Conventionally, hydrogen-induced cracking resistance (HIC resistance) and sulfide stress corrosion cracking resistance (SSCC resistance), and the like have been required for materials for line pipes. Steel excellent in these properties are called HIC resistant steel, sour-resistant steel, and the like.
Up to now, an inclusions-morphology control technology by Ca treatment has been developed to improve this HIC resistance performance. The initial object of Ca treatment was to inhibit HIC attributable to MnS by morphing MnS as sulfide into Ca-type inclusions. However, it came to light that HIC is attributed to Ca-type oxide and sulfide inclusions (oxysulfide inclusions) other than MnS, for example, inclusions represented by Ca—Al—O—S, Ca—S, and Ca—S—O. And, the need for morphology control of Ca-type oxysulfides in addition to MnS has been recognized. Thus, many technologies that attempt to control inclusions-morphology have been developed. For instance, Japanese Patent Application Publication No. 56-98415, etc. discloses steel production methods that decrease the number of inclusions.
In addition, as the environment of pipes in use become hostile, further enhancement of sour-resistance performance and higher strength are demanded and the development of inclusions-morphology control technology is also conducted to satisfy the demand. Japanese Patent Application Publication No. 06-330139 discloses a method of controlling inclusions that involves adding Ca, Al and Si so as to satisfy a specified relational expression for steel types of X42 to 65 grades of API Standards.
Meanwhile, in recent years, much higher sour-resistance performance and strength in steel have been demanded and more advanced technology development has been pursued. Japanese Patent Application Publication No. 2005-60820 discloses a technology that improves sour-resistance performance by attempting the dispersion of carbonitrides for a steel grade equal to or higher than the X65 grade of API Standards. In addition, Japanese Patent application Publication No. 2003-313638 discloses steel obtained by dispersing and depositing precipitates including Ti and W for a similar steel type which is equal to or higher than the X65 grade of API Standards. Moreover, Japanese Patent Application Publication No. 2001-11528 discloses a method for melting and refining steels that controls the composition of Ca—Al—O—S-type inclusions by adjusting the amount of Ca addition such that the Ca concentration satisfies a predetermined relation according to the S and O concentrations in molten steel.
Then, the present inventors found that bulky TiN-type inclusions exceeding 30 μm in size become the initiation point of HIC and proposed steel in which these are reduced and a method of controlling the size of TiN to 10 to 30 μm by use of Ca—Al-type inclusions in WO2005/075694.
As described above, the morphology control technology for inclusions by Ca treatment has been upgraded according to performance demand for steel, and the technology has been developed from simple addition of Ca to inhibiting CaS generation and improving cleanliness to controlling composition of Ca-type inclusions and further to the fine dispersion and precipitation of carbonitride-type inclusions.
Incidentally, recently, higher sour-resistance performance and strength have been demanded as previously described. For these demands, following problems are present. A first problem is to address the instability of sour-resistance performance. In other words, the technology intended for high-strength steel is for the dispersion of carbonitrides and the composition control of Ca-type inclusions. Although the technology can control the generation of HIC to the low level, HIC still happened to generate in some cases. In addition, a second problem is to cope with the difficulty of completely inhibiting the generation of HIC even by applying rigorous conditions in Ca treatment. The prior art has been primarily directed to optimization of Ca treatment conditions. However, though the Ca treatment conditions are rigorously managed in high strength steel, there is still a problem in that the complete inhibition of HIC generation is difficult.
Although the above-mentioned problems imply the possibility of the presence of proper production conditions to be controlled other than proper conditions for Ca treatment, their detailed contents and approaches have been quite uncertain and solutions of these problems has been difficult.