Method for improving toughness performance of low-alloy high-strength steel by adding rare earth cerium-iron alloy and application of method

Abstract

The invention discloses a method for improving the toughness performance of low-alloy high-strength steel by adding rare earth cerium-iron alloy and application of the method, and belongs to the technical field of steel rolling and product development. According to the method, narrow window control is conducted on converter steelmaking, LF refining and RH vacuum treatment in an existing wide and thick plate low-alloy high-strength toughness steel production technological process, a cerium-iron alloy is added after alloying in the RH vacuum treatment process is finished and circulated, temperature measurement and oxygen determination are conducted before the cerium-iron alloy is added, and the active oxygen is smaller than or equal to 3 ppm. The method is applied to an RH vacuum treatment step of industrial production of wide and thick plate Q690E steel and comprises the following steps: adding a titanium-iron alloy and a ferroboron alloy in sequence for alloying, circulating after alloying is finished, and adding a cerium-iron alloy; and carrying out soft blowing after the RH vacuum treatment is finished. After the rare earth cerium-iron alloy is added, the average size of low-alloy high-strength steel inclusions is remarkably reduced, the average grain size is greatly small, the average width of a martensite lath bundle is narrowed by less than 200nm, and the toughness performance is remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of steel rolling and product development technology, specifically relating to a method for improving the strength and toughness of low-alloy high-strength steel by adding rare earth cerium-iron alloy and its application. Background Technology

[0002] Low-alloy high-strength steel is widely used in large-scale engineering projects such as ore mining, large-scale construction, bridges, shipbuilding, and infrastructure construction. It is required to be suitable for various harsh service conditions and have good comprehensive mechanical properties to meet the quality requirements and practicality of steel under special operations. Its manufacturing technology is one of the important indicators for measuring the level of special steel production and technological development.

[0003] Currently, some scholars are dedicated to the research of ultra-high strength and toughness steels, but most of this research is at the forefront of exploration. Due to the demanding experimental conditions, further exploration of new control methods is needed to achieve industrial application. To improve the strength and toughness of steel and achieve lightweight applications, traditional design concepts mainly focus on increasing the amount of alloying elements added, such as C, Si, and Mn for solid solution strengthening, or Nb, V, and Ti for compound precipitation strengthening and grain refinement. However, in industrial applications, the strict requirements on carbon equivalent for the weldability of steel limit the amount of alloying elements that can be added, thus restricting the technology of continuously increasing alloy content to improve product strength and toughness.

[0004] Many scholars have proposed using rare earth elements to control cleanliness and have conducted research on improving strength. However, ensuring continuous production and casting while maintaining a high rare earth yield has always been a challenge in transforming rare earth steel from laboratory to industrial production. There are no reported cases of achieving a rare earth yield of over 50% in the industrial application of heavy plate production. Summary of the Invention

[0005] To address the problems existing in the prior art, the present invention aims to provide a method for improving the strength and toughness of low-alloy high-strength steel by adding rare earth cerium-iron alloy, and to apply it to the industrial trial production of low-alloy high-strength and toughness steel for special operation vehicle floor plates in a heavy plate production line. Through systematic processing, the invention studies the high-yield rare earth addition process of low-alloy high-strength steel Q690E in the heavy plate process and the influence of rare earth on the microstructure, inclusions, and properties of the billet and finished plate, thereby further improving the strength and toughness of the product.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] In a first aspect, the present invention provides a method for improving the strength and toughness of low-alloy high-strength steel by adding rare earth cerium-iron alloy. In the existing production process of low-alloy high-strength and tough steel for wide and thick plates, a narrow window control is implemented for converter steelmaking, LF refining and RH vacuum treatment. After the alloying process is completed and the cycle is completed in the RH vacuum treatment process, cerium-iron alloy is added. Before adding cerium-iron alloy, the oxygen content is measured by temperature and oxygen activity is ≤3ppm.

[0008] Furthermore, the steel used for thick plates with low alloy strength and high toughness is Q690E.

[0009] Furthermore, the narrow window control involves controlling the dephosphorization rate to >60% and the slag discharge rate to >50% during the early stage of converter smelting. When the converter taps 1 / 4 to 1 / 3 of the steel, 2.5 kg / t steel to 3.3 kg / t steel of fine-grained quicklime is added to dilute and adjust the slag, thereby reducing the FeO and MnO content in the slag and increasing the basicity of the slag. After LF refining and before RH vacuum treatment, 1.1 kg / t steel to 1.8 kg / t steel of aluminum modifier is added to the surface of the top slag of the ladle to remove oxygen from the slag, and the Ca / Al mass ratio is controlled at 1.8%-2.0%.

[0010] Preferably, the aluminum modifier contains 40wt.%-60wt.% aluminum.

[0011] Secondly, this invention provides a method for improving the strength and toughness of low-alloy high-strength steel by adding rare earth cerium-iron alloys, and its application in the industrial production of Q690E steel for thick plates.

[0012] Furthermore, it includes the following steps:

[0013] (1) Desulfurization pretreatment of molten iron;

[0014] (2) In a single carbon removal operation, the converter steelmaking process involves adding ferromanganese and ferrochrome for alloying, followed by slag blocking and steel tapping.

[0015] (3) LF refining involves temperature adjustment and desulfurization, and the addition of ferromolybdenum and ferrochrome for alloying;

[0016] (4) RH vacuum treatment, add ferro-titanium alloy and ferro-boron alloy in sequence for alloying, circulate after alloying, add ferro-cerium alloy; after RH vacuum treatment, perform soft blowing.

[0017] (5) Intermediate package process;

[0018] (6) Under the protection measures to prevent secondary oxidation, nitrogen and oxygen increase are prevented, and continuous casting is carried out to obtain the billet;

[0019] (7) Rolling and heat treatment to obtain finished low-alloy high-strength steel plates with refined tempered sorbite structure.

[0020] Furthermore, in step (1), the area of ​​molten iron slag removal must be >95%, and the sulfur content of the molten iron entering the converter must be <0.002%.

[0021] Furthermore, in step (4), the vacuum degree of RH vacuum treatment is ≤0.266Kpa, the vacuum treatment time is 20±3min, the circulation time is 3±1min, and the soft blowing time is ≥15min, so as to ensure that the mixed inclusions of Al2O3 and CeAlO3 are fully floated.

[0022] After the cycle is completed, the cerium content in the added ferrocerium alloy is 25wt.%-35wt.%, and the amount of ferrocerium alloy added is based on the cerium content in the prepared Q690E steel composition being 0.0017wt.%-0.0040wt.%.

[0023] Furthermore, the protective measures to prevent secondary oxidation in step (6) are to use a hollow plug rod, introduce argon gas at a flow rate ≥1.5L / min, and use argon gas sealing between the slide plates for the immersion nozzle;

[0024] The continuous casting machine adopts a secondary cooling zone electromagnetic stirring process and a dynamic light reduction process to improve the internal quality of the billet.

[0025] Furthermore, step (7) rolling includes rough rolling and finish rolling, and heat treatment includes quenching and tempering.

[0026] Furthermore, the furnace exit temperature before rolling is 1170℃-1190℃, the roughing rolling start temperature is 1140℃-1160℃, the intermediate billet thickness is 30±5mm, the finishing rolling temperature is 805℃-825℃, the final pass reduction rate is 14%-16%, the quenching temperature is 870℃-890℃, the quenching holding time is 20±3min, the tempering temperature is 560℃-580℃, and the tempering holding time is 25min-35min.

[0027] Advantages and effects of the present invention:

[0028] Compared to existing Q690E steel, the addition of rare earth cerium-iron alloy in this invention results in a lower Gibbs free energy for the combination of Ce with active O and S in the steel, inhibiting the combination of S with Mn and Ca, and transforming Al2O3 inclusions into spherical CeAlO3 inclusions. Simultaneously, the composite inclusions of AlN, sulfides, and oxides are modified to precipitate as TiN alone, spheroidizing the inclusions and significantly reducing their average size and dispersion. Ce plays a role in desulfurization, deoxidation, and other purification functions of the molten steel. The addition of rare earth Ce effectively increases the number of nucleation sites in the liquid phase, improving the nucleation rate and expanding the equiaxed crystal refinement region while limiting the formation of columnar crystals. The average grain size of the cast slab is significantly smaller, with the proportion of large grains larger than 50 μm being less than 28%. All cast slabs have a low magnification rating of C2.0 or higher, achieving a refined solidification structure and thus mitigating the banded structure of the hot-rolled plate. Due to the inherited nature of the upstream refined structure, and the effect of strain-induced precipitation of CeS, CeO2, Ce2O3, Ce2O3S, and Ca compounds, the recrystallization of deformed austenite and the growth of grains after recrystallization are suppressed, achieving a refined grain structure and resulting in a refined tempered sorbite structure. The addition of rare earth Ce narrows the average width of the martensite lath bundles to less than 200 nm, significantly improving the product's strength and toughness mechanical properties. The rare earth recovery rate of low-alloy high-strength steel after the addition of rare earth cerium-iron alloy in this invention can reach over 56.8%. Attached Figure Description

[0029] Figure 1 This is a schematic diagram illustrating the timing of adding cerium-iron alloys during the smelting process in Examples 1 to 8;

[0030] Figure 2 The images show the inclusion morphology of the finished steel plate from Example 1 as detected by scanning electron microscopy, where: (a) is a spherical inclusion, (b) is an elliptical inclusion, and (c) is a spindle-shaped inclusion;

[0031] Figure 3 This is a graph showing the carbon segregation index of the rare earth billet at various positions along its entire length and at its full thickness, as shown in Example 4.

[0032] Figure 4 This is a graph showing the carbon segregation index of the rare earth billet at 1 / 4 of its width, corresponding to the full thickness position, in Example 4.

[0033] Figure 5 This is a graph showing the carbon segregation index of the rare earth billet at 3 / 4 of its width, corresponding to the full thickness position, in Example 4.

[0034] Figure 6 The images shown are grain images of rare earth cast billets under an optical microscope in Example 5, where: (a) is the morphology image of field I, (b) is the morphology image of field II, and (c) is the morphology image of field III;

[0035] Figure 7 Transmission electron microscopy (TEM) image of microscopic tissue observation in Example 7;

[0036] Figure 8 Example 8 shows the low-temperature ductile-brittle transition curve detected laterally;

[0037] Figure 9 Example 8 shows the longitudinal detection of the low-temperature ductile-brittle transition curve;

[0038] Figure 10 This is a diagram showing the carbon segregation index of the rare earth billet at various positions along its entire length and at its full thickness, as shown in Comparative Example 1.

[0039] Figure 11 The carbon segregation index diagram of the rare earth billet at 1 / 4 of its width, corresponding to the full thickness position, is shown in Comparative Example 1.

[0040] Figure 12 The carbon segregation index diagram of the rare earth billet at 3 / 4 of its width, corresponding to the full thickness position, is shown in Comparative Example 1.

[0041] Figure 13 The images shown are grain images of rare earth cast billets under an optical microscope in Comparative Example 2, where: (a) is the morphology image of field I, (b) is the morphology image of field II, and (c) is the morphology image of field III;

[0042] Figure 14 The image shows the morphology of inclusions detected by scanning electron microscopy in the finished steel plate of Comparative Example 3.

[0043] Figure 15 This is a transmission electron microscope (TEM) image of the microstructure observed in Comparative Example 4.

[0044] Figure 16 Comparative Example 5: Low-temperature ductile-brittle transition curve detected laterally;

[0045] Figure 17 The low-temperature ductile-brittle transition curve was measured longitudinally for comparative example 5. Detailed Implementation

[0046] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples.

[0047] A method for enhancing the strength and toughness of low-alloy high-strength steel by incorporating rare-earth cerium-iron alloys involves narrow-window control of the converter steelmaking, LF refining, and RH vacuum treatment processes in the existing production flow of Q690E low-alloy high-strength and toughness steel for wide and thick plates. Specifically, during the early stage of converter smelting, the dephosphorization rate is controlled to be >60%, and the slag discharge rate is >50%. When the steel is tapped from the converter to 1 / 4-1 / 3 of its length, fine-grained quicklime (2.5 kg / t steel - 3.3 kg / t steel) is added to dilute and condition the slag, thereby reducing the ferrophosphate content in the slag. The content of eO and MnO and the basicity of the slag are increased; after LF refining and before RH vacuum treatment, 1.1 kg / t steel-1.8 kg / t steel aluminum modifier (containing 40wt.%-60wt.% aluminum) is added to the surface of the ladle top slag to remove oxygen from the slag, and the Ca / Al mass ratio is controlled at 1.8%-2.0%; after the alloying process is completed and the cycle is completed in the RH vacuum treatment process, cerium-iron alloy is added. Before adding cerium-iron alloy, the oxygen content is measured by temperature and the active oxygen is ≤3ppm.

[0048] The application of a method for improving the strength and toughness of low-alloy high-strength steel by adding rare earth cerium-iron alloys in the industrial production of heavy plate Q690E steel includes the following steps:

[0049] (1) Hot metal desulfurization pretreatment: In order to meet the cleanliness requirements of low sulfur content of the product, hot metal desulfurization pretreatment is adopted. The area of ​​hot metal slag removal is required to be greater than 95%, and the sulfur content of the hot metal entering the converter is required to be less than 0.002%;

[0050] (2) In a single carbon removal operation, the converter steelmaking process involves adding ferromanganese and ferrochrome for alloying, followed by slag blocking and steel tapping.

[0051] (3) LF refining involves temperature adjustment and desulfurization, and the addition of ferromolybdenum and ferrochrome for alloying;

[0052] (4) The vacuum degree of RH vacuum treatment is ≤0.266Kpa, the RH vacuum treatment time is 20±3min, and titanium iron alloy and boron iron alloy are added in sequence for alloying. After alloying, the mixture is circulated for 3±1min, and cerium iron alloy is added. The cerium content in the cerium iron alloy is 25wt.%-35wt.%, and the amount of cerium iron alloy added is based on the cerium content in the prepared Q690E steel composition being 0.0017wt.%-0.0040wt.%. After RH vacuum treatment, soft blowing is performed for ≥15min to ensure that the mixed inclusions of Al2O3 and CeAlO3 are fully floated.

[0053] (5) Intermediate package process;

[0054] (6) Measures such as using hollow plug rods, argon flow rate ≥1.5L / min and argon sealing between slide plates for submerged nozzles are adopted to prevent secondary oxidation and nitrogen and oxygen increase. Electromagnetic stirring process in the secondary cooling zone and dynamic light pressure process are used for continuous casting to improve the internal quality of the billet and obtain the billet.

[0055] (7) Rolling and heat treatment. Rolling includes roughing and finishing. The furnace exit temperature before rolling is 1170℃-1190℃, the roughing start temperature is 1140℃-1160℃, the intermediate billet thickness is 30±5mm, the finishing temperature is 805℃-825℃, and the reduction rate of the last pass is 14%-16%. Heat treatment includes quenching and tempering. The quenching temperature is 870℃-890℃, the quenching holding time is 20±3min, the tempering temperature is 560℃-580℃, and the tempering holding time is 25min-35min. The finished low alloy high strength steel plate is obtained. The low alloy high strength steel plate has a refined tempered sorbite structure and the average width of the martensite lath bundles is less than 200nm.

[0056] The specific timing of adding the cerium-iron alloys in Examples 1 to 8 during the smelting process is as follows: Figure 1 As shown.

[0057] Example 1

[0058] This invention discloses a method for enhancing the strength and toughness of low-alloy high-strength steel by adding rare earth cerium-iron alloy. In the existing production process of Q690E low-alloy high-strength and toughness steel for thick plates, a hot metal desulfurization pretreatment is adopted. Narrow window control is implemented for converter steelmaking, LF refining, and RH vacuum treatment. Specifically, in the early stage of converter smelting, the dephosphorization rate is controlled to be >60%, and the slag discharge rate to be >50%. When the converter tapps 1 / 4 to 1 / 3 of the steel, 3.1 kg / t of steel-grade quicklime is added for slag dilution and conditioning to reduce the FeO and MnO content in the slag and enhance the slag basicity. After LF refining and before RH vacuum treatment, 1.55 kg / t of steel-grade aluminum modifier is added to the surface of the ladle top slag to remove oxygen from the slag. The aluminum modifier contains 55 wt.% aluminum, and the Ca / Al mass ratio is controlled at 1.85%. After alloying and cycling in the RH vacuum treatment process, cerium-iron alloy is added. Before adding the cerium-iron alloy, the oxygen content is measured at temperature, and the active oxygen is 2 ppm.

[0059] A method for enhancing the strength and toughness of low-alloy high-strength steel by incorporating rare-earth cerium-iron alloys has been applied to the industrial production of Q690E steel for heavy plates, comprising the following steps:

[0060] (1) Hot metal desulfurization pretreatment, the area of ​​hot metal slag removal is 98%, and the sulfur content of hot metal entering the converter is 0.001%; ​​the scrap steel is selected from the heavy scrap steel of the head and tail of the wide and thick plate casting billet with a sulfur content of less than 0.050wt.%.

[0061] (2) Strong deoxidation system and calcium treatment measures are adopted to carry out steelmaking in a single carbon removal operation in the converter. The dephosphorization rate is controlled at 65% and the slag discharge rate is 55% in the early stage of converter smelting. Ferromanganese and ferrochrome (fine adjustment) are added for alloying and slag blocking is used for tapping. When 1 / 3 of the steel is tapped from the converter, 3.1 kg / t steel fine-grained lime is added to dilute and adjust the slag, so as to reduce the FeO and MnO content in the slag and enhance the basicity of the slag. Strong deoxidation system and calcium treatment are adopted in the converter process to strictly control the harmful elements and inclusions in the steel in order to obtain high-purity molten steel.

[0062] (3) LF refining involves temperature adjustment and desulfurization, and the addition of ferromolybdenum and ferrochrome for alloying. Harmful elements and inclusions in the steel are strictly controlled to obtain high-purity molten steel. The LF refining process adopts strong deoxidation system, calcium treatment and other technical measures to strictly control harmful elements and inclusions in the steel to obtain high-purity molten steel. After the LF refining process leaves the station, 1.55 kg / t steel containing 55 wt.% aluminum modifier is added to the surface of the top slag of the ladle to remove oxygen from the slag, and the Ca / Al mass ratio is controlled at 1.85%.

[0063] (4) The vacuum degree of RH vacuum treatment is ≤0.266Kpa. Because rare earth has strong oxidizing properties, in order to avoid it being oxidized by oxygen brought in by other alloys during RH alloying, and to ensure the effective B content in the steel, titanium iron alloy and boron iron alloy are added in sequence for alloying. After alloying, the cycle is 3.5min, the temperature is measured and the oxygen is determined. The active oxygen is 2ppm. 0.165kg / t steel of cerium iron alloy (cerium content 30wt.%) is added. RH vacuum treatment is completed after 18min. After RH vacuum treatment, soft blowing is carried out. During soft blowing, the slag surface of the ladle is kept calm and no molten steel surface is exposed to the air. The soft blowing time is ≥15min to ensure that the inclusions float up fully.

[0064] (5) Intermediate package process.

[0065] (6) A hollow stopper rod is used, and the argon flow rate is ≥1.5L / min. The immersion nozzle is sealed with argon gas in the slide plate to prevent nitrogen and oxygen from increasing. Continuous casting is carried out. The casting machine adopts the electromagnetic stirring process in the secondary cooling zone and dynamic light pressure to improve the internal quality of the billet and obtain the billet with a billet size of 1950 × 250mm.

[0066] (7) Rolling and heat treatment: the furnace exit temperature before rolling is 1185℃, the rough rolling start temperature is 1148℃, the intermediate billet thickness is 32mm, the finishing rolling temperature is 822℃, the last pass reduction rate is 15.5%, the quenching temperature is 885℃, the quenching holding time is 21min, the tempering temperature is 573℃, and the tempering holding time is 29min; the finished low alloy high strength steel plate is obtained, and the thickness of the finished low alloy high strength steel plate is 10mm.

[0067] Performance Analysis:

[0068] (1) Composition analysis:

[0069] The chemical composition of the finished low-alloy high-strength steel with improved strength and toughness prepared in Example 1 is shown in Table 1, and the rare earth recovery rate is 56.80%.

[0070] Table 1. Chemical composition (wt.%) of the finished low-alloy high-strength steel from Example 1

[0071]

[0072] (2) Analysis of the degree of segregation:

[0073] Segregation degree analysis was performed using low-magnification etching. After polishing the cross-section of the billet, it was etched with a cold etching reagent. The low-magnification condition of the billet was compared and observed. The low-magnification rating of the billet reached C1.5.

[0074] (3) Analysis of the grain refinement degree of the cast billet:

[0075] Metallographic specimens measuring 20mm (rolling direction) × 20mm (transverse direction) × 20mm (thickness) were cut at 1 / 4 of the width and thickness of the cast billet. After grinding and polishing, the specimens were etched with a 4% nitric acid alcohol solution. Grain size was statistically compared using Image-J, an Axiovert.A1 optical microscope software. The rule was to select four different fields of view at 100x magnification for grain size statistics. Grains located at the boundaries of the field of view were not included in the statistics because their size could not be accurately measured as some were inside and some were outside the field of view. The average grain size of the cast billet was measured to be 31.15μm, with large grains larger than 50μm accounting for 20.66%.

[0076] (4) Strength and toughness analysis:

[0077] The mechanical properties of the product were analyzed using a Z150 electronic universal testing machine. After testing, the Q690E steel plate with added rare earth elements showed a yield strength of 796 MPa, a tensile strength of 831 MPa, an elongation of 15.4%, and a strength-ductility product of 12797, demonstrating high strength and toughness.

[0078] (5) Tissue performance analysis:

[0079] The finished plate was wire-cut into metallographic samples of 20mm (rolling direction) × 20mm (transverse direction) × thickness. The microstructure was observed using a FEI Tecnai G2 F20 transmission electron microscope. The average width of the martensite lath bundles was measured to be 185nm.

[0080] (6) Inclusion analysis:

[0081] The composition and morphology of nanoscale precipitates in the finished steel plate were analyzed using a German ZEISSEVO 18 scanning electron microscope and small-sample electrolysis. The cleanliness of the steel plate was also evaluated. The average size of the inclusions was 1.7 μm. Figure 2 As shown, the inclusions in the rare earth steel samples exhibit spherical, elliptical, and spindle-shaped morphologies, which are beneficial to the continuity of the steel's microstructure.

[0082] Example 2

[0083] A method for enhancing the strength and toughness of low-alloy high-strength steel by incorporating rare-earth cerium-iron alloys has been applied to the industrial production of Q690E steel for heavy plates, comprising the following steps:

[0084] (1) Hot metal desulfurization pretreatment, the area of ​​hot metal slag removal is 97%, the sulfur content of hot metal entering the converter is 0.0015%, and the scrap steel is the heavy scrap steel from the head and tail of the wide and thick plate billet with a sulfur content of less than 0.050wt.%.

[0085] (2) Strong deoxidation system and calcium treatment measures are adopted to carry out steelmaking in a single carbon removal operation in the converter. The dephosphorization rate is controlled at 67% and the slag discharge rate is 53% in the early stage of converter smelting. Ferromanganese and ferrochrome (fine adjustment) are added for alloying and slag blocking is used for tapping. When 1 / 3 of the steel is tapped from the converter, 2.9 kg / t steel fine-grained lime is added to dilute and adjust the slag, so as to reduce the FeO and MnO content in the slag and enhance the basicity of the slag. Strong deoxidation system and calcium treatment are adopted in the converter process to strictly control the harmful elements and inclusions in the steel in order to obtain high-purity molten steel.

[0086] (3) LF refining involves temperature adjustment and desulfurization, and the addition of ferromolybdenum and ferrochrome for alloying. Harmful elements and inclusions in the steel are strictly controlled to obtain high-purity molten steel. The LF refining process adopts strong deoxidation system, calcium treatment and other technical measures to strictly control harmful elements and inclusions in the steel to obtain high-purity molten steel. After the LF refining process leaves the station, 1.5 kg / t steel containing 40 wt.% aluminum modifier is added to the surface of the top slag of the ladle to remove oxygen from the slag, and the Ca / Al mass ratio is controlled at 1.8%.

[0087] (4) The vacuum degree of RH vacuum treatment is ≤0.266Kpa. Because rare earth has strong oxidizing properties, in order to avoid it being oxidized by oxygen brought in by other alloys during RH alloying, and to ensure the effective B content in the steel, titanium iron alloy and boron iron alloy are added in sequence for alloying. After alloying, the cycle is 4min, the temperature is measured and the oxygen is determined. The active oxygen is 2ppm. Cerium iron alloy (cerium content 30wt.%) is added at 0.14kg / t steel. RH vacuum treatment is completed after 18min. After RH vacuum treatment, soft blowing is carried out. During soft blowing, the slag surface of the ladle is kept calm and no molten steel surface is exposed to the air. The soft blowing time is 15min to ensure that the inclusions float up fully.

[0088] (5) In the intermediate batch process, a hollow stopper rod is used, and the argon gas flow rate is ≥1.5L / min. The immersion nozzle is sealed with argon gas in the slide plate to prevent nitrogen and oxygen from increasing.

[0089] (6) Continuous casting is carried out. The casting machine adopts the electromagnetic stirring process in the second cooling zone and dynamic light pressure to improve the internal quality of the billet and obtain the billet with a size of 1950 × 250mm.

[0090] (7) Rolling and heat treatment: the furnace exit temperature before rolling is 1190℃, the rough rolling start temperature is 1140℃, the intermediate billet thickness is 30mm, the finishing rolling temperature is 825℃, the last pass reduction rate is 15%, the quenching temperature is 880℃, the quenching holding time is 23min, the tempering temperature is 575℃, and the tempering holding time is 25min; the finished low alloy high strength steel plate is obtained with a thickness of 10mm.

[0091] Performance Analysis:

[0092] (1) Composition analysis:

[0093] The chemical composition of the finished low-alloy high-strength steel with improved strength and toughness prepared in Example 2 is shown in Table 2, and the rare earth recovery rate is 61.5%.

[0094] Table 2. Chemical composition (wt.%) of the finished low-alloy high-strength steel from Example 2

[0095]

[0096] (2) Analysis of the degree of segregation:

[0097] Using the analytical method of Example 1, the low-magnification rating of the cast billet reached C0.5.

[0098] (3) Analysis of the grain refinement degree of the cast billet:

[0099] Using the analysis method of Example 1, the average grain size of the cast billet was measured to be 32.75 μm, with large grains larger than 50 μm accounting for 21.86%.

[0100] (4) Strength and toughness analysis:

[0101] Using the analytical method of Example 1, the Q690E steel with added rare earth elements was tested and found to have a yield strength of 792 MPa, a tensile strength of 839 MPa, an elongation of 15.7%, and a strength-ductility product of 13172, demonstrating high strength and toughness.

[0102] (5) Tissue performance analysis:

[0103] Using the analytical method of Example 1, the average width of the martensitic lath bundles was measured to be 197 nm.

[0104] (6) Inclusion analysis:

[0105] Using the analytical method of Example 1, the average size of the inclusions was 1.68 μm. The inclusion morphology of the rare earth steel sample was spherical, elliptical and spindle-shaped, which is beneficial to the continuity of the steel structure.

[0106] Example 3

[0107] A method for enhancing the strength and toughness of low-alloy high-strength steel by incorporating rare-earth cerium-iron alloys has been applied to the industrial production of Q690E steel for heavy plates, comprising the following steps:

[0108] (1) Hot metal desulfurization pretreatment, the area of ​​hot metal slag removal is 96%, the sulfur content of hot metal entering the converter is 0.0008%, and the scrap steel is the heavy scrap steel from the head and tail of the wide and thick plate billet with a sulfur content of less than 0.050wt.%.

[0109] (2) Strong deoxidation system and calcium treatment measures are adopted to carry out steelmaking in a single carbon removal operation in the converter. The dephosphorization rate is controlled at 69% and the slag discharge rate is 60% in the early stage of converter smelting. Ferromanganese and ferrochrome (fine adjustment) are added for alloying and slag blocking is used for tapping. When 1 / 4 of the steel is tapped from the converter, 2.5 kg / t steel fine-grained lime is added to dilute and adjust the slag, so as to reduce the FeO and MnO content in the slag and enhance the basicity of the slag. Strong deoxidation system and calcium treatment are adopted in the converter process to strictly control the harmful elements and inclusions in the steel in order to obtain high-purity molten steel.

[0110] (3) LF refining involves temperature adjustment and desulfurization, and the addition of ferromolybdenum and ferrochrome for alloying. Harmful elements and inclusions in the steel are strictly controlled to obtain high-purity molten steel. The LF refining process adopts strong deoxidation system, calcium treatment and other technical measures to strictly control harmful elements and inclusions in the steel to obtain high-purity molten steel. After the LF refining process leaves the station, 1.1 kg / t steel containing 60 wt.% aluminum modifier is added to the surface of the top slag of the ladle to remove oxygen from the slag, and the Ca / Al mass ratio is controlled at 2.0%.

[0111] (4) The vacuum degree of RH vacuum treatment is ≤0.266Kpa. Because rare earth has strong oxidizing properties, in order to avoid it being oxidized by oxygen brought in by other alloys during RH alloying, and to ensure the effective B content in the steel, titanium iron alloy and boron iron alloy are added in sequence for alloying. After alloying, the cycle is 4min, the temperature is measured and the oxygen is determined. The active oxygen is 2ppm. Cerium iron alloy (cerium content 30wt.%) is added at 0.145kg / t steel. RH vacuum treatment is completed after 19min. After RH vacuum treatment, soft blowing is carried out. During soft blowing, the slag surface of the ladle is kept calm and no molten steel surface is exposed to the air. The soft blowing time is ≥15min to ensure that the inclusions float up fully.

[0112] (5) In the intermediate batch process, a hollow stopper rod is used, and the argon gas flow rate is ≥1.5L / min. The immersion nozzle is sealed with argon gas in the slide plate to prevent nitrogen and oxygen from increasing.

[0113] (6) Continuous casting is carried out. The casting machine adopts the electromagnetic stirring process in the second cooling zone and dynamic light pressure to improve the internal quality of the billet and obtain the billet with a size of 1950 × 250mm.

[0114] (7) Rolling and heat treatment: the furnace exit temperature before rolling is 1170℃, the rough rolling start temperature is 1160℃, the intermediate billet thickness is 33mm, the finishing rolling temperature is 815℃, the last pass reduction rate is 16%, the quenching temperature is 885℃, the quenching holding time is 19min, the tempering temperature is 560℃, and the tempering holding time is 32min; the finished low alloy high strength steel plate is obtained, and the thickness of the finished low alloy high strength steel plate is 12mm.

[0115] Performance Analysis:

[0116] (1) Composition analysis:

[0117] The chemical composition of the finished low-alloy high-strength steel with improved strength and toughness prepared in Example 3 is shown in Table 3, and the rare earth recovery rate is 63.5%.

[0118] Table 3. Chemical composition (wt.%) of the finished low-alloy high-strength steel from Example 3

[0119]

[0120] (2) Analysis of the degree of segregation:

[0121] Using the analysis method of Example 1, the low-magnification rating of the cast billet reached C1.0.

[0122] (3) Analysis of the grain refinement degree of the cast billet:

[0123] Using the analysis method of Example 1, the average grain size of the cast billet was measured to be 30.75 μm, with large grains larger than 50 μm accounting for 21.23%.

[0124] (4) Strength and toughness analysis:

[0125] Using the analytical method of Example 1, the Q690E steel plate with added rare earth elements was tested and found to have a yield strength of 788 MPa, a tensile strength of 896 MPa, an elongation of 16.2%, and a strength-ductility product of 14515, demonstrating high strength and toughness.

[0126] (5) Tissue performance analysis:

[0127] Using the analytical method of Example 1, the average width of the martensitic lath bundles was measured to be 183 nm.

[0128] (6) Inclusion analysis:

[0129] Using the analytical method of Example 1, the average size of the inclusions was 1.58 μm. The inclusion morphology of the rare earth steel sample was spherical, elliptical and spindle-shaped, which is beneficial to the continuity of the steel structure.

[0130] Example 4

[0131] A method for enhancing the strength and toughness of low-alloy high-strength steel by incorporating rare-earth cerium-iron alloys has been applied to the industrial production of Q690E steel for heavy plates, comprising the following steps:

[0132] (1) Hot metal desulfurization pretreatment, the area of ​​hot metal slag removal is 98%, the sulfur content of hot metal entering the converter is 0.0015%, and the scrap steel is the heavy scrap steel from the head and tail of the wide and thick plate billet with a sulfur content of less than 0.040wt.%.

[0133] (2) Strong deoxidation system and calcium treatment measures are adopted for one-time carbon removal operation of converter steelmaking. The dephosphorization rate is controlled at 62% and the slag discharge rate is 56% in the early stage of converter smelting. Ferromanganese and ferrochrome (fine adjustment) are added for alloying and slag blocking is used for steel tapping. When 1 / 3 of the steel is tapped from the converter, 2.65 kg / t steel fine-grained lime is added to dilute and adjust the slag, so as to reduce the FeO and MnO content in the slag and enhance the basicity of the slag. Strong deoxidation system and calcium treatment and other technical measures are adopted in the converter process to strictly control the harmful elements and inclusions in the steel in order to obtain high-purity molten steel.

[0134] (3) LF refining involves temperature adjustment and desulfurization, and the addition of ferromolybdenum and ferrochrome for alloying. Harmful elements and inclusions in the steel are strictly controlled to obtain high-purity molten steel. The LF refining process adopts strong deoxidation system, calcium treatment and other technical measures to strictly control harmful elements and inclusions in the steel to obtain high-purity molten steel. After the LF refining process leaves the station, 1.15 kg / t steel containing 60 wt.% aluminum modifier is added to the surface of the top slag of the ladle to remove oxygen from the slag, and the Ca / Al mass ratio is controlled at 1.9%.

[0135] (4) The vacuum degree of RH vacuum treatment is 0.235 kPa. Because rare earth has strong oxidizing properties, in order to avoid it being oxidized by oxygen brought in by other alloys during RH alloying, and to ensure the effective B content in the steel, titanium iron alloy and boron iron alloy are added in sequence for alloying. After alloying, the cycle is 3 min, the temperature is measured and the oxygen is determined. The active oxygen is 2 ppm. Cerium iron alloy (cerium content 30 wt.%) is added at 0.15 kg / t steel. RH vacuum treatment is completed after 19 min. After RH vacuum treatment, soft blowing is carried out. During soft blowing, the slag surface of the ladle is kept calm and no molten steel surface is exposed to the air. The soft blowing time is 16 min to ensure that the inclusions float up fully.

[0136] (5) In the intermediate batch process, a hollow stopper rod is used and argon gas is introduced at a flow rate of 1.5 L / min. The immersion nozzle is sealed with argon gas in the slide plate to prevent nitrogen and oxygen from being added.

[0137] (6) Continuous casting is carried out. The casting machine adopts the electromagnetic stirring process in the second cooling zone and dynamic light pressure to improve the internal quality of the billet and obtain the billet with a specification of 1700 × 250mm.

[0138] (7) Rolling and heat treatment: the furnace exit temperature before rolling is 1180℃, the rough rolling start temperature is 1150℃, the intermediate billet thickness is 25mm, the finishing rolling temperature is 805℃, the last pass reduction rate is 15.5%, the quenching temperature is 887℃, the quenching holding time is 22min, the tempering temperature is 560℃, and the tempering holding time is 32min; the finished low alloy high strength steel plate is obtained, and the thickness of the finished low alloy high strength steel plate is 10mm.

[0139] Performance Analysis:

[0140] (1) Composition analysis:

[0141] The chemical composition of the finished low-alloy high-strength steel with improved strength and toughness prepared in Example 4 is shown in Table 4, and the rare earth recovery rate reached 71.5%.

[0142] Table 4. Chemical composition (wt.%) of the finished low-alloy high-strength steel from Example 4

[0143]

[0144] (2) Analysis of the degree of segregation:

[0145] Using the analytical method of Example 1, the low-magnification rating of the cast billet reached C1.5; carbon segregation analysis was performed on Example 4, such as... Figures 3 to 5 As shown, the average carbon segregation values ​​at various positions along the entire length, and at 1 / 4 and 3 / 4 of the width of the rare earth-added billet, corresponding to the full thickness positions, are 1.048, 1.029, and 1.035, respectively. This indicates that the degree of carbon segregation is relatively small in the thickness and width directions, and the degree of segregation inside the billet is well controlled.

[0146] (3) Analysis of the grain refinement degree of the cast billet:

[0147] Using the analysis method of Example 1, the average grain size of the cast billet was measured to be 37.65 μm, with large grains larger than 50 μm accounting for 22.96%.

[0148] (4) Strength and toughness analysis:

[0149] Using the analytical method of Example 1, the Q690E steel with added rare earth elements was tested and found to have a yield strength of 798 MPa, a tensile strength of 829 MPa, an elongation of 15.6%, and a strength-ductility product of 12932, demonstrating high strength and toughness.

[0150] (5) Tissue performance analysis:

[0151] Using the analytical method of Example 1, the average width of the martensitic lath bundles was measured to be 173 nm.

[0152] (6) Inclusion analysis:

[0153] Using the analytical method of Example 1, the average size of the inclusions was 1.78 μm. The inclusion morphology of the rare earth steel sample was spherical, elliptical and spindle-shaped, which is beneficial to the continuity of the steel structure.

[0154] Example 5

[0155] A method for enhancing the strength and toughness of low-alloy high-strength steel by incorporating rare-earth cerium-iron alloys has been applied to the industrial production of Q690E steel for heavy plates, comprising the following steps:

[0156] (1) Hot metal desulfurization pretreatment, the area of ​​hot metal slag removal is 97%, the sulfur content of hot metal entering the converter is 0.0018%, and the scrap steel is the heavy scrap steel from the head and tail of the wide and thick plate billet with a sulfur content of less than 0.045wt.%.

[0157] (2) Strong deoxidation system and calcium treatment measures are adopted for one-time carbon removal operation of converter steelmaking. The dephosphorization rate is controlled at 61% and the slag discharge rate is 55% in the early stage of converter smelting. Ferromanganese and ferrochrome (fine adjustment) are added for alloying and slag blocking is used for tapping steel. When 1 / 3 of the steel is tapped from the converter, 2.63 kg / t steel fine-grained lime is added to dilute and adjust the slag, so as to reduce the FeO and MnO content in the slag and enhance the basicity of the slag. Strong deoxidation system and calcium treatment and other technical measures are adopted in the converter process to strictly control the harmful elements and inclusions in the steel in order to obtain high-purity molten steel.

[0158] (3) LF refining involves temperature adjustment and desulfurization, and the addition of ferromolybdenum and ferrochrome for alloying. Harmful elements and inclusions in the steel are strictly controlled to obtain high-purity molten steel. The LF refining process adopts strong deoxidation system, calcium treatment and other technical measures to strictly control harmful elements and inclusions in the steel to obtain high-purity molten steel. After the LF refining process leaves the station, 1.25 kg / t steel containing 60 wt.% aluminum modifier is added to the surface of the top slag of the ladle to remove oxygen from the slag, and the Ca / Al mass ratio is controlled at 1.88%.

[0159] (4) The vacuum degree of RH vacuum treatment is 0.223 kPa. Because rare earth has strong oxidizing properties, in order to avoid it being oxidized by oxygen brought in by other alloys during RH alloying, and to ensure the effective B content in the steel, titanium iron alloy and boron iron alloy are added in sequence for alloying. After alloying, the cycle is 4 min, the temperature is measured and the oxygen is determined. The active oxygen is 1 ppm. Cerium iron alloy (cerium content 30 wt.%) is added at 0.17 kg / t steel. RH vacuum treatment is completed after 19 min. After RH vacuum treatment, soft blowing is carried out. During soft blowing, the slag surface of the ladle is kept calm and no molten steel surface is exposed to the air. The soft blowing time is 16 min to ensure that the inclusions float up fully.

[0160] (5) In the intermediate batch process, a hollow stopper rod is used and the argon gas flow rate is 1.45 L / min. The immersion nozzle is sealed with argon gas in the slide plate to prevent nitrogen and oxygen from increasing.

[0161] (6) Continuous casting is carried out. The casting machine adopts the electromagnetic stirring process in the second cooling zone and dynamic light pressure to improve the internal quality of the billet and obtain the billet with a specification of 1650 × 250mm.

[0162] (7) Rolling and heat treatment: the furnace exit temperature before rolling is 1184℃, the rough rolling start temperature is 1154℃, the intermediate billet thickness is 31mm, the finishing rolling temperature is 825℃, the last pass reduction rate is 15.8%, the quenching temperature is 870℃, the quenching holding time is 20min, the tempering temperature is 580℃, and the tempering holding time is 35min; the finished low alloy high strength steel plate is obtained, and the thickness of the finished low alloy high strength steel plate is 12mm.

[0163] Performance Analysis:

[0164] (1) Composition analysis:

[0165] The chemical composition of the finished low-alloy high-strength steel with improved strength and toughness prepared in Example 5 is shown in Table 5, and the rare earth recovery rate reached 73.4%.

[0166] Table 5. Chemical composition (wt.%) of the finished low-alloy high-strength steel from Example 5

[0167]

[0168] (2) Analysis of the degree of segregation:

[0169] Using the analysis method of Example 1, the low-magnification rating of the cast billet reached C2.0.

[0170] (3) Analysis of the grain refinement degree of the cast billet:

[0171] The analytical method of Example 1 is used, such as Figure 6 The diagrams show different field-of-view topography, among which Figure 6 (a) Small grain morphology is visible, 6(b) Rounded grain morphology is visible, 6(c) The overall grain morphology is relatively uniform. The average grain size of the billet is 30.85μm, and large grains with a size of 50μm or more account for 26.13%.

[0172] (4) Strength and toughness analysis:

[0173] Using the analytical method of Example 1, the Q690E steel plate with added rare earth elements was tested and found to have a yield strength of 742 MPa, a tensile strength of 886 MPa, an elongation of 16.8%, and a strength-ductility product of 14884, demonstrating high strength and toughness.

[0174] (5) Tissue performance analysis:

[0175] Using the analytical method of Example 1, the average width of the martensitic lath bundles was measured to be 192 nm.

[0176] (6) Inclusion analysis:

[0177] Using the analytical method of Example 1, the average size of the inclusions was 1.65 μm. The inclusion morphology of the rare earth steel sample was spherical, elliptical and spindle-shaped, which is beneficial to the continuity of the steel structure.

[0178] Example 6

[0179] A method for enhancing the strength and toughness of low-alloy high-strength steel by incorporating rare-earth cerium-iron alloys has been applied to the industrial production of Q690E steel for heavy plates, comprising the following steps:

[0180] (1) Hot metal desulfurization pretreatment, the area of ​​hot metal slag removal is 96%, the sulfur content of hot metal entering the converter is 0.001%, and the scrap steel is the heavy scrap steel from the head and tail of the wide and thick plate billet with a sulfur content of less than 0.030wt.%.

[0181] (2) Strong deoxidation system and calcium treatment measures are adopted for one-time carbon removal operation of converter steelmaking. The dephosphorization rate is controlled at 67% and the slag discharge rate is 52% in the early stage of converter smelting. Ferromanganese and ferrochrome (fine adjustment) are added for alloying and slag blocking is used for tapping steel. When 1 / 4 of the steel is tapped from the converter, 2.25 kg / t steel fine-grained lime is added to dilute and adjust the slag, so as to reduce the FeO and MnO content in the slag and enhance the basicity of the slag. Strong deoxidation system and calcium treatment and other technical measures are adopted in the converter process to strictly control the harmful elements and inclusions in the steel in order to obtain high-purity molten steel.

[0182] (3) LF refining involves temperature adjustment and desulfurization, and the addition of ferromolybdenum and ferrochrome for alloying. Harmful elements and inclusions in the steel are strictly controlled to obtain high-purity molten steel. The LF refining process adopts strong deoxidation system, calcium treatment and other technical measures to strictly control harmful elements and inclusions in the steel to obtain high-purity molten steel. After the LF refining process leaves the station, 1.17 kg / t steel containing 60 wt.% aluminum modifier is added to the surface of the top slag of the ladle to remove oxygen from the slag, and the Ca / Al mass ratio is controlled at 1.9%.

[0183] (4) The vacuum degree of RH vacuum treatment is ≤0.255Kpa. Because rare earth has strong oxidizing properties, in order to avoid it being oxidized by oxygen brought in by other alloys during RH alloying, and to ensure the effective B content in the steel, titanium iron alloy and boron iron alloy are added in sequence for alloying. After alloying, the cycle is 4min, the temperature is measured and the oxygen is determined. The active oxygen is 2ppm. Cerium iron alloy (cerium content 30wt.%) is added at 0.155kg / t steel. RH vacuum treatment is completed after 17min. After RH vacuum treatment, soft blowing is carried out. During soft blowing, the slag surface of the ladle is kept calm and no molten steel surface is exposed to the air. The soft blowing time is 15min to ensure that the inclusions float up fully.

[0184] (5) In the intermediate batch process, a hollow stopper rod is used and the argon gas flow rate is 1.55L / min. The immersion nozzle is sealed with argon gas in the slide plate to prevent nitrogen and oxygen from increasing.

[0185] (6) Continuous casting is carried out. The casting machine adopts the electromagnetic stirring process in the second cooling zone and dynamic light pressure to improve the internal quality of the billet and obtain the billet with a specification of 1650 × 250mm.

[0186] (7) Rolling and heat treatment: the furnace exit temperature before rolling is 1170℃, the rough rolling start temperature is 1160℃, the intermediate billet thickness is 35mm, the finishing rolling temperature is 816℃, the last pass reduction rate is 16%, the quenching temperature is 883℃, the quenching holding time is 18.5min, the tempering temperature is 563℃, and the tempering holding time is 33min; the finished low alloy high strength steel plate is obtained with a thickness of 14mm.

[0187] Performance Analysis:

[0188] (1) Composition analysis:

[0189] The chemical composition of the finished low-alloy high-strength steel with improved strength and toughness prepared in Example 6 is shown in Table 6, and the rare earth recovery rate reached 67.2%.

[0190] Table 6. Chemical composition (wt.%) of the finished low-alloy high-strength steel from Example 6

[0191]

[0192] (2) Analysis of the degree of segregation:

[0193] Using the analytical method of Example 1, the low-magnification rating of the cast billet reached C0.5.

[0194] (3) Analysis of the grain refinement degree of the cast billet:

[0195] Using the analysis method of Example 1, the average grain size of the cast billet was measured to be 35.65 μm, with large grains larger than 50 μm accounting for 27.46%.

[0196] (4) Strength and toughness analysis:

[0197] Using the analytical method of Example 1, the Q690E steel with added rare earth elements was tested and found to have a yield strength of 765 MPa, a tensile strength of 844 MPa, an elongation of 16.93%, and a strength-ductility product of 14288, demonstrating high strength and toughness.

[0198] (5) Tissue performance analysis:

[0199] Using the analytical method of Example 1, the average width of the martensitic lath bundles was measured to be 188 nm.

[0200] (6) Inclusion analysis:

[0201] Using the analytical method of Example 1, the average size of the inclusions was 1.73 μm. The inclusion morphology of the rare earth steel sample was spherical, elliptical and spindle-shaped, which is beneficial to the continuity of the steel structure.

[0202] Example 7

[0203] A method for enhancing the strength and toughness of low-alloy high-strength steel by incorporating rare-earth cerium-iron alloys has been applied to the industrial production of Q690E steel for heavy plates, comprising the following steps:

[0204] (1) Hot metal desulfurization pretreatment, the area of ​​hot metal slag removal is 98%, the sulfur content of hot metal entering the converter is 0.0017%, and the scrap steel is the heavy scrap steel from the head and tail of the wide and thick plate billet with a sulfur content of less than 0.035wt.%.

[0205] (2) Strong deoxidation system and calcium treatment measures are adopted for one-time carbon removal operation of converter steelmaking. The dephosphorization rate is controlled at 69% and the slag discharge rate is 58% in the early stage of converter smelting. Ferromanganese and ferrochrome (fine adjustment) are added for alloying and slag blocking is used for tapping steel. When 1 / 3 of the steel is tapped from the converter, 2.22 kg / t steel fine powder lime is added to dilute and adjust the slag, so as to reduce the FeO and MnO content in the slag and enhance the basicity of the slag. Strong deoxidation system and calcium treatment and other technical measures are adopted in the converter process to strictly control the harmful elements and inclusions in the steel in order to obtain high-purity molten steel.

[0206] (3) LF refining involves temperature adjustment and desulfurization, and the addition of ferromolybdenum and ferrochrome for alloying. Harmful elements and inclusions in the steel are strictly controlled to obtain high-purity molten steel. The LF refining process adopts strong deoxidation system, calcium treatment and other technical measures to strictly control harmful elements and inclusions in the steel to obtain high-purity molten steel. After the LF refining process leaves the station, 1.16 kg / t steel containing 60 wt.% aluminum modifier is added to the surface of the top slag of the ladle to remove oxygen from the slag, and the Ca / Al mass ratio is controlled at 1.8%.

[0207] (4) The vacuum degree of RH vacuum treatment is ≤0.245Kpa. Because rare earth has strong oxidizing properties, in order to avoid it being oxidized by oxygen brought in by other alloys during RH alloying, and to ensure the effective B content in the steel, titanium iron alloy and boron iron alloy are added in sequence for alloying. After alloying, the cycle is 4min, the temperature is measured and the oxygen is determined. The active oxygen is 1ppm. Cerium iron alloy (cerium content 30wt.%) is added at 0.159kg / t steel. RH vacuum treatment is completed after 16min. After RH vacuum treatment, soft blowing is carried out. During soft blowing, the slag surface of the ladle is kept calm and no molten steel surface is exposed to the air. The soft blowing time is 15min to ensure that the inclusions float up fully.

[0208] (5) In the intermediate batch process, a hollow stopper rod is used and the argon gas flow rate is 1.52 L / min. The immersion nozzle is sealed with argon gas in the slide plate to prevent nitrogen and oxygen from increasing.

[0209] (6) Continuous casting is carried out. The casting machine adopts the electromagnetic stirring process in the second cooling zone and dynamic light pressure to improve the internal quality of the billet and obtain the billet with a specification of 2200 × 250mm.

[0210] (7) Rolling and heat treatment: the furnace exit temperature before rolling is 1175℃, the rough rolling start temperature is 1155℃, the intermediate billet thickness is 32mm, the finishing rolling temperature is 815℃, the last pass reduction rate is 16%, the quenching temperature is 881℃, the quenching holding time is 18.2min, the tempering temperature is 561℃, and the tempering holding time is 31min; the finished low alloy high strength steel plate is obtained with a thickness of 12mm.

[0211] Performance Analysis:

[0212] (1) Composition analysis:

[0213] The chemical composition of the finished low-alloy high-strength steel with improved strength and toughness prepared in Example 7 is shown in Table 7, and the rare earth recovery rate reached 63.6%.

[0214] Table 7. Chemical composition (wt.%) of the finished low-alloy high-strength steel from Example 7

[0215]

[0216] (2) Analysis of the degree of segregation:

[0217] Using the analysis method of Example 1, the low-magnification rating of the cast billet reached C2.0.

[0218] (3) Analysis of the grain refinement degree of the cast billet:

[0219] Using the analysis method of Example 1, the average grain size of the cast billet was measured to be 32.85 μm, with large grains larger than 50 μm accounting for 24.63%.

[0220] (4) Strength and toughness analysis:

[0221] Using the analytical method of Example 1, the Q690E steel plate with added rare earth elements was tested and found to have a yield strength of 768 MPa, a tensile strength of 852 MPa, an elongation of 16.6%, and a strength-ductility product of 14143, demonstrating high strength and toughness.

[0222] (5) Tissue performance analysis:

[0223] Using the analytical method of Example 1, the average width of the martensitic lath bundles was measured to be 195 nm. Figure 7 The TEM image shows that the martensite lamellar spacing is relatively fine, the precipitation of carbide particles inside the grains is relatively small, the continuous precipitation at grain boundaries is small, and the structure is dense with increased dislocation density. The lath boundaries contain a large number of fine carbide dispersed precipitation, which can play a role in precipitation strengthening and is conducive to the steel plate having better low-temperature impact toughness.

[0224] (6) Inclusion analysis:

[0225] Using the analytical method of Example 1, the average size of the inclusions was 1.79 μm. The inclusion morphology of the rare earth steel sample was spherical, elliptical and spindle-shaped, which is beneficial to the continuity of the steel structure.

[0226] Example 8

[0227] A method for enhancing the strength and toughness of low-alloy high-strength steel by incorporating rare-earth cerium-iron alloys has been applied to the industrial production of Q690E steel for heavy plates, comprising the following steps:

[0228] (1) Hot metal desulfurization pretreatment, the area of ​​hot metal slag removal is 98%, the sulfur content of hot metal entering the converter is 0.0016%, and the scrap steel is the heavy scrap steel from the head and tail of the wide and thick plate billet with a sulfur content of less than 0.035wt.%.

[0229] (2) Strong deoxidation system and calcium treatment measures are adopted to carry out steelmaking in a single carbon removal operation in the converter. The dephosphorization rate is controlled at 66% and the slag discharge rate is 57% in the early stage of converter smelting. Ferromanganese and ferrochrome (fine adjustment) are added for alloying and slag blocking is used for tapping. When 1 / 4 of the steel is tapped from the converter, 3.3 kg / t steel fine-grained lime is added to dilute and adjust the slag, so as to reduce the FeO and MnO content in the slag and enhance the basicity of the slag. Strong deoxidation system and calcium treatment are adopted in the converter process to strictly control the harmful elements and inclusions in the steel in order to obtain high-purity molten steel.

[0230] (3) LF refining involves temperature adjustment and desulfurization, and the addition of ferromolybdenum and ferrochrome for alloying. Harmful elements and inclusions in the steel are strictly controlled to obtain high-purity molten steel. The LF refining process adopts strong deoxidation system, calcium treatment and other technical measures to strictly control harmful elements and inclusions in the steel to obtain high-purity molten steel. After the LF refining process leaves the station, 1.8 kg / t steel containing 60 wt.% aluminum modifier is added to the surface of the top slag of the ladle to remove oxygen from the slag, and the Ca / Al mass ratio is controlled at 1.95%.

[0231] (4) The vacuum degree of RH vacuum treatment is ≤0.265Kpa. Because rare earth has strong oxidizing properties, in order to avoid it being oxidized by oxygen brought in by other alloys during RH alloying, and to ensure the effective B content in the steel, titanium iron alloy and boron iron alloy are added in sequence for alloying. After alloying, the cycle is 2min, the temperature is measured and the oxygen is determined. The active oxygen is 1ppm. Cerium iron alloy (cerium content 30wt.%) is added at 0.153kg / t steel. RH vacuum treatment is completed after 23min. After RH vacuum treatment, soft blowing is carried out. During soft blowing, the slag surface of the ladle is kept calm and no molten steel surface is exposed to the air. The soft blowing time is 15min to ensure that the inclusions float up fully.

[0232] (5) In the intermediate batch process, a hollow stopper rod is used and the argon gas flow rate is 1.52 L / min. The immersion nozzle is sealed with argon gas in the slide plate to prevent nitrogen and oxygen from increasing.

[0233] (6) Continuous casting is carried out. The casting machine adopts the electromagnetic stirring process in the second cooling zone and dynamic light pressure to improve the internal quality of the billet and obtain the billet with a specification of 2000 × 250mm.

[0234] (7) Rolling and heat treatment: the furnace exit temperature before rolling is 1171℃, the rough rolling start temperature is 1160℃, the intermediate billet thickness is 35mm, the finishing rolling temperature is 805℃, the last pass reduction rate is 14%, the quenching temperature is 890℃, the quenching holding time is 17min, the tempering temperature is 580℃, and the tempering holding time is 35min; the finished low alloy high strength steel plate is obtained, and the thickness of the finished low alloy high strength steel plate is 15mm.

[0235] Performance Analysis:

[0236] (1) Composition analysis:

[0237] The chemical composition of the finished low-alloy high-strength steel with improved strength and toughness prepared in Example 8 is shown in Table 8, and the rare earth recovery rate reached 68.5%.

[0238] Table 8. Chemical composition (wt.%) of the finished low-alloy high-strength steel from Example 8

[0239]

[0240] (2) Analysis of the degree of segregation:

[0241] Using the analysis method of Example 1, the low-magnification rating of the cast billet reached C1.5.

[0242] (3) Analysis of the grain refinement degree of the cast billet:

[0243] Using the analytical method of Example 1, the average grain size of the cast billet was measured to be 31.95 μm, with large grains larger than 50 μm accounting for 25.43%.

[0244] (4) Strength and toughness analysis:

[0245] Using the analytical method of Example 1, the Q690E steel plate with added rare earth elements, after testing, showed a yield strength of 778 MPa, a tensile strength of 897 MPa, an elongation of 15.82%, and a strength-ductility product of 14190, demonstrating high strength and toughness. Example 8 also conducted a low-temperature impact toughness test, as shown... Figure 8 , Figure 9 The transverse and longitudinal low-temperature ductile-brittle transition curves shown indicate that it has good low-temperature impact toughness, and the ductile-brittle transition temperature has reached a control level below -60℃.

[0246] (5) Tissue performance analysis:

[0247] Using the analytical method of Example 1, the average width of the martensitic lath bundles was measured to be 193 nm.

[0248] (6) Inclusion analysis:

[0249] Using the analytical method of Example 1, the average size of the inclusions was 1.56 μm. The inclusion morphology of the rare earth steel sample was spherical, elliptical and spindle-shaped, which is beneficial to the continuity of the steel structure.

[0250] Comparative Example 1

[0251] The only difference from Example 4 is that no cerium-iron alloy was added, resulting in a Q690E steel wide and thick plate billet.

[0252] (1) Composition analysis:

[0253] The chemical composition of the finished low-alloy high-strength steel prepared in Comparative Example 1 is shown in Table 9.

[0254] Table 9. Chemical composition (wt.%) of the finished low-alloy high-strength steel of Comparative Example 1

[0255]

[0256] (2) Analysis of the degree of segregation:

[0257] The low-magnification rating of the cast billet is B2.0, such as Figures 10 to 12 As shown, the average carbon segregation values ​​at various positions along the entire length, and at 1 / 4 and 3 / 4 of the width of the un-rare earth cast billet, corresponding to the full thickness positions, are 1.063, 1.056, and 1.061, respectively. The experimental results indicate that the degree of segregation in the cast billet is poorly controlled, which is detrimental to the uniformity and mechanical properties of the product.

[0258] Comparative Example 2

[0259] The only difference from Example 5 is the absence of cerium-iron alloy, resulting in a Q690E steel wide and thick plate billet. The average grain size of the billet is 45.76 μm, and the proportion of large grains (50 μm and above) is 40.41%. Figure 13 The morphology images shown under different fields of view reveal that the grain morphology is mainly triangular, knife-edge, and polygonal, with relatively sharp edges. The grains are also relatively slender and large in size. The coarse cast grains are not conducive to the mechanical properties of the product.

[0260] Comparative Example 3

[0261] The difference from Example 1 is that cerium-iron alloy was not added. Additionally, after LF refining, an aluminum modifier containing 60 wt.% aluminum (1.08 kg / t steel) was added to the surface of the ladle top slag to remove oxygen, with a Ca / Al mass ratio of 2.1%. This failed to achieve effective deoxidation and rare earth alteration inclusion removal. Figure 14 The SEM image shown shows that the average size of the inclusions in the steel plate product is 10.32 μm. The inclusions in the rare earth steel sample are elongated with obvious sharp corners at the head. The tips of such inclusions are the source of easy crack initiation and propagation, which will seriously impair the strength and toughness of the steel.

[0262] Comparative Example 4

[0263] The difference from Example 7 is that no cerium-iron alloy was added, the final pass reduction rate was 13%, and the quenching temperature was 865℃-890℃. This failed to leverage the role of rare earth elements in steel and the refining control of the steel plate microstructure through large reduction and narrow-window temperature tempering. The average width of the martensite lath bundles was measured to be 533nm. The final steel plate obtained under the process of Comparative Example 4 is as follows... Figure 15 As shown, TEM analysis revealed that carbide particles precipitated on the grain surface and at grain boundaries, and the number of particles was relatively large. The martensite lamellar spacing was also large, which is not conducive to improving the mechanical properties of the product through grain refinement.

[0264] Comparative Example 5

[0265] The difference from Example 8 is that the quenching temperature was 892℃, the quenching holding time was 15 min, and the tempering holding time was 16 min; the effective time for grain refinement and favorable microstructure transformation during the heat treatment of the steel plate was not guaranteed. Mechanical property testing of the finished steel plate showed that the low-temperature impact toughness of the steel plate processed in Comparative Example 5 was relatively low. Figure 16 , Figure 17 As shown, the ductile-brittle transition temperature control only reaches a control level of -40℃.

[0266] Comparative Example 6

[0267] The difference from Example 1 is that no rare earth was added. The furnace exit temperature before rolling was 1168°C, the rough rolling start temperature was 1136°C, and the reduction rate of the last pass was 13%. It failed to achieve the effect of rare earth quenching and tempering in removing inclusions and refining grains. In addition, the grain refinement of the steel plate was insufficient due to the insufficient rolling temperature. After testing, the strength-ductility product of the Q690E steel plate without rare earth was 10685, which failed to meet the requirements of high strength and toughness for service.

[0268] As can be seen from Examples 1 to 8 and Comparative Examples 1 to 6, the method of adding rare earth cerium-iron alloy to improve the strength and toughness of low-alloy high-strength steel according to the present invention is applied to the industrial production of Q690E steel in thick plates. The degree of segregation inside the billet is well controlled; it can obtain refined as-cast grains, which is beneficial to the fine grain strengthening effect of the final product; the inclusion morphology of rare earth steel samples is more conducive to the continuity of steel structure; the martensite lamellar spacing is relatively fine, the precipitation of carbide particles inside the grains is relatively small, the continuous precipitation at grain boundaries is small, and the structure is dense with increased dislocation density. The lath boundaries contain a large number of fine carbide dispersed precipitation, which can play a precipitation strengthening role and is conducive to the steel plate having better low-temperature impact toughness; it has good low-temperature impact toughness.

Claims

1. A method for improving the strength and toughness of low-alloy high-strength steel by adding rare earth cerium-iron alloy, characterized in that, In the existing production process of low-alloy high-strength and high-toughness steel for thick plates, narrow window control is applied to converter steelmaking, LF refining and RH vacuum treatment. After the alloying process is completed and the cycle is completed in the RH vacuum treatment process, cerium-iron alloy is added. Before adding cerium-iron alloy, the oxygen content is measured by temperature and oxygen activity is ≤3ppm.

2. The method for improving the strength and toughness of low-alloy high-strength steel by adding rare earth cerium-iron alloy as described in claim 1, characterized in that, The steel used for the wide and thick plates is Q690E, which is a low-alloy high-strength and high-toughness steel.

3. The method for improving the strength and toughness of low-alloy high-strength steel by adding rare earth cerium-iron alloy as described in claim 2, characterized in that, Narrow window control involves controlling the dephosphorization rate to >60% and the slag discharge rate to >50% in the early stage of converter smelting. When the converter taps 1 / 4 to 1 / 3 of the steel, 2.5 kg / t steel to 3.3 kg / t steel of fine-grained quicklime is added to dilute and adjust the slag, so as to reduce the FeO and MnO content in the slag and enhance the basicity of the slag. After LF refining and before RH vacuum treatment, 1.1 kg / t steel to 1.8 kg / t steel of aluminum modifier is added to the surface of the top slag of the ladle to remove oxygen from the slag, and the Ca / Al mass ratio is controlled at 1.8%-2.0%.

4. The application of the method for improving the strength and toughness of low-alloy high-strength steel by adding rare earth cerium-iron alloy as described in claim 1 in the industrial production of heavy plate Q690E steel.

5. The application of the method for improving the strength and toughness of low-alloy high-strength steel by adding rare earth cerium-iron alloy as described in claim 4 in the industrial production of heavy plate Q690E steel, characterized in that, Includes the following steps: (1) Desulfurization pretreatment of molten iron; (2) In a single carbon removal operation, the converter steelmaking process involves adding ferromanganese and ferrochrome for alloying, followed by slag blocking and steel tapping. (3) LF refining involves temperature adjustment and desulfurization, and the addition of ferromolybdenum and ferrochrome for alloying; (4) RH vacuum treatment, add ferro-titanium alloy and ferro-boron alloy in sequence for alloying, circulate after alloying, add ferro-cerium alloy; after RH vacuum treatment, perform soft blowing. (5) Intermediate package process; (6) Under protective measures to prevent secondary oxidation, continuous casting is carried out to obtain a cast billet; (7) Rolling and heat treatment to obtain finished low-alloy high-strength steel plates with refined tempered sorbite structure.

6. The application of the method for improving the strength and toughness of low-alloy high-strength steel by adding rare earth cerium-iron alloy as described in claim 5 in the industrial production of heavy plate Q690E steel, characterized in that, Step (1) The area of ​​molten iron slag removal should be >95%, and the sulfur content of molten iron entering the converter should be <0.002%.

7. The application of the method for improving the strength and toughness of low-alloy high-strength steel by adding rare earth cerium-iron alloy as described in claim 5 in the industrial production of heavy plate Q690E steel, characterized in that, Step (4) The vacuum degree of RH vacuum treatment is ≤0.266Kpa, the vacuum treatment time is 20±3min, the circulation time is 3±1min; the soft blowing time is ≥15min; After the cycle is completed, the cerium content in the added ferrocerium alloy is 25wt.%-35wt.%, and the amount of ferrocerium alloy added is based on the cerium content in the prepared Q690E steel composition being 0.0017wt.%-0.0040wt.%.

8. The application of the method for improving the strength and toughness of low-alloy high-strength steel by adding rare earth cerium-iron alloy as described in claim 5 in the industrial production of heavy plate Q690E steel, characterized in that, Step (6) The protective measures to prevent secondary oxidation are to use a hollow plug rod, introduce argon gas at a flow rate ≥1.5L / min, and use argon gas sealing between the slide plates for the immersion nozzle; The continuous casting machine adopts a two-cooling-zone electromagnetic stirring process and a dynamic light reduction process.

9. The application of the method for improving the strength and toughness of low-alloy high-strength steel by adding rare earth cerium-iron alloy as described in claim 5 in the industrial production of heavy plate Q690E steel, characterized in that, Step (7) Rolling includes rough rolling and finish rolling, and heat treatment includes quenching and tempering.

10. The application of the method for improving the strength and toughness of low-alloy high-strength steel by adding rare earth cerium-iron alloy as described in claim 9 in the industrial production of heavy plate Q690E steel, characterized in that, The furnace exit temperature before rolling is 1170℃-1190℃, the roughing rolling temperature is 1140℃-1160℃, the intermediate billet thickness is 30±5mm, the finishing rolling temperature is 805℃-825℃, the reduction rate of the last pass is 14%-16%, the quenching temperature is 870℃-890℃, the quenching holding time is 20±3min, the tempering temperature is 560℃-580℃, and the tempering holding time is 25min-35min.

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