Metallurgical factors affecting SSC resistance of low-alloy steel having yield strength of 600 MPa to 900 MPa were investigated. A new characteristic value of SSC resistance, namely an SSC critical strength, σc, defined in the text, was introduced on the basis of experimental results and some analysis. σc approximately coincides with a peak strength, σypeak, also defined in the text. σc is influenced by metallurgical factors, that is, σc decreases with an increase in Mn and P contents and increases with an increase in Mo content and a refinement of grain size, while these metallurgical factors have no pronounced effects on SSC threshold stress, σth, at yield strength below σc. Further, σc is linearly related to a newly derived compositional parameter, an A-value, which is Mn + 4.3P + 17.0Mn. P. By introducing the new parameter, σc, effects of metallurgical factors on SSC resistance could be explained consistently. As the results of fundamental investigations, principles of material design of SSC-resistant, high-strength steel were obtained as follows:
(1) Steel should have enough hardenability to transform into a fully martensitic structure. (2) Chemical compositions and grain size should be so selected as to satisfy the condition that σc exceeds the maximum yield strength to be required. σc can be estimated from the chemical composition and the grain size.
Based on these concepts derived from fundamental investigation, high-strength, SSC-resistant tubulars, the minimum yield strength of which are 100 ksi (686 MPa) and 105 ksi (720 MPa), were developed. Their SSC resistance are excellent; σth was above 90% of specified minimum yield strength.
