A process for manufacturing compact coils of ultra-fine grained martensite-free steel rods

Abstract

A process for manufacturing compact coils of ultra-fine grained martensite-free steel bars, the process comprising the following phases: a) rolling a billet by means of a roughing mill (1) of the steel bar; b) performing at least one first cooling stage (2) so that the steel bar has a surface temperature higher than the martensite start temperature (Ms) and at least one first equalization stage in air; c) rolling the steel bar by means of at least one intermediate mill (3); d) performing at least one second cooling stage (4) always keeping the surface temperature higher than the martensite start temperature (Ms) and at least one second equalization stage in air; e) rolling the steel bar by means of a finishing mill (5) in the non-recrystallization temperature range, keeping the entire cross section of the steel bar in said non-recrystallization temperature range and wherein the total reduction is between 25% and 50% with respect to the cross section of the steel bar at the inlet of the finishing mill, so as to obtain an ultra-fine grained austenitic matrix; f) winding the steel bar into compact coils by means of at least one winding device (7) so that the ultra-fine grained austenitic matrix transforms into a mixture of ferrite and pearlite. After the winding operation is completed, the compact coils can be conveyed to a storage area by means of a conveying device (8), for example a walking beam, in which the coils are subjected to natural cooling or forced cooling or delayed cooling.

Description

Invention Field

[0001] This invention relates to a process for manufacturing compact coils of ultrafine-grained martensitic-free steel bars.

[0002] background

[0003] Compared to straight bar production, the process of winding ribbed steel bars into compact coils represents a significant advancement in terms of storage, transportation, and handling.

[0004] The winding process can be applied to ribbed steel bars with diameters ranging from 6 mm to 40 mm, for example from 6 mm to 32 mm, as well as smooth round steel bars.

[0005] The compact roll is unwound by means of a machine that can straighten ribbed bars at room temperature and make steel stirrups.

[0006] The winding process is performed as shown in Figures 1 and 2, in which cooling is applied to the product from the final finishing rolling step.

[0007] This cooling can be applied in two different solutions.

[0008] The most common solution (Figures 1 and 8) anticipates a single cooling step with a high cooling rate after the final finishing pass to achieve quenching in the surface region of the bar. This process is often referred to as QTS (Quenching and Tempering in a Winding Machine) and represents the typical solution for medium to high tensile grades (e.g., British Standard grades B500B, B500C; American Standard grades Gr60, Gr80, Gr100).

[0009] Because the high cooling rate inhibits the diffusion transformation of austenite, surface quenching results in a mixed martensite-bainite structure.

[0010] Following the quenching step, the bar undergoes an equalization process in air, during which the heat from the core diffuses toward the surface region, tempering the martensitic-bainitic structure. The bar is then wound into a winding machine to form a compact coil.

[0011] The final product of this process solution is a compromise between mechanical strength (which is mainly contributed by the surface area) and toughness (which is mainly contributed by the mixed ferrite and pearlite in the core).

[0012] Composite structures obtained through quenching and self-tempering allow for mechanical properties established by various national and international standards for reinforcing bars, where the chemical composition of the billet is inferior to that typically used in the production of ribbed bars without quenching.

[0013] On another front, the high hardness of the surface area during unwinding, compared to unhardened materials, leads to greater wear on the straightening equipment and thus a rougher straightening operation.

[0014] Another common process (Figure 2) is called soft hardening (SQ) and represents an ideal solution for high ductility weldable grades (such as European grades like B450C, B500B, and B500C).

[0015] The goal of the SQ process is to optimize the microstructure of different steel grades, avoiding unwanted particle growth and rapid cooling, which would result in a thick martensite layer.

[0016] In the SQ process, material cooling is divided into multiple steps after the last finishing mill stand and before the winding mill. These cooling steps are separated by an equalization space.

[0017] The final microstructure obtained by the SQ process differs from that of the conventional QTS process. For soft quenching, compared to QTS, the presence of a mixture of martensite and bainite tempered on the surface is reduced, but not eliminated, maintaining a mixture of ferrite and pearlite that gradually transitions to the core.

[0018] However, international market analysis indicates that the use of quenched steel for reinforcing bars is not accepted in all markets. This is due both to local regulatory laws (e.g., China) and because some markets do not accept quenched bars even in the absence of specific regulations (e.g., Japan). This stems from the fact that reinforcing bars are widely used in civil engineering construction as reinforcement for concrete. In earthquake-prone regions such as China and Japan, these countries require bars with high ductility, which is provided by the microstructure lacking the presence of brittle phases typically associated with quenching, such as martensite and bainite.

[0019] In particular, the newly revised Chinese national standard GB / T 1499.2018 (Steel for Reinforcing Concrete - Part 2: Hot-rolled Ribbed Bars) has made significant changes to the manufacture and supply of steel reinforcing bars for reinforcing concrete in China, directly addressing the problems caused by using substandard steel reinforcement materials.

[0020] To ensure compliance with stringent standards or, in any case, to meet certain market requirements, additional alloying elements such as niobium (Nb), vanadium (V), and titanium (Ti) are necessary. However, adding these alloying elements inevitably leads to a significant increase in production costs. Invention Overview

[0022] The purpose of this invention is to develop a process for manufacturing compact coils of steel bars that allows for the production of ultrafine-grained, martensitic-free, and highly ductile wound steel bars at a lower production cost without the addition (or minimization) of microalloying elements (Nb, V, Ti).

[0023] Another object of the present invention is to produce coils of steel bars having a microstructure having a particle size equal to or greater than 9 according to standard ASTM E112, and wherein the hardness difference (HV, preferably HV 0.5, i.e. Vickers hardness measured under a load of 4.903 N) between the surface and the core of the steel bar is less than or equal to 40 HV, for example in the range of 10 HV to 40 HV.

[0024] The present invention achieves these and other objectives by means of a process for manufacturing ultrafine-grained martensitic-free steel bars, including the stage of claim 1, as will become apparent from the description of the invention.

[0025] According to another aspect of the invention, an apparatus is provided for manufacturing compact coils of ultrafine-grained martensitic-free steel bars, the apparatus being adapted to carry out the process, the apparatus comprising:

[0026] - A roughing mill used to roll steel billets to obtain steel bars;

[0027] - At least one first cooling device for cooling the steel bar and at least one first equalization space for performing at least one first equalization in the air;

[0028] - At least one intermediate rolling mill for rolling steel bars;

[0029] - At least one second cooling device for cooling the steel bar and at least one second equalization space for performing at least one second equalization in the air;

[0030] - A finishing mill used for rolling steel bars;

[0031] - At least one winding device for winding steel bars into a compact coil.

[0032] Advantageously, the steel bars treated with the process of the present invention do not possess the characteristic microstructure obtained by means of quenching surface processes (i.e., an outer ring of martensite with a ferrite and pearlite core), but instead exhibit a microstructure consisting only of a mixture of ferrite and pearlite uniformly distributed across the entire cross-section of the bar. The mechanical properties are achieved through austenite grain refinement resulting from alternating cooling-equilibrium-rolling stages, which can be combined into thermomechanical rolling. Small austenite grains rapidly transform into a finer ferrite-pearlite texture. Compared to classically hot-rolled products, with the same chemical composition, the ultra-fine-grained product exhibits better mechanical properties, particularly higher ductility. These cooling-equilibrium-rolling stages can be repeated multiple times using a variable number of cooling devices, such as cooling boxes or water tanks, which, depending on the equipment's production volume, allow the desired bar surface temperature to be reached at the inlet of the finishing mill assembly.

[0033] The heat treatment according to the invention is particularly suitable for producing compact coils of ribbed steel bars for concrete reinforcement, wherein the yield stress is in the range of 200 MPa to 1200 MPa, for example, from 400 MPa to 1000 MPa, or from 400 MPa to 700 MPa for the most common low-carbon / medium-carbon steel composition range.

[0034] Optionally, additional contributions to the mechanical properties may come from subsequent unfolding and straightening operations (work hardening), as well as possible natural aging. Thus, in this particular embodiment, the mechanical properties of the final product are obtained by means of a combination of thermomechanical rolling, heat treatment on a cooling line, straightening, and possible aging.

[0035] The following are some additional advantages of the solution of the present invention compared to the prior art:

[0036] - It is suitable for all grades of concrete-reinforced ribbed bars;

[0037] - The absence of martensite and bainite ensures better earthquake resistance and less wear on the straightening machine;

[0038] - The fine-grained microstructure allows for the reduction or absence of microalloying elements, thus saving production costs.

[0039] Further features and advantages of the invention will become more apparent from the detailed description of the illustrative but non-exclusive embodiments.

[0040] The dependent claims describe specific embodiments of the invention. Brief description of the attached diagram

[0042] Reference is made to accompanying diagrams in the description of the invention, which are given by way of non-limiting examples, wherein:

[0043] Figure 1 shows a first schematic layout of an apparatus for quenching and tempering in a winding machine according to the prior art;

[0044] Figure 2 shows a second schematic layout of an apparatus for soft hardening according to the prior art;

[0045] Figure 3 A first embodiment of an apparatus on which the process of the present invention is performed is shown;

[0046] Figure 4 A second embodiment of an apparatus on which the process of the present invention is performed is shown;

[0047] Figure 5 A third embodiment of an apparatus on which the process of the present invention is performed is shown;

[0048] Figure 6 A fourth embodiment of an apparatus on which the process of the present invention is performed is shown;

[0049] Figure 7 A schematic Fe-C diagram is shown, highlighting the carbon and temperature range suitable for thermomechanical rolling;

[0050] Figure 8 shows the cooling profiles (surface temperature, average temperature, and core temperature) of a steel bar that has undergone a known heat treatment along the layout of Figure 1.

[0051] Figure 9 It is shown that, according to the invention, along Figure 3 The layout of the heat-treated steel bars shows the cooling curves (surface temperature, average temperature, and core temperature).

[0052] Figure 10 It is shown that, according to the invention, along Figure 4 The layout of the heat-treated steel bars shows the cooling curves (surface temperature, average temperature, and core temperature).

[0053] Figure 11 It is shown that, according to the invention, along Figure 5 The layout of the heat-treated steel bars shows the cooling curves (surface temperature, average temperature, and core temperature).

[0054] Figure 12 It is shown that, according to the invention, along Figure 6 The layout of the steel bar undergoing heat treatment shows the cooling curves (surface temperature, average temperature, and core temperature).

[0055] Detailed description of some illustrative embodiments of the present invention

[0056] This invention relates to a process for manufacturing compact coils of steel bars, which allows for the production of ultra-fine-grained, martensitic-free, and highly ductile wound steel bars without the addition of microalloying elements or by minimizing the addition of microalloying elements, resulting in lower production costs.

[0057] In this specification, the term "compact roll" means a roll having a fill factor of 65% or higher, preferably in the range of 65%-74%, which, taking into account the volume of the roll, is defined as the ratio between the density of the roll and the theoretical density of the steel.

[0058] Conversely, the term "ultrafine-grained" refers to a microstructure having an average particle size equal to or greater than 9 according to standard ASTM E112.

[0059] The coiled steel bars produced by the process of the present invention can have dimensions (i.e. diameter) preferably in the range of 8 mm to 40 mm.

[0060] The weight of the roll is in the range of 1.0 ton to 10.0 ton, preferably in the range of 2.0 ton to 8.0 ton.

[0061] Figures 3-6 Some embodiments of the equipment layout that can perform the process of the present invention are shown.

[0062] In all embodiments of the present invention, the process for manufacturing compact coils of ultrafine-grained martensitic-free steel bars includes the following steps:

[0063] a) Steel bars are produced by rolling a steel billet with an initial surface temperature of 850°C-1200°C, preferably 900°C-1100°C, using a roughing mill 1;

[0064] b) Perform at least one first cooling stage 2 to give the steel bar a surface temperature higher than the martensite initiation temperature Ms, and perform at least one first equalization stage in air to minimize the temperature difference between the core and surface of the steel bar until a surface temperature in the range of 850°C-920°C is reached.

[0065] c) Rolling steel bars by means of at least one intermediate rolling mill 3, for example, only one intermediate rolling mill;

[0066] d) Perform at least one second cooling stage 4 to always keep the surface temperature above the martensite initiation temperature Ms, and perform at least one second equalization stage in air to minimize the temperature difference between the core and surface of the steel bar until a surface temperature in the range of 700°C-900°C, preferably 750°C-840°C or 750°C-820°C is reached.

[0067] e) The steel bar is rolled in the non-recrystallization temperature range by means of the finishing mill 5, the entire cross section of the steel bar is maintained in the non-recrystallization temperature range, and wherein the total reduction relative to the cross section of the steel bar at the entrance of the finishing mill 5 is between 25% and 50% in order to obtain an ultra-fine austenitic matrix.

[0068] f) At a winding temperature in the range of 500°C to 800°C, preferably 500°C to 750°C or 650°C to 730°C, the steel bar is wound into a compact coil by means of at least one winding device 7, such as two winding devices 7, so that the ultrafine austenitic matrix is ​​transformed into a mixture of ferrite and pearlite.

[0069] In the specific case of low carbon steel / medium carbon steel, the process of the present invention can be performed in a device with a production capacity of 90t / h-120t / h according to the steps mentioned above.

[0070] The billet from step a) enters the roughing mill 1. The billet comes from a reheating furnace, such as a gas furnace or induction heater, or directly from a continuous casting machine (not shown). At the entrance of the first set of rolling mill stands, i.e., the roughing mill 1, the surface temperature of the steel bar is in the range of 850°C-1200°C, preferably 900°C-1100°C.

[0071] Preferably, the steel billet is a low-carbon steel or medium-carbon steel billet.

[0072] The low-carbon / medium-carbon steel comprises or is composed of the following: less than or equal to 0.28% carbon, less than or equal to 0.80% silicon, less than or equal to 1.60% manganese by weight, with the remainder being iron and unavoidable or possible impurities.

[0073] Preferably, the low-carbon steel / medium-carbon steel comprises or is composed of the following: carbon in the range of 0.20%-0.25% by weight, silicon in the range of 0.20%-0.70%, manganese in the range of 0.80%-1.30%, possible vanadium in the range of 0.020%-0.050%, with the remainder being iron and unavoidable or possible impurities.

[0074] For steel bars with dimensions (diameter) of 8mm-40mm, two non-limiting examples of steel compositions are disclosed in the table below.

[0075]

[0076] In the implementation of the process, after the roughing mill 1, the steel bar is cooled by at least one first cooling stage 2, such that the surface of the steel bar does not reach the martensite initiation temperature (Ms), which can be calculated according to the following formula.

[0077] Ms(℃)=512-453*C-16.9*Ni+15*Cr-9.5*Mo+217*C2-71.5*(C*Mn)-67.6*(C*Cr),

[0078] Or simply, Ms(℃)=512-453*C+217*C2-71.5*(C*Mn).

[0079] An air equalization space is provided between the at least one first cooling stage 2 and the subsequent intermediate rolling mill 3.

[0080] In the variant, only one first cooling stage 2 is set (e.g., Figures 3-6 (As shown in the diagram), and only one first equalization stage in air is provided between the first cooling stage 2 and the intermediate mill 3. Alternatively, at least two first cooling stages 2 are provided, and a first equalization stage in air is provided between at least two first cooling stages 2 and between the last first cooling stage 2 and the intermediate mill 3. For example, two first cooling stages 2 are provided, and corresponding first equalization stages in air are provided between two subsequent first cooling stages 2 and between the last first cooling stage 2 and the intermediate mill 3.

[0081] At least two second cooling stages 4 are provided after the intermediate mill 3 to reduce the surface temperature of the bar, but always keep the surface temperature above Ms. A second equalization stage in air is provided between the at least two second cooling stages 4 and between the final second cooling stage 4 and the finishing mill 5. In step d), no microstructural change occurs, and both the surface and core of the bar remain completely in the austenitic phase.

[0082] Preferably, two or three second cooling stages 4 are provided, and corresponding second equalization stages in air are provided between two subsequent second cooling stages 4 and between the final second cooling stage 4 and the finishing mill 5. Therefore, if two second cooling stages 4 are provided, there will be two second equalization stages in air. Alternatively, if three second cooling stages 4 are provided, there will be three second equalization stages in air.

[0083] Optionally, the equalization space between the two subsequent second cooling stages 4 can vary from 8m to 25m depending on the equipment production rate; alternatively, the equalization space between the final second cooling stage and the subsequent finishing mill 5 can vary between 25m and 50m depending on the equipment production rate.

[0084] Preferably, the cooling stage-equilibrium stage-intermediate rolling stage can be repeated multiple times, and the number of second cooling stages 4 can vary depending on the equipment production volume, in order to achieve the desired bar surface temperature at the entrance of the finishing mill 5. In this case, more than one intermediate rolling mill 3 is provided. Additional intermediate rolling mills 3 are provided between two corresponding subsequent second cooling stages 4, particularly between the equilibrium space after the cooling stage 4 and the subsequent cooling stages 4.

[0085] Due to the cooling applied in the second cooling stage 4, the surface temperature gradually decreases until it reaches a range of 700°C-900°C, preferably 750°C-840°C or 750°C-820°C at the entrance of the finishing mill 5.

[0086] During all finishing rolling passes, the surface temperature of the bar is maintained within the non-recrystallization range (see example). Figure 7 For example, 750℃-850℃, 750℃-840℃, or 750℃-820℃. This means that the austenite particle size is reduced by applying a high reduction rate (25%-50% total reduction rate on the finishing mill set), and the recrystallization and growth of austenite are inhibited due to the lack of available heat energy. Figure 7 A schematic Fe-C diagram is shown in particular, highlighting the carbon and temperature range (C zone) suitable for thermomechanical rolling without recrystallization.

[0087] Advantageously, the number of finishing rolling passes should be less than or equal to four. A higher number of rolling passes may cause the temperature inside the rolled bar to rise, which may jeopardize the microstructure processing.

[0088] At the exit of finishing mill 5, as a result of austenite grain size refinement and subsequent possible controlled cooling, the final grain size is ultrafine, resulting in a value equal to or greater than 9 according to standard ASTM E112.

[0089] The absence of brittle phases, such as martensite and bainite, has been confirmed by the reduced hardness difference (HV, preferably HV 0.5, i.e., Vickers hardness measured under a load of 4.903 N) measured between the surface and core of the steel bar. Such a difference is advantageously less than or equal to 40 HV, preferably in the range of 10 HV to 40 HV.

[0090] Preferably, between the finishing rolling step e) and the winding step f), at least one third cooling stage 6 and at least one third equalization stage in air are provided to minimize the temperature difference between the core and surface of the steel bar, and to always avoid martensite formation until the predetermined winding temperature is reached.

[0091] In the variant, at least two third cooling stages 6 are provided, and a third equalization stage in air is provided between the at least two third cooling stages 6 and between the last third cooling stage 6 and at least one winding device 7.

[0092] In summary, one or more third cooling stages 6 are optional. These cooling stages 6 can be avoided if the surface temperature of the bars from the finishing mill 5 is suitable for the winding operation.

[0093] Preferably, when configured, the number of third cooling stages 6 includes two to six.

[0094] The number of cooling stages and the distance between the two subsequent cooling stages 6 depend on the equipment production rate. These distances can always be equal (e.g., ...). Figures 3-6 (as shown in the image) or different.

[0095] One or more cooling stages 6 can be used to obtain different winding temperatures along the same steel bar, so as to make the cooling profile of different layers uniform and to limit the diffusion of mechanical properties as much as possible. Optionally, in the winding step, the first and last layers are wound at a temperature 20°C-50°C higher than the remaining layers. The reference temperature range for the winding operation is 500°C-800°C, preferably 650°C-730°C, including the higher temperature of the first and last layers.

[0096] exist Figure 3 In the first embodiment shown, cooling stage 6 is not provided. Only one cooling stage 2 and two cooling stages 4 are provided.

[0097] exist Figure 4 In the second embodiment shown, three cooling stages 6 are provided.

[0098] exist Figure 5 In the third embodiment shown, five cooling stages 6 are provided.

[0099] exist Figure 6 In the fourth embodiment shown, six cooling stages 6 are provided.

[0100] exist Figures 3-6 In these implementation schemes, only one cooling stage 2 and two (optionally three) cooling stages 4 are provided.

[0101] Figure 9 , Figure 10 , Figure 11 and Figure 12 The invention is shown along the following lines. Figure 3 , Figure 4 , Figure 5 and Figure 6The layout of the heat-treated steel bars has cooling curves 20, 21, and 22 (surface temperature, average temperature, and core temperature).

[0102] The horizontal dashed line represents the martensite initiation temperature Ms at approximately 500°C.

[0103] Throughout all steps of the process of this invention, the surface temperature of the steel bar is always kept above the martensite initiation temperature Ms, which is different from the cooling curve of the steel bar undergoing QTS heat treatment along the layout of Figure 1 (see Figure 8).

[0104] Preferably, at least one first cooling stage 2 is carried out by means of a corresponding first cooling device, at least one second cooling stage 4 is carried out by means of a corresponding second cooling device, and at least one possible third cooling stage 6 is carried out by means of a corresponding third cooling device.

[0105] As an example, the first, second, and third cooling stages are water-cooled stages, and the first, second, and third cooling devices are cooling tanks, such as water-cooled tanks. Preferably, the operating pressure used in all cooling stages 2, 4, and 6 is in the range of 0.2 MPa to 0.6 MPa.

[0106] The distance between the two subsequent cooling boxes can vary from 8m to 25m depending on the equipment's production capacity; while the distance between the final cooling box and the subsequent rolling mill can vary between 25m and 50m depending on the equipment's production capacity.

[0107] The number of cooling boxes and the distance between them, as well as the distance between the last cooling box and the next set of rolling mill stands, depend on the production line's output and the grade of steel to be processed. Downstream of the last cooling box of the equipment, two or more winding devices 7 are provided for winding the processed material onto, for example, spools.

[0108] Optionally, in all embodiments, the surface temperature of the steel bar can be monitored by means of sensors, such as pyrometers, installed at both the inlet and outlet of each of the roughing mill 1, intermediate mill 3, and finishing mill 5. Based on the readings of said sensors, the operating parameters of the at least one first cooling stage 2, the at least one second cooling stage 4, and possibly the at least one third cooling stage 6 can be managed by closed-loop automatic control, which operates through both feedforward and feedback control.

[0109] The number of cooling stages along the entire production line allows the intensity of cooling to be adapted to the chemical composition of the steel bar and the mechanical properties required for the final product. In the same way, the chemical composition can be used to balance the need for higher mechanical properties without exceeding the cooling intensity within the cooling stage or lowering the rolling temperature. Microalloyed billets can be used for this purpose.

[0110] During and shortly after the winding process, the ultrafine-grained austenitic matrix transforms into a fine mixture of ferrite and pearlite. As a result, given the same final product yield strength, the material exhibits greater ductility compared to wound steel bars with a quenched surface.

[0111] After the winding operation is completed, the compact roll can be conveyed to the storage area by a conveying device 8, such as a stepping beam, where the roll is subjected to natural cooling, forced cooling, or delayed cooling.

[0112] Preferably, when loaded on the conveyor 8, the surface temperature of the roll is in the range of 600°C-700°C.

[0113] Along the conveyor 8, the coil can be cooled by natural air convection, or its cooling profile can be altered using suitable equipment. Cooling can be accelerated, for example, by blowing air or mist along the conveyor 8, or cooling can be delayed, for example, by using a hood covering the conveyor or an active soaking furnace. Altering the coil's cooling profile can be a useful tool for further influencing the morphology of the ferrite-pearlite mixture.

[0114] Optionally, after cooling to room temperature in the storage area, the coil can be unwound and straightened. This operation results in an increase (to a lesser extent) in yield strength and tensile strength, and a decrease in elongation at break. Work hardening can be applied to varying degrees by adjusting the straightening parameters. Overall, the ductility of the steel bar remains satisfactory.

[0115] As examples, and to better understand the nature of the invention, some typical mechanical properties are provided below, which can be obtained according to GB 1499-2:2018 standard - grade HRBF400E:

[0116]

[0117] in

[0118] YS = Yield stress;

[0119] UTS = Ultimate tensile stress;

[0120] El = Elongation at break.

[0121] The ratio between ultimate tensile stress and yield stress gives the concept of a material's ductility.

Claims

1. A process for manufacturing compact coils of ultrafine-grained, martensitic-free steel bars, said steel bars having a microstructure with an actual grain size equal to or greater than 9 according to standard ASTM E112, and wherein the hardness HV difference measured between the surface and core of said steel bars is less than or equal to 40 HV, said process comprising the following stages: a) Using a roughing mill (1) for producing steel bars, roll steel billets with an initial surface temperature of 850°C-1200°C; The steel billet mentioned above is a carbon steel billet, and the carbon steel is composed of the following: By weight percentage, it contains 0.20%-0.25% carbon, 0.20%-0.70% silicon, 0.80%-1.30% manganese, with the remainder being iron and unavoidable impurities. b) Perform at least one first cooling stage (2) to give the steel bar a surface temperature above the martensite initiation temperature, and perform at least one first equalization stage in air to minimize the temperature difference between the core and surface of the steel bar until the surface temperature is reached in the range of 850°C-920°C. c) The steel bar is rolled by means of at least one intermediate rolling mill (3); d) Perform at least one second cooling stage (4) to always keep the surface temperature above the martensite initiation temperature, and perform at least one second equalization stage in air to minimize the temperature difference between the core and surface of the steel bar until the surface temperature is reached in the range of 700°C-900°C. e) The steel bar is rolled in a non-recrystallization temperature range by means of a finishing mill (5), the entire cross section of the steel bar is kept in the non-recrystallization temperature range, and wherein the total reduction is between 25% and 50% relative to the cross section of the steel bar at the entrance of the finishing mill, in order to obtain an ultra-fine austenitic matrix. f) The steel bar is wound into a compact coil by means of at least one winding device (7) at a winding temperature in the range of 500°C to 800°C, so that the ultrafine austenitic matrix is ​​transformed into a mixture of ferrite and pearlite.

2. The process according to claim 1, wherein the carbon steel further comprises vanadium in the range of 0.020%-0.050% by weight.

3. The process according to claim 1 or 2, wherein in step d), at least two second cooling stages (4) are provided, and a second equalization stage in air is provided between the at least two second cooling stages (4) and between the last second cooling stage (4) and the finishing mill (5).

4. The process according to any one of claims 1-3, wherein in step b), at least two first cooling stages (2) are provided, and a first equalization stage in air is provided between the at least two first cooling stages (2) and between the last first cooling stage (2) and the at least one intermediate mill (3).

5. The process according to any one of claims 1-4, wherein between step e) and step f), at least one third cooling stage (6) and at least one third equalization stage in air are provided to minimize the temperature difference between the core and surface of the steel bar until the winding temperature is reached.

6. The process according to claim 5, wherein at least two third cooling stages (6) are provided, and a third equalization stage in air is provided between the at least two third cooling stages (6) and between the last third cooling stage (6) and the at least one winding device (7).

7. The process according to claim 6, wherein a third cooling stage (6) is provided, comprising two to six such stages.

8. The process according to any one of claims 5-7, wherein the at least one third cooling stage (6) is carried out by means of a corresponding third cooling device.

9. The process according to claim 8, wherein the operating pressure of the third cooling device is in the range of 0.2 MPa-0.6 MPa.

10. The process according to any one of claims 1-9, wherein the at least one first cooling stage (2) is carried out by means of a corresponding first cooling device, and the at least one second cooling stage (4) is carried out by means of a corresponding second cooling device.

11. The process according to claim 10, wherein the operating pressure of the first cooling device and the second cooling device is in the range of 0.2 MPa to 0.6 MPa.

12. The process according to any one of claims 1-11, wherein in step e), the number of finishing rolling passes is less than or equal to four.

13. The process according to any one of claims 1-12, wherein in step f), the first and last layers are wound at a temperature 20°C-50°C higher than the remaining layers.

14. The process according to any one of claims 1-4 and 10-13, wherein the surface temperature of the steel bar is monitored by means of sensors installed at both the inlet and outlet of each of the roughing mill (1), the intermediate mill (3) and the finishing mill (5), and the operating parameters of the at least one first cooling stage (2) and the at least one second cooling stage (4) are managed by closed-loop automatic control based on the readings of the sensors, the closed-loop automatic control operating by both feedforward control and feedback control.

15. The process according to any one of claims 5-9, wherein the surface temperature of the steel bar is monitored by means of sensors installed at both the inlet and outlet of each of the roughing mill (1), the intermediate mill (3) and the finishing mill (5), and the operating parameters of the at least one first cooling stage (2) and the at least one second cooling stage (4) are managed by closed-loop automatic control based on the readings of the sensors, the closed-loop automatic control operating by both feedforward control and feedback control.

16. The process according to claim 15, wherein the operating parameters of the at least one third cooling stage (6) are further managed by the closed-loop automatic control.

17. The process according to any one of claims 1-16, wherein after step f), the compact roll is conveyed to the storage area by a conveying device (8), and the roll is subjected to natural cooling, forced cooling, or delayed cooling along the conveying device (8).

18. The process according to claim 17, wherein when loaded on the conveying device (8), the surface temperature of the roll is in the range of 600°C-700°C.

19. The process according to claim 17 or 18, wherein after the storage area is cooled to room temperature, the roll is unrolled and straightened.

20. The process of claim 19, wherein after the roll is unrolled and straightened, the steel bar is subjected to natural aging at room temperature.

21. The process according to any one of claims 1-20, wherein the billet enters the roughing mill (1), and the billet comes from a reheating furnace or directly from a continuous casting machine.

22. A coil of steel bar, produced by the process according to any one of claims 1-21, having a microstructure with an actual particle size equal to or greater than 9 according to standard ASTM E112, and wherein the difference in hardness HV measured between the surface and the core of the steel bar is less than or equal to 40 HV.

23. The coil of steel bar according to claim 22, wherein the difference in hardness HV measured between the surface and the core of the steel bar is in the range of 10 HV to 40 HV.

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