Method for analyzing cause of unqualified v-type impact toughness of hot-rolled sectional steel

Abstract

The application provides a method for analyzing the reason for unqualified impact toughness of hot-rolled sectional steel, which comprises the following steps: analyzing a batch of impact energy detection values, calculating the percentage of unqualified detection values, the average value of unqualified detection values and the average value of qualified detection values; and performing microscopic analysis and fracture analysis according to the comparison of the above values. The application has important significance for analyzing the reason for unqualified toughness of steel and guiding the production of high-toughness steel, and can shorten the analysis time of the reason for unqualified impact toughness of hot-rolled sectional steel to less than one day, which is efficient and accurate.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of process analysis and adjustment of steel metallurgical materials, and particularly relates to a method for analyzing the unqualified reason of low-temperature impact toughness of hot-rolled section steel. BACKGROUND

[0002] Toughness refers to the ability of a material to absorb deformation force when deformed, and the ability of absorbing energy and performing plastic deformation before the material breaks. In contrast to brittleness, the material has a large deformation before breaking, and the fracture surface often shows an extension deformation. The stress-deformation relationship is nonlinear, and the fracture energy consumed is large. The impact strength and the percentage of crystalline fracture are usually used to measure toughness. Toughness is the ability of a material to absorb energy during plastic deformation and fracture. The better the toughness, the less likely the material is to break in a brittle manner. Tough materials are relatively soft, have a large tensile elongation, high impact strength, and relatively small hardness, tensile strength, and tensile elastic modulus. Rigid materials have high hardness and tensile strength, and low elongation and impact strength. The tensile elastic modulus is large. The bending strength reflects the rigidity of the material. The greater the bending strength, the greater the rigidity of the material, and vice versa.

[0003] Impact toughness is an important indicator of low-temperature steel performance. The strength of steel materials is a relatively stable indicator of material performance, but impact toughness changes with temperature. In the steel industry, this value representing material toughness is defined as the ductile-brittle transition temperature, and the sudden and significant reduction in material toughness at a certain temperature is called brittle transition. This temperature represents the ductile-brittle transition temperature of steel. Brittle transition of steel materials can cause sudden failure of materials in the application industry, and even lead to serious accidents. Common accidents related to material toughness failure include low-temperature fracture of bridges, low-temperature fracture and collapse of wind turbine towers, and material toughness failure. However, the determination of the ductile-brittle transition temperature of materials is complex, so the impact energy value at a certain temperature becomes an indicator of material toughness at that temperature. The symbol is defined as A K . Because toughness is related to the low-temperature safety performance of steel structures such as buildings and equipment, it is highly valued by steel producers and users, especially components used in low-temperature environments.

[0004] The low temperature toughness of steel material is a very strict performance required for the material, and its qualification or not is closely related to the microstructure of the steel, the cleanliness of the molten steel, the sampling method of the impact sample, the processing method of the sample, the test control and other links, and is an important performance in the steel metallurgy industry throughout the whole link of material design, production processing and test inspection. In recent years, the low temperature toughness of steel has been particularly valued by the manufacturers due to the influence of the international trade relationship with the Nordic countries. However, because there are too many related factors, it has become a hot issue in the industry to find the key factors affecting the low temperature impact and to control them. In the samples with unqualified low temperature impact toughness, it is of great significance for the technical personnel to quickly find the analysis method and analyze the main reasons for the unqualified low temperature impact. SUMMARY

[0005] The purpose of the present application is to provide an analysis method for the unqualified reasons of the impact toughness of hot-rolled steel, which can reduce the traditional analysis time of 2-3 days to within 1 day through a set of effective analysis of the hot-rolled steel sample with unqualified low temperature impact test, can quickly find and determine the root cause of the unqualified impact performance, and can directly find the process cause of the unqualified impact performance and make corresponding process adjustment according to the optimized working path.

[0006] In order to achieve the above purpose, the technical scheme adopted by the present application is as follows:

[0007] An analysis method for the unqualified reasons of the impact toughness of hot-rolled steel, the analysis method comprising the following steps:

[0008] 1) Take a number of impact energy detection values, calculate the percentage of unqualified detection values, the average value of unqualified detection values and the average value of qualified detection values;

[0009] 2) If the percentage of unqualified detection values is less than 30%, and the difference between the average value of unqualified detection values and the average value of qualified detection values is greater than or equal to 50% of the average value of qualified detection values, then the unqualified sample of the batch of hot-rolled steel is subjected to the first fracture analysis method;

[0010] If the percentage of unqualified detection values is less than 30%, and the difference between the average value of unqualified detection values and the average value of qualified detection values is less than 50% of the average value of qualified detection values, then the unqualified sample of the batch of hot-rolled steel is subjected to the first microstructure analysis;

[0011] 3) If the percentage of unqualified detection values is 30%-60%, and the difference between the average value of unqualified detection values and the average value of qualified detection values is greater than or equal to 50% of the average value of qualified detection values, then the unqualified sample of the batch of hot-rolled steel is subjected to the second fracture analysis method;

[0012] If the percentage of unqualified test values is between 30% and 60%, and the difference between the average value of unqualified test values and the average value of qualified test values is less than 50% of the average value of qualified test values, then the unqualified sample of the batch of hot-rolled section steel is subjected to a second microscopic structure analysis;

[0013] 4) If the percentage of unqualified test values is greater than 60%, and the difference between the average value of unqualified test values and the product requirement value is less than 30% of the product requirement value, then the unqualified sample of the batch of hot-rolled section steel is subjected to a third microscopic structure analysis;

[0014] If the percentage of unqualified test values is greater than 60%, and the difference between the average value of unqualified test values and the product requirement value is greater than or equal to 30% of the product requirement value, it can be judged that the main reason for the unqualified impact energy of the material is that the design performance of the material does not meet the standard or the processing of the test block is not standardized.

[0015] First microscopic structure analysis:

[0016] The unqualified sample of the batch of hot-rolled section steel is subjected to microscopic detection of metallographic structure. If the steel grain size rating is less than 5, then the unqualified impact energy is caused by coarse grains and excessively high heating temperature. If the steel grain size rating is greater than or equal to 6, and the proportion of Widmanstatten structure abnormal to the matrix is between 10% and 20%, then the unqualified impact energy is caused by abnormal structure due to segregation or excessively fast cooling speed. If the grain size difference between a certain area and the matrix of most areas is between 1.5 and 2, then the unqualified impact energy is caused by mixed grains due to segregation or insufficient heating.

[0017] The improved process is: (1) reduce or eliminate Widmanstatten structure, extend the heating time at low temperature by more than 20 minutes to ensure complete remelting of segregated elements, and control the cooling speed after rolling to be less than 0.5℃ / S. (2) reduce the grain size difference: avoid rolling in the two-phase region, and ensure that the final deformation is greater than or equal to 10%.

[0018] Second microscopic structure analysis:

[0019] The unqualified sample of the batch of hot-rolled section steel is subjected to microscopic detection of metallographic structure. If the steel grain size rating is less than 5, then the unqualified impact energy is caused by coarse grains and excessively high heating temperature. If the steel grain size rating is greater than or equal to 6, and the proportion of Widmanstatten structure abnormal to the matrix is between 10% and 20%, then the unqualified impact energy is caused by abnormal structure due to excessively fast cooling speed. If the grain size difference between a certain area and the matrix of most areas is between 2 and 3, then the unqualified impact energy is caused by mixed grains due to insufficient heating.

[0020] The improved process is: (1) refining the grain: reducing the heating temperature by 20-30°C; (2) reducing or eliminating the widmanstatten structure, prolonging the heating time at the low-temperature heating section by more than 30 minutes to ensure complete remelting of the segregated elements, and controlling the cooling speed after rolling to be lower than 0.4°C / S; (3) reducing the grain size difference: avoiding rolling in the two-phase region while ensuring that the final pass deformation is greater than 15%.

[0021] The third microstructure analysis:

[0022] If the steel material of the batch of hot-rolled steel is unqualified, the metallographic microstructure is detected, and if the grain size rating is less than 5, the unqualified impact energy is caused by coarse grains and excessively high heating temperature; if the grain size rating is greater than or equal to 6 and the proportion of widmanstatten structure deviating from the matrix is greater than or equal to 30%, the unqualified impact energy is caused by abnormal structure due to excessively fast cooling speed; if the grain size difference between a certain area and the matrix in most areas is greater than or equal to 3.0, the unqualified impact energy is caused by mixed grains due to excessively high heating temperature and insufficient time.

[0023] The improved process is: (1) refining the grain: reducing the heating temperature by more than 30°C; (2) reducing or eliminating the widmanstatten structure, prolonging the heating time at the low-temperature heating section by more than 40 minutes to ensure complete remelting of the segregated elements, and controlling the cooling speed after rolling to be lower than 0.25°C / S; (3) reducing the grain size difference: avoiding rolling in the two-phase region while ensuring that the final pass deformation is greater than 20%.

[0024] If there is no grain size rating less than 5, no proportion of widmanstatten structure deviating from the matrix greater than or equal to 30%, and no grain size difference between a certain area and the matrix in most areas greater than or equal to 3.0, the third fracture analysis method is required.

[0025] The first fracture analysis method:

[0026] The unqualified sample of the batch of hot-rolled steel is detected by scanning electron microscopy of the impact fracture, and if the impact fracture is a cleavage fracture, the unqualified impact energy is caused by the existence of inclusions greater than 20μm in cross section; the inclusions greater than 20μm in cross section belong to non-steady-state slag entrapment during continuous casting, and the corresponding continuous casting billet should be rejected, downgraded or directly discarded as waste. If the impact fracture is a quasi-dissociation fracture, the unqualified impact energy is caused by the existence of inclusions with a size of 10-20μm in cross section, and the inclusions greater than 10-20μm in cross section are caused by erosion and falling of refractory materials during casting, which is occasional. According to the impact energy recheck rules, the same material is sampled and rechecked, and if it is qualified, the product is qualified, and if it is unqualified, the corresponding rolled material of the whole continuous casting billet is downgraded.

[0027] The second fracture analysis method:

[0028] If the impact fracture is quasi-cleavage fracture, the unqualified impact work is caused by the existence of a few inclusions with a size greater than 10 μm in the cross section; it indicates that the slag flows into the tundish at the end of the ladle, and the current continuous casting billet should be removed.

[0029] If the impact fracture is quasi-cleavage fracture, the unqualified impact work is caused by the existence of a few inclusions with a size greater than 10 μm in the cross section; it indicates that the slag flows into the tundish at the end of the ladle, and the current continuous casting billet should be removed.

[0030] If the impact fracture is quasi-cleavage fracture, the unqualified impact work is caused by the existence of a few inclusions with a size greater than 10 μm in the cross section; it indicates that the slag flows into the tundish at the end of the ladle, and the current continuous casting billet should be removed.

[0031] The third fracture analysis method is:

[0032] If the impact fracture is quasi-cleavage fracture, the unqualified impact work is caused by the existence of a few inclusions with a size greater than 10 μm in the cross section; it indicates that the slag flows into the tundish at the end of the ladle, and the current continuous casting billet should be removed.

[0033] If the impact fracture is quasi-cleavage fracture, the unqualified impact work is caused by the existence of a few inclusions with a size greater than 10 μm in the cross section; it indicates that the slag flows into the tundish at the end of the ladle, and the current continuous casting billet should be removed.

[0034] The present application determines the analysis method and analysis steps according to the numerical performance of the unqualified impact sample, determines the main reason for the brittle fracture of the material through the most short analysis process.

[0035] The present application mainly includes the judgment of the unqualified detection value of impact work, the metallographic analysis of steel, the micro-analysis of impact fracture and the optimal overall process.

[0036] For the judgment of the high and low values of the test and retest values and the distinction of the obvious degree of data differentiation, the difference between the numerical mean level and the extreme value. Specifically, it includes several characteristic values: the overall mean of the impact work value, the specific value of the unqualified detection value, the difference between the above two values, the number of unqualified values, the number of qualified values, and the percentage of unqualified values in the total value.

[0037] The grain size level is determined, the difference between the maximum value and the minimum value of the grain, and the control proportion of the special phase in the grain, specifically, the lowest level of the grain size level should be 5, the grain size difference is limited by 1.5, and the proportion of the phase different from ferrite and pearlite in the steel is limited by 10%.

[0038] For the distinction of impact fracture being cleavage fracture, quasi-cleavage fracture and ductile fracture, under the observation of high-power scanning electron microscope, the basis for judgment is whether the fracture texture belongs to cleavage plane shape, brittle river pattern, or ductile dimple shape and various shapes.

[0039] The present application has the following beneficial effects:

[0040] The present application can quickly judge the cause of unqualified impact value and find the mechanism, guide the inspection and production practice. BRIEF DESCRIPTION OF DRAWINGS

[0041] Figure 1 It is the framework diagram of the analysis method of the present application;

[0042] Figure 2 It is the framework diagram of the microstructure analysis of the present application;

[0043] Figure 3 It is the framework diagram of the fracture analysis method of the present application;

[0044] Figure 4 It is the electron microscope graph of example 1 of the present application;

[0045] Figure 5 It is the electron microscope graph of example 2 of the present application;

[0046] Figure 6 It is the metallographic structure graph of example 3 of the present application;

[0047] Figure 7 It is the electron microscope graph of example 4 of the present application. DETAILED DESCRIPTION

[0048] The technical solutions of the present application will be described in detail below in combination with the drawings and examples.

[0049] Because there are many influencing factors of impact test, when unqualified samples appear, analysis is carried out according to certain analysis ideas, the product qualification can be quickly judged, and basis is provided for subsequent judgment and production. The present application formulates an analysis method according to the following method through long-term analysis process of low-temperature impact performance. First, the following method is carried out in the data analysis stage, and the analysis process is shown in Figures 1-3 .

[0050] 1. Performance result data 30% or less unqualified and value is low, unqualified test value and the average value of qualified test value difference ≥ 50%, the batch of this unqualified sample should be analyzed by the first fracture analysis method, unqualified test value and the average value of qualified test value difference < 50%, the batch of test should be analyzed by the first microstructure analysis.

[0051] 2. Performance result data 30%-60% or less unqualified and value is low, unqualified test value and the average value of qualified test value difference ≥ 50%, the batch of this unqualified sample should be analyzed by the second fracture analysis method, the value and the average value of qualified test value difference < 50%, the batch of test should be analyzed by the second microstructure analysis.

[0052] 3. If the percentage of unqualified test value is greater than 60%, the average value of unqualified test value and the difference of product requirement value < 30% of product requirement value, the third microstructure analysis is performed on the unqualified sample of hot-rolled steel material; if the percentage of unqualified test value is greater than 60%, the average value of unqualified test value and the difference of product requirement value ≥ 30% of product requirement value, it can be judged that the main reason for the unqualified impact energy of the material is that the material design performance does not meet the standard or the test block processing is not standardized. The test block processing can be observed by a low-power body microscope that there is a clear crack at the bottom of the V-shaped notch, which is caused by the dulling of the tool during the notch processing.

[0053] Secondly, in the microstructure analysis, the following methods are used. The analysis process is shown in Figure 2

[0054] The first microstructure analysis: observe the grain size of the steel, the grain size rating is less than 5 levels, the unqualified impact energy is caused by coarse grain, the grain size rating is ≥ 6 levels, observe abnormal organization, the proportion of abnormal organization to the matrix is 10%-20%, which causes the unqualified impact energy. Secondly, observe the size difference between the grains, such as the grain size difference between a certain area and the matrix is 1.5-2 levels, which causes the unqualified impact energy caused by mixed crystal, the technical personnel can make corresponding adjustment from the rolling process.

[0055] The second microstructure analysis: observe the grain size of the steel, the grain size rating is less than 5 levels, the unqualified impact energy is caused by coarse grain, the grain size rating is ≥ 6 levels, observe abnormal organization, the proportion of abnormal organization to the matrix is 10%-20%, which causes the unqualified impact energy. Secondly, observe the size difference between the grains, such as the grain size difference between a certain area and the matrix is 2-3 levels, which causes the unqualified impact energy caused by mixed crystal, the technical personnel can make corresponding adjustment from the rolling process.

[0056] Third microstructure analysis: observe the grain size of the steel, the grain size rating is less than 5, the unqualified reason of impact energy is caused by the coarse grain, the grain size rating is greater than or equal to 6, observe abnormal organization, the proportion of abnormal organization in the matrix is greater than or equal to 30%, the unqualified reason of impact energy is caused by abnormal organization. Secondly, observe the size difference between the grains, if the grain size difference between a certain area and most of the matrix area is greater than or equal to 3.0, if there is no above phenomenon, then the third fracture analysis method is needed.

[0057] Third, in the present application, the fracture analysis is analyzed according to the following structure:

[0058] The first fracture analysis method: the fracture is cleavage fracture, the main reason for the unqualified impact energy is that there are more than 20 μm inclusions in the cross section, the fracture is quasi-cleavage fracture, the main reason for the unqualified impact energy is that there are inclusions with a size of 10-20 μm in the cross section.

[0059] The second fracture analysis method: the fracture is cleavage fracture, the main reason for the unqualified impact energy is that there are more inclusions with a size greater than 10 μm in the cross section, the fracture is quasi-cleavage fracture, the main reason for the unqualified impact energy is that there are fewer inclusions with a size greater than 10 μm in the cross section. The fracture is ductile fracture, the main reason for the unqualified impact energy is that there are many inclusions with a small size but a large number, which are full of dimples.

[0060] The third fracture analysis method: the fracture is cleavage fracture, which is brittle precipitate on the grain boundary or hydrogen exceeding the standard of the steel, the fracture is quasi-cleavage fracture, the main reason for the unqualified impact energy is that the trace amount of low melting point elements in the steel is rich in the grain boundary.

[0061] Example 1

[0062] The following is the Q345E-40℃ impact energy detection value, only one group of data is unqualified in 6 groups of data, according to the analysis method given by the present application, the first fracture analysis method should be used for analysis.

[0063] Impact work 1 Impact work 2 Impact work 3 Impact work 4 Impact work 5 Impact work 6 Required value 198 207 188 30 234 206 34

[0064] The sample of impact energy 4 is detected by scanning electron microscope, the electron microscope graph is as shown in Figure 4 , it can be seen from Figure 4 that the unqualified impact energy 4 fracture finds that there are more than 20 μm inclusions in the cross section. Through electron microscope identification, the inclusions are calcium aluminate, which is caused by unstable tundish liquid level during molten steel casting, and the subsequent unstable tundish liquid level is downgraded and rejudged.

[0065] Example 2

[0066] The following is the Q345E-40℃ impact energy detection value, only two groups of data in 6 groups of data are unqualified, according to the analysis method given by the present application, should be analyzed according to the second kind of fracture analysis method.

[0067] Impact work 1 Impact work 2 Impact work 3 Impact work 4 Impact work 5 Impact work 6 Required value 165 23 174 19 158 166 34

[0068] The sample of impact energy 2, 4 is detected by scanning electron microscope, and the electron microscope graph is shown in Figure 5 It can be seen from Figure 5 that the unqualified impact energy 4 fracture is found to have a large number of inclusions less than 10um in cross section. Through energy spectrum analysis, such inclusions are inclusions gathered caused by sulfide segregation. In production, the desulfurization process is strengthened, and the S in the steel is reduced to less than 0.006%, so that the impact energy is not qualified.

[0069] Example 3

[0070] The following is the Q235D-20℃ impact energy detection value, 3 groups of data in 6 groups of data are lower, which cannot meet the required value, according to the analysis method given by the present application, should be analyzed according to the second kind of microstructure analysis.

[0071] Impact work 1 Impact work 2 Impact work 3 Impact work 4 Impact work 5 Impact work 6 Required value 45 19 39 26 21 40 27

[0072] Select one sample of unqualified sample, carry out metallographic structure detection, as shown in Figure 6 It can be seen from Figure 6 that the unqualified impact energy metallographic phase produces mixed crystal organization greater than 2 level in rolling direction metallographic phase. In actual rolling process, the heating holding time is prolonged by 30 min, and the final pass compression ratio is adjusted to be greater than or equal to 15%. The impact energy is all qualified.

[0073] Example 4

[0074] The following is the 20MnSiV 0℃ impact energy detection value, 6 groups of data are low, according to the analysis method given by the present application, should be analyzed according to the third kind of fracture analysis method.

[0075] Impact work 1 Impact work 2 Impact work 3 Impact work 4 Impact work 5 Impact work 6 Required value 21 39 37 18 22 30 60

[0076] The sample of impact energy 4 is detected by scanning electron microscope, and the electron microscope graph is shown in Figure 7 It can be seen from Figure 7 that according to the analysis method given by the present application, the third kind of fracture analysis method is carried out, and it is observed that there is a crack at the bottom of V port of impact sample (in white line frame), which is the machining crack caused by dull machining tool. After reprocessing the sample, the retest is qualified.

[0077] The upper and lower limits of the process parameters (such as temperature, time, etc.) and the interval values of the present application can all achieve the method, and examples are not listed here.

[0078] The contents not described in detail in the present application can adopt the conventional technical knowledge in the art.

[0079] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present application and are not limiting. Although the present application has been described in detail with reference to the examples, those skilled in the art should understand that modifications or equivalent replacements to the technical solutions of the present application do not deviate from the spirit and scope of the technical solutions of the present application, and they should all be covered in the scope of the claims of the present application.

Claims

1. A method for analyzing the cause of unqualified impact toughness of hot-rolled section steel, the method comprising the following steps: 1) Taking a number of impact energy test values, calculating the percentage of unqualified test values, the average of unqualified test values and the average of qualified test values; 2) If the percentage of unqualified test values is less than 30%, and the difference between the average of unqualified test values and the average of qualified test values is greater than or equal to 50% of the average of qualified test values, then performing first fracture analysis on the unqualified samples of the batch of hot-rolled section steel; First fracture analysis: performing scanning electron microscope detection on the impact fracture of the unqualified samples of the batch of hot-rolled section steel, if the impact fracture is cleavage fracture, the cause of unqualified impact energy is the existence of inclusions greater than 20 μm in cross section; if the impact fracture is quasi-cleavage fracture, the cause of unqualified impact energy is the existence of inclusions with size between 10-20 μm in cross section; If the percentage of unqualified test values is less than 30%, and the difference between the average of unqualified test values and the average of qualified test values is less than 50% of the average of qualified test values, then performing first microstructure analysis on the unqualified samples of the batch of hot-rolled section steel; First microstructure analysis: performing metallographic micro-detection on the unqualified samples of the batch of hot-rolled section steel, if the steel grain size rating is less than 5, then the cause of unqualified impact energy is coarse grain; if the steel grain size rating is greater than or equal to 6, and the proportion of abnormal Widmanstatten structure in the matrix is 10%-20%, then the cause of unqualified impact energy is abnormal structure; if the difference in grain size between a certain area and the matrix is 1.5-2, then the cause of unqualified impact energy is mixed crystal, and the rolling process should be adjusted accordingly; 3) If the percentage of unqualified test values is between 30%-60%, and the difference between the average of unqualified test values and the average of qualified test values is greater than or equal to 50% of the average of qualified test values, then performing second fracture analysis on the unqualified samples of the batch of hot-rolled section steel; Second fracture analysis: performing scanning electron microscope detection on the impact fracture of the unqualified samples of the batch of hot-rolled section steel, if the impact fracture is cleavage fracture, the cause of unqualified impact energy is the existence of more inclusions with size greater than 10 μm in cross section; if the impact fracture is quasi-cleavage fracture, the cause of unqualified impact energy is the existence of less inclusions with size greater than 10 μm in cross section; if the impact fracture is ductile fracture, the cause of unqualified impact energy is the existence of more inclusions with small size but large quantity in cross section, which fill the dimples; If the percentage of unqualified test values is between 30%-60%, and the difference between the average of unqualified test values and the average of qualified test values is less than 50% of the average of qualified test values, then performing second microstructure analysis on the unqualified samples of the batch of hot-rolled section steel; The second microstructure analysis: the microstructure of the unqualified sample of the batch of hot-rolled steel is detected, if the grain size rating of the steel is less than 5, the unqualified impact energy is caused by coarse grain; if the grain size rating of the steel is greater than or equal to 6, and the proportion of abnormal widmanstatten structure in the matrix is 10%-20%, the unqualified impact energy is caused by abnormal structure; if the difference between the grain size of a certain area and the grain size of the matrix is 2-3 levels, the unqualified impact energy is caused by mixed crystal, and the rolling process is adjusted accordingly; 4) if the percentage of unqualified test values is greater than 60%, and the difference between the average value of unqualified test values and the required value of the product is less than 30% of the required value of the product, the third microstructure analysis is performed on the unqualified sample of the batch of hot-rolled steel; The third microstructure analysis: the microstructure of the unqualified sample of the batch of hot-rolled steel is detected, if the grain size rating of the steel is less than 5, the unqualified impact energy is caused by coarse grain; if the grain size rating of the steel is greater than or equal to 6, and the proportion of abnormal widmanstatten structure in the matrix is greater than or equal to 30%, the unqualified impact energy is caused by abnormal structure; if the difference between the grain size of a certain area and the grain size of the matrix is greater than or equal to 3.0 levels, the unqualified impact energy is caused by mixed crystal, and the rolling process is adjusted accordingly; if none of the above phenomena occurs, the third fracture analysis is required; The third fracture analysis: the impact fracture of the unqualified sample of the batch of hot-rolled steel is detected by scanning electron microscope, if the impact fracture is cleavage fracture, the unqualified impact energy is caused by brittle precipitates in grain boundary or excessive hydrogen in steel; if the impact fracture is quasi-cleavage fracture, the unqualified impact energy is caused by trace low-melting-point elements in grain boundary; If the percentage of unqualified test values is greater than 60%, and the difference between the average value of unqualified test values and the required value of the product is greater than or equal to 30% of the required value of the product, it can be judged that the main reason for the unqualified impact energy of the material is that the design performance of the material does not meet the standard or the processing of the test block is not standardized.

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