SEAWATER CORROSION-RESISTANT MARINE ENGINEERING STEEL AND PREPARATION METHOD THEREOF

Abstract

A seawater corrosion-resistant marine engineering steel and a preparation method thereof are provided. The seawater corrosion-resistant marine engineering steel consists of the following chemical compositions in percentage by mass: C: 0.011-0.069%, Si: 0.11-0.29%, Cr: 1.51-1.99%, Nb: 0.02-0.05%, Zr: 0.01-0.02%, RE: 0.0034-0.02%, and the balance of Fe and inevitable impurities. The mass percentages of Zr element and RE element also satisfy the following formulas: 0.01%<Zr+RE<0.02%, and Zr/RE=1-3. The marine engineering steel is designed with cheap chemical compositions of low carbon, low silicon and medium chromium, and is completely free of valuable corrosion-resistant metal elements such as Ni and Cu. Instead of the traditional Al deoxidization technology, Si deoxidization assisted by ZrRE composite deoxidization is used to form a fine, dispersed and uniform composite oxysulfide.

Claims

1. A seawater corrosion-resistant marine engineering steel, comprising the following chemical compositions in percentage by mass: C: 0.011-0.069%, Si: 0.11-0.29%, Cr: 1.51-1.99%, Nb: 0.02-0.05%, Zr: 0.01-0.02%, RE: 0.0034-0.02%, and a balance of Fe and inevitable impurities; wherein mass percentages of Zr element and RE element further satisfy the following formulas: 0.01%<Zr+RE<0.02% and Zr/RE=1-3.

2. The seawater corrosion-resistant marine engineering steel according to claim 1, wherein the seawater corrosion-resistant marine engineering steel comprises the following chemical compositions in percentage by mass: C: 0.029-0.068%, Si: 0.15-0.28%, Cr: 1.55-1.78%, Nb: 0.025-0.049%, Zr: 0.01-0.0117%, RE: 0.0039-0.0099%, and the balance of Fe and inevitable impurities.

3. The seawater corrosion-resistant marine engineering steel according to claim 1, wherein the seawater corrosion-resistant marine engineering steel comprises the following chemical compositions in percentage by mass: C: 0.04%, Si: 0.20%, Cr: 1.75%, Nb: 0.035%, Zr: 0.012%, RE: 0.006%, and the balance of Fe and inevitable impurities.

4. The seawater corrosion-resistant marine engineering steel according to claim 1, wherein in the other inevitable impurities, a mass percentage of S element satisfies: S0.0010%.

5. The seawater corrosion-resistant marine engineering steel according to claim 1, wherein the RE element comprises lanthanum and cerium, and a weight ratio of the lanthanum element to the cerium element is (70-90):(10-30).

6. The seawater corrosion-resistant marine engineering steel according to claim 1, wherein a microstructure type of the seawater corrosion-resistant marine engineering steel is acicular ferrite and polygonal grain boundary ferrite, and a quantity ratio of the polygonal grain boundary ferrite to the acicular ferrite is 4-8.

7. The seawater corrosion-resistant marine engineering steel according to claim 1, wherein a density of corrosion-active inclusions in the seawater corrosion-resistant marine engineering steel is less than or equal to 5/mm.sup.2.

8. The seawater corrosion-resistant marine engineering steel according to claim 1, wherein a saturation current density of the seawater corrosion-resistant marine engineering steel is less than or equal to 6.0 mA under a condition that a static electrode potential is equal to 300 mV; and a corrosion rate of the seawater corrosion-resistant marine engineering steel is less than or equal to 0.04/mm.Math.a under a condition that a mass content of NaCl in a seawater solution is 3.5%.

9. A method for preparing the seawater corrosion-resistant marine engineering steel according to claim 1, comprising following steps: 1) smelting and refining molten steel in turn, carrying out vacuum treatment, and continuously casting the molten steel into a slab to obtain a casting slab; 2) heating and soaking the casting slab to obtain a heat-treated casting slab; and 3) continuously rolling the heat-treated casting slab, controlling a final rolling temperature to 750-850 C., cooling by water to 410-550 C. after rolling, and naturally cooling to a room temperature to obtain the seawater corrosion-resistant marine engineering steel.

10. The method for preparing the seawater corrosion-resistant marine engineering steel according to claim 9, wherein a method for the smelting and refining in step 1) comprises following steps: steelmaking molten iron and/or scrap steel by using a converter or an electric arc furnace, and adjusting a temperature and compositions to obtain the molten steel; bringing the molten steel into a steel ladle and stirring the molten steel with fine argon bubbling, and pre-deoxidizing the molten steel in the steel ladle by using FeSi alloy or FeSiMn alloy to adjust a free oxygen content in the molten steel to 19-99 ppm; and stirring the molten steel with fine argon bubbling and carrying out final deoxidization by a composite additive, and carrying out ladle furnace (LF) refining, vacuum degassing (VD) refining or Ruhrstahl-Heraeus (RH) refining on the molten steel after the final deoxidization.

11. The seawater corrosion-resistant marine engineering steel according to claim 2, wherein in the other inevitable impurities, a mass percentage of S element satisfies: S0.0010%.

12. The seawater corrosion-resistant marine engineering steel according to claim 3, wherein in the other inevitable impurities, a mass percentage of S element satisfies: S0.0010%.

13. The seawater corrosion-resistant marine engineering steel according to claim 2, wherein the RE element comprises lanthanum and cerium, and a weight ratio of the lanthanum element to the cerium element is (70-90):(10-30).

14. The seawater corrosion-resistant marine engineering steel according to claim 3, wherein the RE element comprises lanthanum and cerium, and a weight ratio of the lanthanum element to the cerium element is (70-90):(10-30).

15. The seawater corrosion-resistant marine engineering steel according to claim 2, wherein a microstructure type of the seawater corrosion-resistant marine engineering steel is acicular ferrite and polygonal grain boundary ferrite, and a quantity ratio of the polygonal grain boundary ferrite to the acicular ferrite is 4-8.

16. The seawater corrosion-resistant marine engineering steel according to claim 3, wherein a microstructure type of the seawater corrosion-resistant marine engineering steel is acicular ferrite and polygonal grain boundary ferrite, and a quantity ratio of the polygonal grain boundary ferrite to the acicular ferrite is 4-8.

17. The seawater corrosion-resistant marine engineering steel according to claim 2, wherein a density of corrosion-active inclusions in the seawater corrosion-resistant marine engineering steel is less than or equal to 5/mm.sup.2.

18. The seawater corrosion-resistant marine engineering steel according to claim 3, wherein a density of corrosion-active inclusions in the seawater corrosion-resistant marine engineering steel is less than or equal to 5/mm.sup.2.

19. The seawater corrosion-resistant marine engineering steel according to claim 2, wherein a saturation current density of the seawater corrosion-resistant marine engineering steel is less than or equal to 6.0 mA under a condition that a static electrode potential is equal to 300 mV; and a corrosion rate of the seawater corrosion-resistant marine engineering steel is less than or equal to 0.04/mm.Math.a under a condition that a mass content of NaCl in a seawater solution is 3.5%.

20. The seawater corrosion-resistant marine engineering steel according to claim 3, wherein a saturation current density of the seawater corrosion-resistant marine engineering steel is less than or equal to 6.0 mA under a condition that a static electrode potential is equal to 300 mV; and a corrosion rate of the seawater corrosion-resistant marine engineering steel is less than or equal to 0.04/mm.Math.a under a condition that a mass content of NaCl in a seawater solution is 3.5%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] FIG. 1 shows a potentiodynamic polarization result of a marine engineering steel prepared in Embodiment 1 in a 3.5% NaCl solution according to the present invention;

[0038] FIG. 2 shows a potentiodynamic polarization result of a comparative steel in a 3.5% NaCl solution according to the present invention;

[0039] FIGS. 3A-3D show an alternating current (AC) impedance result of a marine engineering steel prepared in Embodiment 1 in a 3.5% NaCl solution according to the present invention; wherein FIG. 3A is a Nyquist diagram, FIGS. 3B and 3C are Bode diagrams, and FIG. 3D is an equivalent circuit diagram;

[0040] FIGS. 4A-4D show an AC impedance result of a comparative steel in a 3.5% NaCl solution according to the present invention; wherein FIG. 4A is a Nyquist diagram, FIGS. 4B and 4C are Bode diagrams, and FIG. 4D is an equivalent circuit diagram;

[0041] FIG. 5 shows a potentiostatic polarization result of a marine engineering steel prepared in Embodiment 1 in a chloride ion medium according to the present invention;

[0042] FIG. 6 shows a potentiostatic polarization result of a comparative steel in a chloride ion medium according to the present invention;

[0043] FIG. 7 is a micrograph of corrosion-active inclusions in a comparative steel according to the present invention; and

[0044] FIG. 8 is a micrograph of corrosion-active inclusions in a marine engineering steel prepared in Embodiment 1 according to the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0045] The principles and features of the present invention are described below, and the examples given are only for explaining the present invention, not for limiting the scope of the present invention. Where the specific technologies or conditions are not specified in the embodiments, the technologies or conditions described in the literature in the art or product specifications shall be followed. The reagents or instruments used are all conventional products purchasable through regular channels if manufacturers are not indicated thereon.

[0046] The following composite additive is a composition of zirconium, lanthanum and cerium, and a weight ratio of zirconium, lanthanum and cerium is 8:4.4:1.1; and comparative steel Q345 is obtained by performing final deoxidization with conventional aluminum blocks, aluminum particles or aluminum wires to form coarse and clustered alumina, composite oxides thereof and the like.

Embodiment 1

[0047] The present embodiment relates to a seawater corrosion-resistant marine engineering steel, which consists of the following chemical compositions in percentage by mass: 0.04% of C, 0.20% of Si, 1.75% of Cr, 0.035% of Nb, 0.012% of Zr, 0.006% of RE, 0.0008% of S, and the balance of Fe and inevitable impurities. The RE includes lanthanum and cerium, and a weight ratio of the lanthanum element to the cerium element is 90:10.

[0048] The present embodiment relates to a method for preparing a seawater corrosion-resistant marine engineering steel, which includes the following steps: [0049] 1) Molten steel is smelted and refined in turn, then vacuum treating is carried out, and then the molten steel is continuously cast into a slab to obtain a casting slab.

[0050] A specific method of the smelting and refining in step 1) is as follows: after performing steelmaking on molten iron by using a converter, the temperature and compositions of the molten steel are adjusted, wherein the tapping temperature is adjusted to 1,615 C., and the free oxygen content in the molten steel is 205 ppm; after entering a steel ladle, the molten steel is stirred for 7 min with fine argon bubbling, and then pre-deoxidized by using FeSi alloy or FeSiMn alloy in the steel ladle, so that the free oxygen content in the molten steel is adjusted to 60 ppm; stirring is performed for 5 min with fine argon bubbling, and then the final deoxidization is performed with the composite additive; the composite additive is added into the molten steel in the form of blocky alloy or a cored wire, and a particle size of the composite additive is 12 mm; an addition amount of the composite additive is 1.9 kg per ton of molten steel; and then LF refining and RH refining are performed on the molten steel according to the conventional process.

Lf Refining:

[0051] The viscosity of refining slag is controlled to 1.525-1.938 Pa.Math.s, so as to improve the inclusion adsorbing ability of a slag system, thereby increasing the cleanliness of the molten steel; the alkalinity R of white slag in a refining furnace is controlled so that 5.25R7.45, which is conductive to increasing the desulfurization rate, increasing the cleanliness of the molten steel and decreasing oxide inclusions in the molten steel; the MI slag index (=a ratio of CaO/SiO.sub.2:Al.sub.2O.sub.3) is controlled so that MI>0.158, the distribution coefficient of sulfur is greatly increased, thereby controlling the proper fluidity of refining slag at a certain alkalinity; and the retention time of the white slag is 14.35 min, the refining period is 39.47 min, and the soft blowing time is enabled to be >4.58 min, so as to control the outlet [O] content.

RH Vacuum Treating:

[0052] The air pressure in a vacuum chamber is pumped below 66.67 kPa for 12.13-14.45 min, and the bottom argon blowing flow rate is 10.35-19.58 m.sup.3/h, so as to realize circulation of the molten steel for 4 times; the types and weights of added alloys are strictly controlled, alloys with higher grades, such as low-carbon ferromanganese, metal manganese, low-carbon ferrosilicon and ferrotitanium, are used to ensure that the compositions of the molten steel are completely qualified, and the vacuum is kept for more than 5.37 min after the alloys are added to obtain more pure molten steel; at the same time, a suitable molten steel temperature is provided for continuous casting, which ensures that the superheat of a tundish is 10.28-29.19 C. above the liquidus.

[0053] Then the refined molten steel is continuously cast according to the conventional process: the temperature of the continuous casting tundish is 1,542 C., and the pulling speed is 1.23 m/s.

[0054] 2) Conventional heating treatment and soaking treatment are carried out on the casting slab at the temperature of 1,190 C. for 3.4 hours, and a heat-treated casting slab is obtained.

[0055] 3) The heat-treated casting slab is continuously rolled, a final rolling temperature is controlled to be 810 C., and the casting slab is cooled by water to 490 C. after rolling, and then naturally cooled to a room temperature to obtain the marine engineering steel.

[0056] A specific rolling method in step 3) includes following steps: heating at 1,190 C. for 3.4 hours; continuously rolling the casting slab into a product steel plate, controlling the final rolling temperature to 810 C., and cooling by water to 490 C. after rolling; and naturally cooling to the room temperature for later use.

[0057] The microstructure type of the marine engineering steel plate obtained according to the above compositions and preparation process is acicular ferrite and polygonal grain boundary ferrite, and a quantity ratio of the polygonal grain boundary ferrite to the acicular ferrite is 6. The density of corrosion-active inclusions in the marine engineering steel plate is 4/mm.sup.2. The saturation current density of the marine engineering steel plate at a static electrode potential (E=300 mV) is 5.5 mA. The corrosion rate of the marine engineering steel plate in a simulated seawater solution (3.5 wt % NaCl solution) is 0.035/mm.Math.a.

Embodiment 2

[0058] The present embodiment relates to a seawater corrosion-resistant marine engineering steel, which consists of the following chemical compositions in percentage by mass: 0.068% of C, 0.28% of Si, 1.78% of Cr, 0.049% of Nb, 0.0117% of Zr, 0.0039% of RE, 0.0010% of S, and the balance of Fe and inevitable impurities. The RE includes lanthanum and cerium, and a weight ratio of the lanthanum element to the cerium element is 80:20.

[0059] The present embodiment relates to a method for preparing a seawater corrosion-resistant marine engineering steel, which includes the following steps:

[0060] 1) Molten steel is smelted and refined in turn, then vacuum treatment is carried out, and then the molten steel is continuously cast into a slab to obtain a casting slab.

[0061] A method of the smelting and refining in step 1) is as follows: after performing steelmaking on molten iron by using a converter, the temperature and compositions of the molten steel are adjusted, wherein the tapping temperature is adjusted to 1,670 C., and the free oxygen content in the molten steel is 380 ppm; after entering the steel ladle, the molten steel is stirred for 9 min with fine argon bubbling, and then pre-deoxidized by using FeSi alloy or FeSiMn alloy in the steel ladle, so that the free oxygen content in the molten steel is adjusted to 90 ppm; stirring is performed for 6 min with fine argon bubbling, and then the final deoxidization is performed with the composite additive; the composite additive is added into the molten steel in the form of blocky alloy or a cored wire, and a particle size of the composite additive is 18 mm; an addition amount of the composite additive is 3.8 kg per ton of molten steel; and then LF refining and RH refining are performed on the molten steel according to the conventional process.

Lf Refining:

[0062] The viscosity of refining slag is controlled to 1.535-1.931 Pa.Math.s, so as to improve the inclusion adsorbing ability of a slag system, thereby increasing the cleanliness of the molten steel; the alkalinity R of white slag in a refining furnace is controlled so that 5.22R7.48, which is conductive to increasing the desulfurization rate, increasing the cleanliness of the molten steel and decreasing oxide inclusions in the molten steel; the MI slag index (=a ratio of CaO/SiO.sub.2:Al.sub.2O.sub.3) is controlled so that MI>0.157, the distribution coefficient of sulfur is greatly increased, thereby controlling the proper fluidity of refining slag at a certain alkalinity; and the retention time of the white slag is 14.43 min, the refining period is 39.51 min, and the soft blowing time is enabled to be >4.59 min, so as to control the outlet [O] content.

RH Vacuum Treatment:

[0063] The air pressure in a vacuum chamber is pumped below 66.57 kPa for 12.11-14.23 min, and the bottom argon blowing flow rate is 10.31-19.59 m.sup.3/h, so as to realize circulation of the molten steel for 5 times; the types and weights of added alloys are strictly controlled, alloys with higher-grades, such as low-carbon ferromanganese, metal manganese, low-carbon ferrosilicon and ferrotitanium are used to ensure that the compositions of the molten steel are completely qualified, and the vacuum is kept for more than 5.31 min after the alloys are added to obtain more pure molten steel; at the same time, a suitable molten steel temperature is provided for continuous casting, which ensures that the superheat of a tundish is 10.23-29.29 C. above the liquidus.

[0064] Then the refined molten steel is continuously cast according to the conventional process: the temperature of the continuous casting tundish is 1,541 C., and the pulling speed is 1.21 m/s.

[0065] 2) Conventional heating treatment and soaking treatment are carried out on the casting slab at the temperature of 1,185 C. for 3.45 hours, and a heat-treated casting slab is obtained.

[0066] 3) The heat-treated casting slab is continuously rolled, a final rolling temperature is controlled to be 840 C., and the casting slab is cooled by water to 540 C. after rolling, and then naturally cooled to a room temperature to obtain the marine engineering steel.

[0067] A rolling method in step 3) includes following steps: heating the casting slab at 1,180 C. for 3.5 hours; continuously rolling the casting slab into a product steel plate, controlling the final rolling temperature to 840 C., and cooling by water to 540 C. after rolling; and naturally cooling to the room temperature for later use.

[0068] The microstructure type of the marine engineering steel plate obtained according to the above compositions and preparation process is acicular ferrite and polygonal grain boundary ferrite, and a ratio of the polygonal grain boundary ferrite to the acicular ferrite is 5. The density of corrosion-active inclusions in the steel plate is 4.2/mm.sup.2. The saturation current density of the steel plate at a static electrode potential (E=300 mV) is 5.8 mA. The corrosion rate of the steel plate in a simulated seawater solution (3.5% NaCl solution) is 0.033/mm.Math.a.

Embodiment 3

[0069] The present embodiment relates to a seawater corrosion-resistant marine engineering steel, which consists of the following chemical compositions in percentage by mass: 0.029% of C, 0.15% of Si, 1.55% of Cr, 0.025% of Nb, 0.01% of Zr, 0.0099% of RE, 0.0008% of S, and the balance of Fe and inevitable impurities. The RE includes lanthanum and cerium, and a weight ratio of the lanthanum element to the cerium element is 70:30.

[0070] The present embodiment relates to a method for preparing a seawater corrosion-resistant marine engineering steel, which includes the following steps:

[0071] 1) Molten steel is smelted and refined in turn, then vacuum treatment is carried out, and then the molten steel is continuously cast into a slab to obtain a casting slab.

[0072] A method of the smelting and refining in step 1) is as follows: after performing steelmaking on molten iron by using a converter, the temperature and compositions of the molten steel are adjusted, wherein the tapping temperature is adjusted to 1,590 C., and the free oxygen content in the molten steel is 150 ppm; after entering the steel ladle, the molten steel is stirred for 4 min with fine argon bubbling, and then pre-deoxidized by using FeSi alloy or FeSiMn alloy in the steel ladle, so that the free oxygen content in the molten steel is adjusted to 30 ppm; stirring is performed for 4 min with fine argon bubbling, and then the final deoxidization is performed with the composite additive; the composite additive is added into the molten steel in the form of blocky alloy or a cored wire, and a particle size of the composite additive is 6 mm; an addition amount of the composite additive is 0.89 kg per ton of molten steel; and then LF refining and RH refining are performed on the molten steel according to the conventional process.

Lf Refining:

[0073] The viscosity of refining slag is controlled to 1.513-1.927 Pa.Math.s, so as to improve the inclusion adsorbing ability of a slag system, thereby increasing the cleanliness of the molten steel; the alkalinity R of white slag in a refining furnace is controlled so that 5.22R7.43, which is conductive to increasing the desulfurization rate, increasing the cleanliness of the molten steel and decreasing oxide inclusions in the molten steel; the MI slag index (=a ratio of CaO/SiO.sub.2:Al.sub.2O.sub.3) is controlled so that MI>0.153, the distribution coefficient of sulfur is greatly increased, thereby controlling the proper fluidity of refining slag at a certain alkalinity; and the retention time of the white slag is 14.37 min, the refining period is 39.51 min, and the soft blowing time is enabled to be >4.63 min, so as to control the outlet [O] content, RH vacuum treatment:

[0074] The air pressure in a vacuum chamber is pumped below 66.67 kPa for 12.24-14.36 min, and the bottom argon blowing flow rate is 10.17-19.43 m.sup.3/h, so as to realize circulation of the molten steel for 6 times; the types and weights of added alloys are strictly controlled, alloys with higher-grades, such as low-carbon ferromanganese, metal manganese, low-carbon ferrosilicon and ferrotitanium, are used to ensure that the compositions of the molten steel are completely qualified, and the vacuum is kept for more than 5.31 min after the alloys are added to obtain more pure molten steel; at the same time, a suitable molten steel temperature is provided for continuous casting, which ensures that the superheat of a tundish is 10.31-29.25 C. above the liquidus.

[0075] Then the refined molten steel is continuously cast according to the conventional process: the temperature of the continuous casting tundish is 1,540 C., and the pulling speed is 1.25 m/s.

[0076] 2) Conventional heating treatment and soaking treatment are carried out on the casting slab at the temperature of 1,187 C. for 3.5 hours, and a heat-treated casting slab is obtained.

[0077] 3) The heat-treated casting slab is continuously rolled, a final rolling temperature is controlled to be 770 C., and the casting slab is cooled by water to 430 C. after rolling, and then naturally cooled to a room temperature to obtain the marine engineering steel.

[0078] A rolling method in step 3) includes following steps: heating the casting slab at 1,210 C. for 3.1 hours; continuously rolling the casting slab into a product steel plate, controlling the final rolling temperature to 770 C., and cooling by water to 430 C. after rolling; and naturally cooling to the room temperature for later use.

[0079] The microstructure type of the marine engineering steel plate obtained according to the above compositions and preparation process is acicular ferrite and polygonal grain boundary ferrite, and a ratio of the polygonal grain boundary ferrite to the acicular ferrite is 4. The density of corrosion-active inclusions in the steel plate is 4.7/mm.sup.2. The saturation current density of the steel plate at a static electrode potential (E=300 mV) is 5.5 mA. The corrosion rate of the steel plate in a simulated seawater solution (3.5% NaCl solution) is 0.038/mm.Math.a.

Test Case

[0080] The corrosion resistance of the marine engineering steel with excellent seawater corrosion resistance prepared in Embodiment 1 is analyzed and tested.

1. Test Method

(1) Electrochemical Corrosion Experiment

[0081] The environment of an electrochemical corrosion experiment is at a room temperature, and the corrosive solution is a 3.5% NaCl solution, which simulates a corrosion environment. The electrode is implemented by a classic tri-electrode system: a sample is a working electrode, a platinum electrode is an auxiliary electrode, and a saturated calomel electrode (SCE) is a reference electrode. An electrochemical device is a ZAHNER electrochemical workstation. Parameters are set by the Thales electrochemical software, and the workstation is connected to a computer for data display.

[0082] The electrochemical corrosion experiment is carried out at the room temperature, and a potentiodynamic polarization curve (Tafel) and an electrochemical impedance (EIS) of a small sheet sample are tested. Before the test, the sample is soaked in the corrosive solution for 40 min, and then electrochemical impedance and potentiodynamic polarization tests are carried out after an open circuit potential (OCP) is stable. The transition signal of sine waves applied by the electrochemical impedance is 10 mV, and a test scanning range is 10 mHz to 10 KHz. A scanning rate of the potentiodynamic polarization curve is 0.5 mV/s and a scanning range is-600 mV to 1.2V. The potentiodynamic polarization curve and an electrochemical impedance curve are fitted by the Origin and Zsimpwin softwares respectively.

[0083] An AC impedance method disturbs the electrode system with small-amplitude sine waves of different frequencies, an equivalent circuit of the electrodes is inferred through the relationship between a response of the electrode system and a disturbance signal, and the parameters of various elements in the equivalent circuit are fitted, so as to obtain corrosion kinetic parameters of the material, and directly and quantitatively analyze factors affecting the corrosion resistance of the material and further understand corrosion behaviors of the material.

[0084] The corrosion electrochemical experiment is carried out in the classical tri-electrode system, the electrochemical sample to be tested serves as the working electrode, the saturated calomel electrode (SCE) serves as the reference electrode, a platinum sheet serves as a counter electrode, and the test temperature is normal temperature 25 C. At the room temperature, for a weld metal in an as-welded state, the OCP is tested by soaking the sample in the corrosive solution, the test time is 40 min, and then the electrochemical impedance test is started after the OCP is stable. The amplitude of the sine waves applied by the electrochemical AC impedance is 10 mV, the scanning frequency range is 10 mHz to 10 kHz, and the scanning time is 40 min.

(2) Determination of the Density of the Corrosion-Active Inclusions

[0085] A sample is cut into a size of 101010 mm, the surface is mechanically ground to 1500 meshes and then polished to a mirror surface, a corrosive reagent is prepared according to the following proportion: every 100 mL of ethanol solution contains 4.5-5.5 mL of concentrated hydrochloric acid, 0.08-0.15 g of CuCl.sub.2, 0.03-0.08 g of SnCl.sub.2 and 2.6-3.4 g of FeCl.sub.3, the corrosive reagent is dropped on the surface of the sample for treatment for 5-10 s, then the surface is rinsed with alcohol and blow-dried, and the density of the corrosion-active inclusions is counted by placing the sample under a metallographic microscope at a magnification of 100.

2. Test Results and Analysis

[0086] FIG. 1 shows a potentiodynamic polarization result of the marine engineering steel prepared in Embodiment 1 in the 3.5% NaCl solution. As can be seen from FIG. 1, the marine engineering steel prepared in Embodiment 1 has a corrosion potential of 0.535V and a corrosion current density of 2.01210.sup.5 A.Math.cm.sup.2.

[0087] FIG. 2 shows a potentiodynamic polarization result of a comparative steel in the 3.5% NaCl solution. As can be seen from FIG. 2, the comparative steel has a corrosion potential of 0.5619V and a corrosion current density of 8.50910.sup.5 A.Math.cm.sup.2.

[0088] FIGS. 3A-3D show an AC impedance test result of the marine engineering steel prepared in Embodiment 1 in the 3.5% NaCl solution. Table 1 shows an AC impedance fitting result. The constant phase angle index n in Table 1 reflects the deviation between an actual capacitance and an ideal capacitance. The greater the value of n, the smaller the deviation between the actual capacitance and the ideal capacitance. Generally, the value of n of an electric double-layer capacitance of a corroding electrode is between 0.5 and 1. As can be seen from the results in FIGS. 3A-3D and Table 1, a charge transfer resistance of the marine engineering steel prepared in Embodiment 1 is 808.6 .Math.cm.sup.2.

TABLE-US-00001 TABLE 1 AC impedance fitting result of marine engineering steel prepared in Embodiment 1 in 3.5% NaCl solution Constant phase R.sub.s, Y.sub.0, angle R.sub.ct, ( .Math. cm.sup.2) (S .Math. sec.sup.n .Math. cm.sup.2) index n ( .Math. cm.sup.2) Marine 33.69 0.0009014 0.7782 808.6 engineering steel

[0089] FIG. 4A-4D show the AC impedance test result of the marine engineering steel prepared in Embodiment 1 in the 3.5% NaCl solution. Table 2 shows the AC impedance fitting result. As can be seen from the results in FIGS. 4A-4D and Table 2, the charge transfer resistance of the comparative steel is 233.7 .Math.cm.sup.2.

TABLE-US-00002 TABLE 2 AC impedance fitting result of comparative steel in 3.5% NaCl solution Constant phase R.sub.s, Y.sub.0, angle R.sub.ct, ( .Math. cm.sup.2) (S .Math. sec.sup.n .Math. cm.sup.2) index n ( .Math. cm.sup.2) Q345 36.24 0.001118 0.683 233.7

[0090] FIG. 5 shows that the saturation current density of the marine engineering steel prepared in Embodiment 1 acquired by a potentiostatic polarization test is 3.25 mA/cm.sup.2. It is generally considered that if the saturation current density is 6 mA/cm.sup.2 and 8 mA/cm.sup.2, the corrosion resistance to a chloride ion aqueous medium is considered to be better. If the saturation current density is 6 mA/cm.sup.2, the corrosion resistance to the chloride ion aqueous medium is considered to be excellent. The above results show that the marine engineering steel according to the present invention has excellent corrosion resistance to the chloride ion aqueous medium.

[0091] FIG. 6 shows that the saturation current density of the comparative steel obtained by the potentiostatic polarization test is 5.71 mA/cm.sup.2. The above result show that the comparative steel has good corrosion resistance to the chloride ion aqueous medium.

[0092] FIG. 7 is a micrograph of corrosion-active inclusions in the comparative steel. The measurement result shows that the density of the corrosion-active inclusions in the comparative steel is 15/mm.sup.2.

[0093] FIG. 8 is a micrograph of the corrosion-active inclusions in the marine engineering steel prepared in Embodiment 1. The measurement result show that the density of the corrosion-active inclusions in the marine engineering steel prepared in Embodiment 1 is 4/mm.sup.2. The density of the corrosion-active inclusions in the marine engineering steel prepared in Embodiment 1 is less than 50% of that in the comparative steel.

[0094] As can be seen from the above test results, the order of the corrosion current density is: comparative steel>marine engineering steel prepared in Embodiment 1, the order of the charge transfer resistance is: comparative steel<marine engineering steel prepared in Embodiment 1, and the order of the saturation current density is: comparative steel>marine engineering steel prepared in Embodiment 1, which indicate that the corrosion resistance to the chloride ion aqueous medium of the marine engineering steel prepared in Embodiment 1 is excellent and obviously superior to that of the comparative steel.

[0095] In summary, the marine engineering steel according to the present invention is designed with cheap chemical compositions of low carbon, low silicon and medium chromium, and is completely free of precious corrosion-resistant metal elements such as Ni and Cu, thereby greatly reducing the material cost; instead of the traditional Al deoxidization technology, Si deoxidization assisted by ZrRE composite deoxidization is used to form a fine, dispersed and uniform composite oxysulfide, which greatly decreases the density of the corrosion-active inclusions and significantly improves the seawater corrosion resistance. Such a steel is especially suitable for a marine environment steel such as a marine engineering steel, a warship and ship steel, an offshore fixed wind power steel, an offshore floating wind power steel, a coastal cross-sea bridge steel, an iron tower steel and a track steel, and can obviously improve the corrosion resistance to the seawater mediums rich in chloride ions.

[0096] In descriptions of the description, the descriptions referring to the terms one embodiment. some embodiments, examples, specific examples or some examples mean that specific features, structures, materials or characteristics described in conjunction with the embodiments or examples are included in at least one embodiment or example according to the present invention. In the description, the schematic expressions of the above terms are not necessarily aimed at the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in an appropriate way. In addition, different embodiments or examples and features of different embodiments or examples described in the description without contradicting each other can be combined by those skilled in the art.

[0097] Although the embodiments according to the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and cannot be understood as limitations to the present invention, and those skilled in the art can make variations, modifications, substitutions and transformations to the above embodiments within the scope of the present invention.