Method for manufacturing thin strip continuously cast 700MPa-grade high strength weather-resistant steel

Abstract

A method for manufacturing thin strip continuously cast 700 Mpa grade high strength weather-resistant steel, the method comprising the following steps: 1) casting a 1-5 mm thick cast strip in a double roller continuous casting machine, the cast strip comprising the following chemical compositions by weight percent: C 0.03-0.1%, Si0.4%, Mn 0.75-2.0%, P 0.07-0.22%, S0.01%, N0.012%, and Cu 0.25-0.8%, further comprising more than one of Nb, V, Ti and Mo: Nb 0.01-0.1%, V 0.01-0.1%, Ti 0.01-0.1%, and Mo 0.1-0.5%, and the balance being Fe and unavoidable impurities; 2) cooling the cast strip at a rate greater than 20 C./s; 3) hot rolling the cast strip under a temperature of 1050-1250 C. at a reduction rate of 20-50% and a deformation rate greater than 20 s1; then conducting austenite online recrystallization, the thickness of the hot rolled strip being 0.5-3.0 mm; 4) cooling at a rate of 10-80 C./s; and 5) rolling up under a temperature of 500-650 C. The obtained steel strip microstructure mainly consists of uniformly distributed bainites and needle-shaped ferrites.

Claims

1. A manufacturing method of a continuous strip cast weather-resistant steel having a high-strength of 700 MPa-grade, the method sequentially comprising the following steps: 1) casting a cast strip having a thickness of 15 mm by using a twin-roller continuous casting mill, wherein the cast strip has a chemical composition by weight percentage as follows: C 0.030.1%, Si0.4%, Mn 0.752.0%, P 0.070.22%, 0<S0.01%, 0<N0.012% and Cu 0.250.8%, and at least one microalloy element selected from Nb, V, Ti, and Mo having a content of Nb 0.010.1%, V 0.010.1%, Ti 0.010.1%) and Mo 0.10.5%, and balance being Fe and inevitable impurities; 2) cooling the cast strip after the casting the cast strip, wherein the cooling rate is 23 C./sec. to 42 C./sec.; 3) online hot rolling the cast strip after cooling the cast strip under a hot rolling temperature of 1,0501,250 C., a reduction rate of 2050%, and a deformation rate of >20 s.sup.1, wherein the thickness of the steel strip after hot rolling is 0.53.0 mm, and online austenite recrystallization occurs upon the hot rolling of the cast strip; 4) cooling the hot-rolled strip after online hot rolling the cast strip, wherein the cooling rate is 14 C./sec. to 79 C./sec.; 5) coiling the hot-rolled strip after cooling the hot-rolled strip, wherein the coiling temperature of the hot-rolled strip is controlled to be 500650 C.; and wherein the final resulting steel strip has a microstructure substantially consisting of homogeneous bainite and acicular ferrite conferring a strength property and an elongation property to the steel strip.

2. The manufacturing method of claim 1, wherein, in step 1), the content of each of Nb, V and Ti is 0.010.05% by weight percentage.

3. The manufacturing method of claim 1, wherein, in step 1), the content of Mo is 0.10.25% by weight percentage.

4. The manufacturing method of claim 1, wherein, in step 3), the hot rolling temperature is in the range of 11001250 C.

5. The manufacturing method of claim 1, wherein, in step 3), the hot rolling temperature is in the range of 11501250 C.

6. The manufacturing method of claim 1, wherein, in step 3), the reduction rate of hot rolling is in the range of 30-50%.

7. The manufacturing method of claim 1, wherein, in step 3), the deformation rate of hot rolling is 28 s.sup.1 to 76 s.sup.1.

8. The manufacturing method of claim 1, wherein, in step 4), the cooling rate of the hot-rolled strip is in the range of 23 C./sec. to 52 C./sec.

9. The manufacturing method of claim 1, wherein, in step 5), the coiling temperature is in the range of 500600 C.

10. The manufacturing method of claim 1, wherein, the thickness of said steel strip is less than 3 mm.

11. The manufacturing method of claim 1, wherein, the thickness of said steel strip is less than 2 mm.

12. The manufacturing method of claim 1, wherein, the thickness of said steel strip is less than 1 mm.

13. The manufacturing method of claim 1, wherein, said steel strip has a yield strength of 700 MPa or above, a tensile strength of 780 MPa or above, and an elongation of 18% or above.

Description

BRIEF DESCRIPTION OF DRAWING

(1) FIG. 1 provides the schematic diagram of the continuous strip casting process flow.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(2) Referring to FIG. 1, the continuous strip casting process flow of the present invention is described below: The molten steel in the large steel ladle is introduced through the long nozzle 2, tundish 3 and submersed nozzle 4 to the molten pool 7 formed by a pair of relatively rotating and internally water-cooling casting rollers (5a and 5b) and the side dams (6a and 6b), and forms the cast strip 11 15 mm in size through cooling by the water-cooling casting rollers; the steel strip then goes through the secondary cooling device 8 in the airtight chamber 10 to control its cooling rate, and is then delivered to the hot rolling mill 13 through the swinging guide plate 9 and pinch roller 12; the hot-rolled strip 0.53 mm in size formed after hot rolling then goes through the third cooling device 14, and then goes into the coiling machine 15. The steel coil is then taken down from the coiling machine for natural cooling to room temperature.

(3) In all the examples of the present invention, the molten steel is produced through electric furnace smelting; see the specific chemical composition in Table 1 below. Table 2 provides the thickness and cooling rate of the cast strip produced after the continuous strip casting, the temperature, reduction rate and deformation rate of hot rolling, the thickness and cooling rate of the hot-rolled strip, the coiling temperature and other process parameters, as well as the tensile performance and bending performance of the hot-rolled strip after cooling down to room temperature.

(4) It can be seen from Table 2 that, the steel strip of the present invention has a yield strength of 700 MPa or above, a tensile strength of 780 MPa or above, an elongation of 18% or above and a qualified bending performance of 180, as well as a superior strength and plasticity matching.

(5) TABLE-US-00001 TABLE 1 Chemical composition of the molten steel in the examples (wt. %) Example C Si Mn P S N Cu Nb V Ti Mo 1 0.032 0.29 1.80 0.22 0.005 0.0080 0.36 0.010 0.098 2 0.048 0.33 1.25 0.08 0.006 0.0052 0.74 0.048 0.050 3 0.054 0.34 0.96 0.13 0.005 0.0063 0.40 0.028 0.033 0.012 4 0.063 0.36 0.78 0.12 0.006 0.0060 0.55 0.097 0.10 5 0.077 0.29 0.80 0.18 0.003 0.0072 0.26 0.100 0.12 6 0.098 0.32 0.65 0.16 0.001 0.0045 0.50 0.026 0.50 7 0.058 0.28 1.46 0.20 0.008 0.0120 0.80 0.018 0.050 0.033 0.25 8 0.030 0.40 2.00 0.15 0.003 0.0096 0.68 0.011 0.078

(6) TABLE-US-00002 TABLE 2 Process parameters and product performance of the examples 180 C. Tem- Reduc- bending, Thick- Cooling pera- tion Thickness Cooling Flexural ness rate of ture rate Deformation of rate of center of cast cast of hot of hot rate of hot-rolled hot-rolled Coiling Yield Tensile Elonga- diameter strip, strip, rolling, rolling, hot rolling, strip, strip, temperature, strength, strength, tion, a = strip Example mm C./sec. C. % s.sup.1 mm C./sec. C. MPa MPa % thickness 1 4.5 25 1,236 35 33 2.9 79 500 735 813 18 Qualified 2 1.1 30 1,098 50 46 0.6 23 618 708 788 22 Qualified 3 3.3 36 1,057 25 38 2.5 25 600 712 789 21 Qualified 4 2.2 24 1,212 35 40 1.4 36 580 716 798 21 Qualified 5 2.8 28 1,168 40 35 1.7 14 650 703 782 23 Qualified 6 2.4 24 1,245 30 28 1.7 52 535 722 806 20 Qualified 7 5.0 23 1,250 48 31 2.6 43 630 710 790 19 Qualified 8 1.0 42 1,150 50 76 0.5 32 640 715 803 20 Qualified