Nano-precipitation strengthened easy-to-weld high-strength steel, shipbuilding steel plate and preparation method

Abstract

The present invention relates to the technical field of iron and steel metallurgy, and in particular to nano-precipitation strengthened easy-to-weld high-strength steel, ship steel plate and preparation method. The composition of the steel material includes 0.04-0.08% C, 2.4-2.8% Ni, 0.25-0.50% Si, 0.6-1.2% Mn, 1.0-2.0% Cu, 0.01-0.02% Ti, 0.4-0.6% Al, and the balance is Fe and impurities. The steel material promotes the precipitation of Cu+NiAl-rich nano-composite phases by adding Cu, Ni and Al. These nano-composite precipitates in a semi-coherent / incoherent relationship with the ferrite matrix can not only improve the strength of the heat-affected zone through the precipitation strengthening effect, but also inhibit the expansion of microcracks, thereby improving the strength and toughness of the heat-affected zone, making it particularly suitable for use as a ship steel plate.

Description

Technical Field

[0001] The present invention relates to the technical field of iron and steel metallurgy, and in particular to nano-precipitation strengthened easy-to-weld high-strength steel, a ship steel plate and a preparation method thereof. Background Art

[0002] As my country's marine development gradually moves from nearshore to offshore, the use of steel for large ships and thick ship plates is also increasing, which also puts higher requirements on the mechanical properties of steel for ship plates, such as strength and low-temperature toughness.

[0003] For thick ship plate steel, in order to improve welding efficiency, high-line energy welding processes such as submerged arc welding and electroslag welding are usually used for welding. The coarsening of the original austenite grains and the formation of granular bainite and martensite-austenite (MA) components under low cooling rate conditions will seriously damage the strength and toughness of the heat-affected zone, especially the coarse-grained heat-affected zone. Therefore, the design and preparation of thick ship steel plates need to take into account the strength and toughness matching of the base material and the welding heat-affected zone.

[0004] In the prior art, there have been relevant studies on the strength and toughness of steel plates for ships. For example, the patent with publication number CN104264047B discloses "an extra-thick steel plate for container ships and its preparation method", which refines the structure by adding trace elements such as Ti and Nb, and has a yield strength greater than 460MPa, a tensile strength of 570-720MPa, and a low-temperature impact toughness of -40°C greater than 75J. The patent with publication number CN113174536B discloses "an economical low-temperature toughness E-grade high-strength ship steel plate and its manufacturing method", which refines the parent material structure by adding elements such as N, Al, Ti, and Nb to form nitrides, and has a steel plate yield strength greater than 355MPa, a tensile strength greater than 520MPa, and a -40°C impact toughness greater than 150J. The patent with publication number CN106086650B discloses "a polar ship steel plate capable of large line energy welding and its preparation method", by adding a trace amount of Ti to form Ti2O3 and TiN particles, the steel plate has a yield strength of 370-420MPa, a tensile strength of 530-560MPa, and an impact toughness of more than 200J at -60℃. After welding with a line energy of 200kJ / cm, the weld joint strength is between 540 and 600MPa, and the test breaks at the parent material position. The patent with publication number CN115287431 discloses "a low-temperature ship steel plate with excellent plasticity and its manufacturing method", through a three-stage cooling method of slow cooling-accelerated cooling-slow cooling, a two-phase structure of soft ferrite and hard bainite is obtained, with a yield strength greater than 355MPa, a tensile strength greater than 490MPa, and an impact toughness greater than 200J at -60℃.

[0005] The above patented technology forms nitride precipitation phase by adding microalloying elements such as Ti and Nb, or forms two structures by optimizing the cooling process. Although the steel plate has good low-temperature toughness, the yield strength of its base material can only reach about 350-450MPa. The level of this yield strength is very low and it is difficult to apply and promote on a large scale. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide nano-precipitation strengthened easy-to-weld high-strength steel, ship steel plate and preparation method, so as to solve the problem in the prior art that it is difficult to balance the strength and toughness matching between the base material and the welding heat affected zone.

[0007] The technical solution of the present invention to solve the above technical problems is as follows:

[0008] Based on the above technical solution, the present invention can also be improved as follows.

[0009] The invention provides a nano-precipitation strengthened easy-to-weld high-strength steel. The steel comprises, by mass percentage, 0.04-0.08% C, 2.4-2.8% Ni, 0.25-0.50% Si, 0.6-1.2% Mn, 1.0-2.0% Cu, 0.01-0.02% Ti, 0.4-0.6% Al, and the remainder is Fe and impurities.

[0010] Furthermore, the microstructure of the steel is lath bainite.

[0011] Furthermore, the yield strength of the steel is 830-900 MPa, the tensile strength is 860-930 MPa, the elongation is 15-20%, and the impact energy at -60°C is 140-220 J.

[0012] Furthermore, after welding with a maximum heat input of 200 kJ / cm, the tensile strength of the coarse-grained heat-affected zone of the steel is greater than 800 MPa, and the impact toughness at -60°C is greater than 70 J.

[0013] The present invention also provides a ship steel plate, which is made of the above-mentioned nano-precipitation strengthened easy-to-weld high-strength steel, and the thickness of the steel plate is 25 to 50 mm.

[0014] The present invention also provides a method for preparing the above-mentioned shipbuilding steel plate, comprising the following steps:

[0015] 1) Smelting: Smelting molten steel according to the composition of the nano-precipitation strengthened weldable high-strength steel and casting it into a casting billet;

[0016] 2) hot rolling: heating the ingot, and then sequentially performing rough rolling, finish rolling and quenching to obtain a steel plate;

[0017] 3) Tempering: The steel plate is tempered and then cooled to obtain the shipbuilding steel plate.

[0018] Furthermore, in the step 2), the temperature of heating the ingot is 1200-1300° C., and the ingot is kept warm after heating.

[0019] Furthermore, in the step 2), the starting rolling temperature of the rough rolling is 1050-1150° C., and the cumulative reduction ratio is greater than 40%.

[0020] Furthermore, in the step 2), after the rough rolling, the steel plate is air-cooled to 900-980°C, and then the finish rolling is performed; the cumulative reduction rate of the finish rolling is greater than 60%, and the final rolling temperature of the finish rolling is 750-850°C; after the finish rolling, the steel plate is quenched and cooled to room temperature.

[0021] Furthermore, in the step 3), the tempering temperature is 550-650° C., and after being kept at this temperature for 1-3 hours, the steel plate is air-cooled to room temperature to obtain the shipbuilding steel plate.

[0022] The beneficial effects of the present invention are:

[0023] (1) The nano-precipitation strengthened weldable high-strength steel of the present invention promotes the precipitation of Cu+NiAl-rich nano-composite phases by adding Cu, Ni and Al. These nano-composite precipitates in a semi-coherent / incoherent relationship with the ferrite matrix can not only increase the strength of the heat-affected zone through the precipitation strengthening effect, but also inhibit the extension of microcracks, thereby improving the strength and toughness of the heat-affected zone;

[0024] (2) The nano-precipitation strengthened weldable high-strength steel of the present invention does not require the addition of microalloying elements such as N and Nb. The steel and the heat-affected zone under high-energy conditions have good low-temperature toughness while maintaining high strength;

[0025] (3) The nano-precipitation strengthened weldable high-strength steel of the present invention has fewer types of alloying elements and does not require the addition of micro-alloying elements such as Ca, Zr, and Mg. The composition and process control is simpler and can avoid problems such as coarsening of inclusions and embrittlement of coarse-grained heat-affected zones caused by improper control of micro-alloying elements.

[0026] (4) The shipbuilding steel plate of the present invention is made of the nano-precipitation strengthened easy-to-weld high-strength steel of the present invention, which can take into account the strength and toughness of the base material and the welding heat affected zone, and is therefore more suitable for use as a shipbuilding steel plate;

[0027] (5) The shipbuilding steel plate of the present invention has an average Charpy impact energy of greater than 60 J at a temperature of -60°C under the condition of an input line energy of 200 kJ / cm, and can be widely used in the field of marine equipment such as deep-sea ships and offshore drilling platforms;

[0028] (6) The method for preparing the ship steel plate of the present invention has simple process, low cost and is easy to promote. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The nano-precipitation strengthened weldable high-strength steel of the present invention is a microstructure diagram of the steel obtained in Example 1 observed under an optical microscope at 400 times magnification;

[0030] Figure 2 The nano-precipitation strengthened easy-to-weld high-strength steel of the present invention is a microstructure diagram of the heat-affected zone of the steel obtained in Example 1 observed under an optical microscope at 400 times magnification;

[0031] Figure 3 The nano-precipitation strengthened easy-to-weld high-strength steel of the present invention is a high-magnification bright field microstructure image of the heat-affected zone of the steel obtained in Example 1 observed under a transmission electron microscope;

[0032] Figure 4 The nano-precipitation strengthened easy-to-weld high-strength steel of the present invention is a high-magnification dark field microstructure diagram of the steel welding heat affected zone obtained in Example 1 observed under a transmission electron microscope. DETAILED DESCRIPTION

[0033] The principles and features of the present invention are described below in conjunction with the accompanying drawings. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0034] The nano-precipitation strengthened easy-to-weld high-strength steel of the present invention comprises, by mass percentage, 0.04-0.08% C, 2.4-2.8% Ni, 0.25-0.50% Si, 0.6-1.2% Mn, 1.0-2.0% Cu, 0.01-0.02% Ti, 0.4-0.6% Al, and the remainder is Fe and impurities.

[0035] The nano-precipitation-strengthened weldable high-strength steel of the present invention promotes the precipitation of Cu+NiAl-rich nano-composite phases by adding Cu, Ni, and Al. These nano-composite precipitation phases that are semi-coherent / incoherent with the ferrite matrix can not only improve the strength of the heat-affected zone through the precipitation strengthening effect, but also inhibit the expansion of microcracks, thereby improving the strength and toughness of the heat-affected zone. At the same time, the present invention does not need to add micro-alloying elements such as N and Nb. The steel and the heat-affected zone under large line energy conditions have good low-temperature toughness while maintaining high strength. In addition, in the nano-precipitation-strengthened weldable high-strength steel of the present invention, there are fewer types of alloy elements, and there is no need to add micro-alloying elements such as Ca, Zr, and Mg. The composition process control is simpler, and the problems of coarsening of inclusions and embrittlement of coarse-grained heat-affected zones caused by improper control of micro-alloying elements can be avoided.

[0036] The nano-precipitation strengthened easy-to-weld high-strength steel of the present invention has a microstructure of lath bainite.

[0037] Preferably, the nano-precipitation strengthened weldable high-strength steel of the present invention has a yield strength of 830-900 MPa, a tensile strength of 860-930 MPa, an elongation of 15-20%, and an impact energy of 140-220 J at -60°C. After actual welding with a heat input of 200±5 kJ / cm, the tensile strength of the coarse-grained heat-affected zone is 840-890 MPa, and the impact energy at -60°C is 70-100 J.

[0038] Although the strength and toughness of the coarse-grained heat-affected zone are lower than those of steel, it still has a good match between strength and toughness. It can be seen that the present invention reduces the addition amount of hardenability elements such as Cr and Mo, adds a trace amount of Ti to inhibit grain coarsening, and adds an appropriate amount of Al to promote ferrite phase transformation. Under the condition of input line energy of 200Kj / cm, the coarse-grained heat-affected zone is fine quasi-polygonal ferrite, and the ferrite grain boundaries divide the coarse original austenite grains, which increases the crack extension work and is conducive to improving the low-temperature toughness of the heat-affected zone.

[0039] The specific functions of the components contained in the nano-precipitation strengthened weldable high-strength steel of the present invention are as follows:

[0040] C: An important solid solution strengthening element, but too high a carbon content will cause the ship plate steel to produce a brittle martensite structure during welding, greatly damaging the low-temperature toughness of the heat-affected zone and deteriorating the weldability of the steel. In order to improve the weldability of ship plate steel, the present invention adopts a low-carbon design with a C content of 0.04-0.08%.

[0041] Ni: It is beneficial to the formation of lath bainite and significantly improves the toughness of the matrix structure of the parent material. It is also an important forming element of nano-NiAl, and further improves the strength of the parent material and heat-affected zone through nano-precipitation strengthening.

[0042] Si: Mainly plays the role of solid solution strengthening.

[0043] Mn: Solid solution strengthening element, participates in the formation of nano-Cu-rich phase and NiAl, reduces nucleation energy, and accelerates the precipitation of nano-composite precipitation phase during tempering and welding.

[0044] Cu: The main element forming the nano Cu-rich phase, which can effectively compensate for the strength loss caused by the reduction of C content and other alloying elements.

[0045] Ti: can combine with C atoms to form TiC, which has the effect of refining grains. In order to avoid the formation of large-sized TiN, in the present invention, its content is controlled within 0.02%.

[0046] Al: As a ferrite-forming element, it can promote the formation of quasi-polygonal ferrite in the heat-affected zone. At the same time, as the main forming element of NiAl phase, it enhances the precipitation strengthening effect of nano-NiAl in the base material and the heat-affected zone.

[0047] The ship steel plate of the present invention is made of the nano-precipitation strengthened easy-to-weld high-strength steel as mentioned above.

[0048] Preferably, the shipbuilding steel plate of the present invention has a thickness of 25 to 50 mm.

[0049] The method for preparing a marine steel plate of the present invention comprises the following steps:

[0050] 1) Smelting: Smelting molten steel according to the composition of nano-precipitation strengthened weldable high-strength steel and casting into ingots;

[0051] 2) Hot rolling: After the ingot is heated, rough rolling, finish rolling and quenching are carried out in sequence to obtain a steel plate.

[0052] Specifically, the hot rolling process is to heat the ingot obtained in step 1) to 1200-1300°C and keep it warm, then use a reversible rough rolling unit for rough rolling, the start rolling temperature is 1050-1150°C, and the cumulative reduction rate of rough rolling is greater than 40%. After rough rolling, the ingot is air-cooled to 900-980°C, and then a reversible finishing mill is used for finishing rolling, the cumulative reduction rate of finishing rolling is greater than 60%, and the final rolling temperature is controlled to be 750-850°C. The steel plate after finishing rolling is cooled to room temperature by direct quenching.

[0053] Tempering: The steel plate is tempered and then cooled to obtain the shipbuilding steel plate.

[0054] Preferably, the tempering temperature is 550-650° C., and after keeping the temperature for 1-3 hours, the steel plate is air-cooled to room temperature to obtain a shipbuilding steel plate.

[0055] The present invention is described in detail below through specific examples.

[0056] The specific metal composition ratios of the nano-precipitation strengthened weldable high-strength steels prepared in Examples 1 to 4 and Comparative Examples 1 to 8 are shown in Table 1, and the parameters of the preparation processes are shown in Table 2.

[0057] Table 1

[0058]

[0059] Table 2

[0060]

[0061] The microstructure of the steel plate obtained in Example 1 was observed. Figures 1 to 4 shown.

[0062] Figure 1 This is a microstructure diagram observed under an optical microscope at 400 times. Figure 1 It can be seen that after hot rolling, direct quenching and tempering, the microstructure of the steel in Example 1 is lath bainite. The fine bainite lath boundaries can not only hinder dislocation slip and improve the strength of the steel, but also the bainite laths can hinder crack propagation, thereby improving the low-temperature toughness of the steel.

[0063] Figure 2 The microstructure of the heat affected zone of the welding of Example 1 observed under an optical microscope at 400 times magnification is shown in FIG. 1 . The welding process is a welding process with a heat input of 200 kJ / cm. Figure 2 It can be seen that the coarse-grained heat-affected zone of the steel is mainly composed of quasi-polygonal ferrite, with a small amount of granular bainite.

[0064] Figure 3 and Figure 4 They are respectively the high-magnification bright field microstructure image and the high-magnification dark field microstructure image of the heat affected zone of welding in Example 1 observed under a transmission electron microscope. Figure 3 and Figure 4 It can be seen that dispersed nano-scale Cu-rich phase and NiAl phase are precipitated in the coarse-grained heat affected zone of Example 1. The quasi-polygonal ferrite matrix ensures that the coarse-grained heat affected zone has good low-temperature toughness, and the nano-Cu+NiAl composite precipitation phase ensures that the coarse-grained heat affected zone has high strength.

[0065] The steel obtained in each embodiment and comparative example was tested, and the strength and low-temperature toughness of the test are shown in Table 3. At the same time, the weldability test was carried out on the ship steel plate obtained in each embodiment and comparative example, the welding heat input was 200±5kJ / cm, no preheating before welding and post-weld heat treatment were performed, the welding groove was a single-sided V-shaped, and the strength and low-temperature toughness of the coarse-grained heat-affected zone were shown in Table 4.

[0066] Table 3

[0067]

[0068] Table 4

[0069]

[0070] It can be seen from Table 3 that the yield strength of the steels of Examples 1 to 4 is 830 to 900 MPa, the tensile strength is 860 to 930 MPa, the elongation is 15 to 20%, and the impact energy at -60°C is 140 to 220 J, indicating that each steel has a good match between strength and toughness.

[0071] It can be seen from Table 4 that after actual welding with a heat input of 200±5 kJ / cm, the tensile strength of the coarse-grained heat-affected zone of the steel plates of Examples 1 to 4 is 840 to 890 MPa, and the impact energy at -60°C is 70 to 100 J. The strength and toughness of the coarse-grained heat-affected zone of Examples 1 to 4 are lower than those of the steel, but still have a good match between strength and toughness, and the steel plate production process is stable and has strong operability.

[0072] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to 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 a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0073] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A nano-precipitation strengthened weldable high-strength steel, characterized in that: The composition of the steel, by mass percentage, includes 0.07% C, 2.5% Ni, 0.480% Si, 1.1% Mn, 1.2% Cu, 0.02% Ti, 0.4% Al, and the balance is Fe and impurities; The steel has a yield strength of 830-900 MPa, a tensile strength of 860-930 MPa, an elongation of 15-20%, and an impact energy of 140-220 J at -60°C.

2. The nano-precipitation strengthened weldable high-strength steel according to claim 1, characterized in that: The microstructure of the steel is lath bainite.

3. The nano-precipitation strengthened weldable high-strength steel according to claim 1, characterized in that: After welding with a maximum heat input of 200 kJ / cm, the tensile strength of the coarse-grained heat-affected zone of the steel is greater than 800 MPa, and the impact toughness at -60°C is greater than 70 J.

4. A marine steel plate, characterized in that: The steel plate is made of the nano-precipitation strengthened weldable high-strength steel as claimed in any one of claims 1 to 3, and the thickness of the steel plate is 25 to 50 mm.

5. A method for preparing a shipbuilding steel plate as claimed in claim 4, characterized in that: The following steps are involved: 1) Smelting: Smelting molten steel according to the composition of the nano-precipitation strengthened weldable high-strength steel and casting it into a casting billet; 2) hot rolling: heating the ingot, and then sequentially performing rough rolling, finish rolling and quenching to obtain a steel plate; 3) Tempering: The steel plate is tempered and then cooled to obtain the shipbuilding steel plate.

6. The method for preparing a ship steel plate according to claim 5, characterized in that: In the step 2), the temperature of the ingot is heated to 1200-1300° C. and the ingot is kept warm after heating.

7. The method for preparing a shipbuilding steel plate according to claim 6, characterized in that: In the step 2), the starting rolling temperature of the rough rolling is 1050-1150° C., and the cumulative reduction rate is greater than 40%.

8. The method for preparing a shipbuilding steel plate according to claim 7, characterized in that: In the step 2), after the rough rolling, the steel plate is air-cooled to 900-980° C., and then the finish rolling is performed; the cumulative reduction rate of the finish rolling is greater than 60%, and the final rolling temperature of the finish rolling is 750-850° C.; after the finish rolling, the steel plate is quenched and cooled to room temperature.

9. The method for preparing a ship steel plate according to claim 5, characterized in that: In the step 3), the tempering temperature is 550-650° C., and after being kept at this temperature for 1-3 hours, the steel plate is air-cooled to room temperature to obtain the shipbuilding steel plate.

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