Steelmakers have developed and improved new processes for making low-carbon, low-impurity, high-strength steel plates with high toughness and improved weldability at low cost. This general type of steel is described in the new API 2Wsteel specification. This new API specification was developed to take advantageof the new technology of steelmaking.
The use of these steels, referred to as Thermo mechanical ControlProcess (TMCP) steels, can be an effective cost-saving means forfabrication of offshore structures. This report describes the characteristicsof the TMCP steels. These steels can be expected to provide improved weldability, greater strength, and toughness than conventional carbon manganesenormalized steels at the same or lower cost.
High-strength structural steels for offshore fabrication can be generallyclassified as follows:
Carbon steel (carbon-manganese) heat treated for enhanced properties
Low-alloy, high-strength steel (probably heat-treated)
Precipitation-hardening steel (A710) (probably heat-treated)
Controlled Rolled Steels
Thermo mechanical Controlled Process (TMCP) steel
This report will concentrate on the TMCP type of steel. The Thermomechanical Control Process (TMCP)* has evolved from the controlled rollingprocess which has been used for a number of years. TMCP is a new generation ofcontrolled rolling usually combined with controlled cooling from controlledwater sprays or direct quenching. The TMCP produces a fine-grain steel by acombination of carefully selected and controlled chemical composition andcontrol of the manufacturing process from slab reheating to post rollingcooling, thereby achieving the required mechanical properties. Generally alarge proportion of the rolling reduction occurs at a temperature close to orbelow (Ar3) (=800°C) the temperature at which austenite begins to transform toferrite during cooling.
TMCP steels were developed in the late 1970searly 1980s and are characterizedby very fine grain (ASTM 10-12) microstructures with higher strength andtoughness than conventional carbon manganese, micro alloyed, heat-treatedsteels. The cost should be less when compared with conventional C-Mn normalizedstructural steels because the cost for heat treatment more than offsets theadditional cost for the controlled rolling and accelerated cooling.
Normally the TMCP steels processing route could be described as a combinationof controlled rolling and controlled (accelerated) cooling. Figure 1 showsschematic diagrams of conventional, conventional controlled rolled (CCR), andTMCPsteel processing. More specific detail for some individual manufacturers'processes are shown in Figure 2.
One significant characteristic is the low carbon content of this steel--usuallyless than 0.10 weight percent.
