subject: Combined mechanical and thermal processing method for production of seamless steel pipe [print this page] Combined mechanical and thermal processing method for production of seamless steel pipe
This invention relates to a mechanical and thermal processing method for production of seamless steel pipe having homogeneous martensitic structure with a combination of high strength and toughness and with minimized distortion, and moreparticularly to a process for producing such steel pipes at a high thermal efficiency.
In producing seamless steel pipes of high quality with respect to strength and toughness, it has been the prior art practice to carry out either or both of the adjustment of the alloying elements of the steel itself and the heat treatment of thesteel pipe of final gage in a manner to control within predetermined limits, the final properties of the steel pipe. Where the heat treatment is employed to control the final properties, the resultant conventional process for producing steel pipes ischaracterized by the separate and independent application of the forming and heat treating steps. In other words, the pipe forming operation is not correlated to the heat-treating operation involving the quenching and tempering. This permits the use ofa heat-treating apparatus as arranged independently of the pipe producing apparatus so that the steel pipe in the as-formed condition is cooled down to room temperature before the application of the heat treatment thereto.
Such an independently operating mechanical and thermal processing method for improving quality characteristics of steel pipes has various disadvantages. One of these is that the heat energy retained in the steel pipe at the forming step is lostwith no effect on the heat treating step as the steel pipe is cooled during the time period intervening the forming and heat treating steps. Another disadvantage is biased on the remarkable reduction of the productivity of steel pipes due to theinterruption of a production run thereof at a point between the forming and heat treating steps. Still another disadvantage is that the heat treatment requires an additional amount of heat energy as the steel pipe is re-heated from room temperature toand maintained at a temperature at which the heat treatment is performed.
Such scale adhered to the pipe surfaces leads to the reduction of the cooling rate in the quenching step with the resulting slack quenching, which is the main factor in giving rise to increasing the degree of distortion of te quenched pipe.
The present invention has as its general object to overcome the above-mentioned conventional drawbacks and to provide a combined mechanical and thermal processing method for production of seamless steel pipes having a homogeneous martensiticstructure with excellent strength and toughness and with minimized distortion at a high thermal efficiency compared with the prior art. This has been accomplished by the following findings: The heat energy of the steel pipe resulted from the hot workingoperation can be utilized as a part of the heat energy necessary for the steel pipe to be austenitized. After a hollow billet or bloom is hot rolled to an intermediate gate, de-scaling is performed at the outside surface of the steel pipe to an extentsufficient to assist in uniform cooling of the steel pipe when quenched. The subsequent diameter reducing operation causes sufficient removal of scale from the inside surface of the steel pipe provided that the reduction, measured in terms of equivalentstrain () as defined by the following formula, is more than 0.02.
The basic equipment for performing the primary hot working step consists generally of three pieces of equipment, namely, a piercing machine, a roll stand and a reeling machine, if necessary, followed by a sizing mill, these pieces of equipmentbeing arranged along the same production line of pipes, while the basic equipment for producing pipes of final dimensions from the mother tubes supplied from the primary hot working step consists of only a single piece of equipment, such as, a sizingmill and a stretch reducing mill capable of working the mother tube with a controlled reduction of the pipe diameter as specified above.
As far as is known, the direct quenching method which is characterized by a remarkable economy in heat energy cost has been brought into practice with the production of thick plates, but not with the production of pipes. This is because pipesare very susceptible to distortion when quenched as compared with plates, and because this problem has thus far been considered very difficult to solve on the industrial scale. As stated above, however, the present invention has established thepractical utilization of the direct quenching method in producing seamless steel pipe by the sequence of the descaling step and the secondary hot working step with a specified pipe diameter reduction.
