A method for improving the surface quality of a double stand rolling engineering machinery steel

Abstract

This invention relates to a control method for improving the surface quality of steel used in engineering machinery produced by twin-stand rolling, belonging to the field of metal rolling technology. The process route includes continuous casting, direct billet feeding, heating, rough rolling, finish rolling, straightening, and cooling. In the continuous casting process, the superheat of the casting is controlled at 5–25°C, the casting speed is controlled at 1.10–1.20 m / min, and the center temperature of the upper surface of the billet exiting the fan-shaped section reaches 1100–1200°C. In the direct billet feeding process, after the billet is cut, it is directly and rapidly fed hot to the heating furnace via a roller conveyor with an insulation cover. The hot feeding time is controlled at 3–10 minutes, and the average hot feeding and charging temperature reaches above 850°C. This invention can effectively reduce surface defects such as blemishes and pits in steel used in engineering machinery, thereby improving the surface quality of the steel.

Description

Technical Field

[0001] This invention relates to a control method for improving the surface quality of steel used in engineering machinery rolled by a two-stand mill, belonging to the field of metal rolling technology. Background Technology

[0002] Construction machinery steel is widely used in industries such as heavy machinery, presses, mining machinery, cranes, ship equipment, and marine engineering equipment. Because it needs to withstand complex and variable loads and usually requires further coating to improve the appearance quality of the product, the steel plate is required to have high strength and toughness, low yield strength ratio, easy welding, fatigue resistance and other characteristics, as well as good surface quality.

[0003] Due to the rapid rolling rhythm and close coordination of processes, double-stand rolling mills frequently encounter problems such as surface blemishes, thick iron oxide scale, and pits on rolled steel plates, continuously hindering the development of high-quality surface steel for engineering machinery. Surface blemishes refer to uneven oxidation of the billet during heating and rolling, resulting in a wrinkled appearance and alternating red and blue color variations. Surface pits are caused by iron oxide scale being pressed into the steel substrate, forming surface pit defects. These defects are flat on the surface of hot-rolled steel plates before shot blasting, making them difficult to detect through manual online quality inspection. After shot blasting, small pits become visible on the surface.

[0004] Chinese patent CN110976534B discloses a "method to reduce surface spots on steel used in engineering machinery in a single-stand wide and thick plate rolling mill". The method involves heating the billet for more than 200 minutes and using high-temperature homogenization at 1230-1270℃. Prolonged high-temperature heating will lead to severe over-oxidation of the billet surface and thick iron oxide scale. Therefore, at least two descaling processes are required to ensure that the furnace-grown iron oxide scale is completely removed, resulting in billet temperature loss. Furthermore, it is not suitable for the fast-paced two-stand rolling mode.

[0005] Chinese patent CN 105624383A discloses "A production method for improving the surface quality of high-strength medium-thick plate products". This invention mainly solves the problem of pit defects generated in steel plates during the straightening process by optimizing the cooling process and controlling the microstructure of iron oxide scale in high-strength medium-thick plate products online. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for improving the surface quality of steel for engineering machinery rolled by double stand rolling, which can effectively reduce surface defects such as spots and pits in the steel for engineering machinery and improve the surface quality of the steel for engineering machinery.

[0007] The technical solution to the above technical problem is as follows:

[0008] A method for improving the surface quality of steel for engineering machinery produced by twin-stand rolling, the process route including continuous casting, direct feeding of billet, heating, rough rolling, finish rolling, straightening, and stacking cooling;

[0009] In the continuous casting process, the superheat of the casting is controlled at 5-25℃, the casting speed is controlled at 1.10-1.20M / min, and the center temperature of the upper surface of the billet exiting the fan-shaped section reaches 1100-1200℃.

[0010] The billet direct delivery process refers to the process where, after the billet is cut, it is directly and quickly delivered to the heating furnace via a roller conveyor with an insulation cover. The hot delivery time is controlled between 3 and 10 minutes, and the average hot delivery and hot charging temperature reaches above 850°C.

[0011] The above-mentioned method for improving the surface quality of steel for engineering machinery produced by double-stand rolling includes a four-stage precision heating control method for the heating process. The first stage heating temperature is 1100~1150℃, the second stage is 1200±30℃, the third stage is 1250±20℃, and the soaking stage is 1220±20℃. The temperature difference between two adjacent billets along the length is controlled within ≤10℃. Each heating stage is held for 15~25 minutes, and the soaking stage for 30~40 minutes, with a total time controlled between 80-100 minutes. The heating raw material is pure blast furnace gas with a stable calorific value of 740~760 kcal / m³. The air-fuel ratio is precisely controlled within the range of 0.75-0.80.

[0012] The above-mentioned method for controlling the surface quality of steel for engineering machinery produced by double-stand rolling involves the following steps: in the direct feeding process of the billet, the average temperature of hot feeding and hot charging reaches 850℃ or higher, the minimum temperature of the corner of the billet is ≥800℃, and the temperature of the center of the billet surface is ≥900℃.

[0013] The above-mentioned method for improving the surface quality of steel for engineering machinery rolled by double-stand rolling includes the following steps: in the rough rolling process, the temperature of the first descaling pass is controlled to be >1173℃, and the temperature in the rough rolling stage is above 1000℃; and a spray cooling process for the intermediate billet after rolling is adopted to improve the cooling rate of the intermediate billet and improve the rolling efficiency.

[0014] In the finishing rolling process, the initial rolling temperature is controlled below 930℃; a two-pass, multi-stage high-pressure water descaling process is adopted, with a descaling pressure ≥20MPa, primarily to meet the final rolling temperature requirements; the finishing rolling speed is 5~6m / s; when the target steel plate thickness is 20mm~40mm, the final rolling temperature is controlled at 820-850℃; when the target steel plate thickness is greater than or equal to 12mm and less than 20mm, the final rolling temperature is controlled at 840-870℃; when the target steel plate thickness is greater than or equal to 10mm and less than 12mm, the final rolling temperature is controlled at 860-900℃.

[0015] The above-mentioned method for controlling the surface quality of steel for engineering machinery rolled by double-stand rolling involves a straightening process in which external cooling water is not used in the straightening machine, and the straightening reduction is controlled to be ≤5mm and the straightening force is controlled to be ≤20000KN, so as to control the plastic deformation rate of the steel plate to exceed 70%, so that the steel plate can obtain sufficient plastic deformation and reduce the residual stress of the steel plate.

[0016] The above-mentioned method for controlling the surface quality of steel for engineering machinery produced by twin-stand rolling, wherein the continuous casting process uses a 260*1700~1900mm cross section, and the chemical composition of the billet by weight percentage is: C 0.05~0.20%, Si ≤0.30%, Mn 0.80~1.60%, P≤0.045%, S≤0.045%, Als 0.005~0.040%, Nb ≤0.02%, V ≤0.015%, Ti ≤0.02%, with the balance being iron and unavoidable impurities.

[0017] C: Steel used in engineering machinery is widely used in heavy machinery, therefore, there are special requirements for the strength and weldability of the material; the material is required to have good weldability while meeting the strength requirements; if its content is less than 0.05%, the strength requirements cannot be met; if its content is greater than 0.20%, the good weldability of the material cannot be met; therefore, the C content in the steel of this invention is designed to be 0.05-0.20%.

[0018] Si is an important reducing agent and deoxidizer. It can dissolve in ferrite and austenite to increase the hardness and strength of steel. At the same time, it can promote the coarsening of ferrite grains. When silicon-containing steel is heated in an oxidizing atmosphere, a layer of FeSiO4 with a melting point of 1173℃ will form on the surface, which increases the difficulty of surface dephosphorization. Therefore, from the perspective of improving surface quality, Si ≤ 0.30% should be controlled to improve the surface quality of steel.

[0019] Mn is a major solid solution strengthening element that can significantly improve the strength of steel. It is also an element that forms carbides. When the content does not exceed 2%, Mn will enter the cementite and replace some iron atoms, thereby lowering the critical transformation temperature of austenite to pearlite and increasing the stability of austenite. Its ability to stabilize austenite structure is second only to nickel.

[0020] P and S: These are harmful elements. Excessively low P and S content will increase the cost of smelting and removal. By controlling the P and S content according to the grade of steel used in engineering machinery for different purposes, production costs can be saved while ensuring the stable performance of the steel.

[0021] Als (Al₂S): As a deoxidizer or alloying element added to steel, aluminum has a much stronger deoxidizing ability than silicon and manganese. The main role of aluminum in steel is to refine grains and fix nitrogen in the steel, thereby significantly improving the impact toughness and reducing the tendency for cold brittleness and aging. At the same time, a high Al content can improve the high-temperature oxidation resistance and surface quality of the steel. If its content is less than 0.005%, it cannot achieve a good deoxidizing effect. When the aluminum content is high, it will reduce the high-temperature strength and toughness of the steel and bring some difficulties to smelting and casting. Therefore, the Al content is controlled between 0.005% and 0.040%.

[0022] Nb+V+Ti: These are selectively added elements in this scheme. After adding microalloying elements such as Nb, V, and Ti, the trace elements form highly dispersed carbon and nitrogen compound particles, which can "pin" the austenite grain boundaries, thereby preventing austenite grain migration, inhibiting austenite grain growth, improving grain refinement, and enhancing the stability of steel plate performance.

[0023] The parameter control mechanism of each process in this invention is analyzed as follows:

[0024] Continuous casting process: The 5-25℃ low superheat casting process ensures a high casting speed of 1.10~1.20M / mi. At the same time, the low superheat casting can suppress the formation of columnar crystals and promote the growth of equiaxed crystals inside the billet, thereby improving the quality of the billet at high casting speed. In addition, increasing the casting speed has a significant effect on the backward movement of the liquid core of the billet, which helps to increase the temperature of the billet exiting the fan-shaped section to above 1100℃.

[0025] In the direct billet delivery process: when the average hot charging temperature is greater than 850℃, the minimum temperature at the corners of the billet is ≥800℃, and the temperature at the center of the billet surface is ≥900℃, the core temperature is much higher than 850℃ due to the latent heat inside the billet (actually measured 900-1080℃). 850℃ is the critical temperature for the austenite-ferrite phase transformation. At this temperature, the microstructure of each part of the billet is single austenite, and no phase transformation occurs, ensuring that even if the steel plate is heated at high temperatures, no large thermal stress will be generated inside. Below this temperature, austenite is unstable and will gradually precipitate ferrite, entering the austenite-ferrite two-phase region. Heating at this temperature and during subsequent rolling processes can easily lead to surface crack defects in the steel plate.

[0026] Four-stage heating process: When the average temperature of hot charging and hot delivery exceeds 850℃, the internal structure of the billet is still austenitic without phase transformation. At this time, adjusting the heating temperature of each stage and appropriately increasing the heating rate will not affect the internal quality of the billet. The reason for the steady temperature increase of each heating stage within a small range of 30~50℃ is that the billet is in the second brittle zone of steel in the range of 1000~1300℃. The characteristic of this range is that the growth of austenitic grains and the dissolution process of substances such as (Fe,Mn)S and (Fe,Mn)O that precipitate first at the austenitic grain boundaries occur simultaneously. Rapid heating will cause asynchronous deformation at different locations, increase grain boundary brittleness, and easily lead to surface cracks after rolling. Controlling the holding time of each heating section to 15-25 minutes ensures minimal surface burn-off and uniform internal temperature of the billet. Controlling the soaking heat section to 30-40 minutes, with a total time of 80-100 minutes, ensures the billet's soaking temperature remains stable at 1200-1240℃, and that the temperature difference between adjacent billets along the length is controlled within ≤10℃. Furthermore, it increases the heating rate of the high-temperature direct-feed billet, effectively guaranteeing the performance stability of the rolled plate during rolling. This provides favorable conditions for reducing rolling passes and increasing the rolling rhythm. By reducing the heating time of the billet at high temperatures, it minimizes billet oxidation and burn-off, improving the surface quality of the rolled steel plate.

[0027] Controlling the air-fuel ratio within the range of 0.75 to 0.8 improves combustion efficiency because most domestic heating furnaces currently use a dual regenerative combustion method with pure blast furnace gas supply. Since the calorific value of pure blast furnace gas is relatively stable, ranging from 740 to 760 kcal / m³, heating personnel can strictly adjust and control the air-fuel ratio within this range. Simultaneously, by using a laser oxygen content detection and analysis device in the exhaust branch pipe to monitor the oxygen content in the flue gas in real time, operators can be guided to perform self-optimization and correction of the specific air-fuel ratio value. This ensures complete combustion of fuel within the furnace while minimizing billet oxidation loss and improving billet descaling.

[0028] The roughing rolling temperature is above 1000℃. At this temperature, the steel has a single austenitic structure, exhibiting optimal plasticity. Within the single-phase austenitic region, the resistance to deformation is low, and residual stress after rolling is minimal, facilitating improved rolling stability. Simultaneously, the iron oxide scale exhibits better plasticity and toughness at high temperatures. The "high-temperature, high-speed rolling" process avoids iron oxide scale breakage and reduces its thickness. Post-rolling, an intermediate billet spray cooling process is employed. Spraying the intermediate billet achieves rapid cooling, quickly lowering the finishing rolling start temperature to below 930℃. This reduces the thickness of the iron oxide scale on the steel plate surface during air cooling, effectively thinning the intermediate billet's iron oxide scale during the warm-up process. This process design, mentioned in multiple documents, is a non-critical but essential part of this invention.

[0029] In the finishing rolling stage, the initial rolling temperature is controlled below 930℃, and the rolling speed is 5~6m / s, continuing the "high temperature, high speed rolling" concept. This avoids iron oxide scale breakage and reduces the thickness of the iron oxide scale. Setting the initial rolling temperature below 930℃ ensures that deformation always occurs within the non-recrystallization temperature range, thus avoiding part of the recrystallization temperature range. Repeated rolling deformation of the steel plate in the non-recrystallization region helps to form a large number of dislocations and deformation bands inside the steel plate; these accumulated dislocations and deformation bands become ferrite phase deformation nuclei, contributing to the formation of finer ferrite grains later. This process design has been mentioned in several documents and is a non-critical but essential process part of this invention. The finishing rolling speed is controlled at 5~6 m / s. This high-speed rolling process reduces the surface temperature drop of the steel plate, ensuring that the final rolling temperature is 820~850℃ when the target steel plate thickness is 20mm~40mm; 840~870℃ when the target steel plate thickness is greater than or equal to 12mm and less than 20mm; and 860~900℃ when the target steel plate thickness is greater than or equal to 10mm and less than 12mm. Controlling the final rolling temperature within the range of 820~900℃ ensures that the steel plate deforms at a lower temperature, reducing phenomena such as recovery at high temperatures that lower dislocation density, thus improving the stability of the steel plate's performance.

[0030] In the straightening process, external cooling water is not used on the straightener to prevent the iron oxide scale on the steel plate surface from cracking upon contact with water and further penetrating into the steel plate matrix. Controlling the plastic deformation rate of the steel plate to over 70%, using a smaller straightening reduction (≤5mm) and a straightening force (≤20000KN) avoids iron oxide scale cracking during straightening while ensuring sufficient plastic deformation and reducing residual stress. In the SMS hot straightener model, OVS represents the degree of plastic deformation of the steel plate. OVS is the elastic-plastic ratio, equal to the ratio of the thickness *he* of the steel plate undergoing plastic deformation to the thickness *hp* undergoing elastic deformation, OVS = *he* / *hp*. A larger OVS value indicates a greater degree of plastic deformation during straightening and bending; OVS = 3 indicates a plastic deformation rate of 70%, and OVS = 4 indicates a plastic deformation rate of 80%.

[0031] The beneficial effects of this invention are as follows:

[0032] This invention reduces the oxidation time of the billet in the furnace by high-temperature rapid charging of the billet, improves combustion efficiency by adjusting the air-fuel ratio in the heating furnace to the range of 0.75~0.8, and reduces the degree of oxidation on the surface of the billet. Based on this, the rolling process is further optimized to control the thickness of the secondary iron oxide scale generated during the rolling stage. The appearance of the steel plate changes from the previously randomly distributed, loose reddish-brown iron oxide scale to a dense, bluish-black iron oxide scale, resulting in excellent surface quality and avoiding surface pits and spots. After adopting the production process of this invention, the inspection pass rate for high surface quality has increased from 50% to over 90%. The steel plates produced by the method of this invention not only have excellent surface quality but also effectively improve operating efficiency, shorten the process flow and reduce production costs. The process is simple and feasible. Attached Figure Description

[0033] Figure 1 A surface quality diagram (10×) of the steel plate produced in Example 1.

[0034] Figure 2 Surface quality diagram of the steel plate produced in Example 2 (10×);

[0035] Figure 3 A surface quality diagram (10×) of the steel plate produced in Example 3;

[0036] Figure 4 A surface quality diagram (10×) of the steel plate produced in Example 4.

[0037] Figure 5 A surface quality diagram (10×) of the steel plate produced in Example 5. Detailed Implementation

[0038] This invention discloses a method for controlling the surface quality of steel for engineering machinery produced by twin-stand rolling. The process route includes continuous casting, direct billet feeding, heating, rough rolling, finish rolling, straightening, and stacking cooling.

[0039] Continuous casting uses a cross-section of 260*1700~1900mm, with the superheat of the casting controlled at 5~25℃, and the casting speed controlled at 1.10~1.20M / min. The center temperature of the upper surface of the fan-shaped section of the billet reaches 1100~1200℃. After the billet is cut, it is directly and quickly hot-fed to the heating furnace through a roller conveyor with a heat insulation cover. The hot-fed time is controlled at 3~10min, and the average hot-fed and hot-charged temperature reaches above 850℃. The minimum temperature of the corner of the billet is ≥800℃, and the center temperature of the billet surface is ≥900℃.

[0040] In the heating process, the raw material for the heating furnace is pure blast furnace gas with a stable calorific value of 740~760 kcal / m3 and an air-fuel ratio of 0.75-0.80. The heating temperature is 1100~1150℃ in the first heating stage, 1170~1230℃ in the second heating stage, 1220~1270℃ in the third heating stage, and 1200~1240℃ in the soaking stage. The temperature difference between two adjacent billets along the length is controlled within ≤10℃. The holding time for each heating stage is 15~25 min, and the soaking stage is held for 30~40 min. After a total of 80~100 min in the furnace, the billets are removed from the furnace.

[0041] The billet is immediately descaled with high-pressure water at a pressure of 22 MPa and a temperature of >1173℃ after exiting the furnace. The initial rolling temperature during the roughing stage is 1040-1060℃, and the final rolling temperature is 1000-1020℃. Intermediate billet is cooled by spraying. The initial rolling temperature during the finishing stage is controlled to be ≤930℃, and the finishing rolling speed is 5~6m / s. The second rolling pass uses a multi-pass high-pressure water descaling process with a pressure of 20~22 MPa. The final rolling temperature is controlled at 820-900℃.

[0042] For high-temperature controlled rolling steel plates, the corresponding ACC controlled cooling process is adopted. The steel plates are transported to the hot straightener via roller conveyor. At the same time, no external cooling water is used in the straightener. A straightening force of 3000KN~20000KN and a reduction of 0.5mm~5.0mm are used. At this time, the OVS display of the straightening system is 3~4, which means that the plastic deformation rate of the steel plate during straightening and bending is ≥70%.

[0043] The straightened steel plates are slowly cooled to ≤60℃ on a cooling bed before being finished.

[0044] The present invention will be further described below through specific embodiments 1-5:

[0045] The above method was used to test finished engineering machinery steel with a thickness of 10~40mm.

[0046] The specific process parameters are as follows:

[0047]

[0048]

[0049]

[0050]

[0051] Under the process methods of Examples 1-5, the surface quality of the steel plate is as follows: Figures 1-5 As shown. From Figures 1-5 It can be seen that there is no residual iron oxide scale, spots, pits or other defects on the surface of the steel plate.

Claims

1. A method for controlling the surface quality of steel for engineering machinery produced by twin-stand rolling mills, the process route including continuous casting, direct billet feeding, heating, rough rolling, finish rolling, straightening, and stacking cooling; characterized in that: In the continuous casting process, the superheating temperature is controlled at 5-25℃, the casting speed is controlled at 1.10-1.20 m / min, and the center temperature of the upper surface of the billet exiting the fan-shaped section reaches 1100-1200℃; a cross-section of 260*1700-1900mm is adopted, and the chemical composition of the billet by weight percentage is: C 0.05-0.20%, Si≤0.30%, Mn 0.80-1.60%, P≤0.045%, S≤0.045%, Als 0.005-0.040%, Nb≤0.02%, V≤0.015%, Ti≤0.02%, with the balance being iron and unavoidable impurities; The billet direct delivery process refers to the process where, after the billet is cut, it is directly and quickly delivered to the heating furnace via a roller conveyor with a heat insulation cover. The hot delivery time is controlled at 3 to 10 minutes, and the average hot delivery and hot charging temperature reaches more than 850°C. In the heating process, the raw material used for heating is pure blast furnace gas, with a stable calorific value of 740~760 kcal / m³. 3 The air-fuel ratio should be controlled within the range of 0.75-0.

80. In the rough rolling process, the temperature of the first dephosphorization pass is controlled to be >1173℃, and the temperature of the rough rolling stage is above 1000℃. In the finishing rolling process, the initial rolling temperature is controlled to be below 930℃; and a two-pass, multi-stage high-pressure water descaling process is adopted, with a descaling pressure ≥20MPa; the finishing rolling speed is 5~6m / s. When the target steel plate thickness is between 20mm and 40mm, the final rolling temperature is controlled at 820-850℃; when the target steel plate thickness is greater than or equal to 12mm and less than 20mm, the final rolling temperature is controlled at 840-870℃; when the target steel plate thickness is greater than or equal to 10mm and less than 12mm, the final rolling temperature is controlled at 860-900℃.

2. The control method for improving the surface quality of steel used in twin-stand rolled engineering machinery as described in claim 1, characterized in that: In the billet direct delivery process, the average temperature of hot delivery and hot charging reaches 850℃ or higher, the minimum temperature of the billet corner is ≥800℃, and the temperature of the billet surface center is ≥900℃.

3. The control method for improving the surface quality of steel used in twin-stand rolling of engineering machinery as described in claim 1, characterized in that: The heating process employs a four-stage precision heating control method, wherein the first stage heating temperature is 1100~1150℃, the second stage heating temperature is 1200±30℃, the third stage heating temperature is 1250±20℃, and the soaking stage temperature is 1220±20℃. The temperature difference between two adjacent billets along the length is controlled within ≤10℃. The holding time for each heating stage is 15~25min, the soaking stage is 30~40min, and the total time is controlled within 80-100min.

4. The control method for improving the surface quality of steel used in twin-stand rolling for engineering machinery as described in claim 1, characterized in that: The straightening process does not use external cooling water for the straightening machine. The straightening reduction is controlled to be ≤5mm and the straightening force to be ≤20000KN, so as to control the plastic deformation rate of the steel plate to exceed 70%, so that the steel plate can obtain sufficient plastic deformation and reduce the residual stress of the steel plate.

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