Method for obtaining optimal low-temperature impact toughness and welded body

Abstract

The invention provides a method for obtaining the optimal low-temperature impact toughness and a welded body, and the method comprises the following steps: S1, obtaining chemical components of sample steel, and calculating the cooling rate corresponding to the bainite phase change curve nose tip temperature of the sample steel; s2, the cooling rate serves as the key cooling rate of the welding heat cycle process, and the corresponding welding heat input amount is calculated; s3, cutting a plurality of cuboid thermal simulation initial samples of which the length directions are parallel to the rolling direction of the steel plate from the sample steel; s4, two-pass welding thermal simulation cycle experiments at different temperatures are set; s5, two thermocouples are welded to the center of the cuboid thermal simulation initial sample for temperature measurement, and a welding simulation experiment is completed; and S6, size machining is conducted on the welding sample, a low-temperature impact experiment is conducted, impact energy under different technologies is measured, and the optimal impact toughness of CGHAZ and ICCGHAZ is determined. The method can be used for developing and optimizing the welding process with insufficient low-temperature impact toughness of the submerged-arc welding heat affected zone of the low alloy steel.

Description

Technical Field

[0001] The present invention relates to the technical field of steel material welding processes, and particularly to a method for obtaining optimal low-temperature impact toughness and a welded body.

Background Art

[0002] In the fields of equipment manufacturing such as pipeline engineering, offshore platforms, offshore wind power, and construction machinery, submerged arc welding is widely used due to its high efficiency, low cost, and simple process. However, since the steel plates usually used are low-alloy steels with relatively large thickness specifications (15 mm and above), double-sided submerged arc welding and multi-layer multi-pass welding have become the most commonly used welding methods. This makes the welding heat-affected zone of the steel plate base material relatively complex, and local areas will be affected by the heat cycles of the second pass or subsequent multiple welding passes, thereby making the structure of the heat-affected zone complex and the performance fluctuate greatly. Among them, the area with the worst low-temperature toughness appears in the intercritical coarse-grained heat-affected zone (ICCGHAZ). The reason is that ICCGHAZ is formed when the coarse-grained heat-affected zone (CGHAZ) formed by the first heat cycle is again subjected to the critical zone temperature (between Ac1 - Ac3) of the second-pass welding heat cycle. Coarse austenite grains and large-sized M-A islands (chain-like martensite-austenite components) continuously distributed along the austenite grain boundaries are formed in the ICCGHAZ area, significantly reducing the crack initiation work and propagation work of the ICCGHAZ cracks, and the low-temperature impact toughness often being less than 27 J.

[0003] Although the area percentage of ICCGHAZ in the entire welding heat-affected zone is very small (about 1 - 3 mm2), before actual engineering applications, low-temperature impact toughness tests will be carried out on simulated actual welded joints, including positions such as weld metal, equivalent fusion line, and equivalent heat-affected zone. This makes it extremely easy for the V-notch position of the impact test sample (10×10×55 mm) to exactly contain the ICCGHAZ to become the crack initiation source. Once the crack initiates, it will quickly propagate in a cleavage or quasi-cleavage manner and fracture. The work absorbed by crack initiation and propagation is very low, and the impact toughness is extremely poor. Therefore, it is particularly important to optimize the low-temperature impact toughness of ICCGHAZ.

[0004] Therefore, it is necessary to study a method for obtaining optimal low-temperature impact toughness and a welded body to address the deficiencies of the prior art and solve or mitigate one or more of the above problems.

Summary of the Invention

[0005] In view of this, the present invention provides a method for obtaining optimal low-temperature impact toughness and a welded body, which can develop and optimize the welding process with insufficient low-temperature impact toughness in the heat-affected zone of low-alloy steel submerged arc welding.

[0006] On the one hand, the present invention provides a method for obtaining the best low-temperature impact toughness, which is used to improve the low-temperature impact toughness of CGHAZ and ICCGHAZ in submerged arc welding of low-alloy steel. The method for obtaining the best low-temperature impact toughness includes the following steps:

[0007] S1: Obtain the chemical composition of the sample steel and calculate the cooling rate corresponding to the nose temperature of the bainite phase transformation curve thereof;

[0008] S2: Take the cooling rate in S1 as the key cooling rate in the welding thermal cycle process, and calculate the welding heat input corresponding to this key cooling rate;

[0009] S3: Cut several cuboid thermal simulation initial specimens on the sample steel with the length direction parallel to the rolling direction of the steel plate;

[0010] S4: Set two-pass welding thermal simulation cycles at different temperatures with the welding heat input and the thickness of the sample steel as parameters;

[0011] S5: Weld two thermocouples at the center of the cuboid thermal simulation initial specimen in S3 for temperature measurement, and complete the welding simulation experiments of the coarse-grained heat-affected zone and the critical coarse-grained heat-affected zone according to the steps of S4;

[0012] S6: Process the welded specimens obtained through the welding simulation experiment in S5 and conduct low-temperature impact experiments, measure the impact energy under different processes, and determine the optimal impact toughness of CGHAZ and ICCGHAZ.

[0013] In the above-mentioned aspect and any possible implementation manner, a further implementation manner is provided. The specific calculation method of the cooling rate in S1 is as follows: Calculate the continuous cooling transformation CCT curve of the sample steel through its chemical composition, and determine the cooling rate corresponding to the nose temperature of the bainite phase transformation curve through the continuous cooling transformation CCT curve.

[0014] In the above-mentioned aspect and any possible implementation manner, a further implementation manner is provided. The sample steel in S1 is a 550 MPa grade low-alloy steel for offshore engineering.

[0015] In the above-mentioned aspect and any possible implementation manner, a further implementation manner is provided. The specific calculation method of the welding heat input in S2 is as follows: Take the cooling rate determined in S1 as the key cooling rate in the welding thermal cycle process, and calculate the welding heat input corresponding to this cooling rate in combination with the thickness of the sample steel and the Rykalin analytical formula.

[0016] For the aspects and any possible implementation manners as described above, a further implementation manner is provided. Specifically, S4 is as follows: Set up a two-pass welding thermal simulation cycle experiment on a resistance heating type thermal simulation testing machine system. For the first pass, input the plate thickness of the sample steel, a peak temperature of 1350°C, and the welding heat input calculated in S2. For the second pass, input the plate thickness of the sample steel, a peak temperature of 780°C, and the welding heat input calculated in S2.

[0017] For the aspects and any possible implementation manners as described above, a further implementation manner is provided. In S3, the initial dimensions of the cuboid thermal simulation specimen are 60mm×11mm×11mm, without notches, and the cutting position is at the 1 / 2 thickness position of the sample steel.

[0018] For the aspects and any possible implementation manners as described above, a further implementation manner is provided. Specifically, S5 is as follows: Install the initial thermal simulation specimen on a resistance heating type thermal simulation testing machine, and weld two thermocouples at the center position in the length direction of the initial thermal simulation specimen for temperature measurement. Then, complete the welding simulation experiments for the coarse grain heat affected zone (CGHAZ, one thermal cycle) and the critical coarse grain heat affected zone (ICCGHAZ, two thermal cycles) according to S4, and repeat each process 3 times.

[0019] For the aspects and any possible implementation manners as described above, a further implementation manner is provided. Specifically, S6 is as follows: Further prepare the welded specimens obtained in S5 into standard impact specimens of 55mm×10mm×10mm, and complete the -40°C low-temperature impact experiment on an impact testing machine to measure the impact energy under each process and determine the accuracy of obtaining the optimal impact toughness for CGHAZ and ICCGHAZ.

[0020] For the aspects and any possible implementation manners as described above, a further implementation manner is provided. In S5, the welded critical coarse grain heat affected zone obtains a duplex structure of lath bainite with high-density large-angle grain boundaries and discontinuous blocky M-A divided by intragranular blocks along the austenite grain boundaries.

[0021] For the aspects and any possible implementation manners as described above, a further implementation manner is provided. A welded body is obtained after determining the welding process by the method described above. The average -40°C impact energy of the welded body in the welded critical coarse grain heat affected zone can be increased from 23.8J to 51.4J or more.

[0022] Compared with the prior art, the present invention can achieve the following technical effects:

[0023] 1. The present invention uses the cooling rate corresponding to the lath bainite obtained by low alloy steel at the nose temperature of the bainite transformation curve as the critical cooling rate (t8 / 5) during the welding thermal cycle, so as to refine the microstructure of the coarse-grained heat affected zone, refine the matrix microstructure of the critical coarse-grained heat affected zone, and control the segmentation of the chain-like M-A at the original austenite grain boundary, avoid the formation of large and chain-like continuous M-A, and optimize the low-temperature impact toughness of the coarse-grained heat affected zone and the critical coarse-grained heat affected zone;

[0024] 2. Without the need for a trial-and-error method to determine the optimal welding heat input, the method of the present invention can determine the optimal welding heat input required for the experimental steel only by alloy composition calculation and critical cooling rate, so as to achieve the highest low-temperature impact toughness in the welded coarse-grained heat affected zone and the critical coarse-grained heat affected zone. This method is both efficient and economical, and is applicable to the submerged arc welding method of low alloy steel in offshore engineering, wind power, pipelines, etc.

[0025] Of course, it is not necessary for any product implementing the present invention to achieve all the above-described technical effects simultaneously.

Description of the Drawings

[0026] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0027] Figure 1 It is a microstructural diagram of the welded simulated coarse-grained heat affected zone (CGHAZ) and the critical coarse-grained heat affected zone (ICCGHAZ) under the condition of the optimal heat input of the embodiment of the present invention;

[0028] Figure 2 It is a -40°C impact energy and impact fracture morphology diagram of the welded simulated coarse-grained heat affected zone (CGHAZ) and the critical coarse-grained heat affected zone (ICCGHAZ) under the condition of the optimal heat input of the embodiment of the present invention.

Detailed Embodiments

[0029] In order to better understand the technical solutions of the present invention, the embodiments of the present invention will be described in detail below with reference to the drawings.

[0030] It should be clear that the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0031] The terms used in the embodiments of the present invention are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The singular forms "a", "the" and "said" used in the embodiments of the present invention and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0032] The present invention provides a method for obtaining the best low-temperature impact toughness, which is used to improve the low-temperature impact toughness of CGHAZ and ICCGHAZ in submerged arc welding of low-alloy steel. The method for obtaining the best low-temperature impact toughness includes the following steps:

[0033] S1: Obtain the chemical composition of the sample steel and calculate the cooling rate corresponding to the nose temperature of its bainite transformation curve.

[0034] S2: Take the cooling rate in S1 as the key cooling rate of the welding thermal cycle process, and calculate the welding heat input corresponding to this key cooling rate.

[0035] S3: Cut several cuboid thermal simulation initial specimens with the length direction parallel to the rolling direction of the steel plate on the sample steel.

[0036] S4: Set two-pass welding thermal simulation experiments at different temperatures with the welding heat input and the thickness of the sample steel as parameters.

[0037] S5: Weld two thermocouples at the center of the cuboid thermal simulation initial specimen in S3 for temperature measurement, and complete the welding simulation experiments of the coarse-grained heat-affected zone and the critical coarse-grained heat-affected zone according to the steps of S4.

[0038] S6: Process the size of the welding specimens obtained through the welding simulation experiments in S5 and conduct low-temperature impact experiments, measure the impact energy under different processes, and determine the optimal impact toughness of CGHAZ and ICCGHAZ.

[0039] The specific calculation method of the cooling rate in S1 is as follows: Calculate the continuous cooling transformation CCT curve of the sample steel through its chemical composition, and determine the cooling rate corresponding to the nose temperature of its bainite transformation curve through the continuous cooling transformation CCT curve.

[0040] The sample steel in S1 is a 550MPa grade low-alloy steel for offshore engineering.

[0041] The specific calculation method of the welding heat input in S2 is as follows: Take the cooling rate determined in S1 as the key cooling rate of the welding thermal cycle process, and calculate the welding heat input corresponding to this cooling rate in combination with the thickness of the sample steel and the Rykalin analytical formula.

[0042] The specific content of S4 is as follows: Set two-pass welding thermal simulation cycle experiments on the resistance heating type thermal simulation testing machine system; for the first pass, input the plate thickness of the sample steel, the peak temperature of 1350 °C, and the welding heat input calculated by S2, and for the second pass, input the plate thickness of the sample steel, the peak temperature of 780 °C, and the welding heat input calculated by S2.

[0043] In S3, the size of the cuboid thermal simulation initial specimen is 60 mm × 11 mm × 11 mm, without notches, and the cutting position is at the 1 / 2 thickness position of the sample steel.

[0044] The specific content of S5 is as follows: Install the thermal simulation initial specimen on the resistance heating type thermal simulation testing machine, and weld two thermocouples at the central position in the length direction of the thermal simulation initial specimen for temperature measurement. Then, complete the welding simulation experiments of the coarse grain heat affected zone (CGHAZ as the primary thermal cycle) and the critical coarse grain heat affected zone (ICCGHAZ as the secondary thermal cycle) according to S4, and repeat each process 3 times.

[0045] The specific content of S6 is as follows: Further prepare the welded specimens obtained in S5 into 55 mm × 10 mm × 10 mm standard impact samples, and complete the -40 °C low-temperature impact experiment on the impact testing machine to measure the impact energy under each process, and determine the accuracy of obtaining the optimal impact toughness in CGHAZ and ICCGHAZ.

[0046] In S5, the welding critical coarse grain heat affected zone obtains a duplex structure of lath bainite with high-density large-angle grain boundaries and discontinuous blocky M-A divided by intragranular blocks along the austenite grain boundaries.

[0047] The present invention also provides a welded body, which is obtained after determining the welding process by the method described above. The average -40 °C impact energy of the welded body in the welding critical coarse grain heat affected zone can be increased from 23.8 J to 51.4 J or more.

[0048] Example 1:

[0049] The present invention aims to propose a method for obtaining the best low-temperature impact toughness in the welding critical coarse grain heat affected zone (ICCGHAZ) of low alloy steel. This method is applicable to the submerged arc welding method of low alloy steel. The key technical problem to be solved is to reverse design the welding heat input by calculating the best cooling rate for obtaining the fine lath bainite structure, and then the problem of insufficient low-temperature impact toughness in CGHAZ and ICCGHAZ during the submerged arc welding of low alloy steel can be solved simultaneously. By using the method proposed in the present invention, the average -40 °C impact energy of the welding ICCGHAZ of 36 mm thick 550 MPa grade offshore engineering steel can be increased from 23.8 J to 51.4 J or more, and the average -40 °C impact energy of CGHAZ reaches more than 170 J.

[0050] To achieve the above object, the present invention first uses the material thermodynamics software Thermo-Calc to calculate the chemical composition of the experimental steel to obtain the CCT curve of its cooling phase transformation process, and determines the cooling rate corresponding to the lath bainite obtained at the nose temperature of the bainite phase transformation curve. Then, in combination with the general Rykalin analytical formula for welding thermal simulation and the reverse calculation of the steel plate thickness, the welding heat input corresponding to this cooling rate is calculated. Finally, through the Gleeble welding simulation experiment, the phase transformation microstructure of the CGHAZ and ICCGHAZ and the -40°C impact energy obtained at this cooling rate are compared and verified to determine the technical reliability of the present invention.

[0051] The technical solution of the present invention is: to propose a method for obtaining the best low-temperature impact toughness in the critical coarse-grained heat-affected zone (ICCGHAZ) of low-alloy steel welding, and to develop and optimize the welding process with insufficient low-temperature impact toughness in the heat-affected zone of submerged arc welding of low-alloy steel.

[0052] A method for obtaining the best low-temperature impact toughness in the critical coarse-grained heat-affected zone (ICCGHAZ) of low-alloy steel welding, the key is to determine the cooling rate corresponding to the lath bainite obtained at the nose temperature of the bainite phase transformation curve, and in combination with welding thermal simulation calculation and the actual thickness of the steel plate, perform reverse calculation to determine the optimal welding heat input. Finally, through the Gleeble thermal simulation experiment, welding simulation impact samples under the condition of this heat input are obtained, and further verified by the -40°C low-temperature impact experiment to determine the level of its impact toughness.

[0053] Specifically, it includes the following steps:

[0054] Step 1: Calculate the phase transformation CCT curve of the 550 MPa grade offshore engineering steel with a thickness of 36 mm during continuous cooling according to its chemical composition, and determine the cooling rate corresponding to the nose temperature of the bainite phase transformation curve in the CCT curve;

[0055] Step 2: Take the cooling rate determined in Step 1 as the key cooling rate (t8 / 5 cooling rate) of the welding thermal cycle process, and calculate the welding heat input corresponding to this cooling rate in combination with the steel plate thickness (36 mm) and the Rykalin analytical formula;

[0056] Step 3: Cut several cuboid thermal simulation initial specimens with the length direction parallel to the rolling direction of the steel plate at the 1 / 2 thickness position of the 36 mm thick 550 MPa grade offshore engineering steel plate. The specimen size is 60 mm × 11 mm × 11 mm, without notches;

[0057] Step 4: Set up a two-pass welding thermal simulation cycle experiment on the resistance heating type thermal simulation testing machine system. For the first pass, input the plate thickness (36 mm), peak temperature of 1350 °C, and the calculated heat input. For the second pass, input the plate thickness (36 mm), peak temperature of 780 °C (between Ac1 - Ac3), and the calculated heat input;

[0058] Step 5: Install the initial thermal simulation specimen on the resistance heating type thermal simulation testing machine, and weld two thermocouples at the central position in the length direction of the sample for temperature measurement. Then, complete the welding simulation experiments for the coarse grain heat affected zone (CGHAZ: single thermal cycle) and the critical coarse grain heat affected zone (ICCGHAZ: double thermal cycle) according to Step 4, and repeat each process 3 times;

[0059] Step 6: Further prepare the specimens obtained in Step 5 into standard impact samples of 55 mm × 10 mm × 10 mm, and complete the -40 °C low-temperature impact experiment on the impact testing machine to measure the impact energy under each process;

[0060] Step 7: To compare and verify that the heat input obtained in Step 2 is the optimal value, conduct comparative experiments on heat inputs higher and lower than this value respectively, and repeat Steps 3 to 6.

[0061] A method for obtaining the best low-temperature impact toughness in the critical coarse grain heat affected zone (ICCGHAZ) of low alloy steel welding, which can increase the average -40 °C impact energy of the ICCGHAZ of 36 mm thick 550 MPa grade offshore engineering steel welding from 23.8 J to 51.4 J or more, and the average -40 °C impact energy of the CGHAZ reaches more than 170 J.

[0062] The specific experimental process is as follows:

[0063] Calculate the continuous cooling transformation CCT curve of the 550 MPa grade offshore engineering steel according to its alloy composition, and determine that the cooling rate corresponding to the nose temperature of the bainite transformation curve in the CCT curve is 37.5 °C / s. When the cooling rate is higher or lower than 37.5 °C / s, the transformation microstructure is in the martensite transformation range and the transition range between lath bainite and granular bainite or the granular bainite transformation range respectively.

[0064] Use the obtained cooling rate as the critical cooling rate (t 8 / 5Cooling rate), and then combined with the steel plate thickness (36 mm) and the Rykalin analytical formula to calculate the welding heat input corresponding to this cooling rate as 14 kJ / cm. A single-pass thermal cycle (peak temperature of 1350 °C) and a two-pass thermal cycle (peak temperatures of 1350 °C and 780 °C respectively) welding simulation experiments under this heat input condition were completed through a Gleeble thermal simulation testing machine. Further, a -40 °C impact experiment was carried out on the thermal simulation samples, and Figure 1 and Figure 2 the CGHAZ and ICCGHAZ microstructures, as well as the impact fracture surface and impact energy shown were obtained.

[0065] To compare and verify that the heat input obtained in step 2 is the optimal value, comparative experiments with heat inputs higher and lower than this value were respectively carried out. 10 kJ / cm, 20 kJ / cm, and 30 kJ / cm were selected, and the welding thermal simulation experiment and the -40 °C impact experiment were repeated. The comparison results are shown in Table 1.

[0066] A method for obtaining the best low-temperature impact toughness in the intercritical coarse-grained heat-affected zone (ICCGHAZ) of low-alloy steel welding proposed by the present invention. The key is to obtain the best cooling rate of lath bainite phase transformation according to the alloy composition system, and use this as the key cooling rate (t 8 / 5 Cooling rate) during the welding thermal cycle of the experimental steel. From this, the optimal heat input required for welding is calculated inversely, and then a fine lath bainite structure is obtained in the CGHAZ, and a duplex structure of discontinuous distributed M-A and fine lath bainite divided by fine lath bainite is obtained in the ICCGHAZ, and finally the low-temperature impact toughness in the CGHAZ and ICCGHAZ reaches the maximum value.

[0067] Table 1 -40 °C impact experiment results under different heat input conditions

[0068]

[0069]

[0070] The present invention provides a method for obtaining the best low-temperature impact toughness in the intercritical coarse-grained heat-affected zone (ICCGHAZ) of low-alloy steel welding. First, based on the composition of the low-alloy steel, phase transformation kinetics is calculated to determine the cooling rate required to obtain lath bainite at the nose temperature of the bainite phase transformation curve. Then, combined with the general Rykalin analytical formula of welding thermal simulation and the thickness of the steel plate, the welding heat input corresponding to this cooling rate is inversely calculated. Finally, through a single welding thermal cycle experiment, a fine lath bainite structure is first obtained in the CGHAZ, and then a secondary thermal cycle experiment at the critical zone temperature is carried out on this kind of structure. The obtained ICCGHAZ structure is lath bainite with a high density of large-angle grain boundaries and discontinuous blocky M-A divided by intragranular blocks along the austenite grain boundaries. Its low-temperature impact toughness at -40 °C can reach 51.4 J or above, which is much higher than the low-temperature impact toughness obtained under other cooling conditions.

[0071] The above has introduced in detail a method for obtaining the best low-temperature impact toughness and the welded body provided by the embodiments of the present application. The description of the above embodiments is only used to help understand the method and its core idea of the present application; at the same time, for those of ordinary skill in the art, according to the idea of the present application, there will be changes in the specific implementation manners and application scopes. In summary, the content of this specification should not be construed as a limitation to the present application.

[0072] As used in the specification and claims, certain terms are used to refer to specific components. Those skilled in the art should understand that hardware manufacturers may use different terms to refer to the same component. The specification and claims do not use the difference in names as a way to distinguish components, but use the difference in functions of components as the criterion for distinction. As mentioned throughout the specification and claims, "comprising" and "including" are open-ended terms, so they should be interpreted as "comprising / including but not limited to". "Substantially" means within an acceptable error range. Those skilled in the art can solve the technical problem within a certain error range and basically achieve the technical effect. The subsequent description of the specification is for the purpose of describing the preferred embodiments of the present application, but the description is for the purpose of explaining the general principles of the present application and is not used to limit the scope of the present application. The protection scope of the present application shall be determined by the scope defined by the appended claims.

[0073] It should also be noted that the term "including", "comprising" or any other variant thereof is intended to cover non-exclusive inclusion, so that a commodity or system including a series of elements not only includes those elements, but also includes other elements not explicitly listed, or also includes elements inherent in such a commodity or system. Without further limitation, the element defined by the statement "including one..." does not exclude the existence of another identical element in the commodity or system including the said element.

[0074] It should be understood that the term "and / or" used herein is merely a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: the situation where A exists alone, the situation where both A and B exist simultaneously, and the situation where B exists alone. Additionally, the character " / " in this text generally indicates that the associated objects before and after are in an "or" relationship.

[0075] The above description shows and describes several preferred embodiments of the present application. However, as mentioned above, it should be understood that the present application is not limited to the form disclosed herein, should not be regarded as excluding other embodiments, but can be used in various other combinations, modifications, and environments, and can be changed within the scope of the application concept described herein through the above teachings or the technology or knowledge in the relevant field. And any changes and alterations made by those skilled in the art without departing from the spirit and scope of the present application shall fall within the protection scope of the appended claims of the present application.

Claims

1. A method for obtaining optimal low-temperature impact toughness, for improving the low-temperature impact toughness of CGHAZ and ICCGHAZ in submerged arc welding of low-alloy steel, characterized in that: The method for obtaining the best low temperature impact toughness comprises the following steps: S1: Obtain the chemical composition of the sample steel and calculate the cooling rate corresponding to the nose temperature of its bainite transformation curve; S2: Take the cooling rate in S1 as the critical cooling rate in the welding thermal cycle process and calculate the welding heat input corresponding to this critical cooling rate; S3: Cut a number of rectangular thermal simulation initial specimens from the sample steel, with the length direction parallel to the rolling direction of the steel plate; S4: Taking welding heat input and sample steel thickness as parameters, set up two-pass welding heat simulation cycle experiments at different temperatures; S5: Weld two thermocouples at the center of the rectangular thermal simulation initial specimen in S3 to measure the temperature, and complete the welding simulation experiment of the coarse-grained heat affected zone and the critical coarse-grained heat affected zone according to the steps of S4; S6: The welding specimens obtained through the S5 welding simulation experiment are processed in size and subjected to low-temperature impact tests to measure the impact energy under different processes and determine the optimal impact toughness of CGHAZ and ICCGHAZ.

2. The method for obtaining optimal low temperature impact toughness according to claim 1, characterized in that: The calculation method of the S1 cooling rate is specifically as follows: calculating the CCT curve of the continuous cooling process phase transformation of the sample steel according to the chemical composition thereof, and determining the cooling rate corresponding to the nose temperature of the bainite phase transformation curve according to the CCT curve of the continuous cooling process phase transformation.

3. The method for obtaining optimal low temperature impact toughness according to claim 1, characterized in that: The sample steel in S1 is 550MPa grade low alloy steel for marine engineering.

4. The method for obtaining optimal low temperature impact toughness according to claim 1, characterized in that: The calculation method of the welding heat input in S2 is specifically: taking the cooling rate determined by S1 as the key cooling rate of the welding thermal cycle process, combining the thickness of the sample steel and the Rykalin analytical formula to calculate the welding heat input corresponding to this cooling rate.

5. The method for obtaining optimal low temperature impact toughness according to claim 1, characterized in that: The S4 is specifically: setting up two welding thermal simulation cycle experiments on the resistance heating thermal simulation testing machine system; in the first pass, the plate thickness of the sample steel, the peak temperature of 1350°C and the welding heat input calculated by S2 are input, and in the second pass, the plate thickness of the sample steel, the peak temperature of 780°C and the welding heat input calculated by S2 are input.

6. The method for obtaining optimal low temperature impact toughness according to claim 1, characterized in that: The initial size of the S3 medium-rectangular thermal simulation specimen is 60 mm×11 mm×11 mm, without a notch, and the cutting position is 1 / 2 thickness of the sample steel.

7. The method for obtaining optimal low temperature impact toughness according to claim 5, characterized in that: The S5 is specifically as follows: installing the thermal simulation initial sample on a resistance heating thermal simulation testing machine, and welding two thermocouples at the center position of the length direction of the thermal simulation initial sample for temperature measurement, and then completing the welding simulation experiment of the coarse-grained heat affected zone, CGHAZ as a first thermal cycle, and the critical coarse-grained heat affected zone, ICCGHAZ as a second thermal cycle according to S4, and each process is repeated 3 times.

8. The method for obtaining optimal low temperature impact toughness according to claim 1, characterized in that: Specifically, S6 is as follows: the welding sample obtained in S5 is further prepared into a 55mm×10mm×10mm standard impact sample, and a -40°C low-temperature impact test is completed on an impact testing machine, the impact energy under each process is measured, and the accuracy of obtaining the optimal impact toughness of CGHAZ and ICCGHAZ is determined.

9. The method for obtaining optimal low temperature impact toughness according to claim 7, characterized in that: The critical coarse-grained heat-affected zone of welding in S5 obtains a complex phase structure of lath bainite with high-density and large-angle grain boundaries and discontinuous blocky MA divided by intracrystalline blocks along the austenite grain boundaries.

10. A welded body, characterized in that: The weld body is obtained after the welding process is determined by the method described in one of claims 1 to 9. In the welding of 550MPa grade marine engineering steel, the average impact energy of ICCGHAZ at -40°C of the weld body can be increased from 23.8J to 51.4J or above, and the average impact energy of CGHAZ at -40°C reaches more than 170J.

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