800MPa-grade high-strength easy-to-weld ultra-fine grain steel and preparation method thereof

Abstract

The invention discloses 800MPa-grade high-strength easy-to-weld ultra-fine grain steel and a preparation method thereof, and relates to the technical field of structural steel, the 800MPa-grade high-strength easy-to-weld ultra-fine grain steel comprises the following chemical components: 0.06-0.12% of C, 0.20-0.50% of Si, 0.8-1.5% of Mn, less than or equal to 0.01% of P, less than or equal to 0.005% of S, 2.0-3.5% of Ni, 0.01-0.02% of N, 0.10-0.20% of Ti, 0.05-0.10% of V, and the balance Fe and inevitable impurity elements thereof. An alloy element Ti is adopted to replace an alloy element Cu added in traditional easily-welded steel, and the content of an alloy element Ni is set to be high, so that the steel is suitable for special application fields such as nuclear power and nuclear submarines; and by optimizing the preparation method, controlling parameters such as temperature and deformation quantity of warm rolling, directly quenching a plate blank after warm rolling, and then carrying out tempering treatment, the strength of the steel is greatly improved, and the steel can achieve relatively high strength while the weldability of the steel is ensured, the production cost is low, and the application field is wider.

Description

Technical Field

[0001] The present invention relates to the technical field of structural steel, and in particular to an 800MPa-grade high-strength and easy-to-weld ultrafine-grained steel and a preparation method thereof. Background Art

[0002] With the rapid development of the construction machinery industry, the demand for high-strength and easy-to-weld steel is increasing both at home and abroad. However, traditional high-strength steel mainly obtains high strength and toughness by adding carbon and alloys to steel. As carbon and alloy elements increase, the carbon equivalent of steel increases, and weldability decreases accordingly. Figure 1 As shown. In order to meet the manufacturing needs of low-cost and easy welding, the United States took the lead in the world to develop HSLA80 and HSLA100 steels. The main principle is to adopt a low C component design, add about 1.0-2.0% Cu, and use the large amount of fine dispersed ε-Cu precipitated during the aging treatment of Cu to produce strong precipitation strengthening to compensate for the strength loss caused by the reduction of C content. Inspired by this, the Iron and Steel Research Institute, Wuhan Iron and Steel and other domestic research institutes and enterprises began to develop Cu-containing easy-to-weld steel in the 1980s and successfully launched the mature steel grade A710 steel (its composition is similar to HSLA80 steel), and formed a series of Cu-containing easy-to-weld steels with yield strengths of 550MPa, 590MPa, 785MPa, and 890MPa.

[0003] Although the above-mentioned Cu-containing easy-to-weld steel can solve the welding problem, due to the low melting point of Cu (about 1083°C), it is easy to exist in the form of liquid phase at the grain boundary when the ingot is heated, resulting in Cu brittleness, poor surface quality of the steel plate after rolling, and a sharp decline in forming performance; and after a certain amount of Cu is added to the steel, the residual Cu during converter smelting is not easy to remove, thereby contaminating other smelting steel grades; and in some special application places, such as nuclear power, nuclear submarines and other fields, due to radiation, the Cu-containing steel will gradually age and cause performance deterioration, so the application field of the Cu-containing steel will be limited. Therefore, a 800MPa-grade high-strength easy-to-weld ultrafine-grained steel and its preparation method are needed to solve the above problems. Summary of the invention

[0004] The object of the present invention is to provide an 800MPa grade high-strength easy-to-weld ultrafine-grained steel and a preparation method thereof, so as to solve the problems existing in the prior art mentioned in the above background technology.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] An 800MPa grade high-strength and easy-to-weld ultrafine-grained steel comprising the following chemical components:

[0007] 0.06-0.12% C, 0.20-0.50% Si, 0.8-1.5% Mn, ≤0.01% P, ≤0.005% S, 2.0-3.5% Ni, 0.01-0.02% N, 0.10-0.20% Ti, 0.05-0.10% V, the rest are Fe and its inevitable impurity elements.

[0008] Preferably, the thickness specification of the ultrafine grain steel is 10-60 mm.

[0009] Preferably, the yield strength R of the ultrafine grain steel is el ≥800MPa, tensile strength Rm≥890MPa, impact energy KV2 at -84℃≥150J, tensile strength after welding Rm≥710MPa, impact energy KV2 at -40℃ of fusion line ≥60J, and the steel plate performance is uniform and the welding performance is good.

[0010] A method for preparing 800MPa grade high-strength and easy-to-weld ultrafine-grained steel comprises the following steps:

[0011] S1: Heating of ingot: For ingots with a thickness of 250-400 mm, heat to 1150±20℃ and keep warm for 4-6 hours;

[0012] S2: Rough rolling: The starting temperature of rough rolling is ≥1000℃, the final rolling temperature is ≥960℃, and the intermediate billet with a thickness of 50-100mm is rolled;

[0013] S3: Warm rolling: After the rough rolling, the intermediate billet is cooled by high pressure water for 1-5 times in the finishing mill, and is quickly cooled to near the Bs temperature, the starting rolling temperature is Bs+(50-70)℃, and the final rolling temperature is Bs+(10-30)℃;

[0014] S4: Cooling: After rolling, the steel plate is immediately water-cooled online to room temperature to obtain martensite with high dislocation density;

[0015] S5: Tempering: The tempering temperature is 520-580℃, and the tempering time is 1.5-6.0min / mm;

[0016] S6: After tempering treatment, the final structure is lath martensite plus a small amount of residual austenite.

[0017] Preferably, in S3, the rolling rate is 1.0-1.5 m / min, the rolling deformation is 40%-80%, and the steel plate is rolled into a thickness of 10-60 mm.

[0018] Preferably, in S6, the width of the lath martensite is ≤2 um.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] The present invention optimizes the chemical composition and proportion of the steel, adopts the alloy element Ti to replace the alloy element Cu added in the traditional easy-to-weld steel, and sets the content of the alloy element Ni to a higher level, so that the steel is suitable for special application fields such as nuclear power and nuclear submarines; and by optimizing the preparation method, the temperature, deformation amount and other parameters of the warm rolling are controlled, and the slab is directly quenched after the warm rolling and then tempered, so that the strength of the steel is greatly improved. While ensuring the easy-to-weld performance of the steel, the steel can also achieve higher strength, has low production cost, and has a wider application field. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a graph showing the relationship between carbon equivalent and weldability.

[0022] Figure 2 It is the CCT curve of the steel grade of the present invention.

[0023] Figure 3 It is the Fe-C equilibrium phase diagram of the steel grade of the present invention.

[0024] Figure 4 This is the organizational diagram obtained in Example 1 of the present invention.

[0025] Figure 5 This is the organizational diagram obtained in Example 2 of the present invention.

[0026] Figure 6 This is the organizational diagram obtained in Example 3 of the present invention.

[0027] Figure 7 This is the organizational diagram of Example 4 of the present invention after welding. DETAILED DESCRIPTION

[0028] In order to make the technical means, creative features, objectives and effects achieved by the present invention easy to understand, the present invention is further explained below in conjunction with specific implementation methods.

[0029] See also Figure 1-7 , the present invention provides the following technical solutions:

[0030] An 800MPa grade high-strength and easy-to-weld ultrafine-grained steel comprising the following chemical components:

[0031] 0.06-0.12% C, 0.20-0.50% Si, 0.8-1.5% Mn, ≤0.01% P, ≤0.005% S, 2.0-3.5% Ni, 0.01-0.02% N, 0.10-0.20% Ti, 0.05-0.10% V, the rest are Fe and its inevitable impurity elements.

[0032] The thickness specification of the ultrafine grain steel is 10-60 mm; the yield strength R of the ultrafine grain steel is el≥800MPa, tensile strength Rm≥890MPa, impact energy KV2 at -84℃≥150J, tensile strength after welding Rm≥710MPa, impact energy KV2 at -40℃ of fusion line ≥60J, and the steel plate performance is uniform and the welding performance is good.

[0033] In order to make up for the insufficient strength caused by the decrease in C content, the present invention does not use the alloy element Cu added to the traditional easy-to-weld steel, but adds a small amount of alloy element Ti, and uses the precipitation of TiN, TiC, and Ti(CN) particles to make up for part of the strength decrease caused by the C content. In addition, in the present invention, the content of the alloy element Ni is relatively high, because the Ni element can effectively shift the CCT curve to the right, hinder the formation of ferrite during cooling, and obtain a sub-temperature austenite phase region with a larger range, such as Figure 2 As shown, warm rolling therefore has a larger processing window.

[0034] The present invention also provides a method for preparing 800MPa grade high-strength and easy-to-weld ultrafine-grained steel, comprising the following steps:

[0035] S1: Heating of ingot: For ingots with a thickness of 250-400 mm, heat to 1150±20℃ and keep warm for 4-6 hours;

[0036] S2: Rough rolling: The starting temperature of rough rolling is ≥1000℃, the final rolling temperature is ≥960℃, and the intermediate billet with a thickness of 50-100mm is rolled;

[0037] S3: Warm rolling: After the rough rolling, the intermediate billet is subjected to 1-5 passes of high-pressure water cooling in the finishing mill and rapidly cooled to near the Bs temperature, which is the starting temperature of the transformation of austenite to bainite during cooling. The starting rolling temperature is Bs+(50-70)℃, and the final rolling temperature is Bs+(10-30)℃; the rolling rate is 1.0-1.5m / min, the rolling deformation is 40%-80%, and the steel plate is rolled into a thickness of 10-60mm.

[0038] In the traditional warm rolling process, the rolling temperature range is generally r1 -Ms points. However, in the present invention, the start and final rolling temperatures of warm rolling are both above the Bs temperature, that is, the start rolling temperature is Bs+(50-70)°C, and the final rolling temperature is Bs+(10-30)°C. This can ensure, on the one hand, that during warm rolling deformation, the metastable austenite does not undergo phase transformations such as bainite, thereby ensuring that all martensite is obtained after quenching; on the other hand, when rolling is above the Bs temperature, the strength of the metastable austenite is lower than that of bainite and martensite, thereby reducing the rolling force of the rolling mill, reducing the wear of the rolls, and delaying the life of the rolls. The start and final rolling temperatures of warm rolling are both measured in terms of Bs. This is mainly because the designed steel type does not have an A1 temperature line, such as Figure 3The equilibrium phase diagram shown, therefore only Bs can be used to determine the start and end rolling temperatures.

[0039] In the present invention, the cooling rate before warm rolling, the start rolling temperature, the final rolling temperature, the deformation amount and the cooling rate after rolling are all controlled. This is because the temperature drop from rough rolling to warm rolling is large, and some intermediate billets are thick and the cooling rate is slow. In order to improve the efficiency of the rolling mill, the intermediate billets are cooled by high-pressure water 1-5 times to speed up the temperature drop process. The rolling rate is controlled at 1.0-1.5m / min. This is mainly because slow rolling can allow the rolling deformation to be fully transmitted to the core of the billet, which is beneficial to the refinement of the grains in the core of the billet. On the other hand, it can also avoid the increase of the billet temperature due to excessively high rolling rate, ensuring the warm rolling effect.

[0040] The deformation amount is 40%-80%. This is because if the deformation amount is less than 40%, the metastable austenite cannot be fully deformed and the purpose of grain refinement cannot be achieved. If the deformation amount exceeds 80%, the temperature drop during rolling is too large, the hardness of the steel plate increases, and the rolling difficulty increases. Therefore, the deformation amount of 40%-80% can introduce a higher density of dislocations in the metastable austenite structure, providing sufficient nucleation sites for martensite crystallization during the subsequent quenching process, thereby refining the martensite grains. At the same time, the dislocations in the metastable austenite can be inherited to the martensite after phase transformation, increasing the dislocation density in the martensite structure after phase transformation, that is, the appropriate deformation amount can fully refine the original austenite grain size, make the martensite structure after phase transformation finer, and improve the mechanical properties of steel through grain refinement strengthening and dislocation strengthening.

[0041] S4: Cooling: After rolling, the steel plate is immediately water-cooled online to room temperature to obtain martensite with high dislocation density;

[0042] After warm rolling, it is immediately cooled to room temperature, that is, direct quenching, which can ensure that the austenite is not easy to form ferrite, pearlite and bainite structures during cooling, and most of the dislocations in the warm rolling deformation process can be retained, and new dislocations can be generated through martensitic phase transformation to increase the dislocation density.

[0043] S5: Tempering: The tempering temperature is 520-580℃, and the tempering time is 1.5-6.0min / mm;

[0044] S6: After tempering, a structure of lath martensite plus a small amount of residual austenite is finally obtained, and the width of the lath martensite is ≤2um.

[0045] In the present invention, the steel grade has obtained higher strength and toughness, which is mainly because the slab is directly quenched after warm rolling and then tempered to obtain fine-grained tempered martensite with a lath size of ≤2um. At the same time, the elements such as Ti and V dissolved in the steel can be combined with N, C, etc., and the large number of dislocations generated during warm rolling and the obtained fine-grained martensite can greatly improve the strength of the steel. The improvement in toughness is mainly attributed to the refinement of twin martensite, the increase in the content of residual austenite and the formation of ultrafine lamellar structure, that is, the formation of ultrafine lamellar structure can effectively hinder the expansion of cracks in the tensile specimen and improve the toughness and plasticity of the steel.

[0046] Generally speaking, the finer the grains in the steel, the worse the performance after welding due to the abnormal growth of the grain structure. In the present invention, since the steel has a certain content of Ti and N elements, TiN particles are formed during the rolling process. Due to the high melting point (greater than 1600°C), they cannot be completely dissolved during welding, which can effectively hinder the growth of grains during welding, thereby improving the welding performance.

[0047] Example 1

[0048] The chemical composition of the selected steel is: 0.06% C, 0.20% Si, 0.8% Mn, ≤0.01% P, ≤0.005% S, 2.0% Ni, 0.015% N, 0.105% Ti, 0.10% V, the balance is Fe, and its Bs temperature is 685℃; the steel plate rolling thickness is 10mm, and its specific rough rolling, warm rolling and tempering processes are shown in Table 1. After tempering, the microstructure and mechanical properties of the steel plate are as follows Figure 4 And as shown in Table 2. After the steel plate is gas shielded welded, its mechanical properties are shown in Table 2.

[0049] Example 2

[0050] The chemical composition of the selected steel is: 0.10% C, 0.50% Si, 1.5% Mn, ≤0.01% P, ≤0.005% S, 3.5% Ni, 0.01% N, 0.20% Ti, 0.08% V, the balance is Fe, and its Bs temperature is 560℃; the steel plate rolling thickness is 60mm, and its specific rough rolling, warm rolling and tempering processes are shown in Table 1. After tempering, the microstructure and mechanical properties of the steel plate are as follows Figure 5 And as shown in Table 2. After the steel plate is gas shielded welded, its mechanical properties are shown in Table 2.

[0051] Example 3

[0052] The chemical composition of the selected steel is: 0.12% C, 0.30% Si, 1.0% Mn, ≤0.01% P, ≤0.005% S, 3.0% Ni, 0.02% N, 0.15% Ti, 0.05% V, the balance is Fe, and its Bs temperature is 627℃; the steel plate rolling thickness is 45mm, and its specific rough rolling, warm rolling and tempering processes are shown in Table 1. After tempering, the microstructure and mechanical properties of the steel plate are as follows Figure 6 And as shown in Table 2. After the steel plate is gas shielded welded, its mechanical properties are shown in Table 2.

[0053] Example 4

[0054] The chemical composition of the selected steel is: 0.08% C, 0.42% Si, 1.2% Mn, ≤0.01% P, ≤0.005% S, 2.5% Ni, 0.015% N, 0.17% Ti, 0.06% V, the balance is Fe, and its Bs temperature is 630℃; the steel plate rolling thickness is 32mm, and its specific rough rolling, warm rolling and tempering processes are shown in Table 1. After tempering, the mechanical properties of the steel plate are shown in Table 2. After gas shielded welding of the steel plate, its structure is as shown in Table 2. Figure 7 The mechanical properties are shown in Table 2.

[0055] Example 5

[0056] The chemical composition of the selected steel is: 0.07% C, 0.35% Si, 1.35% Mn, ≤0.01% P, ≤0.005% S, 2.68% Ni, 0.014% N, 0.18% Ti, 0.09% V, the balance is Fe, and its Bs temperature is 620°C; the steel plate rolling thickness is 20mm, and its specific rough rolling, warm rolling and tempering processes are shown in Table 1. After tempering, the mechanical properties of the steel plate are shown in Table 2. After gas shielded welding of the steel plate, its mechanical properties are shown in Table 2.

[0057] Table 1 Steel plate production process of Examples 1-5

[0058]

[0059] Table 2 Mechanical properties of steel plates in Examples 1-5

[0060]

[0061] It can be seen from the results obtained in Examples 1-5 that the easy-to-weld ultrafine-grained steel prepared by the method of the present invention has a yield strength Rel≥820MPa, a tensile strength Rm≥890MPa, an impact energy KV2≥160J at -84°C, a tensile strength Rm≥712MPa after welding, and an impact energy KV2≥65J at -40°C of the fusion line. The steel plate has uniform performance and good welding performance, and can meet the requirements of special application places such as nuclear power, nuclear submarines and other fields for high and ultra-high strength steel plates.

[0062] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. An 800MPa grade high-strength and easy-to-weld ultrafine-grained steel, characterized in that: Includes the following chemical components: 0.06-0.12% C, 0.20-0.50% Si, 0.8-1.5% Mn, ≤0.01% P, ≤0.005% S, 2.0-3.5% Ni, 0.01-0.02% N, 0.10-0.20% Ti, 0.05-0.10% V, the rest are Fe and its inevitable impurity elements.

2. The 800MPa grade high-strength and easy-to-weld ultrafine-grained steel according to claim 1, characterized in that: The thickness specification of the ultrafine grain steel is 10-60 mm.

3. The 800MPa grade high-strength and easy-to-weld ultrafine-grained steel according to claim 1, characterized in that: The yield strength R of ultrafine grain steel el ≥800MPa, tensile strength Rm≥890MPa, impact energy KV2 at -84℃≥150J, tensile strength after welding Rm≥710MPa, impact energy KV2 at -40℃ of fusion line ≥60J, and the steel plate performance is uniform and the welding performance is good.

4. A method for preparing 800MPa grade high-strength and easy-to-weld ultrafine-grained steel as claimed in any one of claims 1 to 3, characterized in that: The following steps are involved: S1: Heating of ingot: For ingots with a thickness of 250-400 mm, heat to 1150±20℃ and keep warm for 4-6 hours; S2: Rough rolling: The starting temperature of rough rolling is ≥1000℃, the final rolling temperature is ≥960℃, and the intermediate billet with a thickness of 50-100mm is rolled; S3: Warm rolling: After the rough rolling, the intermediate billet is cooled by high pressure water for 1-5 times in the finishing mill, and is quickly cooled to near the Bs temperature, the starting rolling temperature is Bs+(50-70)℃, and the final rolling temperature is Bs+(10-30)℃; S4: Cooling: After rolling, the steel plate is immediately water-cooled online to room temperature to obtain martensite with high dislocation density; S5: Tempering: The tempering temperature is 520-580℃, and the tempering time is 1.5-6.0min / mm; S6: After tempering treatment, the final structure is lath martensite plus a small amount of residual austenite.

5. The method for preparing 800MPa grade high-strength and easy-to-weld ultrafine-grained steel according to claim 4, characterized in that: In the step S3, the rolling rate is 1.0-1.5 m / min, the rolling deformation is 40%-80%, and the steel plate is rolled into a steel plate with a thickness of 10-60 mm.

6. The method for preparing 800MPa grade high-strength and easy-to-weld ultrafine-grained steel according to claim 4, characterized in that: In S6, the width of the lath martensite is ≤2 um.

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