Method for controlling bainite structure components of a steel

Abstract

The application discloses a method for controlling the bainite structure component of steel, which comprises the following steps: S1, controlling the parameters of the hot rolling process of the steel billet, including reducing the opening rolling temperature by 50-100 DEG C, reducing the rolling speed by 3-10%, and water cooling between the rolling mill racks to form a low final rolling temperature; and S2, after the final rolling is completed, forcibly cooling the rolled piece at a cooling speed of 50 DEG C / s-300 DEG C / s, so that the temperature of the rolled piece rapidly enters the two-phase region or even below the Ac1 line, thereby slowing down or inhibiting the dynamic recrystallization process of the deformed austenite, and making the cooling curve "chase" the continuously right-moving pro-eutectoid ferrite line. By adopting the technical scheme of the application, a significant amount of ferrite component can be obtained for the bainite steel under the hot rolling condition, that is, a ferrite+bainite complex phase structure with different proportions is formed, so that the strength and the toughness and plasticity of the steel can be regulated in a large range.

Description

Technical Field

[0001] The invention relates to the field of hot-rolled steel microstructure control technology, and particularly to a method for controlling the bainitic microstructure composition of steel. Background Technology

[0002] To ensure the various physicochemical and mechanical properties of steel, it is often necessary to add different types of alloying elements. For example, to improve the corrosion resistance of steel, a certain amount of elements such as Cu, Cr, P, Ni, and Mo are often added. Similarly, for welding wire used in welding steel materials, to ensure the chemical and / or mechanical properties of the weld metal, elements such as Mn, Si, Mo, Ti, and Cr are often added to the welding wire steel. Alloying elements have a significant impact on the supercooled austenite transformation of steel. When steel contains a certain amount of elements such as Mo, Cr, and Cu, the microstructure formed after air cooling or hot rolling often contains partial bainite, or even forms a fully bainitic microstructure. Steel with a predominantly bainitic microstructure in the air-cooled or rolled state is generally called bainitic steel.

[0003] Compared to ferrite-pearlite microstructures, bainitic steels exhibit higher strength but generally lower toughness or plasticity. To improve toughness or plasticity, it is sometimes desirable to eliminate bainite or form a ferrite-bainite dual-phase microstructure in the steel. The conventional method is slow cooling of the steel, controlling the cooling rate below the critical cooling rate for ferrite formation, thereby ensuring a certain proportion of ferrite in the room-temperature microstructure. However, in actual industrial production processes, such as hot rolling, suitable slow cooling conditions are sometimes unavailable—either the cooling rate cannot be low enough, or the temperature is already very low when slow cooling begins, causing the bainitic transformation to occur. Therefore, slow cooling is not always effective in ensuring a certain proportion of ferrite in the room-temperature microstructure to improve toughness and plasticity.

[0004] It is well known that subjecting high-temperature austenite to deformation can lead to the premature formation of proeutectoid ferrite, a phenomenon known as deformation-induced ferrite (DIF). The temperature at which the deformation-induced ferrite phase transformation occurs can be in a single austenite region or in a two-phase region. For ordinary ferrite-pearlite hypoeutectoid steels, the essence of deformation-induced ferrite formation is an increase in the proeutectoid ferrite formation temperature. In the terminology of the steel's C-curve, this means that the proeutectoid ferrite line rises due to austenite deformation (e.g., ...). Figure 1 (As shown); Furthermore, due to the simultaneous occurrence of dynamic recovery and dynamic recrystallization processes, the previously rising proeutectoid ferrite line will subsequently decline.

[0005] For bainitic steel, a more accurate description of the effect of deformation on the C-curve is that austenitic deformation causes the proeutectoid ferrite line to shift to the upper left (e.g., ...). Figure 2(As shown); Furthermore, due to the simultaneous occurrence of dynamic recovery and dynamic recrystallization processes, the previously upper-left proeutectoid ferrite line will subsequently move to the lower-right direction and coincide with the original proeutectoid ferrite line after complete recrystallization.

[0006] During hot rolling, the temperature of the steel during deformation (e.g., 900~1150℃) is often higher than the deformation-induced ferrite formation temperature. Furthermore, the rolling deformation temperature is generally much higher than the austenite recrystallization temperature, causing dynamic recovery and dynamic recrystallization of austenite to occur simultaneously. This negates the thermodynamic and kinetic conditions for deformation-induced ferrite formation. Therefore, deformation-induced ferrite phase transformation is usually difficult to occur during ordinary hot rolling. For bainitic steels, the possibility of deformation-induced ferrite phase transformation during hot rolling is even smaller because the proeutectoid ferrite transformation is significantly delayed.

[0007] Therefore, the applicant proposes a method for controlling the bainitic microstructure composition of steel to solve the above-mentioned problems. Summary of the Invention

[0008] The purpose of this invention is to provide a method for controlling the bainitic microstructure composition of steel. Based on the theory of deformation-induced ferrite phase transformation and dynamic recrystallization coupling effect proposed by the applicant, bainitic steel can obtain a significant amount of ferrite composition under hot rolling conditions, that is, form a ferrite + bainite multiphase microstructure with different proportions, thereby enabling the strength and toughness of steel to be controlled over a wide range.

[0009] To achieve the above objectives, the solution of the present invention is: a method for controlling the bainitic microstructure composition of steel, specifically:

[0010] S1: Parameters for controlling the hot rolling process of steel billets, including reducing the initial rolling temperature by 50~100℃, reducing the rolling rate by 3~10%, and using water cooling between the mill stands to achieve a low final rolling temperature.

[0011] S2: After the final rolling is completed, the rolled piece is forced to cool at a cooling rate of 50℃ / s to 300℃ / s, so that the temperature of the rolled piece quickly enters the two-phase region or even below the Ac1 line, in order to slow down or suppress the dynamic recrystallization process of deformed austenite and make the cooling curve "catch up" with the proeutectoid ferrite line that is constantly shifting to the right.

[0012] Furthermore, the deformation amount is allocated according to the maximum rolling capacity in the final rolling stand or pass.

[0013] Furthermore, in terms of steel alloying, microalloying elements that strongly inhibit the dynamic recrystallization of austenite, such as Nb, V, and Al, can be added to the steel. In this case, the role of these microalloying elements is to "soften" the steel rather than to "strengthen" it as usual.

[0014] Furthermore, the steel is a corrosion-resistant steel bar. The method for controlling the rolled microstructure of the corrosion-resistant steel bar is as follows: reduce the roughing rolling temperature and rolling rate, and perform intense water cooling after rolling to significantly reduce the temperature of the upper cooling bed. Specifically, the rolling rate is controlled at 25m / s-30m / s, the roughing rolling temperature is 950℃~1000℃, the pre-finishing rolling temperature is 900℃~960℃, the finishing rolling temperature is 860℃~900℃, and the rolled piece is forcibly cooled at a cooling rate of 30~150℃ / s to reduce the temperature of the upper cooling bed to 650℃~850℃.

[0015] Furthermore, the steel is an alloy welding wire rod, and the rolling microstructure control method of the alloy welding wire rod is as follows: reduce the roughing rolling temperature, specifically: roughing rolling temperature 980℃, finishing rolling temperature 860℃, and sizing temperature 750℃. In addition, water forced cooling is performed before finishing rolling, sizing, and wire drawing to reduce the wire drawing temperature to 720℃, thereby inhibiting the dynamic recrystallization of deformed austenite.

[0016] Furthermore, the steel is 800MPa grade hot-rolled TRIP steel with the following composition: C: 0.12~0.18%, Si: 1.00~1.50%, Mn: 1.20~1.70%, Mo: 0.3~0.4%, Nb: 0.03~0.04%. The rolling microstructure control process of the 800MPa grade hot-rolled TRIP steel is as follows: soaking zone temperature 1220℃, roughing rolling start temperature 1150℃, finishing rolling finish temperature 850℃, exiting ultra-fast cooling zone temperature 680℃, exiting laminar flow cooling zone temperature 500℃, and coiling temperature 480℃.

[0017] Furthermore, the steel is a 1000MPa grade hot-rolled TRIP steel with the following composition: C: 0.17~0.23%, Si: 1.00~1.50%, Mn: 1.20~1.70%, Cr: 0.45~0.55%, Mo: 0.3~0.4%, Nb: 0.03~0.04%, V: 0.045~0.055%. The rolling microstructure control process for the 1000MPa grade hot-rolled TRIP steel is as follows: soaking zone temperature 1220℃, roughing rolling start temperature 1150℃, finishing rolling finish temperature 850℃, exiting the ultra-fast cooling zone temperature 680℃, exiting the laminar flow cooling zone temperature 550℃, and coiling temperature 520℃.

[0018] After adopting the above solution, the beneficial effects of the present invention are as follows:

[0019] During the hot rolling process of steel, when deformation is completed, the dynamic recovery and dynamic recrystallization of austenite do not necessarily occur simultaneously. Therefore, the thermodynamic and kinetic conditions for deformation-induced ferrite phase transformation are not completely eliminated. This invention utilizes these characteristics to propose a theory of the coupling effect between deformation-induced ferrite phase transformation and dynamic recrystallization. It employs low-temperature hot rolling, i.e., reducing the initial rolling temperature, rolling rate, or water cooling methods in conventional hot rolling processes to achieve a low final rolling temperature. After final rolling, the rolled piece is immediately subjected to forced cooling, causing the steel temperature to rapidly enter the two-phase region or even below the Ac1 line. This slows down or inhibits the dynamic recrystallization process of deformed austenite and causes the cooling curve to "catch up" with the continuously right-shifting proeutectoid ferrite line, thus inducing deformation-induced ferrite phase transformation.

[0020] like Figure 3 As shown, when the rolled piece is cooled at different rates after deformation, the combined effect of deformation-induced ferrite phase transformation and dynamic recrystallization results in different proportions of deformation-induced ferrite being formed at different cooling rates, as detailed below:

[0021] First, after hot rolling deformation, the proeutectoid ferrite line F moves to the upper left due to the deformation-induced ferrite phase transformation, forming the DIF line; subsequently, due to dynamic recovery and recrystallization, the DIF line will continuously move to the lower right (represented by DIF1 line and DIF2 line), and after complete recrystallization, it will coincide with the original F line.

[0022] After the workpiece is rolled and deformed, it is naturally cooled in the air, i.e., the case of cooling curve ①. Due to the slow cooling, the austenite is completely recrystallized. Cooling curve ① does not intersect with line F, deformation-induced ferrite phase transformation fails to occur, and finally a full bainite structure is obtained.

[0023] If the rolled piece is rapidly cooled to a certain temperature and then air-cooled after rolling deformation, i.e., the case of cooling curve ②, the rapid cooling inhibits the recrystallization of deformed austenite. Cooling curve ② intersects with the DIF2 line, thus generating a certain proportion of deformation-induced ferrite.

[0024] Furthermore, if the workpiece is cooled to a lower temperature at a faster cooling rate (50℃ / s~300℃ / s) after rolling deformation and then air-cooled, i.e., the case of cooling curve ③, the recrystallization of deformed austenite is further suppressed, and cooling curve ③ intersects with the DIF1 line further to the left, resulting in a larger proportion of deformation-induced ferrite formation.

[0025] For bainitic steel, its microstructure under hot rolling conditions is generally dominated by bainite. The technical solution of this invention allows bainitic steel to obtain a significantly larger amount of ferrite composition under hot rolling conditions, i.e., forming a multiphase structure of ferrite + bainite with different proportions, thereby enabling a wider range of control over the steel's strength and toughness. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the dynamic CCT curve of ferrite-pearlite hypoeutectoid steel, showing that hot rolling deformation causes the proeutectoid ferrite line to rise and form the DIF line;

[0027] Figure 2 This is a schematic diagram of the dynamic CCT curve of bainitic steel, showing that hot rolling deformation causes the proeutectoid ferrite line to shift to the upper left to form the DIF line;

[0028] Figure 3 This is a schematic diagram of the dynamic CCT curve of bainitic steel, showing that when cooled at different rates after deformation, the deformation-induced ferrite phase transformation and dynamic recrystallization work together to form different proportions of deformation-induced ferrite at different cooling rates.

[0029] Figure 4 This is a metallographic diagram of the corrosion-resistant steel of the present invention, which forms 100% granular bainite during slow cooling;

[0030] Figure 5 This invention relates to a corrosion-resistant steel that forms a multiphase structure of ferrite (approximately 35%) and granular bainite during rapid cooling.

[0031] Figure 6 The present invention describes a corrosion-resistant steel that forms a multiphase structure of ferrite (approximately 65%) and granular bainite when cooled at a faster rate.

[0032] Figure 7 The room temperature microstructure of an 800MPa grade TRIP steel according to the present invention is: ferrite (50%) + bainite / retained austenite (50%).

[0033] Figure 8 The present invention describes the room temperature microstructure of a 1000MPa grade TRIP steel: ferrite (25%) + bainite / retained austenite (75%). Detailed Implementation

[0034] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0035] This invention provides a method for controlling the bainitic microstructure composition of steel, specifically:

[0036] S1: Parameters for controlling the hot rolling process of steel billets, including reducing the initial rolling temperature by 50~100℃, reducing the rolling rate by 3~10%, and using water cooling between the mill stands to achieve a low final rolling temperature.

[0037] S2: After the final rolling is completed, the rolled piece is forced to cool at a cooling rate of 50℃ / s to 300℃ / s, so that the temperature of the rolled piece quickly enters the two-phase region or even below the Ac1 line, in order to slow down or suppress the dynamic recrystallization process of deformed austenite and make the cooling curve "catch up" with the proeutectoid ferrite line that is constantly shifting to the right, thereby causing deformation-induced ferrite phase transformation.

[0038] Furthermore, this invention proposes to allocate the deformation amount according to the maximum rolling capacity in the final mill stand or pass, that is, to provide the workpiece with the deformation amount according to the maximum rolling force that the mill can withstand, so as to suppress the dynamic recovery and dynamic recrystallization of deformed austenite.

[0039] Furthermore, this invention proposes that, in order to suppress the dynamic recovery and recrystallization of deformed austenite, microalloying elements that strongly inhibit the dynamic recrystallization of austenite, including Nb, V, and Al, can be added to the steel. While microalloying elements such as Nb, V, and Al are typically considered "strengthening" elements, this invention breaks with conventional understanding and overcomes technical bias, using them as "softening" elements to achieve the effect of reducing the strength of bainitic steel while improving its toughness and plasticity.

[0040] The following section provides a detailed explanation of the methods for controlling the rolled microstructure of several types of steel.

[0041] Example 1: Rolled microstructure control of corrosion-resistant steel bars.

[0042] To ensure corrosion resistance, this steel incorporates a significant amount of elements such as Cu, Ni, Cr, and Mo, which are added during the hot rolling process into bars (e.g., ...). After hot rolling (14mm thick), conventional air cooling resulted in a fully bainitic microstructure at room temperature, leading to excessively high strength and insufficient plasticity in the bars, failing to meet the product's mechanical property requirements. An attempt was made to address this issue using a slow cooling approach: implementing slow cooling (avoiding water cooling) during hot rolling and maximizing the collection temperature of the hot-rolled bars before immediate stack cooling. However, due to the effects of alloying elements, the slow cooling measures available under the on-site process conditions did not significantly reduce the steel's strength or improve its plasticity. Metallographic examination showed that the slow-cooled microstructure remained fully bainitic.

[0043] After adopting the control method of the present invention, the microstructure of the steel changed, as shown in Tables 1 and 2:

[0044] Table 1. Different hot rolling processes for a corrosion-resistant steel bar.

[0045]

[0046] Table 2. Microstructure and mechanical properties of a corrosion-resistant steel bar under different hot rolling processes.

[0047]

[0048] In Tables 1 and 2, Process 1 is a slow cooling process with a relatively high roughing rolling temperature. No water cooling is performed after rolling, and the upper cooling bed temperature is high, reaching 880℃. The cooling process cannot keep up with the DIF line, which is constantly shifting to the right due to dynamic recrystallization (see...). Figure 3 The cooling curve ① does not intersect with line F, indicating that the room temperature microstructure of this bar is a fully bainitic structure (e.g., Figure 4 As shown, it has high strength but low plasticity.

[0049] In Process 2, the roughing rolling temperature is slightly lower than that in Process 1, the rolling speed is reduced to some extent, and after rolling, water cooling with a certain intensity is performed at a rate of 30~150℃ / s to obtain a lower upper cooling bed temperature. That is, the cooling process, to some extent, catches up with the DIF line that is constantly shifting to the right due to dynamic recrystallization (see...). Figure 3 The cooling curve ② intersects with DIF2, and the room temperature microstructure of this bar shows approximately 35% ferrite (e.g., Figure 5 As shown in the figure, the strength decreases while the plasticity increases.

[0050] Compared to process 2, process 3 further reduces the rolling speed and roughing rolling temperature, and performs intense water cooling at a rate of 50~300℃ / s after rolling, resulting in a significant reduction in the upper cooling bed temperature. This means the cooling process further catches up with the DIF line, which is continuously shifting to the right due to dynamic recrystallization (see...). Figure 3 The cooling curve ③ intersects with DIF1, and the room temperature microstructure of the bar material obtained about 60% ferrite. Correspondingly, the strength further decreased while the plasticity continued to increase.

[0051] Example 2: Rolled microstructure control of high-speed alloy welding wire.

[0052] The steel grade is ER69-1, containing alloying elements such as Mn (approximately 1.25%), Si (approximately 0.5%), Mo (approximately 0.4%), and Ni (approximately 1.4%). The steel is hot-rolled into wire rod coils on a high-speed wire rod mill. After being drawn to 5.5mm and subjected to conventional air cooling, the resulting room temperature microstructure is fully granular bainite, leading to high strength but insufficient ductility. Welding wire manufacturers typically require intermediate annealing during drawing, and wire breakage is common during multi-pass drawing. Attempts to reduce strength and improve ductility using slow cooling have been made, but the effect is not significant, and the microstructure remains predominantly granular bainite.

[0053] After adopting the control method of the present invention, the microstructure of the steel changed, as shown in Tables 3 and 4:

[0054] Table 3 Different hot rolling processes for an alloy welding wire.

[0055]

[0056] Table 4. Microstructure and mechanical properties of an alloy welding wire under different hot rolling process conditions

[0057]

[0058] In Tables 3 and 4, process 1 is a conventional process. Normal water quenching is performed before entering the reducing and sizing mill and before spinning. Normal cooling is performed on the Stellmore air-cooled line with a cover to obtain a fully granular bainite room temperature structure.

[0059] Process 2 is a traditional slow cooling process. It controls the water cooling intensity before entering the reducing and sizing mill and before spinning to increase the spinning temperature. It also controls the running speed of the Stellmore air-cooling line to extend the cooling time in the cover. As a result, a small amount of ferrite appears in the room temperature microstructure, and the strength is slightly reduced while the plasticity is slightly increased.

[0060] Process 3 is the rapid cooling process of this invention. Its roughing rolling start temperature is relatively low, and forced water cooling is performed before finishing rolling, sizing, and wire drawing, resulting in a significant drop in wire drawing temperature. This suppresses the dynamic recrystallization of deformed austenite, allowing the cooling process to catch up with the rightward-shifting DIF line (see...). Figure 3 The Stellmore air-cooled line, on the other hand, is cooled by a cover at normal running speed. Its room temperature microstructure is a ferrite (about 60%) + bainite multiphase microstructure. Correspondingly, its strength is more than 100 MPa lower than that of the conventional process in process 1, while its elongation and reduction of area are significantly increased. This creates favorable conditions for anneal-free drawing or reducing the wire breakage rate during drawing.

[0061] Example 3: Rolled microstructure control of a high-strength hot-rolled TRIP steel.

[0062] TRIP steel, or transformation-induced plasticity steel, is typically composed of C-Mn steel with the addition of elements such as Si to inhibit carbide precipitation. By controlling cooling and allowing partial ferrite precipitation, the untransformed austenite becomes carbon-rich and further stabilized. Subsequently, through incomplete bainitic transformation, a multiphase microstructure of ferrite, bainite, and retained austenite is formed. Due to the transformation-induced plasticity effect of the retained austenite, TRIP steel generally possesses a much higher strength-ductility product than ordinary steel, allowing for the stamping of complex, high-strength components. It also exhibits high impact absorption performance, making it frequently used in the manufacture of automotive safety parts.

[0063] Early TRIP steels had lower strength; for example, 600 MPa (tensile strength) grade TRIP steel typically had a composition of 0.15% C, 1.3% Si, and 1.5% Mn. It could be produced through cold rolling and continuous annealing or through hot rolling of strip steel. For 600 MPa grade hot-rolled TRIP steel, its typical room temperature microstructure is 50% ferrite + 40% bainite + 10% retained austenite; typical properties are: yield strength 465 MPa, tensile strength 680 MPa, and elongation 28%.

[0064] As the requirements for lightweighting in automobiles continue to increase, the strength levels of automotive steel sheets are also gradually improving. Therefore, higher-strength TRIP steels (such as 800MPa or even 1000MPa) are gaining favor in the automotive industry. While increasing the carbon content can certainly improve strength, this leads to a decrease in weldability. Therefore, it is generally necessary to supplement this with the addition of other alloying elements, such as increasing the manganese content and adding elements like Cr and V.

[0065] Mo, a highly effective strengthening element, is often excluded from high-strength hot-rolled TRIP steels because it is a bainite-forming element. This reduces ferrite precipitation, consequently decreasing the amount of retained austenite and thus affecting the TRIP effect. In fact, for TRIP steel produced by hot continuous rolling of strip steel, the time required for proeutectoid ferrite formation at higher temperatures is already insufficient. Adding Mo to the steel significantly delays the proeutectoid ferrite line to the right on the CCT curve, making ferrite formation even more difficult.

[0066] However, according to the theory of the coupling effect of deformation-induced ferrite phase transformation and dynamic recrystallization proposed by the present invention, it is feasible to add the strengthening element Mo to the steel. It is also preferable to add the microalloying element Nb to suppress the dynamic recrystallization of deformation austenite, and to immediately perform rapid cooling after hot rolling deformation to further suppress the dynamic recrystallization of deformation austenite, so that the cooling process can catch up with the DIF line that continuously shifts to the right due to dynamic recrystallization (see...). Figure 3 This process induces a certain amount of ferrite through deformation-induced ferrite phase transformation, ultimately forming a multiphase structure of ferrite + bainite + retained austenite in appropriate proportions.

[0067] The following lists the composition, production process, and microstructure properties of two types of high-strength TRIP steel:

[0068] 1. 800MPa grade hot-rolled TRIP steel:

[0069] Typical composition: C: 0.15%, Si: 1.30%, Mn: 1.50%, Mo: 0.35%, Nb: 0.035%.

[0070] Hot rolling process for strip steel: soaking zone temperature 1220℃, roughing rolling start temperature 1150℃, finishing rolling finish temperature 850℃, exiting the ultra-fast cooling zone temperature 680℃, exiting the laminar flow cooling zone temperature 500℃, coiling temperature 480℃.

[0071] Microstructure and properties: 50% ferrite, 50% bainite + retained austenite (typical room temperature microstructure as shown in the image). Figure 7 As shown), the yield strength is 604 MPa, the tensile strength is 825 MPa, and the elongation is 23%.

[0072] 2. 1000MPa grade hot-rolled TRIP steel:

[0073] Typical composition: C: 0.20%, Si: 1.30%, Mn: 1.50%, Cr: 0.50%, Mo: 0.35%, Nb: 0.035%, V: 0.05%.

[0074] Hot rolling process for strip steel: soaking zone temperature 1220℃, roughing rolling start temperature 1150℃, finishing rolling finish temperature 850℃, exiting ultra-fast cooling zone temperature 680℃, exiting laminar flow cooling zone temperature 550℃, coiling temperature 520℃.

[0075] Microstructure and properties: 25% ferrite, 75% bainite + retained austenite (typical room temperature microstructure as shown in the figure). Figure 8 As shown), the yield strength is 825 MPa, the tensile strength is 1080 MPa, and the elongation is 17%.

[0076] Tables 5 and 6 list the temperature control parameters for the hot rolling process of the two types of hot-rolled TRIP steels, as well as the resulting rolled microstructure and mechanical properties.

[0077] Table 5 Hot rolling process of TRIP steel with different strength grades

[0078]

[0079] Table 6. Microstructure and mechanical properties of TRIP steels with different strength grades

[0080]

[0081] The above description is only a preferred embodiment of the present invention and is not intended to limit the design of this case. All equivalent changes made based on the key design features of this case shall fall within the protection scope of this case.

Claims

1. A method for controlling the bainitic microstructure composition of steel, characterized in that: Specifically: S1: Parameters for controlling the hot rolling process of steel billets, including reducing the initial rolling temperature by 50~100℃, reducing the rolling rate by 3~10%, and using water cooling between the mill stands to achieve a low final rolling temperature. S2: After the final rolling is completed, the rolled piece is forced to cool at a cooling rate of 50℃ / s to 300℃ / s, so that the temperature of the rolled piece quickly enters below the Ac1 line. After entering below the Ac1 line, the cooling rate is maintained to continue cooling, so as to slow down or suppress the dynamic recrystallization process of deformed austenite and make the cooling curve catch up with the proeutectoid ferrite line that is constantly shifting to the right. The steel is a corrosion-resistant steel bar. The method for controlling the rolled microstructure of the corrosion-resistant steel bar is as follows: reduce the roughing rolling temperature and rolling rate, and perform strong water cooling after rolling to significantly reduce the temperature of the upper cooling bed. Specifically, the rolling rate is controlled at 25 m / s to 27 m / s, the roughing rolling temperature is 950℃ to 960℃, the pre-finishing rolling temperature is 900℃ to 920℃, the finishing rolling temperature is 860℃ to 880℃, and the rolled piece is forcibly cooled at a cooling rate of 50℃ / s to 300℃ / s to reduce the temperature of the upper cooling bed to 650℃ to 670℃.

2. The method for controlling the bainitic microstructure composition of steel as described in claim 1, characterized in that: The deformation is allocated according to the maximum rolling capacity in the final mill stand or pass.

3. The method for controlling the bainitic microstructure composition of steel as described in claim 1, characterized in that: Adding microalloying elements, including Nb, V, and Al, to steel strongly inhibits the dynamic recrystallization of austenite. In other words, microalloying elements are used to soften rather than strengthen the steel.

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