345MPa-grade bridge steel plate and production method thereof

Abstract

The invention discloses a 345MPa-grade bridge steel plate and a production method thereof. The steel plate comprises the following components in percentage by weight: 0.09 to 0.13 percent of C, 0.11 to 0.21 percent of Si, 1.37 to 1.45 percent of Mn, 0.06 to 0.14 percent of Ni and 0.012 to 0.020 percent of Nb. The production method comprises the steps that first-stage rolling is conducted, the rolling reduction of each pass is smaller than or equal to 30 mm, and the rolling reduction of a non-broadening blooming pass is larger than or equal to 20 mm; cooling the intermediate billet, wherein the final cooling temperature is Ar < 3 + > (90-110) DEG C; in the second-stage rolling, the initial rolling temperature is Ar3 + (80-100) DEG C, the final rolling temperature is Ar3 + (40-60) DEG C, and stopping is conducted for 8-10 s before the last-pass rolling; the water inlet temperature is Ar3 + (10-30) DEG C, and the water outlet temperature is Bs + (40-80) DEG C; the temperature of an upper cooling bed is Bs + (-150-200) DEG C, and the temperature of a lower cooling bed does not exceed Ms-150 DEG C.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of steel material preparation, and relates to a 345MPa-grade bridge steel plate and a production method thereof. BACKGROUND

[0002] In addition to focusing on mechanical properties and low-temperature properties, the generation of residual stress is an unavoidable problem during the manufacturing and use of bridge steel plates.

[0003] Residual stress refers to the self-balancing stress existing inside the material without external force, which is mainly derived from the processing of bridge steel plates, such as hot processing, cold forming, welding, etc.

[0004] Residual stress has a significant impact on the performance and service life of bridge structures. First, residual stress can cause deformation of the bridge structure, affecting its geometric shape and dimensional accuracy, and further affecting the overall performance and safety of the bridge. Second, the existence of residual stress reduces the fatigue strength of the bridge structure, increases the initiation and propagation of fatigue cracks, and thus shortens the service life of the bridge. In addition, residual stress can also cause local stress concentration, further exacerbating the damage and destruction risk of the structure. SUMMARY

[0005] In order to solve the problem of residual stress in the prior art, the purpose of the present application is to provide a 345MPa-grade bridge steel plate and a production method thereof.

[0006] To achieve the above-mentioned purpose, an embodiment of the present application provides a 345MPa-grade bridge steel plate. The chemical composition of the steel plate includes, in mass percent: C 0.09~0.13%, Si 0.11~0.21%, Mn 1.37~1.45%, Ni 0.06~0.14%, Nb 0.012~0.020%, Ti 0.010~0.018%, Al 0.024~0.048%, P≤0.0200%, S≤0.0050%, O≤0.0025%, N≤0.0048%, H≤0.00018%, and the rest is iron and unavoidable impurities. The thickness d of the steel plate is 6~100mm, R m ≥490MPa, R eL ≥345MPa, impact energy KV2 at -20℃≥220J, residual stress≤40MPa, and the residual stress difference between any two of the head, middle, tail, upper and lower parts is≤15MPa.

[0007] Thus, in one embodiment of the present invention, a low-carbon silicon-manganese alloy with trace amounts of nickel is designed in terms of chemical composition. By combining precise control of each element, the alloy elements are optimized to promote the microstructure and properties, and the deterioration of properties by the elements is avoided. This ensures that excellent mechanical properties and low-temperature toughness are obtained in the end, and lays the foundation for easy rolling, cooling and other processes. This ensures that the residual stress of the final steel plate is low and has small fluctuations, and achieves the effect of low cost overall.

[0008] To achieve the above objectives, one embodiment of the present invention provides a method for producing 345MPa grade bridge steel plates. The chemical composition, microstructure, and properties of the steel plate are as described above. The production method includes a sequential process of billet heating, hot rolling, controlled water cooling, and air-cooled self-tempering, wherein: The hot rolling process includes first-stage rolling, intermediate billet cooling, and second-stage rolling. During the first-stage rolling, the initial rolling temperature is T. nr +(60~100)℃, final rolling temperature T nr +(10~50)℃, the reduction in each pass is ≤30mm, and the reduction in the non-broadening initial rolling pass is ≥20mm; during intermediate billet cooling, the intermediate billet thickness is 2.5d~3.0d, and the final cooling temperature is A r3 +(90~110)℃, cooling time ≥60s; during the second stage of rolling, the initial rolling temperature A r3 +(80~100)℃, final rolling temperature A r3 +(40~60)℃, hold for 8~10s before the last rolling pass, and the thickness d of the rolled steel plate is 6~100mm; Controlling the inlet water temperature A in the water cooling process r3 +(10~30)℃, outlet water temperature B s +(40~80)℃; The air-cooled self-tempering process is carried out on a cooling bed, with the upper cooling bed temperature being B. s +(-150~200)℃, the temperature of the lower cooling bed does not exceed M. s -150℃.

[0009] Among them, T nr The minimum temperature for austenite recrystallization is given by formula T. nr =887+464C-357Si+6445Nb-644 +890Ti+363Al calculation.

[0010] A r3 The temperature at which ferrite (α-Fe) begins to precipitate from austenite (γ-Fe) upon cooling is given by formula A. r3 =910-203 Calculation of -11Si-15.2Mn-13Ni.

[0011] B s The temperature at which the bainitic phase transformation begins is given by formula B. s =550-160C-10Mn-10Ni calculation.

[0012] M S The starting temperature for the austenite-to-martensite phase transformation is given by formula M. S =539-423C-11Si-30.4Mn-17.7Ni, calculated in °C.

[0013] In this application, the element symbols in these formulas represent the mass percentage of the corresponding element in the cast billet. For example, if the content of element C in the steel plate is 0.05%, then "C" in the formula represents a mass percentage of 0.05.

[0014] Thus, the production method according to one embodiment of the present invention has the following beneficial effects: First, rolling at a higher temperature range reduces the resistance to rolling deformation and increases the rolling reduction, which facilitates deformation penetration into the core of the billet and improves defects such as core segregation, porosity, and banded structure. Second, it ensures that the billet can be rolled in the recrystallization zone, avoiding mixed crystals. At the same time, the precipitation of Nb compounds inhibits the growth of recrystallized grains and refines the recrystallized grains. Moreover, by controlling the reduction of each pass and having a larger reduction in the initial rolling pass (non-widening passes), the as-cast structure can be fully broken down to obtain refined recrystallized grains, while avoiding a sharp increase in internal stress. Secondly, by controlling the final cooling temperature and cooling time during the intermediate billet cooling process, not only can production efficiency be guaranteed, but also the rapid growth of recrystallized grains during the cooling process can be avoided, which would lead to a decrease in low-temperature toughness. More importantly, it allows for sufficient recrystallization and releases certain internal stresses at high temperatures. Third, during the second stage of rolling, the microstructure can be optimized by controlling the initial rolling temperature and the final rolling temperature. In addition, by pausing for 8 to 10 seconds before the last rolling pass, the stress accumulated in the steel can be fully released, thereby ensuring that low residual stress is ultimately obtained. Fourth, in the controlled water cooling process, by controlling the temperature and cooling rate, the supercooled austenite phase in the steel plate can be cooled to cause a phase transformation. By using weak water and slow cooling rate, it transforms into ferrite / bainite, thereby ensuring a good match and balanced development of strength and toughness; avoiding the formation of martensite; and ensuring low and uniform distribution of residual stress. Fifth, after water cooling, air cooling self-tempering is carried out, combined with temperature control on the cooling bed. On the one hand, the steel plate can release phase transformation stress and thermal stress during air cooling self-tempering; on the other hand, air cooling self-tempering can decompose the bainite or MA generated in the phase transformation, improve toughness and local hard spots, and further release internal stress.

[0015] Therefore, based on low-cost chemical composition, the production method achieves precise control of the steel plate's microstructure through parameter settings and specific operations in each process, thereby obtaining a steel plate with excellent comprehensive performance, including mechanical strength, low-temperature toughness, residual stress, and other aspects. Moreover, the production method is simple and the process cost is low. Attached Figure Description

[0016] Fig. 1 This is a metallographic diagram of the steel plate of Embodiment 1 of the present invention; Fig. 2 This is a metallographic diagram of the steel plate of Embodiment 2 of the present invention. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them.

[0018] One embodiment of the present invention provides a 345MPa grade bridge steel plate.

[0019] The chemical composition of the steel plate, by mass percentage, includes: C 0.09~0.13%, Si 0.11~0.21%, Mn 1.37~1.45%, Ni 0.06~0.14%, Nb 0.012~0.020%, Ti 0.010~0.018%, Al 0.024~0.048%, P≤0.0200%, S≤0.0050%, O≤0.0025%, N≤0.0048%, H≤0.00018%, with the remainder being iron and unavoidable impurities.

[0020] The following is a detailed analysis and explanation of the main functions of each element and the selection of its dosage.

[0021] C: Carbon is the most economical strengthening element in steel, providing solid solution strengthening. It also forms carbides with niobium, titanium, chromium, molybdenum, etc., resulting in precipitation strengthening. Increased carbon content increases the material's hardness and strength, but reduces ductility and toughness. During quenching or rapid cooling, the formation of carbides and martensite causes significant volume expansion, thus increasing residual stress. Higher carbon content leads to greater hardenability of the steel and a greater tendency for residual stress to concentrate. In this invention, the carbon content is controlled at 0.09~0.13%.

[0022] Preferably, the carbon content can be controlled at any one of 0.09%, 0.10%, 0.11%, and 0.13%.

[0023] Silicon (Si): As a deoxidizer and solid solution strengthening element, silicon can improve the strength and hardness of steel, but it reduces plasticity and toughness. The addition of silicon increases the elastic limit and yield strength of steel, but has little direct impact on residual stress. High silicon content increases grain boundary segregation of elements such as phosphorus and sulfur, reducing low-temperature toughness and weldability. Furthermore, excessive silicon easily forms Fe2SiO4 on the surface of continuously cast billets, which is detrimental to the control of steel plate surface quality. In one embodiment, the silicon content is controlled at 0.11~0.21%.

[0024] Preferably, the silicon content can be controlled at any one of 0.11%, 0.12%, 0.13%, 0.14%, 0.15%, 0.16%, 0.17%, 0.18%, 0.19%, 0.20%, and 0.21%.

[0025] Mn: Manganese plays a solid solution strengthening role in steel, improving its hardenability and strength. However, increasing manganese content increases residual stress, especially during rapid cooling. Excessive manganese can lead to center segregation in the cast billet, negatively impacting toughness and increasing the banding level, resulting in a more uneven microstructure. Furthermore, manganese readily combines with sulfur to form manganese sulfide inclusions. Center segregation, banding, and inclusions all increase residual stress in the steel. In one embodiment, the manganese content is controlled at 1.37~1.45%.

[0026] Preferably, the manganese content can be controlled at any one of 1.37%, 1.39%, 1.40%, and 1.45%.

[0027] Ni: Nickel plays a solid solution strengthening role in steel, increasing its strength without significantly increasing its hardness. Nickel expands the austenite phase region, thereby reducing residual stress; furthermore, nickel can also improve the toughness of steel and reduce stress concentration. In this application, the nickel content is controlled at 0.06~0.14%.

[0028] Preferably, the nickel content can be controlled at any one of 0.06%, 0.08%, 0.10%, 0.11%, 0.12%, and 0.14%.

[0029] Niobium (Nb) is an important grain-refining element in steel. During hot rolling, niobium strongly inhibits austenite recrystallization and precipitation within austenite, pinning austenite grain boundaries and refining recrystallized grains. During cooling, dissolved niobium can continue to precipitate as niobium carbonitrides, significantly refining the microstructure after phase transformation and further improving the strength and toughness of the steel. Niobium can reduce residual stress through grain refinement and precipitation strengthening. In this application, the niobium content is controlled at 0.012~0.020%.

[0030] Preferably, the niobium content can be controlled at any one of 0.012%, 0.015%, 0.018%, 0.019%, or 0.020%.

[0031] Ti: Titanium is a nitrogen-fixing element in steel. It can form dispersed carbonitrides, which inhibit austenite grain coarsening and refine grains during billet heating and hot rolling, thereby reducing residual stress. However, when the titanium content is high, large carbonitride precipitates are easily formed in the core of the billet, affecting the low-temperature toughness of the steel plate. Furthermore, large TiN particles can easily lead to stress concentration. In this application, the titanium content is controlled at 0.010~0.018%.

[0032] Preferably, the titanium content can be controlled at any one of 0.010%, 0.014%, 0.015%, 0.016%, and 0.018%.

[0033] Al: Aluminum is a deoxidizing element in steel. Excessive aluminum can increase the number of Al2O3 inclusions in steel, affecting its low-temperature toughness. Furthermore, large inclusions lead to discontinuous microstructure and stress concentration. In this application, the aluminum content is controlled at 0.024~0.048%.

[0034] Preferably, the aluminum content can be controlled at any one of 0.024%, 0.025%, 0.028%, 0.030%, 0.033%, 0.035%, 0.038%, 0.040%, 0.042%, 0.045%, and 0.048%.

[0035] P, S, N, O, and H are all impurity elements in steel that can cause deterioration in the properties of steel plates, including but not limited to increased residual stress. In this application, P is controlled to be ≤0.0200%, S ≤0.0050%, O ≤0.0025%, N ≤0.0048%, and H ≤0.00018%.

[0036] Thus, in one embodiment of the present invention, a low-carbon silicon-manganese alloy with trace amounts of nickel is designed in terms of chemical composition. By combining precise control of each element, the alloy elements are optimized to promote the microstructure and properties, and the deterioration of properties by the elements is avoided. This ensures that excellent mechanical properties and low-temperature toughness are obtained in the end, and lays the foundation for easy rolling, cooling and other processes. This ensures that the residual stress of the final steel plate is low and has small fluctuations, and achieves the effect of low cost overall.

[0037] Preferably, the P content is controlled at 0.0005% or more, or even 0.0100% or more.

[0038] Preferably, the sulfur content is controlled at 0.0005% or more, or even 0.0010% or more.

[0039] Preferably, the nitrogen content is controlled at 0.0022% or more.

[0040] Preferably, the O content is controlled at 0.0011% or more.

[0041] Preferably, the H content is controlled at 0.00005% or more.

[0042] Thus, even with high P and S content, the mechanical properties and low-temperature toughness of the steel plate can still be guaranteed. The performance degradation effect caused by these two elements is greatly reduced, and the difficulty of steelmaking is also greatly reduced.

[0043] Furthermore, the chemical composition of the steel plate, by mass percentage, also satisfies any one or more of the following: CEV is 0.322~0.381, Pcm is 0.163~0.212, and 5C+Si+Mn+Ni is 1.99~2.45.

[0044] Wherein, CEV = C + Mn / 6 + Ni / 15; Pcm = C + Si / 30 + Mn / 20 + Ni / 60.

[0045] In the formula, the element symbols represent the mass percentage of the corresponding element. For example, if the content of element C in the steel plate is 0.05%, then "C" in the formula represents a mass percentage of 0.05.

[0046] Furthermore, the thickness d of the steel plate is 6~100mm, which is a medium-thick plate, meeting the requirements of most bridge projects for steel plate thickness.

[0047] Furthermore, the R of the steel plate m ≥490MPa, R eL ≥345MPa. Thus, the steel plate exhibits excellent mechanical properties, with yield strength and tensile strength superior to existing technologies with higher alloy content.

[0048] Furthermore, the steel plate has an impact energy (KV2) of ≥220J at -20℃. Thus, the steel plate possesses excellent low-temperature toughness.

[0049] Furthermore, the residual stress of the steel plate is ≤40MPa. Thus, the residual stress of the steel plate is low.

[0050] Furthermore, the residual stress difference between any two points at the head, middle, tail, upper, and lower parts of the steel plate is ≤15MPa. This demonstrates that the residual stress fluctuation across the entire steel plate is small and the distribution is uniform.

[0051] Furthermore, the steel plate also satisfies one or more of the following: A≥21%; R eL / Rm ≤0.82; Z-direction reduction of area ≥40%.

[0052] Impact energy at 0℃ KV2≥250J; Impact energy at -40℃ KV2≥200J; -60℃ impact energy KV2≥180J; At 0℃, the crack tip opening displacement of CTOD is ≥0.6mm; At -20℃, the crack tip opening displacement of CTOD is ≥0.4mm; At -40℃, the crack tip opening displacement of CTOD is ≥0.3mm; Unevenness ≤ 1 mm / m.

[0053] In other words, the steel plate has very small yield strength ratio and unevenness, and overall, its strength, toughness and plate shape are excellent.

[0054] In this application, the steel can be sampled and subjected to residual stress testing in accordance with GB / T 31310-2014 "Determination of Residual Stress in Metallic Materials - Drilling Strain Method".

[0055] In this application, the steel can be sampled and its mechanical properties tested in accordance with GB / T 2975-2018 "Sampling location and specimen preparation for mechanical property testing of steel and steel products" and GB / T 228.1-2021 "Metallic materials, tensile testing - Part 1: Room temperature test method".

[0056] In this application, the steel can be sampled and its low-temperature performance tested in accordance with GB / T 2975-2018 "Sampling Location and Sample Preparation for Mechanical Property Testing of Steel and Steel Products" and GB / T 229-2020 "Charpy Pendulum Impact Test Method for Metallic Materials".

[0057] Furthermore, the microstructure of the steel plate is composed of polygonal ferrite + pearlite + tempered bainite + martensite-austenite, with the total volume percentage of polygonal ferrite, pearlite, tempered bainite, and martensite-austenite being 100%.

[0058] Preferably, the volume fraction of polygonal ferrite is 65-75%, the volume fraction of pearlite is 20-25%, the volume fraction of tempered bainite is 0-8%, and the volume fraction of martensite-austenite components is 0-7%. Such a microstructure can greatly improve the overall performance of the steel plate.

[0059] Preferably, the average grain size of the steel plate is 12~20μm.

[0060] To achieve the above objectives, one embodiment of the present invention provides a method for producing 345MPa grade bridge steel plates. The chemical composition, microstructure, and properties of the steel plate are as described above. The production method includes a sequential process of billet heating, hot rolling, controlled water cooling, and air-cooled self-tempering, wherein: The hot rolling process includes first-stage rolling, intermediate billet cooling, and second-stage rolling. During the first-stage rolling, the initial rolling temperature is T. nr +(60~100)℃, final rolling temperature T nr +(10~50)℃, the reduction in each pass is ≤30mm, and the reduction in the non-broadening initial rolling pass is ≥20mm; during intermediate billet cooling, the intermediate billet thickness is 2.5d~3.0d, and the final cooling temperature is A r3 +(90~110)℃, cooling time ≥60s; during the second stage of rolling, the initial rolling temperature A r3 +(80~100)℃, final rolling temperature A r3 +(40~60)℃, hold for 8~10s before the last rolling pass, and the thickness d of the rolled steel plate is 6~100mm; Controlling the inlet water temperature A in the water cooling process r3 +(10~30)℃, outlet water temperature B s +(40~80)℃; The air-cooled self-tempering process is carried out on a cooling bed, with the upper cooling bed temperature being B. s +(-150~200)℃, the temperature of the lower cooling bed does not exceed M. s -150℃.

[0061] Among them, T nr The minimum temperature for austenite recrystallization is given by formula T. nr =887+464C-357Si+6445Nb-644 +890Ti+363Al calculation.

[0062] A r3 The temperature at which ferrite (α-Fe) begins to precipitate from austenite (γ-Fe) upon cooling is given by formula A. r3 =910-203 Calculation of -11Si-15.2Mn-13Ni.

[0063] B s The temperature at which the bainitic phase transformation begins is given by formula B. s =550-160C-10Mn-10Ni calculation.

[0064] M S The starting temperature for the austenite-to-martensite phase transformation is given by formula M. S=539-423C-11Si-30.4Mn-17.7Ni, calculated in °C.

[0065] In this application, the element symbols in these formulas represent the mass percentage of the corresponding element in the cast billet. For example, if the content of element C in the steel plate is 0.05%, then "C" in the formula represents a mass percentage of 0.05.

[0066] Thus, the production method according to one embodiment of the present invention has the following beneficial effects: First, rolling at a higher temperature range reduces the resistance to rolling deformation and increases the rolling reduction, which facilitates deformation penetration into the core of the billet and improves defects such as core segregation, porosity, and banded structure. Second, it ensures that the billet can be rolled in the recrystallization zone, avoiding mixed crystals. At the same time, the precipitation of Nb compounds inhibits the growth of recrystallized grains and refines the recrystallized grains. Moreover, by controlling the reduction of each pass and having a larger reduction in the initial rolling pass (non-widening passes), the as-cast structure can be fully broken down to obtain refined recrystallized grains, while avoiding a sharp increase in internal stress. Secondly, by controlling the final cooling temperature and cooling time during the intermediate billet cooling process, not only can production efficiency be guaranteed, but also the rapid growth of recrystallized grains during the cooling process can be avoided, which would lead to a decrease in low-temperature toughness. More importantly, it allows for sufficient recrystallization and releases certain internal stresses at high temperatures. Third, during the second stage of rolling, the microstructure can be optimized by controlling the initial rolling temperature and the final rolling temperature. In addition, by pausing for 8 to 10 seconds before the last rolling pass, the stress accumulated in the steel can be fully released, thereby ensuring that low residual stress is ultimately obtained. Fourth, in the controlled water cooling process, by controlling the temperature and cooling rate, the supercooled austenite phase in the steel plate can be cooled to cause a phase transformation. By using weak water and slow cooling rate, it transforms into ferrite / bainite, thereby ensuring a good match and balanced development of strength and toughness; avoiding the formation of martensite; and ensuring low and uniform distribution of residual stress. Fifth, after water cooling, air cooling self-tempering is carried out, combined with temperature control on the cooling bed. On the one hand, the steel plate can release phase transformation stress and thermal stress during air cooling self-tempering; on the other hand, air cooling self-tempering can decompose the bainite or MA generated in the phase transformation, improve toughness and local hard spots, and further release internal stress.

[0067] Therefore, based on low-cost chemical composition, the production method achieves precise control of the steel plate's microstructure through parameter settings and specific operations in each process, thereby obtaining a steel plate with excellent comprehensive performance, including mechanical strength, low-temperature toughness, residual stress, and other aspects. Moreover, the production method is simple and the process cost is low.

[0068] Preferably, the billet used in the billet heating process can be a continuously cast billet or a die-cast billet, which is not limited in this application. The thickness of the billet is preferably above 200 mm, for example, 220~320 mm.

[0069] Furthermore, in the billet heating process, the soaking temperature is T. NbC +(100~150)℃, heat soaking time ≥25min.

[0070] Among them, T NbC The initial precipitation temperature of NbC is given by the formula T. NbC =7700 / (3.18-lg(Nb×C 0.87 The formula ))-273.15 is used for calculation, where the element symbols in the formula represent the mass percentage of the corresponding element in the billet.

[0071] Thus, by controlling the homogenization temperature and duration, on the one hand, excessive growth of austenite grains can be avoided, preparing for precipitation in subsequent hot rolling processes; on the other hand, and more importantly, it can ensure that Nb precipitates are completely dissolved, reducing residual stress.

[0072] Preferably, the heat spreader temperature is T. NbC +100℃, T NbC +110℃, T NbC +115℃, T NbC +120℃, T NbC +125℃, T NbC Any value of +150℃.

[0073] Preferably, this process can employ segmented heating, such as including heat recovery, preheating, primary heating, secondary heating, and homogenization. However, it is not limited to this.

[0074] The heat recovery temperature is ≤850℃, the preheating temperature is ≤950℃, the first heating temperature is 1020~1080℃, the second heating temperature is 1120~1180℃, and the total heating time is 0.019~0.023h / mm billet thickness. In this way, by segmenting the heating process and controlling the temperature of each segment, the billet can be heated slowly and uniformly, reducing the temperature difference between the beginning, middle, and end of the billet, as well as the surface and center, thereby avoiding cracking caused by thermal stress.

[0075] Furthermore, the furnace entry temperature of the billet is ≥ M S -300℃.

[0076] Preferably, during the first stage of rolling, the initial rolling temperature is T. nr +60℃, T nr +65℃, T nr +70℃, T nr +75℃, T nr +80℃, Tnr +85℃, T nr +90℃, T nr +95℃, T nr Any value within +100℃.

[0077] Preferably, during the first stage of rolling, the final rolling temperature is T. nr +10℃, T nr +20℃, T nr +30℃, T nr +40℃, T nr Any value within +50℃.

[0078] Preferably, during the cooling of the intermediate billet, the final cooling temperature is A. r3 +90℃, A r3 +100℃, A r3 Any value within +110℃.

[0079] More preferably, the intermediate billet can be cooled by air cooling.

[0080] Preferably, during the second stage of rolling, the initial rolling temperature is A. r3 +80℃, A r3 +90℃, A r3 Any value within +100℃.

[0081] Preferably, during the second stage of rolling, the final rolling temperature is A. r3 +40℃, A r3 +50℃, A r3 Any value within +60℃.

[0082] Preferably, during the second stage of rolling, the reduction in each pass is ≤20mm, and the reduction in the last pass is ≤3mm.

[0083] Thus, by using small to medium reduction deformation, a fine microstructure can be obtained during the subsequent cooling process, thereby improving the low-temperature toughness of the steel plate; furthermore, through the synergistic effect of the final reduction and dwell time, the plate shape can be improved, the temperature difference between plates can be reduced, and residual stress can be further reduced.

[0084] Preferably, in the water cooling process, the inlet water temperature is A. r3 +10℃, A r3 +20℃, A r3 Any value within +30℃.

[0085] Preferably, in the water cooling process, the outlet water temperature is B. s +40℃, B s +60℃, B s Any value within +80℃.

[0086] Preferably, in the water cooling process, the cooling rate is controlled at 8~20℃ / s.

[0087] Preferably, in the water cooling process, the cooling rate is any value among 8℃ / s, 10℃ / s, 12℃ / s, 14℃ / s, 15℃ / s, 16℃ / s, 18℃ / s, and 20℃ / s.

[0088] Preferably, in the controlled water cooling process, the steel plate is cooled by water cooling on an ultra-fast cooling system. The roller speed of the ultra-fast cooling system is preferably controlled at 0.6~1.8m / s, the water pressure at 0.15~0.25MPa, and the water-to-water ratio at 0.88~0.94.

[0089] Furthermore, in an optional embodiment, the cooling manifold in the ultrafast cooling system can be controlled in an intermittent manner or with head-to-tail shielding. Of course, this application is not limited to this.

[0090] The intermittent method is specifically exemplified by the ultra-fast cooling system, which includes more than 24 sets of cooling manifolds distributed along the roller conveyor. Furthermore, the cooling manifolds are controlled in such a way that, along the roller conveyor, for every 1 to 2 sets of cooling manifolds that are turned on, there are 1 to 2 sets of cooling manifolds that are turned off at intervals.

[0091] The head and tail shielding method is specifically as follows: the steel plate is divided into Xn head low-temperature zones, a middle zone, and Xm tail low-temperature zones from head to tail, where Xn and Xm are positive integers greater than 1; using the cooling water volume Q per unit length in the middle zone as a standard, the cooling water volume per unit length in the Xn head low-temperature zones is controlled to increase sequentially from the head, and each is k times Q. X1 ~k Xn The K values ​​are: times, and the cooling water volume per unit length of the Xm tail-end low-temperature zones increases sequentially from the tail end, with each value being Q. X1 ~K Xm times; where k X1 ~k Xn All values ​​range from 0.55 to 0.85, K X1 ~K Xm The values ​​are all between 0.50 and 0.80.

[0092] For example, Xn and Xm can both be 3, and correspondingly, k X1 k X2 k X3 K takes values ​​of 0.55~0.65, 0.65~0.75, and 0.75~0.85 respectively. X1 K X2 K X3 The values ​​are taken as 0.50~0.60, 0.60~0.70, and 0.70~0.80 respectively. Preferably, in the air-cooled self-tempering process, the temperature of the upper cooling bed is B. s-150℃, B s -140℃, B s -120℃, B s -100℃, B s -80℃, B s -60℃, B s -50℃, B s -40℃, B s -10℃, B s B s +10℃, B s +30℃, B s +50℃, B s +60℃, B s +70℃, B s +80℃, B s +90℃, B s +100℃, B s +130℃, B s +150℃, B s +160℃, B s +180℃, B s Any value within +200℃.

[0093] Preferably, in the air-cooled self-tempering process, the temperature of the lower cooling bed is M. s -(260~150)℃.

[0094] More preferably, the temperature of the lower cooling bed for the steel plate is M. s -260℃, M s -240℃, M s -210℃, M s -200℃, M s -190℃, M s -180℃, M s -170℃, M s -160℃, M s Any value within -150℃.

[0095] In a preferred embodiment, the production method further includes a stacking process after the air-cooled self-tempering process: the steel plate is sandwiched between two auxiliary steel plates and stacked in a layered manner of auxiliary steel plate-steel plate-auxiliary steel plate, the stacking temperature M of the auxiliary steel plates is... s +(-80~50)℃, the stacking temperature of the steel plates is M s -(270~160)℃, destacking temperature not exceeding M s -(370~240)℃.

[0096] Thus, the stacking temperature of the steel plate is lower than that of the auxiliary steel plate. The heat from the auxiliary steel plate can be used to generate a tempering effect on the steel plate. On the one hand, this improves the uniformity of the steel plate's structure and properties. On the other hand, it can further release internal stress, reduce the residual stress of the steel plate, and improve the uniformity of residual stress distribution.

[0097] Preferably, the stacking temperature of the steel plates is M. s -270℃, M s -260℃, M s -250℃, M s -240℃, M s -230℃, M s -220℃, M s -210℃, M s -200℃, M s -190℃, M s Any value within -160℃.

[0098] Furthermore, the length, width, and thickness of the steel plate are all smaller than the length, width, and thickness of the auxiliary steel plate, respectively.

[0099] Preferably, the stacking time in the stacking process is 12 to 18 hours.

[0100] Preferably, the production method further includes a surface treatment process after the air-cooled self-tempering process: after the steel plate cools to room temperature, the steel plate is surface treated by shot blasting or shot peening, during which the shot peening or shot blasting meets any one or more of the following conditions: t = k × d + C; P = k1 × d + C1; S = k2 × d + C2; Where t is the shot blasting or shot peening time in min, k is 0.1~0.2 min / mm, and C is 5~7 min; P is the shot blasting or shot peening rate in kg / min, k1 is 0.4~0.6 kg / (min·mm), and C1 is 160~200 kg / min; S is the shot blasting or shot peening velocity in m / s, k2 is 0.1~0.3 m / (s·mm), and C2 is 65~75 m / s.

[0101] Thus, by shot blasting or shot peening the steel plate cooled to room temperature, surface compressive stress can be achieved. This surface compressive stress can counteract the residual tensile stress inside the steel plate, thereby reducing residual stress and improving its distribution. Furthermore, by jointly controlling the time t, shot blasting or shot peening amount P, speed S, and plate thickness d, a uniform compressive stress layer can be formed on the surface of the steel plate. The resulting compressive stress can greatly eliminate residual tensile stress, thereby reducing residual stress and improving its distribution.

[0102] Preferably, in the surface treatment process, for the steel plate with a thickness d of 6~20mm, the shot used for shot blasting or shot peening is a mixture of shot with a diameter of 0.5mm and shot with a diameter of 0.7mm in a mass ratio of (7~8):5; for the steel plate with a thickness d of 20~50mm, the shot used for shot blasting or shot peening is a mixture of shot with a diameter of 0.7mm and shot with a diameter of 1.0mm in a mass ratio of (9~11):5; for the steel plate with a thickness d>50mm, the shot used for shot blasting or shot peening is a mixture of shot with a diameter of 1.0mm and shot with a diameter of 1.2mm in a mass ratio of (4~6):5.

[0103] Optionally, the shot used may be cast steel shot with a hardness of 35-50 HRC. The specific material of the cast steel shot is not limited in this application.

[0104] Preferably, the angle of shot blasting or shot peening is 45°~80°, and the coverage is 100%.

[0105] Preferably, the production method further includes a tempering heat treatment step after the air-cooled self-tempering step, specifically after the stacking step and the surface treatment step. In the tempering heat treatment step: the thickness d of the steel plate is 6~20mm, and the tempering temperature T is B. s +(35~55)℃; or, the thickness d of the steel plate is 20~50mm, and the tempering temperature T is B. s +(15~35)℃; or, if the thickness d of the steel plate is greater than 50mm, the tempering temperature T is B. s +(-5~15)℃; and control the tempering holding time t1 according to the formula (T+273.15)×(20+lg(t1 / 60)) / 1000≥13.7, where t1 is in min.

[0106] In this way, by coordinating and controlling the tempering temperature T and the tempering holding time t1, and further precisely controlling the tempering temperature T according to different thicknesses, the microstructure and various mechanical properties of the steel plate can be optimized. While ensuring sufficient strength, the plasticity and toughness of the steel plate, such as impact toughness, can be improved. Poor core-surface properties of the steel plate, such as hardness, can be reduced. Moreover, residual stress can be significantly reduced and its distribution improved, ensuring that the steel plate is less prone to deformation and cracking during subsequent pipe manufacturing. Furthermore, the problem of excessive energy consumption and production costs caused by the introduction of tempering heat treatment can be avoided.

[0107] Preferably, the tempering heat treatment process is as follows: the tempering holding time t1 is controlled according to the formula (T+273.15)×(20+lg(t1 / 60)) / 1000≥16.0, where t1 is in min.

[0108] Preferably, the heating rate is 8~15℃ / min.

[0109] Preferably, in this process, tempering heat treatment can be carried out in a tempering furnace, where a nitrogen atmosphere can be maintained.

[0110] Thus, under a nitrogen atmosphere, not only can oxidation or decarburization of the steel plate be prevented during tempering heat treatment, thus protecting the surface quality of the steel plate, research has also found that this can further optimize the uniformity of residual stress.

[0111] Preferably, the production method further includes a post-tempering stacking process after the tempering heat treatment process: after reaching the tempering holding time t1, the tempering is ended and stacking is carried out directly, with a stacking temperature B. s -(210~110)℃, cooling rate 10~30℃ / h, destacking temperature ≤B s -450℃. Thus, rapid unloading and stacking, along with controlled cooling rates, helps to homogenize the distribution of residual stress.

[0112] Preferably, the stacking time is 12~24h.

[0113] Compared with the prior art, the beneficial effects of one embodiment of this application are at least as follows: In terms of chemical composition, a low-carbon silicon-manganese alloy with trace amounts of nickel was adopted. By precisely controlling each element, the optimal effect of the alloy elements on the microstructure and properties was achieved, while avoiding the deterioration of properties. This ensured excellent mechanical properties and low-temperature toughness, and laid the foundation for easy rolling and cooling processes. As a result, the residual stress of the final steel plate was low and the fluctuation was small, and the overall cost was reduced. Based on low-cost chemical composition, the production method achieves precise control of the steel plate's microstructure through parameter settings and specific operations in each process, thereby obtaining a steel plate with excellent comprehensive performance, including mechanical strength, low-temperature toughness, residual stress, and other aspects. Furthermore, the production method is simple and has low process cost.

[0114] Based on the above technical approach, the production method of the present invention has multiple implementations, such as the basic implementation of billet heating-hot rolling-controlled water cooling-air cooling self-tempering, and the preferred implementation of billet heating-hot rolling-controlled water cooling-air cooling self-tempering-optional stacking process-optional surface treatment-optional tempering heat treatment-optional post-tempering stacking. However, it is not limited to these.

[0115] The foregoing has described the technical principles and basic details of various embodiments of the present invention. Several embodiments are provided below to demonstrate the beneficial effects of this application. Of course, these embodiments are only a part of the numerous variations contained in this invention, and not all of them.

[0116] Each of these embodiments provides a steel plate, the chemical composition of which is shown in Table 1.

[0117] [Table 1]

[0118] The production methods for steel plates in these embodiments are illustrated in Tables 2 and 3, with the process routes and some key process parameters listed. Other process parameters not shown in the tables are implemented according to the methods described above. The " / " symbols in the tables indicate that the corresponding process was not performed.

[0119] [Table 2]

[0120] [Table 3]

[0121] [Table 4]

[0122] The microstructure and performance of the steel plates in each embodiment were tested. Figs. 1-2 The metallographic structures of some embodiments are shown, and the specific test results are shown in Tables 4 and 5.

[0123] [Table 4]

[0124] [Table 5]

Claims

1. A method for producing 345MPa grade bridge steel plates, characterized in that, The chemical composition of the steel plate, by mass percentage, includes: C 0.09~0.13%, Si 0.11~0.21%, Mn 1.37~1.45%, Ni 0.06~0.14%, Nb 0.012~0.020%, Ti 0.010~0.018%, Al 0.024~0.048%, P≤0.0200%, S≤0.0050%, O≤0.0025%, N≤0.0048%, H≤0.00018%, with the remainder being iron and unavoidable impurities; The production method includes, in sequence, a billet heating process, a hot rolling process, a controlled water cooling process, and an air-cooled self-tempering process, wherein: The hot rolling process includes first-stage rolling, intermediate billet cooling, and second-stage rolling. During the first-stage rolling, the initial rolling temperature is T. nr +(60~100)℃, final rolling temperature T nr +(10~50)℃, T nr =887+464C-357Si+6445Nb-644 +890Ti+363Al, the reduction in each pass is ≤30mm, and the reduction in the non-broadening initial rolling pass is ≥20mm; during intermediate billet cooling, the intermediate billet thickness is 2.5d~3.0d, and the final cooling temperature is A. r3 +(90~110)℃, cooling time ≥60s, A r3 =910-203 -11Si-15.2Mn-13Ni; During the second stage of rolling, the initial rolling temperature A r3 +(80~100)℃, final rolling temperature A r3 +(40~60)℃, hold for 8~10s before the last rolling pass, and the thickness d of the rolled steel plate is 6~100mm; Controlling the inlet water temperature A in the water cooling process r3 +(10~30)℃, outlet water temperature B s +(40~80)℃, B s =550-160C-10Mn-10Ni; The air-cooled self-tempering process is carried out on a cooling bed, with the upper cooling bed temperature being B. s +(-150~200)℃, the temperature of the lower cooling bed does not exceed M. s -150℃, M S =539-423C-11Si-30.4Mn-17.7Ni; The element symbols in each formula represent the mass percentage of the corresponding element in the cast billet.

2. The method for producing 345MPa grade bridge steel plates according to claim 1, characterized in that, The production method further includes a stacking process after the air-cooled self-tempering process: the steel plate is sandwiched between two auxiliary steel plates and stacked in a layered manner of auxiliary steel plate-steel plate-auxiliary steel plate, with the stacking temperature M of the auxiliary steel plates being... s +(-80~50)℃, the stacking temperature of the steel plates is M s -(270~160)℃, destacking temperature not exceeding M s -(370~240)℃.

3. The method for producing 345MPa grade bridge steel plates according to claim 1, characterized in that, The production method further includes a surface treatment process following the air-cooled self-tempering process: after the steel plate cools to room temperature, the steel plate is surface-treated by shot blasting or shot peening, during which the shot peening or shot blasting meets one or more of the following conditions: t = k × d + C; P = k1 × d + C1; S = k2 × d + C2; Where t is the shot blasting or shot peening time in min, k is 0.1~0.2 min / mm, and C is 5~7 min; P is the shot blasting or shot peening rate in kg / min, k1 is 0.4~0.6 kg / (min·mm), and C1 is 160~200 kg / min; S is the shot blasting or shot peening velocity in m / s, k2 is 0.1~0.3 m / (s·mm), and C2 is 65~75 m / s.

4. The method for producing 345MPa grade bridge steel plates according to claim 1, characterized in that, The production method further includes a tempering heat treatment process after the air-cooled self-tempering process: the thickness d of the steel plate is 6~20mm, and the tempering temperature T is B. s +(35~55)℃; or, the thickness d of the steel plate is 20~50mm, and the tempering temperature T is B. s +(15~35)℃; or, if the thickness d of the steel plate is greater than 50mm, the tempering temperature T is B. s +(-5~15)℃; and control the tempering holding time t1 according to the formula (T+273.15)×(20+lg(t1 / 60)) / 1000≥13.7, where t1 is in min.

5. The method for producing 345MPa grade bridge steel plates according to claim 4, characterized in that, Tempering heat treatment process: The tempering holding time t1 is controlled according to the formula (T+273.15)×(20+lg(t1 / 60)) / 1000≥16.0, where t1 is in min.

6. The method for producing 345MPa grade bridge steel plates according to claim 4, characterized in that, The production method further includes a post-tempering stacking process following the tempering heat treatment process: after reaching the tempering holding time t1, tempering is ended and stacking is carried out directly, with a stacking temperature B. s -(210~110)℃, cooling rate 10~30℃ / h, destacking temperature ≤B s -450℃.

7. The method for producing 345MPa grade bridge steel plates according to claim 4, characterized in that, In the tempering heat treatment process: the heating rate is 8~15℃ / min, and the tempering heat treatment is carried out in a tempering furnace while maintaining a nitrogen atmosphere in the tempering furnace.

8. The method for producing 345MPa grade bridge steel plates according to claim 1, characterized in that, In the billet heating process: the soaking temperature is T NbC +(100~150)℃, heat soaking time ≥25min; where, T NbC =7700 / (3.18-lg(Nb×C 0.87 ))-273.

15.

9. A 345MPa grade bridge steel plate, characterized in that, The chemical composition of the steel plate, by mass percentage, includes: C 0.09~0.13%, Si 0.11~0.21%, Mn 1.37~1.45%, Ni 0.06~0.14%, Nb 0.012~0.020%, Ti 0.010~0.018%, Al 0.024~0.048%, P≤0.0200%, S≤0.0050%, O≤0.0025%, N≤0.0048%, H≤0.00018%, with the remainder being iron and unavoidable impurities; The thickness d of the steel plate is 6~100mm, R m ≥490MPa, R eL ≥345MPa, impact energy at -20℃ KV2≥220J, residual stress≤40MPa, and the residual stress difference between any two points in the head, middle, tail, upper and lower parts is ≤15MPa.

10. The 345MPa grade bridge steel plate according to claim 9, characterized in that, The microstructure of the steel plate is composed of polygonal ferrite + pearlite + tempered bainite + martensite-austenite, with the total volume percentage of polygonal ferrite, pearlite, tempered bainite, and martensite-austenite being 100%.

11. The 345MPa grade bridge steel plate according to claim 10, characterized in that, The volume fraction of polygonal ferrite is 65-75%, pearlite is 20-25%, tempered bainite is 0-8%, and martensite-austenite is 0-7%.

12. The 345MPa grade bridge steel plate according to claim 10, characterized in that, The average grain size of the steel plate is 12~20μm.

13. The 345MPa grade bridge steel plate according to claim 9, characterized in that, The steel plate also satisfies one or more of the following: A≥21%； R eL / R m ≤0.82; Z-direction reduction of area ≥40%. Impact energy at 0℃ KV2≥250J; Impact energy at -40℃ KV2≥200J; -60℃ impact energy KV2≥180J; At 0℃, the crack tip opening displacement of CTOD is ≥0.6mm; At -20℃, the crack tip opening displacement of CTOD is ≥0.4mm; At -40℃, the crack tip opening displacement of CTOD is ≥0.3mm; Unevenness ≤ 1 mm / m.

14. The 345MPa grade bridge steel plate according to claim 9, characterized in that, The chemical composition of the steel plate, by mass percentage, also meets one or more of the following conditions: CEV is 0.322~0.381, Pcm is 0.163~0.212, and 5C+Si+Mn+Ni is 1.99~2.

45. Where CEV=C+Mn / 6+Ni / 15, Pcm=C+Si / 30+Mn / 20+Ni / 60, the element symbols in the formulas represent the mass percentage of the corresponding elements.

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