The delivery state of
seamless steel pipe (smls) refers to the last process of seamless steel pipe before leaving the factory. This process is of great significance. It determines the performance and quality of seamless steel pipe. Usually, the delivery state of seamless steel pipe is divided into hot rolled state, cold drawn (rolled) state, normalized state, annealed state, high temperature tempered state and solution treatment state. Their characteristics are as follows:
1. Hot rolled state
The seamless steel pipe is no longer subjected to special heat treatment after hot rolling or forging, and it is delivered directly after cooling, which is called hot rolling or hot rolling state.
The termination temperature of hot-rolled seamless steel pipe is generally 800~900°C, and then it is generally cooled naturally in the air, so the hot-rolled (forging) state is equivalent to normalizing treatment. The difference is that because the termination temperature of hot rolling (forging) varies from high to low, it is not as strict as the normalizing heating temperature control, so the microstructure and performance of the seamless pipe fluctuate more than normalizing. At present, many iron and steel enterprises adopt controlled rolling. Because the temperature control of the final rolling is very strict, and forced cooling measures are taken after the final rolling, the crystal material of the steel is refined, and the seamless steel pipes delivered have higher comprehensive performance. This is the reason why the twist-free cold and hot-rolled wire rod has better performance than ordinary hot-rolled wire rod. The seamless steel pipe delivered in the hot-rolled state has a certain corrosion resistance because the surface is covered with a layer of oxide scale, and the storage, transportation and storage requirements are not as strict as the seamless steel pipe delivered in the cold-drawn (rolled) state. In general, large-diameter or thick-walled seamless steel pipes can be stored in the open yard.
2. Cold drawn (rolled) state
The seamless steel pipe formed by cold drawing, cold rolling, etc., without any heat treatment, is directly delivered, which is called cold drawing or cold rolling state. Compared with the hot-rolled (forged) state, the seamless steel pipe in the cold-drawn (rolled) state has high dimensional accuracy, good surface quality, small surface roughness value, and high mechanical properties.
Since the surface of the seamless steel pipe delivered in the cold-drawn (rolled) state is not covered by scale, and there is a large internal stress, it is very easy to be corroded or rusted. There is a comparison
Strict requirements generally need to be kept in the warehouse, and attention should be paid to the temperature and humidity control in the warehouse.
3. Normalized state
The seamless steel pipe is subjected to normalizing heat treatment before leaving the factory, and this delivery state is called the normalizing state.
Since the normalizing heating temperature (Ac3+ 30~50°C for hypoeutectoid steel and Accm+ 30~50°C for hypereutectoid steel) is strictly controlled than the hot rolling termination temperature, the microstructure and properties of seamless steel pipes are uniform.
Compared with the steel in the annealed state, since the normalizing heating temperature is strictly controlled than the hot rolling termination temperature, the structure and properties of the seamless steel pipe are uniform. Compared with the seamless steel pipe in the annealed state, due to the faster cooling rate of normalizing, the number of pearlite in the structure of the seamless steel pipe increases, the pearlite layer and the grain of the steel are refined, so it has higher comprehensive mechanical properties. And it is beneficial to improve the Widmanstatten structure of low carbon steel and the cementite network of hypereutectoid steel, which can prepare the structure for further heat treatment of finished products. Seamless steel pipes are often delivered in the normalizing state.
4. Annealed state
The seamless steel pipe is annealed and heat-treated before leaving the factory. This delivery state is called the annealed state.
The purpose of annealing is mainly to eliminate and improve the organizational defects and internal stress left over from the previous process, and to make preparations for the structure and performance of the subsequent process. Alloy structural steel and ferritic stainless heat-resistant steel seamless steel pipe are usually delivered in the annealed state.
5. High temperature tempered state
The seamless steel pipe is treated with high temperature tempering before leaving the factory. This delivery state is called the high temperature tempering state. The high temperature tempering temperature is high, which is conducive to completely eliminating internal stress and improving the plasticity and toughness of the seamless steel pipe.
Carbon structural steel pipes, alloy steel pipes, and structural steels with guaranteed hardenability can be delivered in a high-temperature tempered state. Some martensitic high-strength
stainless steel seamless pipes, due to their high hardenability and the strengthening effect of alloying elements, are often subjected to a high-temperature tempering after quenching (or tempering) to properly aggregate carbides in the steel. , a tempered sorbite structure with relatively coarse carbide particles (similar to the spheroidizing annealed structure) is obtained. Therefore, the steel in this delivery state has good machinability.
6. Solution treatment state
The steel has been solution treated before leaving the factory, and this delivery state is called the solution treatment state. This state is mainly suitable for the treatment of austenitic stainless steel before leaving the factory. Through solution treatment, a single-phase austenite structure is obtained to improve the toughness and plasticity of the steel, to create conditions for further cold working (cold rolling or cold drawing), and to prepare for further precipitation hardening.
