Steel Tubes and Profiles with Multi-Wall Thickness (TMT)
|Publication Date:||1 September 2011|
This specification covers the requirements for heat-treatable or non-heat-treatable extruded low carbon steel tubes with multi-wall thickness (TMT) and profiles used in the manufacture of structural parts requiring low, middle and high strength. The usage of this standard is indicated for applications which require low weight and high performance. Typical applications include door impact beams, cross car beam, bumper impact beams, reinforcements for impact structures, bull bars and general tubular parts, and specific cases of reduction or elimination of stamping tools by employment of split techniques of symmetrical TMT profiles according to 188.8.131.52.
Material Description. The particular application shall determine the extrusion operations to produce the finished steel tube and profile.
Resistance Welded Mechanical Tubing. Primary tubes and profiles supplied to this specification are roll formed from steel strip to the desired shape and dimension, plasma or electrical resistance welded into a tube, and then extrusion process to achieve the desired profile and variable thickness, the heat treated may be made to reach the strength mechanical properties.
TMT Extruded Tubes. Tubes manufactured by seamless process or rolled steel strip according to item 1.1.1 are submitted in cold extrusion process to construct a desired profile with variable wall thickness. The cold extrusion process causes a work hardening and a mechanical strength improving, for higher strength grades, an induction heat treatment is performed later to achieve the final mechanical property requirements. The specified grades are low strength with multi-wall thickness (LSTMT) type ferrite + pearlite (FP), medium strength with multi-wall thickness (MSTMT) and high strength with multi-wall thickness (HSTMT) for the three types of multi-phases (MP) such as ferrite and pearlite, bainite and/or martensite. The TMT tube final external profile may be circular, square or any other form specified by Product Engineering. The microstructure of the finished tubes may consist of ferrite and pearlite after extrusion operation and martensite and/or bainite for heat-treatable tubes.
Material Identification. The GMW base metal specification number defines the material designation, steel product type, grade, and finish type. The sheet steel product type for manufactured primary tubes are cold rolled (CR) or hot rolled (HR) products. The steel grades will correspond to the chemical and mechanical requirements as shown in Tables 1 through 3. The finish type, an alpha character, will be designated with the finish unexposed (U). The descriptive items are used together to form the coding system. Examples of the coding system for engineering part drawings, electronic math data files, and/or manufacturing engineering documents are shown in Section 8.
Typical Applications. Typical applications are uncoated or pre-coated thin gauge tubular structural parts or other design products derivates from tubes requiring yield and tensile strengths exceeding 775 MPa and 787 MPa respectively.