Online normalization process of steel pipe

Online normalization is a heat treatment process developed in recent years, namely deformation and normalization. It is a steel pipe after rolling the continuous rolling pipe, cooled to a recrystallization temperature on a cold bed, and then reheated the furnace. After heating the steel pipe to a temperature of Ac3 or above in the furnace, it is kept for a period of time to make the metallographic phase of the steel. The structure is transformed into austenite, and then discharged, and the austenite structure is transformed into pearlite by the sizing machine or the reduction machine after air reduction or air cooling, fog cooling, etc., thereby realizing the performance of the steel pipe, which is modern control. Rolling scheme.

Online normalization performance requirements:

While achieving the strength requirements, it achieves excellent matching of toughness and strength, has good weldability, and has excellent low temperature toughness.

The purpose of online normalization:

• (1) making the structure of the steel uniform and grain refining;
• (2) Improve the mechanical properties of some steel grades;
• (3) Improve the metallographic structure and properties of low carbon steel and low alloy steel to create conditions for the diffusion of alloying elements.

The ferrite plus pearlite structure is exhibited under a metallographic microscope. If it is a microalloyed steel, a precipitated phase and a second phase particle can be observed under a transmission electron microscope.

On-line normalization is to insert a normalizing process in the middle of the rolling process, and use part of the rolling residual heat to shorten the normalizing heating time. Microalloyed on-line normalized steel replaces off-line normalizing or modulating steel, reducing costs associated with heat treatment, finishing, energy, decarburization, and iron loss (oxidation). In short, online normalization not only simplifies the process but also saves energy.

Compared with quenched and tempered steel, on-line normalized steel has better cutting performance because the ferrite pearlite structure has better cutting performance than the tempered sorbite structure.

Online process description:

The rolled steel is cooled to below Ar1, and all of the austenite is transformed into a ferrite pearlite structure, that is, reheated, and all transformed into austenite, and air-cooled to obtain a relatively fine ferrite pearlite structure.

According to the size of the workpiece, determine the cooling time before entering the reheating furnace, ensure that the temperature is below the Ar1 temperature, determine the time of the workpiece in the reheating furnace, ensure that the alloy dissolves, the microstructure is transformed into austenite, and homogenized. Set the reheater temperature.

In order to dissolve all the gold elements into the austenite and homogenize the austenite, the workpiece in the furnace should be kept in the specified heating temperature range for a suitable period of time, the length of the workpiece and the effective thickness of the workpiece, the steel grade, and the loading. The furnace method, the amount of furnace charge, the temperature of the furnace, the performance of the furnace and the degree of sealing are related.

Different cooling methods mean different cooling speeds of the workpiece, such as stack cooling, side-by-side cooling, workpiece spacing, cold, single cooling, and cooling rate from small to large. Generally, the grain size decreases, the percentage of pearlite increases, and pearlescence The spacing of the body sheets is reduced, resulting in an increase in strength and impact toughness.

The holding time of the workpiece in the reheating furnace is an indispensable control parameter. The holding time is insufficient, the strength is not up to the requirement, the holding time is too long, energy is wasted, the production efficiency is lowered, and the surface of the workpiece may be decarburized.

Before the online normalizing steel enters the reheating furnace, it undergoes a staged rolling process, and each part of the workpiece must complete the transformation of the austenite to the entire microstructure of the ferrite pearlite, so the part where the workpiece is cooled relatively slowly is also Must be cold below Ar1.

The control of the temperature of the reheating furnace is critical to the performance of the steel. The reheating furnace heats the workpiece, not only to ensure the necessary tissue transformation and diffusion, but also to dissolve all the alloying elements in the austenite. The temperature is low, the strength is insufficient, the temperature is high, the toughness is lowered, and the temperature is too high and the strength is also lowered.

Microalloying elements are added to steel for two purposes, namely grain refinement and precipitation strengthening. Both effects are caused by the precipitation of microalloyed carbides, nitrides or carbonitrides.

Online normalization causes bending:

In the (reheating furnace) cold bed bending, the steel tube itself is inconsistent in structural transformation, which is the cause of bending. Some steels not only obtain ferrite pearlite at a certain cooling rate, but also produce a part of bainite structure, bainite. The tissue stress is large, and the steel pipe is partially cooled first, and the portion where the bainite is first generated may be bent.

Bending occurs after entering the reheating furnace. This is because the inlet of the steel tube in the reheating furnace is difficult to maintain the temperature of other parts. When the temperature of the tube is too low, the higher furnace temperature in the furnace will increase the thermal stress of the tube sharply. When the furnace condition is not ideal, the tube is not ideal. The different parts will have a large difference in thermal stress due to the inconsistent temperature inside the furnace. This causes the tube to bend.

Source: China Steel Pipes Manufacturer - Yaang Pipe Industry Co., Limited (www.yaang.com)

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