Control method for improving elongation of low-carbon cold heading material

Abstract

The invention relates to the technical field of metal material hot working, and discloses a control method for improving the elongation of a low-carbon cold heading material, which comprises the steps of billet heating, hot rolling forming and spinning of a rolled wire onto a Stelmor air cooling line for controlled cooling. The cooling control step comprises the following steps of: controlling the running of a conveying roller way on the Stelmor air cooling line, so that a plurality of coil rings formed after spinning form a close packing state on the air cooling line; and when the coil rings are in a close packing state, carrying out heat preservation and slow cooling on the close packing coil rings through a heat preservation facility on the air cooling line. A stable and long-time-history online heat treatment environment is constructed by controlling close packing of coils and executing segmented collaborative slow cooling; the environment can ensure that austenite is fully transformed and a microscopic structure beneficial to plasticity improvement is obtained, so that the elongation of the low-carbon cold heading material is directly and stably improved on line on the premise of not depending on off-line annealing.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of hot working of metal materials, in particular to a control method for improving the elongation of low-carbon cold-upset material. BACKGROUND

[0002] Low-carbon cold-upset steel is a key raw material for manufacturing standard parts such as bolts, nuts, screws and various special-shaped fasteners, and its final use performance depends largely on its cold-forming performance. Excellent cold-forming performance is directly related to the plasticity of the material, which is usually characterized by the core mechanical property index of elongation after fracture. In order to ensure that the steel does not crack during subsequent complex and large deformation multi-station cold-upset processing, it must have high enough plasticity, which corresponds to the microstructure of the steel being composed of coarse ferrite and lamellar pearlite in metallography. This softened microstructure is the organizational guarantee for obtaining high elongation.

[0003] In the existing wire rod production process, the wire rod after hot rolling is usually cooled online by Stelmor air cooling line. However, the design idea and control target of the traditional online controlled cooling process are often focused on refining the grain by controlling the cooling speed to meet the strength or hardness requirements of the material. Therefore, the conventional operation is usually to make the wire coil rings discharged by the wire rod machine to be pulled apart from each other on the air cooling conveying line, to maintain a large distance, and to be supplemented by a cooling fan for forced or controllable air cooling. This control method aimed at promoting uniform cooling and grain refinement inevitably suppresses the full growth of ferrite grains during the decomposition of austenite, and the fine-grained structure obtained can meet certain strength levels, but the plasticity potential is not fully explored, and it is difficult to stably reach the high elongation level required by high-end or complex cold-upset products.

[0004] In order to make up for the inherent limitations of the conventional online cooling process in plasticity control, the industry generally adopts a remedial measure of adding an offline spheroidizing annealing or recrystallization annealing heat treatment process to the wire rod product. That is, the wire rod cooled into a coil is sent back into a heat treatment furnace for reheating and long-time soaking and slow cooling, in order to obtain a softened microstructure. Although this method can effectively improve the elongation of the material, it also has significant defects. It not only greatly prolongs the total production cycle of the product, increases the additional energy consumption, equipment investment and labor cost, but also inevitably causes decarburization and oxidation loss on the surface of the material during the secondary heating process, resulting in a long production process, high comprehensive cost and low production efficiency. SUMMARY

[0005] In view of the deficiencies of the prior art, the application provides a control method for improving the elongation of low-carbon cold upsetting material, which solves the problem that the online cooling process of low-carbon cold upsetting steel cannot directly obtain high elongation and must rely on an offline annealing process with high cost and low efficiency to improve the plasticity.

[0006] To achieve the above object, the application is implemented by the following technical scheme: a control method for improving the elongation of low-carbon cold upsetting material, comprising the steps of heating a billet, hot rolling, and controlling cooling of the wire after rolling to a Stelmor air cooling line, wherein the step of controlling cooling comprises:

[0007] The operation of the conveying roller of the Stelmor air cooling line is controlled to form a close-piled state of the multiple coil rings formed after wire drawing;

[0008] The close-piled coil rings are kept warm and slowly cooled by the heat preservation facilities on the air cooling line while the coil rings are in the close-piled state.

[0009] Preferably, the close-piled state is formed by controlling the operation of the conveying roller, specifically by setting at least 1 to 4 segments of the conveying roller in the front region of the Stelmor air cooling line in a synchronous operation mode without speed difference.

[0010] Preferably, the close-piled coil rings are kept warm and slowly cooled, specifically by closing the heat preservation cover above the region corresponding to the close-piled coil rings on the air cooling line and stopping the operation of the cooling fan below the region.

[0011] Preferably, the step of controlling cooling further comprises controlling the wire drawing temperature of the wire drawn to the Stelmor air cooling line to be in the range of 880-930 DEG C.

[0012] Preferably, the total conveying time of the warm and slow cooling and the wire drawing temperature are set in coordination, so that in the temperature range, a lower wire drawing temperature corresponds to a longer total conveying time, and the total conveying time is not less than 700 seconds.

[0013] Preferably, the step of controlling cooling is divided into at least two stages, wherein in the second stage where the core phase change region of the austenite phase change is located, a micro-negative speed gradient is applied to the conveying roller of the region while the coil rings are kept warm and slowly cooled, so that the close-piled coil rings are more compactly packed by being axially extruded.

[0014] Preferably, the micro-negative speed gradient is that the speed difference between two adjacent segments of the roller is in the range of 0.01-0.05 m / s.

[0015] Preferably, the control cooling step further comprises a third stage of coil organization stabilization and coiling temperature control, which is located after the second stage, and is operated as follows: on the basis of maintaining the tight packing state of the coil ring, the last 1 to 2 heat preservation covers at the end of the air cooling line are opened in a stepped manner.

[0016] The application provides a control method for improving the elongation of low-carbon cold upsetting material.

[0017] 1. The application establishes a cooperative control relationship between the spinning temperature and the total slow cooling time, ensuring that the wire can obtain a long enough online heat treatment time matched with the high initial temperature. This control method of coupling the two key process parameters of temperature and time ensures that the phase transition process from austenite to ferrite and pearlite can be fully carried out, laying a solid thermodynamic and kinetic foundation for forming a microstructure conducive to improving plasticity, thereby helping to stably improve the elongation after fracture of low-carbon cold upsetting material.

[0018] 2. The application uses the mutual thermal radiation between the tightly packed coil rings to build an efficient self-insulation and soaking environment before the phase transition of the wire starts. This can significantly improve the temperature uniformity of the wire rod, creating ideal initial conditions for subsequent overall and synchronous organization transformation, which is the key to obtaining uniform final microstructure and mechanical properties and improving the overall elongation of the material.

[0019] 3. The application builds a complete online heat treatment path composed of high-temperature soaking, extreme slow cooling and organization stabilization through cooperative control of the air-cooled line in different regions and by multiple means. Especially in the core phase transition area, through the cooperative action of coil densification and external insulation facilities, the most favorable conditions for the formation of coarse ferrite are created. This precise shaping technique of the cooling curve can effectively regulate the final microstructure morphology and promote it to change in a direction conducive to improving the elongation of low-carbon cold upsetting material. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 The method steps of the application are shown in the figure. DETAILED DESCRIPTION

[0021] The technical solutions in the embodiments of the application will be described clearly and completely below with reference to the drawings in the specification of the application. Obviously, the described embodiments are only part of the embodiments of the application, not all. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the application.

[0022] Please refer to the attached Figure 1The embodiment of the present application provides a control method for improving the elongation of low-carbon cold upsetting material, which comprises the following steps: billet heating, hot rolling forming, and controlling cooling of the wire rod after rolling by discharging the wire rod to a Stelmor air cooling line.

[0023] By controlling the operation of the conveying roller way on the Stelmor air cooling line, the multiple coil rings formed after discharging the wire rod are in a close packing state on the air cooling line.

[0024] When the coil rings are in the close packing state, the close-packed coil rings are kept warm and slowly cooled by the heat preservation facilities on the air cooling line.

[0025] In the embodiment, a control method for improving the elongation of low-carbon cold upsetting material is described. The method starts from the steps of billet preparation and hot rolling forming, which provides an initial wire rod with a physical and organizational state meeting the requirements for the subsequent on-line controlled cooling process.

[0026] The first operation of the step is billet heating. A low-carbon cold upsetting steel billet with chemical composition meeting the predetermined standard is selected, and an example of a preferred option is an ML08 grade billet. The billet is conveyed to a heating device, preferably a walking beam type heating furnace.

[0027] In the heating device, the billet is heated, and the target temperature of the heating is controlled in the range of 1050-1150°C. A preset holding time is applied in the temperature range. The technical purpose of the heating and holding operation is to ensure that the temperature distribution from the core to the surface of the billet cross section reaches a uniform state, and at the same time, the metallographic structure is completely converted into a single austenite phase. A uniform initial temperature field and complete austenitization are the prerequisite conditions for obtaining a uniform and predictable phase change structure in the subsequent controlled cooling step.

[0028] The second operation of the step is hot rolling forming. After completing the heating and holding and confirming that the billet reaches the predetermined state, it is taken out of the heating device and immediately sent to a hot rolling mill train arranged in series.

[0029] The billet passes through a rough rolling mill train, a medium rolling mill train, a pre-precision rolling mill train, and a precision rolling mill train in sequence. In each mill train, the billet is subjected to continuous plastic deformation by multiple rolling passes. The technical purpose of the series of rolling operations is to gradually reduce the cross-sectional size of the billet and finally form a high-temperature wire rod meeting the target specification size. At the same time, the multi-pass hot deformation process also helps to further refine the austenite grains.

[0030] After the final pass of the finishing mill train is completed, the output product of this step is a high-temperature wire rod. This wire rod has a preset diameter and cross-sectional shape in physical terms, and is in a high-temperature, fully austenitic state in terms of organization. This high-temperature austenitic wire rod is the direct processing object of the subsequent step, i.e., the spooling temperature setting and soaking time coordination calculation step.

[0031] In this embodiment, after the billet preparation and hot rolling forming steps are completed, the spooling temperature setting and soaking time coordination calculation step is performed. This step sets a set of interrelated core process parameters in advance for the subsequent physical cooling process, and the technical purpose is to quantify the thermodynamic conditions required for the target organization transformation into executable control instructions.

[0032] The first operation of this step is the setting and control of the spooling temperature. The high-temperature wire rod output from the finishing mill train is precisely controlled in temperature before entering the spooling machine. Preferably, this temperature control is achieved through the pre-finishing water tank (0# water tank) located upstream of the finishing mill train. By adjusting the cooling water flow of this water tank, the temperature of the wire rod entering the final pass of the finishing mill is intervened, so that the temperature of the wire rod when it leaves the spooling machine to form a coil, i.e., the spooling temperature, is controlled within the target interval of 880°C to 930°C.

[0033] The technical purpose of controlling the spooling temperature in this higher interval is to ensure that the wire rod enters the subsequent Stelmor air cooling line in a fully recrystallized austenitic state and carries a higher initial heat. This initial heat provides the necessary energy conditions for atomic diffusion and completion of solid-state phase transformation in the subsequent long-term soaking process.

[0034] The second operation of this step is the coordination calculation of the soaking time. After the actual spooling temperature is determined or measured, according to the temperature value, the minimum total conveying time required for the wire rod on the subsequent Stelmor air cooling line is calculated and set. This calculation is performed through a preset functional relationship to establish a quantitative relationship between the initial temperature condition and the required soaking time.

[0035] The technical purpose of this coordination calculation operation is to couple the two process parameters of temperature and time, which were originally independent of each other. Through this calculation, a process time that matches the actual initial temperature condition and is sufficient to complete the expected organization transformation can be determined, so that the entire control scheme has corresponding adjustment ability to fluctuations in the initial conditions.

[0036] The two core parameters output by this step, i.e., the target spooling temperature and the calculated minimum total conveying time, will be directly input as control instructions into the subsequent Stelmor air cooling line segment coordination soaking step.

[0037] In this embodiment, after the aforementioned step of temperature setting of the spinning process and the step of calculating the slow cooling time are completed, the step of Stelmor air cooling line segment coordination slow cooling is performed immediately. This step is the physical implementation process of the preset process parameters, and the technical purpose is to actively build a preset cooling curve by applying a series of precise, regional, and multi-method coordinated control operations to the wire cooling path, so as to induce the formation of the target microstructure.

[0038] This step divides the entire conveying process of the Stelmor air cooling line into three continuous functional areas according to different stages of metallurgical phase change, and different control strategies are applied in each area.

[0039] First stage: high temperature soaking and coil dense packing

[0040] This stage is performed when the wire is just spun to the front area of the air cooling line, preferably corresponding to the first to fourth segment conveying roller of the air cooling line.

[0041] In this stage, the control operation is to set all conveying roller in this area to synchronous running mode without speed difference. That is, the running speed of each segment roller in this area is set to the same value. This operation makes the coil ring formed after spinning to form a dense packing state on the roller according to its natural pitch without being stretched by external force.

[0042] In order to quantitatively control this dense packing state, the coil packing density is introduced as a process control index. By setting the speed value of synchronous running, the coil packing density is controlled in the target range of 15 to 25 rings per meter.

[0043] The technical purpose of this stage is to utilize the sensible heat carried by the wire in the high temperature austenite state, to reduce the heat dissipation surface area by the close packing between coil rings, and to form a self-insulation physical environment. This environment helps to eliminate the temperature gradient between the head and tail, the core and surface layer of the wire that may be generated during rolling and conveying, so as to obtain an austenite structure with uniform temperature field before entering the critical phase change zone.

[0044] Second stage: extreme slow cooling and densification in the phase change zone

[0045] This stage is performed after the first stage when the wire is conveyed to the middle area of the air cooling line, preferably corresponding to the fifth to twelfth segment conveying roller of the air cooling line. This area is the main interval for the transformation of austenite to ferrite and pearlite.

[0046] In this stage, its control operation includes two actions performed in coordination. The first action is environmental isolation: close all the heat insulation covers above this area, and at the same time, stop running all the cooling fans below the corresponding heat insulation covers. This operation aims to minimize the forced convection and heat radiation exchange between the wire and the external environment, and to build a static air insulation layer.

[0047] The second action is extrusion compaction: a micro-negative speed gradient is applied on the conveying roller of this area. The speed difference is set to 0.01-0.05 m / s. The application of this speed gradient will generate a weak but continuous axial extrusion force on the tightly packed coil group formed in the previous stage, making it more compact.

[0048] The technical purpose of this stage is to create an extremely slow cooling condition for the austenite decomposition process through the dual action of external thermal insulation and internal extrusion compaction. This condition provides sufficient time for the nucleation and growth of ferrite crystal nuclei and the coarsening of cementite lamellae in pearlite, which is the key to forming the target soft structure.

[0049] Third stage: organization stabilization and temperature control

[0050] This stage is the last stage of controlled cooling, which is performed when the wire is delivered to the rear area of the air cooling line, preferably corresponding to the 13th to 18th conveying roller of the air cooling line.

[0051] In this stage, the control operation is to open the last 1-2 heat insulation covers at the end of the air cooling line in a step-by-step manner while maintaining the tight packing and slow conveying state of the coil.

[0052] The technical purpose of this stage is to adjust the end of the cooling curve. At this time, the phase change process that determines the main mechanical properties of the material has been basically completed. By introducing a gentle and controllable cooling rate increase at the end of the cooling process, it can prevent unintended changes in the grain size, such as excessive coarsening, that may occur due to prolonged adiabatic cooling. At the same time, this operation helps to control the final temperature of the wire when it leaves the air cooling line within a pre-set, lower range, to reduce high-temperature surface oxidation during subsequent coiling, bundling and storage.

[0053] The final output of this step is the low-carbon cold heading steel wire rod that has completed the pre-set online heat treatment path.

[0054] Although embodiments of the present application have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made therein without departing from the principles and spirit of the application, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A control method for improving the elongation of low-carbon cold-upset material, comprising the steps of heating a billet, hot-rolling the billet, and performing controlled cooling by drawing the rolled wire onto a Stelmor air cooling line, characterized in that, The control cooling step comprises: By controlling the operation of the conveying roller way on the Stelmor air cooling line, the multiple coil rings formed after the wire rod is drawn are formed into a compact stacking state on the air cooling line; While the coil rings are in the compact stacking state, the compact stacked coil rings are kept warm and slowly cooled by the heat preservation facilities on the air cooling line.

2. The control method for improving elongation of low-carbon cold- upset material according to claim 1, characterized by, The compact stacking state is formed by controlling the operation of the conveying roller way, specifically, at least 1-4 sections of the conveying roller way in the front region of the Stelmor air cooling line are set to a synchronous operation mode without speed difference.

3. The control method for improving elongation of low carbon cold upset material according to claim 1, wherein The compact stacked coil rings are kept warm and slowly cooled, specifically, the heat preservation cover above the corresponding region of the compact stacked coil rings on the air cooling line is closed, and the cooling fan below the heat preservation cover is stopped.

4. The control method for improving elongation of low carbon cold upset material according to claim 1, wherein The control cooling step further comprises controlling the wire rod drawing temperature in the range of 880-930℃ when the wire rod is drawn onto the Stelmor air cooling line.

5. The control method for improving elongation of low carbon cold upset material according to claim 4, wherein The total conveying time of the warm and slow cooling and the wire rod drawing temperature are set in coordination, so that in the temperature range, a lower wire rod drawing temperature corresponds to a longer total conveying time, and the total conveying time is not less than 700 seconds.

6. The control method for improving elongation of low carbon cold upset material according to claim 1, wherein The control cooling step is divided into at least two stages, and in the second stage where the core phase transformation region of the austenite phase transformation is located, a micro negative speed gradient is applied to the conveying roller way in the region while the coil rings are kept warm and slowly cooled, so that the compact stacked coil rings are more compact due to axial extrusion.

7. The control method for improving elongation of low-carbon cold- upset material according to claim 6, wherein The micro negative speed gradient is that the speed difference between adjacent two sections of the roller way is in the range of 0.01-0.05 m / s.

8. The control method for improving elongation of low carbon cold upset material according to claim 1, wherein The control cooling step further comprises a third stage of organization stabilization and coiling temperature control, which is located after the second stage, and the operation is to open the last 1-2 heat preservation covers at the end of the air cooling line in a stepped manner on the basis of maintaining the compact stacking state of the coil rings.

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