STEEL RESISTANT TO SEAWATER CORROSION AND MANUFACTURING METHOD THEREFOR

Abstract

The present invention discloses a seawater-corrosion-resistant steel, the mass percentage of the chemical elements thereof being: 0.03-0.05% of C, 0.04-0.08% of Si, 0.8-1.2% of Mn, 0.1-0.2% of Cu, 2.5-5.5% of Cr, 0.05-0.15% of Ni, 0.15-0.35% of Mo, 1.5-3.5% of Al, 0.01-0.02% of Ti, 0.0015-0.003% of Ca, and the balance being Fe and other inevitable impurities. The present invention further discloses a method for manufacturing the seawater-corrosion-resistant steel. The method includes the following steps: (1) smelting and casting; (2) reheating: reheating a casting blank to 1200° C.-1260° C.; (3) rough rolling; (4) finish rolling; (5) coiling; and (6) cooling to room temperature. The seawater-corrosion-resistant steel has good seawater corrosion resistance and excellent mechanical properties.

Claims

1. Seawater-corrosion-resistant steel, characterized in chemical elements by mass as follows: 0.03-0.05% of C, 0.04-0.08% of Si, 0.8-1.2% of Mn, 0.1-0.2% of Cu, 2.5-5.5% of Cr, 0.05-0.15% of Ni, 0.15-0.35% of Mo, 1.5-3.5% of Al, 0.01-0.02% of Ti, 0.0015-0.003% of Ca, and the balance being Fe and other inevitable impurities.

2. The seawater-corrosion-resistant steel according to claim 1, wherein the mass percentage of elements Cr and Al further satisfies the following: the ratio of Cr/Al is 0.8-4, and Cr+Al≤7.0%.

3. The seawater-corrosion-resistant steel according to claim 1, wherein the mass percentage of Cr is 3.0-4.5%, and/or the mass percentage of Al is 1.5-2.2%.

4. The seawater-corrosion-resistant steel according to claim 1, wherein, as said other inevitable impurities, elements P, S and N satisfy at least one of mass percentages as follows: P≤0.015%, S≤0.004%, and N≤0.005%.

5. The seawater-corrosion-resistant steel according to claim 4, wherein mass percentages of Ca and S further satisfies Ca/S≥0.65.

6. The seawater-corrosion-resistant steel according to claim 1, characterized by microstructure of bainite and ferrite.

7. The seawater-corrosion-resistant steel according to claim 1, wherein said steel has a yield strength of 450 MPa or above, tensile strength of 550 MPa or above, and an annual average corrosion rate of no more than 0.1 mm/a when fully immersed in seawater.

8. A method for manufacturing the seawater-corrosion-resistant steel of claim 1, comprising the steps of: (1) smelting and casting; (2) reheating, wherein a casting blank is reheated to 1200° C.-1260° C.; (3) rough rolling; (4) finish rolling; (5) coiling; and (6) cooling to room temperature.

9. The manufacturing method according to claim 8, wherein, in step (3), an ending temperature of step of rough rolling is controlled within a range of 950° C.-1150° C.; when the thickness of a steel plate does not exceed 12 mm, the accumulative deformation in step of rough rolling is controlled to be 80% or above; and when the thickness of the steel plate exceeds 12 mm, the accumulative deformation in step of rough rolling is controlled to be 70% or above.

10. The manufacturing method according to claim 8, wherein, in step (4), an ending temperature of step of finish rolling is controlled to be not lower than 800° C.; when the thickness of a steel plate does not exceed 12 mm, a deformation ratio in step of finish rolling is controlled to be 5 or above; and when the thickness of the steel plate exceeds 12 mm, the deformation ratio in step of finish rolling is controlled to be 3.5 or above.

11. The manufacturing method according to claim 8, wherein, in step (5), the finish-rolled steel plate is water-cooled to 550-650° C. for coiling.

12. The seawater-corrosion-resistant steel of claim 2, wherein said steel has a yield strength of 450 MPa or above, tensile strength of 550 MPa or above, and an annual average corrosion rate of no more than 0.1 mm/a when fully immersed in seawater.

13. The seawater-corrosion-resistant steel of claim 3, wherein said steel has a yield strength of 450 MPa or above, tensile strength of 550 MPa or above, and an annual average corrosion rate of no more than 0.1 mm/a when fully immersed in seawater.

14. The seawater-corrosion-resistant steel of claim 4, wherein said steel has a yield strength of 450 MPa or above, tensile strength of 550 MPa or above, and an annual average corrosion rate of no more than 0.1 mm/a when fully immersed in seawater.

15. The seawater-corrosion-resistant steel of claim 5, wherein the steel has a yield strength of 450 MPa or above, tensile strength of 550 MPa or above, and an annual average corrosion rate of no more than 0.1 mm/a when fully immersed in seawater.

16. The seawater-corrosion-resistant steel of claim 6, wherein the steel has a yield strength of 450 MPa or above, tensile strength of 550 MPa or above, and an annual average corrosion rate of no more than 0.1 mm/a when fully immersed in seawater.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0048] FIG. 1 shows a microstructure of a seawater-corrosion-resistant steel in embodiment 1.

DETAILED DESCRIPTION OF THE INVENTION

[0049] A seawater-corrosion-resistant steel and a manufacturing method therefor provided by the present invention will be further explained and described below in conjunction with the specific embodiments and the accompanying drawings of the specification. However, the explanation and description do not construct improper limitations to the technical solutions of the present invention.

Embodiments 1-6

[0050] The mass percentage (mass %) of each of chemical elements in seawater-corrosion-resistant steel in the embodiments 1-6 is listed in table 1.

TABLE-US-00001 TABLE 1 (mass %, the balance being Fe and other inevitable impurity elements other than P, S and N) Serial number C Si Mn P S Al Cu Ni Cr Ti N Ca Mo Cr/Al Cr +Al Ca/S Embodiment 1 0.035 0.045 1.1  0.0081 0.0032 1.52 0.2  0.148 5.4  0.012 0.0035 0.0022 0.35 3.56 6.92 0.69 Embodiment 2 0.032 0.058 0.82  0.00808 0.0021 3.18 0.12 0.052 3.8  0.018 0.004 0.0024 0.32 1.19 6.98 1.14 Embodiment 3 0.041 0.064 0.94 0.0075 0.0022 2.72 0.18 0.134 2.54 0.019 0.0047 0.0017 0.21 0.93 5.26 0.77 Embodiment 4 0.048 0.078 0.98 0.0087 0.0034 3.42 0.15 0.098 3.18 0.017 0.0043 0.0028 0.28 0.93 6.6  0.82 Embodiment 5 0.05  0.052 1.18 0.0084 0.0029 2.64 0.16 0.127 4.16 0.016 0.0041 0.0019 0.33 1.58 6.8  0.66 Embodiment 6 0.048 0.042 1.15 0.0084 0.0028 2.16 0.16 0.137 4.62 0.016 0.0041 0.0029 0.34 2.14 6.78 1.04

[0051] It can be seen from table 1, compared with the prior art, each embodiment of the present invention lies in that a Cu—Cr—Mo component system in the prior art is not adopted, and P, S, C and Si with relatively high contents are not adopted too. Provided by the present invention, the design of a Cr—Al—Mo component system is actually adopted. Due to the cooperated addition of alloy elements Cr and Al, the seawater corrosion resistance is improved. Due to the addition of Mo, pitting corrosion is inhibited, a “reverse” effect of Cr with a relatively high content in inhibiting corrosion under a seawater environment is eliminated, and therefore, the seawater corrosion resistance is further improved.

[0052] A method for manufacturing the seawater-corrosion-resistant steel in the embodiments 1-6 includes the following steps: [0053] (1) Smelting and casting: smelting is performed on a 500 kg vacuum induction furnace according to chemical element components as shown in table 1, and casting is performed to obtain a casting blank. [0054] (2) Reheating: the casting blank is reheated to 1200° C.-1260° C. [0055] (3) Rough rolling: an ending temperature of step of rough rolling is controlled at 950° C.-1150° C. When the thickness of a steel plate does not exceed 12 mm, the accumulative deformation in step of rough rolling is controlled to be 80% or above. When the thickness of the steel plate exceeds 12 mm, the accumulative deformation in step of rough rolling can be controlled to be 70% or above. [0056] (4) Finish rolling: an ending temperature of step of finish rolling is controlled to be not lower than 800° C. When the thickness of the steel plate does not exceed 12 mm, a deformation ratio in step of finish rolling is controlled to be 5 or above. When the thickness of the steel plate exceeds 12 mm, the deformation ratio in step of finish rolling is controlled to be 3.5 or above. [0057] (5) Coiling: the finish-rolled steel plate is water-cooled to 550-650° C. for coiling. [0058] (6) Cooling to room temperature.

[0059] Specific process parameters related to the method for manufacturing the seawater-corrosion-resistant steel in the embodiments 1-6 are listed in table 2.

TABLE-US-00002 TABLE 2 Step (2) Step (3) Step (4) Step (5) Heating Accumulative Ending Ending Coiling Specification temperature deformation temperature of temperature of Deformation temperature Serial number (mm) (° C.) (%) rough rolling (° C.) finish rolling (° C.) ratio (° C.) Embodiment 1 6 1258 86.0 952 880 7.0 600 Embodiment 2 8 1244 85.3 980 865 5.5 580 Embodiment 3 12 1236 80.0 1102 858 5.0 580 Embodiment 4 14 1203 83.3 1150 852 3.6 560 Embodiment 5 18 1218 77.3 1109 848 3.8 560 Embodiment 6 20 1227 73.3 986 830 4.0 550

[0060] Various properties of the seawater-corrosion-resistant steel in the embodiments 1-6 are tested, and test results are listed in table 3.

TABLE-US-00003 TABLE 3 Serial Rp0.2 Rm Elongation Impact energy number (MPa) (MPa) (%) at 40° C. (J) Microstructure Embodiment 546 674 23.5 84 Bainite + ferrite 1 Embodiment 584 695 24.5 76 Bainite + ferrite 2 Embodiment 493 586 28.5 102 Bainite + ferrite 3 Embodiment 466 589 21.5 148 Bainite + ferrite 4 Embodiment 473 610 21.5 162 Bainite + ferrite 5 Embodiment 530 632 23.5 158 Bainite + ferrite 6

[0061] It can be seen from table 3, the seawater-corrosion-resistant steel of each embodiment shows excellent mechanical properties, and tensile properties of a test steel are tested according to a “room-temperature tensile test method in the first part of a tensile test for a metal material” in GB/T 228.1-2010, wherein the yield strength is 450 MPa-600 MPa, and the tensile strength is 550 Mpa-700 MPa. In addition, the seawater-corrosion-resistant steel of each embodiment also shows good low-temperature toughness and elongation, wherein the elongation can reach 21.5-28.5%, and the impact energy at −40° C. is 76 J or above.

[0062] In addition, the seawater-corrosion-resistant steel in all embodiments 1˜4 are compared with comparative examples 1 and 2 in terms of seawater corrosion resistance test, wherein Q345B is adopted in the comparative example 1, and Q345C-NHY3 is adopted in the comparative example 2.

[0063] Adopted in the seawater corrosion resistance test is a fully immersible testing machine manufactured by Research Institute No. 725 of China Shipbuilding Group, and the corrosion resistance is tested under the condition of full immersion in seawater in a laboratory by reference to a JB/T7901-1999 standard. Specimens have sizes of 100×30×3 mm, the surface roughness is designed according to GB1031, and the maximum allowable value of Ra is 3.2 μm. Three parallel samples are taken. Before the test, greasy dirt on the surfaces of the specimens are removed by adopting a degreasant, the specimens are cleaned with anhydrous alcohol and are blow-dried by using a dryer, the sizes of the specimens are measured, and original weights of the specimens are weighed.

[0064] A 3.5% NaCl solution is used as a test medium. The movement speeds of the specimens in a corrosive medium are 1 m/s, and the test is performed at 30° C. for 30 d. The corrosion rate is calculated as follows:

[00001]$\mathrm{Cr}=\frac{87600{\Delta }_m}{t\rho S}$

[0065] In the formula, Cr represents an annual average corrosion rate and has a dimension expressed by mm/a; Δ.sub.m represents weight loss ratios of the specimens before and after the test and has a dimension expressed by g; S represents the total surface area of the specimens and has a dimension expressed by cm.sup.2; p represents the density of the specimens, and p=7.85 g/cm.sup.3; and t represents corrosion time and has a dimension expressed by h.

[0066] Corrosion rates and relative weight loss ratios of the seawater-corrosion-resistant steel in the embodiments 1-4 and the comparative examples 1 and 2 are listed in table 4. The relative weight loss ratio is obtained by calculating a relative ratio of a corrosion rate (Cr, mm/a) obtained by calculating the corrosion weight loss of each specimen to a corrosion rate in the comparative example 1.

TABLE-US-00004 TABLE 4 Corrosion rate Relative weight Serial number (mm/a) loss ratio (%) Comparative 0.187 100 example 1 Comparative 0.135 72.4 example 2 Embodiment 1 0.068 36.63 Embodiment 2 0.07 37.64 Embodiment 3 0.068 36.34 Embodiment 4 0.071 38.22 Embodiment 5 0.070 37.41 Embodiment 6 0.069 36.79

[0067] It can be seen from table 4, each embodiment of the present invention shows higher seawater corrosion resistance than comparative examples 1-2, and the annual average corrosion thickness is lower than 0.1 mm/a.

[0068] FIG. 1 shows a microstructure of a seawater-corrosion-resistant steel in an embodiment 1. As shown in FIG. 1, the seawater-corrosion-resistant steel in the embodiment 1 has a microstructure of bainite and ferrite.

[0069] Compared with the prior art, the seawater-corrosion-resistant steel and the manufacturing method therefor provided by the present invention have the following advantages and beneficial effects: the seawater-corrosion-resistant steel provided by the present invention not only has good seawater corrosion resistance, but also has high strength and excellent toughness and welding property so as to be very suitable for a marine steel structure.

[0070] In addition, the seawater-corrosion-resistant steel provided by the present invention adopts the design of a Cr—Al—Mo component system. Due to the cooperated addition of the alloy elements Cr and Al, the seawater corrosion resistance is improved. Due to the addition of Mo, pitting corrosion is inhibited, a “reverse” effect of Cr with a relatively high content in inhibiting corrosion under a seawater environment is eliminated, and therefore, the seawater corrosion resistance is further improved.

[0071] In addition, the seawater-corrosion-resistant steel provided by the present invention is good in forming property, capable of meeting the subsequent cold machining requirement of the steel plate, easy to weld and capable of meeting the requirement on welding without preheating at a temperature higher than 0° C.

[0072] The manufacturing method provided by the present invention also has the above-mentioned advantages and beneficial effects. In addition, a controlled rolling and cooling production process is adopted in the manufacturing method provided by the present invention. In this way, heat treatment is not needed after rolling, and the steel can be supplied directly in a hot rolling state, thereby effectively shortening the supply period and reducing the production cost.

[0073] It should be noted that the prior art within the protective scope of the present invention is not limited to the embodiments given in the present application, and all of the prior art not contradicting the solutions of the present invention, including, but not limited to previous patent documents, previous public publications, previous public applications and the like should fall within the protective scope of the present invention.

[0074] In addition, combination modes of all technical features in the present invention are not limited to combination modes recorded in claims of the present invention or combination modes recorded in the specific embodiments, and all the technical features recorded in the present invention can be freely combined for incorporated in any mode, unless mutual contradiction occurs.

[0075] It should be further noted that the embodiments illustrated as above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or transformations thereof can be directly obtained or easily envisioned by those skilled in the art according to the contents disclosed by the present invention, and shall fall within the protective scope of the present invention.