Low-yield-ratio bridge steel with low-temperature impact toughness and production method thereof

Abstract

The invention discloses low-temperature impact toughness bridge steel with a low yield ratio and a production method thereof, and belongs to the technical field of bridge steel plate production, the bridge steel comprises the following chemical components in percentage by mass: 0.04 to 0.06 percent of C, 0.1 to 0.20 percent of Si, 1.50 to 1.60 percent of Mn, 0.20 to 0.30 percent of Cr, 0.05 to 0.08 percent of Nb, 0.03 to 0.05 percent of Al, 0.01 to 0.03 percent of Ti and the balance of Fe and inevitable impurities. According to the bridge steel, the requirements for low segregation component design, bonding strength and steel plate weldability are met, Cr, Mn and microalloy strengthening are adopted, and through the means of KR desulfurization, LF refining, continuous casting tail end soft reduction and the like, a good macrostructure is obtained, the defects of center porosity, cracks and the like are relieved, guarantee is provided for obtaining good steel plate performance, and the production cost is reduced. And good low-temperature toughness welding performance and application prospects are achieved.

Description

Technical Field

[0001] The invention relates to the technical field of bridge steel plate production, and in particular to a low-yield-to-strength ratio bridge steel with low-temperature impact toughness and a production method thereof. Background Art

[0002] In the construction of cross-river and cross-sea bridge projects, safety is the primary issue that needs to be considered in bridge design, and the yield strength ratio is a key indicator for measuring safety. When a bridge fails due to excessive strength service during service, the material is required to have a large space from yielding to complete fracture in order to provide high safety guarantees. Therefore, bridge steel has strict requirements on the yield strength ratio (the ratio of yield strength to tensile strength).

[0003] In recent years, the industry has continued to conduct detailed analysis and improvement on the composition, process, and production process of bridge steel. Q370qE-level bridge steel can achieve stable and large-scale production by generating and controlling acicular ferrite structure while ensuring strength, low-temperature impact toughness, and yield strength ratio. For example, 370MPa-level bridge steel is used in large projects such as the Hutong Bridge, Pingtan Bridge, and Ma'anshan Bridge. When the strength continues to increase to Q420qE, because the increase rate of soft phase strength is much greater than the increase rate of hard phase strength, traditional bridge steel is often difficult to effectively control the low yield strength ratio while maintaining high strength, resulting in a decrease in product qualification rate and yield rate, limiting the large-scale production and application of Q420qE.

[0004] At present, the strength of bridge steel is mainly improved by solid solution strengthening, fine grain strengthening and precipitation strengthening. The solid solution elements C and Mn can effectively improve the strength, but reduce the welding performance and impact performance of the steel; increasing the content of precious alloy Ni can improve the strength without losing or less toughness, but it will lead to a significant increase in cost; by increasing the Nb and V content to enhance precipitation strengthening, or by increasing the alloy composition to enhance solid solution strengthening, the strength is improved, but the low-temperature toughness will be lost; through the combined effect of grain refinement and precipitation strengthening, the strength of the steel plate can be effectively improved, but it often leads to a sharp increase in the yield strength ratio of the steel plate, and the steel plate is scrapped because the yield strength ratio exceeds 0.85. In addition, as the thickness of the bridge steel plate decreases, the requirements for various properties of the steel plate tend to be strict, and its production difficulty increases significantly. Summary of the invention

[0005] Technical problem to be solved: In view of the problems existing in the production process of bridge steel in the prior art, the present invention provides a low-yield ratio bridge steel with low-temperature impact toughness and a production method thereof, wherein the content of polygonal ferrite and acicular ferrite is controlled according to the thickness, the yield ratio and tensile strength are effectively controlled, and the yield ratio of the obtained high-strength bridge steel is less than 0.85; the composition design is simple, the rolling process is convenient to control, the performance is stable and it has good welding performance.

[0006] Technical solution: The present invention discloses a low-temperature impact toughness and low-yield ratio bridge steel, wherein the chemical composition of the bridge steel comprises, by mass percentage, 0.04-0.06% C, 0.1-0.20% Si, 1.50-1.60% Mn, 0.20-0.30% Cr, 0.05-0.08% Nb, 0.03-0.05% Al, 0.01-0.03% Ti, and the rest is Fe and unavoidable impurities.

[0007] Preferably, the unavoidable impurities include H≤0.00015%, P≤0.010%, S≤0.002%, O≤0.0025%, and N≤0.004%.

[0008] Preferably, the initial thickness of the ingot of the bridge steel is 220~320mm; the final thickness of the obtained bridge steel plate is 10~28mm, the center segregation is ≦C0.5, and the center porosity is ≦0.5 level.

[0009] Preferably, the carbon equivalent C of the obtained bridge steel plate eq The calculation formula is: C eq (%)=(%C)+(%Mn) / 6+(%Mo+%Cr+%vV) / 5+(%Ni+%Cu) / 15; The element symbol in brackets is the mass percentage of the corresponding element, and the % element symbol represents the mass percentage of the corresponding element multiplied by 100.

[0010] Preferably, the bridge steel plate has a tensile strength of 540-660 MPa, a yield strength ≥420 MPa, a yield strength ratio ≤0.85, a carbon equivalent ≤0.45%, and has good welding performance.

[0011] Preferably, the flaw detection of the bridge steel plate meets the Level 1 standard of NB / T47013, and the impact value of the center of the bridge steel plate thickness is ≥120J under -40°C conditions, there is no crack defect in the center and it has good low-temperature toughness.

[0012] The present invention also discloses a production method of low yield ratio bridge steel, which comprises the following steps: Step 1: Smelting is carried out according to the chemical composition design scheme above, the slab is cut twice and loaded into the furnace, and the loading temperature is 300~600℃; Step 2: uniformly heat through the heat recovery section, preheating section, first heating section, second heating section, and soaking section of the heating furnace; the preheating temperature of the heat recovery section is 850~900℃, and the heating time is 80min; the preheating temperature of the preheating section is 1000~1050℃, and the heating time is 60min; the heating temperature of the first heating section is 1100~1150℃, and the heating time is 60min; the heating temperature of the second heating section is 1150~1200℃, and the heating time is 50min; the soaking temperature of the soaking section is 1150~1200℃, and the heating time is 50min; Step 3: After the heating furnace is taken out, dephosphorization is carried out to ensure that the iron oxide scale on the surface of the ingot is completely removed; Step 4: The rolling is divided into two stages: the recrystallization zone and the non-recrystallization zone. The first stage of rolling is carried out after the surface temperature of the ingot is cooled to 1030-1080°C, and the red-returning temperature after rolling is ≥1010°C; the intermediate ingot is rapidly cooled at a rate of 3-5°C / s, and the second stage of rolling is carried out after cooling to 770-820°C. The second stage rolling start temperature is ≤900°C, and the final rolling temperature of the ingot is Ar3±10°C; Step 5: After the final rolling, the ingot is relaxed for 10 to 30 seconds, cooled to a temperature of 200 to 400°C at a cooling rate of 20 to 40°C / min, and the bridge steel plate is obtained.

[0013] Preferably, the calculation formula of Ar3 in step 4 is: Ar3=910-310*(%C)-80*(%Mn)-20*(%Cu)-15*(%Cr)-55*(%Ni)-80*(%Mo); The element symbol in brackets is the mass percentage of the corresponding element, and the % element symbol represents the mass percentage of the corresponding element multiplied by 100.

[0014] Preferably, in step 5, a bridge steel plate having a polygonal ferrite + acicular ferrite structure is obtained, wherein the polygonal ferrite structure accounts for 50-70% and the acicular ferrite structure accounts for 30-50%.

[0015] Preferably, the first stage rolling in step 4 is ensured to be rolled in the complete recrystallization zone, and the reduction ratio of at least 2 passes is ≥ 18% to ensure the core penetration; the intermediate billet thickness is ≥ 2.5t; wherein t is the final thickness of the steel plate / mm; The starting rolling temperature of the second stage rolling is guaranteed to be in the non-recrystallization zone, and the final rolling is guaranteed to be at Ar3±30℃.

[0016] Compared with the prior art, the present invention has at least the following beneficial effects: 1. The composition design of the present invention is simple, the production process is easy to control, and the obtained bridge steel plate has stable performance and good welding performance; 2. This production process controls the content of polygonal ferrite and acicular ferrite according to the different thickness requirements of bridge steel plates. By subdividing the rolling process, adopting low final rolling, relaxation time control, and low final cooling temperature, the final required polygonal ferrite + acicular ferrite structure is obtained, and the yield strength ratio and tensile strength of the steel plate are effectively controlled. The yield strength ratio of the obtained high-strength bridge steel is less than 0.85. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a schematic diagram of the production process of the bridge steel of the present invention; Figure 2 This is the crystal phase diagram of the bridge steel plate of Example 1 of the present invention; Figure 3 This is the crystal phase diagram of the bridge steel plate of Example 2 of the present invention; Figure 4 This is the crystal phase diagram of the bridge steel plate of Example 3 of the present invention; Figure 5 This is the crystal phase diagram of the bridge steel plate of Example 4 of the present invention; Figure 6 This is the crystal phase diagram of the bridge steel plate of Example 5 of the present invention; Figure 7 This is the crystal phase diagram of the bridge steel plate of Example 6 of the present invention; Figure 8 This is the crystal phase diagram of the bridge steel plate of Example 7 of the present invention. DETAILED DESCRIPTION

[0018] To make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the following will be combined with the attached Figures 1 to 8 The technical solutions of the embodiments of the present invention are described clearly and completely. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. All other embodiments obtained by ordinary technicians in this field based on the described embodiments of the present invention belong to the scope of protection of the present invention.

[0019] The invention discloses a bridge steel with low yield ratio and low temperature impact toughness. The chemical composition of the bridge steel includes, by mass percentage, 0.04-0.06% C, 0.1-0.20% Si, 1.50-1.60% Mn, 0.20-0.30% Cr, 0.05-0.08% Nb, 0.03-0.05% Al, 0.01-0.03% Ti, and the rest is Fe and inevitable impurities. The inevitable impurities include H≤0.00015%, P≤0.010%, S≤0.002%, O≤0.0025%, and N≤0.004%. The initial thickness of the bridge steel ingot is 220-320 mm, and the final thickness of the obtained bridge steel plate is 10-28 mm, the center segregation is ≤C0.5, and the center porosity is ≤0.5.

[0020] The carbon equivalent C of the bridge steel plate of the present invention is eq The calculation formula is: C eq (%)=(%C)+(%Mn) / 6+(%Mo+%Cr+%vV) / 5+(%Ni+%Cu) / 15; where the element symbol in brackets is the mass percentage of the corresponding element, and the % element symbol represents the mass percentage of the corresponding element multiplied by 100.

[0021] The components of the present invention play a role in preparing low yield ratio bridge steel: C and Mn are important components for improving the strength and hardenability of steel plates. Bridge steel plates need to be welded, and have very high requirements for carbon equivalent. In order to take into account the strength and good welding performance of steel plates, the C content is controlled between 0.04 and 0.06%, and the Mn content is controlled between 1.50 and 1.60%, so that bridge steel can obtain higher strength and welding performance. Si, as a deoxidizing element, can effectively reduce the yield ratio by improving the strength of the material through solid solution strengthening, but the Si element has an adverse effect on the removal of iron oxide scale on the surface of the slab, so it is appropriate to control the Si content to 0.1 to 0.20%. The Cr element is beneficial to improving the hardenability of the steel plate and expanding the dual-phase structure process window, but too high a content of the Cr element will reduce the toughness of the welding heat affected zone, so it is appropriate to control the Cr content to 0.20 to 0.30%. Ti and Nb are microalloying elements. Ti can significantly inhibit the growth of grains during heating and weaken the generation of transverse microcracks on the surface. At the same time, as the nucleation point of Nb, it has a positive effect on the precipitation strengthening of Nb. Nb plays an important role in refining grains after rolling and promoting the formation of acicular ferrite during welding. However, when the Nb content is too high, it will have an adverse effect on the elongation and yield strength ratio. Therefore, it is appropriate to control the Nb content at 0.05~0.08% and the Ti content at 0.01~0.03%.

[0022] like Figure 1 As shown, the production method of bridge steel of the present invention comprises the following steps: Step 1: Smelting is carried out according to the chemical composition design scheme as above. The slab is cut twice and loaded into a furnace with a loading temperature of 300-600°C (process parameters are not control parameters, i.e., the loading temperature may be any value in this range).

[0023] Step 2: Evenly heat through the heat recovery section, preheating section, first heating section, second heating section, and soaking section of the heating furnace; the preheating temperature of the heat recovery section is 850~900℃, and the heating time is 80min; the preheating temperature of the preheating section is 1000~1050℃, and the heating time is 60min; the heating temperature of the first heating section is 1100~1150℃, and the heating time is 60min; the heating temperature of the second heating section is 1150~1200℃, and the heating time is 50min; the soaking temperature of the soaking section is 1150~1200℃, and the heating time is 50min. During this process, the heating temperature is gradually increased, which can improve the temperature uniformity of the slab and avoid surface cracks caused by thermal stress during the heating process; the maximum heating temperature is controlled at 1150~1200℃ to ensure that the alloy elements are fully dissolved and at the same time avoid rapid grain growth; the pinning effect of TiN can refine the high-temperature austenite grains, which is beneficial to the final grain size control; the carbonitrides are fully dissolved, and the subsequently precipitated carbonitrides are finer and more dispersed, which will have a favorable effect on the tensile and impact toughness of the final product.

[0024] Step 3: After the slab comes out of the heating furnace, dephosphorization is carried out to ensure that the iron oxide scale on the surface of the ingot is completely removed, which provides a guarantee for the subsequent uniform cooling of the slab.

[0025] Step 4: Rolling is divided into two stages: recrystallization zone and non-recrystallization zone. The first stage of rolling is carried out after the surface temperature of the ingot is cooled to 1030~1080℃, and the red return temperature after rolling is ≥1010℃; the intermediate billet is quickly cooled, and the rapid cooling rate is 3~5℃ / s (the process parameter is not a control parameter, that is, the cooling rate may be any value in this range). After cooling to 770~820℃, the second stage of rolling is carried out. The second stage rolling temperature is ≤900℃, and the final rolling temperature of the ingot is Ar3±10℃. The first stage of rolling ensures rolling in the complete recrystallization zone, and the reduction rate is ≥18% for at least 2 passes to ensure core penetration; the intermediate billet thickness is ≥2.5t, and t is the final thickness of the steel plate / mm. The starting rolling temperature of the second stage rolling is guaranteed to be in the non-recrystallization zone, and the final rolling is guaranteed to be at Ar3±30℃. The starting cooling temperature is controlled by the relaxation time to ensure the formation of a dual-phase structure, that is, when the steel plate enters water cooling, 20~30% proeutectoid ferrite structure is guaranteed to be generated. Subsequently, the ACC roller speed, water volume and final cooling temperature are controlled to obtain the final required polygonal ferrite + acicular ferrite structure of bridge steel.

[0026] Among them, the calculation formula of the final rolling temperature Ar3 is: Ar3=910-310*(%C)-80*(%Mn)-20*(%Cu)-15*(%Cr)-55*(%Ni)-80*(%Mo); where the element symbol in brackets is the mass percentage of the corresponding element, and the % element symbol represents the mass percentage of the corresponding element multiplied by 100. It should be noted that due to the different thickness of the steel plate, the steel plate has different roller transportation process and relaxation process after rolling, so the final rolling temperature varies according to the thickness, as shown in Table 1.

[0027] Table 1 Production process parameters of bridge steel plates with different final thicknesses: .

[0028] Step 5: After the final rolling of the ingot, the ingot is relaxed for 10 to 30 seconds, cooled to a temperature of 200 to 400°C, and cooled at a rate of 20 to 40°C / min (the process parameter is not a control parameter, that is, the cooling rate may be any value in this range), and the bridge steel plate can be obtained. The bridge steel plate produced by this processing technology has a variable ferrite + acicular ferrite structure, in which the polygonal ferrite structure accounts for 50 to 70% and the acicular ferrite structure accounts for 30 to 50%. The bridge steel with this structure effectively reduces the yield strength ratio of the steel plate while ensuring strength.

[0029] Example 1: The slab number of the bridge steel of Example 1 of the present invention is 2482706269041, the steel grade is Q420qE, and the chemical composition of the slab includes, by mass percentage: 0.051% C, 0.144% Si, 1.58% Mn, 0.0114% P, 0.0039% S, 0.259% Cr, 0.0661% Nb, 0.04% Al sol (acid soluble aluminum), 0.0162% Ti, 0.00375% N, 0.368% C eq (Carbon equivalent), the rest is Fe. The initial thickness of the bridge steel ingot is 220~320mm, the intermediate thickness is 42mm, and the final thickness is 14mm.

[0030] The production method of bridge steel in Example 1 of the present invention comprises the following steps: Step 1: Smelt according to the chemical composition design scheme above, cut the slab twice and load it into the furnace, the loading temperature of the furnace is 300~600℃.

[0031] Step 2: Evenly heat through the heat recovery section, preheating section, first heating section, second heating section, and equalizing section of the heating furnace; the preheating temperature of the heat recovery section is 850℃, and the heating time is 80min; the preheating temperature of the preheating section is 1010℃, and the heating time is 60min; the heating temperature of the first heating section is 1100℃, and the heating time is 60min; the heating temperature of the second heating section is 1150℃, and the heating time is 50min; the equalizing temperature of the equalizing section is 1150℃, and the heating time is 50min.

[0032] Step 3: After the slab comes out of the heating furnace, dephosphorization is carried out to ensure that the iron oxide scale on the surface of the ingot is completely removed, which provides a guarantee for the subsequent uniform cooling of the slab.

[0033] Step 4: Divide the rolling into two stages: recrystallization zone and non-recrystallization zone. The first stage of rolling is carried out after the surface temperature of the ingot is cooled to 1053°C, and the red-returning temperature after rolling is ≥1010°C; the intermediate ingot is rapidly cooled at a rate of 3-5°C / s, and the second stage of rolling is carried out after cooling to 770°C. The starting temperature of the second stage rolling is ≤900°C, the final rolling temperature Ar3 is 766°C, and the actual final rolling temperature of the ingot is 763°C.

[0034] Step 5: After the final rolling, the ingot is relaxed, the relaxation time is 10s, the actual start cooling temperature is 714℃, the actual final cooling temperature is 328℃, and the cooling rate is 20~40℃ / min, and the bridge steel plate can be obtained.

[0035] Example 2: The slab number of the bridge steel of Example 2 of the present invention is 2481705605031, the steel grade is Q420qE, and the chemical composition of the slab includes, by mass percentage: 0.045% C, 0.117% Si, 1.58% Mn, 0.0102% P, 0.0033% S, 0.27% Cr, 0.0636% Nb, 0.039% Al sol (acid soluble aluminum), 0.0154% Ti, 0.0027% N, 0.3642% C eq (Carbon equivalent), the rest is Fe. The initial thickness of the bridge steel ingot is 220~320mm, the intermediate ingot thickness is 84mm, and the final thickness is 28mm.

[0036] The production method of bridge steel in Example 2 of the present invention comprises the following steps: Step 1: Smelt according to the chemical composition design scheme above, cut the slab twice and load it into the furnace, the loading temperature of the furnace is 300~600℃.

[0037] Step 2: Evenly heat through the heat recovery section, preheating section, first heating section, second heating section, and equalizing section of the heating furnace; the preheating temperature of the heat recovery section is 860℃, and the heating time is 80min; the preheating temperature of the preheating section is 1015℃, and the heating time is 60min; the heating temperature of the first heating section is 1115℃, and the heating time is 60min; the heating temperature of the second heating section is 1155℃, and the heating time is 50min; the equalizing temperature of the equalizing section is 1150℃, and the heating time is 50min.

[0038] Step 3: After the slab comes out of the heating furnace, dephosphorization is carried out to ensure that the iron oxide scale on the surface of the ingot is completely removed, which provides a guarantee for the subsequent uniform cooling of the slab.

[0039] Step 4: Divide the rolling into two stages: recrystallization zone and non-recrystallization zone. The first stage of rolling is carried out after the surface temperature of the ingot is cooled to 1030°C, and the red-returning temperature after rolling is ≥1010°C; the intermediate ingot is rapidly cooled at a rate of 3-5°C / s, and the second stage of rolling is carried out after cooling to 775°C. The starting temperature of the second stage rolling is ≤900°C, the final rolling temperature Ar3 is 762°C, and the actual final rolling temperature of the ingot is 762°C.

[0040] Step 5: After the final rolling, the ingot is relaxed, the relaxation time is 10s, the actual start cooling temperature is 730℃, the actual final cooling temperature is 254℃, and the cooling rate is 20~40℃ / min, and the bridge steel plate can be obtained.

[0041] Example 3: The slab number of the bridge steel of Example 3 of the present invention is 2482706457022, the steel grade is Q420qE, and the chemical composition of the slab includes, by mass percentage: 0.049% C, 0.117% Si, 1.56% Mn, 0.0075% P, 0.0041% S, 0.258% Cr, 0.0676% Nb, 0.034% Al sol (acid soluble aluminum), 0.0128% Ti, 0.0042% N, 0.3623% C eq (Carbon equivalent), the rest is Fe. The initial thickness of the bridge steel ingot is 220~320mm, the intermediate thickness is 60mm, and the final thickness is 20mm.

[0042] The production method of bridge steel in Example 3 of the present invention comprises the following steps: Step 1: Smelt according to the chemical composition design scheme above, cut the slab twice and load it into the furnace, the loading temperature of the furnace is 300~600℃.

[0043] Step 2: Evenly heat through the heat recovery section, preheating section, first heating section, second heating section, and equalizing section of the heating furnace; the preheating temperature of the heat recovery section is 850℃, and the heating time is 80min; the preheating temperature of the preheating section is 1025℃, and the heating time is 60min; the heating temperature of the first heating section is 1125℃, and the heating time is 60min; the heating temperature of the second heating section is 1160℃, and the heating time is 50min; the equalizing temperature of the equalizing section is 1160℃, and the heating time is 50min.

[0044] Step 3: After the slab comes out of the heating furnace, dephosphorization is carried out to ensure that the iron oxide scale on the surface of the ingot is completely removed, which provides a guarantee for the subsequent uniform cooling of the slab.

[0045] Step 4: Divide the rolling into two stages: recrystallization zone and non-recrystallization zone. The first stage of rolling is carried out after the surface temperature of the ingot is cooled to 1075°C, and the red-returning temperature after rolling is ≥1010°C; the intermediate ingot is quickly cooled at a rate of 3-5°C / s, and the second stage of rolling is carried out after cooling to 785°C. The starting temperature of the second stage rolling is ≤900°C, the final rolling temperature Ar3 is 762°C, and the actual final rolling temperature of the ingot is 770°C.

[0046] Step 5: After the final rolling, the ingot is relaxed, the relaxation time is 20s, the actual start cooling temperature is 762℃, the actual final cooling temperature is 358℃, and the cooling rate is 20~40℃ / min, and the bridge steel plate can be obtained.

[0047] Example 4: The slab number of the bridge steel of Example 4 of the present invention is 2381704740011, the steel grade is Q420qE, and the chemical composition of the slab includes, by mass percentage: 0.059% C, 0.139% Si, 1.56% Mn, 0.014% P, 0.0041% S, 0.27% Cr, 0.0462% Nb, 0.047% Al sol (acid soluble aluminum), 0.0132% Ti, 0.00335% N, 0.3954% C eq (Carbon equivalent), the rest is Fe. The initial thickness of the bridge steel ingot is 220~320mm, the intermediate thickness is 42mm, and the final thickness is 14mm.

[0048] The production method of bridge steel in Example 4 of the present invention comprises the following steps: Step 1: Smelt according to the chemical composition design scheme above, cut the slab twice and load it into the furnace, the loading temperature of the furnace is 300~600℃.

[0049] Step 2: Evenly heat through the heat recovery section, preheating section, first heating section, second heating section, and equalizing section of the heating furnace; the preheating temperature of the heat recovery section is 870℃, and the heating time is 80min; the preheating temperature of the preheating section is 1035℃, and the heating time is 60min; the heating temperature of the first heating section is 1135℃, and the heating time is 60min; the heating temperature of the second heating section is 1170℃, and the heating time is 50min; the equalizing temperature of the equalizing section is 1170℃, and the heating time is 50min.

[0050] Step 3: After the slab comes out of the heating furnace, dephosphorization is carried out to ensure that the iron oxide scale on the surface of the ingot is completely removed, which provides a guarantee for the subsequent uniform cooling of the slab.

[0051] Step 4: The rolling is divided into two stages: recrystallization zone and non-recrystallization zone. The first stage of rolling is carried out after the surface temperature of the ingot is cooled to 1061°C, and the red-returning temperature after rolling is ≥1010°C; the intermediate ingot is quickly cooled at a rate of 3-5°C / s, and the second stage of rolling is carried out after cooling to 790°C. The starting temperature of the second stage rolling is ≤900°C, the final rolling temperature Ar3 is 766°C, and the actual final rolling temperature of the ingot is 768°C.

[0052] Step 5: After the final rolling, the ingot is relaxed, the relaxation time is 20s, the actual start cooling temperature is 690℃, the actual final cooling temperature is 200℃, and the cooling rate is 20~40℃ / min, and the bridge steel plate can be obtained.

[0053] Example 5: The slab number of the bridge steel of Example 5 of the present invention is 2482706269042, the steel grade is Q420qE, and the chemical composition of the slab includes, by mass percentage: 0.051% C, 0.144% Si, 1.58% Mn, 0.0114% P, 0.0039% S, 0.259% Cr, 0.0661% Nb, 0.04% Al sol (acid soluble aluminum), 0.0162% Ti, 0.00375% N, 0.368% C eq (Carbon equivalent), the rest is Fe. The initial thickness of the bridge steel ingot is 220~320mm, the intermediate thickness is 48mm, and the final thickness is 16mm.

[0054] The production method of bridge steel in Example 1 of the present invention comprises the following steps: Step 1: Smelt according to the chemical composition design scheme above, cut the slab twice and load it into the furnace, the loading temperature of the furnace is 300~600℃.

[0055] Step 2: Evenly heat through the heat recovery section, preheating section, first heating section, second heating section, and equalizing section of the heating furnace; the preheating temperature of the heat recovery section is 880℃, and the heating time is 80min; the preheating temperature of the preheating section is 1045℃, and the heating time is 60min; the heating temperature of the first heating section is 1145℃, and the heating time is 60min; the heating temperature of the second heating section is 1180℃, and the heating time is 50min; the equalizing temperature of the equalizing section is 1180℃, and the heating time is 50min.

[0056] Step 3: After the slab comes out of the heating furnace, dephosphorization is carried out to ensure that the iron oxide scale on the surface of the ingot is completely removed, which provides a guarantee for the subsequent uniform cooling of the slab.

[0057] Step 4: The rolling is divided into two stages: recrystallization zone and non-recrystallization zone. The first stage of rolling is carried out after the surface temperature of the ingot is cooled to 1048°C, and the red-returning temperature after rolling is ≥1010°C; the intermediate ingot is quickly cooled at a rate of 3-5°C / s, and the second stage of rolling is carried out after cooling to 800°C. The starting temperature of the second stage rolling is ≤900°C, the final rolling temperature Ar3 is 753°C, and the actual final rolling temperature of the ingot is 755°C.

[0058] Step 5: After the final rolling, the ingot is relaxed, the relaxation time is 20s, the actual start cooling temperature is 707℃, the actual final cooling temperature is 279℃, and the cooling rate is 20~40℃ / min, and the bridge steel plate can be obtained.

[0059] Example 6: The slab number of the bridge steel of Example 6 of the present invention is 2482705900022, the steel grade is Q420qE, and the chemical composition of the slab includes, by mass percentage: 0.05% C, 0.131% Si, 1.6% Mn, 0.0102% P, 0.0043% S, 0.267% Cr, 0.0643% Nb, 0.027% Al sol (acid soluble aluminum), 0.014% Ti, 0.00427% N, 0.372% C eq (Carbon equivalent), the rest is Fe. The initial thickness of the bridge steel ingot is 220~320mm, the intermediate thickness is 48mm, and the final thickness is 16mm.

[0060] The production method of bridge steel in Example 6 of the present invention comprises the following steps: Step 1: Smelt according to the chemical composition design scheme above, cut the slab twice and load it into the furnace, the loading temperature of the furnace is 300~600℃.

[0061] Step 2: Evenly heat through the heat recovery section, preheating section, first heating section, second heating section, and equalizing section of the heating furnace; the preheating temperature of the heat recovery section is 890℃, and the heating time is 80min; the preheating temperature of the preheating section is 1050℃, and the heating time is 60min; the heating temperature of the first heating section is 1150℃, and the heating time is 60min; the heating temperature of the second heating section is 1190℃, and the heating time is 50min; the equalizing temperature of the equalizing section is 1190℃, and the heating time is 50min.

[0062] Step 3: After the slab comes out of the heating furnace, dephosphorization is carried out to ensure that the iron oxide scale on the surface of the ingot is completely removed, which provides a guarantee for the subsequent uniform cooling of the slab.

[0063] Step 4: The rolling is divided into two stages: recrystallization zone and non-recrystallization zone. The first stage of rolling is carried out after the surface temperature of the ingot is cooled to 1045°C, and the red-returning temperature after rolling is ≥1010°C; the intermediate billet is quickly cooled at a rate of 3-5°C / s, and the second stage of rolling is carried out after cooling to 810°C. The starting temperature of the second stage rolling is ≤900°C, the final rolling temperature Ar3 is 764°C, and the actual final rolling temperature of the ingot is 762°C.

[0064] Step 5: After the final rolling, the ingot is relaxed, the relaxation time is 30s, the actual start cooling temperature is 685℃, the actual final cooling temperature is 200℃, and the cooling rate is 20~40℃ / min, and the bridge steel plate can be obtained.

[0065] Example 7: The slab number of the bridge steel of Example 7 of the present invention is 2482705900023, the steel grade is Q420qE, and the chemical composition of the slab includes, by mass percentage: 0.05% C, 0.131% Si, 1.6% Mn, 0.0102% P, 0.0043% S, 0.267% Cr, 0.0643% Nb, 0.027% Al sol (acid soluble aluminum), 0.014% Ti, 0.00427% N, 0.372% C eq (Carbon equivalent), the rest is Fe. The initial thickness of the bridge steel ingot is 220~320mm, the intermediate thickness is 72mm, and the final thickness is 24mm.

[0066] The production method of bridge steel in Example 7 of the present invention comprises the following steps: Step 1: Smelt according to the chemical composition design scheme above, cut the slab twice and load it into the furnace, the loading temperature of the furnace is 300~600℃.

[0067] Step 2: Evenly heat through the heat recovery section, preheating section, first heating section, second heating section, and equalizing section of the heating furnace; the preheating temperature of the heat recovery section is 900℃, and the heating time is 80min; the preheating temperature of the preheating section is 1005℃, and the heating time is 60min; the heating temperature of the first heating section is 1150℃, and the heating time is 60min; the heating temperature of the second heating section is 1200℃, and the heating time is 50min; the equalizing temperature of the equalizing section is 1200℃, and the heating time is 50min.

[0068] Step 3: After the slab comes out of the heating furnace, dephosphorization is carried out to ensure that the iron oxide scale on the surface of the ingot is completely removed, which provides a guarantee for the subsequent uniform cooling of the slab.

[0069] Step 4: The rolling is divided into two stages: recrystallization zone and non-recrystallization zone. The first stage of rolling is carried out after the surface temperature of the ingot is cooled to 1037°C, and the red-returning temperature after rolling is ≥1010°C; the intermediate ingot is quickly cooled at a rate of 3-5°C / s, and the second stage of rolling is carried out after cooling to 820°C. The starting temperature of the second stage rolling is ≤900°C, the final rolling temperature Ar3 is 762°C, and the actual final rolling temperature of the ingot is 767°C.

[0070] Step 5: After the final rolling, the ingot is relaxed, the relaxation time is 30s, the actual start cooling temperature is 737℃, the actual final cooling temperature is 254℃, and the cooling rate is 20~40℃ / min, and the bridge steel plate can be obtained.

[0071] The performance and crystal phase analysis of the bridge steel plates prepared in Examples 1 to 7 of the present invention, the physical and chemical properties corresponding to each group of examples are shown in Table 2, and the crystal phase structure of each group of examples is shown in Table 2. Figures 2 to 8 shown.

[0072] Table 2 Physical and chemical properties of the bridge steel plates of Examples 1 to 7 of the present invention: .

[0073] It can be seen from Table 2 that the bridge steel plates of Examples 1 to 7 of the present invention are designed with low segregation components, combined with the requirements of strength and weldability of the steel plates, and strengthened with Cr, Mn and microalloys, and through KR desulfurization, LF refining, and light reduction at the end of continuous casting, etc., a good low-magnification structure is obtained to reduce defects such as central looseness and cracks, thereby providing a guarantee for obtaining good steel plate performance; the tensile strength of the obtained bridge steel plate can reach 540~660MPa, the yield strength ≥420MPa, the yield strength ratio ≤0.85, and the carbon equivalent ≤0.45%. The flaw detection of the bridge steel plate meets the Level 1 standard in NB / T47013. At -40°C, the impact value of the center of the bridge steel plate thickness is ≥120J, and there is no crack defect in the center, which has good low-temperature toughness welding performance and application prospects.

[0074] Depend on Figure 2As shown, Example 1 obtains a bridge steel plate with polyhedral ferrite + acicular ferrite structure, in which the polygonal ferrite structure accounts for 55% and the acicular ferrite structure accounts for 45%.

[0075] Depend on Figure 3 As shown, Example 2 obtains a bridge steel plate with polyhedral ferrite + acicular ferrite structure, in which the polygonal ferrite structure accounts for 50% and the acicular ferrite structure accounts for 50%.

[0076] Depend on Figure 4 As shown, Example 3 obtains a bridge steel plate with polyhedral ferrite + acicular ferrite structure, in which the polygonal ferrite structure accounts for 70% and the acicular ferrite structure accounts for 30%.

[0077] Depend on Figure 5 As shown, Example 4 obtains a bridge steel plate with polyhedral ferrite + acicular ferrite structure, in which the polygonal ferrite structure accounts for 52% and the acicular ferrite structure accounts for 48%.

[0078] Depend on Figure 6 As shown, Example 5 obtains a bridge steel plate with polyhedral ferrite + acicular ferrite structure, in which the polygonal ferrite structure accounts for 55% and the acicular ferrite structure accounts for 45%.

[0079] Depend on Figure 7 As shown, Example 6 obtains a bridge steel plate with polyhedral ferrite + acicular ferrite structure, in which the polygonal ferrite structure accounts for 50% and the acicular ferrite structure accounts for 50%.

[0080] Depend on Figure 8 As shown, Example 7 obtains a bridge steel plate with polyhedral ferrite + acicular ferrite structure, in which the polygonal ferrite structure accounts for 60% and the acicular ferrite structure accounts for 40%.

[0081] The above are preferred embodiments of the present invention. It should be pointed out that, for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A low yield strength ratio bridge steel with low temperature impact toughness, characterized in that: The chemical composition of the bridge steel includes, by mass percentage, 0.04-0.06% C, 0.1-0.20% Si, 1.50-1.60% Mn, 0.20-0.30% Cr, 0.05-0.08% Nb, 0.03-0.05% Al, 0.01-0.03% Ti, and the rest is Fe and unavoidable impurities.

2. The low-temperature impact toughness and low yield strength ratio bridge steel according to claim 1, characterized in that: The unavoidable impurities include H≤0.00015%, P≤0.010%, S≤0.002%, O≤0.0025%, and N≤0.004%.

3. The low yield strength ratio bridge steel with low temperature impact toughness according to claim 1, characterized in that: The initial thickness of the ingot of the bridge steel is 220~320mm; the final thickness of the obtained bridge steel plate is 10~28mm, the center segregation is ≤C0.5, and the center porosity is ≤0.5 level.

4. The low yield strength ratio bridge steel with low temperature impact toughness according to claim 3, characterized in that: The carbon equivalent C of the bridge steel plate obtained eq The calculation formula is: C eq (%)=(%C)+(%Mn) / 6+(%Mo+%Cr+%vV) / 5+(%Ni+%Cu) / 15; The element symbol in brackets is the mass percentage of the corresponding element, and the % element symbol represents the mass percentage of the corresponding element multiplied by 100.

5. The low yield strength ratio bridge steel with low temperature impact toughness according to claim 1, characterized in that: The bridge steel plate has a tensile strength of 540-660 MPa, a yield strength of ≥420 MPa, a yield strength ratio of ≤0.85, a carbon equivalent of ≤0.45%, and has good welding performance.

6. The low yield strength ratio bridge steel with low temperature impact toughness according to claim 5, characterized in that: The flaw detection of the bridge steel plate meets the Level 1 standard of NB / T47013. At -40°C, the center impact value of the bridge steel plate thickness is ≥120J, there is no crack defect in the center and it has good low-temperature toughness.

7. A method for producing low yield ratio bridge steel according to any one of claims 1 to 6, characterized in that: The following steps are involved: Step 1: Smelting is carried out according to the chemical composition design scheme above, the slab is cut twice and loaded into the furnace, and the loading temperature is 300~600℃; Step 2: uniformly heat through the heat recovery section, preheating section, first heating section, second heating section, and soaking section of the heating furnace; the preheating temperature of the heat recovery section is 850~900℃, and the heating time is 80min; the preheating temperature of the preheating section is 1000~1050℃, and the heating time is 60min; the heating temperature of the first heating section is 1100~1150℃, and the heating time is 60min; the heating temperature of the second heating section is 1150~1200℃, and the heating time is 50min; the soaking temperature of the soaking section is 1150~1200℃, and the heating time is 50min; Step 3: After the heating furnace is taken out, dephosphorization is carried out to ensure that the iron oxide scale on the surface of the ingot is completely removed; Step 4: The rolling is divided into two stages: the recrystallization zone and the non-recrystallization zone. The first stage of rolling is carried out after the surface temperature of the ingot is cooled to 1030-1080°C, and the red-returning temperature after rolling is ≥1010°C; the intermediate ingot is rapidly cooled at a rate of 3-5°C / s, and the second stage of rolling is carried out after cooling to 770-820°C. The second stage rolling start temperature is ≤900°C, and the final rolling temperature of the ingot is Ar3±10°C; Step 5: After the final rolling, the ingot is relaxed for 10 to 30 seconds, cooled to a temperature of 200 to 400°C at a cooling rate of 20 to 40°C / min, and the bridge steel plate is obtained.

8. The method for producing low yield ratio bridge steel according to claim 7, characterized in that: The calculation formula for Ar3 in step 4 is: Ar3=910-310*(%C)-80*(%Mn)-20*(%Cu)-15*(%Cr)-55*(%Ni)-80*(%Mo); The element symbol in brackets is the mass percentage of the corresponding element, and the % element symbol represents the mass percentage of the corresponding element multiplied by 100.

9. The method for producing low yield ratio bridge steel according to claim 8, characterized in that: In step 5, a bridge steel plate having a polygonal ferrite + acicular ferrite structure is obtained, wherein the polygonal ferrite structure accounts for 50-70% and the acicular ferrite structure accounts for 30-50%.

10. The method for producing low yield ratio bridge steel according to claim 9, characterized in that: In step 4, the first stage rolling is ensured to be rolled in the complete recrystallization zone, and the reduction rate of at least 2 passes is ≥ 18% to ensure the core penetration; the intermediate billet thickness is ≥ 2.5t; where t is the final thickness of the steel plate / mm; The starting rolling temperature of the second stage rolling is guaranteed to be in the non-recrystallization zone, and the final rolling is guaranteed to be at Ar3±30℃.

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