A process for controlled rolling and controlled cooling of high carbon alloy tool steel large coil without annealing

Abstract

The present application belongs to the field of tool steel rolling technology, and particularly relates to a controlled rolling and controlled cooling process of high-carbon alloy tool steel large coil without annealing. The production of high-carbon alloy tool steel large coil without annealing takes continuous casting billets as raw materials, adopts a regenerative heating furnace for heating, completes rough and medium rolling and pre-precision rolling through a flat-stand alternation type two-roller mill, uses a three-roller mill to finish rolling to the finished product specification, then uses a hot coiler to roll and collect, and finally controls cooling through a heat preservation tunnel. Through the innovative application of the controlled rolling and controlled cooling process of the large coil rolling line, the present application makes the hot-rolled high-carbon alloy tool steel large coil obtain a pearlite structure, greatly improves the plasticity, and at the same time ensures that the decarburization layer depth meets the requirements, satisfies the downstream users to draw hexagonal steel wire and cold bending forming without annealing, and realizes the high-carbon alloy tool steel large coil without annealing.

Description

Technical Field

[0001] This invention belongs to the field of tool steel rolling technology, and relates to a controlled rolling and controlled cooling process for large coils of high-carbon alloy tool steel without annealing. Background Technology

[0002] Tool steel is used to manufacture tools such as wrenches, knives, drill bits, taps, measuring tools, and saw blades. Its special applications require high hardness and wear resistance while maintaining a certain level of toughness. Based on different chemical compositions, tool steel is generally divided into three main categories: carbon tool steel, alloy tool steel, and high-speed tool steel, with alloy tool steel having the widest range of applications. Alloy tool steel typically refers to a type of steel made by adding alloying elements such as chromium, molybdenum, tungsten, vanadium, and niobium to carbon steel to improve its hardness, wear resistance, heat resistance, toughness, and hardenability. Currently, most commonly used alloy tool steels in China are medium-carbon or high-carbon steels, with carbon content mainly concentrated between 0.40% and 0.80%. Those with a carbon content of 0.60% or higher are generally referred to as high-carbon alloy tool steel. Due to its higher hardness and wear resistance, as well as better hardenability, high-carbon alloy tool steel has become the mainstream material for manufacturing high-end hexagonal wrenches, while hot-rolled bars and wires have become the main product forms of tool steel.

[0003] High-carbon alloy tool steels, due to their high carbon content, also have high silicon and chromium content to improve hardness and wear resistance. Their chemical composition dictates that their hot-rolled metallographic structure is prone to low-temperature structures such as martensite and bainite, reducing material plasticity. Furthermore, the high carbon and silicon content significantly increases the steel's decarburization sensitivity. Decarburization easily occurs during the heating and cooling processes of bar and wire rod rolling, and tool steels have stringent requirements for decarburization. Therefore, controlling the metallographic structure and decarburization has always been a challenge for steel mills rolling tool steel bars and wire rods. This problem also limits the processing technology of downstream users. Because hot-rolled bars and wire rods have poor plasticity, direct drawing or cold bending can cause cracking or fracture. Therefore, downstream users must perform 1-2 annealing treatments before processing, but the annealing process also easily leads to decarburization. CN202010267215.5 describes a controlled rolling and cooling process for reducing the decarburization layer depth of high-carbon alloy tool steel wire rod. Through a scientific combination of controlled rolling and controlled cooling, it effectively suppresses decarburization in the two-phase region. However, the entire process involves low-temperature heating, which easily leads to the presence of martensite in the microstructure, causing material fracture during drawing and cold bending. This makes it unsuitable for applications requiring non-annealing drawing of hexagonal steel wire and cold bending, especially for materials with higher Cr and Si content and more severe elemental segregation. Furthermore, the finishing mills used are all two-roll mills, and the applicable specifications produced using a two-roll mill + wire drawing machine currently do not exceed Φ26.0mm. The post-rolling controlled cooling uses a wire drawing + Stellmore controlled cooling line, employing a combination of rapid cooling with a water mist fan followed by slow cooling under an insulation cover. This primarily addresses decarburization and reduces martensite precipitation, but it cannot achieve the desired pearlite effect. CN201911311641.8 discloses a method for producing hot-rolled high-carbon steel, the product being hot-rolled steel plate of high-carbon steel grade. Under these product morphology and steel grade conditions, pearlite can be obtained using conventional rolling processes, unlike the production process for hot-rolled large coils of high-carbon alloy steel. The difference in product morphology and steel grade leads to significantly different levels of difficulty in obtaining pearlite microstructure. Under the production process conditions of hot-rolled large coils of high-carbon alloy steel, the conventional process inevitably results in the presence of martensite microstructure, making non-annealed drawing and cold bending impossible. In the field of high-carbon alloy tool steel rolling, obtaining pearlite microstructure is a technical challenge.

[0004] In summary, developing a suitable controlled rolling and cooling process to improve the metallographic structure and decarburization of hot-rolled bars and wires of high-carbon alloy tool steel, enhance material plasticity, eliminate the annealing process to reduce the risk of decarburization, and meet the processing needs of downstream users for non-annealed hexagonal steel wire drawing and cold bending is an urgent problem for steel mills to solve. Summary of the Invention

[0005] To address the aforementioned issues, this invention utilizes an innovative controlled rolling and cooling process in large coil rolling lines to achieve a pearlitic microstructure in hot-rolled high-carbon alloy tool steel coils, significantly enhancing their plasticity while ensuring the required decarburization layer depth. This satisfies the downstream user's need for non-annealed hexagonal wire drawing and cold bending, enabling the anneal-free use of high-carbon alloy tool steel coils.

[0006] A controlled rolling and controlled cooling process for large coils of high-carbon alloy tool steel without annealing is described below, along with the controlled rolling and controlled cooling process and chemical composition.

[0007] Controlled rolling and controlled cooling process:

[0008] The rolling process is as follows: continuous casting billet heating—roughing and intermediate rolling—pre-finishing rolling—finishing rolling—hot coiling—insulated tunnel cooling—bundling.

[0009] Heating of continuously cast billets:

[0010] The continuously cast billet is heated in a regenerative heating furnace. The billet cross-section is 160mm*160mm. The heating furnace is divided into a preheating section, a second preheating section, a heating section, and a soaking section. The heating temperature is between 900 and 1200℃, with the first preheating section at 900–950℃, the second preheating section at 1000–1050℃, the heating section at 1150–1200℃, and the soaking section at 1000–1050℃. The high-temperature heating of 1150–1200℃ in the heating section is beneficial for obtaining a pearlitic microstructure. This heating scheme ensures diffusion effect, reduces decarburization, and achieves a relatively low temperature after high-temperature diffusion, providing conditions for subsequent rolling temperature.

[0011] Roughing, intermediate rolling and pre-finishing rolling:

[0012] The roughing, intermediate, and pre-finishing mills employ alternating horizontal and vertical two-roll mills to roll heated steel billets into intermediate pieces. The roughing and intermediate mills consist of 14 stands, and the pre-finishing mill has 4 stands. The rolling temperature for both mills is between 900 and 980℃. During the basic deformation stages of roughing, intermediate, and pre-finishing, only the rolling temperature needs to meet the requirements throughout the entire process.

[0013] Finishing rolling:

[0014] The finishing mill uses five three-high stands to roll intermediate workpieces to finished specifications. The rolling temperature of the finishing mill is 800–840℃. Cooling water is turned on between the five three-high stands to suppress the temperature rise of the workpieces, reduce the temperature difference between the workpieces, and ensure controlled rolling. The applicable specifications for large coils are Φ25.0–42.0mm.

[0015] Hot rolling:

[0016] After rolling, the large coils are collected using a hot coiler at a temperature of 700–730℃, and then immediately transferred to an insulated tunnel for controlled cooling. This minimizes the time spent transferring the coils from hot coil to the insulated tunnel and ensures the temperature drop of the large coils is ≤50℃. The coiling temperature directly affects the subsequent microstructure transformation of the steel.

[0017] Insulated tunnel cooling control:

[0018] The temperature of the large coils entering the insulation tunnel is controlled at 650–680℃. The large coils enter the insulation tunnel at intervals, meaning there is a break between each large coil. The insulation hood of the insulation tunnel is fully open, and the stepping rhythm of the insulation tunnel is adjusted to ensure that the large coils spend 30–40 minutes in the insulation tunnel, with a cooling rate of 0.20–0.22℃ / s. The temperature of the large coils exiting the insulation hood is controlled below 200℃. This invention employs hot coiling + insulation tunnel for post-rolling controlled cooling. The controlled cooling scheme is slow cooling throughout, increasing the insulation and cooling time, mainly addressing the phase transformation problem of the steel, enabling the steel to obtain a pearlitic structure, avoiding martensite precipitation, and effectively suppressing decarburization while achieving the microstructure control objective.

[0019] Bundling:

[0020] After the controlled cooling is completed, the large coils are transferred to a baler for baling. The baling temperature is <100℃. A steel strip baler is used, and the baling pressure is 10-15 tons.

[0021] Chemical composition:

[0022] The main chemical composition range of the non-annealing high-carbon alloy tool steel large coils described in this invention, by weight percentage, is: C: 0.69-0.75%, Si: 1.40-1.60%, Mn: 0.50-0.70%, Cr: 1.00-1.20%, Ni: 0.10-0.20%, V: 0.15-0.20%, Al: 0.015-0.030%, P≤0.025%, S≤0.025%, Cu≤0.20%, with the remainder being iron and unavoidable impurities.

[0023] The high-carbon alloy tool steel of this invention has a carbon content of 0.7%, a Si content exceeding 1.40%, and a Cr content exceeding 1.0%. Additionally, Ni and V, two microalloying elements, are added, along with a small amount of Al. This composition design effectively improves the material's hardness and wear resistance, and achieves high hardenability, ensuring downstream processing requirements while meeting the product requirements of high-end hexagonal wrenches. Other elements are controlled as residual or harmful elements.

[0024] This invention, through the innovative application of controlled rolling and controlled cooling process in large coil rolling lines, enables hot-rolled high-carbon alloy tool steel large coils to obtain a pearlitic structure, significantly improving plasticity while ensuring that the decarburization layer depth meets the requirements. This satisfies the downstream users' need for non-annealed drawing of hexagonal steel wire and cold bending, realizing the anneal-free use of high-carbon alloy tool steel large coils. Attached Figure Description

[0025] Figure 1 The metallographic structure diagram of the wire rod produced in Example 1.

[0026] Figure 2 The metallographic structure diagram of the wire rod produced in Example 2.

[0027] Figure 3 The metallographic structure of the wire rod produced for Comparative Example 1 is shown in the image.

[0028] Figure 4 The metallographic structure of the wire rod produced for Comparative Example 2 is shown in the image.

[0029] Figure 5 The metallographic structure of the wire rod produced in Comparative Example 3 is shown in the diagram. Detailed Implementation

[0030] The specific chemical composition of the high-carbon alloy tool steel coil 70SiCrV (which does not require annealing) is as follows:

[0031] Wt, %

[0032] C Si Mn Cr Ni V Al P, S Cu 0.69-0.75 1.40-1.60 0.50-0.70 1.00-1.20 0.10-0.20 0.15-0.20 0.015-0.030 ≤0.025 ≤0.20

[0033] The wire rod manufacturing process is as follows: converter primary refining—RH vacuum refining—continuous casting of steel billets—bill rolling—wire rod rolling—controlled cooling of wire rod.

[0034] The applicable specifications for large coils of this invention are Φ25.0~42.0mm, and the embodiment uses Φ26.0mm as an example.

[0035] Example 1

[0036] 1. Chemical composition

[0037] The chemical composition, by weight percentage, is: C: 0.70%, Si: 1.45%, Mn: 0.68%, Cr: 1.09%, Ni: 0.14%, V: 0.16%, Al: 0.016%, P: 0.010%, S: 0.006%, Cu: 0.05%, with the remainder being iron and unavoidable impurities.

[0038] 2. Controlled rolling and controlled cooling process:

[0039] The rolling process is as follows: continuous casting billet heating—roughing and intermediate rolling—pre-finishing rolling—finishing rolling—hot coiling—insulated tunnel cooling—bundling.

[0040] Heating of continuously cast billets:

[0041] The continuously cast billet is heated in a regenerative heating furnace. The cross-section of the continuously cast billet is 160mm*160mm. The heating furnace is divided into a preheating section 1, a preheating section 2, a heating section, and a soaking section. The heating temperature is 905~1190℃, of which the temperature of the preheating section 1 is 905~940℃, the temperature of the preheating section 2 is 1010~1038℃, the temperature of the heating section is 1160~1190℃, and the temperature of the soaking section is 1015~1031℃.

[0042] Roughing, intermediate rolling and pre-finishing rolling:

[0043] The roughing, intermediate, and pre-finishing mills use alternating horizontal and vertical two-roll mills to roll heated steel billets into intermediate rolled products. The roughing and intermediate mills have 14 stands, and the pre-finishing mills have 4 stands. The rolling temperature of the roughing, intermediate, and pre-finishing mills is between 907 and 969°C.

[0044] Finishing rolling:

[0045] The finishing mill uses five three-roll stands to roll intermediate workpieces to finished specifications. The rolling temperature of the finishing mill is between 802 and 832°C. Cooling water is turned on between the five three-roll stands to suppress the temperature rise of the workpieces, reduce the temperature difference between the workpieces, and ensure controlled rolling.

[0046] Hot rolling:

[0047] After rolling, the large coils are collected using a hot coiler at a temperature of 705–728°C, and then immediately transferred to an insulated tunnel for controlled cooling. This reduces the operation time from hot coiling to the insulated tunnel and controls the temperature drop of the large coils to ≤50°C.

[0048] Insulated tunnel cooling control:

[0049] The temperature of the large coil entering the insulation tunnel is controlled at 661-670℃. The large coil enters the insulation tunnel in an intermittent manner, that is, a break step is set between each large coil. The insulation cover of the insulation tunnel is fully opened, and the stepping rhythm of the insulation tunnel is adjusted to ensure that the large coil spends 35 minutes in the insulation tunnel, with a cooling rate of 0.20℃ / s. The temperature of the large coil exiting the insulation cover is controlled at 180℃.

[0050] Bundling:

[0051] After the controlled cooling process, the large coils are transferred to a baler for baling. The baling temperature is <100℃. A steel strap baler is used with a baling pressure of 12 tons.

[0052] Example 2

[0053] 1. Chemical composition

[0054] The chemical composition, by weight percentage, is: C: 0.75%, Si: 1.58%, Mn: 0.55%, Cr: 1.15%, Ni: 0.18%, V: 0.19%, Al: 0.022%, P: 0.012%, S: 0.010%, Cu: 0.09%, with the remainder being iron and unavoidable impurities.

[0055] 2. Controlled rolling and controlled cooling process:

[0056] The rolling process is as follows: continuous casting billet heating—roughing and intermediate rolling—pre-finishing rolling—finishing rolling—hot coiling—insulated tunnel cooling—bundling.

[0057] Heating of continuously cast billets:

[0058] The continuously cast billet is heated in a regenerative heating furnace. The cross-section of the continuously cast billet is 160mm*160mm. The heating furnace is divided into a preheating section 1, a preheating section 2, a heating section, and a soaking section. The heating temperature is 911~1198℃, of which the temperature of the preheating section 1 is 911~949℃, the temperature of the preheating section 2 is 1022~1046℃, the temperature of the heating section is 1158~1193℃, and the temperature of the soaking section is 1012~1044℃.

[0059] Roughing, intermediate rolling and pre-finishing rolling:

[0060] The roughing, intermediate, and pre-finishing mills use alternating horizontal and vertical two-roll mills to roll heated steel billets into intermediate rolled products. The roughing and intermediate mills have 14 stands, and the pre-finishing mills have 4 stands. The rolling temperature of the roughing, intermediate, and pre-finishing mills is between 911 and 972℃.

[0061] Finishing rolling:

[0062] The finishing mill uses five three-roll stands to roll intermediate workpieces to finished specifications. The rolling temperature of the finishing mill is between 808 and 833℃. Cooling water is turned on between the five three-roll stands to suppress the temperature rise of the workpieces, reduce the temperature difference between the workpieces, and ensure controlled rolling.

[0063] Hot rolling:

[0064] After rolling, the large coils are collected using a hot coiler at a temperature of 703–725°C, and then immediately transferred to an insulated tunnel for controlled cooling. This reduces the operation time from hot coiling to the insulated tunnel and controls the temperature drop of the large coils to ≤50°C.

[0065] Insulated tunnel cooling control:

[0066] The temperature of the large coil entering the insulation tunnel is controlled at 659-668℃. The large coil enters the insulation tunnel in an intermittent manner, that is, a break step is set between each large coil. The insulation cover of the insulation tunnel is fully opened, and the stepping rhythm of the insulation tunnel is adjusted to ensure that the large coil spends 36 minutes in the insulation tunnel, with a cooling rate of 0.21℃ / s. The temperature of the large coil exiting the insulation cover is controlled at 158℃.

[0067] Bundling:

[0068] After the controlled cooling process, the large coils are transferred to a baler for baling. The baling temperature is <100℃. A steel strap baler is used with a baling pressure of 13 tons.

[0069] Comparative Example 1

[0070] In Example 1, step 2, the hot rolling temperature is replaced with 900-930°C. Due to the adjustment of the rolling temperature, the temperature of the large coil entering the insulation tunnel becomes 832-870°C, and the temperature of the large coil exiting the insulation cover becomes 405°C. Other conditions are the same as in Example 1.

[0071] Comparative Example 2

[0072] Replace the cooling scheme of step 2 in Example 1 with "the temperature of the large coil entering the insulation tunnel is controlled at 661-670℃, the large coil enters the insulation tunnel in a continuous unloading manner, that is, there should be no gaps between each large coil, the insulation cover of the insulation tunnel is completely closed, the stepping rhythm of the insulation tunnel is adjusted to ensure that the time of the large coil in the insulation tunnel is 35 minutes, the cooling rate is 0.10℃ / s, and the temperature of the large coil exiting the insulation cover is controlled at 455℃", other conditions are the same as in Example 1.

[0073] Comparative Example 3

[0074] Replace the hot rolling and thermal insulation tunnel cooling scheme in step 2 of Example 1 with "cooling after rolling using a wire spinning + Stellmore cooling line, wire spinning temperature 705~728℃, cooling rate 0.2℃ / s, cooling time 19min, and cooling end temperature 492℃", with other conditions the same as in Example 1.

[0075] The embodiments of this invention and the comparative examples produce high-carbon alloy tool steel coils / wires (Comparative Example 3 produces wire rods). The embodiments use a Φ26.0mm specification as an example. Using the coils / wires as raw materials, the steel undergoes a processing procedure of "pickling—phosphating—drawing external hexagonal steel wire—cold bending 90°—turning—quenching and tempering heat treatment—surface treatment" to produce hexagonal wrenches. The comparison of inspection results and usage results of the high-carbon alloy tool steel coils / wires produced using different methods and the processed hexagonal wrenches is shown in Table 1 below:

[0076] Table 1

[0077]

[0078] The metallographic structures of high-carbon alloy tool steel coils / bars produced using different methods are as follows: Figures 1-5 .

Claims

1. A method of controlled rolling and controlled cooling of a high carbon alloy tool steel large coil without annealing, characterized in that: The rolling process is continuous casting billet heating-rough intermediate rolling-pre-precision rolling-precision rolling-hot coiling-tunnel temperature control-cooling-bundling; The continuous casting billet is heated by a regenerative heating furnace, the cross section of the continuous casting billet is 160mm*160mm, and the heating furnace is divided into a preheating first section, a preheating second section, a heating section and a soaking section; the heating temperature is 900-1200℃, wherein the preheating first section temperature is 900-950℃, the preheating second section temperature is 1000-1050℃, the heating section temperature is 1150-1200℃, and the soaking section temperature is 1000-1050℃; The rough intermediate rolling and pre-precision rolling mill group adopts a flat-stand alternating two-roller mill to roll the heated steel billet into an intermediate rolling piece; the rolling temperature of the rough intermediate rolling and pre-precision rolling mill group is 900-980℃; The precision rolling mill group adopts a five-roller three-roller mill to roll the intermediate rolling piece into a finished product specification, and the rolling temperature of the precision rolling mill group is 800-840℃; After the rolling is completed, the large coil is coiled and collected by a hot coiling machine, and the coiling temperature is 700-730℃, and then it is immediately transferred to the temperature control cooling tunnel; The temperature of the large coil entering the temperature control cooling tunnel is controlled to be 650-680℃, the large coil enters the temperature control cooling tunnel in an interval offline manner, i.e. an empty step is arranged between each large coil, the temperature control cooling tunnel cover is fully opened, the temperature control cooling tunnel step pace is adjusted to ensure that the time of the large coil in the temperature control cooling tunnel is 30-40min, the cooling rate is 0.20-0.22℃ / s, and the temperature of the large coil out of the temperature control cooling tunnel cover is controlled to be <200℃; After the temperature control cooling is completed, the large coil is transferred to a bundling machine for bundling, the bundling temperature is <100℃, a steel belt bundling machine is used, and the bundling pressure is 10-15 tons.

2. The process for controlled rolling and controlled cooling of high carbon alloy tool steel large coil without annealing as claimed in claim 1 wherein: The main chemical component range of the high-carbon alloy tool steel large coil for non-annealing use, in terms of weight percentage, is C: 0.69-0.75%, Si: 1.40-1.60%, Mn: 0.50-0.70%, Cr: 1.00-1.20%, Ni: 0.10-0.20%, V: 0.15-0.20%, Al: 0.015-0.030%, P≤0.025%, S≤0.025%, Cu≤0.20%, and the rest is iron and unavoidable impurities.

3. Use of the annealing-free high-carbon-alloyed tool steel large coil prepared according to the method of any one of claims 1-2, characterized in that, The metallurgical structure of the high-carbon alloy tool steel large coil for non-annealing use is pearlite, the decarburization layer depth is ≤0.10mm, and the processing application of non-annealing drawing hexagonal steel wire and cold bending forming is realized.

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