Manufacturing method for strip casting 700 MPa-grade high strength atmospheric corrosion-resistant steel

Abstract

A manufacturing method for strip casting 700 MPa-grade high strength atmospheric corrosion-resistant steel, comprising the following steps: 1) smelting, where the chemical composition of a molten steel in terms of weight percentage is that C is between 0.03-0.1%, Si≦0.4%, Mn is between 0.75-2.0%, P is between 0.07-0.22%, S≦0.01%, N≦0.012%, Cu is between 0.25-0.8%, Cr is between 0.3-0.8%, and Ni is between 0.12-0.4%, additionally, also comprised is at least one micro-alloying element among Nb, V, Ti, and Mo, where Nb is between 0.01-0.1%, V is between 0.01-0.1%, Ti is between 0.01-0.1%, and Mo is between 0.1-0.5%, and where the remainder is Fe and unavoidable impurities; 2) strip casting, where a 1-5 mm-thick cast strip is casted directly; 3) cooling the cast strip, where the cooling rate is greater than 20° C./s; 4) online hot rolling the cast strip, where the hot rolling temperature is between 1050-1250° C., where the reduction rate is between 20-50%, and where the deformation rate is >20s.sup.−1; austenite online recrystallizing after hot rolling, where the thickness of the hot rolled strip is between 0.5-3.0 mm; and, 5) cooling and winding, where the cooling rate is between 10-80° C./s, and where the winding temperature is between 520-670° C. The microscopic structure of a steel strip acquired is primarily constituted by evenly distributed bainite and acicular ferrite.

Claims

1. A manufacturing method of a continuous strip cast atmospheric corrosion-resistant steel having a high-strength of 700 MPa-grade, the method sequentially comprising the following steps: 1) smelting, wherein the molten steel has a chemical composition by weight percentage as follows: C 0.03˜0.1%, Si≦0.4%, Mn 0.75˜2.0%, P 0.07˜0.22%, O<S≦0.01%, O<N≦0.012%, Cu 0.25˜0.8%, Cr 0.3˜0.8%, Ni 0.12˜0.4%, and at least one microalloy element selected from Nb, V, Ti, and Mo having a content of Nb 0.01˜0.1%, V 0.01˜0.1%, Ti 0.01˜0.1% and Mo 0.1˜0.5%, and balance being Fe and inevitable impurities; 2) continuous strip casting, wherein the molten steel is introduced into a molten pool formed by a pair of relatively rotating and internally water-cooled casting rollers and side dams, and is directly cast into a cast strip having a thickness of 1˜5 mm through rapid solidification; 3) cooling the cast strip after the continuous strip casting, wherein after being continuous cast and coming out of the casting rollers, the cast strip goes through an airtight chamber for cooling, the cooling rate is 22° C./sec. to 38° C./sec.; 4) online hot rolling the cast strip after cooling the cast strip under a hot rolling temperature of 1,050˜1,250° C., a reduction rate of 20˜50%, and a deformation rate of >20 s.sup.−1, wherein the thickness of the steel strip after hot rolling is 0.5˜3.0 mm, and online austenite recrystallization occurs upon the hot rolling of the cast strip; 5) cooling and coiling after the online hot rolling the cast strip, wherein the cooling rate of the hot-rolled strip is controlled to be 16° C./sec to 78° C./sec., and the coiling temperature of the hot-rolled strip is controlled to be 520˜670° 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 by weight percentage is 0.01˜0.05%, and the content of Mo is 0.1˜0.25% by weight percentage.

3. The manufacturing method of claim 1, wherein, in step 4), the hot rolling temperature is in the range of 1,100˜1,250° C.

4. The manufacturing method of claim 1, wherein, in step 4), the reduction rate of the hot rolling is 30˜50%.

5. The manufacturing method of claim 1, wherein, in step 4), the deformation rate of hot rolling is 22 s.sup.−1 to 48 s.sup.−1.

6. The manufacturing method of claim 1, wherein, in step 5), the cooling rate of the hot-rolled strip is in the range of 24° C./sec to 63° C./sec.

7. The manufacturing method of claim 1, wherein, in step 5), the coiling temperature is in the range of 520˜620° C.

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

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

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

11. The manufacturing method of claim 1, wherein, said steel strip has 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 DRAWINGS

(1) FIG. 1 is a schematic diagram showing 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 1 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 1˜5 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 0.5˜3 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 property 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 qualified 180° bending property, 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 No. C Si Mn P S N Cu Cr Ni Nb V Ti Mo 1 0.032 0.31 1.80 0.20 0.004 0.0067 0.34 0.44 0.31 0.014 0.098 2 0.046 0.30 1.27 0.09 0.003 0.0053 0.80 0.31 0.35 0.050 0.048 0.10 3 0.053 0.32 0.92 0.14 0.007 0.0065 0.42 0.70 0.26 0.026 0.035 0.012 4 0.065 0.35 0.76 0.12 0.005 0.0048 0.56 0.80 0.13 0.096 5 0.055 0.40 1.56 0.22 0.008 0.0113 0.60 0.53 0.30 0.010 0.050 0.36 6 0.078 0.33 0.82 0.17 0.006 0.0071 0.25 0.60 0.24 0.100 0.13 7 0.094 0.28 0.83 0.16 0.004 0.0042 0.50 0.37 0.40 0.023 0.50 8 0.039 0.36 2.00 0.19 0.007 0.0085 0.66 0.62 0.34 0.010 0.050 0.028 0.25

(6) TABLE-US-00002 TABLE 2 Process parameters and product performance of the examples Defor- 180° Ex- Thick- Cooling Temper- mation Thick- Coiling Elon- bending, am- ness rate of ature of Reduction rate of hot ness of Cooling temper- Yield Tensile ga- Flexural center ple of cast cast strip, hot rolling, rate of hot rolling, hot-rolled rate of ature, strength, strength, tion, diameter a = No. strip, mm ° C./sec. ° C. rolling, % s.sup.−1 strip, mm hot-rolled ° C. MPa MPa % strip thickness 1 4.8 25 1,236 36 35 3.0 78 520 756 839 20 Qualified 2 1.2 30 1,092 50 48 0.6 24 620 733 806 21 Qualified 3 3.5 38 1,055 28 36 2.5 26 600 735 812 22 Qualified 4 2.0 24 1,210 35 40 1.3 37 585 737 816 22 Qualified 5 2.7 28 1,175 42 37 1.6 16 645 725 805 23 Qualified 6 2.5 24 1,240 33 30 1.7 55 530 742 832 18 Qualified 7 1.0 35 1,150 50 46 0.5 63 650 728 809 20 Qualified 8 3.2 22 1,250 20 22 2.6 40 670 720 800 22 Qualified