Hot rolling composite process for improving strength of low-alloy high-strength steel / nickel-based alloy bimetal composite plate

Abstract

The invention relates to a hot rolling composite process for improving the strength of a low-alloy high-strength steel / nickel-based alloy bimetal composite plate, and belongs to the technical field of materials. Firstly, a welded assembly is subjected to heat preservation for 2-4 h at the temperature of 1250 DEG C, large-deformation rolling is adopted at the temperature higher than 1000 DEG C after discharging, the rolling reduction of each pass is larger than or equal to 20%, and in the stage, a composite plate is rolled to be 70% or above of the target thickness; and when the temperature is 900-1000 DEG C, rolling to a target thickness by adopting small deformation, and then cooling to room temperature in air. Effective metallurgical compounding of the bimetallic material can be realized by using the hot rolling compounding process, the obtained bimetallic compound interface is flat and straight and has no metallurgical defects such as pores and cracks, the interface of a sample subjected to three-point bending is still well compounded, and cracks are not generated macroscopically and microcosmically. According to the hot rolling composite technology, the back stress strengthening effect of a composite interface can be fully exerted, meanwhile, the proportion of dynamic recrystallization grains in the nickel-based alloy is reduced, and the strength of a nickel-based alloy layer is improved.

Description

Technical Field

[0001] The present invention relates to a hot rolling composite process for improving the strength of a low alloy high strength steel / nickel base alloy bimetal composite plate, belonging to the field of materials technology. Background Art

[0002] A hot rolled bimetal composite plate is a laminated structure plate formed by laminating two different metal materials through a hot rolling process at high temperature. It combines the performance advantages of both materials while reducing costs and is widely used in the petrochemical field. The base material of the bimetal composite plate is a low alloy high strength steel to ensure strength, and the corrosion resistant layer material is a nickel base alloy to improve corrosion resistance. Combining the strength of the low alloy high strength steel and the good corrosion resistance of the nickel base alloy can be applied to a variety of extremely complex working environments. Due to the differences in the properties of the bimetal materials, hot rolling at high temperature is usually used to improve the thermal deformation coordination of the bimetal materials. However, too high a rolling temperature will cause the strength of the bimetal composite plate to decrease. A more reasonable process is needed to prepare the bimetal composite plate so that it has excellent mechanical properties while having high interfacial bonding strength to expand the engineering applications in extreme environments. Summary of the Invention

[0003] The purpose of the present invention is to provide a hot rolling composite process for improving the strength of a low alloy high strength steel / nickel base alloy bimetal composite plate, adopting a hot rolling composite process of symmetric billet assembling, two-stage rolling and large reduction rolling. By precisely controlling the hot rolling process, the yield strength and tensile strength are improved without reducing the interfacial bonding strength.

[0004] To achieve the above purpose, the technical solution of the present invention is as follows: A hot rolling composite process for improving the strength of a low alloy high strength steel / nickel base alloy bimetal composite plate, with the low alloy high strength steel located on the outside as the base material and the nickel base alloy located on the inside as the corrosion resistant layer, adopting a hot rolling composite process of symmetric billet assembling, two-stage rolling and large reduction rolling. The base material of the bimetal composite plate is a low alloy high strength steel to ensure strength, and the corrosion resistant layer of the bimetal composite plate is a Ni-Cr-Mo-Ni-Ti series nickel base alloy to improve corrosion resistance; by weight percentage, the chemical composition of the low alloy high strength steel is: Cr 0.3 - 0.45%, Mo 0.3 - 0.5%, Mn 1.6 - 1.8%, Nb 0.07 - 0.1%, Ti 0.015 - 0.018%, and the balance is Fe; by weight percentage, the chemical composition of the nickel base alloy is: Cr 20 - 23%, Mo 8 - 10%, Nb 3.15 - 4.15%, Ti 0.15 - 0.2%, C 0.06 - 0.1%, and the balance is Ni.

[0005] The hot rolling composite process for improving the strength of a low alloy high strength steel / nickel base alloy bimetal composite plate includes the following steps: (1)The billets are grouped in the order of low-alloy high-strength steel, nickel-based alloy, nickel-based alloy, and low-alloy high-strength steel, and are welded and fixed around by argon arc welding process. The welded billets are placed inside a heating furnace and evacuated to a pressure below 1.0×10 -3 Pa, and the billets are heated in the furnace to 1200℃~1250℃ and held for 2~4h; (2)During the first-stage rolling after taking out of the furnace, it is required that the rolling temperature is higher than 1000℃, the number of rolling passes is 3~5 times, and the reduction rate per pass is ≥20%. In this stage, the bimetal composite plate is rolled to 30%~40% of the initial thickness; (3)Subsequently, the second-stage rolling is carried out. At this time, the rolling temperature is 900~1000℃, and the number of rolling passes is 2~4 times. In this stage, the bimetal composite plate is rolled to the target thickness, and then air-cooled to room temperature.

[0006] For the hot-rolling composite process for improving the strength of low-alloy high-strength steel / nickel-based alloy bimetal composite plate, the total reduction of the bimetal composite plate rolling is ≥80%.

[0007] For the hot-rolling composite process for improving the strength of low-alloy high-strength steel / nickel-based alloy bimetal composite plate, the interfacial bonding strength of the bimetal composite plate prepared according to this hot-rolling composite process is ≥432MPa. After three-point bending test, the interface is still well bonded.

[0008] For the hot-rolling composite process for improving the strength of low-alloy high-strength steel / nickel-based alloy bimetal composite plate, the proportion of the dynamically recrystallized grains of the nickel-based alloy is controlled below 10%, and the maximum value of the initial back stress strengthening generated at the interface is 340MPa.

[0009] For the hot-rolling composite process for improving the strength of low-alloy high-strength steel / nickel-based alloy bimetal composite plate, the performance indexes of the bimetal composite plate prepared according to this hot-rolling composite process are: yield strength ≥520MPa, tensile strength ≥670MPa, elongation ≥30%, achieving good strength-ductility matching.

[0010] The design concept of the present invention is: In order to ensure that the bimetallic composite plate has good interfacial bonding strength, tensile properties, and three-point bending properties, the present invention adopts a two-stage rolling process, that is, large deformation rolling is carried out at a temperature higher than 1000°C, and then small deformation rolling is carried out at 900 - 1000°C, and finally a bimetallic composite plate with the target thickness is obtained. Carrying out large deformation rolling at high temperature (>1000°C) can effectively reduce the difference in thermal deformation resistance between the bimetallic materials, improve the thermal deformation coordination, and promote the composite between the two metals. Carrying out small deformation rolling at 900 - 1000°C can effectively prevent excessive warping of the bimetallic composite plate caused by uncoordinated deformation. This hot rolling composite process can give full play to the back stress strengthening effect of the composite interface, and at the same time control the proportion of dynamically recrystallized grains in the nickel-based alloy layer below 10%, improving the strength of the nickel-based alloy layer.

[0011] During the plastic deformation process of the bimetallic composite material, due to uncoordinated deformation, a certain amount of dislocations will accumulate at the interface of the bimetallic material, and the dislocations will produce an obvious back stress strengthening effect. Back stress strengthening is a very important strengthening mechanism in the bimetallic composite plate. Usually, the greater the performance difference between the bimetallic materials, the more dislocations accumulate at the interface, and the stronger the back stress strengthening effect. The back stress strengthening value can be measured according to the cyclic loading tensile test. The present invention precisely controls the rolling process to maximize the back stress strengthening effect at the interface, thereby improving the overall strength of the bimetallic composite plate while ensuring the interfacial bonding strength.

[0012] The advantages and beneficial effects of the present invention are as follows: 1. The present invention adopts a two-stage rolling process: in the first stage, high-temperature (>1000°C) large reduction (≥20% per pass) rolling is carried out to reduce the difference in thermal deformation resistance; in the second stage, medium-temperature (900 - 1000°C) small reduction rolling is carried out to inhibit dynamic recrystallization (grain ratio ≤10%) and utilize back stress strengthening (initial value ≥317 MPa), realizing the synergistic optimization of interfacial properties and mechanical properties. By controlling the rolling parameters, the yield strength of the hot-rolled bimetallic composite plate prepared according to the present invention is ≥520 MPa, the tensile strength is ≥670 MPa, the elongation is ≥30%, and the interfacial bonding strength is ≥432 MPa. Compared with the bimetallic composite plate completely rolled at high temperature (>1000°C), the strength can be increased by nearly 100 MPa while the interfacial bonding strength does not decrease. Compared with the bimetallic composite plate with a rolling temperature of 800 - 900°C, the thermal deformation coordination of the bimetallic material is improved, the interfacial bonding strength is high, and the composite plate shape is better. The said hot-rolled bimetallic composite plate has a wider application range and higher structural safety, and is suitable for more demanding and extremely complex working environments.

[0013] 2. For the combination of low-alloy high-strength steel and nickel-based alloy, through the processes of two-stage rolling and large reduction rolling, the present invention not only improves the interfacial bonding strength, but also significantly increases the yield strength and tensile strength. By precisely controlling the rolling temperature and reduction rate, the present invention gives full play to the back stress strengthening effect of the composite interface, and at the same time controls the proportion of dynamically recrystallized grains in the nickel-based alloy layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a schematic diagram of symmetrical blank assembly for the preparation method of the present invention.

[0015] Figure 2 It is a distribution diagram of dynamically recrystallized grains in the nickel-based alloy layer of the bimetallic composite plate obtained in Example 1.

[0016] Figure 3 It is the microstructure morphology near the interface of the bimetallic composite plate obtained in Example 1.

[0017] Figure 4 It is the macroscopic morphology of the bimetallic composite plate obtained in Comparative Example 3 at different angles.

[0018] Figure 5 It is the back stress strengthening value of different bimetallic composite plates measured by cyclic loading tensile test. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0019] The following non-limiting embodiments can enable those of ordinary skill in the art to more comprehensively understand the present invention, but do not limit the present invention in any way.

[0020] In the specific implementation process, by weight percentage, the composition of the base material low-alloy high-strength steel (X65) used in the present invention is: C 0.08%, Si 0.27%, Mn 1.62%, P 0.025%, S 0.034%, Cr 0.42%, Mo 0.39%, Nb 0.08%, Ti 0.016%, and the balance is Fe; the chemical composition of the corrosion-resistant layer nickel-based alloy (Inconel625) is: C 0.06%, Si 0.23%, Mn 0.36%, P 0.014%, S 0.012%, Cr 21.46%, Mo 9.13%, Nb 3.71%, Ti 0.18%, and the balance is Ni.

[0021] The surfaces of the low-alloy high-strength steel and the nickel-based alloy are polished with sandpaper and cleaned with alcohol and acetone. Subsequently, the low-alloy high-strength steel (X65), nickel-based alloy (Inconel625), nickel-based alloy (Inconel625), and low-alloy high-strength steel (X65) are assembled into a blank in sequence, and the periphery is welded and fixed by argon arc welding process ( Figure 1 ). Subsequently, the internal vacuum degree of the blank is pumped to 1.0×10 -3Below Pa, the billet thickness is 45 mm, the thicknesses of the low-alloy high-strength steel and the nickel-based alloy are 20 mm and 2.5 mm respectively, and the thickness ratio is 8:1.

[0022] In the test methods described in the following examples, unless otherwise specified, they are all conventional methods; the reagents and materials, unless otherwise specified, can all be obtained from commercial channels.

[0023] Example 1:

[0024] In this example, a bimetallic composite plate with a finishing rolling temperature of 930 °C and a reduction of 80% is prepared, and the process steps are as follows: The billet is heated in the furnace to 1250 °C and held for 4 h. After the billet is taken out of the furnace, it first undergoes the first-stage rolling. The first-stage rolling consists of 3 passes, and the thicknesses after each pass of rolling are 35 mm, 22 mm, and 16 mm respectively, and the temperature after rolling is 1100 °C. When the temperature of the composite plate drops to 980 °C, the second-stage rolling is carried out. The second-stage rolling consists of 2 passes, and the thicknesses after each pass of rolling are 13 mm and 9 mm respectively. The final rolling temperature is 930 °C, the total reduction is 80%, and the steel plate after hot rolling is air-cooled to room temperature.

[0025] After mechanical property testing, the yield strength of the bimetallic composite plate prepared in Example 1 is 530 MPa, the tensile strength is 686 MPa, the elongation is 30%, the interfacial bonding strength is 432 MPa, and the composite of the specimen after three-point bending is good without cracks. According to the statistics of electron backscatter diffraction (EBSD), the proportion of dynamically recrystallized grains inside the nickel-based alloy is 6.4%, as Figure 2 shown. The difference in Vickers hardness between the low-alloy high-strength steel and the nickel-based alloy is 158 HV. The initial value of the back stress at the composite interface is 336 MPa determined by the cyclic loading tensile test.

[0026] Example 2:

[0027] In this example, a bimetallic composite plate with a finishing rolling temperature of 980 °C and a reduction of 80% is prepared, and the process steps are as follows: The billet is heated in the furnace to 1250 °C and held for 4 h. After the billet is taken out of the furnace, it first undergoes the first-stage rolling. The first-stage rolling consists of 3 passes, and the thicknesses after each pass of rolling are 35 mm, 22 mm, and 16 mm respectively, and the temperature after rolling is 1100 °C. When the temperature of the composite plate drops to 1000 °C, the second-stage rolling is carried out. The second-stage rolling consists of 2 passes, and the thicknesses after each pass of rolling are 13 mm and 9 mm respectively. The final rolling temperature is 980 °C, the total reduction is 80%, and the steel plate after hot rolling is air-cooled to room temperature.

[0028] After mechanical property testing, the yield strength of the bimetallic composite plate prepared in Example 2 was 520 MPa, the tensile strength was 670 MPa, the elongation was 30%, the interfacial bonding strength was 436 MPa, and the specimens after three-point bending were well bonded without cracks. According to EBSD statistics, the proportion of dynamically recrystallized grains in the nickel-based alloy was 9.8%. The difference in Vickers hardness between the low-alloy high-strength steel and the nickel-based alloy was 104 HV. The initial value of the back stress at the composite interface was 317 MPa as determined by cyclic loading tensile testing.

[0029] Comparative Example 1: In this comparative example, the bimetallic composite plate was prepared without two-stage rolling and completely by hot rolling at a high temperature (>1000 °C). The final rolling temperature was 1100 °C and the reduction ratio was 80%. The process steps were as follows: The assembled billet was heated in the furnace to 1250 °C and held for 4 h. After the assembled billet was taken out of the furnace, it was directly rolled to the end. The number of rolling passes was 5. The final rolling temperature was 1100 °C. The thicknesses after each rolling pass were 35 mm, 22 mm, 16 mm, 13 mm, and 9 mm respectively. The total reduction ratio was 80%. After hot rolling, the steel plate was air-cooled to room temperature.

[0030] After mechanical property testing, the yield strength of the bimetallic composite plate prepared in Comparative Example 1 was 448 MPa, the tensile strength was 598 MPa, the elongation was 29%, the interfacial bonding strength was 442 MPa, and the specimens after three-point bending were well bonded without cracks. According to EBSD statistics, the proportion of dynamically recrystallized grains in the nickel-based alloy was 98%. The difference in Vickers hardness between the low-alloy high-strength steel and the nickel-based alloy was 52 HV. The initial value of the back stress at the composite interface was 302 MPa as determined by cyclic loading tensile testing.

[0031] Comparative Example 2: In this comparative example, the bimetallic composite plate was prepared without two-stage rolling and completely by hot rolling at a high temperature (>1000 °C). The final rolling temperature was 1050 °C and the reduction ratio was 80%. The process steps were as follows: The assembled billet was heated in the furnace to 1250 °C and held for 4 h. After the assembled billet was taken out of the furnace, it was directly rolled to the end. The number of rolling passes was 5. The intermittent time for each pass was controlled to make the final rolling temperature 1050 °C. The thicknesses after each rolling pass were 35 mm, 22 mm, 16 mm, 13 mm, and 9 mm respectively. The total reduction ratio was 80%. After hot rolling, the steel plate was air-cooled to room temperature.

[0032] After mechanical property testing, the yield strength of the bimetallic composite plate prepared in Comparative Example 2 was 417 MPa, the tensile strength was 599 MPa, the elongation was 32%, the interfacial bonding strength was 440 MPa, and the specimens after three-point bending were well bonded without cracks. According to EBSD statistics, the proportion of dynamically recrystallized grains in the nickel-based alloy was 97%, as Figure 3As shown in the figure. The Vickers hardness difference between the low-alloy high-strength steel and the nickel-based alloy is 53 HV. The initial value of the back stress at the composite interface was measured to be 310 MPa through cyclic loading tensile tests.

[0033] Comparative Example 3: This comparative example involved two-stage rolling with a rolling temperature in the second stage below 900 °C to prepare a bimetallic composite plate. The process steps for preparing a bimetallic composite plate with a final rolling temperature of 850 °C and a reduction of 80% are as follows: The assembled billet was heated in the furnace to 1250 °C and held for 4 h. After the assembled billet was taken out of the furnace, it first underwent the first-stage rolling. The first-stage rolling consisted of a total of 3 passes, and the thickness after each pass was 35 mm, 22 mm, and 16 mm respectively, and the temperature after rolling was 1100 °C. When the temperature of the composite plate dropped to 890 °C, the second-stage rolling was carried out. The second-stage rolling consisted of a total of 2 passes, and the thickness after each pass was 13 mm and 9 mm respectively. The final rolling temperature was 850 °C, the total reduction was 80%, and the steel plate after hot rolling was air-cooled to room temperature.

[0034] As Figure 4 shown, due to the relatively low rolling temperature and the relatively large hot deformation resistance of the bimetallic material, the bimetallic composite plate prepared in Comparative Example 3 showed a large degree of bending. Therefore, the mechanical properties of the samples in this comparative example were not tested.

[0035] Both Comparative Example 1 and Comparative Example 2 adopted direct rolling to the end after taking out of the furnace, and the final rolling temperature was higher than 1000 °C. As Figure 5 shown, since dynamic recrystallization almost completely occurred in the nickel-based alloy layer in Comparative Example 1 and Comparative Example 2, the strength of the nickel-based alloy layer decreased significantly. At the same time, the back stress strengthening effect provided by the composite interface also decreased compared with the examples, which made the strength of the bimetallic composite plates prepared in Comparative Example 1 and Comparative Example 2 relatively low.

[0036] Comparative Example 3 adopted two-stage rolling, but the rolling temperature in the second stage was relatively low (<900 °C), and there was a relatively large hot deformation resistance between the bimetallic materials. At this time, the composite plate showed obvious bending, which brought unnecessary trouble to subsequent processing and use.

[0037] Table 1 Yield strength / MPa Tensile strength / MPa Elongation / % Interface bonding strength / MPa Volume ratio of dynamically recrystallized grains of nickel-based alloy / % Hardness difference / HV Initial value of back stress / MPa Example 1 530 686 30 432 6.4 158 336 Example 2 520 670 30 436 9.8 104 317 Comparative example 1 448 598 29 442 98 52 302 Comparative example 2 417 599 32 440 97 53 310 As can be seen from Table 1, for the bimetallic composite plate obtained by two-stage rolling, the proportion of dynamically recrystallized grains in the nickel-based alloy layer is ≤10%, retaining most of the hot-rolling deformation energy storage. At the same time, the composite interface provides an obvious back stress strengthening effect, improving the overall strength. The yield strength of the bimetallic composite plate prepared in the final example is ≥520 MPa, the tensile strength is ≥670 MPa, the elongation is ≥30%, and the strength-ductility product reaches more than 20 GPa·%. Compared with the bimetallic composite plate prepared in the comparative example, the interface bonding strength remains stable at 432 MPa.

[0038] The implementation results show that the hot-rolling composite process of the present invention for bimetallic materials can achieve effective metallurgical bonding. The obtained bimetallic composite interface is flat, without metallurgical defects such as pores and cracks. The interface of the specimen remains well bonded after three-point bending, and no cracks appear macroscopically or microscopically. Compared with full high-temperature rolling (Comparative Examples 1-2), the strength is increased by nearly 100 MPa, and the interface bonding strength is not reduced; compared with low-temperature rolling (Comparative Example 3), the warping problem is avoided, and high deformation coordination is maintained.

[0039] The above embodiments are only used to illustrate the technical concept and features of the present invention, and their purpose is to enable those familiar with this technology to understand the content of the present invention and implement it accordingly, and should not be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit and essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A hot rolling composite process for improving the strength of a low alloy high strength steel / nickel-based alloy bimetallic composite plate, characterized in that: The invention adopts a low-alloy high-strength steel on the outside as a substrate and a nickel-based alloy on the inside as a corrosion-resistant layer, adopts a symmetrical billet assembly, a two-stage rolling and a hot rolling composite process with large reduction rolling, the substrate of the bimetallic composite plate adopts low-alloy high-strength steel to ensure strength, and the corrosion-resistant layer of the bimetallic composite plate adopts Ni-Cr-Mo-Ni-Ti series nickel-based alloy to improve corrosion resistance; the chemical composition of the low-alloy high-strength steel is 0.3-0.45% by weight, Mo 0.3-0.5%, Mn 1.6-1.8%, Nb 0.07-0.1%, Ti 0.015-0.018%, and the balance is Fe; the chemical composition of the nickel-based alloy is 20-23% by weight, Mo 8-10%, Nb 3.15-4.15%, Ti 0.15-0.2%, C 0.06-0.1%, and the balance is Ni.

2. A hot rolling composite process for improving the strength of low alloy high strength steel / nickel-based alloy bimetallic composite plates according to claim 1, characterized in that: The steps include: (1) The billets are assembled in the order of low alloy high strength steel, nickel-based alloy, nickel-based alloy, and low alloy high strength steel. The surrounding parts are welded and fixed by argon arc welding. The welded billets are placed in a heating furnace and evacuated to a pressure of 1.0×10 -3 Pa, heat the assembly to 1200℃~1250℃ and keep it warm for 2~4h; (2) During the first stage of rolling after the furnace is discharged, the rolling temperature is required to be higher than 1000°C, the rolling passes are 3 to 5 times, and the reduction rate of each pass is ≥20%. In this stage, the bimetallic composite plate is rolled to 30% to 40% of the initial thickness; (3) Then the second stage of rolling is carried out, at which the rolling temperature is between 900 and 1000 °C, and the rolling passes are 2 to 4 times. In this stage, the bimetallic composite plate is rolled to the target thickness and then air-cooled to room temperature.

3. A hot rolling composite process for improving the strength of a low alloy high strength steel / nickel-based alloy bimetallic composite plate according to claim 2, characterized in that: The total rolling reduction of bimetallic composite plates is ≥80%.

4. A hot rolling composite process for improving the strength of a low alloy high strength steel / nickel-based alloy bimetallic composite plate according to claim 2, characterized in that: The interface bonding strength of the bimetallic composite plate prepared according to this hot rolling composite process is ≥432MPa, and after a three-point bending test, the interface is still well composited.

5. A hot rolling composite process for improving the strength of low alloy high strength steel / nickel-based alloy bimetallic composite plates according to claim 2, characterized in that: The dynamic recrystallization grain ratio of the nickel-based alloy is controlled below 10%, and the initial back stress strengthening value generated at the interface is up to 340MPa.

6. A hot rolling composite process for improving the strength of low alloy high strength steel / nickel-based alloy bimetallic composite plates according to claim 2, characterized in that: The performance indicators of the bimetallic composite plate prepared according to this hot rolling composite process are: yield strength ≥520MPa, tensile strength ≥670MPa, elongation ≥30%, achieving a good match between strength and plasticity.

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