JPH0241400B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to the creation of a method for manufacturing a clad steel plate with high toughness and high corrosion resistance, and which uses stainless steel to achieve the high corrosion resistance required for line pipe materials used in environments such as sour weather, as well as high toughness and high shear strength. The object of the present invention is to obtain a method that can appropriately manufacture a non-tempered stainless steel clad steel sheet having the following characteristics. As the conditions under which steel plates are used have become more severe, the use of clad steel plates has increased in recent years from the viewpoint of high corrosion resistance, but in this case, not only corrosion resistance but also high toughness is often required. For example, line pipe materials used in environments such as sour gas are strongly required to have both such high corrosion resistance and high toughness. However, in the production of rolled clad steel sheets, it is common to perform rolling at high temperatures and pressures in order to obtain good bondability, and rolling conditions such as rolling temperature, pass schedule, and pass reduction ratio are mainly determined from the viewpoint of bondability. Only the number of points has been determined. Therefore, when the base material is required to have severe low-temperature toughness or high ductility, heat treatment is usually performed after clad rolling. However, such heat treatment has the disadvantage of increasing production costs and decreasing production efficiency. In addition, when the composite material is austenitic stainless steel, σ phase embrittlement and sensitization (embrittlement due to precipitation of Cr carbide due to slow cooling) occur.
Due to various embrittlement phenomena such as embrittlement phenomena, there are major restrictions on heat treatment methods, such as the inability to perform tempering treatment for a sufficient period of time, and it is difficult to obtain sufficient low-temperature toughness and high shear strength. The present invention was devised after repeated studies in view of the above-mentioned circumstances, and in producing a clad steel plate by stacking and rolling members made of carbon steel and austenitic stainless steel, the carbon content of the carbon steel is 0.12% or less, and the carbon content of the austenitic stainless steel is 0.05% or less, and in the temperature range of 1000 to 1250°C, rolling is performed at a welding rough reduction rate that satisfies the relationship of the following formula,
Further, the method for producing a high toughness and high corrosion resistance clad steel sheet is characterized in that rolling is carried out at a cumulative reduction rate that satisfies the relationship of the following formula in the temperature range of 650 to 950°C. Cumulative rough reduction rate (%) ≧ 15% + 0.2 × [CR rate (%)]...
20% ≦ CR rate ≦ [90% − 200 × {carbon content of carbon steel (w
t%)}]... However, in the above formula, the CR rate is 650 to 950℃
This shows the cumulative rolling reduction rate in the temperature range. To further explain the present invention, a controlled rolling method is known as a method for manufacturing high-toughness steel sheets as rolled, and this controlled rolling method is used to manufacture the above-mentioned austenitic stainless steel clad steel sheets. During the rolling process (950 to 650°C), chromium carbide precipitation becomes noticeable, reducing the corrosion resistance of the stainless steel itself, and reducing the bonding strength between the carbon steel and stainless steel interface. It is considered difficult to easily adopt this controlled rolling technology. However, as a result of a detailed study of the rolling pattern, the inventors of the present invention have succeeded in suppressing the precipitation of chromium carbide in controlled rolled stainless steel clad steel sheets and achieving sufficient shear strength at the carbon steel-stainless steel interface (for example, according to JIS standards). 20 specified
Kg/mm ² or more), the following factors are controlled: C content in carbon steel, C content in austenitic stainless steel, Total reduction rate between 1250 and 1000℃, and Total reduction rate in the temperature range below 950℃. It has been found that this shear strength can be appropriately secured by doing this. That is, in the present invention, a high-toughness, non-heat-refined stainless clad steel is obtained by employing a special pass schedule during hot rolling. To further explain the technical content of the invention as described above, in the production of rolled clad steel sheets, etc., carbon steel billets containing 0.12% C and carbon
When combining 0.05% austenitic stainless steel slabs and rolling them at a temperature range of less than 1250°C and more than 650°C, the lower limit of the joint crude reduction rate between temperatures of less than 1250°C and more than 1000°C is determined by the following formula. Rolling is performed at a reduction rate or higher. Crude joint reduction rate (%) ≧ 15% + 0.2 × [cumulative reduction rate (%) below 950℃]... Also, following the above, the cumulative reduction rate (CR rate) in the temperature range below 950℃ is as follows: Rolling is performed within the range shown by the formula. 20% ≦ CR rate (%) ≦ [90% - 200 × {C content of carbon steel; wt
%}]... Use a rolling pattern like this. In other words, the rolling conditions indicated by this are as shown by hatching in Fig. 1, and this rolling pattern controls the precipitation of chromium carbide in the controlled rolling process and improves the stability at the carbon steel-stainless steel interface.
High shear strength of 20Kg/mm2 or ^more can be obtained. It should be noted that there is no need to particularly limit the heating of the steel billet that is carried out prior to rolling as described above.
Preferably, the temperature is 1050-1300°C. However, to explain the reason for specifying the crude joining reduction rate as mentioned above, in order to obtain high shear strength at the interface between carbon steel and stainless steel, it is necessary to apply a large reduction above a certain level in the high temperature range as mentioned above. Although it is clear that it is fundamentally important, when performing controlled rolling in a low temperature range, it is necessary to increase the joint rough reduction rate in this high temperature range in accordance with the magnitude of the CR ratio. In other words, controlled rolling in a low temperature region cumulatively gives deformation strain near the interface,
This is because the shear strength is gradually reduced. The reason for specifying the upper limit of the CR rate as mentioned above is that under high CR rate conditions, the residence time of steel in the low temperature range (950 to 650°C) where chromium carbide precipitation is noticeable increases, and the deformation strain near the interface increases. By doing so, chromium carbide precipitation is promoted. That is, according to the results of studies conducted by the present inventors, it is impossible to maintain the shear strength at 20 Kg/mm ² or more with a CR rate greater than the upper limit shown in the formula. Also, this upper limit CR
The CR rate depends on the carbon content of the base material, and if the above-mentioned rolling is performed above the upper limit CR rate, chromium carbide will precipitate at the interface between carbon steel and stainless steel, resulting in a significant decrease in shear strength. Theoretically the above
It is thought that the CR rate depends on the carbon content of both the base material carbon steel and the stainless steel composite material, but the carbon content of the base material carbon steel is usually several times higher, and the upper limit CR rate is essentially this base material carbon content. ruled by quantity. Also, the reason for specifying the lower limit of the CR rate above is that a CR rate below this is sufficient to maintain the toughness level of carbon steel (for example, 85% in the DWTT test).
SATT≦0℃) is not possible. Furthermore, the reason for specifying the upper limit carbon content for base material carbon steel and stainless steel composite material is that the base material carbon content is
In a region exceeding 0.12%, the precipitation of chromium carbides increases rapidly, and the above equation no longer holds true, making it impossible to obtain sufficient joint strength even if rolling is performed according to the present invention. If the carbon content of the stainless steel composite material exceeds 0.052%, the corrosion resistance of the controlled rolled stainless steel surface will decrease. Further, when the rolling temperature is lower than 650°C, the deformation resistance of both pieces of steel, especially carbon steel, increases significantly, making rolling extremely difficult, and the adhesive strength between the pieces of steel decreases. Furthermore, if the rolling temperature is 1250° C. or higher, the austenite grains will become coarse, making it impossible to obtain the high toughness clad steel sheet targeted by the present invention. Therefore, the rolling temperature needs to be 650 to 1250°C. During rolling, the polymerization occurs as carbon steel - stainless steel - stainless steel - carbon steel, and a separator is interposed between the stainless steels to facilitate separation between the austenitic stainless steels after rolling. The carbon steel plate size is larger than the intermediate stainless steel plate, and in order to prevent the stainless steel-carbon steel joint surface from oxidizing during heating prior to rolling, a sandwich-shaped body is prepared by welding the four circumferences of the upper and lower carbon steel plates. This material is then heated and rolled. Specific embodiments according to the present invention will be described below. The composition of the test steel specifically adopted by the present inventors is shown in Table 1 below, and both the base material and the mating material were used with their surfaces shot blasted. The thickness of the laminated material was 3 mm, and it was prepared according to the Sanderuch method as described above.

[Table] The heating temperature was 1250℃, and then the original thickness was rolled using various combinations of base materials and composite materials as shown in Table 2 under the rolling conditions shown in Table 2. The mechanical properties are also shown in this Table 2. In each pass schedule, the finishing temperature was 700°C, the finishing thickness was 10 mm, and the tensile strength and shear strength test pieces were taken from the direction perpendicular to the rolling direction.

【table】

[Table] Figure 2 shows the shear strength when the rough joint reduction rate and the cumulative reduction ^rate (CR rate) from 950 to 650℃ are changed. There is a lower limit crude reduction rate and an upper limit cumulative reduction rate necessary for this purpose. In the case of No. 15 in Table 2 above, the standard rolling of the present invention was performed in a combination in which the base material carbon content exceeded the upper limit carbon content, and a significant decrease in shear strength was observed. No. 6, No. 10 and No. 14 have rough joint reduction ratio,
Even though the CR ratio is high and the toughness is excellent, the shear strength is significantly inferior. This also applies to Nos. 16 and 17, or Nos. 21 and 22.
On the other hand, although Nos. 18 and 19 or Nos. 24 and 25 have excellent shear strength, it is clear that they are inferior in toughness, and neither of them can achieve the object of the present invention. Nos. 4, 5, Nos. 7 to 9, which satisfied the conditions of the present invention,
Nos. 11 to 13 and No. 20 all have a toughness of -℃ and a shear strength of 25 kg/mm ² or more, which indicates that they are excellent in both toughness and shear strength, making them desirable cladding materials. It's obvious. Note that the method of the present invention is applicable to cases where (a) insert material such as Ni is not included between carbon steel and stainless steel, and (b)
It can be applied to both cases where the final product contains an insert material such as Ni with a thickness of 0 to 50 μm, and even if the final product contains an insert material such as Ni with a thickness of this level, chromium carbide at the interface of stainless steel There is no significant difference in the precipitation behavior compared to the case without insert material.
Furthermore, the austenitic stainless steel referred to in the present invention refers to a generally recognized stainless steel with no particular specification other than carbon content. That is,
Si: 2% or less, Mn: 5% or less, Ni: 6 to 50%,
The basic composition is steel consisting of Cr: 10 to 30%, Al: 1% or less, the balance being iron and unavoidable impurities, and if necessary, further Ti: 2% or less, Nb: 2% or less,
Steels containing one or more of Cu: 4% or less and Mo: 10% or less are included. Also, the carbon steel referred to in the present invention does not need to be specified except for the carbon content, but Si: 0.10 to 0.70%, Mn: 0.20 to 2.00%.
%, so1.The basic composition is steel with Al: 0.07% or less and the balance consisting of iron and unavoidable impurities, and if necessary, Nb: 0.20% or less, V: 0.30% or less, Zr:
0.20% or less, Ti: 0.30% or less, Ta: 0.10% or less,
This refers to steel containing one or more of the following: B: 0.002% or less, Mo: 0.6% or less, Cu: 1.0% or less, and Ni: 3.0% or less. It has been fully confirmed experimentally that the present invention can be similarly applied to these austenitic stainless steels and carbon steels and can exhibit the desired effects. According to the present invention as explained above, a clad steel plate having high toughness that can withstand severe usage conditions and suitably having high corrosion resistance due to stainless steel is rolled using a controlled rolling method, suppressing precipitation of chromium carbide, and achieving sufficient bonding. This invention has great industrial effects because it can be suitably manufactured as a cladding material with strength, and as a result, various desirable products suitable for use in environments such as sour gas are required. It is.

[Brief explanation of the drawing]

The drawings show the technical contents of the present invention. Figure 1 is a chart showing the range of the method of the present invention in relation to the rough joint reduction rate and CR rate, and Figure 2 is a graph showing the range of the method of the present invention in relation to the rough joint reduction rate and CR rate. This is a chart showing the range of shear strength of 20 Kg/mm ² or more with respect to CR rate.

Claims (1)

[Scope of Claims] 1. In producing a clad steel plate by stacking and rolling members made of carbon steel and austenitic stainless steel, the carbon content of the carbon steel is set to 0.12% or less, and the carbon content of the austenitic stainless steel is The amount should be 0.052% or less, and 1000 to 1250
In the temperature range of 650 to 950 degrees Celsius, rolling is performed with a joint rough reduction rate that satisfies the relationship of the following formula, and in the temperature range of 650 to 950 degrees Celsius, rolling is performed with a cumulative reduction rate that satisfies the relationship of the following formula. A method for manufacturing a high-toughness, high-corrosion-resistant clad steel sheet. Rough joint reduction rate (%) ≧ 15% + 0.2 × [CR rate (%)]...
20% ≦ CR rate ≦ [90% − 200 × {carbon content of carbon steel (w
t%)}]... However, in the above formula, the CR rate is 650 to 950℃
This shows the cumulative rolling reduction rate in the temperature range.