CN114231826B - Production method of Q420qE bridge structural steel plate - Google Patents

Abstract

The Q420qE bridge structure steel plate is produced with the components in weight percentage including C = 0.08-0.10, Si = 0.20-0.30, Mn = 1.50-1.60, P ≤ 0.015, S ≤ 0.003, Al = 0.02-0.05, Ti = 0.008-0.020, Nb = 0.080-0.090, Mo = 0.20-0.30, Cr = 0.20-0.50, and the balance Fe and inevitable impurity elements. The method can shorten the heating time of the Q420qE steel with the thickness of 6-30 mm in the furnace for 20-30 min, improve the production efficiency by more than 20%, ensure that the yield strength of the steel plate is 440-500 Mpa, the tensile strength is 560-620 Mpa, the yield ratio is less than or equal to 0.85, the impact energy at-40 ℃ is more than or equal to 250J, and ensure that the percent of pass of the steel plate is more than 99%.

Description

Production method of Q420qE bridge structural steel plate

Technical Field

The invention belongs to the technical field of metallurgy, and relates to a production method for rolling a Q420qE bridge structural steel plate at a high temperature.

Background

The bridge structural steel is mainly used for steel structures of bridge engineering projects and has the comprehensive properties of high bearing capacity, good ductility and toughness, excellent welding performance and the like. With the progress of science and technology, bridges in a new era are developed in the directions of large span, heavy load, light weight and the like, and higher requirements such as higher strength, higher toughness and the like are also put forward on bridge structural steel. At present, the Q420Q strength-grade bridge structural steel is a main variety of public railway bridges and is widely applied, such as Nanjing Dasheng Guangxiang bridge, Hutong Changjiang grand bridge, Gangzhao bridge and the like.

Q420Q belongs to low-alloy high-strength bridge steel, and the production process of each steel mill mostly adopts TMCP or TMCP + T. The alloy content is high, the heating time is long, the production efficiency is low, and the method is one of main factors influencing the production efficiency and the ton steel cost of medium plate plants. It is especially necessary to develop a process technology for rapidly heating, tapping and rapidly rolling Q420Q steel.

Chinese patent CN201910208082.1 'a thin low-carbon steel and a manufacturing method thereof' discloses a hot-rolled ferritic steel, the yield strength of the product is 204-214 MPa, the tensile strength is 316-334 MPa, and the elongation is more than or equal to 48%. Chinese patent CN201810436196.7 'a low carbon steel plate with excellent forming performance in CSP process and a manufacturing method thereof', discloses a hot-rolled ferritic steel, the invention adopts C-Si-Mn component and high-temperature rolling and post-rolling laminar cooling process design, the purpose is to reduce the thickness of the iron scale of the steel plate, the integral strength of the produced product is within 400Mpa, and the Q420 strength level can not be satisfied. Chinese patent CN106282789A 'a low-carbon ultra-thick TMCP type Q420qE bridge steel and a manufacturing method thereof' discloses a TMCP type low-carbon Q420qE bridge steel and a manufacturing method thereof, the rolling method adopts the working procedures of casting blank slow cooling, continuous casting blank heating, rolling, pre-straightening, controlled cooling, hot straightening, stacking slow cooling, cutting and finishing, etc. to complete the invention, the invention adopts the design of C-Si-Mn-Nb-Cr components, the performance of the steel plate is excellent, but the working procedures are complicated, and the production efficiency is very low. Chinese patent CN102051525A 'a production method of a low-cost Q420qE steel plate for a bridge', discloses a production method of a low-cost Q420qE steel plate for a bridge, the invention adopts a C-Si-Mn-Nb component design, and the rolling method adopts the procedures of slab continuous casting → casting blank cleaning → slab heating → 4300 rolling mill rolling → ACC rapid cooling → pile cooling → flaw detection → finished product sampling, inspection → warehousing, delivery and the like, and the rolling method still adopts the traditional TMCP mode, has large influence on the rolling rhythm and the slab type, has low production efficiency, and is not beneficial to high-yield production.

Disclosure of Invention

Aiming at the problems in the prior art, the invention aims to provide a Q420qE bridge structural steel plate and a production method thereof, the method adopts a high-Nb microalloy design, and applies high-temperature fast sintering and fast rolling to produce Q420qE steel with the thickness of 6-30 mm, the yield strength of the steel plate is 440-500 Mpa, the tensile strength is 560-620 Mpa, the yield ratio is less than or equal to 0.85, and the impact energy at-40 ℃ is more than or equal to 250J.

The technical scheme of the invention is as follows:

a Q420qE bridge structural steel plate is composed of C = 0.08-0.10, Si = 0.20-0.30, Mn = 1.50-1.60, P ≤ 0.015, S ≤ 0.003, Al = 0.02-0.05, Ti = 0.008-0.020, Nb = 0.080-0.090, Mo = 0.20-0.30, Cr = 0.20-0.50, and the balance of Fe and unavoidable impurity elements. The key process steps comprise:

(1) plate blank inserting: smelting the components in a converter and casting the components into a steel billet, wherein the off-line temperature of the steel billet is 600-750 ℃;

(2) heating the plate blank at a high temperature: after a steel billet is taken off line, a hot delivery and hot charging mode is adopted, the surface of the casting blank is cooled by water to below 550 ℃ and then is fed into a furnace, the heating furnace is divided into a preheating section, an I adding section, an II adding section and a soaking section for controlled heating, the temperature of the preheating section is 800-1200 ℃, the temperature of the I adding section is 1200-1300 ℃, the temperature of the II adding section is 1300-1350 ℃, the soaking section is 1300-1350 ℃, the total in-furnace time is 160-190 min, the heating rate is 3-8 min/cm, and the tapping temperature is 1210-1250 ℃;

(3) Rough rolling: after the plate blank is taken out of the furnace, primarily removing phosphorus by using high-pressure water until the plate blank is in front of a roughing mill, wherein the initial rolling temperature is less than or equal to 1240 ℃, the final three-pass average reduction rate after broadening is more than or equal to 15%, rolling is carried out for 5-7 passes, the finishing temperature of roughing rolling is less than or equal to 1050 ℃, the rolling speed is 2.5-3 m/s, and the phosphorus is removed by using the high-pressure water for 2-3 passes;

(4) finish rolling: and (3) cooling the intermediate blank by an intermediate roller way until the intermediate blank is rolled directly before a finishing mill, wherein the rolling speed is 6-7 m/s, the initial rolling temperature is more than or equal to 960 ℃, the rolling time is 7-9 times, the average reduction rate is more than or equal to 10%, the final rolling temperature is 840-880 ℃, and phosphorus is removed by high-pressure water for 1-4 times.

The design principle of the components of the steel grade of the invention is as follows:

carbon: carbon is an effective element for increasing the strength of steel, but the toughness, the plasticity and the weldability of the bridge structural steel are all reduced along with the increase of the carbon content, the welding performance is obviously deteriorated when the carbon content exceeds 0.11 percent, but if the carbon content is too low, the strength of the bridge structural steel is influenced, and the smelting is difficult for industrial production. Therefore, the invention adopts the design of medium and low carbon, and the carbon content is determined to be 0.08-0.10%.

Silicon: silicon dissolves in ferrite in steel, and increases the strength and hardness of steel, but decreases the plasticity and toughness. Since silicon is a deoxidizer which can be combined with FeO in molten steel to form a silicate slag having a low density and removed, silicon is an element useful for improving the purity of molten steel. However, an excessively high silicon content is disadvantageous in controlling the surface quality and inclusions of the steel, and also in toughness and weldability of the steel. The invention determines the range of the silicon content to be 0.20-0.30%.

Manganese: manganese is an important element with high toughness and can reduce the phase change Ar of gamma → alpha in steel₃Temperature, thereby promoting bainite transformation. Mn causes solid solution strengthening by dissolving into ferrite, and improves the strength of steel. Manganese promotes grain refinement, thereby improving the toughness of the steel while enhancing the strength. However, manganese is also easily segregated, and forms a band-shaped structure such as MnS in combination with S, which affects low-temperature impact and thickness-tensile properties. The invention determines the manganese content to be 1.50% -1.60%.

Phosphorus: in general, phosphorus is a harmful element in steel, increases cold brittleness of steel, deteriorates weldability, and reduces plasticity. Therefore, the invention adopts a low-phosphorus design and limits the phosphorus content to be not more than 0.015 percent.

Sulfur: sulfur is a harmful impurity in steel, and reduces ductility, toughness and weldability of steel, and high-sulfur steel is susceptible to brittle fracture when subjected to press working at high temperature. In addition, S can form MnS segregation with Mn, which seriously affects the comprehensive performance of the steel. Therefore, the invention adopts a low-sulfur design and limits the sulfur content to be not more than 0.003 percent.

Aluminum: the aluminum is a deoxidizer in the steel, and the proper amount of Al and Ca is compounded, so that the quantity of the inclusions is reduced, the forms of the inclusions are changed, and the aluminum is favorable for the internal quality, the plasticity and the toughness of the bridge steel. Al has a certain effect of refining grains, improves impact toughness and reduces the ductile-brittle transition temperature of steel. The invention determines the aluminum content to be 0.02% -0.05%.

Titanium: the titanium can refine the grain structure of the steel, thereby improving the strength and toughness of the steel, reducing aging sensitivity and cold brittleness and improving welding performance. TiN formed by micro-titanium treatment can effectively pin austenite grain boundaries and is beneficial to controlling the growth of austenite grains, but large-size liquated TiN is easily generated when the titanium content is higher, and the mechanical property of steel is influenced. The invention determines the range of the titanium content to be 0.008% -0.020%.

Niobium: niobium is a microalloying element and has two functions of solid solution strengthening and fine grain strengthening. The solid solution strengthening means that after the steel is dissolved in austenite, the hardenability of the steel can be obviously improved, and the strength and the impact toughness of the steel are improved. Nb and C, N can combine to generate high melting point and high dispersion Nb (C, N), the carbide has strong austenite recrystallization inhibiting effect, and has the characteristics of separating out and pinning austenite grain boundary in gamma and refining original austenite grain, so that the material obtains obviously refined structure after phase transformation, and the grain size is smaller. The high Nb content, the austenite stability is stronger, the recrystallization termination temperature Tnr can be greatly improved, and the bridge steel can be processed at a higher rolling temperature to obtain the effect of refining grains. The method adopts a high-Nb design, and the Nb content is determined to be 0.080-0.090%.

Chromium: chromium plays a multi-element and important role in bridge steel, can form relatively stable and fine chromium carbide, is uniformly distributed in the volume of the steel, and plays a role in refining the structure and improving the strength and the wear resistance. Chromium and nickel form stable compounds, can play a role in oxidation resistance and corrosion resistance, and has a good effect of protecting the surface quality of bridge steel. The invention determines the range of the chromium content to be 0.20-0.50%.

Molybdenum: the combination of molybdenum, nickel and chromium can obviously improve the hardenability of the steel, play a role in refining grains and comprehensively improve the comprehensive performance of the steel, particularly the tensile strength and the hardness of the steel. Generally, the content of molybdenum in the low-alloy steel is selected to be 0.10% -0.30%, and the content of molybdenum which is too high can increase brittle phases in the structure, so that the toughness of the steel is deteriorated, and the content of molybdenum in the scheme is selected to be 0.20% -0.30%.

The production process of the invention is set according to the following steps:

according to the invention, the Q420qE steel is rapidly heated at high temperature in the heating furnace, so that the effects of casting blank temperature homogenization and full solid solution of alloy can be achieved in a short time, and the abnormal growth of original austenite grains can be inhibited by adopting a high-Nb component design and utilizing the pinning grain boundary effect of a large amount of dispersed solid solution Nb (C, N) at high temperature; the rough rolling stage is all rolled in a recrystallization temperature range, and the original austenite grains are fully refined by matching with high reduction; the addition of the high-Nb and Mo alloy enables Tnr (recrystallization termination temperature) to be increased by 70-100 ℃, the rolling start temperature of finish rolling is more than or equal to 960 ℃, the rolling control process window in the finish rolling stage is greatly increased, and the intermediate billet can be rolled from a rough rolling outlet to finish rolling without waiting for temperature; the alloy component Mo enables pearlite to transform and move to the right, and formation of pro-eutectoid ferrite and carbide is delayed, so that the transformation cooling rate of bainite is greatly reduced, acicular ferrite and grains B can be formed by finish rolling of a steel plate after air cooling (0.5-1.0 m/s) of a matrix structure, then fine grain effect of high Nb deformation induction precipitation is added, and finally the grain size of Q420qE under the air cooling condition of the process can reach more than 11 grades.

The invention has the beneficial effects that: the heating furnace time of the Q420qE steel with the thickness of 6-30 mm produced by the method can be shortened by 20-30 min, the rolling can realize the rolling of an intermediate billet without waiting for warm, the production efficiency can be improved by more than 20%, the yield strength of the steel plate is 440-500 Mpa, the tensile strength is 560-620 Mpa, the yield ratio is less than or equal to 0.85, the impact power at-40 ℃ is more than or equal to 250J, and the percent of pass of the steel plate is more than 99%.

Drawings

FIG. 1 is a process flow diagram of example 2 of the present invention.

Fig. 2 is an optical microscope photograph of Q420qE bridge steel 1/4 at thickness according to example 2 of the present invention.

Detailed Description

The invention is further illustrated by the following set of examples.

Example 1: production method of Q420qE steel plate with thickness of 6mm

The chemical composition of the steel is shown in table 1. The thickness of the casting blank is 180mm, and the thickness of the steel plate is 6 mm. The key production process parameters are as follows:

the compositions of example 1 according to table 1 were smelted in a converter and cast into steel slabs, followed by (1) off-line of the slab: smelting the components in a converter and casting the components into a steel billet, wherein the offline temperature of the steel billet is 700-750 ℃; (2) heating the plate blank at a high temperature: after steel billets are taken off line, a hot delivery and hot charging mode is adopted, the surfaces of the casting billets are cooled to below 550 ℃ by water and then are fed into a furnace, the heating furnace is divided into a preheating section, an I adding section, an II adding section and a soaking section for controlled heating, the temperature of the preheating section is 800-1200 ℃, the temperature of the I adding section is 1200-1300 ℃, the temperature of the II adding section is 1300-1350 ℃, the temperature of the soaking section is 1300-1350 ℃, the total in-furnace time is 160min, the heating rate is 3-6 min/cm, and the tapping temperature is 1230-1250 ℃; (3) rough rolling: primarily removing phosphorus from the plate blank after the plate blank is taken out of the furnace by high-pressure water to the front of a roughing mill, wherein the rolling starting temperature is 1210 ℃, the average reduction rate of three times is 22% after the plate blank is widened, the rolling time is 5 times, the final rolling temperature of the roughing mill is 1050 ℃, the rolling speed is 3m/s, and the phosphorus is removed by the high-pressure water for the first time and the third time; (4) finish rolling: and (3) cooling the intermediate blank by an intermediate roller way until the intermediate blank is directly rolled before a finishing mill, wherein the rolling speed is 7m/s, the initial rolling temperature is 1030 ℃, the rolling is performed for 7 times, the average reduction rate is 18%, the final rolling temperature is 880 ℃, and the high-pressure water dephosphorization is performed for the second time.

Example 2: production method of Q420qE steel plate with thickness of 20mm

The chemical composition of the steel is shown in table 1. The thickness of the casting blank is 260mm, and the thickness of the steel plate is 20 mm. The key production process parameters are as follows: the components of example 1 according to table 1 were smelted in a converter and cast into a steel slab, and subsequently (1) the slab was taken off-line: smelting the components by a converter and casting the components into a steel billet, wherein the off-line temperature of the steel billet is 650-700 ℃; (2) heating the plate blank at a high temperature: after steel billets are taken off line, a hot delivery and hot charging mode is adopted, the surfaces of the casting billets are cooled to below 550 ℃ by water and then are fed into a furnace, the heating furnace is divided into a preheating section, an I adding section, an II adding section and a soaking section for controlled heating, the temperature of the preheating section is 800-1200 ℃, the temperature of the I adding section is 1200-1300 ℃, the temperature of the II adding section is 1300-1350 ℃, the temperature of the soaking section is 1300-1350 ℃, the total in-furnace time is 180min, the heating rate is 4-7 min/cm, and the tapping temperature is 1220-1240 ℃; (3) rough rolling: primarily removing phosphorus from the plate blank after the plate blank is taken out of the furnace by high-pressure water to the front of a roughing mill, wherein the rolling starting temperature is 1200 ℃, the average reduction rate of three times is 20% after the plate blank is widened, the rolling time is 7 times, the roughing rolling finishing temperature is 1040 ℃, the rolling speed is 2.8m/s, and phosphorus is removed in the first, third and fifth times; (4) finish rolling: and (3) cooling the intermediate blank by an intermediate roller way until the intermediate blank is directly rolled before a finishing mill, wherein the rolling speed is 6.5m/s, the initial rolling temperature is 1000 ℃, the rolling time is 9 times, the average reduction rate is 16%, the final rolling temperature is 860 ℃, and phosphorus is removed in the second, fourth and sixth times.

When the microstructure of the steel sheet of example 2 was observed, the microstructure of the steel sheet at 1/4 thickness was 50% acicular ferrite +45% granular bainite +5% pearlite as shown in fig. 2.

Example 3: production method of Q420qE steel plate with thickness of 30mm

The chemical composition of the steel is shown in table 1. The thickness of the continuous casting billet is 260mm, and the thickness of the steel plate is 30 mm. The key production process parameters are as follows:

the components of example 3 according to table 1 were smelted in a converter and cast into a steel slab, and subsequently (1) the slab was taken off-line: smelting the components in a converter and casting the components into a steel billet, wherein the off-line temperature of the steel billet is 600-650 ℃; (2) heating the plate blank at a high temperature: after steel billets are taken off line, a hot delivery and hot charging mode is adopted, the surfaces of the casting billets are cooled to below 550 ℃ by water and then are fed into a furnace, the heating furnace is divided into a preheating section, an I adding section, an II adding section and a soaking section for controlled heating, the temperature of the preheating section is 800-1200 ℃, the temperature of the I adding section is 1200-1300 ℃, the temperature of the II adding section is 1300-1350 ℃, the temperature of the soaking section is 1300-1350 ℃, the total in-furnace time is 190min, the heating rate is 5-8 min/cm, and the tapping temperature is 1210-1230 ℃; (3) rough rolling: after the plate blank is taken out of the furnace, primarily removing phosphorus by high-pressure water to the front of a roughing mill, wherein the rolling starting temperature is 1190 ℃, the average reduction rate of the last three times is 15% after widening, the rolling is carried out for 7 times, the finishing temperature of the roughing mill is 1020 ℃, the rolling speed is 2.5m/s, and the phosphorus is removed by the high-pressure water in the first, third and fifth times; (4) finish rolling: the intermediate billet is cooled by an intermediate roller way to be rolled directly before a finishing mill, the rolling speed is 6.0m/s, the initial rolling temperature is 960 ℃, the rolling is carried out for 9 times, the average reduction rate is 10%, the final rolling temperature is 840 ℃, and the high-pressure water dephosphorization is carried out for the first, third, fifth and seventh times.

The steel sheets of the above three examples were subjected to tensile, impact, and cold bending tests, and the properties thereof are shown in Table 2.

TABLE 1 chemical composition (%)

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TABLE 2 tensile Properties of the steel sheets of the examples

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Claims (1)

1. A production method of a Q420qE bridge structural steel plate is characterized by comprising the following steps: the steel comprises C = 0.08-0.10, Si = 0.20-0.30, Mn = 1.50-1.60, P ≤ 0.015, S ≤ 0.003, Al = 0.02-0.05, Ti = 0.008-0.020, Nb = 0.080-0.090, Mo = 0.20-0.30, Cr = 0.20-0.50, and the balance of Fe and unavoidable impurity elements; the key process steps comprise:

(1) plate blank inserting: smelting the components in a converter and casting the components into a steel billet, wherein the off-line temperature of the steel billet is 600-750 ℃;

(2) heating the plate blank at a high temperature: after steel billets are taken off line, a hot delivery and hot charging mode is adopted, the surfaces of the casting billets are cooled to below 550 ℃ by water and then are fed into a furnace, the heating furnace is divided into a preheating section, an I adding section, an II adding section and a soaking section for controlled heating, the temperature of the preheating section is 800-1200 ℃, the temperature of the I adding section is 1200-1300 ℃, the temperature of the II adding section is 1300-1350 ℃, the temperature of the soaking section is 1300-1350 ℃, the total in-furnace time is 160-190 min, the heating rate is 3-8 min/cm, and the tapping temperature is 1210-1250 ℃;

(3) rough rolling: primarily removing phosphorus from the plate blank after the plate blank is taken out of the furnace by using high-pressure water to the front of a roughing mill, wherein the rolling starting temperature is less than or equal to 1240 ℃, the average reduction rate of the three times after widening is more than or equal to 15%, the rolling is carried out for 5-7 times, the finishing temperature of roughing mill is less than or equal to 1050 ℃, the rolling speed is 2.5-3 m/s, and the phosphorus is removed by using the high-pressure water for 2-3 times;

(4) Finish rolling: and (3) cooling the intermediate blank by an intermediate roller way until the intermediate blank is rolled directly before a finishing mill, wherein the rolling speed is 6-7 m/s, the initial rolling temperature is more than or equal to 960 ℃, the rolling time is 7-9 times, the average reduction rate is more than or equal to 10%, the final rolling temperature is 840-880 ℃, and phosphorus is removed by high-pressure water for 1-4 times.

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