Method for modifying, spheroidizing, refining and liquating TiN second-phase large-particle inclusions by using rare earth

Abstract

The invention discloses a method for modifying, spheroidizing, refining and liquating TiN second-phase large-particle inclusions by using rare earth, which comprises the following steps: adding trace rare earth Ce into economical Ti microalloyed (containing 0.050-0.100% by mass of Ti) hot-rolled high-strength steel to give play to the good chemical activity of the rare earth Ce in high-temperature molten metal, firstly modifying an oxygen product mixed with Al2O3 to be CeAlO3, and then modifying the oxygen product mixed with Al2O3 to be CeAlO3; ceAlO3 is used as a heterogeneous nucleating agent of TiN inclusions to form ellipsoidal CeAlO3-TiN composite inclusions, so that the purpose of modifying, refining and liquating large-particle TiN inclusions is achieved, and the toughness of the economical Ti microalloyed hot-rolled high-strength steel plastic is improved.

Description

Technical Field

[0001] The present invention belongs to the field of metallurgical material technology, and specifically relates to a method for utilizing rare earth to modify spheroidization and refinement of liquid TiN second phase large particle inclusions, and in particular to a method for utilizing rare earth element Ce to spheroidize and refine high temperature liquid large size TiN inclusions of Ti microalloyed high strength steel, which is helpful to improve the plastic toughness of Ti microalloyed high strength steel. The materials involved are mainly used in the engineering machinery industry, and the product thickness specification is 3.0 to 16.0 mm. The implementation of this method will further expand the processing and application field of Ti microalloyed high strength steel, and provide strong support for the development of economical high strength steel in my country. Background Art

[0002] As the demand for steel products gradually develops towards lightweight and long life, the gradual strengthening of products has become the direction of research and development of steel enterprises. However, with the increasing competitive pressure in the steel market and the decreasing profits of products, how to reduce the production cost of products while ensuring product quality has become the research and development theme of various steel mills. After more than 20 years of research and practice, the use of (Nb, V, Ti) microalloy strengthening on the basis of the composition of ordinary carbon steel has made its products have high strength and high toughness characteristics, which has been trusted by the majority of users. However, with the continuous changes in the prices of microalloy elements, at present, Ti alloys with cost advantages have gradually become the main direction of component optimization and cost reduction for various steel enterprises. The characteristics of Ti in strong carbide formation in steel are utilized to exert the precipitation strengthening effect of TiC and improve the strength and toughness of the material. However, with the increase of Ti alloy, the size of the multi-angular TiN second phase of high-temperature liquid precipitation increases accordingly, and it is easy to concentrate in the thickness center of the finished product, resulting in reduced plastic toughness and weldability of the product, which seriously affects the cold forming performance. Based on this, the active chemical properties of rare earth Ce in molten steel are used to achieve the role of modification, refinement and spheroidization of high-temperature liquid precipitation TiN second phase, which is of great significance to improve the cold formability and weldability of high-Ti microalloyed high-strength steel.

[0003] Patent CN 109252087 A discloses "alloy process for adding Ce to bearing steel to inhibit the formation of TiN composite inclusions". By adding rare earth Ce (0.15-0.75 kg / (ton of steel)) to bearing steel, MgAl2O4 in the steel is modified into a specific type of rare earth oxide (Ce2O3) or rare earth oxysulfide (Ce2O2S), which inhibits the heterogeneous nucleation and precipitation of TiN on MgAl2O4 during solidification; the generated rare earth oxide or rare earth oxysulfide is small in size and has a hardness much lower than that of MgAl2O4 inclusions; the obtained single TiN inclusions are small in size and evenly distributed, which can improve the fatigue life of bearing steel. The present invention adds a certain amount of rare earth (Ce) alloy, first Ce combines with the deoxidation product Al2O3 to form CeAlO3, and uses CeAlO3 as a heterogeneous nucleating agent for TiN inclusions to form ellipsoidal CeAlO3-TiN composite inclusions, thereby achieving the purpose of modifying and refining the liquid precipitated large-particle TiN inclusions.

[0004] The study of the document "The Effect of Zirconium Content on Austenite Structure of High-Strength and Toughness Ti-Zr Microalloyed Steel" shows that zirconium, as an element of the same family and adjacent to titanium, has similar physical and chemical properties to titanium, and also has a strong chemical affinity for elements such as O, N, S, and C, and can refine and modify inclusions in titanium steel. The present invention adds a certain amount of rare earth (Ce) alloy, firstly modifies the oxygen product inclusion Al2O3 into CeAlO3, and uses CeAlO3 as a heterogeneous nucleating agent for TiN inclusions to form ellipsoidal CeAlO3-TiN composite inclusions, thereby achieving the purpose of modifying and refining the large-particle TiN inclusions in the liquid phase.

[0005] The study of the literature "Effect of rare earth Ce on TiN precipitation in Ti microalloyed steel" shows that the addition of 100ppm rare earth Ce promotes the refinement of the second phase TiN in the steel, plays a certain role in purifying the molten steel, and promotes the formation of spherical Ce-containing inclusions in the steel. Al2O3, MgO and CeO2 can serve as the nucleation core of TiN. As the N content increases, the number of TiN particles generated increases, the size increases, and a textured structure is generated at the central nucleation position. CeO2, TiO, MgO and Al2O3 are mixed and intertwined. A Ce content of 200ppm will promote the growth of TiN clusters, increase the number of large-sized inclusions in the steel, and be harmful to the performance of the steel. The article studies the addition of a high rare earth content of 100ppm to 200ppm. On the one hand, the industrial production of high rare earth content continuous casting process is easy to flocculate the water nozzle, resulting in discontinuous and unsmooth production; on the other hand, the heterogeneous nucleating agents that refine TiN inclusions are mainly Al2O3, MgO and CeO2. The present invention adds a certain content (10-30ppm) of rare earth (Ce) alloy and uses rare earth inclusion CeAlO3 as a heterogeneous nucleating agent for TiN inclusions to form ellipsoidal CeAlO3-TiN composite inclusions, thereby achieving the purpose of modifying and refining liquid precipitated large-particle TiN inclusions. Summary of the invention

[0006] Based on the above technical background, based on economical Ti microalloying (containing Ti mass percentage: 0.050-0.100%) hot-rolled high-strength steel, due to the large presence of large-particle, angular liquid-phase TiN inclusions, the plastic toughness of high-strength steel is reduced, limiting the scope of high-strength steel processing and application. The present invention adds a trace amount of rare earth Ce to give full play to its good chemical activity in high-temperature molten metal, firstly transforms the oxygen product inclusion Al2O3 into CeAlO3, and uses CeAlO3 as a heterogeneous nucleating agent for TiN inclusions to form ellipsoidal CeAlO3-TiN composite inclusions, thereby achieving the purpose of modifying and refining the large-particle liquid-phase TiN inclusions. Improve the plastic toughness of economical Ti microalloying hot-rolled high-strength steel. Industrial production application and evaluation have been achieved, and it has good promotion value.

[0007] The specific technical solutions of the present invention are as follows:

[0008] One aspect of the present invention provides a method for utilizing rare earth to modify, spheroidize and refine liquid TiN second phase large particle inclusions, which comprises adding a trace amount of rare earth Ce to economical Ti microalloyed (containing Ti mass percentage: 0.050-0.100%) hot-rolled high-strength steel to give full play to its good chemical activity in high-temperature molten metal, firstly modifying the oxygen product inclusion Al2O3 to CeAlO3, using CeAlO3 as a heterogeneous nucleating agent for TiN inclusions, forming ellipsoidal CeAlO3-TiN composite inclusions, thereby achieving the purpose of modifying and refining liquid TiN large particle inclusions, and improving the plastic toughness of the economical Ti microalloyed hot-rolled high-strength steel.

[0009] In some embodiments, the method comprises the following processes: a smelting and continuous casting process and a controlled rolling and controlled cooling process; wherein:

[0010] The smelting and continuous casting process is: molten iron → KR desulfurization → converter steelmaking → LF refining → RH degassing → slab continuous casting; the converter smelting adopts KR pre-desulfurized molten iron, and after the smelting, ferrosilicon and ferromanganese are used for deoxidation alloying, ferroniobium is added at the end of LF alloying, and ferrotitanium is added at the end of RH, and rare earth iron alloy (20% Ce-Fe) is added within 3 minutes before vacuum re-pressing, and the oxygen content in the molten steel is measured before adding the rare earth alloy to ensure that T[O] in the molten steel is ≤15ppm; the molten steel is blown with argon and stirred throughout the whole process; the vacuum degree is ≤2mbar, the processing time is ≥10min, and the continuous casting superheat is controlled by 5℃~10℃ higher than that in conventional steel, and its superheat ΔT is ≤35~45℃;

[0011] The controlled rolling and controlled cooling process is as follows: the heating temperature and coiling temperature of the Ti microalloyed high-strength steel are controlled during the hot rolling stage, the heating temperature is controlled at 1220-1270°C, and the heating time is 120-180 minutes; the coiling temperature is designed to be within the range of 560°C-600°C according to the product thickness.

[0012] In some embodiments, the chemical composition of the economical Ti microalloyed hot-rolled high-strength steel has a Ti content of 0.05-0.10% by mass, a rare earth Ce content of 0.0010%-0.0030%, and the gas in the steel is controlled to be H≤2ppm, O≤25ppm, and N≤50ppm.

[0013] In some embodiments, the chemical composition of the economical Ti microalloyed hot-rolled high-strength steel is calculated by mass percentage as follows: C 0.05-0.07%, Si 0.05-0.20%, Mn 0.60-1.60%, P≤0.015%, S≤0.005%, Als 0.020-0.040%, Nb 0.005-0.020%, Ti 0.050-0.100%, H≤2ppm, O≤25ppm, N≤50ppm, and the rest is Fe and unavoidable impurities.

[0014] In some embodiments, if the thickness h of the economical Ti microalloyed hot-rolled high-strength steel satisfies: 3.0 mm ≤ h ≤ 6.0 mm, the content of Mn is 0.60 - 0.90%.

[0015] In some embodiments, if the thickness h of the economical Ti microalloyed hot-rolled high-strength steel satisfies: 6.0 mm < h ≤ 12.0 mm, the content of Mn is 0.90 - 1.20%.

[0016] In some embodiments, if the thickness h of the economical Ti microalloyed hot-rolled high-strength steel satisfies: 12.0 mm < h ≤ 16.0 mm, the content of Mn is 1.20 - 1.60%.

[0017] On the other hand, the present invention provides an economical Ti microalloyed hot-rolled high-strength steel obtained by the above method.

[0018] The advantages of the present invention are as follows:

[0019] The present invention provides a manufacturing method for spheroidizing and refining large liquid-phase TiN particles by using trace (0.0010% - 0.0030%) rare earth Ce. The key is that the addition range of rare earth content enables industrial large-scale and stable continuous casting production, and the effect of rare earth modification on refining TiN is obvious, improving the inherent defect of "strong but not tough" of the economical Ti microalloyed high-strength steel, further expanding the application field of the product. Production practice has been achieved on a 2250 mm hot-rolling production line, and it has good promotion value. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Shows the morphology of TiN under SEM observation (Ti: 0.075%, N: 0.0040%, Ce: 0).

[0021] Figure 2 Shows the morphology of TiN-CeAlO3 under SEM observation (Ti: 0.072%, N: 0.0045%, Ce: 0.0015%).

[0022] Figure 3 Shows the morphology of TiN-CeAlO3 and compositional surface scanning under SEM observation (Ti: 0.060%, N: 0.0045%, Ce: 0.0020%).

[0023] Figure 4 Shows the morphology of TiN-CeAlO3 and compositional surface scanning under SEM observation (Ti: 0.085%, N: 0.0035%, Ce: 0.0030%).

[0024] Figure 5The morphology and composition surface scan of TiN-CeAlO3 under SEM observation (Ti: 0.090%, N: 0.0050%, Ce: 0.0025%) are shown.

[0025] Figure 6 The morphology and composition surface scan of the ultra-large TiN-CeAlO3 under SEM observation are shown (Ti: 0.090%, N: 0.0070%, Ce: 0.0020%).

[0026] Figure 7 The graph shows the change of TiN particle size of hot-rolled steel strip with a tensile strength of 700 MPa as the rare earth Ce is added (Ti: 0.085%, N: 0.0045%). DETAILED DESCRIPTION

[0027] The present invention designs a hot-rolled steel strip of engineering machinery high-strength steel and commercial vehicle beam steel with a C-Si-Mn-Ti (0.05% to 0.105) component system, and performs micro-alloying treatment by adding rare earth to achieve the effect of modifying and spheroidizing large-particle TiN inclusions, thereby improving the plasticity, toughness and fatigue resistance of the material.

[0028] The purpose of the present invention is to achieve through the following measures:

[0029] The smelting and continuous casting process is: molten iron → KR desulfurization → converter steelmaking → LF refining → RH degassing → slab continuous casting. KR pre-desulfurized molten iron is used for converter smelting. After smelting, ferrosilicon and ferromanganese are used for deoxidation alloying. Niobium iron is added at the end of LF alloying. After ferrotitanium is added at the end of RH, rare earth iron alloy (20% Ce-Fe) is added within 3 minutes before vacuum re-pressing. Before adding rare earth alloy, the oxygen content in the molten steel should be measured to ensure that T[O] in the molten steel is ≤15ppm. The molten steel is stirred with argon blowing throughout the process; the vacuum degree is ≤2mbar, and the processing time is ≥10min. The continuous casting superheat is controlled by 5℃~10℃ higher than that of conventional steel, and its superheat ΔT is ≤35~45℃.

[0030] Material composition control: For rare earth treated Ti microalloyed high-strength steel, the focus is on product composition design with Ti content of 0.05% to 0.10% and rare earth Ce of 0.0010% to 0.0030%. Gas in steel is controlled to be H≤2ppm, O≤25ppm, and N≤50ppm. Modification and refinement are carried out around liquid TiN.

[0031] Control of controlled rolling and controlled cooling process: The key control of heating temperature and coiling temperature in the hot rolling stage of Ti micro-alloyed high-strength steel is the heating temperature is controlled at 1220-1270℃, and the heating time is 120-180min; the coiling temperature is designed to be in the range of 560℃-600℃ according to the product thickness.

[0032] The present invention is described in detail below with specific examples. It should be understood that the examples are only for explaining the present invention, and are not intended to limit the content of the present invention.

[0033] 1. Material smelting

[0034] 1.1 Composition design: Taking the high-strength steel hot-rolled strip with a tensile strength of 700 MPa as an example, its composition design is shown in Table 1.

[0035] Table 1: Composition design (mass percentage)

[0036]

[0037] 1.2 Converter smelting: The pretreated molten iron (S≤0.002%) enters the converter, where oxygen is blown for decarburization and the temperature is raised. In the later stage of smelting, ferrosilicon and ferromanganese are added for deoxidation and alloying. The P and S components are controlled to prevent overoxidation of the molten steel and the amount of steel tapping is controlled to prevent slag from entering the molten steel. The tapping temperature is 1600-1650℃, and the converter tapping has [P]≤0.013% and [S]≤0.006%.

[0038] 1.3 Refining outside the furnace: Adopt LF+RH full-process argon blowing process, maintain good reducing atmosphere during the refining process, use Al wire for deoxidation, alloying in the later stage of LF, add titanium iron at the end of RH, and add rare earth iron alloy (20% Ce-Fe) within 3 minutes before vacuum re-pressing. Before adding rare earth alloy, measure the oxygen content in the molten steel to ensure that T[O] in the molten steel is ≤15ppm. The molten steel is stirred with argon blowing throughout the process; vacuum degree ≤2mbar, and the processing time is ≥10min.

[0039] 1.4 Slab continuous casting: The continuous casting superheat is controlled to be 5℃~10℃ higher than that of conventional steel, and its superheat ΔT≤35~45℃.

[0040] 1.5 Full nitrogen control: N≤50ppm in the tundish of molten steel, scrap ratio≤15%, converter tapping temperature≥1600℃, bottom blowing argon stirring in the converter, bottom blowing stirring mode with argon and nitrogen switching alternately cannot be used. The ladle is emptied with argon, argon is used to purge the tundish for ≥10min before continuous casting and heat exchange to exhaust the air in the ladle, and when the tundish pours 30t, the argon blowing tube is removed to reduce nitrogen absorption by molten steel, and argon blowing is used to protect the casting during the normal casting process.

[0041] 2. Controlled rolling and controlled cooling process

[0042] The ingot is heated in a walking-beam heating furnace (see Table 2 for the heating process). The rough rolling adopts single-stand R1 and R2 reciprocating rolling. The rough rolling modes adopted are 1+5, 3+3 and 3+5. The finishing rolling adopts F1~F7 continuous rolling process. The coiling temperature is designed in the range of 560℃~600℃ according to the product thickness.

[0043] Table 2: Billet heating system

[0044]

[0045] 3. Example Analysis

[0046] 3.1 According to the above smelting technical requirements, high-strength steel hot-rolled steel strip ingots with a tensile strength of 700 MPa and the following compositions are smelted. The specific compositions are shown in Table 3 below.

[0047] Table 3: Chemical composition of each example (mass percentage: %)

[0048]

[0049] 3.2 According to the above composition design and hot rolling process, the hot-rolled steel strips of each example were obtained, and SEM imaging was performed on the hot-rolled steel strips obtained from each example, and the morphology and particle size of TiN / TiN-CeAlO3 in the hot-rolled steel strips of each example were analyzed.

[0050] Figure 1 The TiN morphology of the hot-rolled steel strip obtained in Example 1 under SEM observation is shown. Figure 2 The SEM morphology of the hot-rolled steel strip obtained in Example 2 is shown. Figure 3-5 The morphology and composition scanning results of TiN-CeAlO3 of the hot-rolled steel strips obtained in Examples 3-5 under SEM observation are shown respectively. Figure 6 The morphology and composition surface scanning results of the ultra-large TiN-CeAlO3 of the hot-rolled steel strip obtained in Example 6 under SEM observation are shown. Figure 7 The variation of TiN particle size in hot-rolled steel strips obtained in Examples 7-9 with the addition of rare earth Ce is shown, where Example 7 corresponds to 0ppm Ce, Example 8 corresponds to 9ppm Ce, and Example 9 corresponds to 20ppm Ce. The above results show that adding a trace amount (0.0010% to 0.0030%) of rare earth Ce and controlling the N content in the hot-rolled steel strip to ≤50ppm can modify the spheroidization and refine the size of the liquid second phase TiN particles, which helps to improve the inherent defect of "strong but not tough" of economical Ti microalloyed high-strength steel, and further expand the application field of the product.

[0051] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein by equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for utilizing rare earth to modify the spheroidization and refinement of liquid phase TiN second phase large particle inclusions, characterized in that: The method includes adding a small amount of rare earth Ce to an economical Ti microalloyed hot-rolled high-strength steel (Ti mass percentage: 0.050 - 0.100%). By taking advantage of its good chemical activity in high-temperature molten metal, it first modifies the oxygen product inclusion Al2O3 into CeAlO3. Using CeAlO3 as a heterogeneous nucleating agent for TiN inclusions, an ellipsoidal CeAlO3-TiN composite inclusion is formed, thereby achieving the purpose of modifying and refining large liquid segregation TiN inclusions and improving the plasticity and toughness of the economical Ti microalloyed hot-rolled high-strength steel.

2. The method according to claim 1, characterized in that The method includes the following processes: smelting and continuous casting process and controlled rolling and controlled cooling process; among which: The smelting and continuous casting process is: hot metal → KR desulfurization → converter steelmaking → LF refining → RH degassing → slab continuous casting; Converter smelting uses KR pre-desulfurized hot metal. After smelting, ferrosilicon and ferromanganese are used for deoxidation alloying. Ferro-niobium is added at the end of LF alloying, and ferro-titanium is added at the end of RH. Before vacuum recompression within 3 minutes, rare earth ferroalloy (20% Ce-Fe) is added. The oxygen content in the molten steel should be measured before adding the rare earth alloy to ensure that T[O] in the molten steel ≤ 15 ppm; The molten steel is stirred with argon throughout the process; The vacuum degree ≤ 2 mbar, the treatment time ≥ 10 min, and the continuous casting superheat is controlled to increase by 5°C - 10°C compared with conventional steel, and its superheat ΔT ≤ 35 - 45°C; The controlled rolling and controlled cooling process is: For Ti microalloyed high-strength steel, the heating temperature and coiling temperature are controlled during the hot rolling stage. The heating temperature is controlled at 1220 - 1270°C, and the heating time is 120 - 180 min; The coiling temperature is designed according to the product thickness in the range of 560°C - 600°C.

3. The method according to claim 2, characterized in that The chemical composition of the economical Ti microalloyed hot-rolled high-strength steel, by mass percentage, the Ti content is 0.05 - 0.10%, the rare earth Ce content is 0.0010% - 0.0030%, and the gases in the steel are controlled as H ≤ 2 ppm, O ≤ 25 ppm, N ≤ 50 ppm.

4. The method according to any one of claims 1 to 3, characterized in that The chemical composition of the economical Ti microalloyed hot-rolled high-strength steel, by mass percentage, is: C 0.05 - 0.07%, Si 0.05 - 0.20%, Mn 0.60 - 1.60%, P ≤ 0.015%, S ≤ 0.005%, Als 0.020 - 0.040%, Nb 0.005 - 0.020%, Ti 0.050 - 0.100%, H ≤ 2 ppm, O ≤ 25 ppm, N ≤ 50 ppm, and the rest is Fe and inevitable impurities.

5. The method according to any one of claims 1 to 4, characterized in that If the thickness h of the economical Ti microalloyed hot-rolled high-strength steel satisfies: 3.0 mm ≤ h ≤ 6.0 mm, then the Mn content is 0.60 - 0.90%; If the thickness h of the economical Ti microalloyed hot-rolled high-strength steel satisfies: 6.0 mm < h ≤ 12.0 mm, then the Mn content is 0.90 - 1.20%; If the thickness h of the economical Ti microalloyed hot-rolled high-strength steel satisfies: 12.0 mm < h ≤ 16.0 mm, then the Mn content is 1.20 - 1.60%.

6. An economical Ti microalloyed hot-rolled high-strength steel obtained by the method according to any one of claims 1 to 5.

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