3.5 Ni seamless steel tube is of moderate strength at low temperature and good low temperature toughness, is widely applied to the working temperature of 45 ~ 101 ℃ in - oil, chemical industry, fertilizer and other industries of equipment manufacturing in low temperature. As the material requirements of the development of related industries not only confined to the basic requirements, such as mechanical property, welding performance of customers also puts forward the low temperature pipe H2S stress corrosion resistance requirements.

A famous domestic steel pipe manufacturing enterprises based on the new technical requirements, starting from the design of steel grade, for 3.5 Ni seamless steel pipe steel at low temperature, rolling, heat treatment process was carried out strict control, to ensure that the 3.5 Ni seamless steel tube to obtain a good strong toughness match at low temperature and H2S stress corrosion resistance. Based on the customer technical requirements of seamless steel tube at low temperature, to comprehensively consider the production organization, and the cost of steel pipe, seamless steel tube at low temperature of the component design ensures the product with proper strength, high toughness at low temperature, convenient heat treatment craft, excellent welding performance, the ability of resistance to hydrogen induced cracking.

Seamless steel tube steelmaking process at low temperature control measures are as follows:

(1) control C content. Match the electric furnace and refining is the effective means to realize accurate control C content, at the end of the C content, strict control of electric furnace smelting furnace in VD and RH refining outside the furnace tube diameter and the increase of argon flow rate and immersion on the vacuum takes off C in the form of oxygen requirements to ensure the precision of pipeline steel C content;

(2) reduce the content of P, S. Through the use of high quality steel scrap and the composition of appropriate refining slag to meet S need, through electric furnace and high basicity slag under the strict control of the steel slag to reduce P content in the steel.

(3) ensure that the Ca content in the steel and the weak stirring time, in order to strengthen the management of cohesion between continuous casting furnace times up and down, pouring and protection, can effectively reduce the non-metallic inclusions in steel.

Rolling process control. Because customers purchasing the low temperature of the specifications of the seamless steel pipe for Φ 60 ~ 720 mm x 4 ~ 10 mm, for product geometry with thin wall thickness precision will directly influence the customers in the construction site welding quality and efficiency, so the MPM unit adopts the PQF rolling, and adjust the tube billet heating temperature and time, reduce the eccentricity of the perforation process, reduce the uneven wall thickness.

Heat treatment process control. Seamless steel tube heat treatment is based on the reasonable temperature range, the temperature range by CCT curve of steel. For design of steel 840 ℃ austenitizing CCT curve, 3 min insulation test, the cooling speed was observed and metallographic organization relationship, to determine the production of heat treatment process.

According to the CCT curve of steel and equipment conditions, seamless steel tube by using normalized or best at low temperature tempering process, such production organization is easier. Due to the specification including Φ 60 mm x 4 mm long and thin tubes, if use water quenching and tempering process, prone to bending, therefore, by using normalized or normalizing + tempering process has more applicability.

Seamless steel tube heat treatment need to be considered at low temperature heating speed and time of heat preservation, avoid burning loss and the production of iron oxide, steel and austenitic grain size control to improve the steel pipe matching strong toughness and corrosion resistance.

Through the above process measures developed - 100 ℃ low temperature seamless steel pipe has the following features:

(1) the size range is wide, from 60 mm to 720 mm diameter;

(2) has excellent low temperature toughness and corrosion resistance, corrosion resistance that can pass strict evaluation test;

(3) to bend after heat treatment, bend bend each performance has improved.