A low-cost, acid-free structural steel plate with a strength of 700MPa and its manufacturing method.

Abstract

A low-cost, acid-free structural steel of 700MPa grade and its manufacturing method are disclosed. The chemical composition (mass percentage) is as follows: C: 0.04–0.16%, Si ≤ 0.05%, Mn: 0.4–1.3%, P ≤ 0.02%, S ≤ 0.01%, Ti: 0.03–0.16%, Nb: 0.006–0.035%, Mo: 0.06–0.3%, with the balance including Fe and other unavoidable impurities. Furthermore, it must simultaneously satisfy: 0.251Ti + 1.12Nb + 0.115Mo ≥ 0.05; C + Mn / 6 ≤ 0.25. This invention achieves excellent steel plate performance while maintaining good surface characteristics (smooth, dense oxide scale, and good resistance to deformation and peeling). Its yield strength is ≥ 600MPa, tensile strength is ≥ 700MPa, elongation is ≥ 14%, cold bending performance is D = 2a bending at 180°, and acid washing is eliminated.

Description

Technical Field

[0001] This invention relates to the field of structural steel, and in particular to a 700MPa grade low-cost acid-free structural steel and its manufacturing method. Background Technology

[0002] With the increasing stringent environmental protection requirements of the country, green and high-quality development has become the main theme of various industries, and users have also put forward requirements for steel products such as high performance, high quality, and low cost. Many hot-rolled structural steel products usually need to be pickled to remove the iron oxide scale on the surface of the steel plate before painting and use, and the waste acid generated therefrom seriously pollutes the environment.

[0003] To address the aforementioned problems, researchers have considered improving the state and structure of the oxide scale on the steel surface to achieve a state where pickling is unnecessary, thereby reducing environmental pollution from waste acid emissions. Currently, there are numerous technologies for controlling hot-rolled oxide scale, such as:

[0004] Chinese patent CN201010235928.X discloses a "Production Method of Environmentally Friendly High Surface Quality Acid-Free Automotive Beam Steel", which introduces a method to produce high-surface-quality acid-free automotive beam steel by controlling the oxide scale structure on the surface of automotive beam steel through high-temperature heating, high-temperature rolling, and low-temperature coiling.

[0005] Chinese patent CN201110307238.5 discloses a "temperature control method for solving the black ash on the surface of hot-rolled automotive frame steel plates". It introduces a method to solve the black ash on the surface of frame steel by reducing the slab heating temperature, adopting double descaling treatment, and increasing the final rolling temperature and coiling temperature to improve the oxide scale composition.

[0006] Chinese patent CN201210176422.5 discloses "Acid-free steel coil with uniform surface iron scale and its manufacturing method", which introduces a method for producing acid-free steel coils with uniform surface iron scale by using micro-wave and lubricated fine rolling, coating the two ends of the steel coil with anti-oxidation glass powder after rolling, and forced cooling of the steel coil.

[0007] Chinese patent CN201310057003.4 discloses "High-strength steel for beams with stable oxide layer and pickling-free method thereof", which introduces a method for producing steel for beams with stable oxide layer and pickling-free method by means of post-cooling: after the strip exits the finishing mill, it is relaxed and cooled for 10 to 30 seconds, then cooled to 420 to 550°C at a cooling rate of 10 to 40°C / s and coiled. After coiling, the steel coil is kept at the temperature for 6 to 10 hours and then cooled to room temperature.

[0008] The aforementioned patents all employ special processes to improve the uniformity of oxide scale on the surface of steel plates, primarily emphasizing production methods without considering the steel plate composition design or the economic efficiency and convenience of production. For example, Chinese patent CN201110307238.5 requires a double descaling process, and Chinese patent CN201210176422.5 requires coating both ends of the rolled steel coil with anti-oxidation glass powder, both of which affect normal production efficiency and increase production costs. Furthermore, Chinese patent CN201310057003.4 requires the use of an insulation cover to insulate the coiled steel, equipment not available in conventional hot rolling mills, and also hinders the convenient production of acid-free steel.

[0009] Chinese patent CN201610486480.6 discloses "A high surface quality pickling-free hot-rolled strip steel and its manufacturing method", which introduces a method to produce high surface quality pickling-free hot-rolled strip steel by controlling the Si and Mn element content in the steel and high coiling temperature, and utilizing the internal oxidation of Si and Mn after coiling to form a pure iron layer on the surface of the strip steel.

[0010] Chinese patent CN201810003351.6 discloses a method for preventing the powdery peeling of iron oxide scale from hot-rolled, pickled-free automotive frame steel. The method involves controlling the Si content (0.28wt% to 0.3wt%) in the steel, adding 0.2wt% to 0.225wt% Cr, and adding a fan during the coiling process to increase the cooling rate, thereby adjusting the composition ratio of Fe3O4 and FeO in the oxide scale to prevent the powdery peeling of iron oxide scale from the frame steel.

[0011] The two patents mentioned above control the structure of the oxide scale by adding specific amounts of Si and Mn or Cr to the steel, so as to improve the quality of the oxide scale on the surface of the steel plate.

[0012] Other patents include Chinese Patent CN201510778750.6, which discloses "400MPa grade acid-free hot-rolled steel plate for automotive structures and its production method"; Chinese Patent CN201010298939.2, which discloses "490MPa grade acid-free hot-rolled steel plate and its production method"; Chinese Patent CN201711190855.5, which discloses "510MPa grade hot-rolled acid-free automotive structural steel plate and its production method"; Chinese Patent CN201711190852.1, which discloses "560MPa grade hot-rolled acid-free automotive structural steel plate and its production method"; and Chinese Patent CN201711190853.6, which discloses "610MPa grade hot-rolled acid-free automotive structural steel plate and its production method". These patents respectively describe the production methods of hot-rolled acid-free steel plates with tensile strength levels of 400MPa, 490MPa, 510MPa, 560MPa, and 610MPa.

[0013] These patented technologies share similar characteristics. Their composition employs a C, Si, Mn+(Nb, Ti) design, with C content ranging from 0.04% to 0.11%, Si content from 0.05% to 0.27%, Mn content from 0.7% to 1.5%, and Ti addition below 0.03%. The process involves controlling the heating temperature at 1210–1240℃, the heating time at 1.2–1.8 hours, the final rolling temperature at 860–890℃, and the coiling temperature at 560–590℃. Through the combination of composition and process, hot-rolled pickled-free steel sheets of different strength grades are ultimately obtained. Summary of the Invention

[0014] The purpose of this invention is to provide a low-cost, acid-free structural steel of grade 700MPa and its manufacturing method. This invention achieves excellent performance of the steel plate while maintaining good surface characteristics (smooth, dense oxide scale, and good resistance to deformation and peeling). Its yield strength is ≥600MPa, tensile strength is ≥700MPa, elongation is ≥14%, and it has excellent cold bending performance (D=2a bending 180°). Moreover, it eliminates the need for acid washing.

[0015] To achieve the above objectives, the technical solution of the present invention is as follows:

[0016] A low-cost, acid-free structural steel has the following chemical composition by mass percentage: C: 0.04–0.16%, Si ≤ 0.05%, Mn: 0.4–1.3%, P ≤ 0.02%, S ≤ 0.01%, Ti: 0.03–0.16%, Nb: 0.006–0.035%, Mo: 0.06–0.3%, with the balance including Fe and other unavoidable impurities. Furthermore, it must simultaneously satisfy the following:

[0017] 0.251Ti+1.12Nb+0.115Mo≥0.05;

[0018] C+Mn / 6≤0.25.

[0019] Furthermore, it also contains one or more of the following: Cr≤0.2%, V≤0.15%, Ni≤0.1%, and Ca≤0.015%.

[0020] Preferably, the microstructure of the structural steel is polygonal ferrite and / or quasi-polygonal ferrite plus a small amount of pearlite.

[0021] The structural steel described in this invention has a yield strength ≥600MPa, tensile strength ≥700MPa, elongation ≥14%, and cold bending performance D=2a for bending 180°.

[0022] In the composition design of the structural steel described in this invention:

[0023] C is the most important strengthening element in this invention. As an interstitial atom in steel, C plays a crucial role in improving the strength of the steel. In this invention, C can also form precipitates with Ti, increasing the yield strength and tensile strength of the steel. However, a high C content will deteriorate the weldability and low-temperature toughness of the steel, and will form more pores in the oxide scale. Therefore, the C content needs to be controlled within a suitable range. In this invention, the C content is controlled at 0.04% to 0.16%.

[0024] Si: Although Si can improve the strength of steel through solid solution strengthening and inhibit or delay the formation of Fe3C, excessive Si content can lead to the formation of Fe2SiO4 during furnace heating, resulting in incomplete descaling and a deterioration in the surface quality of the steel plate. Furthermore, increased Si content can cause poor uniformity of the oxide scale. Therefore, in this invention, the Si content is controlled below 0.05%.

[0025] Mn: Mn is an important strengthening and toughening element. It can expand the austenite phase region, stabilize austenite, delay the transformation of austenite to pearlite, reduce the critical cooling rate of martensite transformation, and greatly improve hardenability. Mn also has a certain solid solution strengthening effect, which is beneficial to improving the strength of steel plates. However, excessively high Mn content can lead to excessively high hardenability in steel, which is not conducive to precise material control. Furthermore, with increasingly stringent environmental regulations, the price of Mn raw materials has risen sharply, and for cost considerations, it is not advisable to add too much Mn to steel. Therefore, in this invention, the Mn content is controlled at 0.4%–1.3%.

[0026] P: P is an impurity element in steel. P readily segregates to the grain boundaries and forms a low-melting-point eutectic compound Fe2P with Fe, reducing the plasticity and toughness of steel. Therefore, its content should be as low as possible, generally controlled below 0.02%.

[0027] S: S is an impurity element in steel. S in steel usually combines with Mn to form MnS inclusions. MnS has a certain plasticity and deforms along the rolling direction during the rolling process, which destroys the continuity of the matrix and reduces the transverse properties of the steel plate. Therefore, the lower the S content in steel, the better. In actual production, it is usually controlled to be within 0.01%.

[0028] Ti: Ti is a strong carbonitride forming element. Undissolved Ti carbonitrides can effectively pin austenite grain boundaries during steel heating, controlling austenite grain growth. During rough rolling in the high-temperature austenite region, the precipitated TiN and TiC can effectively inhibit austenite grain growth, while the fine TiC second-phase particles precipitated in ferrite can prevent dislocation movement, thus greatly improving the strength of the steel plate. Therefore, the Ti content in this invention is controlled at 0.03–0.16%.

[0029] Nitrogen (Nb): Nitrogen is a strong carbonitride forming element that enhances the strength of steel through grain refinement and precipitation strengthening. Nib can raise the austenite recrystallization temperature, enabling non-recrystallization rolling of austenite at higher temperatures. This allows the rolled piece to undergo rolling deformation at higher temperatures while simultaneously obtaining a fine microstructure. However, Nib is relatively expensive; therefore, the Nib content in this invention is controlled at 0.006–0.035%.

[0030] Mo: Mo effectively improves hardenability and inhibits pearlite formation. Simultaneously, Mo has a strong affinity for C, and compared to steels with only Ti added, (Ti,Mo)C has higher thermal stability than TiC. Therefore, during high-temperature coiling, the coarsening of (Ti,Mo)C is much less than that of TiC, allowing for better precipitation strengthening of the second phase. However, Mo is a valuable alloying element, and a high content will significantly increase the manufacturing cost of the steel. Therefore, this invention controls the Mo content to 0.06–0.3%.

[0031] This invention employs a composition design with low C, low Si, low Mn, plus Ti, Nb, and a small amount of Mo.

[0032] Low C and low Mn content (C+Mn / 6≤0.25) is used to reduce the hardenability of the steel, facilitating precise control of the steel plate microstructure. If the C and Mn content is too high, the hardenability of the steel is high, and during the mesocooling process after finishing rolling, the ferrite phase transformation is suppressed, resulting in the inability to obtain polygonal ferrite and / or quasi-polygonal ferrite. This also hinders the precipitation of second phases such as TiC and NbC within the ferrite grains, thus preventing precipitation strengthening. Simultaneously, lower C content reduces the formation of voids in the oxide scale, increasing its density. Higher oxide scale density results in better toughness, which is more conducive to achieving pickling-free use. Lower Mn content helps reduce alloy costs.

[0033] In this invention, because the contents of C, Si, and Mn in the steel are very low, a certain amount of Ti and Nb are added to ensure that the steel plate meets the required strength indicators. The strength of the steel plate is improved through precipitation strengthening of second-phase particles such as TiC and NbC. The more numerous and smaller the precipitated particles, the more significant the precipitation strengthening effect. To ensure sufficient precipitation of the Ti and Nb-containing second phases and to prevent the coarsening and growth of the precipitated particles, the contents of Ti, Nb, and Mo in the steel must satisfy: 0.251Ti + 1.12Nb + 0.115Mo ≥ 0.05.

[0034] The method for manufacturing low-cost, acid-free structural steel according to the present invention includes the following steps:

[0035] 1) Smelting and casting

[0036] The above-mentioned components are smelted, refined, and continuously cast into billets or ingots;

[0037] 2) Heating of billets or ingots

[0038] The billet or ingot is heated to 1250–1300℃ for 2–5 hours.

[0039] 3) Hot rolling

[0040] Before roughing, the descaling water in the descaling box is opened to remove the furnace-grown oxide scale, i.e., the primary oxide scale. Descaling is introduced during the roughing pass, and the exit temperature of the roughing mill is ≤1010℃. Then, the steel is rolled into finished coils through a six- or seven-stand finishing mill. Before finishing, the descaling water in the descaling box is opened to remove the secondary oxide scale. Descaling water is introduced between the F1 and F2 stands of the finishing mill. The final rolling temperature is 870~920℃, and the threading speed is ≥7m / s. After rolling, a pre-cooling process is used for cooling, and the coiling temperature is 600~650℃.

[0041] 4) Cooling

[0042] After the steel coil is unloaded, it is cooled by a fan at a cooling rate of ≥2℃ / s to 520~550℃. Then, the steel coil is sent to an insulated wall or hot coil stack to be slowly cooled to room temperature at a cooling rate of ≤0.5℃ / s.

[0043] In the method for manufacturing low-cost, acid-free structural steel described in this invention:

[0044] This invention employs a higher heating temperature to ensure that the billet (ingot) is fully austenitized and that the Ti and Nb second phases in the billet are fully dissolved. The dissolved Ti and Nb precipitate in the form of TiC, NbC and other second phases during subsequent rolling and laminar cooling processes to improve the strength of the steel plate.

[0045] More descaling water is used during rough rolling to fully remove primary and secondary oxide scale, while ensuring a lower rough rolling exit temperature and reducing the formation of oxide scale during billet stacking.

[0046] The use of descaling water in finishing rolling is to reduce the formation of tertiary oxide scale. The main purpose of using descaling water in the above process is to reduce the formation of oxide scale and control its thickness. When the oxide scale is too thick, the thermal stress in the oxide scale is large, which can easily lead to cracks, reduce the toughness of the oxide scale and its adhesion to the matrix, and create conditions for achieving pickling-free final steel plates.

[0047] The purpose of using a higher final rolling temperature is to utilize the plasticity of the oxide scale at high temperatures, so that the oxide scale can be fully extended and deformed during rolling without being crushed, thereby improving the uniformity, toughness and bonding force of the oxide scale with the steel matrix, and creating conditions for the final steel plate to be pickled-free.

[0048] The present invention employs a faster strip threading speed and a pre-cooling process to cool the strip steel to a lower temperature as quickly as possible, reduce post-rolling oxidation, and decrease the thickness of the oxide scale on the final steel plate surface.

[0049] The pre-cooling process after rolling, also known as the front-stage cooling process, means that the strip enters the laminar flow roller table and is cooled by water from the first valve. The system model automatically calculates and determines the laminar flow roller table length and coiling temperature target value based on the final rolling temperature, strip speed, laminar flow roller table length, and coiling temperature target value.

[0050] The corresponding process is post-cooling, where the strip enters the laminar flow roller table without water and is air-cooled. The system model automatically calculates and determines which valve to start continuously opening from the last valve based on the final rolling temperature, strip speed, laminar flow roller table length, and coiling temperature target value.

[0051] A higher winding temperature is used to ensure that the oxide scale on the strip surface remains mainly in the form of FeO during winding.

[0052] After coiling, the coil is rapidly cooled to 520–550℃ (the decomposition temperature of FeO is 570℃). Rapid cooling is used to suppress FeO decomposition during the cooling process. The relatively low rapid cooling stop temperature ensures a certain degree of supercooling for FeO decomposition, increasing the driving force for FeO decomposition. After reaching the stop cooling temperature, the coil is transported to an insulated wall or the middle of a hot coil stack to reduce the cooling rate of the coil, allowing it to remain at 520–550℃ for a sufficient time to allow FeO to fully decompose into Fe and Fe3O4. See also... Figure 1 The final oxide scale on the steel plate surface is mainly composed of two phases: Fe and Fe3O4. These two phases have the best toughness among all Fe-O compounds and the strongest bonding force with the iron matrix. The oxide scale composed of these two phases also has a good bonding force with the steel matrix, allowing the oxide scale to deform along with the steel matrix without breaking or peeling off, thus achieving the purpose of using the steel plate without pickling.

[0053] The difference between this invention and the prior art is that:

[0054] Chinese patent CN201610486480.6 discloses "A high surface quality pickling-free hot-rolled strip steel and its manufacturing method", which introduces a method to produce high surface quality pickling-free hot-rolled strip steel by controlling the Si and Mn element content in the steel and high coiling temperature, and utilizing the internal oxidation of Si and Mn after coiling to form a pure iron layer on the surface of the strip steel.

[0055] Chinese patent CN201810003351.6 discloses a method for preventing the powdery peeling of iron oxide scale from hot-rolled, pickled-free automotive frame steel. The method involves controlling the Si content (0.28wt% to 0.3wt%) in the steel, adding 0.2wt% to 0.225wt% Cr, and adding a fan during the coiling process to increase the cooling rate, thereby adjusting the composition ratio of Fe3O4 and FeO in the oxide scale to prevent the powdery peeling of iron oxide scale from the frame steel.

[0056] The two patents mentioned above control the structure of the oxide scale by adding specific amounts of Si and Mn or Cr to the steel, so as to improve the quality of the oxide scale on the surface of the steel plate.

[0057] This invention does not contain Cr, and the Si content is lower than that of the two patents mentioned above. The technical solution used to control the phase composition in the oxide scale is also different from that of the aforementioned patents.

[0058] Other Chinese patents, such as CN201510778750.6, CN201010298939.2, CN201711190855.5, CN201711190852.1 "560MPa", and CN201711190853.6, respectively describe the production methods of hot-rolled pickling-free steel plates with tensile strength grades of 400MPa, 490MPa, 510MPa, 560MPa, and 610MPa.

[0059] The aforementioned patented technologies share similar characteristics, employing a composition of C, Si, Mn+(Nb, Ti), with C content ranging from 0.04% to 0.11%, Si content from 0.05% to 0.27%, Mn content from 0.7% to 1.5%, and Ti addition below 0.03%. The heating temperature is controlled at 1210–1240℃, with a heating time of 1.2–1.8 hours, a final rolling temperature of 860–890℃, and a coiling temperature of 560–590℃. Through the combination of composition and process, hot-rolled pickled-free steel sheets of different strength grades are ultimately obtained.

[0060] Compared with the aforementioned patents, the present invention controls the Si content to below 0.05%, the Ti addition to above 0.03%, the heating time to 2 to 5 hours, the winding temperature to 600 to 650°C, and adopts post-winding cooling control, resulting in a significantly different technical solution.

[0061] Other Chinese patents, such as CN201010235928.X, CN201110307238.5, CN201210176422.5, and CN201310057003.4, all employ special processes to improve the uniformity of oxide scale on the steel plate surface. They primarily emphasize the production process method, without considering the steel plate composition design or the economic efficiency and convenience of production. Specifically, Chinese patent CN201110307238.5 requires a double descaling process, and Chinese patent CN201210176422.5 requires coating both ends of the rolled steel coil with anti-oxidation glass powder. These processes negatively impact normal production efficiency and increase production costs. Furthermore, Chinese patent CN201310057003.4 requires an insulation cover to keep the coiled steel insulated, equipment not available in conventional hot rolling mills, which also hinders the convenient production of acid-free steel.

[0062] This invention takes into account both the economic efficiency of the composition design and the convenience of the steel plate production process, which is significantly different from the aforementioned patents.

[0063] Compared with existing patents, the pickling-free structural steel of this invention adopts a low-cost composition design. By controlling the rolling process and post-coiling cooling process, the phase composition and morphology of the iron oxide scale on the steel plate surface are regulated to improve the toughness of the iron oxide scale and its bonding force with the steel matrix, thereby achieving the purpose of pickling-free operation. This is something that other known patents do not possess, as detailed below:

[0064] 1. The present invention adds a small variety of microalloying elements in low amounts, with only small amounts of Nb and Mo added, and no expensive alloying elements such as V, Ni, and Cr added, which greatly reduces production costs.

[0065] 2. This invention fully utilizes the precipitation strengthening of Ti and Mo, resulting in a steel plate with a yield strength ≥600MPa, tensile strength ≥700MPa, elongation ≥14%, and excellent cold bending performance; it can be bent 180° without cracking when D=2a.

[0066] 3. The steel grade of this invention has a uniform, dense oxide scale on its surface with good anti-peeling properties, which can reduce peeling and dust generation during processing and use, and improve the user's working environment.

[0067] 4. The steel grade of this invention has a simple production process, a wide process window, and a simple manufacturing process, and can be directly produced on existing hot continuous casting production lines. Attached Figure Description

[0068] Figure 1 This is the Fe-O phase diagram.

[0069] Figure 2 This is a photo showing the poor resistance of oxide scale to bending and peeling.

[0070] Figure 3 The photo shows the average resistance of the oxide scale to bending and peeling.

[0071] Figure 4 Photograph showing good resistance to bending and peeling of oxide scale. Detailed Implementation

[0072] The present invention will be further described below with reference to the embodiments.

[0073] The chemical composition of the steel plates of the present invention in the embodiments and comparative examples is shown in Table 1, with the balance including Fe and unavoidable impurities. Components numbered I to IX are within the composition range designed in this invention, while components numbered X and XI are not within the composition range designed in this invention.

[0074] The steel production process of this invention is as follows: hot metal is deeply desulfurized and then smelted in a converter, refined in a ladle, continuously cast into slabs, hot-rolled, and cooled; specific parameters are shown in Table 2, wherein...

[0075] 1. In all embodiments and comparative examples, during steel production, two sets of descaling water are fully open before rough rolling, three sets of descaling water are open before rough rolling, two sets of descaling water are fully open before finishing rolling, and the descaling water between F1 and F2 stands is fully open.

[0076] 2. Post-coiling cooling adopts "rapid cooling + slow cooling", which means that after the steel coil is unloaded, it is rapidly cooled (≥2℃ / s) to 520~550℃ by a fan, and then the steel coil is sent to an insulated wall or hot coil stack for slow cooling to room temperature; "natural cooling" means that after the steel coil is unloaded, it is transported to the steel coil warehouse for natural cooling.

[0077] As can be seen from Tables 1 to 3, the steel plate composition and process of Examples 1 to 13 of the present invention are within the scope of the present invention, the steel plate performance meets the requirements, and the surface quality of the steel plate is also "good", achieving the surface quality requirements of pickling-free steel.

[0078] In Comparative Examples 1 and 2, the steel plate composition was within the design range of this invention. However, the coiling temperature in Comparative Example 1 was lower than the design range of this invention, resulting in the final tensile strength of the steel plate not reaching 700 MPa, and the surface quality grade of the steel plate being "general". In Comparative Example 2, the rolling process was within the design range of this invention, but the cooling rate after coiling was not controlled. Although the performance of the steel plate met the requirements, the surface quality was "poor".

[0079] In Comparative Examples 3 and 4, the steel plate composition was not within the scope of the present invention, and the process of Comparative Example 3 was also different from the process of the present invention. The steel plate performance met the requirements, but the surface quality of the steel plate was "poor".

[0080] Although the process of Comparative Example 4 falls within the process range designed in this invention and the performance of the steel plate meets the requirements, the composition does not fall within the composition range designed in this invention, and the surface quality of the steel plate is "average" and does not meet the surface quality requirements of pickled steel.

[0081] Table 1 Units, wt%

[0082]

[0083]

[0084] Table 2

[0085]

[0086]

[0087] Table 3

[0088]

[0089]

[0090] Note: Whether a steel plate can be used without pickling can be evaluated by the resistance of the oxide scale to bending and peeling. The resistance of the oxide scale to bending and peeling is measured by examining the degree of peeling of the oxide scale on the outer surface of the steel plate after bending. The specific procedure is as follows:

[0091] The steel plate with oxide scale is bent at 90° with d=2a. After bending, transparent tape is applied to the outer R-corner and then peeled off and transferred to white paper to observe the extent of oxide scale removal.

[0092] See Figures 2-4 Based on the degree of oxide scale peeling, the bending and peeling resistance of oxide scale is divided into three levels: "good", "average", and "poor". Steel plates with "good" bending and peeling resistance can be used without pickling, while steel plates with "average" or "poor" resistance are recommended to be pickled or shot blasted before use.

Claims

1. A 700MPa grade low-cost, acid-free structural steel, with the following chemical composition by mass percentage: C: 0.04~0.16%, Si≤0.048%, Mn: 0.4~1.3%, P≤0.02%, S≤0.01%, Ti: 0.03~0.16%, Nb: 0.006~0.035%, Mo: 0.06~0.3%, with the balance being Fe and other unavoidable impurities. Furthermore, it must simultaneously satisfy the following: 0.251Ti+1.12Nb+0.115Mo≥0.05; C + Mn / 6 ≤ 0.25; The oxide scale of the acid-free structural steel exhibits good resistance to bending and peeling; and it is manufactured using the following method, including: 1) Smelting and casting The components are smelted, refined, and continuously cast into billets or ingots according to the stated composition. 2) Heating of billets or ingots The billet or ingot is heated to 1250~1300℃ for 2~5 hours. 3) Hot rolling Before rough rolling, descaling is performed to remove the furnace-grown oxide scale, i.e., the primary oxide scale. Descaling is introduced during the rough rolling pass, and the exit temperature of the rough rolling mill is ≤1010℃. Then, the product is rolled into finished steel coils through a multi-stand finishing mill. Before finishing rolling, descaling is performed to remove the secondary oxide scale. Descaling water is introduced between the F1 and F2 stands of the finishing mill. The final rolling temperature is 870~920℃, and the threading speed is ≥7m / s. After rolling, a pre-cooling process is used for cooling, and the coiling temperature is 600~650℃. 4) Cooling After the steel coil is unloaded, it is cooled by a fan at a cooling rate of ≥2℃ / s to 520~550℃. Then, the steel coil is sent to an insulated wall or hot coil stack to be slowly cooled to room temperature at a cooling rate of ≤0.5℃ / s.

2. The 700MPa grade low-cost acid-free structural steel as described in claim 1, characterized in that, It also contains one or more of the following: Cr≤0.2%, V≤0.15%, Ni≤0.1%, and Ca≤0.015%.

3. The 700MPa grade low-cost acid-free structural steel as described in claim 1 or 2, characterized in that, The microstructure of the structural steel is polygonal ferrite and / or quasi-polygonal ferrite plus a small amount of pearlite.

4. The 700MPa grade low-cost acid-free structural steel as described in claim 1 or 2, characterized in that, The structural steel has a yield strength ≥600MPa, tensile strength ≥700MPa, elongation ≥14%, and cold bending performance D=2a for bending 180°.

5. The 700MPa grade low-cost acid-free structural steel as described in claim 3, characterized in that, The structural steel has a yield strength ≥600MPa, tensile strength ≥700MPa, elongation ≥14%, and cold bending performance D=2a for bending 180°.

6. The method for manufacturing 700MPa grade low-cost acid-free structural steel as described in any one of claims 1 to 5, characterized in that, Includes the following steps: 1) Smelting and casting The components are smelted, refined, and continuously cast into billets or ingots according to the stated composition. 2) Heating of billets or ingots The billet or ingot is heated to 1250~1300℃ for 2~5 hours. 3) Hot rolling Before rough rolling, descaling is performed to remove the furnace-grown oxide scale, i.e., the primary oxide scale. Descaling is introduced during the rough rolling pass, and the exit temperature of the rough rolling mill is ≤1010℃. Then, the product is rolled into finished steel coils through a multi-stand finishing mill. Before finishing rolling, descaling is performed to remove the secondary oxide scale. Descaling water is introduced between the F1 and F2 stands of the finishing mill. The final rolling temperature is 870~920℃, and the threading speed is ≥7m / s. After rolling, a pre-cooling process is used for cooling, and the coiling temperature is 600~650℃. 4) Cooling After the steel coil is unloaded, it is cooled by a fan at a cooling rate of ≥2℃ / s to 520~550℃. Then, the steel coil is sent to an insulated wall or hot coil stack to be slowly cooled to room temperature at a cooling rate of ≤0.5℃ / s.

7. The method for manufacturing 700MPa grade low-cost acid-free structural steel as described in claim 6, characterized in that, Step 3) Hot rolling finish rolling is performed using six or seven stands to roll finished steel coils.

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