A method for predicting abnormal fluctuation of low-temperature toughness of container steel core

Abstract

The invention discloses a method for predicting abnormal fluctuation of low-temperature toughness of a container steel core, which relates to the technical field of steel production. A hardness test is performed on a normal structure at 1 / 4 of the thickness of a normalized plate, and the arithmetic mean value of the hardness of each point is obtained as HV0. Then, the hardness of N points is tested at 1 / 2 of the plate thickness at intervals of 5 mm in the plate width direction, wherein points with hardness values ​​higher than 1.2HV0 are called abnormally high hardness points, and the number is assumed to be n. When the n / N value is greater than 0, it indicates that a bainite structure with high hardness exists in the core of the steel plate due to a high Mn content, which will cause abnormal fluctuation of its low-temperature impact toughness. The larger the n / N value is, the more bainite structures there are in the core of the steel plate, and the more significant the abnormal fluctuation of its low-temperature impact toughness will be.

Description

Technical Field

[0001] The invention relates to the technical field of steel production, and in particular to a method for predicting abnormal fluctuation of low-temperature toughness of a core of container steel. Background Art

[0002] Low-temperature container steel is mainly used to build low-temperature storage tanks such as liquefied petroleum gas. With the large-scale equipment and the improvement of equipment safety standards, on the one hand, the use of thick steel plates (30-80mm) has gradually increased. On the other hand, users have also put forward more stringent standards for the low-temperature impact toughness of steel plates at -40--60℃, for example, the impact toughness requirements at the 1 / 2 thickness position have been increased. At present, low-cost C-Mn series low-temperature container steel plates have been widely used. The composition range of its main elements is: 0.12%-0.16% C, 0.20%-0.40% Si, 1.3%-1.7% Mn, and it is delivered by normalizing process. However, it is often found in production that after the thick steel plate is heat treated by normalizing, the low-temperature impact value at the 1 / 2 thickness position will be seriously unstable, such as discrete data and abnormal fluctuations, which makes it difficult to meet the stringent technical requirements of users and also affects the full play of the low-temperature performance of the steel plate.

[0003] The main factors affecting the low-temperature impact toughness of steel plates include the inclusion content and abnormal structure caused by element segregation. With the technological progress of iron and steel metallurgy, the control of S, P content and inclusion content in molten steel has reached a high level. On the other hand, the steel ingot solidifies from the outside to the inside, and the enrichment of alloy elements in the center of the ingot, i.e., center segregation, is inevitable, and this segregation is difficult to eliminate through subsequent rolling and conventional heat treatment. Based on the above reasons, it is generally believed that for normalized steel plates for C-Mn low-temperature containers, the main factor affecting the low-temperature impact toughness at the 1 / 2 thickness position is Mn segregation. However, the quantitative relationship between the degree of Mn segregation and its low-temperature impact toughness has not been obtained so far. In fact, practice shows that for the same batch of hot-rolled steel plates, the degree of Mn segregation is the same, and factors such as normalizing process also have a significant effect on the low-temperature impact toughness at the 1 / 2 thickness position.

[0004] When testing the performance of normalized steel plates for C-Mn low-temperature containers, impact specimens (three per group) are usually taken at the 1 / 2 thickness position for testing. If abnormal fluctuations in the impact value are found, the organization and composition of the area near the low impact value fracture will be analyzed, and then the relevant process will be adjusted and tested again. This cycle is time-consuming and labor-intensive on the one hand, and on the other hand, only relevant information of the local area near the fracture can be obtained. It is unknown whether the low-temperature impact value of the wide range area at the 1 / 2 thickness position has abnormal fluctuations, which will bring hidden dangers to the safety of the product. For this reason, there is an urgent need for a simple, efficient and reliable prediction method for whether the low-temperature impact toughness of the core of the normalized steel plate of C-Mn container steel has abnormal fluctuations, so as to provide effective guidance for production process adjustment and performance optimization. Summary of the invention

[0005] The present invention aims at solving the above technical problems, overcomes the shortcomings of the prior art, and provides a method for predicting abnormal fluctuations in low-temperature toughness of a container steel core, comprising the following steps:

[0006] (1) Along the vertical direction of rolling, the cross section of the normalized steel plate is obtained by wire cutting. Samples are taken near 1 / 4 of the plate thickness and metallographic specimens are prepared. The cross-sectional structure is observed using an optical microscope.

[0007] (2) Select the normal structure in step (1), i.e., the ferrite + pearlite region, and measure the hardness of several points using a Vickers hardness tester to obtain the arithmetic mean HV0 of the hardness of each point;

[0008] (3) Samples are taken near the center of the plate thickness. After grinding, hardness tests are performed at a certain interval along the plate width at 1 / 2 of the plate thickness. A total of N points are tested for hardness. Points with hardness values ​​higher than 1.2HV0 are called abnormally high hardness points, indicating that a high-hardness bainite structure is formed at this point due to a high Mn content.

[0009] (4) The number of high hardness points in step (3) is n. When the n / N value is 0, it indicates that there is no bainite structure in the core of the normalized steel plate, and its low-temperature impact toughness will not show abnormal fluctuations. When the n / N value is greater than 0, it indicates that there is bainite structure in the core of the steel plate, and its low-temperature impact toughness will show abnormal fluctuations. The larger the n / N value, the more bainite structure in the core of the steel plate, and the more significant the abnormal fluctuation of its low-temperature impact toughness.

[0010] The technical solution further defined in the present invention is:

[0011] In the aforementioned method for predicting abnormal fluctuation of low-temperature toughness of the core of a container steel, in step (1), the chemical composition of the steel plate is as follows: C: 0.12% to 0.16%, Si: 0.20% to 0.40%, Mn: 1.3% to 1.7%, P≤0.010%, S≤0.005%, Nb: 0.015% to 0.020%, Ti: 0% to 0.015%, V: 0% to 0.020%.

[0012] In the aforementioned method for predicting abnormal fluctuations in low-temperature toughness of the core of container steel, in step (2), the method for measuring hardness with a Vickers hardness tester is: the load is 5000 gf, and the holding time is 10 s.

[0013] In the aforementioned method for predicting abnormal fluctuations in low-temperature toughness of the core of container steel, in step (2), the hardness of 8 to 10 points is measured.

[0014] In the aforementioned method for predicting abnormal fluctuations in low-temperature toughness of the core of container steel, in step (3), N is 15 to 20.

[0015] In the aforementioned method for predicting abnormal fluctuations in low-temperature toughness of the core of container steel, in step (3), hardness testing is performed every 5 mm.

[0016] The beneficial effects of the present invention are:

[0017] ① The essential reason for the abnormal fluctuation of low-temperature impact toughness in the core of normalized steel plate of C-Mn container steel is that a high-hardness bainite structure is formed there, and the distribution is uneven. The traditional metallographic inspection method requires sampling from the cross section of a thick steel plate, grinding and polishing, and observing whether bainite exists with an optical microscope. The method is very cumbersome and costly, and usually can only inspect the metallographic structure of a local range (generally less than 10mm), and it is difficult to conduct a large-scale inspection. The present invention can effectively judge whether there is bainite in the core of the steel plate and the distribution state of bainite (corresponding to the distribution of abnormally high hardness points) in a large range (15 to 20 hardness test points correspond to a width range of 75 to 95mm, and hardness test points can be increased as needed to increase the detection range) by testing the hardness distribution of the core of the steel plate. Therefore, it can be predicted whether the low-temperature impact toughness of the core of the normalized plate has abnormal fluctuations and the strength of the abnormal fluctuations.

[0018] ② Whether bainite structure is formed or not is determined by multiple factors such as the segregation degree of Mn element in the core of the steel plate and the normalizing cooling rate (influencing factors include normalizing temperature, plate thickness and ambient temperature, etc.). The present invention uses a hardness test method to judge whether there is high-hardness bainite in the core of the normalized steel plate, so as to predict whether the low-temperature impact toughness of the core of the steel plate has abnormal volatility. This has an important guiding role in adjusting the alloy composition of C-Mn container steel, optimizing the casting and rolling process and the normalizing heat treatment process to avoid the generation of bainite, so as to obtain stable low-temperature impact toughness in the core of the normalized steel plate.

[0019] ③ For the same batch of hot-rolled steel plates, the average hardness HV0 of the normal structure (ferrite + pearlite) at 1 / 4 of the thickness of the normalized steel plate is basically constant (with a weak correlation with the normalizing process) and can be used as known data (only needs to be tested once). Therefore, when adjusting the normalizing process and daily product quality inspection, it is only necessary to test whether there is an abnormally high hardness point (hardness value higher than 1.2HV0) at 1 / 2 of the plate thickness to predict whether the low-temperature impact toughness of the core of the normalized steel plate has abnormal volatility.

[0020] In summary, the present invention is simple, efficient and reliable, and plays an important guiding role in adjusting the alloy composition, optimizing the casting and rolling process and the heat treatment process of C-Mn series low-temperature container steel to obtain stable low-temperature impact toughness. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of CCT curve of C-Mn container steel;

[0022] Figure 2 The metallographic structure obtained by sampling at the high hardness point in the center of the steel plate in Example 1;

[0023] Figure 3 This is the metallographic structure obtained by sampling at the high hardness point in the center of the steel plate in Example 3. DETAILED DESCRIPTION

[0024] The mechanism by which the present invention can predict whether the low-temperature impact toughness of the core of the normalized steel plate of the C-Mn series container steel has abnormal volatility is:

[0025] ① The normalizing heat treatment structure is determined by the CCT curve of the steel grade and the cooling rate during normalizing. Figure 1 a in the figure is a typical CCT curve diagram of C-Mn container steel (Mn content is set to 1.5%). It can be seen that at the cooling rate V1, the structure is ferrite + pearlite, which is the normal normalized structure; when the cooling rate increases to V3, the structure is ferrite + bainite; the cooling rate V2 is the maximum critical cooling rate in which bainite does not appear during normalizing. Since Mn has a strong effect of stabilizing austenite, when the Mn content increases to 2.5%, the CCT curve will shift significantly to the right ( Figure 1 b). At this time, bainite structure will appear at cooling rate V1, and the bainite content at cooling rates V2 and V3 will continue to increase.

[0026] ② When bainite appears in the matrix, due to its high hardness (the hardness of the ferrite + pearlite structure of the normalized steel plate of C-Mn low-temperature container steel generally varies around 160HV, while the hardness of bainite can be as high as 335HV, see Figure 2 (a) In the figure, cracks can easily expand at the interface between the steel matrix and bainite, which will lead to a sharp decrease in the impact toughness of the steel.

[0027] ③ Due to the complexity of the solidification process of the ingot (the dendrite growth mode from outside to inside, the segregation of Mn between dendrites caused by solute segregation, and electromagnetic stirring, etc.), the segregation of Mn element is not uniform in the macroscopic (central area) and microscopic (segregation between dendrites), and during the normalizing heat treatment, the distribution of bainite structure is also uneven: for example, at the cooling rate V1, the area with normal Mn content (1.5%) will form a normal structure of ferrite + pearlite ( Figure 1 a in the figure), while in the local area where the Mn content is high (2.5%), an abnormal structure of ferrite + bainite will be formed ( Figure 1 b). During hot rolling of the ingot, the dendrites deform along the rolling plane, causing the Mn segregation area to be distributed in a non-uniform band shape in the center of the cross section. Therefore, the bainite structure of the steel plate after normalizing treatment is also distributed in a non-uniform band shape (see Figure 2 a and Figure 3 ).

[0028] ④ The non-uniform distribution of bainite is the fundamental reason for the abnormal fluctuation of the low-temperature impact toughness of the core of the normalized steel plate of C-Mn container steel: the impact toughness of the normal structure (ferrite + pearlite) is good, and the impact toughness of the area with bainite is seriously reduced. Bainite has high hardness and can be characterized by hardness test. Considering the non-uniform distribution of bainite structure in the core of the steel plate, multiple hardness tests over a sufficiently long distance in the width direction of the core of the steel plate must be performed to determine its existence and distribution state.

[0029] ⑤ In actual hardness testing, the hardness of normal normalized structure (ferrite + pearlite) generally fluctuates within the range of HV0±5%. The hardness of bainite is about 1.5 to 2 times that of normal normalized structure, which is related to its morphology and distribution state (see Figure 2 a and Figure 3 ). Considering that the hardness tester indentation often contains both bainite and normal tissue during actual testing, the present invention refers to the test points with hardness values ​​higher than 1.2HV0 as abnormally high hardness points, which is attributed to the existence of high-hardness bainite tissue and is reasonable. Example

[0030] (1) Preparation of normalized steel plates for C-Mn low temperature container steel (SA537L1 steel)

[0031] The chemical composition (wt%) of the steel is: C: 0.16%, Si: 0.30%, Mn: 1.70%, P: 0.009%, S: 0.004%, Cr: 0.040%, Ni: 0.043%, Nb: 0.015%, Ti: 0.010%, V: 0.018%, and the balance is Fe. The steel plate with a thickness of 80 mm was obtained by continuous casting and hot rolling. The normalizing process is: the hot-rolled steel plate is heated to 910°C and kept warm for 30 minutes, and then naturally cooled in the air at an ambient temperature of 20°C.

[0032] (2) Obtain metallographic specimens at 1 / 4 thickness and perform hardness tests

[0033] The cross section of the steel plate was obtained by wire cutting along the vertical direction of the steel plate rolling. The metallographic specimen was sampled near 1 / 4 of the plate thickness and the cross-sectional structure was observed by optical microscope, and it was found that it was composed of ferrite + pearlite. The hardness of 10 points was measured by Vickers hardness tester, the load was 5000gf, and the holding time was 10s. The hardness distribution range was 154.2~162.1HV, and the arithmetic mean HV0 of the hardness of each point was 158.3HV.

[0034] (3) Hardness test along the width of the plate at 1 / 2 of the thickness

[0035] Samples were taken near the thickness center of the steel plate section, and after grinding, hardness tests were performed every 5mm along the width direction at 1 / 2 of the plate thickness. A total of 20 points were tested for hardness, and 4 abnormally high hardness points (with hardness higher than 1.2HV0=190.0HV) were found, with hardnesses of 205.4HV, 323.4HV, 275.2HV and 250.7HV respectively, and the hardness of the remaining points was in the range of 152.4-163.2HV. It can be judged that a high-hardness bainite structure caused by high Mn content is formed at the high-hardness points, and the impact performance of the steel plate will fluctuate abnormally.

[0036] (4) Experimental verification

[0037] a. Take samples at high hardness points and prepare metallographic specimens. It is observed that there are banded abnormal tissues distributed on the matrix of ferrite + pearlite (normal tissue). The hardness test shows that the hardness of normal tissue is 158.1HV, while the hardness at the center of abnormal tissue is 335.3HV (see Figure 2 Scanning electron microscopy analysis showed that the abnormal tissue was distributed in blocks ( Figure 2 c), which is significantly different from normal tissue ( Figure 2b). Through EDS surface scanning analysis, the average Mn content in the abnormal structure is 2.5%. The above analysis shows that the abnormal structure is a typical blocky bainite with high hardness caused by local Mn segregation.

[0038] b. Three groups of impact toughness specimens (9 pieces) were randomly obtained at 1 / 2 of the plate thickness and tested at -50°C. It was found that the impact properties of 3 specimens were significantly lower (32J, 75J and 60J respectively), and the impact values ​​of the remaining 7 specimens were in the range of 125-179J, that is, the impact value of the core of the steel plate showed abnormal fluctuations. Example

[0039] (1) Preparation of normalized steel plates for C-Mn low temperature container steel (SA537L1 steel)

[0040] The Mn content of the steel in Example 1 is reduced to 1.40%. At the same time, the preheating time of the cast ingot before hot rolling is extended by 100 minutes compared with the actual time used in Example 1. The rest of the process is exactly the same as in Example 1.

[0041] (2) Obtain metallographic specimens at 1 / 4 thickness and perform hardness tests

[0042] The cross section of the steel plate was obtained by wire cutting along the vertical direction of the steel plate rolling. The metallographic specimen was sampled near 1 / 4 of the plate thickness and the cross-sectional structure was observed by optical microscope, and it was found that it was composed of ferrite + pearlite. The hardness of 10 points was measured by Vickers hardness tester, the load was 5000gf, and the holding time was 10s. The hardness distribution range was 153.6~162.5HV, and the arithmetic mean HV0 of the hardness of each point was 156.6HV.

[0043] (3) Hardness test along the width of the plate at 1 / 2 of the thickness

[0044] Samples were taken near the thickness center of the steel plate section, and after grinding, hardness tests were performed at 1 / 2 of the plate thickness and every 5 mm along the plate width. A total of 20 points were tested for hardness, and it was found that the hardness values ​​of all points were in the range of 151.6-163.8 HV, and there was no abnormally high hardness point. It can be judged that there is no bainite structure in the steel plate, and the impact performance of the steel plate will not fluctuate abnormally.

[0045] (4) Experimental verification

[0046] Three groups of impact toughness specimens (9 pieces) were randomly obtained at 1 / 2 of the plate thickness and tested at -50°C. The impact values ​​were all in the range of 155 to 184 J, and there was no abnormal fluctuation in the impact values. Example

[0047] (1) Preparation of normalized steel plates of C-Mn low-temperature container steel (Q370R steel)

[0048] The chemical composition (wt%) of the steel is: C: 0.13%, Si: 0.40%, Mn: 1.55%, P: 0.008%, S: 0.003%, Cr: 0.065, Ni: 0.033%, Nb: 0.018%, Ti: 0.015%, V: 0.010%, and the balance is Fe. The steel plate with a thickness of 4 mm was obtained by continuous casting and hot rolling. The normalizing process is: the hot-rolled steel plate is heated to 890°C and kept warm for 20 minutes, and then naturally cooled in the air at an ambient temperature of 10°C.

[0049] (2) Obtain metallographic specimens at 1 / 4 thickness and perform hardness tests

[0050] The cross section of the steel plate was obtained by wire cutting along the vertical direction of the steel plate rolling. The metallographic specimen was sampled near 1 / 4 of the plate thickness and the cross-sectional structure was observed by optical microscope, and it was found that it was composed of ferrite + pearlite. The hardness of 8 points was measured by Vickers hardness tester, the load was 5000gf, the holding time was 10s, and the hardness distribution range was 159.2~163.1HV, and the arithmetic mean HV0 of the hardness of each point was 162.7HV.

[0051] (3) Hardness test along the width of the plate at 1 / 2 of the thickness

[0052] Samples were taken near the thickness center of the steel plate section, and after grinding, hardness tests were performed at 1 / 2 of the plate thickness every 5 mm along the plate width direction. A total of 15 points were tested for hardness, and it was found that there were 2 abnormally high hardness points (their hardness was higher than 1.2HV0=195.2HV), with hardnesses of 221.6HV and 325.3HV respectively, and the hardness of the remaining points was in the range of 160.5-163.9HV. It can be judged that a high-hardness bainite structure caused by high Mn content was formed at the high hardness points, and the impact performance of the steel plate will fluctuate abnormally.

[0053] (4) Experimental verification

[0054] a. Take samples at high hardness points and prepare metallographic specimens. It is observed that there are banded abnormal tissues distributed on the matrix of ferrite + pearlite (normal tissue). The hardness test shows that the hardness of normal tissues is 163.2HV and 162.6HV, while the hardness at the center of abnormal tissues is 334.0HV and 259.0HV (see Figure 3 ). Through EDS surface scanning analysis, the average content of Mn in the abnormal structure is 2.4%. The above analysis shows that the abnormal structure is bainite with high hardness, which is caused by local Mn segregation.

[0055] b. Three groups of impact toughness specimens (9 pieces) were randomly obtained at 1 / 2 of the plate thickness and tested at -50°C. It was found that the impact performance of 2 specimens was low (42J and 58J respectively), and the impact values ​​of the remaining 7 specimens were in the range of 114-159J, that is, the impact value of the core of the steel plate showed abnormal fluctuations. Example

[0056] (1) Preparation of normalized steel plates of C-Mn low-temperature container steel

[0057] The Mn content of the steel in Example 3 is reduced to 1.30%, and the ambient temperature of normalizing cooling in Example 3 is increased to 30°C. The remaining processes are exactly the same as in Example 3.

[0058] (2) Obtain metallographic specimens at 1 / 4 thickness and perform hardness tests

[0059] The cross section of the steel plate was obtained by wire cutting along the vertical direction of the steel plate rolling. The metallographic specimen was sampled near 1 / 4 of the plate thickness and the cross-sectional structure was observed by optical microscope, and it was found that it was composed of ferrite + pearlite. The hardness of 8 points was measured by Vickers hardness tester, the load was 5000gf, the holding time was 10s, and the hardness distribution range was 157.2~161.5HV, and the arithmetic mean HV0 of the hardness of each point was 159.8HV.

[0060] (3) Hardness test along the width of the plate at 1 / 2 of the thickness

[0061] Samples were taken near the thickness center of the steel plate section, and after grinding, hardness tests were performed at 1 / 2 of the plate thickness and every 5 mm along the plate width. A total of 15 points were tested for hardness, and it was found that the hardness values ​​of all points were in the range of 156.8 to 161.7 HV, and there was no abnormally high hardness point. It can be judged that there is no bainite structure in the steel plate, and the impact performance of the steel plate will not fluctuate abnormally.

[0062] (4) Experimental verification

[0063] Three groups of impact toughness specimens (9 pieces) were randomly obtained at 1 / 2 of the plate thickness and tested at -50°C. The impact values ​​were all in the range of 144 to 165J, and there was no abnormal fluctuation in the impact values.

[0064] In addition to the above embodiments, the present invention may also have other implementation modes. Any technical solution formed by equivalent replacement or equivalent transformation falls within the protection scope required by the present invention.

Claims

1. A method for predicting abnormal fluctuations in low-temperature toughness of the core of container steel, characterized by: The following steps are involved: (1) Along the vertical direction of rolling, the cross section of the normalized steel plate is obtained by wire cutting. Samples are taken near 1 / 4 of the plate thickness and metallographic specimens are prepared. The cross-sectional structure is observed using an optical microscope. (2) Select the normal structure in step (1), i.e., the ferrite + pearlite region, and measure the hardness of several points using a Vickers hardness tester to obtain the arithmetic mean HV0 of the hardness of each point; (3) Samples are taken near the center of the plate thickness. After grinding, hardness tests are performed at a certain interval along the plate width at 1 / 2 of the plate thickness. A total of N points are tested for hardness. Points with hardness values ​​higher than 1.2HV0 are called abnormally high hardness points, indicating that a high-hardness bainite structure is formed at this point due to a high Mn content. (4) The number of high hardness points in step (3) is n. When the n / N value is 0, it indicates that there is no bainite structure in the core of the normalized steel plate, and its low-temperature impact toughness will not show abnormal fluctuations. When the n / N value is greater than 0, it indicates that there is bainite structure in the core of the steel plate, and its low-temperature impact toughness will show abnormal fluctuations. The larger the n / N value is, the more bainite structure is in the core of the steel plate, and the more significant the abnormal fluctuations of its low-temperature impact toughness are.

2. A method for predicting abnormal fluctuations in low-temperature toughness of a container steel core according to claim 1, characterized in that: In the step (1), the chemical composition of the steel plate is as follows: C: 0.12% to 0.16%, Si: 0.20% to 0.40%, Mn: 1.3% to 1.7%, P≤0.010%, S≤0.005%, Nb: 0.015% to 0.020%, Ti: 0% to 0.015%, and V: 0% to 0.020%.

3. The method for predicting abnormal fluctuation of low-temperature toughness of the core of container steel according to claim 1, characterized in that: In the step (2), the method for measuring the hardness with a Vickers hardness tester is: the load is 5000 gf and the load holding time is 10 s.

4. The method for predicting abnormal fluctuation of low-temperature toughness of the core of container steel according to claim 1, characterized in that: In the step (2), the hardness of 8 to 10 points is measured.

5. The method for predicting abnormal fluctuation of low-temperature toughness of the core of container steel according to claim 1, characterized in that: In the step (3), N is 15 to 20.

6. The method for predicting abnormal fluctuation of low-temperature toughness of the core of container steel according to claim 1, characterized in that: In the step (3), hardness testing is performed every 5 mm.

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