JPS62208B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a hot coil for a line pipe that has excellent resistance to hydrogen-induced cracking. Conventionally, the manufacturing method for hot coils for sour gas, which requires hydrogen-induced cracking resistance, involves applying treatments such as ultra-low sulfidation (S≦0.002), adding Cu, Ni, and Ca, and then heating the continuous cast slab using the normal process. It was heated in a furnace and manufactured by continuous hot rolling. However, recently, as crude oil containing a large amount of H ₂ S gas has been extracted, there has been a demand for line pipes with even better sour resistance. Hydrogen-induced cracking tests were previously conducted at a pH of about 5.2, but recently tests at a pH of 3.5 to 3.8 are required. When pH=3.5 to 3.8, hydrogen-induced cracking occurs in coils manufactured using conventional methods, and the requirements cannot be met. The reason for this is thought to be that the hardened layer at the center of the thickness of the hot coil plate, which is caused by center segregation of the continuous cast slab, becomes the starting point for cracks. The purpose of the present invention is to improve hydrogen-induced cracking resistance by diffusing the center segregation of a continuously cast slab. Good results were obtained by manufacturing the coil, and this knowledge was completed to manufacture an excellent line pipe coil for sour gas without hydrogen-induced cracking even in tests of PH = 3.5 to 3.8. The present invention is as follows: C≦0.15wt% Si=0.05-0.50wt% Mn=0.50-1.50wt% P≦0.020wt% S≦0.003wt% Al≦0.070wt% Cu=0.10-0.50wt% Ni=0.10-0.50wt % Ca = 0.0010~0.0060wt% A continuously cast slab consisting of the balance Fe and unavoidable impurities was heated at 1250℃~1350℃.
After holding for 5 to 10 hours, the breakdown rate
For line pipes with excellent resistance to hydrogen-induced cracking, characterized by performing breakdown rolling at 20% to 40%, then holding at heating conditions of 1150°C to 1300°C for 3 to 5 hours, and then performing continuous hot rolling. This is a method of manufacturing a hot coil. The hot coil to which the present invention is applied has a thickness of 3.5~
It is 25mm. A thickness of 3.5 mm is the practical lower limit for line pipe material, and a thickness of 25 mm is the upper limit that can currently be manufactured as a hot coil. Breakdown rolling of continuous cast slabs is aimed at reducing center segregation in the heating furnace, and conditions such as soaking temperature, holding time, and breakdown rate during breakdown are important for hydrogen-induced cracking. It is a factor. Here, the breakdown rate is (continuously cast slab thickness - slab thickness after breakdown) ÷ (continuously cast slab thickness) x 100 (%). FIG. 1 shows the influence of the above conditions on the sour characteristics. The heating time before continuous hot rolling was constant at 3 hours under each condition. The results shown in Figure 1 can be summarized as follows. (1) If breakdown rate is 10%, soaking time is 10
If the time is exceeded, sour resistance properties will improve, but cracking cannot be reduced to zero. (2) When the breakdown rate is 20% or 30%, if the soaking temperature is 1250°C to 1300°C, there will be no cracking after soaking for 5 hours or more. (3) When the breakdown rate is 20% or 30%, even if the soaking temperature is 1200℃, there will be no cracking if the product is held for 15 hours or more. By soaking in the heating furnace, C in the center of the slab,
P, Mn, etc. diffuse. In order to promote diffusion, it is possible to lengthen the soaking time and increase the soaking temperature, but diffusion is further promoted by compressing the porosities at the center of the slab. Therefore, breakdown rolling that ensures a breakdown rate of 20% or more in the continuously cast slab significantly reduces center segregation in the heating furnace. However, from the viewpoint of yield and actual operation, the upper limit of the breakdown rate is set at 40%. In addition, soaking the slab before breakdown rolling also promotes the diffusion of C, P, Mn, etc. As is clear from Figure 1, if soaked at 1250°C or higher, 5 hours will be sufficient. In the case of 1200℃, soaking for 15 hours or more will be effective, but soaking for a long time will increase the formation of scale, and there is a concern that scale defects may occur. Due to scale flaws and operational issues, the temperature is
1350℃, maximum time is 10 hours. On the other hand, the influence of heating conditions before continuous hot rolling is second to none.
As shown in the figure. If the heating time is 3 hours or more, the sour resistance is sufficient. The heating temperature is regulated due to mechanical property issues, and the furnace time is regulated due to operational issues. Considering the industrial basis, the appropriate furnace time is 3 to 4 hours. The reasons for regulating chemical components are as follows. C: If it exceeds 0.15wt%, there will be problems with toughness and weldability. Si: Added as a deoxidizing agent, but if it exceeds 0.50wt%, brittleness may increase. Mn: If it is less than 0.50wt%, strength cannot be obtained. If it exceeds 1.50wt%, brittleness becomes large. P: This is an element that is a major factor in hydrogen-induced cracking, and the lower the value, the more advantageous it is, but from the viewpoint of industrial manufacturing
Less than 0.020wt%. It is an essential condition to suppress S: to less than 0.003 wt%, and the lower the S content, the more advantageous it is to hydrogen-induced cracking. Al: Added as a deoxidizing agent like Si.
If it exceeds 0.070wt%, the steel quality will deteriorate. Cu: An effective element against hydrogen-induced cracking, but it is ineffective at less than 0.10wt%, and 0.50wt%
If it exceeds this, weldability will deteriorate. Ni: Added in the same amount as Cu to prevent Cu embrittlement. Ca: Controls the morphology of sulfides and is effective against hydrogen-induced cracking. However, it is necessary to add so that Ca/S≧2.0, and if the upper limit exceeds 0.0060 wt%, CaO will increase and the cleanliness will deteriorate. Others: Nb, V, Ti, etc. are added according to material requirements. Next, Examples of the present invention are summarized in Table 1. Examples 1 to 5 are 15.88 mm thick coils for line pipes with a Mn level of 1.10 wt%, and Examples 6 to 9 have a Mn level of 0.60 wt%, and the chemical components are as follows. Examples 1 to 5 C: 0.07wt%, Si: 0.24wt%, Mn: 1.12wt%, P: 0.013wt%, S: 0.0008wt%, Al: 0.037wt%, Nb: 0.0031wt%, V: - Ca: 0.0036wt% Examples 6 to 9 C: 0.07wt%, Si: 0.16wt%, Mn: 0.62wt%, P: 0.016, S: 0.0009wt%, Al: 0.045wt%, Nb: 0.011wt%, V: - Ca: 0.0033wt% From Table 1, the sizing conditions are heating temperature 1250℃,
When breakdown rolling was performed with a holding time of 5 hours or more and a breakdown rate of 20% or more, and then a heating time of 3 hours, the sour resistance at PH = 3.5 showed good results, clearly demonstrating the excellent effects of the present invention. It is. 【table】

[Brief explanation of the drawing]

FIG. 1 is a graph showing breakdown conditions and sour resistance characteristics of a continuously cast slab, and FIG. 2 is a graph showing soaking time before continuous hot rolling and sour resistance characteristics.

Claims (1)

[Claims] 1 C≦0.15wt% Si=0.05-0.50wt% Mn=0.05-1.50wt% P≦0.020wt% S≦0.003wt% Al≦0.070wt% Cu=0.10-0.50wt% Ni= A continuous cast slab consisting of 0.10 to 0.50wt% Ca = 0.0010 to 0.0060wt% with the balance Fe and unavoidable impurities was heated at 1250℃ to 1350℃.
After holding for 5 to 10 hours, the breakdown rate
For line pipes with excellent resistance to hydrogen-induced cracking, characterized by performing breakdown rolling at 20% to 40%, then holding at heating conditions of 1150°C to 1300°C for 3 to 5 hours, and then performing continuous hot rolling. Method of manufacturing hot coils.