HIGH STRENGTH THIN SPECIFICATION HIGH CORROSION RESISTANCE STEEL AND MANUFACTURING METHOD THEREFOR

Abstract

Disclosed are a high strength thin specification high corrosion resistance steel and a manufacturing method therefor. The chemical ingredients of the steel in percentages by weight are as follows: 0.02-0.06% of C, 0.1-0.5% of Si, 0.4-1.7% of Mn, ≤0.02% of P, 4.0-6.0% of Cr, 1.0-3.0% of Ni, ≤0.007% of S, 0.004-0.010% of N, <0.001% of Als, 0.001-0.006% of B, 0.007-0.020% of total oxygen [O].sub.T, and the balance is Fe and inevitable impurities, and same simultaneously satisfy: comprising one or both elements of 0.01-0.08% of Nb or 0.01-0.08% of V; and Mn/S≥250. In the invention, micro-alloy elements such as Nb/V and a B element are selectively added to steel, 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 a strip is then cast by means of twin-roll thin strip continuous casting, and enters an online rolling mill for hot rolling in closed conditions, and after rolling, the strip steel is cooled by air atomization cooling.

Claims

1. A high-strength, thin-gauge and high-corrosion-resistance steel, comprising the following chemical elements in weight percentages: C: 0.02-0.06%, Si: 0.1-0.5%, Mn: 0.4-1.7%, P≤=0.02%, Cr: 4.0-6.0%, Ni: 1.0-3.0%, S≤0.007%, N: 0.004-0.010%, Als<0.001%, B: 0.001-0.006%, total oxygen [O].sub.T: 0.007-0.020%, and a balance of Fe and unavoidable impurities, and, at the same time, meeting the following conditions: Comprising one or both of Nb: 0.01-0.08% and V: 0.01-0.08%; and/or one or both of Cu: 0.1-0.6% and Sn: 0.005-0.05%; Mn/S≥250.

2. The high-strength, thin-gauge and high-corrosion-resistance steel according to claim 1, wherein the high-strength, thin-gauge and high-corrosion-resistance steel comprises the following chemical elements in weight percentages: C: 0.02-0.06%, Si: 0.1-0.5%, Mn: 0.4-1.7%, P≤=0.02%, Cr: 4.0-6.0%, Ni: 1.0-3.0%, S≤0.007%, N: 0.004-0.010%, Als<0.001%, B: 0.001-0.006%, total oxygen [O].sub.T: 0.007-0.020%, and a balance of Fe and unavoidable impurities, and, at the same time, meets the following conditions: it comprises one or both of Nb: 0.01-0.08% and V: 0.01-0.08%; and Mn/S≥250.

3. The high-strength, thin-gauge and high-corrosion-resistance steel according to claim 1, wherein the high-strength, thin-gauge and high-corrosion-resistance steel comprises the following chemical elements in weight percentages: C: 0.02-0.06%, Si: 0.1-0.5%, Mn: 0.4-1.7%, P≤=0.02%, Cr: 4.0-6.0%, Ni: 1.0-3.0%, S≤0.007%, N: 0.004-0.010%, Als<0.001%, B: 0.001-0.006%, total oxygen [O].sub.T: 0.007-0.020%, and a balance of Fe and unavoidable impurities, and, at the same time, meets the following conditions: it comprises one or both of Nb: 0.01-0.08% and V: 0.01-0.08% and one or both of Cu: 0.1-0.6% and Sn: 0.005-0.05%; and Mn/S≥250.

4. The high-strength, thin-gauge, and high-corrosion-resistance steel according to claim 1, wherein the high-strength, thin-gauge, and high-corrosion-resistance steel has a microstructure of bainite, or acicular ferrite, or a mixed microstructure of bainite+acicular ferrite.

5. The high-strength, thin-gauge, and high-corrosion-resistance steel according to claim 1, wherein the high-strength, thin-gauge, and high-corrosion-resistance steel has a yield strength of ≥480 MPa, a tensile strength of ≥600 MPa, an elongation of ≥22%, and a relative corrosion rate of ≤25%.

6. The high-strength, thin-gauge, and high-corrosion-resistance steel according to claim 1, wherein the high-strength, thin-gauge, and high-corrosion-resistance steel has a thickness of 0.8-2.5 mm.

7. The high-strength, thin-gauge, and high-corrosion-resistance steel according to claim 1, wherein the high-strength, thin-gauge, and high-corrosion-resistance steel has an average corrosion rate of <0.1250 mg/cm.sup.2.Math.h.

8. The high-strength, thin-gauge, and high-corrosion-resistance steel according to claim 1, wherein the high-strength, thin-gauge, and high-corrosion-resistance steel has a yield ratio of less than 0.8.

9. A manufacturing method for the high-strength, thin-gauge, and high-corrosion-resistance steel according to claim 1, comprising the following steps: a) Smelting; wherein smelting is performed on the chemical composition defined in claim 1; wherein a basicity a=CaO/SiO.sub.2 (mass ratio) for slagging in a steelmaking process is controlled at a<1.5, wherein a MnO/SiO.sub.2 ratio (mass ratio) in molten steel for producing a low-melting-point MnO—SiO.sub.2—Al.sub.2O.sub.3 ternary inclusion is controlled at 0.5-2, 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; b) Continuous casting wherein twin-roll thin strip continuous casting is used for the 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, wherein water is supplied to an 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; c) Lower closed chamber protection wherein after a continuously 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 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; d) 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 having a thickness of 0.8-2.5 mm at a rolling temperature of 1100-1250° C. and a hot rolling reduction rate controlled at 10-50%, wherein the rolled strip steel has a thickness of 0.8-2.5 mm; e) Post-rolling cooling of the strip steel wherein the rolled strip steel is cooled, wherein the strip steel is cooled by gas atomization cooling, wherein a cooling rate of the gas atomization cooling is 20-100° C./s; and f) Coiling of the strip steel wherein the hot-rolled strip steel is coiled directly into a coil after the cooling, wherein a coiling temperature is 500-600° C.

10. The manufacturing method for the high-strength, thin-gauge, and high-corrosion-resistance steel according to claim 9, further comprising step g): follow-up treatment, wherein the steel coil is pickled and flattened, and then used as a pickled-flattened coil, or the steel coil is pickled and galvanized, and then used as a galvanized plate.

11. The manufacturing method for the high-strength, thin-gauge, and high-corrosion-resistance steel according to claim 9, wherein in step c), the non-oxidizing gas is N.sub.2, Ar, or CO.sub.2 gas produced by sublimation of dry ice.

12. The manufacturing method for the high-strength, thin-gauge, and high-corrosion-resistance steel according to claim 9, wherein in step e), the gas atomization cooling utilizes a gas-water flow 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, wherein the flow has a unit of m.sup.3/h.

13. The manufacturing method for the high-strength, thin-gauge, and high-corrosion-resistance steel according to claim 9, wherein in step f), the coiling utilizes double-coiler coiling or Carrousel coiling.

14. The manufacturing method for the high-strength, thin-gauge, and high-corrosion-resistance steel according to claim 9, wherein in step f), 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, and the coiling temperature is 500-600° C.

15. The manufacturing method for the high-strength, thin-gauge, and high-corrosion-resistance steel according to claim 9, wherein in step a), an electric furnace is used for smelting to produce molten steel, wherein 100% steel scrap is selected as a 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 a raw material for smelting without pre-screening; wherein the molten steel is then delivered to an LF furnace, VD/VOD furnace or RH furnace for refining.

16. The high-strength, thin-gauge, and high-corrosion-resistance steel according to claim 6, wherein the high-strength, thin-gauge, and high-corrosion-resistance steel has a thickness of 1.0-1.8 mm.

17. The manufacturing method for the high-strength, thin-gauge, and high-corrosion-resistance steel according to claim 9, wherein the basicity a is controlled at a<1.2, or a=0.7-1.0; the MnO/SiO.sub.2 ratio is controlled at 1-1.8; and/or the crystallization rolls have a diameter of 800 mm; the hot rolling reduction rate is controlled at 30-50%; and/or wherein the rolled strip steel has a thickness of 1.0-1.8 mm.

18. The manufacturing method for the high-strength, thin-gauge, and high-corrosion-resistance steel according to claim 9, wherein the high-strength, thin-gauge and high-corrosion-resistance steel comprises the following chemical elements in weight percentages: C: 0.02-0.06%, Si: 0.1-0.5%, Mn: 0.4-1.7%, P≤=0.02%, Cr: 4.0-6.0%, Ni: 1.0-3.0%, S≤0.007%, N: 0.004-0.010%, Als<0.001%, B: 0.001-0.006%, total oxygen [O].sub.T: 0.007-0.020%, and a balance of Fe and unavoidable impurities, and, at the same time, meets the following conditions: it comprises one or both of Nb: 0.01-0.08% and V: 0.01-0.08%; and Mn/S≥250.

19. The manufacturing method for the high-strength, thin-gauge, and high-corrosion-resistance steel according to claim 9, wherein the high-strength, thin-gauge and high-corrosion-resistance steel comprises the following chemical elements in weight percentages: C: 0.02-0.06%, Si: 0.1-0.5%, Mn: 0.4-1.7%, P≤=0.02%, Cr: 4.0-6.0%, Ni: 1.0-3.0%, S≤0.007%, N: 0.004-0.010%, Als<0.001%, B: 0.001-0.006%, total oxygen [O].sub.T: 0.007-0.020%, and a balance of Fe and unavoidable impurities, and, at the same time, meets the following conditions: it comprises one or both of Nb: 0.01-0.08% and V: 0.01-0.08% and one or both of Cu: 0.1-0.6% and Sn: 0.005-0.05%; and Mn/S≥250.

20. The manufacturing method for the high-strength, thin-gauge, and high-corrosion-resistance steel according to claim 9, wherein the high-strength, thin-gauge, and high-corrosion-resistance steel has a microstructure of bainite, or acicular ferrite, or a mixed microstructure of bainite+acicular ferrite, and/or the high-strength, thin-gauge, and high-corrosion-resistance steel has a yield strength of ≥480 MPa, a tensile strength of ≥600 MPa, an elongation of ≥22%, and a relative corrosion rate of ≤25%.

Description

DESCRIPTION OF THE DRAWINGS

[0097] FIG. 1 is a schematic view showing the process layout of a twin-roll thin strip continuous casting process;

[0098] FIG. 2 is a schematic diagram showing the relationship between Sn content and average heat flux;

[0099] FIG. 3 is a schematic diagram showing the relationship between Sn content and cast strip surface roughness;

[0100] FIG. 4 is a ternary phase diagram of MnO—SiO.sub.2—Al.sub.2O.sub.3 (shaded area: low melting point area);

[0101] FIG. 5 is a schematic diagram showing thermodynamic precipitation curves of BN and AlN.

DETAILED DESCRIPTION

[0102] 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.

[0103] 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.

[0104] 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 an inert gas to protect the strip steel, i.e. protecting the strip steel 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 hot rolling mill 13 through a swinging guide plate 9, pinch rolls 12 and a roll table 15. After hot rolling, a hot rolled strip 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 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 after pickling-flattening, or after pickling-hot-dip galvanization.

[0105] The chemical compositions of Examples 1-14 according to the present disclosure using steel scrap as a raw material are shown in Table 2, 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 3, and the mechanical properties of the hot-rolled strips obtained finally are shown in Table 4.

[0106] Corrosion resistance testing on the steel of the Examples: 72 h periodic infiltration and cyclic corrosion experiments were carried out according to Test Method for Periodic Infiltration and Corrosion of Corrosion Resistant Steel (TB/T2375-93), using ordinary carbon steel Q345B and traditional atmospheric corrosion resistant steel SPA-H as comparative samples. The average corrosion rate was obtained by calculating the corrosion weight loss per unit area of a sample, and then the relative corrosion rate of the steel was obtained. The test results are shown in Table 5.

[0107] To sum up, the high-strength and high-corrosion-resistance steel manufactured with the designed steel composition using the thin strip continuous casting process according to the present disclosure has a yield strength of ≥480 MPa, a tensile strength of ≥600 MPa, an elongation of ≥22%, and a yield ratio of less than 0.8, and the cold bendability is qualified. The comparison results of corrosion resistance also show that the relative corrosion rate of the steel according to the present disclosure is ≤25%, and the average corrosion rate is ≤0.1250 mg/cm.sup.2.Math.h.

TABLE-US-00002 TABLE 2 Chemical compositions of the steel Examples (wt. %) Ex. No. C Si Mn P S N O Als Cr Ni Nb V Cu Sn B Ex. 1 0.036 0.27 1.36 0.005 0.005 0.0077 0.0093 0.0009 4.8 1.8 0.01 0.05 0.38 0.003 Ex. 2 0.026 0.10 0.90 0.015 0.003 0.0063 0.0110 0.0006 5.1 2.3 0.03 0.25 0.005 0.001 Ex. 3 0.048 0.39 1.27 0.013 0.004 0.0059 0.0150 0.0004 5.0 3.0 0.04 0.10 0.033 0.004 Ex. 4 0.020 0.26 1.11 0.017 0.004 0.0088 0.0130 0.0008 4.2 2.7 0.03 0.040 0.006 Ex. 5 0.025 0.43 0.66 0.009 0.002 0.0053 0.0120 0.0007 4.1 1.3 0.05 0.44 0.014 0.003 Ex. 6 0.047 0.47 0.65 0.014 0.002 0.0047 0.0070 0.0008 5.8 1.0 0.08 0.53 0.005 Ex. 7 0.045 0.17 0.85 0.017 0.003 0.0040 0.0100 0.0005 6.0 1.8 0.06 0.17 0.035 0.003 Ex. 8 0.048 0.37 1.00 0.016 0.004 0.0100 0.0085 0.0006 4.4 1.4 0.04 0.60 0.015 0.002 Ex. 9 0.038 0.35 0.84 0.018 0.003 0.0088 0.0200 0.0003 5.7 2.5 0.05 0.027 0.004 Ex. 10 0.060 0.46 0.40 0.020 0.001 0.0065 0.0125 0.0004 5.3 2.2 0.04 0.05 0.52 0.016 0.006 Ex. 11 0.053 0.50 0.64 0.010 0.002 0.0090 0.0090 0.0005 4.5 2.7 0.03 0.48 0.003 Ex. 12 0.043 0.25 1.70 0.014 0.0067 0.0075 0.0118 0.0003 5.8 1.7 0.08 0.02 0.35 0.012 0.002 Ex. 13 0.045 0.38 1.37 0.009 0.004 0.0055 0.0132 0.0006 4.6 2.9 0.06 0.038 0.005 Ex. 14 0.035 0.33 1.40 0.008 0.003 0.0044 0.0075 0.0005 5.3 1.8 0.04 0.07 0.27 0.027 0.004

TABLE-US-00003 TABLE 3 Process parameters of the Examples Oxygen Atmosphere concentration Hot Hot-rolled Cast strip in lower in lower Hot rolling rolling strip Post-rolling Coiling thickness closed closed temperature reduction thickness cooling rate temperature mm chamber chamber % ° C. rate/% mm ° C./s ° C. Ex. 1 2.1 N.sub.2 3.5 1180 30 1.48 75 595 Ex. 2 2.4 Ar 4.2 1220 43 1.36 70 600 Ex. 3 2.0 N.sub.2 2.5 1200 33 1.35 59 560 Ex. 4 1.9 CO.sub.2 2.7 1150 34 1.25 20 560 Ex. 5 1.5 Ar 3.5 1185 33 1.0 92 570 Ex. 6 2.6 Ar 2.8 1100 29 1.85 72 550 Ex. 7 2.5 N.sub.2 1.5 1190 30 1.75 65 500 Ex. 8 1.7 CO.sub.2 0.8 1220 26 1.25 40 580 Ex. 9 1.6 N.sub.2 1.5 1250 38 1.0 22 550 Ex. 10 2.0 N.sub.2 1.9 1170 30 1.4 75 560 Ex. 11 2.5 Ar 1.8 1240 28 1.8 100 585 Ex. 12 2.3 N.sub.2 2.6 1170 46 1.25 60 575 Ex. 13 2.0 CO.sub.2 2.4 1180 50 1.0 30 580 Ex. 14 1.6 Ar 2.5 1160 31 1.1 25 560

TABLE-US-00004 TABLE 4 Mechanical properties of the steel products in the Examples Final 180° bend Cast strip product Yield Tensile ddiameter of curve thickness thickness strength strength Elongation Yield center a = strip Ex. No. mm mm MPa MPa % ratio thickness Ex. 1 2.1 1.48 483 627 23 0.77 Pass Ex. 2 2.4 1.36 503 662 29 0.76 Pass Ex. 3 2.0 1.35 482 628 25 0.77 Pass Ex. 4 1.9 1.25 494 665 23 0.74 Pass Ex. 5 1.5 1.0 503 639 29 0.79 Pass Ex. 6 2.6 1.85 489 642 24 0.76 Pass Ex. 7 2.5 1.75 480 638 30 0.75 Pass Ex. 8 1.7 1.25 488 638 28 0.76 Pass Ex. 9 1.6 1.0 515 673 28 0.77 Pass Ex. 10 2.0 1.4 510 658 27 0.78 Pass Ex. 11 2.5 1.8 491 628 24 0.78 Pass Ex. 12 2.3 1.25 501 650 26 0.77 Pass Ex. 13 2.0 1.0 491 633 24 0.78 Pass Ex. 14 1.6 1.1 520 677 27 0.77 Pass

TABLE-US-00005 TABLE 5 Test results of the atmospheric corrosion resistance of the steel Examples Average corrosion Relative corrosion rate, mg/cm.sup.2 .Math. h rate, % Q345B 0.4902 100 SPA-H 0.2148 43.82 Ex. 1 0.1157 23.60 Ex. 2 0.1202 24.52 Ex. 3 0.1164 23.75 Ex. 4 0.1173 23.93 Ex. 5 0.1185 24.17 Ex. 6 0.1108 22.60 Ex. 7 0.1216 24.81 Ex. 8 0.1218 24.85 Ex. 9 0.1137 23.19 Ex. 10 0.1221 24.91 Ex. 11 0.1182 24.11 Ex. 12 0.1158 23.62 Ex. 13 0.1187 24.21 Ex. 14 0.1206 24.60

[0108] The chemical compositions of Examples 15-28 according to the present disclosure without using steel scrap as a raw material are shown in Table 6, 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 7, and the mechanical properties of the hot-rolled strips obtained finally are shown in Table 8.

[0109] Corrosion resistance testing on the steel of the Examples: 72 h periodic infiltration and cyclic corrosion experiments were carried out according to Test Method for Periodic Infiltration and Corrosion of Corrosion Resistant Steel (TB/T2375-93), using ordinary carbon steel Q345B and traditional atmospheric corrosion resistant steel SPA-H as comparative samples. The average corrosion rate was obtained by calculating the corrosion weight loss per unit area of a sample, and then the relative corrosion rate of the steel was obtained. The test results are shown in Table 9.

[0110] To sum up, the high-strength and high-corrosion-resistance steel manufactured with the designed steel composition using the thin strip continuous casting process according to the present disclosure has a yield strength of ≥480 MPa, a tensile strength of ≥600 MPa, an elongation of ≥22%, and a yield ratio of less than 0.8, and the cold bendability is qualified. The comparison results of corrosion resistance also show that the relative corrosion rate of the steel according to the present disclosure is ≤25%, and the average corrosion rate is ≤0.1250 mg/cm.sup.2.Math.h.

TABLE-US-00006 TABLE 6 Chemical compositions of the steel Examples (wt. %) C Si Mn P S N O Als Cr Ni Nb V B Ex. 15 0.036 0.23 1.35 0.008 0.005 0.0074 0.0093 0.0009 4.8 1.9 0.01 0.05 0.003 Ex. 16 0.044 0.10 0.90 0.013 0.003 0.0061 0.0110 0.0006 5.1 2.5 0.02 0.001 Ex. 17 0.048 0.28 1.28 0.015 0.004 0.0058 0.0150 0.0004 5.3 3.0 0.05 0.004 Ex. 18 0.020 0.36 1.10 0.013 0.004 0.0087 0.0130 0.0008 4.5 2.5 0.03 0.006 Ex. 19 0.028 0.45 0.65 0.009 0.002 0.0052 0.0120 0.0007 4.1 2.9 0.04 0.003 Ex. 20 0.038 0.46 0.67 0.012 0.002 0.0046 0.0070 0.0008 5.3 1.0 0.08 0.005 Ex. 21 0.044 0.17 0.85 0.015 0.003 0.0040 0.0100 0.0005 6.0 1.3 0.06 0.003 Ex. 22 0.042 0.38 1.00 0.014 0.004 0.0100 0.0085 0.0006 4.4 1.4 0.04 0.002 Ex. 23 0.026 0.36 0.84 0.018 0.003 0.0078 0.0200 0.0003 5.3 2.5 0.04 0.004 Ex. 24 0.060 0.48 0.40 0.020 0.001 0.0055 0.0125 0.0004 5.5 2.3 0.05 0.05 0.006 Ex. 25 0.047 0.50 0.65 0.010 0.002 0.0090 0.0090 0.0005 4.5 2.8 0.04 0.003 Ex. 26 0.053 0.37 1.70 0.012 0.0067 0.0085 0.0118 0.0003 5.8 1.6 0.08 0.02 0.002 Ex. 27 0.049 0.44 1.37 0.008 0.004 0.0045 0.0132 0.0006 4.7 2.9 0.06 0.005 Ex. 28 0.025 0.28 1.40 0.017 0.003 0.0064 0.0075 0.0005 5.6 1.7 0.04 0.07 0.004

TABLE-US-00007 TABLE 7 Process parameters of the Examples Oxygen Atmosphere concentration Hot-rolled Cast strip in lower in lower Hot rolling Hot rolling strip Post-rolling Coiling thickness closed closed temperature reduction thickness, cooling rate, temperature mm chamber chamber % ° C. rate/% mm ° C./s ° C. Ex. 15 2.2 N.sub.2 3.5 1180 34 1.45 85 595 Ex. 16 2.5 Ar 4.2 1220 46 1.35 30 600 Ex. 17 2.1 N.sub.2 2.5 1200 38 1.3 79 560 Ex. 18 1.8 CO.sub.2 2.7 1150 31 1.25 20 560 Ex. 19 1.6 Ar 3.5 1185 38 1.0 92 570 Ex. 20 2.7 Ar 2.8 1100 31 1.85 72 550 Ex. 21 1.9 N.sub.2 1.5 1190 34 1.25 65 500 Ex. 22 1.5 CO.sub.2 0.8 1220 30 1.05 50 580 Ex. 23 1.7 N.sub.2 1.5 1250 41 1.0 22 550 Ex. 24 2.0 N.sub.2 1.9 1170 30 1.4 75 560 Ex. 25 2.5 Ar 1.8 1240 40 1.5 100 585 Ex. 26 2.2 N.sub.2 2.6 1170 43 1.25 60 575 Ex. 27 2.0 CO.sub.2 2.4 1180 50 1.0 30 580 Ex. 28 1.6 Ar 2.5 1160 31 1.1 25 560

TABLE-US-00008 TABLE 8 Mechanical properties of the steel products in the Examples 180° bend Cast strip Yield Tensile diameter of curve thickness, Final product strength strength Elongation Yield center a = strip mm thickness mm MPa MPa % ratio thickness Ex. 15 2.2 1.45 492 625 26 0.79 Pass Ex. 16 2.5 1.35 513 660 24 0.78 Pass Ex. 17 2.1 1.3 482 622 26 0.77 Pass Ex. 18 1.8 1.25 494 655 27 0.75 Pass Ex. 19 1.6 1.0 500 638 28 0.78 Pass Ex. 20 2.7 1.85 489 642 23 0.76 Pass Ex. 21 1.9 1.25 484 635 26 0.76 Pass Ex. 22 1.5 1.05 485 638 28 0.76 Pass Ex. 23 1.7 1.0 525 673 27 0.78 Pass Ex. 24 2.0 1.4 514 658 24 0.78 Pass Ex. 25 2.5 1.5 496 624 23 0.79 Pass Ex. 26 2.2 1.25 506 650 29 0.78 Pass Ex. 27 2.0 1.0 498 636 25 0.78 Pass Ex. 28 1.6 1.1 526 667 26 0.79 Pass

TABLE-US-00009 TABLE 9 Test results of the atmospheric corrosion resistance of the steel Examples Average corrosion Relative corrosion rate, mg/cm.sup.2 .Math. h rate, % Q345B 0.4902 100 SPA-H 0.2148 43.82 Ex. 15 0.1187 24.21 Ex. 16 0.1218 24.85 Ex. 17 0.1148 23.42 Ex. 18 0.1186 24.19 Ex. 19 0.1163 23.73 Ex. 20 0.1028 20.97 Ex. 21 0.1176 23.99 Ex. 22 0.1224 24.97 Ex. 23 0.1109 22.62 Ex. 24 0.1139 23.24 Ex. 25 0.1185 24.17 Ex. 26 0.1108 22.60 Ex. 27 0.1156 23.58 Ex. 28 0.1203 24.54