DP duplex steel

DP Steel

DP (dual phase) steel is a high strength steel obtained from low carbon steel or low alloy steel by critical zone heat treatment or controlled rolling. Its structure is composed of ferrite matrix and about 20% Island martensite on ferrite grain boundary, also known as martensitic dual phase steel. The basic components of dual phase steel are C and Mn. Sometimes a certain amount of Cr and Mo are added to improve hardenability.

Special performance

Low yield ratio, high elongation

answer      use

Side beam, side member and cross beam of automobile

Special performance

Dual phase steel is a new material developed in the mid-1970s. It has the characteristics of low yield ratio, high elongation and fast initial hardening rate.

application

DP steel is widely used in side beams, side members, beams, pillars, chassis stiffeners, fuel tank supports and structural parts, stiffeners and anti-collision parts of vehicle bodies.

Production process

During the hot working process of DP steel, the austenitized steel can stay in the ferrite transformation area for a long time by reasonably formulating the cooling process, so as to ensure that 80% austenite phase is transformed into ferrite phase, and then the rapid cooling bypasses the bainite formation area and cools below the MS point, so as to form the structure with ferrite + martensite phase.

Next, we take the more typical DP600 as the representative to discuss its production process.

The production process of hot rolled DP steel strip in iron and steel industry is divided into low temperature crimping and medium temperature crimping.

Low temperature crimping process

One is the low-temperature crimping process. The production principle of low-temperature coiling hot-rolled dual phase steel is that most of the A-F phase transformation (about 80%) is completed in the cooling process after final rolling, and the steel strip is quickly cooled to below M8 point in the subsequent rapid cooling stage for coiling to obtain F + M structure. The coiling temperature is generally lower than 300 ℃ to avoid the formation of bainite, the aging of ferrite and the self tempering of martensite (as shown in the cooling curve of low temperature crimping process). It is characterized by reducing the content of alloy elements and reducing the cost, but it has strict requirements for the control of process parameters, especially the cooling process after final rolling, which is difficult to control, and high-power coiling equipment is required. Only Nippon Steel and Kawasaki iron making plants in Japan use this process, and few in China.

Cooling curve of low temperature crimping process

Medium temperature crimping process

The second is the medium temperature coiling process. The principle of the medium temperature coiling type is to properly add Cr, Mo and other alloy elements to improve the stability of austenite in the conventional coiling temperature range. In its continuous cooling process, it is between A_+ There is a metastable austenite transformation free zone in the temperature range of F and A-B transformation. The finished steel plate completes most of a first in the continuous cooling process_+ F transformation (about 80%), and no transformation coiling is carried out at the "coiling window" temperature. After coiling, the residual austenite is transformed into martensite during air cooling to room temperature, and finally the two-phase structure of F and M is obtained. The coiling temperature of this rolling method is 500-600 ℃.