JPS5914538B2 - Steel with low stress relief annealing cracking susceptibility - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a steel in which impurities in the steel are controlled in order to prevent stress relief annealing cracking (hereinafter abbreviated as RSR cracking) in steel materials or welded joints.

Components that require stress relief annealing treatment include forged steel products, cast steel products, machined parts, boilers, pressure vessels, welded joints,
There is a wide range of overlay cladding, etc., and the chemical composition of the steel material is Ss+
MntNi+ CrtMOrV+ There is a wide variety of low carbon steels or medium carbon steels containing Nb, Cu, and other alloying elements, but SR cracking often occurs with this treatment, which can be a major problem in manufacturing steel materials or welded structures. It has become.

SR cracks occur at the final stage of the manufacturing process, and if they occur deep within the steel material, partial repair is impossible. In the past, much research has been conducted to prevent SR cracking in advance, and one direction is to improve the manufacturing process and prevent SR cracking by examining the effects of welding heat, preheating temperature, etc. There is an attempt.

On the other hand, by considering the influence of the chemical composition of steel materials, S
Many attempts are being made to develop steel materials with low susceptibility to R cracking. The main focus of the latter is to elucidate the influence of individual alloying elements on SR cracking susceptibility, and the results of these studies show that intragrain precipitation of secondary hardening elements or grain boundary precipitation of carbides are responsible for SR cracking. It has been said that this is the cause of However, the effects of individual alloying elements on a wide variety of steel materials are not necessarily uniform and universal, and the presence of the above precipitates does not necessarily cause SR cracking. Conversely, even if the composition is designed so that the above-mentioned precipitates are not generated, SR cracking is not necessarily prevented. Furthermore, depending on the use of the steel material, it is often not possible to significantly modify the concentration of alloying elements constituting the chemical composition, and such measures to prevent SR cracking are completely useless. The present invention is based on the following new discovery obtained as a result of various studies conducted from a completely different perspective from the conventional research that impurities may affect SR cracking susceptibility.

That is, as a result of examining the intergranular fracture surface of a steel material with SR cracking by Auger electron analysis, significant segregation of S (sulfur) was observed near the fracture surface, as shown in FIG. FIG. 1 shows the distribution of S (sulfur) in the depth direction from the SR crack fracture surface of steel type A3. The vertical axis is the ratio of the intensity of S (sulfur) to the intensity of iron in the Auger electron spectrum, and corresponds to the concentration of S (sulfur). The horizontal axis is S
This is the depth of the R-crack fracture surface dug by sputtering, and the full scale is about 50A. The depth of this segregation is at most a few + A, and the concentration decreases monotonically from the fracture surface to the inside of the grain, so this segregated S (sulfur) is not an inclusion. It is clear that it is distributed in a solid solution state. Next, FIG. 2 shows the effect of adding Ce (cerium) in the steel manufacturing process in order to eliminate segregation of S (sulfur) in a solid solution state to grain boundaries. In other words, Figure 2 shows steel types A3 and G shown in Tables 1 and 2 below, subjected to welding thermal history reproduction treatment, and then subjected to an initial stress of approximately 40k1mm2 at 610°C and held under constant strain restraint. This figure shows the stress relaxation characteristics of steel type A3 and the fracture time of steel type A3.

As is clear from the figure, steel type A3 to which Ce (cerium) is not added causes SR cracking in a short period of time, whereas Ce (cerium)
Steel type G1 containing cerium) does not cause SR cracking, and stress is sufficiently relaxed by long-term stress-relieving annealing treatment. From these findings, it is clear that grain boundary segregation of solid solution S (sulfur) is the cause of SR cracking. It should be added here that the form of SR cracking is ductile grain boundary fracture, so the effect of grain boundary segregation of solid solution S (sulfur) on causing SR cracking is due to the grain boundary segregation of impurities causing tempering embrittlement. This means that the effect that causes change is completely different. The present invention is based on the above-mentioned new discoveries, and has found a composition system for steel that suppresses the grain boundary segregation of solid solution S (sulfur) and has low SR cracking susceptibility.

-Next, the reasons for the limitations of the present invention will be discussed. C (carbon)
Although 0.05% of Ni is necessary for strengthening steel, the upper limit was set at 0.5% because it deteriorates weldability and workability. S
i (silicon) is added to deoxidize and strengthen steel, and 0.10% or more is necessary, but if it is too much, toughness and weldability deteriorate, so the upper limit was set at 1.0 so. Mn (manganese) is added to steel as a deoxidizer and to add strength and hardenability, but from the viewpoint of difficulty in steelmaking work and workability, the upper limit is 3.0%. And so. S (sulfur) and the element for fixing it are characteristics of the present invention. As shown in Steel Type B in Table 1, it has become clear that SR cracking can be prevented if the total S (sulfur) content in the steel is 0.002% or less.

Therefore, to prevent SR cracking, which is the objective of the present invention, all S(
It is effective to keep the amount below 0.002%. In order to reduce S (sulfur) to 0.002% or less, methods of adding slag and refining, such as CaC2 treatment and ladle refining, are used, but any process can be used to reduce S (sulfur). There is no harm in reducing it. However, even in steel materials in which the total amount of S (sulfur) contained in the steel exceeds 0.002%, S (sulfur) becomes stable in the steel after undergoing thermal history during the manufacturing process of the steel material or welded structure. If it is fixed and S (sulfur) in a solid solution state exceeding 0.002% does not occur as described above, SR cracking will not occur. Steel types C, D, E, F, G, H, I or K in Table 1 are Mg (Magnesium) Tea in the steel making process.
(calcium), Al (aluminum), Y (yttrium), Ce (cerium), Ti (titanium), Zr (
It is clear that SR cracking is prevented by adding a small amount of these elements. All of these elements are strong sulfide-forming elements, and the addition of these elements reduces the total amount of S (sulfur) contained in steel, or contains sulfide or S (sulfur). It is necessary to stably fix S (sulfur) as a composite compound in order to prevent SR cracking. The solubility of sulfides of these elements is extremely low even in molten steel, so the atomic equivalent of S (sulfide) becomes sulfide, and the remainder exists as solid solution S (sulfur). However, in order to be more certain, if the amount of these elements remaining in the steel is 3% compared to the amount of S (sulfur) remaining in the steel,
．． 0 times or more, 1.5 times or more in the case of Ca (calcium), 6.0 times or more in the case of AI (aluminum), Y
(yttrium), 6.0 times or more, Ce (cerium), 9.0 times or more, Ti (titanium), 6.0 times or more, and Zr (zirconium), 6.0 times or more.
．． 0 times or more, or 6 in the case of Hf (hafnium)
.

It is preferable that it is 0 times or more.

Here, the amount of S (sulfur) in a solid solution state is calculated using the following formula. X(%+1(Mg/3.0+Ca/1.5
+AAA. O+Y/6.0・+Ce/9.

0+Ti/6.0+Zr//6.0+Hf/E. O) Amount of S in solid solution (a) = Amount of S in steel (a) - K (% When the left side is negative, it is considered zero.

These elements can be added to steel by injecting powdered additives into molten steel together with gas, by fixing capsules containing additives in molds, or by using wires containing additives. A method such as feeding it into molten steel is used, but it may be added by any method.

However, if too many of these elements are added, the harmful effects of the added elements themselves cannot be ignored, so S(
In steel containing 0.05% or less of sulfur), the content of the above elements should be limited to 0.5% or less. In addition,
These elements may be added as a composite such as ferroalloy or silicon calcium, or two or more of these elements may be added in a composite. In the case of composite addition, the amount (A) remaining in the steel is (Mg/3.0+
Ca/1.5+All/6,0+Y/6.0+Ce/9
．． 0+Ti/6.

0+Zr/6.0+Hf/6.0) should be limited to 0.1% or less, and it is preferable that the above amount of trenched S (sulfur) remaining in the steel is at least 1 Yes.

1) The effects of the present invention occur when the main metal structure of the weld heat affected zone is martensite, paynite, or a mixed structure thereof, and the steel according to the present invention is mainly used in boilers, pressure vessels, and welding. Since it is a structure, etc., in order to meet the required strength, toughness and hardenability, Ni (nickel) of 5% or less and C may be added as necessary.
r (chromium) 3% or less, MO (molybdenum) 3% or less,
V (vanadium) 1% or less, Nb (niobium) 1% or less,
One or more types of Cu (copper) up to 1% can be added.

Ni is often added mainly to improve the toughness of steel materials, but stress relief annealing cracks do not usually occur in currently used steels containing more than 5% Ni. Cr is often added mainly to increase the strength of steel materials, but Cr
Stress relief annealing cracks usually do not occur in steels with a content of more than 3%. MO, V & Nb are often added mainly to increase the strength of steel materials, but these elements are expensive, so MO should be kept below 3%, V below 1%, and Nb below 1%. limited to. Cu is often added to increase the strength and/or corrosion resistance of steel materials, but if the amount added is too large, cracks are likely to occur during welding, so it is limited to 1% or less. Ru. Examples of the present invention are shown in Tables 1 and 2.

In addition, the values of X and solid solution S calculated by the following method when Mg, Ca, AltYyce, Ti, Zr, and Hf were added are also listed in the table. X%) = (Mg/3.0
+Ca7' 1.5+All/6.0+Y/6.0+
Ce/9.0+Ti/6.0+Zr/6.0+Hf/6
．． 0) Amount of S in solid solution (a) = Amount of S in steel (a) - X @ When the left side is negative, it is considered zero.

U] As explained above, according to the present invention, SR cracking can be prevented, and the effect is therefore extremely significant.

[Brief explanation of the drawing]

Figure 1 is a chart showing the relationship between the relative Auger electron strength of S (S) and the depth from the SR crack surface for steel type A3, and Figure 2 shows the load stress of constant strain restraint and the relationship at 610°C for steel types A3 and G1. It is a chart showing the relationship between stress relief annealing time.

Claims (1)

[Claims] 1 C: 0.05-0.5%, Si: 0.10-1.0
%, Mn: 3% or less, Cr: 3% or less, Mo: 3% or less, V: 1% or less, Nb: 1% or less, Cu: 1% or less, and the remainder is substantially For steels that are mainly made of Fe and whose main metallographic structure in the weld heat affected zone is martensite, bainite, or a mixed structure of these, and which tend to crack easily during stress relief annealing treatment, S is further added to 0.002. Steel with low stress relief annealing cracking susceptibility suppressed to less than %. 2C: 0.05-0.5%, Si: 0.10-1.0
%, Mn: 3% or less, Cr: 3% or less, Mo: 3% or less, V: 1% or less, Nb: 1% or less, Cu: 1% or less, and Ni: 5% or less. , the remainder is substantially Fe, and the main metallographic structure of the weld heat affected zone is martensite, bainite, or a mixture of these structures, and is likely to crack during stress relief annealing treatment. A steel with low stress relief annealing cracking susceptibility, with S content suppressed to 0.002% or less. 3C: 0.05-0.5%, Si: 0.10-1.0
%, Mn: 3% or less, Cr: 3% or less, Mo: 3% or less, V: 1% or less, Nb: 1% or less, Cu: 1% or less, and the remainder is substantially In addition, in steels that are primarily Fe and whose main metallographic structure in the weld heat affected zone is martensite, bainite, or a mixed structure of these, and which tend to crack during stress relief annealing treatment, Mg
, Ca, Al, Y, Ce, Ti, Zr and Hf in an amount of 0.5% or less each and (Mg
/3.0+Ca/1.5+Al/6.0+Y/6.0+
Ce/9.0+Ti/6.0+Zr/6.0+Hf/6
．． A steel with low stress relief annealing cracking susceptibility, in which S in solid solution in the steel is suppressed to 0.002% or less by adding 0) in a range not exceeding 0.1%. 4C: 0.05-0.5%, Si: 0.10-1.0
%, Mn: 3% or less, Cr: 3% or less, Mo: 3% or less, V: 1% or less, Nb: 1% or less, Cu: 1% or less, and Ni: 5% or less. In addition, Mg , Ca, Al, Y, Ce, Ti, Zr
and 0.5% each of one or more types of Hf.
and (Mg/3.0+Ca/1.5+Al/6.
0+Y/6.0+Ce/9.0+Ti/6.0+Zr/
6.0+Hf/6.0) is added within a range not exceeding 0.1%, thereby suppressing S in the solid solution state in the steel to 0.002% or less. A steel with low stress relief annealing cracking susceptibility.