NB MICROALLOYED HIGH STRENGTH HIGH HOLE EXPANSION STEEL AND PRODUCTION METHOD THEREFOR

Abstract

Disclosed are a Nb microalloyed high strength high hole expansion steel and a production method therefor. The chemical ingredients of the steel in percentages by weight are as follows: 0.01-0.05% of C, 0.2-0.6% of Si, 0.8-1.5% of Mn, ≤0.02% of P, ≤0.005% of S, ≤0.008% of N, <0.001% of Als, ≤0.0050% of Ca, 0.01-0.08% of Nb, and optionally one or both of 0.1-0.6% of Cu and 0.005-0.04% of Sn, wherein Mn/S>250, total oxygen [O].sub.T is 0.007-0.020%, and the balance is Fe and inevitable impurities. In the present invention, microalloy elements such as Nb are selectively added, and the basicity of slag, the type and melting point of the inclusion in steel, the content of free oxygen in molten steel, and the content of acid-soluble aluminum Als during the smelting process are controlled, and then, a strip is cast by means of twin-roll thin strip continuous casting, and the strip directly enters a lower closed chamber in a non-oxidizing atmosphere and enters an online rolling mill for hot rolling in closed conditions, and after rolling, the strip steel is cooled by air atomization cooling, and finally, the produced steel coil can be used directly as a hot rolled plate or can be used after acid pickling and leveling.

Claims

1. A Nb microalloyed high strength high hole expansion steel comprising the following chemical elements in weight percentages: C: 0.01-0.05%, Si: 0.2-0.6%, Mn: 0.8-1.5%, P≤0.02%, S≤0.005%, N≤0.008%, Als: <0.001%, Ca≤0.0050%, Nb: 0.01-0.08%, optionally one or both of Cu: 0.1-0.6% and Sn: 0.005-0.04%, wherein, Mn/S>250, total oxygen [O].sub.T: 0.007-0.020%, and a balance of Fe and unavoidable impurities.

2. The Nb microalloyed high strength high hole expansion steel according to claim 1, wherein the Nb microalloyed high strength high hole expansion steel comprises the following chemical elements in weight percentages: C: 0.01-0.05%, Si: 0.2-0.6%, Mn: 0.8-1.5%, P≤0.02%, S≤0.005%, N≤0.008%, Als: <0.001%, Ca≤0.0050%, Nb: 0.01-0.08%, Mn/S>250, total oxygen [O].sub.T: 0.007-0.020%, and a balance of Fe and unavoidable impurities, and satisfies: it comprises one or both of Cu: 0.1-0.6% and Sn: 0.005-0.04%.

3. The Nb microalloyed high strength high hole expansion steel according to claim 1 or 2, wherein the high hole expansion steel has a microstructure of ferrite+bainite, wherein the bainite phase has a ratio of ≥15%.

4. The Nb microalloyed high strength high hole expansion steel according to any one of claims 1-3, wherein the high hole expansion steel has a yield strength of ≥440 MPa, a tensile strength of ≥590 MPa, an elongation of ≥19%, and a hole expansion rate of ≥100%.

5. The Nb microalloyed high strength high hole expansion steel according to claim 1, wherein the Nb microalloyed high strength high hole expansion steel has a thickness of 0.8-2.5 mm, preferably, 1.0-1.8 mm.

6. A manufacturing method for the Nb microalloyed high strength high hole expansion steel according to any one of claims 1-5, comprising the following steps: 1) Smelting wherein smelting is performed on the chemical composition of claim 1; wherein during smelting, a basicity a=CaO/SiO.sub.2 (mass ratio) for slagging is controlled at a<1.5, preferably a<1.2, or a=0.7-1.0; wherein a low-melting-point MnO—SiO.sub.2—Al.sub.2O.sub.3 ternary inclusion is required and a MnO/SiO.sub.2 ratio of MnO—SiO.sub.2—Al.sub.2O.sub.3 ternary inclusion is controlled at 0.5-2, preferably 1-1.8; wherein a free oxygen content [O].sub.Free in the molten steel is 0.0005-0.005%; and wherein in the molten steel, Mn/S>250; 2) Continuous casting wherein twin-roll thin strip continuous casting is used for continuous casting, wherein a 1.5-3 mm thick cast strip is formed at the smallest gap between two crystallization rolls; wherein the crystallization rolls have a diameter of 500-1500 mm, preferably 800 mm; wherein water is supplied to the inside of the crystallization rolls for cooling; wherein a casting machine has a casting speed of 60-150 m/min; wherein a two-stage system for dispensing and distributing molten steel is used for molten steel delivery in the continuous casting, i.e., a tundish+a distributor; 3) Lower closed chamber protection wherein after a cast strip exits the crystallization rolls, the cast strip has a temperature of 1420-1480° C., and it enters a lower closed chamber directly, wherein a non-oxidizing gas is supplied to the lower closed chamber, wherein an oxygen concentration (volume) in the lower closed chamber is controlled at <5%; and wherein the cast strip has a temperature of 1150-1300° C. at an outlet of the lower closed chamber; 4) On-line hot rolling wherein the cast strip is delivered through pinch rolls in the lower closed chamber to a rolling mill, and rolled into a rolled strip steel at a rolling temperature of 1100-1250° C. and a hot rolling reduction rate controlled at 10-50%, preferably 30-50%, wherein the rolled steel strip has a thickness of 0.8-2.5 mm, preferably 1.0-1.8 mm; 5) Post-rolling cooling wherein the rolled strip steel is cooled after on-line hot rolling, wherein the strip steel is cooled by gas atomization cooling, wherein a cooling rate of the gas atomization cooling is ≥50° C./s; and 6) Coiling of the strip steel wherein the hot-rolled strip steel is directly coiled into a coil after the cooling, wherein a coiling temperature is 470-570° C.

7. The manufacturing method for the Nb microalloyed high strength high hole expansion steel according to claim 6, wherein, in step 1), electric furnace steelmaking or converter steelmaking is employed for the smelting to obtain molten steel, and then the molten steel enters an LF furnace, a VD/VOD furnace, or an RH furnace for refining.

8. The manufacturing method for the Nb microalloyed high strength high hole expansion steel according to claim 6, wherein, in step 1), an electric furnace is used for smelting to produce molten steel, wherein 100% steel scrap is selected as the raw material for smelting without pre-screening; or a converter is used for smelting to produce molten steel, wherein steel scrap is added to the converter in an amount of ≥20% of the raw material for smelting without pre-screening; then the molten steel is delivered to an LF furnace, VD/VOD furnace or RH furnace for refining.

9. The manufacturing method for the Nb microalloyed high strength high hole expansion steel according to claim 6, wherein, in step 3), the non-oxidizing gas is N.sub.2, Ar, or CO.sub.2 gas produced by sublimation of dry ice.

10. The manufacturing method for the Nb microalloyed high strength high hole expansion steel according to claim 6, wherein, in step 5), the gas atomization cooling utilizes a gas-water ratio of 15:1-10:1, a gas pressure of 0.5-0.8 MPa, and a water pressure of 1.0-1.5 MPa.

11. The manufacturing method for the Nb microalloyed high strength high hole expansion steel according to claim 6, wherein, in step 6), the coiling utilizes double-coiler coiling or Carrousel coiling.

12. The manufacturing method for the Nb microalloyed high strength high hole expansion steel according to claim 6, wherein, in step 5), the cooling rate is 50-75° C./s.

13. The manufacturing method for the Nb microalloyed high strength high hole expansion steel according to claim 6, wherein, in step 6), the hot-rolled and cooled strip steel is directly coiled into a coil after a poor-quality head portion of the strip steel is cut off with a head shear.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0103] FIG. 1 is a schematic view showing the process layout of a twin-roll thin strip continuous casting process;

[0104] FIG. 2 is a schematic diagram showing the influence of Cu on the interface heat flow;

[0105] FIG. 3 is a schematic diagram showing the relationship between Sn content and average heat flux;

[0106] FIG. 4 is a schematic diagram showing the relationship between Sn content and cast strip surface roughness;

[0107] FIG. 5 is a ternary phase diagram of MnO—SiO.sub.2—Al.sub.2O.sub.3 (shaded area: low melting point area); and

[0108] FIG. 6 is the microstructure photograph of the steel in Examples of the present disclosure.

DETAILED DESCRIPTION

[0109] The present disclosure will be further described with reference to the following examples and accompanying drawings, but these examples by no means limit the present disclosure. Any changes made by those skilled in the art in the implementation of the present disclosure under the inspiration of the present specification will fall within the protection scope of the claims in the present disclosure.

[0110] Referring to FIG. 1, the molten steel that conforms to the chemical composition designed according to the present disclosure passes through a ladle 1, a ladle shroud 2, a tundish 3, a submerged nozzle 4 and a distributor 5, and is then directly poured into a molten pool 7 formed with side sealing plate devices 6a, 6b and two counter-rotating crystallization rolls 8a, 8b capable of rapid cooling. The molten steel solidifies on the circumferential surfaces of the rotating crystallization rolls 8a, 8b to form a solidified shell which gradually grows, and then forms a 1.5-3 mm thick cast strip 11 at the minimum gap (nip point) between the two crystallization rolls. The diameter of the crystallization rolls according to the present disclosure is between 500-1500 mm, and water is supplied to the inside of the crystallization rolls for cooling. Depending on the thickness of the cast strip, the casting speed of the casting machine is in the range of 60-150 m/min.

[0111] After the cast strip 11 exits the crystallization rolls 8a and 8b, the temperature of the cast strip is 1420-1480° C., and the cast strip enters a lower closed chamber 10 directly. The lower closed chamber 10 is supplied with a non-oxidizing gas to protect the cast strip, i.e. protecting the cast strip from oxidation. The anti-oxidation protective atmosphere may be N.sub.2, or Ar, or other non-oxidizing gas, such as CO.sub.2 gas obtained by sublimation of dry ice. The oxygen concentration in the lower closed chamber 10 is controlled to be <5%. The anti-oxidation protection provided by the lower closed chamber 10 to the cast strip 11 extends to the inlet of the rolling mill 13. The temperature of the cast strip at the outlet of the lower closed chamber 10 is 1150-1300° C. Then, the cast strip is delivered to the rolling mill 13 through a swinging guide plate 9, pinch rolls 12 and a roll table 15. After hot rolling, a hot rolled strip steel of 0.8-2.5 mm in thickness is formed. The rolled strip steel is cooled by gas atomization cooling with the use of a gas atomization rapid cooling device 14 to improve the temperature uniformity of the strip steel. After the head portion of the strip steel is cut off by a flying shear 16, the cut head portion falls into a flying shear pit 18 along a flying shear guide plate 17, and the hot-rolled strip steel with the head portion cut off enters a coiler 19 for coiling. After the steel coil is taken off the coiler, it is cooled in air to room temperature. Finally, the steel coil produced may be used as hot rolling plate directly, or used after pickling-flattening.

[0112] The chemical compositions of the Examples according to the present disclosure are shown in Table 1, and the balance is Fe and other unavoidable impurities. The process parameters of the manufacturing method according to the present disclosure are shown in Table 2, and the properties of the product obtained finally are shown in Table 3. The hole expansion rate is measured according to the International standard ISO16630:2009.

[0113] To sum up, the high hole expansion steel manufactured with the designed steel composition using the thin strip continuous casting process according to the present disclosure has a yield strength of ≥440 MPa, a tensile strength of ≥590 MPa, an elongation of ≥19%, and a hole expansion rate of ≥100%.

TABLE-US-00001 TABLE 1 Chemical compositions of the steel Examples (wt. %) C Si Mn P S N O Als Nb Ca Ex. 1  0.04 0.27 1.35 0.008 0.004 0.0064 0.0093 0.0009 0.06 0.003 Ex. 2  0.05 0.20 0.90 0.013 0.003 0.0068 0.0110 0.0006 0.05 0.004 Ex. 3  0.02 0.38 1.28 0.015 0.004 0.0048 0.0150 0.0004 0.03 0.005 Ex. 4  0.01 0.22 1.26 0.013 0.005 0.0067 0.0130 0.0008 0.03 0.004 Ex. 5  0.03 0.41 0.85 0.009 0.002 0.0062 0.0120 0.0007 0.01 0.003 Ex. 6  0.04 0.45 0.80 0.012 0.002 0.0046 0.0070 0.0008 0.04 0.001 Ex. 7  0.02 0.28 0.95 0.015 0.003 0.0040 0.0100 0.0005 0.06 0.002 Ex. 8  0.05 0.37 1.30 0.014 0.005 0.0080 0.0085 0.0006 0.05 0.004 Ex. 9  0.04 0.36 0.84 0.018 0.003 0.0078 0.0200 0.0003 0.04 0.004 Ex. 10 0.02 0.45 0.90 0.020 0.001 0.0065 0.0125 0.0004 0.07 0.004 Ex. 11 0.02 0.60 0.85 0.010 0.002 0.0080 0.0090 0.0009 0.04 0.003 Ex. 12 0.03 0.59 1.50 0.012 0.005 0.0075 0.0118 0.0003 0.06 0.001 Ex. 13 0.05 0.45 1.37 0.018 0.004 0.0045 0.0132 0.0006 0.08 0.002 Ex. 14 0.02 0.28 1.40 0.017 0.003 0.0064 0.0075 0.0005 0.03 0.003

TABLE-US-00002 TABLE 2 Process parameters of the Examples Atmosphere Oxygen Hot Hot-rolled Cast strip in lower concentration Hot rolling rolling strip Post-rolling Coiling thickness closed in lower closed temperature reduction thickness cooling rate temperature mm chamber chamber % ° C. rate/% mm ° C./s ° C. Ex. 1  2.7 N.sub.2 3.3 1150 35 1.75 58 495 Ex. 2  2.6 Ar 4.2 1200 37 1.65 60 535 Ex. 3  2.3 N.sub.2 2.3 1110 48 1.2  59 565 Ex. 4  1.8 CO.sub.2 2.5 1150 31 1.25 70 560 Ex. 5  1.7 Ar 3.5 1185 41 1.0  52 570 Ex. 6  3.0 Ar 2.8 1100 40 1.8  52 550 Ex. 7  1.9 N.sub.2 1.5 1190 24 1.45 55 505 Ex. 8  1.6 CO.sub.2 0.6 1120 22 1.25 60 480 Ex. 9  1.5 N.sub.2 1.3 1250 33 1.0  62 550 Ex. 10 2.0 N.sub.2 1.6 1180 30 1.4  75 525 Ex. 11 2.6 Ar 1.8 1140 38 1.6  65 485 Ex. 12 2.3 N.sub.2 2.6 1170 46 1.25 50 475 Ex. 13 2.0 CO.sub.2 2.4 1160 50 1.0  70 480 Ex. 14 1.6 Ar 2.5 1160 31 1.1  55 560

TABLE-US-00003 TABLE 3 Properties of the steel products in the Examples Cast Final strip product Yield Tensile Elon- Hole thickness thickness strength strength gation expansion mm mm MPa MPa % rate % Ex. 1  2.7 1.75 467 596 22 118 Ex. 2  2.6 1.65 468 615 25 117 Ex. 3  2.3 1.2  452 635 22 102 Ex. 4  1.8 1.25 469 658 26 111 Ex. 5  1.7 1.0  485 644 19 108 Ex. 6  3.0 1.8  468 637 23 116 Ex. 7  1.9 1.45 440 598 24 125 Ex. 8  1.6 1.25 496 632 21 122 Ex. 9  1.5 1.0  468 626 27 115 Ex. 10 2.0 1.4  474 617 21 107 Ex. 11 2.6 1.6  455 628 22 114 Ex. 12 2.3 1.25 457 605 25 108 Ex. 13 2.0 1.0  475 624 26 106 Ex. 14 1.6 1.1  468 638 26 114

[0114] The chemical compositions of the Examples according to the present disclosure based on steel scrap are shown in Table 4, and the balance is Fe and other unavoidable impurities. The process parameters of the manufacturing method according to the present disclosure are shown in Table 5, and the properties of the product obtained finally are shown in Table 6.

[0115] To sum up, the high hole expansion steel manufactured with the designed steel composition using the thin strip continuous casting process according to the present disclosure has a yield strength of ≥440 MPa, a tensile strength of ≥590 MPa, an elongation of ≥19%, and a hole expansion rate of ≥100%.

TABLE-US-00004 TABLE 4 Chemical compositions of the steel Examples (wt. %) c Si Mn P S N O Als Nb Cu Sn Ca Ex. 15 0.03 0.26 1.37 0.008 0.004 0.0054 0.0093 0.0009 0.03 0.35 0.022 0.005 Ex. 16 0.01 0.20 0.92 0.013 0.003 0.0071 0.0110 0.0006 0.04 0.16 0.005 0.004 Ex. 17 0.04 0.35 1.28 0.015 0.004 0.0068 0.0150 0.0004 0.06 0.10 0.003 Ex. 18 0.05 0.28 1.26 0.013 0.005 0.0067 0.0130 0.0008 0.04 0.56 0.040 0.003 Ex. 19 0.04 0.45 0.85 0.009 0.002 0.0052 0.0120 0.0007 0.01 0.44 0.014 0.005 Ex. 20 0.05 0.41 0.80 0.012 0.002 0.0046 0.0070 0.0008 0.03 0.023 0.001 Ex. 21 0.03 0.29 0.95 0.015 0.003 0.0040 0.0100 0.0005 0.07 0.38 0.035 0.002 Ex. 22 0.02 0.38 1.30 0.014 0.005 0.0080 0.0085 0.0006 0.05 0.60 0.015 0.005 Ex. 23 0.04 0.33 0.85 0.018 0.003 0.0078 0.0200 0.0003 0.03 0.37 0.002 Ex. 24 0.05 0.43 0.90 0.020 0.001 0.0065 0.0125 0.0004 0.06 0.53 0.016 0.004 Ex. 25 0.02 0.60 0.85 0.010 0.002 0.0080 0.0090 0.0009 0.03 0.038 0.005 Ex. 26 0.05 0.57 1.50 0.012 0.005 0.0055 0.0118 0.0003 0.08 0.35 0.013 0.001 Ex. 27 0.03 0.46 1.38 0.018 0.004 0.0045 0.0132 0.0006 0.07 0.036 0.004 Ex. 28 0.02 0.27 1.40 0.017 0.003 0.0074 0.0075 0.0005 0.04 0.27 0.027 0.003

TABLE-US-00005 TABLE 5 Process parameters of the steel products in the Examples Atmosphere Oxygen Hot Hot-rolled Cast strip in lower concentration Hot rolling rolling strip Post-rolling Coiling thickness closed in lower closed temperature reduction thickness cooling rate temperature mm chamber chamber % ° C. rate/% mm ° C./s ° C. Ex. 15 2.6 Ar 3.3 1130 33 1.75 53 495 Ex. 16 2.5 Ar 4.2 1200 46 1.35 60 535 Ex. 17 2.3 N.sub.2 2.3 1110 43 1.30 59 565 Ex. 18 1.8 CO.sub.2 2.5 1150 31 1.25 70 560 Ex. 19 1.5 Ar 3.5 1185 33 1.00 52 570 Ex. 20 3.0 Ar 2.8 1100 40 1.80 52 550 Ex. 21 1.9 N.sub.2 1.5 1190 21 1.50 55 505 Ex. 22 1.8 CO.sub.2 0.6 1120 31 1.25 60 480 Ex. 23 1.6 N.sub.2 1.3 1250 38 1.00 62 550 Ex. 24 2.0 N.sub.2 1.6 1180 30 1.40 75 525 Ex. 25 2.6 Ar 1.8 1140 38 1.60 65 485 Ex. 26 2.2 N.sub.2 2.6 1170 43 1.25 50 475 Ex. 27 2.0 CO.sub.2 2.4 1160 50 1.00 70 480 Ex. 28 1.7 Ar 2.5 1160 35 1.10 55 560

TABLE-US-00006 TABLE 6 Properties of the steel products in the Examples Cast Final strip product Yield Tensile Elon- Hole thickness thickness strength strength gation expansion mm mm MPa MPa % rate % Ex. 15 2.6 1.75 458 598 21 115 Ex. 16 2.5 1.35 488 612 25 111 Ex. 17 2.3 1.3  460 625 19 112 Ex. 18 1.8 1.25 463 638 28 103 Ex. 19 1.5 1.0  485 645 23 108 Ex. 20 3.0 1.8  467 638 28 112 Ex. 21 1.9 1.5  440 599 22 117 Ex. 22 1.8 1.25 498 636 21 120 Ex. 23 1.6 1.0  463 613 24 113 Ex. 24 2.0 1.4  474 614 20 117 Ex. 25 2.6 1.6  452 625 22 103 Ex. 26 2.2 1.25 469 618 23 105 Ex. 27 2.0 1.0  473 615 27 107 Ex. 28 1.7 1.1  465 642 26 118