JP7671355B2 - Marine engineering steel with corrosion resistance to high humidity and high temperature atmosphere and method for manufacturing same - Google Patents

Description

This disclosure relates to steels and methods for their manufacture, and in particular to steels for marine engineering and methods for their manufacture.

As we all know, the South China Sea contains a great deal of energy and resources. Currently, significant human and material resources are being invested in developing and utilizing the resources in the South China Sea.

The environment of the South China Sea is special. Its high humidity and high temperature environment with strong radiation and high chlorine are easy to cause serious corrosion of steel materials, leading to saponification and aging of coating films, which can cause serious corrosion. ^{Cl- is} easily adsorbed on the metal surface, leading to the occurrence of anodic dissolution and pitting corrosion, which can even cause cracking, and can lead to stress corrosion cracking under the combined action of hydrogen. These problems reduce the mechanical properties and service life of steel for offshore platforms. In addition, since offshore platforms have long service cycles and are far from the coast, regular repair and maintenance are very difficult, so steel for offshore platforms is required to have fairly high corrosion resistance.

In recent years, as China has gradually exercised its sovereignty over the South China Sea and begun to develop and utilize resources in the South China Sea, it has become urgent to develop corrosion-resistant steel suitable for the high humidity and high temperature marine environment in order to overcome the harsh environment of the South China Sea and meet the construction demand for marine engineering facilities.

Currently, many steel companies around the world are developing weathering steels that are resistant to atmospheric corrosion and seawater corrosion-resistant steels that are resistant to seawater corrosion. However, the applications of these steels are not ideal and they cannot be applied well to high humidity and high temperature marine environments.

In the production and development process of existing marine engineering steels, the marine atmospheric corrosion resistance of steel plates was not fully taken into consideration, and the strength and impact properties of steel were the main considerations. In high humidity and high temperature areas such as the South China Sea, the marine atmospheric corrosion resistance of existing marine engineering steels is not good enough, so the service life of the steel plates is not long.

For example, China Patent Publication No. 106756476(A), entitled "High-Strength Weathering Steel for Highly Humid and Hot Marine Atmospheric Environment and Manufacturing Method Therefor," published on May 31, 2017, discloses a high-strength weathering steel for high humidity and high temperature marine atmospheric environments that achieves the goal of improving corrosion resistance by increasing the Ni content, adding a very small amount of Cr element, compounding the addition of Mo, Sn, Sb, RE and other trace alloy elements, and refining the grain structure with Nb trace element. The crux of this patent is the use of Sn and Sb to improve the corrosion resistance of the steel plate. However, Sn and Sb are impurity elements that are strictly controlled in structural steels, and they obviously have a negative impact on the overall mechanical properties of the steel and the safety of marine engineering platforms.

Another example is the article "Steel Plate Having Corrosion Resistance to Highly Humid and Hot Marine Atmosphere, and Manufacturing Method" published on December 9, 2015. China Patent Publication No. 105132832(A), entitled "Therefor," discloses a steel plate having corrosion resistance to high humidity and high temperature marine atmosphere and a manufacturing method thereof, the steel plate being added with 0.5-0.6% Si, 0.5-0.7% Mn, 0.5-0.6% Cu, 0.5-0.6% Ni, 0.3-0.5% Mo, and a high content of Cr (3.00-3.50%), and a combined addition of Sn (0.20-0.30%) and Sb (0.06-0.10%). This patent has the advantages of significantly improving the corrosion resistance of weathering steel in the severe atmospheric corrosion environment of high humidity and high temperature of the South China Sea, and being economical and practical to manufacture.

In another example, China Patent Publication No. 103741056(A), entitled "Corrosion-Resistant Steel Plate for Marine Environment in the South China Sea and Manufacturing Process Therefor," published on April 23, 2014, discloses a corrosion-resistant steel plate for the marine environment of the South China Sea, employing a low carbon composition and adding a lot of Si, Mn, Cu, Cr and Ni, as well as some Sn elements. The steel plate has a single-phase polygonal ferrite structure, an average grain size of 10.17 μm, a steel grade yield strength of 355 MPa, a tensile strength of 490-630 MPa, and a Charpy impact energy at -40°C of more than 34 J.

Considering the shortcomings of the existing technologies mentioned above, it is expected to obtain a new marine engineering steel that not only has excellent toughness but also has excellent fracture and crack resistance properties and corrosion resistance to high humidity and high temperature marine atmosphere.

One of the objectives of the present disclosure is to provide a marine engineering steel that not only has excellent toughness but also has excellent fracture and crack resistance properties and corrosion resistance in high humidity and high temperature marine atmosphere. The marine engineering steel of the present disclosure can be used for ships and marine engineering structures, particularly marine atmosphere structural components of marine engineering structures, and can be used in various sea areas, particularly suitable for high humidity and high temperature sea areas such as the South China Sea, and is expected to have a wide range of applications.

To achieve the above objective, the present disclosure provides a composition comprising the following chemical elements in mass percentages:
The present invention provides a marine engineering steel containing C: 0.01-0.05%, Si: 0.05-0.60%, Mn: 0.50-1.30%, Cr: 0.6-1.20%, Ni: 2.0-3.0%, Al: 0.01-0.06%, Ti: 0.005-0.012%, Mg: 0.0005-0.0015%, 0<Ca≦0.0045%, 0<Cu≦0.5%, and 0<Mo≦0.40%, with the balance being Fe and unavoidable impurities.

In one embodiment, the mass percentages of the chemical elements in the marine engineering steel of the present disclosure are as follows:
C: 0.015-0.04%, Si: 0.15-0.60%, Mn: 0.50-1.30%, Cr: 0.6-0.90%, Ni: 2.0-2.85%, Al: 0.01-0.06%, Ti: 0.005-0.012%, Mg: 0.0005-0.0015%, 0<Ca≦0.0045%, 0<Cu≦0.5%, and 0<Mo≦0.40%, and the balance is Fe and unavoidable impurities.

The marine engineering steel of the present disclosure has ultra-low C and is designed with Mn, Nb, V, and Ti microalloying and a Cr-Ni-Mo-Cu alloying composition system. In the marine engineering steel of the present disclosure, the design principles of the chemical elements are specifically explained as follows:

C: The marine engineering steel of the present disclosure adopts an ultra-low carbon design and utilizes the interstitial strengthening effect of carbon to not only ensure the suitable strength of the steel plate of the present invention, but also effectively prevents excessive carbide precipitation, reduces the potential difference between the parent phase and the carbide phase, and obtains good corrosion resistance, while at the same time providing the steel plate with good low-temperature toughness and welding performance. Therefore, in the marine engineering steel of the present disclosure, the mass percentage of C is controlled to 0.01-0.05%.

Si: In the marine engineering steel of the present disclosure, the Si element is a common weak deoxidizing element in steelmaking and has a certain solid solution strengthening effect. It should be noted that under ^{Cl -} conditions, the Si element forms a composite oxide of FeAlSiO in the rust layer of the steel, filling voids and cracks, thereby playing a role in protecting the steel. For this reason, in the marine engineering steel of the present disclosure, the mass percentage of Si is controlled to 0.05-0.60%.

Mn: In the marine engineering steel of the present disclosure, Mn is the most basic alloying element of low-alloy high-strength steels, and can improve the strength of the steel through solid solution strengthening and compensate for the strength reduction caused by the reduction in the C element content in the steel. However, it should be noted that the Mn element content in the steel should not be too high. If the Mn element content in the steel is too high, segregation is likely to occur in the center of the steel plate, and the low-temperature toughness of the steel will decrease. For this reason, in the marine engineering steel of the present disclosure, the mass percentage of Mn is controlled to 0.50 to 1.30%.

Cr: In the marine engineering steel of the present disclosure, the Cr element can improve the passivation performance of the steel, promote the formation of a dense oxide film on the steel surface, and is enriched in the internal rust layer to easily refine α-iron oxyhydroxide. However, it should be noted that the Cr element content in the steel should not be too high. If too much Cr is added in a Cl ^- environment, the corrosion resistance of the steel will be seriously deteriorated in the later corrosion stage. Therefore, taking into consideration the beneficial and adverse effects of the Cr element comprehensively, the mass percentage of the Cr element is controlled to 0.6-1.20% in the marine engineering steel of the present disclosure.

Ni: In the marine engineering steel of the present disclosure, the Ni element can be infinitely dissolved in the iron base material, and can improve the low-temperature toughness of the steel, especially the impact toughness of the center part of the thick steel plate, and can improve the fracture resistance and crack resistance properties of the steel plate. In addition, an increase in the Ni element content in the steel also plays a major role in improving the corrosion resistance of the steel in the marine environment. Ni can delay the corrosion progression trend of the material over time, and suppress the adverse effect of corrosion and the tendency of pitting corrosion. However, it should be noted that the Ni element content in the steel should not be too high. If the Ni element content in the steel is too high, it is easy for a highly viscous iron oxide scale that is difficult to remove to be generated on the slab surface, which affects the surface quality and fatigue performance of the steel plate. For this reason, in the marine engineering steel of the present disclosure, the mass percentage of Ni is controlled to 2.0-3.0%.

Al: In the marine engineering steel of the present disclosure, Al belongs to the grain refining elements. The Al element is added to the steel for deoxidation. After the deoxidation is completed, the O content in the material is reduced, and the aging properties are improved. In addition, it should be noted that adding an appropriate amount of Al to the steel is also beneficial to refine the grains and improve the toughness of the steel. Therefore, in the marine engineering steel of the present disclosure, the mass percentage of the Al element is controlled to 0.01-0.06%.

Ti: In the marine engineering steel of the present disclosure, the Ti element is a strong N-fixing element, which can effectively suppress the N element content in the steel and prevent the adverse effects on the properties of the steel caused by an excessively high N content. Furthermore, the TiN precipitate phase formed by Ti and N elements can suppress the excessive growth of crystal grains in slabs and steel plates during heating. Therefore, in the marine engineering steel of the present disclosure, the mass percentage of the Ti element added is controlled to 0.005-0.012%.

Mg: In the marine engineering steel of the present disclosure, the Mg element can effectively improve the morphology of sulfides, refine inclusions, and strengthen the corrosion resistance of the steel plate. The Mg element is an important element for realizing the beneficial modification technology of inclusions in the present disclosure. If the Mg element content in the steel is too low, the modification of inclusions cannot be realized. If the Mg element content in the steel is too high, MgO and MgS are easily formed, which may clog the tundish nozzle. Therefore, in the marine engineering steel described in the present invention, the mass percentage of the added Mg element is controlled to 0.0005-0.0015%.

Ca: In the marine engineering steel of the present disclosure, Ca treatment can control the morphology of sulfides in the steel, improve the anisotropy of the steel plate, and improve low-temperature toughness. The Ca element is also an important element for realizing the beneficial inclusion modification technology of the present disclosure, and its content needs to match the Mg content. Therefore, in the marine engineering steel of the present disclosure, the mass percentage of the Ca element added is controlled to 0<Ca≦0.0045%.

Cu: In the marine engineering steel of the present disclosure, the Cu element can appropriately improve the hardenability of the steel and improve the atmospheric corrosion resistance of the steel. However, the Cu element content in the steel should not be too high. If the Cu content in the steel is too high, the welding performance of the steel will deteriorate. Therefore, in the marine engineering steel of the present disclosure, it is preferable to control the mass percentage of Cu to 0<Cu≦0.5%.

Mo: In the marine engineering steel of the present disclosure, the Mo element can effectively improve the pitting corrosion resistance of the steel, but if the Mo content is too high, the tendency of the steel plate to cold crack increases. Therefore, in the marine engineering steel of the present disclosure, the mass percentage of the Mo element can be controlled to 0<Mo≦0.40%.

In one embodiment, the marine engineering steel of the present disclosure further comprises at least one of the following chemical elements: 0<Nb≦0.04%, 0<V≦0.05%, and 0<B≦0.0005%.

In the above technical solutions of the present disclosure, the elements Cu, Mo, Nb, V, and B can all further improve the performance of the marine engineering steel of the present disclosure.

Nb: In the marine engineering steel of the present disclosure, Nb is a strong carbonitride forming element with a strong grain refining effect. By adding an appropriate amount of Nb to the steel to obtain a uniform grain size, it is possible to effectively prevent excessive growth of some of the grains during heating, the formation of a mixed crystal structure, and deterioration of toughness and corrosion resistance. Therefore, in the marine engineering steel of the present disclosure, the mass percentage of the Nb element can be controlled to 0<Nb≦0.04%.

V: In the marine engineering steel of the present disclosure, the V element can contribute to strengthening the steel by forming VN or V(CN) fine precipitate particles together with C and N. In addition, the V element is also beneficial in improving the stability of hardness after quenching and tempering. However, it should be noted that the V element content in the steel should not be too high. If the V element content in the steel is too high, the cost will increase significantly. Therefore, in the marine engineering steel of the present disclosure, the mass percentage of the V element can be controlled to 0<V≦0.05%.

B: In the marine engineering steel of the present disclosure, the B element can improve the hardenability of the steel and affect the low-temperature cracking properties of the steel. Therefore, in the marine engineering steel of the present disclosure, the mass percentage of the B element can be controlled to 0<B≦0.0005%.

It should be noted that the addition of the above-mentioned elements Cu, Mo, Nb, V, and B increases the cost of the material. Therefore, taking into consideration performance and cost reduction in total, it is preferable to add at least one of the above elements in the technical solution of the present disclosure.

In one embodiment, in the marine engineering steel disclosed herein, among the unavoidable impurities, P≦0.015% and/or S≦0.0040%.

In the above technical solutions, P and S are both impurity elements in steel. Therefore, in order to obtain better performance and better quality of steel, the impurity element content in steel should be reduced as much as possible within the limits of technical conditions. If the P and S element content in steel is too high, defects such as segregation and inclusion are likely to occur, and the welding performance, impact toughness and HIC resistance of the steel plate will deteriorate.

Therefore, in the marine platform steel of the present disclosure, it is preferable to control P≦0.015% and S≦0.0040%. It is more preferable to use beneficial modification techniques for inclusions to make their shape spherical, refine their size, and achieve uniform distribution, thereby reducing the impact of inclusions on toughness and corrosion.

In one embodiment, the mass percentages of the chemical elements in the marine engineering steel of the present disclosure are:
1.8≦α≦2.0 (in the formula, α=1.2Cr+5Ni-Cr ² -Ni ² -4.61);
4.2≦β≦7.9 (in the formula,

）；および
３５≦γ≦６５（式中、

) and 35≦γ≦65,

）；
のうちの少なくとも１つをさらに満たす。

).
Further, at least one of the following conditions is satisfied:

For each chemical element, the value before the percent sign for the mass percentage of the chemical element is substituted into the formula above.

In the above technical solution, while controlling the mass percentages of the individual elements in the marine engineering steel of the present disclosure, it is also preferable to control the mass percentages of the chemical elements in the steel to satisfy at least one of 1.8≦α≦2.0, 4.2≦β≦7.9, and 35≦γ≦65 to ensure a balance between the alloying element contents and to obtain in the steel good corrosion resistance to high humidity and high temperature, as well as balanced improved strength and toughness.

In one embodiment, the microstructure of the marine engineering steel disclosed herein is tempered bainite with a phase ratio of 95% or more.

In the above technical solution, the microstructure of the marine engineering steel disclosed herein is a tempered bainite structure, and the phase ratio of tempered bainite is 95% or more, so that the steel has improved strength and toughness in a more balanced manner.

In one embodiment, the marine engineering steel of the present disclosure has a yield strength of 355 MPa or more, a tensile strength of 500-650 MPa, an elongation of 22% or more, an impact energy at -60°C of 100 J or more, a crack tip opening displacement (CTOD) at -60°C of 0.8 mm or more, a non-ductile transition temperature (NDTT) of -65°C or less, and a corrosion rate of 0.85 g/( ^m2 *hr) or less in a humid and hot atmospheric environment.

Correspondingly, another object of the present disclosure is to provide a method for manufacturing marine engineering steel, which is simple to implement. The marine engineering steel produced by this method not only has excellent toughness, but also has excellent fracture and crack resistance properties and corrosion resistance to high humidity and high temperature marine atmosphere.

In one embodiment, the marine engineering steel prepared by the manufacturing method of the present disclosure has a yield strength of 355 MPa or more, a tensile strength of 500-650 MPa, an elongation of 22% or more, an impact energy of 100 J or more at -60°C, a CTOD of 0.8 mm or more at -60°C, an NDTT of -65°C or more, and a corrosion rate of 0.85 g/( ^m2 *hr) or less in high humidity and high temperature atmospheric environment. The marine engineering steel produced by the method of the present disclosure can be used in ships and marine engineering structures and has a wide range of potential applications.

In order to achieve the above object, the present disclosure provides a method for producing a steel for marine engineering, comprising the following steps:
(1) Smelting and casting processes;
(2) a heating step;
(3) a controlled rolling step in which the original austenite grain size after rolling is 20-25 μm;
(4) an air cooling step;
(5) A manufacturing method including a quenching and tempering step, in which the austenite grain size after quenching is 20 to 25 μm.

In one embodiment, in the manufacturing method of the present disclosure, in step (1), hot metal pretreatment, converter smelting, LF refining, RH refining, beneficial treatment of inclusions and continuous casting are sequentially carried out, and in the stage of beneficial treatment of inclusions, composite inclusions containing MgO+Al ₂ O ₃ as a core coated with CaS and MnS are formed, the size of the composite inclusions is 0.2-2.5 μm, and the number of the composite inclusions within the above size range accounts for 95% or more of the total number of inclusions.

In one embodiment, in the manufacturing method of the present disclosure, in step (1), in the converter smelting stage, slag cutoff tapping is performed to control the thickness of the slag layer to less than 30 mm; in the LF refining stage, the sum of the mass percentages of FeO and MnO in the slag is controlled to less than 1%, and (CaO+MgO+MnO)/(SiO ₂ +P ₂ O ₅ )>9 (wherein each substance is substituted by its mass percentage) is satisfied; in the beneficial treatment stage of inclusions, Mg treatment or combined treatment of Mg and Ca is performed; but when combined treatment of Mg and Ca is performed, Ca and Mg need to be supplied simultaneously at a wire feed speed of 150 to 250 m/min.

In the above technical solution, in step (1) of the manufacturing method disclosed herein, slag cut tapping is performed at the converter smelting stage, and the thickness of the slag layer is controlled to less than 30 mm, which reduces the oxidation of the slag in the ladle and prevents an increase in oxygen activity and rephosphorization of the molten steel, contributing to the subsequent generation of white slag and inclusion modification treatment.

In the above technical solution, the (CaO+MgO+MnO)/( _SiO2 + _P2O5 ) ratio is controlled to be greater than 9 during the _LF refining stage to ensure good dephosphorization and desulfurization ability of the slag. In the process of producing white slag from the ladle, the sum of the mass percentages of FeO and MnO in the slag is controlled to be less than 1% to ensure the reduction of the slag, sufficient desulfurization, reduction of the inclusion content in the molten steel, and improvement of the toughness and corrosion resistance of the steel.

In one embodiment, in the manufacturing method of the present disclosure, in step (2), the slab heating temperature, in °C,

となるように制御され、各化学元素について、化学元素の質量百分率のパーセント記号の前の値が式に代入される。

and for each chemical element, the value before the percent sign of the mass percentage of the chemical element is substituted into the formula.

In the above technical solution, in step (2) of the manufacturing method disclosed herein, the purpose of controlling the slab heating temperature to the above value is to ensure sufficient solid solution of microalloyed carbonitrides, promote homogenization of alloying elements, mitigate macro- and micro-segregation in the steel, reduce the formation of primary corrosion cells due to potential differences between different phases and components, and improve the corrosion resistance of the steel sheet.

In one embodiment, in the manufacturing method of the present disclosure, in the step (3), the initial rolling temperature T _sr is controlled to be 0.92T _h - 0.96T _h ; and the final rolling temperature

となるように制御され；初期圧延温度および最終圧延温度の単位は共に℃であり；各化学元素について、化学元素の質量百分率のパーセント記号の前の値が式に代入される。

the units of the initial rolling temperature and the final rolling temperature are both in ° C; for each chemical element, the value before the percent sign of the mass percentage of the chemical element is substituted into the formula.

In the above technical solution, the purpose of controlling the initial rolling temperature T _sr =0.92T _h -0.96T _h is mainly to ensure that the steel sheet is rolled at a relatively high temperature in the recrystallization zone, so as to be sufficiently recrystallized and form uniform and equiaxed austenite grains.

In the above technical solution, the final rolling temperature is

を満たすように制御することで、鋼板が非静的再結晶温度を超える温度で圧延されて、混晶および不均一な結晶粒の発生を防止することを確保することができ；第二に、圧延プロセスにおける温度降下のための十分な空間を確保することができる。

secondly, it can ensure that the steel sheet is rolled at a temperature above the non-static recrystallization temperature to prevent the occurrence of mixed crystals and non-uniform crystal grains; and secondly, it can ensure sufficient space for temperature drop in the rolling process.

In one embodiment, in the manufacturing method disclosed herein, in step (3), the rolling reduction in a single pass is 8 to 12%, and the cumulative rolling reduction is 60% or more.

In the above technical solution of the present disclosure, the purpose of controlling the reduction rate in a single pass to 8-12% is mainly to ensure that the steel sheet has sufficient recrystallization driving force in each pass, and at the same time, to ensure that the number of rolling passes is sufficient to achieve grain homogenization of the steel sheet so that the original austenite grain size after rolling is maintained at 20-25 μm. Furthermore, in step (3) of the present disclosure, the purpose of controlling the cumulative reduction rate to 60% or more is mainly to achieve sufficient recrystallization and sufficient homogenization in the core of the steel sheet to ensure the toughness and fracture- and crack-resistant properties of the core.

Furthermore, in the manufacturing method disclosed herein, in step (5), the quenching temperature

となるように制御され；および／または焼戻し温度

and/or the tempering temperature

となるように制御され；焼入れ温度および焼戻し温度の単位は共に℃であり；各化学元素について、化学元素の質量百分率のパーセント記号の前の値が式に代入される。

the units of the quenching temperature and the tempering temperature are both in ° C; for each chemical element, the value before the percent sign of the mass percentage of the chemical element is substituted into the formula.

In the above technical solution of the present disclosure, the purpose of setting the above quenching temperature is, first, to ensure that the steel sheet is fully austenitized, and second, by austenitizing at a relatively high temperature, sufficient solid solution of carbonitrides can be achieved, the uniform distribution of alloys in the steel can be promoted, and the microscopic electrochemical corrosion caused by segregation can be mitigated. Furthermore, the quenching temperature is not too high, and some of the austenite grains do not grow rapidly to cause mixed crystals. After that, water quenching can be performed, the purpose of which is to ensure that the austenite grain size after quenching is maintained at 20-25 μm by using a high cooling rate to form a single martensite structure.

In the above technical solution of the present disclosure, in step (5) of the present disclosure, the purpose of setting the above tempering temperature is, first, to ensure that the steel plate has good mechanical properties and fracture and crack resistance properties; second, to remove the quenching stress in the steel plate by tempering, thereby preventing corrosion caused by different forces at each position inside the steel plate; and finally, to reduce the microscopic corrosion primary cell caused by multiple phases by allowing the steel plate to obtain a tempered bainite structure after tempering.

In the present disclosure, it should be noted that if the tempering temperature is too high, ferrite structure will form in the steel, reducing the strength and impact properties of the steel plate; if the tempering temperature is too low, the strength of the steel plate will be too high and the impact toughness will be relatively low.

Compared with the prior art, the marine engineering steel and its manufacturing method of the present disclosure have the following advantages and beneficial effects:
By controlling conditions such as composition design, structure adjustment, and manufacturing process, the steel plate of the present disclosure simultaneously achieves suitable strength properties, excellent impact toughness, good fracture and crack resistance properties, and excellent corrosion resistance in high-humidity and high-temperature marine atmospheres.

Compared with the prior art, the manufacturing method of the present disclosure uses unique composition design technology, pure steel smelting technology, inclusion beneficial control technology, steel homogenization technology, grain size control and microstructure adjustment technology to produce a steel type that meets the strength requirements of the 355 MPa level, good low-temperature impact toughness, good fracture and crack resistance properties, and excellent corrosion resistance to high humidity and high temperature atmospheres. The steel plate manufactured by the method of the present disclosure is significantly different from existing steel plates in terms of structure, composition and process design.

The marine engineering steel of the present disclosure can achieve a yield strength of 355 MPa or more, a tensile strength of 500-650 MPa, an elongation of 22% or more, an impact energy of 100 J or more at -60°C, a CTOD of 0.8 mm or more at -60°C, an NDTT of -65°C or less, and a corrosion rate of 0.85 g/( ^m2 *hr) or less in high humidity and high temperature atmospheric environments.

The marine engineering steel disclosed herein can be used for critical components of ships and marine engineering structures, meeting China's current development demand for steel for ships and marine engineering equipment, and is expected to have a wide range of applications.

The specific implementation methods of the present invention are further explained and described below with reference to specific embodiments. However, the explanations and descriptions do not limit the technical solutions of the present disclosure.

Examples 1 to 6 and Comparative Example 1
Both the marine steels of Examples 1 to 6 and the comparative steel of Comparative Example 1 are prepared by the following steps.

(1) Smelting and casting are carried out according to the chemical compositions shown in Tables 1-1 and 1-2 below. Hot metal pretreatment, converter smelting, LF refining, RH refining, beneficial treatment of inclusions and continuous casting are carried out in sequence, and composite inclusions containing MgO+Al ₂ O ₃ as cores coated with CaS and MnS are formed in the stage of beneficial treatment of inclusions. The size of the composite inclusions is 0.2-2.5 μm, and the number of composite inclusions within the above size range accounts for more than 95% of the total number of inclusions.

In the converter smelting stage, slag cut tapping is carried out to control the thickness of the slag layer to less than 30 mm; in the LF refining stage, the sum of the mass percentages of FeO and MnO in the slag is controlled to less than 1%, and (CaO + MgO + MnO) / (SiO ₂ + P ₂ O ₅ ) is controlled to be greater than 9 (wherein each substance is substituted by its mass percentage); in the inclusion beneficial treatment stage, Mg treatment or combined treatment of Mg and Ca is carried out, but if combined treatment of Mg and Ca is carried out, Ca and Mg need to be supplied simultaneously at a wire feed speed of 150 to 250 m/min.

(2) Heating: Slab heating temperature in °C

となるように制御する。

The control is performed so that

(3) Controlled rolling: The original austenite grain size after rolling is maintained at 20-25 μm, and the initial rolling temperature T _sr is controlled to be 0.92T _h -0.96T _h , and the final rolling temperature

となるように制御し、初期圧延温度および最終圧延温度の単位は共に℃であり、シングルパスでの圧下率は８～１２％であり、累積圧下率は６０％以上である。

The units of the initial rolling temperature and the final rolling temperature are both ° C., the rolling reduction in a single pass is 8 to 12%, and the cumulative rolling reduction is 60% or more.

(4) Air-cooled

(5) Quenching + tempering: Quenching temperature

となるように制御し、焼戻し温度

The tempering temperature is controlled so that

となるように制御し、焼入れ後のオーステナイト粒径を２０～２５μｍに維持する。

Thus, the austenite grain size after quenching is maintained at 20 to 25 μm.

It should be noted that in Examples 1-6 of the present disclosure, six different chemical compositions are designed and combined with suitable manufacturing processes to produce steel plates with different thickness specifications. All of the chemical composition designs and associated processes of the marine engineering steels of Examples 1-6 meet the design specification requirements of the present disclosure.

Tables 1-1 and 1-2 show the mass percentages of chemical elements in the marine engineering steels of Examples 1 to 6 and the comparative steel of Comparative Example 1.

Table 2 shows the specific process parameters for producing the marine engineering steels of Examples 1 to 6 and the comparative steel of Comparative Example 1.

The marine engineering steels obtained in Examples 1 to 6 and the comparative steel obtained in Comparative Example 1 were each sampled, and the finished plates of the Examples and Comparative Example were each subjected to a tensile test, a Charpy V-notch impact test, a CTOD test (an index for examining the fracture toughness of the steel plate), an NDTT performance inspection test (an important index for measuring the crack resistance properties of the steel plate), and a corrosion test under high humidity and high temperature conditions. The test results of the Examples and Comparative Example are shown in Table 3.

The test method is described below.

Tensile test: In accordance with GB/T228.1, full-thickness plate tensile test specimens are used for steel plates with a thickness of less than 50 mm, and bar tensile test specimens are used for steel plates with a thickness of more than 50 mm, and then the room temperature tensile properties of the steel plates are measured.

Charpy V-notch impact test: In accordance with GB/T229, a Charpy V-notch impact test piece is used to measure the impact properties of the material at a position t/4 of the plate thickness at -60°C.

CTOD test: Measure the fracture toughness of a material at -60°C using a full thickness CTOD test specimen in accordance with BS7448-1.

NDTT performance inspection test: Measure the non-ductile transition temperature of the material using a P3 test piece in accordance with GB/T6803-2008.

Corrosion test under high humidity and high temperature conditions: In the test process, the temperature of 35°C, 5% NaCl solution is controlled to be used at pH of 6.5-7.2, the average deposition of salt spray is controlled at 1.5mL/( ^80cm2 -hour), and the RH (relative humidity) is controlled at 95%-100%.

Table 3 shows the test results for the marine engineering steels of Examples 1 to 6 and the comparative steel of Comparative Example 1.

As can be seen from Table 3, the overall performance of the marine steels of Examples 1 to 6 is significantly better than that of the comparative steel of Comparative Example 1. In a high humidity and high temperature atmospheric environment, the corrosion rates of the marine steels of Examples 1 to 6 are significantly lower than that of the steel of Comparative Example 1. As can be seen, the marine steels of Examples 1 to 6 have better corrosion resistance to high humidity and high temperatures than the comparative steel of Comparative Example 1.

As shown in Table 3, the marine engineering steels of Examples 1 to 6 have superior toughness, fracture and crack resistance properties, and corrosion resistance to high humidity and high temperature compared to the comparative steel of Comparative Example 1. All of the marine engineering steels of Examples 1 to 6 have a yield strength of 423 MPa or more, a tensile strength of 532 to 595 MPa, an impact energy of 270 J or more at -60°C, an elongation of 22% or more, a CTOD of 0.8 mm or more at -60°C, an NDTT of -65°C or less, and a corrosion rate of 0.83 g/( ^m2 *hr) or less in a high humidity and high temperature atmospheric environment.

In summary, as can be seen, through the rational chemical composition design combined with the optimized process, the marine engineering steel of the present disclosure simultaneously achieves suitable strength properties, excellent impact toughness, good fracture and crack resistance properties, and excellent corrosion resistance to high humidity and high temperature marine atmosphere. The marine engineering steel of the present disclosure can be effectively used in the manufacture of key components of ships and marine engineering structures, offshore wind power platforms, island buildings, etc. The steel meets the current development demand for steel for ships and marine engineering equipment in China, and is expected to have a wide range of applications.

In addition, the combinations of various technical features in this disclosure are not limited to the combinations described in the claims or embodiments. All technical features in this disclosure can be freely combined or incorporated in any manner, as long as they are not mutually inconsistent.

It should be noted that the above-mentioned embodiments are merely specific implementations of the present disclosure. Naturally, the present disclosure is not limited to the above-mentioned embodiments, and modifications or variations similar to the above-mentioned embodiments can be directly obtained or easily conceived by those skilled in the art from the contents of the present disclosure, and all should be included in the protection scope of the present disclosure.

Claims (14)

The following chemical elements by mass percentage:
A marine engineering steel comprising: C: 0.01-0.05%, Si: 0.05-0.60%, Mn: 0.50-1.30%, Cr: 0.6-1.20%, Ni: 2.0-2.85%, Al: 0.01-0.06%, Ti: 0.005-0.012% , Mg : 0.0005-0.0015%; 0<Ca≦0.0045%, 0<Cu≦0.5% , 0 <Mo≦0.40% , P≦0.015%, and S≦0.0040% , with the balance being Fe and other unavoidable impurities. The mass percentages of the above chemical elements are as follows:
C: 0.015-0.04%, Si: 0.15-0.60%, Mn: 0.50-1.30%, Cr: 0.6-0.90%, Ni: 2.0-2.85%, Al: 0.01-0.06%, Ti: 0.005-0.012% , Mg : 0.0005-0.0015%; 0<Ca≦0.0045%, 0<Cu≦0.5% , 0<Mo≦0.40% , P≦0.015%, and S≦0.0040% , with the balance being Fe and other unavoidable impurities.
2. The marine engineering steel according to claim 1. said steel further comprising at least one of the following chemical elements: 0<Nb≦0.04%, 0<V≦0.05%, and 0<B≦0.0005%;
3. Marine engineering steel according to claim 1 or 2. The mass percentages of the above chemical elements are expressed by the following relationship:
1.8≦α≦2.0, where α=1.2Cr+5Ni-Cr ² -Ni ² -4.61; and 35≦γ≦65, where

)
and
3. Marine engineering steel according to claim 1 or 2. The steel has a tempered bainite microstructure with a phase ratio of 95% or more.
3. Marine engineering steel according to claim 1 or 2. The mass percentage of the above chemical elements is
4.2≦β≦7.9 (in the formula,

) further fulfills the above requirements,
4. Marine engineering steel according to claim 3. the steel has a yield strength of 355 MPa or more, a tensile strength of 500-650 MPa, an elongation of 22% or more, an impact energy of 100 J or more at -60°C, a CTOD of 0.8 mm or more at -60°C, an NDTT of -65°C or less, and a corrosion rate of 0.85 g/( ^m2 *hr) or less in a high humidity and high temperature atmospheric environment;
3. Marine engineering steel according to claim 1 or 2. A method for producing a marine engineering steel according to claim 1, comprising the steps of:
(1) Smelting and continuous casting processes;
(2) a heating step;
(3) a controlled rolling step in which the original austenite grain size after rolling is 20-25 μm;
(4) an air cooling step;
(5) A manufacturing method including a quenching and tempering step, in which the austenite grain size after quenching is 20-25 μm. In step (1), hot metal pretreatment, converter smelting, LF refining, RH refining, beneficial treatment of inclusions and continuous casting are carried out in sequence, and in the stage of beneficial treatment of said inclusions, composite inclusions with a size of 0.2-2.5 μm are formed, said composite inclusions contain MgO+Al ₂ O ₃ as a core coated with CaS and MnS, and the number of said composite inclusions within said size range accounts for more than 95% of the total number of inclusions;
The method of claim 8 . In step (1), in the converter smelting stage, slag cut tapping is carried out to control the thickness of the slag layer to less than 30 mm; in the LF refining stage, the sum of the mass percentages of FeO and MnO in the slag is controlled to less than 1%, and (CaO+MgO+MnO)/(SiO ₂ +P ₂ O ₅ )>9 (wherein each substance represents its corresponding mass percentage) is satisfied; in the inclusion beneficial treatment stage, Mg treatment or combined treatment of Mg and Ca is carried out; when combined treatment of Mg and Ca is carried out, Ca and Mg are simultaneously supplied at a wire feed speed of 150-250 m/min;
The method of claim 9 . In step (2), the slab heating temperature T _h is, in ° C.

Fulfilling
The method of claim 9 . In step (3), the initial rolling temperature T _sr satisfies T _sr =0.92T _h -0.96T _h in ° C.; and the final rolling temperature T _fr satisfies

Fulfilling
The method of claim 11 . In step (3), the rolling reduction in a single pass is 8 to 12%, and the cumulative rolling reduction is 60% or more.
The method of claim 11 . In step (5), the quenching temperature _Tq in ° C. is

and/or the tempering temperature _Tt is

Fulfilling
The method of claim 11 .