To increase the operational efficiency of a line pipe, it is necessary to transport petroleum or gas at an increasing amount per hour. For this purpose, it is inevitable to secure a high strength of steel. Also, it has been essential for steel to secure low-temperature toughness as petroleum and gas diggings gradually spread into cold district.
Owing to an increasing demand for large structures such as building structures and offshore structures and an increasing severity of the severe operating conditions (an operational temperature, a connection structure, etc.), there has also been a gradually increasing demand for steel having high strength and high toughness.
In order to facilitate improvement of the steel strength, a technology of improving both hardness and strength of a steel plate has been proposed in the prior art, comprising: adding an element for improving hardenability to form a low-temperature transformation phase during a cooling process. However, the proposed technology has a problem in that, when a low-temperature transformation microstructure such as martensite is formed inside a steel plate, the toughness of the steel plate may be severely deteriorated due to its inner residual stress. That is to say, since the steel plate has two incompatible physical properties, namely strength and toughness, it has been recognized in the art that the toughness of the steel is decreased with strength.
Since then, there has been a continuous attempt to provide a high strength steel with high toughness. As a result of this attempt, a thermo mechanical controlling process (TMCP) was presented and has been used for a high strength steel with high toughness.
The TMCP is the general term for processes of controlling the reduction ratio by rolling and rolling temperature so as to fabricate a steel plate with desired physical properties. Here, the conditions of TMCP may depend on desired physical properties. In this case, the TMCP is generally divided into two steps: a controlled rolling process at a high temperature under strict conditions and an accelerated cooling process at a suitable cooling rate.
The steel plate with TMCP may be composed of fine grains inside a steel plate or have desired microstructure according to conditions of TMCP. Theoretically, therefore, it is possible to easily control physical properties of the steel plate for desired properties.
In order to manufacture a steel plate having a desired strength by means of the accelerated cooling process of the TMCP, it is necessary to form a hard microstructure in the steel plate, as described in the prior art. Therefore, it is still necessary to add an alloying element for improving hardenability in order to form a low-temperature transformation microstructure as a hard microstructure.
This hardenability-improving element has a problem associated with an increase in the manufacturing cost since it is very expensive. Therefore, there have been ardent attempts to enhance the strength of steel in the field of high-strength steel. Also, there have been continuous attempts to secure the low-temperature toughness of steel.
In general, the rolling process of the TMCP is widely divided into two methods according to finish rolling temperature and start cooling temperature. First, one is a single-phase region rolling process in which the finish rolling temperature and cooling are carried out above Ar3 temperature at which austenite is transformed into a ferrite microstructure, and the other is a dual-phase region rolling process in which the finish rolling temperature and cooling are carried out below the Ar3 temperature.
The single-phase region rolling process has advantages in that the load in rolling mill facilities is low since the rolling temperature of the single-phase region rolling process is higher than that of dual-phase region rolling process, and the manufacturing cost may be reduced since the rolling time of the single-phase region rolling process is shorter than that of dual-phase region rolling process. However, the single-phase region rolling process has a lot of problems in that the addition of expensive alloying elements with excellent hardenability is required to improve the steel strength since a transformation microstructure may be formed during a cooling process, but the addition of the alloying elements may impose a heavy burden on the manufacturing cost, and ununiform transformation in an inner part of a prepared steel plate may occur during the cooling process, which leads to a poor flatness of the steel plate.
On the contrary, since hardenable elements add little amount since transformation from austenite to ferrite occurs during rolling process, the dual-phase region rolling process does not have a problem associated with the increase in the cost by the addition of the alloying elements, but the load in the rolling mill facilities is high due to the low rolling temperature, and the manufacturing cost may be increased due to the long manufacturing time.
By making practical application of the conventional TMCP, various methods for manufacturing structural steel have been proposed in the prior art. For example, there is one technology of manufacturing steel having a bainite or martensite microstructure as a low-temperature transformation phase, including; rolling steel a temperature right above Ar3 temperature and performing accelerated cooling of the rolled steel to approximately 150 to 500° C.
However, this technology has a problem in that, since polygonal ferrite in the rolled steel may be formed according to the initial cooling rate, it is not easy to realize a suitable cooling rate according to the alloying components. Also, since the steel is rolled up to the temperature right above Ar3 temperature, the load may be given to the rolling mill facilities, and simultaneously the rolling time may be extended, which leads to the high manufacturing cost.
As another alternative, there is a technology for securing sufficient low-temperature toughness of steel while employing the conventional TMCP, for example, further including: tempering a steel plate below an Ac1 transformation temperature (a temperature where ferrite is transformed into an austenite).
However, this technology should further include a heating operation so as to temper the steel plate after cooling the steel plate. Therefore, the technology still has a problem in that energy for the steel production may be increasingly used, and the manufacturing cost may be high due to the additional tempering process.
Therefore, there is a continuous demand for an epoch-making and stable method for manufacturing a steel plate that may solve the above problems.