A fast spheroidized medium carbon steel bar and its manufacturing method

Abstract

The invention relates to a fast spheroidizing medium carbon steel bar and a manufacturing method thereof. The chemical composition of the steel is C: 0.30-0.60%, Si: ≤0.40%, Mn: 0.50-0.90%, P: ≤0.020%, S: ≤0.030%, Cr: ≤0.30%, Cu and Ni are usually ≤0.25% by mass percentage; in order to adjust the performance, micro alloying elements such as B can also be appropriately added; in order to ensure the grain size, one or more grain refinement elements such as Al, Nb, N, etc. can be added, and the balance is Fe and inevitable impurity elements. The steel is smelted, continuously cast, heated, ultra-low temperature rolled, and fast spheroidized, and the total spheroidization time is 5.5-6.5h, which is equivalent to soft annealing and much lower than the traditional spheroidizing annealing process. The obtained material structure is point spheroidized body + ferrite + a small amount of lamellar cementite. Hardness ≤160HBW, elongation ≥30%, upset rate 80%, no cracking during cold forging, conducive to cold forming processing with large deformation, and realize modern high-precision, energy-saving and emission-reducing green and high-quality production.

Description

Technical Field

[0001] The invention belongs to the field of special steel smelting, and particularly relates to a high-toughness mining circular saw blade steel plate and a manufacturing method thereof. Background Art

[0002] The processing technology of steel parts is changing with each passing day. Cold forming technology is gradually replacing the original hot forming technology to improve the surface quality and dimensional accuracy of forgings; improve the yield rate; reduce the amount of subsequent turning and grinding; reduce production energy consumption; reduce emissions; reduce production costs, etc. Taking automobile parts as an example, the proportion of cold forming has exceeded 20%, and there is a trend of increasing year by year.

[0003] The plastic deformation ability of steel in the cold state is poor compared to that in the hot austenitized state. When producing steel, it is usually necessary to perform heat treatments such as controlled rolling and controlled cooling or offline softening annealing, or even spheroidizing annealing, to reduce the hardness of the material and improve cold deformability. The general rule is that the higher the content of alloying elements, mainly carbon, the higher the hardness of the steel, and the worse the plasticity and cold forging properties. Taking carbon steel as an example, when the carbon content is less than 0.2%, the hot-rolled steel has good plasticity and can be directly cold-formed with a small deformation amount; if the steel is spheroidized, the plasticity is significantly improved, and a larger deformation amount and more complex cold forming process can be performed. However, due to its low strength, the application of low-carbon carbon steel is subject to certain restrictions. As the carbon content increases, the strength of the steel increases significantly. Taking 45 steel as an example, it is widely used in industry.

[0004] Due to the high proportion of pearlite content and low plasticity, 45 hot-rolled bar steel is only suitable for cold forming of parts with small deformation such as shafts. As the parts become more complex and the deformation increases, 45 steel needs to be heat treated before it is suitable for cold forming. The typical heat treatment process is: for cold-processed parts with small deformation, offline softening annealing (5 to 6 hours) is performed after hot rolling; as the deformation increases, the steel needs to be spheroidized offline (usually 12 to 24 hours) to improve the plasticity of the steel, otherwise cold forming cracks are prone to occur. Although the above-mentioned traditional heat treatment before cold forming can effectively solve the problem of cold forming cracks, the heat treatment time is long and the energy consumption is high, which is not suitable for modern green and dual-carbon modern manufacturing.

[0005] To sum up, the invention of a medium-carbon steel bar that can be suitable for large deformation cold forming processing and can be quickly spheroidized can not only shorten the processing cycle, simplify the processing flow, improve production efficiency, and reduce production costs, but also save energy consumption and reduce emissions. It has very significant economic and social value. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a method for manufacturing medium-carbon steel bars that can be suitable for cold forming processing with large deformation and can be quickly spheroidized in response to the above-mentioned existing technology. The medium-carbon steel bars manufactured by this method have good plasticity, are suitable for cold forming processing with large deformation, and have the advantages of short processing time, reduced consumption and reduced emissions.

[0007] The technical solution adopted by the present invention to solve the above problems is: a fast spheroidizing medium carbon steel bar, the series of steels is represented by 45 steel, and the chemical composition is C: 0.30-0.60%, Si: ≤0.40%, Mn: 0.50-0.90%, P: ≤0.020%, S: ≤0.030%, Cr: ≤0.030%, and the rest are Fe and unavoidable impurity elements.

[0008] The residual elements Cu and Ni in the material designed by the present invention are usually ≤0.25%; in order to adjust the performance, micro-alloying elements such as B and Ti can also be appropriately added; in order to prevent abnormal grain growth during subsequent heat treatment of the parts, one or more grain refining elements such as Al, Nb, and N can be added.

[0009] The mechanical properties of the steel of the present invention meet the following requirements: In order to be suitable for cold forming processing with large deformation, the plasticity of the material at room temperature is the key indicator, which is usually characterized by the cold forging upset rate without cracking. The limit upset rate without cracking needs to be ≥80%, that is, when the material is cold forged and the upset rate is 80%, there is no cold forging cracking. The elongation of the steel is greater than 30%; the reference hardness of the steel is ≤160HB.

[0010] The invention discloses a kind of fast spheroidizing medium carbon steel bar. Its core method is to make the cementite of the pearlite layer of the rolled steel fractured through ultra-low temperature rolling, and the steel bar has a large amount of dislocation energy and distortion energy inside. After the rolling is completed and the steel bar is cut off, it is quickly put into the furnace for fast spheroidization. The total spheroidization time is about 6 hours, which is equivalent to ordinary soft annealing and far lower than the traditional spheroidizing annealing method. The steel of the invention has good cold formability and no cracking when the cold heading rate is 80%. The structure of the steel is point-like spheroidized body + ferrite or point-like spheroidized body + ferrite + a small amount of lamellar cementite (the lamellar cementite accounts for less than 10% of the total amount of cementite and is dispersed).

[0011] The design principles of the main elements of the steel of the present invention are as follows:

[0012] C: 0.30-0.60%. The steel of the present invention is essentially a medium carbon steel. The C content mainly determines the proportion of cementite phase in the material, thereby affecting the strength of the material. If the proportion of cementite phase is too low, the strength of the material cannot be guaranteed. The higher the C content, the higher the proportion of cementite phase, which will reduce the cold formability and toughness of the steel. The C content range of the steel of the present invention is set to 0.30-0.60%.

[0013] Si: ≤0.40%. Si is usually dissolved in ferrite and can play a role in solid solution strengthening, significantly improving the strength of ferrite, but at the same time it will reduce the plasticity and toughness of ferrite, and the Si element will promote the segregation of some impurity elements and embrittle the grain boundaries. During the smelting process, Si is added to steel as a common deoxidizing element. The steel of the present invention is suitable for cold forming processing. In view of the above characteristics, Si takes the lower limit, and the setting range is ≤0.40%, and more preferably ≤0.25%.

[0014] Mn: 0.50-0.90%. Mn is also one of the common deoxidizing elements and one of the most effective and economical elements for coordinating the strength of steel. An appropriate amount of Mn can significantly improve the strength of steel without significantly affecting its plasticity. When S is contained in the steel, Mn and S form MnS with good plasticity, which improves the cutting performance of the steel. The Mn content of the steel of the present invention is set to 0.50-0.90%.

[0015] P: ≤0.020%. Except for some special steels where P is added as an alloying element, P in general steels is a typical harmful element that increases the brittleness of the steel. In terms of performance, the lower the P content in the steel of the present invention, the better. However, if the requirement is too strict, the steel smelting cost will also increase. In summary, the P content of the steel of the present invention is set to ≤0.020%.

[0016] S: ≤0.030%. The S element can improve the cutting performance of steel. When the S content in the steel is appropriate and it forms MnS with good deformability with Mn, it has no obvious effect on the cold formability of the steel. If the S content is too high, as the steel is rolled and deformed, when there is too much long strip MnS in the steel, it will cause various anisotropies of the steel and reduce the lateral performance of the steel. The S content range of the steel of the present invention is set to ≤0.030%.

[0017] Cr: ≤0.30%. Cr is also one of the effective and economical elements for coordinating the strength of steel, but the steel of the present invention is a medium carbon series steel. The increase of Cr will significantly affect the structural transformation curve of the steel, the performance of the steel will change, and the spheroidization principle and method will change. The Cr content range of the steel of the present invention is set to ≤0.30%.

[0018] The residual elements Cu and Ni in the material designed by the present invention are usually ≤0.25%; in order to adjust the performance, microalloying elements such as B can also be appropriately added; in order to prevent abnormal grain growth during subsequent heat treatment of the parts, one or more grain refining elements such as Al, Nb, Ti, N, etc. can be added.

[0019] The present invention is a medium carbon steel series. After the chemical composition is well controlled, innovative process control is required in the subsequent rolling and heat treatment process to achieve the purpose of not cracking during large deformation cold forming processing, and ultimately improve the surface quality and dimensional accuracy of cold formed parts, reduce the amount of turning or grinding of parts, increase the yield rate, and omit the heating treatment before forming.

[0020] Another object of the present invention is to provide a method for manufacturing a fast spheroidized medium carbon steel bar, which is also a key control point of the steel of the present invention, and mainly includes the following specific processes:

[0021] Smelting, continuous casting, hot rolling and rapid spheroidization

[0022] During smelting, the chemical composition of this series of steels must be well controlled, especially the control of P. When smelting in a converter or electric furnace, the slag must be quickly made to increase the fluidity of the slag, and the temperature in the furnace must be controlled to be ≤1670℃. Too high a temperature is not good for dephosphorization, because there is no dephosphorization process in the steel production process from the beginning of steelmaking. Once phosphorus exceeds the control target, it cannot be saved. At the same time, the purity of the molten steel must be controlled. Large particle inclusions in the steel, especially those on the surface and under the skin, are prone to cracking during cold heading.

[0023] During continuous casting, the purity of steel is further controlled, and the liquid level fluctuation is controlled to ≤±5mm by means of online automatic monitoring of the crystallizer liquid level. Once the liquid level fluctuates, the protective slag is easily caught by the billet shell to produce subcutaneous inclusions. The superheat is controlled at the lower limit, with a target of 10-25℃, and the cooling water volume of the crystallizer is controlled at the upper limit. The above two points ensure that the primary billet shell is cooled with high intensity, thereby increasing the thickness of the strengthened equiaxed fine grain structure, which is beneficial to increase the surface quality of the billet and subsequent rolled products, reduce the occurrence of surface microcracks, and avoid the expansion of microcracks during cold forming.

[0024] Rolling after continuous casting is one of the core processes of the present invention. Ultra-low temperature rolling is achieved to fracture the cementite in the pearlite lamellae of the rolled steel, increase the distortion energy inside the rolled steel, and facilitate the rapid fracture, dissolution and spheroidization precipitation of cementite during subsequent spheroidization.

[0025] The ingot is heated in a walking beam heating furnace, the preheating section temperature is 650-750°C, the heating section temperature is controlled at 850-950°C, and the soaking forging temperature is controlled at 950-1020°C. To ensure that the billet components are evenly heated, the total heating time is ≥180min. In the heating process, soaking forging temperature control is very important. The steel of the present invention adopts ultra-low temperature rolling, and the soaking section temperature is as low as possible within the rolling capacity. If the heating temperature is high, the subsequent controlled rolling and controlled cooling can reduce the surface temperature of the steel to a limited extent, but the core temperature of the steel is difficult to control, especially when the rolling specification is ≥φ60mm, the technical problem of uneven temperature of the controlled rolling and controlled cooling section is more difficult to solve. After the billet leaves the heating furnace, high-pressure water descaling is carried out, and the rolling temperature after descaling is 880-950°C. Then 25 continuous rolling mills are used for continuous rolling, and the 25 rolling mills are 6 rough rolling mills + 10 medium rolling mills + 4 pre-finishing rolling mills + 5 finishing rolling mills. The intermediate rolling mill, pre-finishing mill, and finishing mill are all equipped with water-through cooling tanks to achieve controlled rolling and controlled cooling. During controlled rolling and controlled cooling, the general process design idea is that the steel should store more distortion energy during the rolling process, so that the cementite is easier to melt during the subsequent spheroidization. During and after rolling, at least four spray water tanks are arranged in sequence for controlled cooling. After intermediate rolling, the 1# water tank is distributed for spray cooling, after pre-finishing rolling, the 2# and 3# water tanks are distributed, and after three-roller finishing, the 4# water tank is distributed. The steel is cooled alternately with strong and weak cooling. The 1# water tank is opened at 45~85% for strong cooling, and the 2# and 3# water tanks are used for weak cooling. The cooling water opening degree is between 5~15%, and the incoming material temperature into the finishing mill is controlled between 730 and 840℃ for two-phase rolling. After leaving the finishing mill, the cooling intensity is appropriately increased, and the cooling water opening degree is increased to between 40 and 60%, ensuring that the temperature of the material is between 720 and 800°C when it is on the cooling bed. Through the above rolling method, it is ensured that the steel has the highest possible internal distortion energy. After rolling, the steel is quickly put on the cooling bed and sheared or cut off.

[0026] After the steel is bundled off the line, it is quickly put into the annealing furnace for rapid spheroidizing annealing treatment. Rapid spheroidization is also one of the core processes of the present invention. Since the steel is rolled at ultra-low temperature, there is a large amount of distortion energy in the rolled steel, and the cementite lamellae in the pearlite are in a metastable state, which is easy to melt when reheated. The bar steel off the line is directly loaded into the annealing furnace for insulation. The insulation temperature is 710-760°C. The spheroidizing insulation temperature is a sensitive process, and the temperature range for the best process effect is very narrow. If the temperature is too low, the cementite lamellae are not easy to melt. If the temperature is too high, a large amount of cementite is dissolved in the austenite, and then re-precipitated during the transformation, which is not conducive to spheroidization. The target value of the insulation temperature is adjusted according to the composition of the steel to achieve the best effect. In order to avoid a large amount of cementite lamellae dissolving into the austenite during insulation, the insulation time should not be too long. The insulation time of the present invention adopts 120-150 minutes. Then it is cooled to 670-720℃ at a rate of 1℃ / min for isothermal transformation, kept at this temperature for 150-180min, and then taken out of the furnace for natural cooling. The entire spheroidizing treatment takes 5.5-6.5h, which is equivalent to the ordinary soft annealing process, but the soft annealing is a lamellar pearlite structure, which is not conducive to large deformation cold forming. Compared with the traditional spheroidizing annealing process (12-24h), the processing time is greatly shortened, which greatly reduces energy consumption.

[0027] Compared with the prior art, the advantages of the present invention are:

[0028] The structure of the fast spheroidized steel obtained by the present invention is point spheroidized body + ferrite or point spheroidized body + ferrite + a small amount of lamellar cementite (the lamellar cementite accounts for less than 10% of the total amount of cementite and is dispersed).

[0029] The fast spheroidizing medium carbon steel bar of the present invention realizes the spheroidization of the structure, reduces the hardness of the steel, greatly improves the cold forming performance, and is conducive to cold forging of complex parts through specific chemical composition design, low temperature heating, controlled rolling and controlled cooling, ultra-low temperature final rolling, fast spheroidization and other processes. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 The microstructure of a rapidly spheroidized medium carbon steel bar in Example 1 of the present invention.

[0031] Figure 2 This is the microstructure of a rapidly spheroidized medium carbon steel bar in Example 2 of the present invention.

[0032] Figure 3 This is the microstructure of a rapidly spheroidized medium carbon steel bar in Example 3 of the present invention. DETAILED DESCRIPTION

[0033] The present invention will be further described below with reference to examples.

[0034] Examples 1-3

[0035] A fast spheroidized medium carbon steel bar in Example 1-3 of the present invention is manufactured according to the following process:

[0036] 1) Smelting: After smelting in a 100-ton steelmaking furnace (converter or electric furnace), refining is carried out outside the furnace, and the molten steel is then vacuum degassed. The mass percentage of each chemical element is strictly controlled as required.

[0037] 2) Continuous casting: Continuously cast into 240mm×240mm square billets, control the tundish superheat at 10-25℃, strengthen the water cooling of the crystallizer to ensure the thickness of the equiaxed fine grain layer of the billet. Advanced end electromagnetic stirring and continuous casting light pressure advanced equipment and processes are used during continuous casting. The chemical composition percentage of the resulting continuous casting billet is shown in Table 1 below:

[0038] Table 1. (wt%, the balance is Fe and other unavoidable impurity elements)

[0039]

[0040] 3) Heating: The billet is heated in a walking beam heating furnace, including a preheating section, a heating section, and a soaking section. The temperature of the preheating section is controlled at 650-750°C, the temperature of the heating section is controlled at 850-950°C, the temperature of the soaking section is controlled at 950-1020°C, and the total heating time is more than 180 minutes.

[0041] 4) Hot rolling: The starting rolling temperature is 880-950℃, the final rolling temperature is 710-790℃, and ultra-low temperature rolling is performed in the two-phase region to ensure that a large amount of distortion energy is stored inside the hot-rolled steel, which is beneficial to the subsequent melting and dissolution of cementite.

[0042] 5) Rapid spheroidization: After hot rolling, it is quickly put into the annealing furnace for 120-150 min at 710-760 ° C, then cooled to 670-720 ° C at a rate of 1 ° C / min, kept for 150-180 min, and then taken out of the furnace and air-cooled to room temperature.

[0043] The specific process parameters of a fast spheroidizing medium carbon steel bar in the above embodiments are shown in Table 2 below:

[0044] Table 2. (Important process parameters such as continuous casting, heating, rolling, spheroidization, etc.)

[0045]

[0046] The rapid spheroidizing medium carbon steel bars in Examples 1-3 were subjected to relevant tests, and the measured mechanical properties are shown in Table 3 below:

[0047]

[0048] As can be seen from Table 3, the hardness in the embodiment meets the target requirement of ≤160HBW. After rapid spheroidization, the tensile strength is 545-582 MPa, and the elongation after rapid spheroidization is 32-35.5%, which is significantly improved compared with the hot rolled state. There is no cracking phenomenon during 80% cold heading. This shows that the steel in the embodiment has good plasticity and structure, which is very conducive to cold forming processing.

[0049] Figure 1-3 The microstructure of the rapidly spheroidized medium carbon steel bar in Examples 1-3 is shown. Figure 1-3 It can be seen that the microstructure of the steel is point spheroidized body + ferrite + a small amount of lamellar cementite. The proportion of lamellar cementite is less than 10%, which is very conducive to large deformation cold forming processing.

[0050] Although the preferred embodiments of the present invention are described in detail above, it should be clearly understood that various modifications and variations are possible for those skilled in the art. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for manufacturing a fast spheroidized medium carbon steel bar, characterized in that: The chemical composition of the steel is C: 0.30-0.60%, Si: ≤0.40%, Mn: 0.50-0.90%, P: ≤0.020%, S: ≤0.030%, Cr: ≤0.30%, Cu, Ni ≤0.25% by mass percentage; the balance is Fe and unavoidable impurity elements; The method comprises the following steps: 1) Smelting To smelt molten steel that meets the chemical composition of steel, when smelting in a converter or electric furnace, it is necessary to quickly make slag to increase slag fluidity, and at the same time control the furnace temperature ≤ 1670℃, control the purity of the molten steel, and reduce large particle inclusions in the steel; 2) Continuous casting During continuous casting, the purity of steel is further controlled, the liquid level fluctuation is controlled to be ≤±5mm, the superheat is controlled at the middle lower limit, the target is 10~25℃, and the cooling water volume of the crystallizer is controlled at the middle upper limit; 3) Heating Heat the billet in a walking beam heating furnace, control the temperature of the preheating section at 650-750°C, the temperature of the heating section at 850-950°C, and the temperature of the soaking section at 950-1020°C, so that the billet is fully and evenly heated, and the total heating time is more than 180 minutes; 4) Rolling After the billet leaves the heating furnace, it is descaled with high-pressure water. After descaling, the rolling temperature is 880-950℃, and the final rolling temperature is controlled between 710-790℃ to achieve final rolling in the two-phase zone. After rolling, the steel is quickly put on the cooling bed and sheared or cut off. The rolling process uses 25 continuous rolling mills, which are 6 roughing mills + 10 intermediate rolling mills + 4 pre-finishing mills + 5 finishing mills. After intermediate rolling, the 1# water tank is used for spray cooling, after pre-finishing rolling, the 2# and 3# water tanks are used, and after three-roller finishing rolling, the 4# water tank is used to perform strong and weak alternating cooling of the steel. The 1# water tank is opened at 45-85%, and the cooling water opening of the 2# and 3# water tanks is 5-15%. The temperature of the incoming material entering the finishing mill is controlled at 730-840℃, and two-phase rolling is performed. The cooling water opening of the 4# water tank is 40-60%, so that the temperature of the material on the cooling bed is 720-800℃. 5) Rapid spherification After the steel is bundled off the production line, it is quickly sent into the annealing furnace for rapid spheroidizing annealing treatment. The entire spheroidizing treatment time is 5.5 to 6.5 hours.

2. The method according to claim 1, characterized in that: The non-cracking limit upsetting rate of the steel is ≥80%, the elongation of the steel is >30%; the reference hardness of the steel is ≤160HB.

3. The method according to claim 1, characterized in that: The structure of the steel is point-shaped spheroidized body + ferrite or point-shaped spheroidized body + ferrite + a small amount of lamellar cementite, wherein the lamellar cementite accounts for less than 10% of the total cementite and is dispersed.

4. The method according to claim 1, characterized in that: The rapid spheroidization process is specifically as follows: the steel bars off the production line are directly loaded into the annealing furnace for insulation at a temperature of 710-760°C for a time of 120-150 minutes. After the insulation is completed, they are cooled to 670-720°C at a speed of 1°C / min for isothermal transformation. After insulation for 150-180 minutes, they are taken out of the furnace for natural cooling.

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