High manganese steel with high toughness and low density and production method thereof

Abstract

The invention belongs to the technical field of high-manganese steel, and discloses high-toughness low-density high-manganese steel and a production method thereof. The high manganese steel with high strength and toughness and low density comprises the following chemical components in percentage by weight: 0.65-1% of C, 0.01-0.05% of Si, 11-22% of Mn, less than or equal to 0.020% of P, less than or equal to 0.005% of S, 6-7% of Al, 0.55-0.8% of Cr and the balance of Fe and inevitable impurities. The high-manganese and high-aluminum component design is adopted, the density of the high-manganese steel is smaller than that of traditional high-manganese steel, the high-strength and light-weight requirements of the downstream industry are met, in the production process, the casting performance degradation phenomenon caused by the high aluminum content is avoided, formation of ferrite, carbide and other multi-phase structures is avoided, and finally uniform and fine austenite is obtained; therefore, the strength, plasticity and toughness are excellent.

Description

Technical Field

[0001] The present invention belongs to the technical field of high manganese steel, and particularly relates to a high-strength, tough and low-density high manganese steel and a production method thereof. Background Art

[0002] As one of the most important wear-resistant materials, high manganese steel is widely used. With the increasing demand for high strength and lightweight in downstream industries, for steel materials, not only high strength, toughness and high wear resistance are required, but also lightweight is required. Some downstream users achieve this by using high strength and thinner thickness. However, after the thickness is reduced, the stiffness of the component will be significantly affected. Therefore, some users hope to achieve weight reduction without reducing the thickness, and there is an urgent need for low-density steel.

[0003] To achieve the lightweight of high manganese steel, aluminum element is considered to be added to high manganese steel. After introducing aluminum, the density can be reduced, but improper regulation will sacrifice some mechanical properties, such as plasticity and impact toughness, resulting in cracking problems during continuous casting and quality problems such as opening and edge cracks during rolling; and aluminum is easy to react with oxygen to form alumina, which affects the surface quality and increases the difficulty of subsequent treatment; it may also promote the formation of multi-phase structures such as austenite, ferrite, and carbide. Since the hardness and toughness of different phases vary greatly, cracks and spalling are likely to occur during the wear process; it may also affect the precipitation behavior of carbide, resulting in uneven distribution of carbide and further reducing wear resistance.

[0004] In summary, although introducing aluminum into high manganese steel can bring the advantage of low density, it will also bring a series of problems, including the decline of strength and toughness, continuous casting and rolling quality problems, increased difficulty in tissue regulation, and increased production cost. Therefore, in practical applications, it is necessary to comprehensively consider the composition, process and performance requirements to achieve the best performance balance. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a high-strength, tough and low-density high manganese steel and a production method thereof in view of the deficiencies of the prior art. By adopting the composition design of high manganese and high aluminum, the density of the high manganese steel is less than that of traditional high manganese steel, meeting the requirements of high strength and lightweight in downstream industries. During the production process, the deterioration of casting performance caused by high aluminum content is solved, the formation of multi-phase structures such as ferrite and carbide is avoided, and finally uniform and fine austenite is obtained, so that the strength, plasticity and toughness are excellent.

[0006] To solve the technical problems proposed by the present invention, the present invention provides a high-strength, tough and low-density high manganese steel, and its chemical composition by weight percentage includes: C: 0.65 - 1%, Si: 0.01 - 0.05%, Mn: 11 - 22%, P ≤ 0.020%, S ≤ 0.005%, Al: 6 - 7%, Cr: 0.55 - 0.8%, and the balance is Fe and inevitable impurities.

[0007] Preferably, the chemical composition of the high-strength, tough and low-density high manganese steel by weight percentage includes: C: 0.75 - 0.90%, Si: 0.02 - 0.04%, Mn: 12 - 17%, P ≤ 0.018%, S ≤ 0.004%, Al: 6.3 - 6.8%, Cr: 0.65 - 0.78%, and the balance is Fe and unavoidable impurities.

[0008] In the above solution, the microstructure of the high-strength, tough and low-density high manganese steel is a homogeneous austenite structure; a very small amount of uniformly distributed fine carbides are precipitated at the austenite grain boundaries, with an area ratio ≤ 0.5% and a size ≤ 7 nm.

[0009] In the above solution, the density of the high-strength, tough and low-density high manganese steel is 6.5 - 7.4 g / cm 3 , the yield strength is 500 - 800 MPa, the tensile strength is 1000 - 1300 MPa, the elongation is 30 - 50%, the impact energy at -40 °C is 120 - 300 J, the initial Brinell hardness is 200 - 300, and under an impact load of 60 - 170 J, work hardening occurs due to dislocation multiplication, and the Brinell hardness increases to 400 - 700.

[0010] The present invention also provides a production method of the high-strength, tough and low-density high manganese steel, adopting a process route of smelting, continuous casting, direct charging, heating, rolling, online water toughening treatment, coiling, bell annealing, and transverse cutting, including the following steps:

[0011] 1) Continuous casting: The tundish superheat is 10 - 20 °C, the cooling water volume of the narrow side of the mold is 400 - 600 L / min, and the cooling water volume of the wide side is 4300 - 4600 L / min;

[0012] 2) Heating: The temperature of the preheating section is 600 - 850 °C, and the time is 30 - 50 min; the temperature of the heating section is 850 - 1180 °C, and the time is 70 - 90 min; the temperature of the soaking section is 1150 - 1180 °C, and the time is 30 - 50 min;

[0013] 3) Rolling: The cumulative reduction ratio of rough rolling is 70 - 80%, and the final rolling temperature of rough rolling is 1050 - 1070 °C; the cumulative reduction ratio of finish rolling is 50 - 60%, and the final rolling temperature of finish rolling is 840 - 900 °C;

[0014] 4) Online water toughening treatment: Immediately cool to the coiling temperature for coiling after exiting the finish rolling, the cooling rate is 100 - 150 °C / s, and the coiling temperature is 10 - 30 °C to achieve online solution treatment;

[0015] 5) Bell annealing: Reheat to 150 - 300 °C, hold for 3 - 9 h, and then air cool to 10 - 30 °C and cut transversely into finished products.

[0016] In the above solution, during the continuous casting process, electromagnetic stirring is provided in the mold, the secondary cooling zone, and the solidification end; among them, the current of the electromagnetic stirring in the mold is 250 - 290 A, and the frequency is 3 - 4 Hz; the current of the electromagnetic stirring in the secondary cooling zone is 500 - 650 A, and the frequency is 6 - 8 Hz; the current of the electromagnetic stirring at the solidification end is 700 - 900 A, and the frequency is 6 - 7 Hz.

[0017] In the above solution, the continuously cast billet is subjected to slow cooling by stacking, with a cooling rate of 1 - 20 °C / h, and slow cooling to above 560 °C.

[0018] In the above solution, an edge heating device or a heat preservation cover is used between the rough rolling outlet and the finish rolling inlet to reduce the edge temperature drop, so that the temperature difference between the edge and the middle of the intermediate billet between the rough rolling outlet and the finish rolling inlet is ≤ 30 °C.

[0019] In the above solution, the rough rolling is carried out by 7 - 9 passes of reversible rolling, and the finish rolling is carried out by multiple passes of rolling.

[0020] In the above solution, the first three passes of rough rolling adopt segmented descaling, with descaling for the head 3 - 5 m in each pass, and the descaling water for the remaining parts is turned off.

[0021] The design key points of the present invention in terms of chemical composition are as follows:

[0022] 1) Carbon is a strong austenite stabilizing element. Carbon and manganese cooperate to stabilize austenite and inhibit martensite transformation. The solid solution strengthening of carbon in austenite can significantly improve strength and hardness, enhance work hardening ability, and increase wear resistance. However, too high carbon content may lead to the precipitation of carbides (such as Fe3C) at grain boundaries or within grains, reducing plasticity and toughness, and may increase casting defects (such as shrinkage cavities, cracks). Considering comprehensively, the C weight percentage content is 0.65 - 1% is sufficient;

[0023] 2) Silicon is a strong deoxidizer, which can reduce the oxygen content in the molten steel and improve purity. In this application, a relatively high Al element is contained, which can reduce the demand for Si deoxidation, reduce the Si content and cost. Silicon is a ferrite forming element. When silicon is relatively high, it may promote the precipitation of ferrite or carbides, increasing the sensitivity to welding hot cracks. Too low silicon content may lead to grain coarsening, reducing the strength and toughness of the steel, weakening the work hardening ability of the steel, and reducing wear resistance. Considering comprehensively, the Si weight percentage content is preferably 0.01 - 0.05%;

[0024] 3) Manganese is a strong austenite stabilizing element, which can inhibit martensite transformation, significantly improve work hardening ability, and enhance wear resistance. Manganese has a relatively low affinity with carbon, which can reduce the precipitation of carbides and improve toughness. The density of manganese is lower than that of iron. Appropriately increasing the manganese content in cooperation with aluminum can further reduce the density and increase strength. Too high manganese content may lead to hot brittleness and increase the risk of hot rolling cracking. Considering comprehensively, the Mn weight percentage content is preferably 11 - 22%;

[0025] 4) The density of aluminum is much lower than that of iron, which can significantly reduce the density. Aluminum can inhibit grain growth, refine grains, has a relatively low affinity with carbon, can reduce carbide precipitation, and improve strength and toughness. Aluminum is a ferrite-forming element. Appropriate aluminum can improve the stability of austenite, but excessive aluminum may promote the precipitation of ferrite or carbides, which may lead to an increase in brittleness, a decrease in plasticity and toughness, and also increase casting defects such as shrinkage cavities and cracks. In addition, Al and Si elements cooperate to reduce the density, can also reduce the content of Si, and narrow the adverse effects of Si. Considering comprehensively, Al is 6-7%;

[0026] 5) Chromium has a relatively high affinity with carbon and can form stable carbides. The solid solution strengthening and carbide precipitation effects can significantly improve strength, hardness and wear resistance. Through the synergistic effects with elements such as manganese, aluminum, and carbon, the stability of austenite can be further optimized, and the work hardening ability, oxidation resistance, etc. can be improved. Manganese is a strong austenite stabilizing element, and chromium is a weak austenite stabilizing element. The synergistic effect of the two can further improve the stability of austenite and inhibit martensite transformation; aluminum has a lower density, and the synergistic effect with chromium can further reduce the overall density; carbon is a strong austenite stabilizing element, and the synergistic effect with chromium can further improve the stability of austenite. Excessive chromium may lead to the precipitation of carbides at grain boundaries and reduce toughness. Considering comprehensively, Cr is 0.55-0.8%;

[0027] 6) P and S are harmful impurity elements and need to be strictly controlled to avoid adverse effects on the structure and properties; phosphorus segregates at grain boundaries, reducing the grain boundary strength and increasing cold brittleness. Sulfur forms sulfides with elements such as manganese and iron, reducing the purity. The precipitation of sulfides at grain boundaries increases hot brittleness and causes cracking during hot working. The sulfides extend along the rolling direction during the rolling process, resulting in anisotropy and reducing the transverse properties. Therefore, the lower the contents of P and S, the better. Considering comprehensively, the contents of P and S in the steel are P≤0.020% and S≤0.005%.

[0028] The design key points of the production method of the present invention are as follows:

[0029] After introducing a relatively high content of Al into high manganese steel, the present invention is prone to the phenomenon of deteriorated casting performance, specifically manifested as a significant reduction in the fluidity of molten steel and a significant increase in the hot cracking tendency; at the same time, the mechanical properties are also abnormal, such as a decrease in toughness and fluctuations in strength. In response to this series of technical problems, the present invention controls the superheat of the tundish within the range of 10-20 °C during continuous casting to reduce the solidification shrinkage stress; then, by controlling the cooling water volume of the mold, the stress concentration caused by uneven cooling is avoided; the electromagnetic stirring technology is adopted to optimize the internal flow state of the molten steel and improve the uniformity of the internal structure of the billet; after continuous casting, the billet is subjected to slow cooling treatment (such as stacking or covering) to reduce the thermal stress; subsequent precise heating and cooling uniformity control means are combined with the coil cover annealing process to adjust the internal temperature field of the material, promote the precipitation of fine carbides, further improve the hardness and wear resistance, and eliminate the internal stress and improve the dimensional stability, successfully overcoming the above technical bottlenecks.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] The present invention adopts the composition design of high manganese and high aluminum, making the density of high manganese steel less than that of traditional high manganese steel, meeting the requirements of high strength and lightweight in the downstream industry; then, through the design of the continuous casting process, the deteriorated casting performance caused by high aluminum content is solved, there is no cracking problem during continuous casting, and there are no quality problems such as openings and edge cracks during the rolling process; subsequent precise heating and cooling uniformity control means are combined with the coil cover annealing process to avoid the formation of multiphase structures such as ferrite and carbides, and finally obtain uniform and fine austenite, without the precipitation of continuous network carbides, and excellent strength, plasticity and toughness. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is the microstructural diagram of the high-strength, tough and low-density high manganese steel of Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0033] In order to better understand the present invention, the content of the present invention will be further clarified below in conjunction with the embodiments, but the content of the present invention is not limited to the following embodiments.

[0034] Examples 1-10

[0035] The chemical compositions and weight percentage contents of the steels in Examples 1-10 and Comparative Examples 1-2 are shown in Table 1.

[0036] Table 1

[0037]

[0038] The preparation methods of the steels in Examples 1-10 include the following steps:

[0039] 1) Continuous casting: The superheat of the tundish is 10 - 20 °C, the cooling water flow rate on the narrow side of the mold is 400 - 600 L / min, and the cooling water flow rate on the wide side is 4300 - 4600 L / min; electromagnetic stirring is set in the mold, secondary cooling zone, and solidification end; among them, the current of electromagnetic stirring in the mold is 250 - 290 A, and the frequency is 3 - 4 Hz; the current of electromagnetic stirring in the secondary cooling zone is 500 - 650 A, and the frequency is 6 - 8 Hz; the current of electromagnetic stirring at the solidification end is 700 - 900 A, and the frequency is 6 - 7 Hz; the continuously cast billet is slowly cooled by stacking, with a cooling rate of 1 - 20 °C / h, and slowly cooled to above 560 °C;

[0040] 2) Heating: The temperature of the preheating section is 600 - 850 °C, and the time is 30 - 50 min; the temperature of the heating section is 850 - 1180 °C, and the time is 70 - 90 min; the temperature of the soaking section is 1150 - 1180 °C, and the time is 30 - 50 min;

[0041] 3) Rolling: The rough rolling uses 7 - 9 passes of reversible rolling, the cumulative reduction ratio of rough rolling is 70 - 80%, and the final rolling temperature of rough rolling is 1050 - 1070 °C; in the first three passes of rough rolling, sectional descaling is adopted, descaling the head 3 - 5 meters for each pass, and the descaling water for the remaining parts is turned off; between the rough rolling exit and the finishing rolling entrance, an edge heating device or a heat preservation cover is used to reduce the edge temperature drop, so that the temperature difference between the edge and the middle of the intermediate billet between the rough rolling exit and the finishing rolling entrance is ≤ 30 °C; the finishing rolling uses 7 passes of rolling, the cumulative reduction ratio is 50 - 60%, and the final rolling temperature of finishing rolling is 840 - 900 °C;

[0042] 4) Online solution treatment: Immediately after the finishing rolling, it is cooled to the coiling temperature for coiling, with a cooling rate of 100 - 150 °C / s, and the coiling temperature is 10 - 30 °C, realizing online solid solution;

[0043] 5) Cover annealing: Reheat to 150 - 300 °C, keep warm for 3 - 9 h, and then air-cool to 10 - 30 °C, and cut transversely into finished products.

[0044] The specific process data of Examples 1 - 10 and Comparative Examples 1 - 2 are shown in Tables 2 - 5.

[0045] Table 2 Continuous casting parameters

[0046]

[0047] Table 3 Heating parameters

[0048]

[0049]

[0050] Table 4 Rolling parameters

[0051]

[0052] Table 5 Online water toughening treatment and cover annealing parameters

[0053]

[0054] The properties of the steels in Examples 1-10 and Comparative Examples 1-2 are shown in Table 6.

[0055] Table 6 Performance test results

[0056]

[0057] While reducing the density of high manganese steel, the present invention avoids the formation of multiphase structures. As Figure 1 shown, the structure of the high-strength, tough and low-density high manganese steel is homogeneous austenite, and a very small amount of uniformly distributed fine carbides are precipitated at the austenite grain boundaries, with an area ratio ≤ 0.5% and a size ≤ 7 nm, enabling the steel to obtain excellent strength, plasticity and toughness.

[0058] The above embodiments are merely examples clearly described and not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation manners here, and the obvious changes or modifications thus extended are still within the protection scope of the present invention.

Claims

1. A high-strength, low-density, high-manganese steel, characterized in that: Its chemical composition includes, by weight percentage: C: 0.65-1%, Si: 0.01-0.05%, Mn: 11-22%, P≤0.020%, S≤0.005%, Al: 6-7%, Cr: 0.55-0.8%, and the balance is Fe and unavoidable impurities.

2. The high-strength, low-density, high-manganese steel according to claim 1, characterized in that: The chemical composition of the high-strength, toughness, low-density high manganese steel includes, by weight percentage: C: 0.75-0.90%, Si: 0.02-0.04%, Mn: 12-17%, P≤0.018%, S≤0.004%, Al: 6.3-6.8%, Cr: 0.65-0.78%, and the remainder is Fe and unavoidable impurities.

3. The high-strength, low-density, high-manganese steel according to claim 1, characterized in that: The microstructure of the high-strength, low-density, high-manganese steel is a uniform austenite structure; a very small amount of uniformly distributed fine carbides are precipitated on the austenite grain boundaries, with an area ratio of ≤0.5% and a size of ≤7nm.

4. The high-strength, low-density, high-manganese steel according to claim 1, characterized in that: The density of the high-strength, low-density, high-manganese steel is 6.5-7.4 g / cm 3 , yield strength is 500-800MPa, tensile strength is 1000-1300MPa, elongation is 30-50%, -40℃ impact energy is 120-300J, initial Brinell hardness is 200-300, under 60-170J impact load, work hardening occurs due to dislocation multiplication, and the Brinell hardness increases to 400-700.

5. A method for producing high-strength, low-density, high-manganese steel as claimed in any one of claims 1 to 4, characterized in that: The following steps are involved: 1) Continuous casting: The superheat of the tundish is 10-20°C, the cooling water volume of the narrow side of the crystallizer is 400-600L / min, and the cooling water volume of the wide side is 4300-4600L / min; 2) Heating: preheating section temperature 600 ~ 850 ℃, time 30 ~ 50 min; heating section temperature 850 ~ 1180 ℃, time 70 ~ 90 min; soaking section temperature 1150 ~ 1180 ℃, time 30 ~ 50 min; 3) Rolling: Rough rolling cumulative reduction rate 70-80%, rough rolling final rolling temperature 1050-1070°C; Finishing rolling cumulative reduction rate 50-60%, finishing rolling final rolling temperature 840-900°C; 4) Online water toughening treatment: Cool to the coiling temperature immediately after finishing rolling, the cooling rate is 100-150℃ / s, the coiling temperature is 10-30℃, and online solid solution is achieved; 5) Hooding: Heat again to 150-300℃, keep warm for 3-9h, then air cool to 10-30℃, and cut into finished products.

6. The method for producing high-strength, low-density, high-manganese steel according to claim 5, characterized in that: In the continuous casting process, electromagnetic stirring is arranged in the crystallizer, the secondary cooling zone and the end of solidification; wherein, the current of the electromagnetic stirring in the crystallizer is 250-290A and the frequency is 3-4Hz; the current of the electromagnetic stirring in the secondary cooling zone is 500-650A and the frequency is 6-8Hz; the current of the electromagnetic stirring at the end of solidification is 700-900A and the frequency is 6-7Hz.

7. The method for producing high-strength, low-density, high-manganese steel according to claim 5, characterized in that: The ingots obtained by continuous casting are slowly cooled by stacking, with a cooling rate of 1 to 20° C. / h, and are slowly cooled to above 560° C.

8. The method for producing high-strength, low-density, high-manganese steel according to claim 5, characterized in that: Use edge heating device or heat preservation cover between rough rolling exit and finishing entrance to reduce edge temperature drop, so that the temperature difference between edge and middle of intermediate billet between rough rolling exit and finishing entrance is ≤30℃.

9. The method for producing high-strength, low-density, high-manganese steel according to claim 5, characterized in that: The rough rolling adopts 7 to 9 reversible rolling passes.

10. The method for producing high-strength, low-density, high-manganese steel according to claim 5, characterized in that: The first three passes of the rough rolling adopt segmented descaling, and the head 3 to 5 meters are descaled in each pass, and the descaling water of the remaining parts is closed.

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