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Vacuum Heat Treatment Furnace

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Heat treatment process for wide and thick plates

Introduction to heat treatment process of wide and thick plates

Heat treatment uses different heating systems, holding times, and cooling rates to cause diffusion and recrystallization of the steel’s composition and structure. It can also promote or prevent the transformation and precipitation of its structure and composition, in order to achieve the required structure and structure. performance.

The purpose of steel plate heat treatment is to increase the strength of the steel plate, improve toughness and weldability, remove internal stress, prevent white spots, and reduce brittleness and surface hardness. When the hot rolling performance is unstable, heat treatment can also adjust its performance and save the losses caused by unqualified performance. Improper heat treatment will cause scratches on the lower surface of the steel plate, and uneven cooling will cause deformation of the steel plate and unevenness of the hardened steel plate. In addition, it will also increase energy consumption and cost.

The main heat treatment methods for medium-thick steel plates include normalizing, hardening and tempering, normalizing + controlled cooling, normalizing + tempering, tempering, annealing, direct hardening (DQ), and direct hardening +Tempering etc.

Among them, the normalizing plate with the largest processing volume, including normalizing + tempering, accounts for about 70% of all heat treatment products; followed by hardened and tempered plates, accounting for about 15%; others such as tempering account for 15%.

Normalizing

Normalizing, also called normalization or normalization, is a heat treatment process in which steel or steel parts are heated to an appropriate temperature above Ac3 (or Accm), kept for an appropriate period of time, and then cooled in the air to obtain a pearlite-like structure. Its purpose is to make the abnormal structures produced in the previous process (such as hypoeutectoid steel structural defects such as coarse ferrite grains, Widmanstatten structure, band structure, non-ferrite + pearlite structure products) through recrystallization , improve the uniform structure (for low carbon steel, it is fine equiaxed ferrite + uniformly distributed massive pearlite structure), thereby improving its mechanical properties and process performance.

Normalizing can be used as a preliminary heat treatment or as a final heat treatment. For structural steel of machined parts, normalizing is mostly used as a preparatory heat treatment to prepare the structure for subsequent cutting and final heat treatment; for low-carbon and low-alloy steel plates, normalizing is mostly used as a final heat treatment to make the steel plates have certain properties. The required organization, so that it has the required mechanical properties and process properties.

After normalizing treatment, the steel plate has fine grains, uniform carbide distribution, and good mechanical properties. The normalizing process can significantly improve the elongation, low-temperature impact toughness and cold bending properties of currently commonly used low (micro) alloy high-strength steel plates with strong carbonitride-forming elements such as Nb, V, and Ti.

However, it is worth noting that while normalizing improves the process performance of hot-rolled low-carbon and low-alloy steel plates, it often reduces the strength of the steel. The yield strength and tensile strength are generally reduced by 20 to 50MPa. For controlled rolling and controlled cooling steel plates, serious risks may occur. Reduced by 80~120MPa. Therefore, in order to ensure the delivery performance of the steel plate, the composition design of the normalized steel plate should be different from that of the general controlled rolling and controlled cooling steel plate. Solid solution strengthening elements such as C and Mn can be appropriately added to improve the strength (note that the carbon equivalent index does not exceed These two indicators are high), although the elongation or impact energy of the steel plate is reduced after hot rolling, it will be greatly improved after normalizing treatment.

Hardening and tempering (vacuum hardening + high temperature tempering)

Vacuum hardening is a heat treatment process in which steel is heated to above the critical point Ac1 or Ac3, kept warm, and then cooled at a rate greater than the critical cooling rate (Vc) to obtain a metastable martensite or lower bainite structure. . Purpose: Improve strength, hardness, and wear resistance.

As the temperature of vacuum hardened steel increases, its internal structure will undergo decomposition of martensite, transformation of retained austenite, transformation of carbides, changes in α phase state, and aggregation and growth of carbides, etc. The series of processes is called tempering. Tempering is divided into low-temperature tempering, medium-temperature tempering and high-temperature tempering according to temperature, and the tempered martensite, tempered troostite and tempered sorbite structures are obtained respectively. Purpose: Reduce or eliminate hardening stress, improve toughness and plasticity, and obtain an appropriate combination of hardness, strength, plasticity and toughness.

Hardening and tempering refers to hardening + high temperature tempering.

In recent years, production technologies such as super steel and ultra-low carbon bainitic steel have been booming. Many steel plates with a yield strength of more than 450MPa produced by traditional hardening and tempering treatment can be produced using ultra-fine grain and other technologies. , but for important structural steel plates such as pressure vessels, oil storage tanks, bridges, and military industries, many standards and users still require steel plates to be delivered in a hardened and tempered state. Therefore, a modern hardened and tempered steel plate production line is indispensable for improving the product quality of medium-thick steel plate plants.

Representative steel types for hardening and tempering include oil tank steel SPV490Q, 14MnNbq, mold steel P20, etc.

Normalizing + controlled cooling (+ tempering)

In order to prevent the strength of many steel plates from being significantly reduced after normalizing, Wuhan Iron and Steel Co., Ltd., Wuhan Iron and Steel Co., Ltd., Chongqing Iron and Steel Co., Ltd., Taiyuan Iron and Steel Co., Ltd., etc. have all developed a normalizing + controlled cooling process, using post-furnace hardening machines or simple water cooling facilities for weak water cooling after normalizing. (Water mist, etc.) process can better compensate for the strength loss of the steel plate. According to Taiyuan Steel’s experience, the strength of the steel plate can be increased by 10MPa. For steel types whose performance fluctuates greatly after controlled cooling or steel types that ASTM (American Society for Testing and Materials) stipulates that accelerated cooling after normalizing must be tempered, the normalizing + controlled cooling + tempering process can also be applied.

Normalizing + Tempering

It is widely used in steel plate processing, especially Cr-Mo steel, which mostly uses normalizing + high-temperature tempering.

Vacuum hardening + low temperature tempering

For steel types that require higher surface hardness, such as wear-resistant steel, hardening + low-temperature tempering is often used.