Production method of rare earth microalloyed high-strength bridge steel

Abstract

The invention discloses a production method of rare earth microalloyed high-strength bridge steel, and belongs to the technical field of hot rolling. The production method comprises the steps of KR desulfurization, converter smelting, LF refining, RH refining, continuous casting, heating, rolling, cooling, hot straightening, shearing, flaw detection, normalizing and sampling inspection. The bridge steel comprises the following chemical components in percentage by mass: 0.14 to 0.16 percent of C, 0.30 to 0.50 percent of S , 1.40 to 1.70 percent of Mn, less than or equal to 0.020 percent of P, less than or equal to 0.010 percent of S, 0.015 to 0.035 percent of Al < s >, 0.03 to 0.06 percent of Nb, 0.04 to 0.07 percent of V, less than or equal to 0.020 percent of T , 0.2 to 0.4 percent of N , less than or equal to 0.0010 percent of Ce and less than or equal to 0.0080 percent of N. Through the production process of slab quenching and hot charging, normalizing heat treatment is adopted, and the product quality is stabilized.

Description

Technical Field

[0001] The invention belongs to the technical field of hot rolling, and in particular relates to a production method of rare earth micro-alloyed high-strength bridge steel. Background Art

[0002] As bridges develop towards larger spans, heavier loads, and higher speeds, the use of traditional bridge steel requires a significant increase in the thickness of the steel plates, which does not conform to the requirements of bridge weight reduction and an economical society. Therefore, it is necessary to develop higher-strength bridge steel. At the same time, the steel plates must also have good low-temperature toughness and weldability, and high-strength bridge steel has emerged.

[0003] High-performance bridge steels have been successfully developed. The basic concept is to use C-Mn steel as the raw material, alloying it with elements such as Nb, V, Ti, Ni, Cr, Cu, Mo, and rare earth elements. Typical production processes include TMCP and controlled rolling and tempering. The chemical composition of the steel plate varies depending on the production process.

[0004] The present invention aims to provide a method for producing rare earth microalloyed high-strength bridge steel with low welding crack sensitivity. The method adopts a normalizing heat treatment process, a medium carbon composition design, and adds appropriate amounts of Mn, Nb, V, Ti and rare earth elements to control the narrow composition of microalloying. The rolling and heat treatment processes are precisely controlled to ensure that the steel plate has low welding crack sensitivity and excellent comprehensive performance, especially the low-temperature toughness meets the high standards of users.

[0005] The document "Research and Application of Shougang's High-Performance Bridge Steel Q420qE" analyzes the composition and process design of Q420qE steel under the controlled rolling + tempering process. The composition not only contains alloys such as Nb, V, and Ti, but also alloys such as Ni, Cr, Cu, and Mo, resulting in a high production cost. This patent provides a normalizing process production method for rare earth micro-alloyed high-strength bridge steel with low welding crack sensitivity, which has low cost, stable performance, and is suitable for mass production.

[0006] Patent CN201310295467, "A Low Yield Ratio High-Strength Bridge Steel and Its Manufacturing Method," provides a production method for 20-32mm thick bridge steel with a yield strength greater than 500 MPa using the TMCP+tempering process. This method incorporates not only Nb, V, and Ti alloys, but also Ni, Cr, Cu, and Mo alloys. This results in high production costs, and the impact energy performance margin is too small compared to the impact energy requirements of the national standard GB / T714-2015 "Structural Steel for Bridges," making it unsuitable for mass production. This method provides a production method for a 30-50mm thick, 420 MPa yield strength, rare earth microalloyed high-strength bridge steel with low weld crack sensitivity using normalizing technology. The addition of rare earths reduces weld crack sensitivity, and the slab quenching and hot-loading process reduces manufacturing costs. Compared to the aforementioned patented processes, this method is more stable and easier to control, with excellent low-temperature toughness and weldability.

[0007] The document "Trial Production and Development of High-Strength Bridge Steel Q420qD" provides a heat treatment normalizing process for a thickness of 16-50mm. The composition uses Mn, V, Ti and alloys, and the impact energy performance does not meet the impact energy requirements of the national standard GB / T 714-2015 "Structural Steel for Bridges"; this patent provides a production method for rare earth micro-alloyed high-strength bridge steel with low welding crack sensitivity and excellent impact toughness. Summary of the Invention

[0008] The purpose of the present invention is to provide a production method of rare earth micro-alloyed high-strength bridge steel. Through the production process of slab quenching + hot charging, the rolling process is precisely controlled, the steel plate is subjected to normalizing heat treatment, the whole process of production process and batch production process are clarified, and the product quality is stabilized.

[0009] As steel plate strength increases, its weldability significantly declines, and weld crack susceptibility increases. In particular, as weld heat input increases, the heat-affected zone (HAZ) performance of conventional low-alloy high-strength steel deteriorates. High-strength steel typically requires welding prior to use. Currently, increasing high-heat-input welding to improve welding efficiency and reduce costs has become a hot trend in the bridge industry. To improve the HAZ performance of thick plate welds, rare earth steels utilize small, dispersed, and compositionally controlled oxide inclusions generated during the steelmaking process to modify the steel's microstructure and grain size, improving weldability. Rare earth oxides delay the bainite transformation during welding and inhibit the formation of upper bainite. Furthermore, fine rare earth oxide inclusions inhibit austenite grain growth. Furthermore, previous studies examining the effects of varying rare earth, oxygen, and sulfur content on inclusions in steel, as well as the effects of rare earth inclusions on austenite grain boundaries, have shown that controlling the formation of rare earth sulfides in steel is more beneficial for improving HAZ toughness than the inclusion of rare earth oxides, rare earth aluminates, and rare earth oxysulfides.

[0010] In view of the above principles, through a large number of experimental studies, it is proposed to reduce the welding crack sensitivity index Pcm of steel plates with low welding crack sensitivity by adding rare earth. The welding crack sensitivity index Pcm is revised as follows:

[0011] Pcm(%)=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B-120Ce.

[0012] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0013] The present invention provides a method for producing rare earth microalloyed high-strength bridge steel, which comprises KR desulfurization → converter smelting → LF refining → RH refining → continuous casting → heating → rolling → cooling → hot straightening → shearing → flaw detection → normalizing → sampling inspection; specifically:

[0014] 1) The tapping temperature of the converter is 1620-1660℃, and the single slag process is adopted for smelting. The converter adopts single slag operation, uses high-quality active lime, and the final slag basicity is controlled at 3.0-3.5; the converter bottom blowing gas adopts the full argon blowing mode, and the end point is hit as much as possible in one time to reduce the nitrogen addition of molten steel due to supplementary blowing;

[0015] 2) Refining accurately controls the composition, and the amount of white ash added is ≥5kg / ton of steel. The slag sample must be dipped during the refining process to ensure that the white slag is quickly formed and the white slag is kept for a certain period of time;

[0016] 3) The molten steel is treated in an RH furnace. The RH vacuum treatment time is not less than 20 minutes, the pure degassing time is not less than 5 minutes, the minimum vacuum degree is 260Pa, and the off-site hydrogen content is less than or equal to 2.0ppm. The RH treated molten steel is fed with wire in an RH furnace. During the feeding period, the ladle adopts weak stirring with bottom argon blowing, so that the molten steel is not exposed. After the feeding is completed, the soft argon blowing time is more than 5 minutes to ensure uniform composition and sufficient floating of inclusions. Then the steel is allowed to stand and steel is added. No further heating treatment or addition of other alloys is allowed after feeding.

[0017] 4) When pouring continuous steel, the superheat should be controlled at 15-26℃, the argon blowing pressure of the long nozzle should be ≥0.1Mpa, the argon blowing pressure gauge flow rate between the submerged nozzle plates of the tundish should reach 6-10L / min, the pressure gauge pressure should be 0.1-0.5Bar, and the casting speed of the casting machine should be 0.9-1.1m / min;

[0018] 5) Optimize and adjust the acceleration time of the roller in the quenching beam to control the surface quenching cooling time of the slab to 180s-250s, and design 16 sets of quenching cooling water beams; the cooling time is slow cooling time ≤ 250min, the slab furnace temperature is 500-600℃, the total furnace time is ≥ 150min, and the slab is heated in a three-stage walking beam furnace. The heating temperature of the first heating section is 1100-1150℃, the heating time is 40-60 minutes; the heating temperature of the second heating section is 1200-1280℃, the heating time is 40-60min; the soaking temperature is 1230-1300℃, the heating time is ≥ 30min, the total heating time is not less than 150min, and the slab furnace discharge temperature is 1160-1210℃;

[0019] 6) Rolling and cooling process: After the slab is heated, it is subjected to two-stage controlled rolling. The first stage of rolling is completed on the roughing mill. The first stage rolling thickness is the slab thickness. The first stage rolling temperature is 1160-1200℃, the rolling speed is 2.0-3.2m / s, and the single pass reduction rate in the high temperature extension stage is not less than 18%; the second stage of rolling is completed on the finishing mill. The second stage rolling temperature is 890-960℃, the second stage rolling thickness is 50-125mm, the second stage final rolling temperature is 820-880℃, and the final pass reduction rate is 5%-10% to ensure the plate shape; after rolling, the steel plate is laminar cooled, the ACC water temperature is 17-20℃, the cooling rate is 5-15℃ / s, and the final cooling temperature is 650-700℃;

[0020] 7) Heat treatment process: The steel plate needs to be shot blasted before normalizing. The normalizing temperature is 870-900℃, the holding time is set to 10 minutes, and the air is cooled to room temperature after leaving the furnace;

[0021] The chemical composition of the bridge steel by weight percentage is (wt%): C: 0.14-0.16, Si: 0.30-0.50, Mn: 1.40-1.70, P: ≤0.020, S: ≤0.010, Als: 0.015-0.035, Nb: 0.03-0.06, V: 0.04-0.07, Ti≤0.020, Ni: 0.2-0.4, Ce: ≤0.0010, N: ≤0.0080; the rest is iron and unavoidable impurities;

[0022] By adding rare earth to reduce the welding crack sensitivity index Pcm of steel plates with low welding crack sensitivity, the welding crack sensitivity index Pcm is revised as follows:

[0023] Pcm(%)=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B-120Ce.

[0024] Furthermore, in order to ensure the quality of slabs with a certain compression ratio, 300mm thick continuous casting slabs are selected, the center segregation of the slab is not greater than B1.0, the center porosity is not greater than level 2.5, and the center crack is not greater than level 1.0.

[0025] Furthermore, by tracking the position of the ingot under different superheat levels and combining it with the continuous temperature measurement system of the tundish, the water volume in the secondary cooling water zone 3-12 is adjusted, so that during the pouring process of molten steel with a superheat level within the range of 10-30°C, the setting parameters of the primary and secondary cooling levels are changed according to different superheat levels, so that the liquid core length remains unchanged and the position of the light pressure remains unchanged, thereby improving the internal quality of the ingot and ensuring a constant pouring speed.

[0026] Furthermore, during the first stage of rolling, the torque was set to 2400 kNm and the reduction was set to 32 mm.

[0027] Furthermore, during the second stage of rolling, the torque is set to 2200 kNm, the rolling force is set to 85 MN, and the reduction is set to 20 mm.

[0028] Furthermore, during laminar cooling: the head is shielded at 0-2.0m, the tail is shielded at 0-2.5m, and the side is shielded at 0-2.0m, and the overall temperature difference after the steel plate returns to red is controlled to be ≤50℃.

[0029] Furthermore, the chemical composition of the bridge steel in mass percentage is: C: 0.15%, Si: 0.40%, Mn: 1.60%, P: 0.012%, S: 0.004%, Als: 0.027%, Nb: 0.50%, V: 0.05%, Ti: 0.010%, Ni: 0.32%, Ce: 0.0004%, N: 0.0052%, and the remainder is Fe and unavoidable impurities.

[0030] Furthermore, the chemical composition of the bridge steel in mass percentage is: C: 0.14%, Si: 0.38%, Mn: 1.65%, P: 0.015%, S: 0.002%, Als: 0.028%, Nb: 0.50%, V: 0.05%, Ti: 0.015%, Ni: 0.36%, Ce: 0.0004%, N: 0.0030%, and the remainder is Fe and unavoidable impurities.

[0031] Furthermore, the chemical composition of the bridge steel in mass percentage is: C: 0.16%, Si: 0.42%, Mn: 1.58%, P: 0.010%, S: 0.003%, Als: 0.030%, Nb: 0.52%, V: 0.06%, Ti: 0.012%, Ni: 0.35%, Ce: 0.0005%, N: 0.0046%, and the remainder is Fe and unavoidable impurities.

[0032] Compared with the prior art, the present invention has the following beneficial technical effects:

[0033] (1) The thickness of the steel plate is 30mm-50mm;

[0034] (2) Proper alloying of the steel plate reduces the alloy cost while ensuring the performance of the steel plate;

[0035] (3) The slab adopts quenching process and is directly warm-loaded, which reduces energy consumption and improves performance stability;

[0036] (4) The addition of appropriate amounts of rare earth elements reduces the welding crack sensitivity coefficient and improves the welding performance of the steel plate;

[0037] The outstanding advantages of the present invention are its use of a C-Mn composition design, the addition of appropriate amounts of Nb, V, Ti, and rare earth elements for narrow composition control, and the adoption of a normalizing process. This method improves the performance stability of thick-gauge bridge steel plates, reduces weld crack sensitivity, and enhances the weldability of the steel plates. This broadens the process design window, improves mass production efficiency, and enables the stable mass production of 30mm-50mm thick rare earth microalloyed high-strength bridge steel with a yield strength of 420 MPa and low weld crack sensitivity. Actual production and testing have demonstrated excellent mechanical properties. The strength of the steel plates in each embodiment meets standard requirements, with an elongation greater than 20%, an impact energy of more than 180 J at -40°C, and a weld crack sensitivity index Pcm less than 0.22%. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The present invention will be further described below with reference to the accompanying drawings.

[0039] Figure 1 These are metallographic photographs of 40mm thick Q420qE bridge steel in rolled and normalized states. DETAILED DESCRIPTION

[0040] The present invention is further described below with reference to the examples.

[0041] Example 1

[0042] The slab surface quenching cooling time was controlled at 195 seconds. The slab was then directly placed warm in a heating furnace for 185 minutes and a soaking time of 36 minutes. The chemical composition of the slab by weight was as follows: C: 0.15, Si: 0.40, Mn: 1.60, P: 0.012, S: 0.004, Als: 0.027, Nb: 0.50, V: 0.05, Ti: 0.010, Ni: 0.32, Ce: 0.0004, N: 0.0052, Pcm: 0.21; the remainder was Fe and unavoidable impurities. The slab was rolled into a 40 mm thick steel plate. The detailed rolling process is shown in Table 1, and the mechanical properties are shown in Table 2.

[0043] Example 2

[0044] The slab surface quenching cooling time was controlled at 200 seconds. The slab was then directly placed warm in a heating furnace for 160 minutes and a soaking time of 41 minutes. The chemical composition of the slab by weight is as follows: C: 0.14, Si: 0.38, Mn: 1.65, P: 0.015, S: 0.002, Als: 0.028, Nb: 0.50, V: 0.05, Ti: 0.015, Ni: 0.36, Ce: 0.0004, N: 0.0030, Pcm: 0.20; the balance is Fe and unavoidable impurities. The slab was rolled into a 45 mm thick steel plate. The detailed rolling process is shown in Table 1, and the mechanical properties are shown in Table 2.

[0045] Example 3

[0046] The slab surface quenching cooling time was controlled at 228 seconds. The slab was then directly placed warm into a heating furnace for 200 minutes and a soaking time of 45 minutes. The chemical composition of the slab by weight is as follows: C: 0.16, Si: 0.42, Mn: 1.58, P: 0.010, S: 0.003, Als: 0.030, Nb: 0.52, V: 0.06, Ti: 0.012, Ni: 0.35, Ce: 0.0005, N: 0.0046, Pcm: 0.20; the balance is Fe and unavoidable impurities. The steel was rolled into a 50 mm thick plate. The detailed rolling and heat treatment processes are shown in Table 1, and the mechanical properties are shown in Table 2.

[0047] A rare earth micro-alloyed high-strength bridge steel with low welding crack sensitivity produced using this composition and process design has chemical composition and mechanical properties that meet the requirements of the national standard GB / T 714-2015 "Structural Steel for Bridges". It has appropriate strength, low yield strength ratio, excellent elongation, and especially outstanding low-temperature impact resistance. Users report excellent welding performance.

[0048] Table 1 Process parameters of Examples 1-3

[0049]

[0050] Table 2 Mechanical properties of Examples 1-3

[0051]

[0052] Microstructure:

[0053] Figure 1Comparison photos of the metallographic structures of Q420qE bridge steel in the as-rolled and normalized states. The as-rolled structure of the 40mm thick plate is ferrite + pearlite, with a grain size of 8-9. After normalizing, the grain size is finer, reaching 10-11. The pearlite is spherical and dispersed.

[0054] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for producing rare earth microalloyed high-strength bridge steel, characterized by: include: 1) The tapping temperature of the converter is 1620-1660℃, and the single slag process is adopted for smelting. The converter adopts single slag operation, uses high-quality active lime, and the final slag basicity is controlled at 3.0-3.5; the converter bottom blowing gas adopts the full argon blowing mode, and the end point is hit as much as possible in one time to reduce the nitrogen addition of molten steel due to supplementary blowing; 2) Refining accurately controls the composition, and the amount of white ash added is ≥5kg / ton of steel. The slag sample must be dipped during the refining process to ensure that the white slag is quickly formed and the white slag is kept for a certain period of time; 3) The molten steel is treated in an RH furnace. The RH vacuum treatment time is not less than 20 minutes, the pure degassing time is not less than 5 minutes, the minimum vacuum degree is 260Pa, and the off-site hydrogen content is less than or equal to 2.0ppm. The RH treated molten steel is fed with wire in an RH furnace. During the feeding period, the ladle adopts weak stirring with bottom argon blowing, so that the molten steel is not exposed. After the feeding is completed, the soft argon blowing time is more than 5 minutes to ensure uniform composition and sufficient floating of inclusions. Then the steel is allowed to stand and steel is added. No further heating treatment or addition of other alloys is allowed after feeding. 4) When pouring continuous steel, the superheat should be controlled at 15-26℃, the argon blowing pressure of the long nozzle should be ≥0.1Mpa, the argon blowing pressure gauge flow rate between the submerged nozzle plates of the tundish should reach 6-10L / min, the pressure gauge pressure should be 0.1-0.5Bar, and the casting speed of the casting machine should be 0.9-1.1m / min; 5) Optimize and adjust the acceleration time of the roller in the quenching beam to control the surface quenching cooling time of the slab to 180s-250s, and design 16 sets of quenching cooling water beams; the cooling time is slow cooling time ≤ 250min, the slab furnace temperature is 500-600℃, the total furnace time is ≥ 150min, and the slab is heated in a three-stage walking beam furnace. The heating temperature of the first heating section is 1100-1150℃, the heating time is 40-60 minutes; the heating temperature of the second heating section is 1200-1280℃, the heating time is 40-60min; the soaking temperature is 1230-1300℃, the heating time is ≥ 30min, the total heating time is not less than 150min, and the slab furnace discharge temperature is 1160-1210℃; 6) Rolling and cooling process: After the slab is heated, it is subjected to two-stage controlled rolling. The first stage of rolling is completed on the roughing mill. The first stage rolling thickness is the slab thickness. The first stage rolling temperature is 1160-1200℃, the rolling speed is 2.0-3.2m / s, and the single pass reduction rate in the high temperature extension stage is not less than 18%; the second stage of rolling is completed on the finishing mill. The second stage rolling temperature is 890-960℃, the second stage rolling thickness is 50-125mm, the second stage final rolling temperature is 820-880℃, and the final pass reduction rate is 5%-10% to ensure the plate shape; after rolling, the steel plate is laminar cooled, the ACC water temperature is 17-20℃, the cooling rate is 5-15℃ / s, and the final cooling temperature is 650-700℃; 7) Heat treatment process: The steel plate needs to be shot blasted before normalizing. The normalizing temperature is 870-900℃, the holding time is set to 10 minutes, and the air is cooled to room temperature after leaving the furnace; The chemical composition of the bridge steel by mass percentage is: C: 0.14-0.16%, Si: 0.30-0.50%, Mn: 1.40-1.70%, P: ≤0.020%, S: ≤0.010%, Als: 0.015-0.035%, Nb: 0.03-0.06%, V: 0.04-0.07%, Ti≤0.020%, Ni: 0.2-0.4%, Ce: ≤0.0010%, N: ≤0.0080%; the remainder is iron and unavoidable impurities; By adding rare earth to reduce the welding crack sensitivity index Pcm of steel plates with low welding crack sensitivity, the welding crack sensitivity index Pcm is revised as follows: Pcm(%)=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B-120Ce.

2. The method for producing rare earth microalloyed high-strength bridge steel according to claim 1, characterized in that: In order to ensure the quality of slab with a certain compression ratio, 300mm thick continuous casting slab is selected, the center segregation of the slab is not greater than B1.0, the center porosity is not greater than level 2.5, and the center crack is not greater than level 1.

0.

3. The method for producing rare earth microalloyed high-strength bridge steel according to claim 1, characterized in that: By tracking the position of the ingot under different superheat levels and combining it with the continuous temperature measurement system of the tundish, the water volume in the secondary cooling water zone 3-12 is adjusted. During the pouring process, the molten steel with a superheat level within the range of 10-30°C is adjusted according to the different superheat levels. The primary and secondary cooling setting parameters are changed to keep the liquid core length unchanged and the light pressure position unchanged, thereby improving the internal quality of the ingot and ensuring a constant casting speed.

4. The method for producing rare earth microalloyed high-strength bridge steel according to claim 1, characterized in that: During the first stage of rolling, the torque was set to 2400 kNm and the reduction was set to 32 mm.

5. The method for producing rare earth microalloyed high-strength bridge steel according to claim 1, characterized in that: During the second stage of rolling, the set torque is 2200 kNm, the set rolling force is 85 MN, and the set reduction is 20 mm.

6. The method for producing rare earth microalloyed high-strength bridge steel according to claim 1, characterized in that: During laminar cooling: head shielding 0-2.0m, tail shielding 0-2.5m, side shielding 0-2.0m, control the overall temperature difference of the steel plate after it returns to red to ≤50℃.

7. The method for producing rare earth microalloyed high-strength bridge steel according to claim 1, characterized in that: The chemical composition of the bridge steel by mass percentage is: C: 0.15%, Si: 0.40%, Mn: 1.60%, P: 0.012%, S: 0.004%, Als: 0.027%, Nb: 0.50%, V: 0.05%, Ti: 0.010%, Ni: 0.32%, Ce: 0.0004%, N: 0.0052%, and the remainder is Fe and unavoidable impurities.

8. The method for producing rare earth microalloyed high-strength bridge steel according to claim 1, characterized in that: The chemical composition of the bridge steel by mass percentage is: C: 0.14%, Si: 0.38%, Mn: 1.65%, P: 0.015%, S: 0.002%, Als: 0.028%, Nb: 0.50%, V: 0.05%, Ti: 0.015%, Ni: 0.36%, Ce: 0.0004%, N: 0.0030%, and the remainder is Fe and unavoidable impurities.

9. The method for producing rare earth microalloyed high-strength bridge steel according to claim 1, characterized in that: The chemical composition of the bridge steel by mass percentage is: C: 0.16%, Si: 0.42%, Mn: 1.58%, P: 0.010%, S: 0.003%, Als: 0.030%, Nb: 0.52%, V: 0.06%, Ti: 0.012%, Ni: 0.35%, Ce: 0.0005%, N: 0.0046%, and the remainder is Fe and unavoidable impurities.

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