Preface
Rolls are one of the most consumed parts in steel production and occupy an important position in metallurgical equipment manufacturing and steel production. Because of its low cost and good adaptability, high chromium cast iron is still one of the most widely used roll materials for hot strip mills. With the improvement of modern energy saving and environmental protection requirements, how to reduce the cost of raw materials has become the most concerned issue of roll enterprises. The advantage of iron and steel materials lies in their reusability. The use of scrap iron and steel can not only realize the utilization of solid waste, but also greatly reduce the cost of raw materials. The production of high chromium cast iron rolls generally adopts electric furnace smelting and centrifugal casting methods. Electric furnace smelting is a smelting method that can maximize the use of scrap iron and steel. The widespread use of scrap rolls and scrap iron and steel has become a common practice in roll manufacturers. However, due to different waste raw materials, the same smelting and pouring may result in different as-cast structures, which will affect the process performance and final structure and performance of the roll. In order to reduce the production cost of high-chromium cast iron rolls, we use a high-chromium cast iron wear-resistant plate (25% Cr) as the main raw material to replace Cr-Fe alloy to produce high-chromium cast iron rolls. However, in the subsequent heat treatment, it was found that although the final composition of the batch of rolls and ferroalloy production rolls is not much different, there are certain differences in the heat treatment process between the two, and the tendency of differential temperature quenching to crack increases, but the reason is not clear. Therefore, how to reasonably use waste high chromium cast iron should also deeply analyze the solidification structure of the two types of rolls and their influence on the transformation of the roll heat treatment process and the final structure and performance.
This paper compares and analyzes the solidification structure of two rolls of the same size produced by the same process: one is made of high-carbon sponge iron, a small amount of scrap steel and ferroalloy raw materials; the other is made of high-chromium cast iron wear-resistant plates as the main raw material. At the same time, the difference in the structure and properties of the two rolls treated by the same heat treatment process is studied. The purpose is to provide test data for the rational use of waste materials and the adjustment of the heat treatment of the cast rolls to ensure the performance of the rolls through the study of the differences in structure and process performance.
Test materials and methods
The test materials were cut and cut on two high chromium cast iron rolls of the same specification. Both rolls are produced using the same smelting and pouring process. The difference is: one uses sponge iron, ferroalloy, and a small amount of scrap steel as raw materials, called Roller A (Roller A); the other uses 58% waste high chromium cast iron The grinding plate is called Roller B. The final composition of the two rolls is shown in Table 1. The chemical composition of the two rolls is not much different. The biggest difference is that the carbon content of roll B is about 0.1% lower than that of roll A.
In order to study the difference in the heat treatment process between the two roll materials, the LINSEISL 78 dilatometer was used to measure the influence of the heating temperature and the cooling rate on the phase change of the cooling process. The sample size is a cylinder of 3 mm×10 mm; the austenitizing temperature is 1020 ℃, and the holding time is 0.5 h; the cooling rate is 10, 3, and 1 ℃/min, respectively. In order to simulate the influence of the actual heat treatment process on the structure and performance of the roll, the sample was quenched and tempered in a muffle furnace. The size of the sample was 10 mm × 10 mm × 8 mm. After the sample is austenitized at 1020 ℃ and held for 1 h, it is cooled to room temperature at a cooling rate similar to that of the actual roll quenching and cooling, and then tempered twice at 400, 450, 500 and 550 ℃ with a holding time of 10 h. After the sample was corroded by 5% nitric acid alcohol, the microstructure was observed with Axiovert 200 MAT optical microscope; the carbide morphology of the sample was observed with S-3400 thermal field emission scanning electron microscope (SEM); D/MAX 2500 PC type X was used The content of retained austenite is measured by XRD; the hardness of each sample is measured by HR-150 C Rockwell hardness tester, and each sample is tested at least 5 times, and the average value is taken.
Test results--Solidification group of rolls
Figure 1 shows the optical microstructure and SEM microstructure of the two rolls. It can be seen from Figures 1 (a) and 1 (b) that the microstructures of the two rolls are not much different. They are both typical hypoeutectic cast iron structures, namely primary austenite dendrites and eutectic ledeburite. There are flaky strips and irregular massive carbides and austenite in the tensite. The XRD diffraction analysis results show (Figure 2) that the two rolls are mainly composed of α-Fe, γ-Fe, M 7 C 3 and a small amount of M 23 C 6 phase. According to the analysis of the microstructure and the characteristics of high chromium cast iron, the carbides in the eutectic ledeburite are M 7 C 3 type carbides mainly composed of Cr, the primary austenite phase and the austenite in the eutectic ledeburite The structure after cooling is martensite + retained austenite11. Because the carbide and retained austenite in eutectic ledeburite are difficult to distinguish under an optical microscope, the relative content of eutectic carbides in colored gold was used for quantitative analysis, as shown in Figure 1 (c) and 1 (d) . The volume fraction of eutectic M 7 C 3 type carbide of roll A without waste high chromium cast iron is 23.1%, while the volume fraction of eutectic M 7 C 3 type carbide of roll B added with high chromium cast iron is It is 25.4%. The difference in the amount of eutectic carbides also affects the room temperature structure of the primary austenite phase and the austenite phase in the eutectic zone.
Test results--The effect of cooling rate on M s point and hardness
Figure 3 shows the cooling curve of two high chromium cast iron roll materials after being austenitized at 1020 ℃ for 0.5 h and then cooled to room temperature at cooling rates of 10, 3, and 1 ℃/min. The two roll materials have no other transformations within the cooling rate range, only low-temperature martensite transformation, showing good hardenability. However, the Ms point of the two roll materials is slightly different. The Ms point of roll A is lower than that of roll B, as shown in Table 2. In addition, as the cooling rate decreases, the Ms point tends to increase. When the cooling rate is reduced from 10 ℃ / min to 1 ℃ / min, the Ms point of roll A increases from 263 ℃ to 327 ℃; roll B increases from 301 ℃ to 346 ℃. Different Ms points also have a certain influence on the hardness, as shown in Table 2. The hardness of roll A is higher than that of roll B, and with the decrease of the cooling rate, the hardness also decreases slightly, but the overall hardness remains above 58 HRC.
Test results--Simulate the structure and hardness of the roll after thermal quenching
Figure 4 shows the structure of the two rolls after quenching and cooling. Compared with the as-cast structure (Figures 1a and 1b), the structure of the ledeburite eutectic zone of the two rolls has little change, but how can the structure of the primary austenite zone change significantly (Figures 4a and 4) c). The structure of the primary austenite zone of roll A is composed of a large number of fine carbides and martensite matrix (Figure 4a); while the amount of carbides in the primary austenite zone of roll B is significantly reduced and coarsened (Figure 4c). This is confirmed by SEM observations (as shown in Figures 4b and 4d). There are two types of carbide particles in the primary austenite zone of roll A: one is larger in size and rod-shaped, and the other is fine. The granular shape of the carbide (Figure 4b); while the amount of carbide particles in roll B is less, the granular carbide particles and the short rod-shaped particles composed of the particles (Figure 4d). Compared with the as-cast structure (Figures 1e and 1f), the shuttle-shaped carbides in roll A disappeared, and the size of some carbides increased significantly, and the number tended to increase (compare Figure 1e and Figure 4b); The fine carbides of roll B disappeared completely and turned into larger carbides (compare Fig. 1f and Fig. 4d). The XRD analysis results show that the phase structure of the roll after simulated differential temperature quenching does not change much, mainly α-Fe, γ-Fe, M 7 C 3 type carbides and a small amount of M 23 C 6 type carbides, that is, the structure is Martensite Body and retained austenite and carbides. Combined with the results of the as-cast structure analysis, the retained austenite is mainly distributed in the eutectic austenite region and around the eutectic carbides. The hardness analysis results show that the hardness is slightly improved after quenching compared with the as-cast state. The hardness of roll A is increased from 59.0 HRC to 61.7 HRC; the hardness of roll B is increased from 59.4 HRC to 62.5 HRC.
Test results--Structure and hardness after tempering
Figures 5 and 6 show the microstructures of the two rolls treated by different tempering processes. The changes of the structure of the two rolls with the tempering temperature are basically the same. Under the condition of one tempering (Figure 5a~5d and Figure 6a~6d), when the tempering temperature is 400 ℃, there is basically no obvious change in the structure, but the austenite in the eutectic ledeburite Fine carbide particles precipitate in the zone; when the temperature reaches 450 ℃, the matrix of the primary austenite zone is tempered, and the quenched structure characteristics disappear; the austenite in the eutectic ledeburite also undergoes significant transformation, and the structure transformation is similar to quenching organization. Combined with the analysis results of the as-cast and quenched structure, there is a large amount of retained austenite in the eutectic austenite zone. Carbides are precipitated from the retained austenite during the tempering and holding process, which reduces the stability of the retained austenite and the tempering cooling process The retained austenite is transformed into martensite. When the tempering temperature is higher than 500 ℃, the structure of the austenite zone in the ledeburite is transformed into a structure similar to that of the primary austenite zone. After the secondary tempering (Figure 5e～5h and Figure 6e～6h), under the same temperature conditions, the tempering is more sufficient, but overall, at a temperature of 400 ℃, the secondary tempering structure changes still Incomplete; when the temperature is higher than 450 ℃, the retained austenite can be completely decomposed after tempering. Comparing the two rolls, the stability of retained austenite and martensite of roll B is higher than that of roll A. Under one tempering condition, when the temperature reaches 500 ℃, the quenched retained austenite in the eutectic zone of roll B can be basically transformed (Figure 6c). Under two tempering conditions, when the tempering temperature is 450 ℃, The retained austenite in the structure can be completely transformed, which is confirmed in the XRD analysis.
Test results--Structure and hardness after tempering
Figures 5 and 6 show the microstructures of the two rolls treated by different tempering processes. The changes of the structure of the two rolls with the tempering temperature are basically the same. Under the condition of one tempering (Figure 5a~5d and Figure 6a~6d), when the tempering temperature is 400 ℃, there is basically no obvious change in the structure, but the austenite in the eutectic ledeburite Fine carbide particles precipitate in the zone; when the temperature reaches 450 ℃, the matrix of the primary austenite zone is tempered, and the quenched structure characteristics disappear; the austenite in the eutectic ledeburite also undergoes significant transformation, and the structure transformation is similar to quenching organization. Combined with the analysis results of the as-cast and quenched structure, there is a large amount of retained austenite in the eutectic austenite zone. Carbides are precipitated from the retained austenite during the tempering and holding process, which reduces the stability of the retained austenite and the tempering cooling process The retained austenite is transformed into martensite. When the tempering temperature is higher than 500 ℃, the structure of the austenite zone in the ledeburite is transformed into a structure similar to that of the primary austenite zone. After the secondary tempering (Figure 5e～5h and Figure 6e～6h), under the same temperature conditions, the tempering is more sufficient, but overall, at a temperature of 400 ℃, the secondary tempering structure changes still Incomplete; when the temperature is higher than 450 ℃, the retained austenite can be completely decomposed after tempering. Comparing the two rolls, the stability of retained austenite and martensite of roll B is higher than that of roll A. Under one tempering condition, when the temperature reaches 500 ℃, the quenched retained austenite in the eutectic zone of roll B can be basically transformed (Figure 6c). Under two tempering conditions, when the tempering temperature is 450 ℃, The retained austenite in the structure can be completely transformed, which is confirmed in the XRD analysis.
Analysis and discussion
It can be seen from the above test results that although the two rolls in this test have little difference in chemical composition (Table 1), only the carbon and chromium content of roll A is slightly higher than that of roll B, which is about 0.1% higher. However, due to the raw material The difference causes a slight difference in the solidification structure of the two rolls (Figure 1 and Figure 2), and this structural difference affects the transformation of the roll during the heat treatment process (Figure 3) and the final structure (Figure 4 to Figure 8) And performance (Table 3). Figure 9 shows the quasi-equilibrium phase diagram of high chromium cast iron with 17.5% Cr calculated using the TCFE 7 database in the Thermal-Cale software. The solidification structure of high chromium cast iron containing 17.5% Cr is pro-eutectic primary austenite and ledeburite composed of eutectic M 7 C 3 and austenite. However, because roll B uses a large amount of waste hypereutectic high chromium cast iron wear-resistant plates, and because there are a large number of hypereutectic M 7 C 3 type carbides and eutectic ledeburite in the structure of the wear-resistant plate (Figure 10 ), in the smelting In the process, the dissolution of the eutectic and eutectic M 7 C 3 carbides requires a higher melting temperature and time to obtain a uniform liquid phase structure. Therefore, under normal smelting conditions, insufficient dissolution of the pro-eutectic M 7 C 3 carbides leads to an increase in the tendency of the uneven concentration of Cr and C in the liquid phase to fluctuate. In the subsequent cooling process, the degree of undercooling of the primary austenite and eutectic structure transformation is reduced, and the eutectic transformation is promoted, resulting in an increase in the amount of eutectic and eutectic M 7 C 3 carbides in the final structure (Figure 1) , Thereby reducing the carbon and alloying element content of austenite in the primary austenite and co-ledite, and the stability of austenite austenite is reduced. In the subsequent cooling process, a small amount of pearlite appears on the roll B (Figure 1 b), and the amount of retained austenite is reduced (Figure 2). The difference in alloys such as C and Cr in the primary austenite also affects the precipitation of secondary carbides during the cooling process. The primary austenite of roll A has two kinds of carbides, M 7 C 3 and granular M 23 C 6, while the primary austenite of roll B has only one kind of granular M 23 C 6 carbide (Figure 1 e And 1 f ). According to the equilibrium phase diagram of high chromium cast iron (Figure 9), during the cooling process after solidification of the high chromium roll, secondary M 7 C 3 carbides should be precipitated; and in the subsequent ferrite region, only M 23 C 6 type Carbides precipitate, but under low C and Cr conditions, M 23 C 6 carbides precipitate in the austenite region during cooling. Related literature studies on 18% Cr hypoeutectic high chromium cast iron have also confirmed that there are two types of secondary carbides in the matrix: one is rhombic and rhombic M 7 C 3 type carbide; the other is face-centered cubic M 23 C 6 type carbide. Under certain conditions, both carbides can become precipitated phases, and this precipitation depends on the content of Cr and C in the matrix. Therefore, there are fusiform M 7 C 3 carbides and granular M 23 C 6 carbides in the primary austenite of roll A (Fig. 1 e ); while there are only mainly granular M 23 in the primary austenite of roll B C6 type carbide (Figure 1e), it is also confirmed that the primary austenite content of roll A is higher than that of roll B. The retained austenite of roll A is also higher than that of roll B after cooling (Figure 2). It is precisely because the raw materials affect the solidification structure of the roll, which affects the subsequent heat treatment process characteristics and the final structure and performance of the roll. During the quenching and heating process of high chromium cast iron, austenitization only occurs in the primary phase and eutectic austenite. The content of alloying elements such as C and Cr in austenite directly affects the transformation of the cooling process. The large amount of hypereutectic carbides in the solidification structure of roll B will inevitably lead to a decrease in the alloy content in austenite. Therefore, the Ms of roll B is higher than that of roll A, and the hardness is also lower than that of roll A. In addition, as the cooling rate decreases, the Ms point of the roll increases and the hardness decreases (Table 2), indicating that carbides precipitate during the cooling process. In the actual heat treatment process of the roll, due to the large size of the roll, the air cooling method is adopted, and the cooling rate of the roll is slow cooling, and when the surface is cooled to about 500 ℃, it directly enters the tempering furnace for heat preservation and then slowly cools.
During this process, supersaturated carbon and alloying elements in the austenite will precipitate, and the precipitation of carbides depends on the degree of supersaturation of the austenite. It can be seen from Figure 4 that after simulated quenching, there are two sizes of carbide particles in roll A, one is a larger block or short rod, and the other is small particles; roll B is mainly granular and The particles constitute chain-like carbides, and the number of carbides is significantly less than that of roll A. High chromium cast iron is in the austenitizing temperature range of austenite and M 7 C 3 equilibrium phase (Figure 9 ), the coarse eutectic M 7 C 3 carbides remain unchanged, but the fine carbides precipitated in the primary austenite The substance dissolves, and the M 7 C 3 carbide spheroidizes and grows up. In high chromium cast iron, there are mainly two kinds of carbides: M 7 C 3 type carbide and M 23 C 6 type carbide. The M 23 C 6 type carbides have lower interfacial energy and can be precipitated as transition phases of the M 7 C 3 carbides. Under low Cr and C conditions, the M 7 C 3 carbides in roll B are formed into chains of particles (Figure 4d). Due to the different contents of C, Cr and other alloying elements in the primary austenite of the two rolls, under the condition of simulating the low cooling rate of the roll, the roll A precipitates fine M 23 C 6 type carbides at low temperature, which causes the roll A to form two sizes of carbonization Objects (Figure 4 b). The quenched structure of high chromium cast iron is undissolved carbide, martensite and retained austenite. In the tempering process, the martensite is tempered, and the retained austenite is decomposed and transformed into martensite in the subsequent cooling process. When the tempering temperature is 400 ℃, due to the low tempering temperature, the quenched structure does not change much, only a small amount of carbides are precipitated in the matrix (Figure 8a and 8c), and martensite tempering mainly occurs (Figure 5a, 5e) And Figure 6a, 6e), the hardness is slightly reduced (Table 3). As the tempering temperature increases, the martensite is tempered and the retained austenite is decomposed (Figure 5b, 5f and Figure 6b, 6f), and fine carbides are precipitated. After tempering and cooling, the retained austenite The body transforms into martensite (Figure 7). The precipitation of carbides and the transformation of martensite produce secondary hardening to increase the hardness (Table 3). When the tempering temperature rises to 500 ℃, the martensite is fully tempered (Figure 5 c ～ 5 d, 5 g ～ 5 h and Figure 6 c ～ 6 d,6 g ~ 6 h), the retained austenite transforms (Figure 7), and the precipitated carbides begin to grow (Figure 8b)And 8 d) to reduce the hardness (Table 3).
Compared with quenching, the number of carbides in the primary austenite matrix is significantly increased, but the coarse block and rod-shaped carbides disappear, and the particles gradually become relatively uniform in size (Figures 8b and 8d). In the low temperature region, the stable phase of chromium carbides is M 23 C 6 type carbides (Figure 9). During the tempering process, on the one hand, M 23 C 6 type carbides, on the other hand, M 7 C 3 transforms to M 23 C 6. This transformation leads to the transformation of the larger M 7 C 3 carbides in the primary austenite matrix to the M 23 C 6 carbides, so that the coarse M 7 C 3 carbides disappear, and finally particles of relatively uniform size are formed in the structure. Shaped carbides (Figure 8 b and 8 d). According to the review, the addition of waste high-chromium cast iron wear-resistant plate rolls has a certain effect on the solidification structure and the structure transformation and performance during heat treatment. This effect is mainly related to the effect of adding high-chromium cast iron wear plate on the solidification structure. Therefore, In order to make reasonable use of high-chromium cast iron wear-resistant plates, it is necessary to thoroughly study the effect of addition amount and smelting temperature and time on the solidification structure.
1) The solidification structures of the two high chromium cast iron rolls are both primary austenite and ledeburite eutectic. The primary austenite and eutectic austenite transform into martensite and retained austenite in the subsequent cooling process. , Retained austenite is mainly distributed in the eutectic austenite and around the eutectic carbides. The raw materials have a certain influence on the solidification structure of the high chromium cast iron rolls. The amount of eutectic carbides in roll A is 23.1%; primary austenite The secondary carbides in the body matrix are shuttle-shaped and granular carbides; while the amount of eutectic carbides in roll B with added waste high chromium cast iron wear-resistant plates is higher than that of roll A, which is 25.4%, and the secondary carbides are Granular carbides, in addition there is a small amount of pearlite in the structure;
2) Under the same austenitizing conditions, the two rolls have a martensitic structure in the range of 10 ～ 1 ℃ / min, but the Ms point of roll A is lower than that of roll B, which is 20 ～ 40 ℃ lower;
3) The structure after quenching of the simulated roll is martensite, retained austenite and carbides. Compared with the as-cast state, the number and morphology of carbides in the primary austenite have changed significantly, and the number of fine carbides has been significantly reduced. However, there are certain differences between the two rolls. The carbides in the primary austenite of roll A are coarse rods and fine particles; while the carbides in the primary austenite with waste high chromium cast iron wear-resistant plate roll B are relatively large particles. , And the number is significantly less than that of roll A, compared with the as-cast state, the hardness after quenching is slightly increased, and the hardness of roll B is slightly higher than that of roll A;
4) After tempering, the two rolls both undergo secondary hardening at 450 ℃, but roll A appears in the primary tempering, and roll B appears in the secondary tempering. This phenomenon is related to the degree of analysis of retained austenite . During the tempering process, the coarse M 7 C 3 type carbides in the primary austenite matrix are transformed into fine M 23 C 6 type carbides. As the tempering temperature increases, the carbides aggregate and grow, and the two types of roll carbides The shape tends to be consistent.
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