Mo: 0.17-0.19% 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; hating 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, 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 following 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 any two or more of the above in any proportion, B: 0.003%-0.004%, Mo: 0.17-0.19%, 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 the slab to be 61 mm to 150 mm, and a casting speed to be 2.8 m/min to 5.5 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 of the soaking furnace in a weak oxidizing atmosphere even if a residual oxygen content in the furnace is 0.5% to 5.0%; step 6: heating the slab, and controlling a temperature of the slab entering the furnace to be 780° C. to 1000° C. and a temperature of the slab leaving the furnace to be 1135° C. to 1149° 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 40-50%, a second pass reduction rate to be 40% to 50% and a final pass reduction rate to be 10-16%, 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 830° C. to 870° 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 635° C. to 665° C.; step 11: performing austenitizing after uncoiling and blanking, controlling an austenitizing temperature to be 930° C. to 980° C., and holding for 12 minutes to 15 minutes; step 12: die punching and deforming, and keeping a pressure for 6 seconds to 9 seconds in a die; and step 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.

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 15-30° C., a thickness of a slab to be 61-150 mm, and the casting speed to be 2.8-5.5 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 780-1000° C. and a temperature of the slab leaving the furnace to be 1135-1165° 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 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 830-870° 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 635-665° C.

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

(18) 12) Perform die stamping forming, and keep a pressure for 6-9 s in a die.

(19) 13) Perform quenching, control a quenching cooling speed to be 50-100° 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 Mo 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 — — 0.19 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 0.17 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 Temperature Quenching Pressure of slab into Tapping Finish 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 1 833-846 1149-1164 858-870 635-646 970 6 100 8 2 791-802 1153-1165 830-842 637-648 980 6 97 6 3  986-1000 1135-1148 852-864 649-660 955 8 85 9 4 966-975 1137-1149 835-847 638-652 975 9 90 7 5 780-792 1145-1157 845-857 636-649 935 12 86 6 6 926-940 1143-1155 856-868 641-654 930 15 62 8 7 870-885 1147-1160 840-851 652-665 945 14 50 9 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 Thickness Yield strength Tensile strength Elongation Component mm R.sub.p0.2 MPa R.sub.m MPa A.sub.80 mm % 1 5.0 1090 1530 6.3 2 7.0 1070 1550 7.2 3 2.1 1120 1625 6.2 4 3.5 1050 1520 7.8 5 4.5 1080 1560 7.4 6 10.0 1060 1540 6.6 7 9.0 1065 1535 6.7 Comparative 3.0 715 750 21 example 1 Comparative 5.5 565 655 22 example 2

(23) As can be seen from Table 3, a short process for directly rolling from a thin slab makes the strength of the steel of the invention up to 1500 MPa, which makes the strength thereof much higher than that of existing short-process products and 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.