JPS633928B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing hot-formed Cr--Mo steel products with excellent mechanical properties. Cr-Mo steel has excellent properties such as high temperature strength and hydrogen resistance, that is, resistance to deterioration of toughness due to hydrogen attack when used under high temperature and high pressure hydrogen. It is widely used as a welded structural material for pressure vessels such as refining vessels, and it complies with the US ASTM standard and Japan's
It is a low-carbon, low-alloy steel that has been established in JIS standards. Cr-Mo steel is classified into several types depending on the Cr and Mo content, but among them, Cr-Mo steel containing about 2.25% Cr and about 1% Mo (ASTM standard A387-22 steel, JIS standard SCMV4 steel) is Cr—
It is most commonly used as a representative of Mo steel, and the present invention is also directed to this steel type. In manufacturing a pressure vessel, some members are often hot formed, ie, simple bending, as well as head plates, pipes, T-pieces, etc., are hot formed. When a steel material undergoes hot working, it inevitably means that it is heated above its transformation point. Therefore, the material before hot working undergoes homogenization treatment by normalizing and tempering to meet specifications. Even if good mechanical properties are maintained, those properties are lost when heated above the transformation point, so in order to guarantee the same mechanical properties as the raw material for molded products, it is necessary to re-normalize and temper them after molding. It has become necessary to do this in the actual work process, and it has become customary to do so in the actual work process. However, those skilled in the art believe that normalizing treatment after molding makes it difficult to obtain uniform mechanical properties because the shape of the molded product is more complex than the material, and the cooling rate varies depending on the part. It is said to be a very undesirable process because it tends to cause defects such as distortion, and it also increases the cost of heat treatment, which is extremely costly. Therefore, it is possible to simplify the heat treatment after forming, in particular omit the normalizing treatment, and perform only reheating at a temperature below the Ac ₁ transformation point, resulting in heat treatment with excellent mechanical properties represented by strength and toughness. It has been a desire of those skilled in the art to obtain preformed products. The purpose of the present invention is to overcome the drawbacks of the conventional manufacturing method for hot-formed products of Cr-Mo steel, simplify the hot treatment process after forming, and provide a manufacturing method that can provide excellent mechanical properties. is to provide. The present inventors conducted detailed research on the effects of hot forming conditions and post-forming heat treatment conditions on the mechanical properties of hot-formed products for various pressure vessel steels including Cr-Mo steel. In other words, the various factors that affect the mechanical properties of hot-formed products include the heating temperature of the material for hot forming during hot forming, the material processing temperature and processing amount during processing forming, and after hot forming, We will discuss various heat treatments such as normalizing, which involves heating above the Ac ₃ transformation point and then cooling, tempering, which involves heating and cooling below the Ac ₁ transformation point, and stress relief annealing, and variously changing these factors. As a result of conducting research on the relationship between the mechanical properties of hot-formed products and the mechanical properties of the hot-formed products from a metallurgical perspective, the following findings were obtained. That is, 2 1/4Cr-1Mo steel (ASTM standard
In the case of hot-formed products made of A387-22 steel (JIS standard SCMV4 steel), if the hot-forming conditions, that is, the heating temperature and processing temperature, are limited to a certain range, then the product must be baked again after hot-forming. By omitting the step of normalizing and simply heating to a temperature below the Ac ₁ transformation point and cooling, the material is subjected to normalizing and tempering, or the material is subjected to normalizing after hot forming. Furthermore, they discovered an epoch-making phenomenon in which strength and toughness are equal to or superior to hot-formed products that are heat-treated by heating to a temperature below the Ac ₁ transformation point and then cooling. .
In other words, conventionally, material → hot forming → normalizing →
The process is (tempering) → welding assembly → stress relief annealing → finished product, but by performing hot forming under the conditions discovered by the inventors, material → hot forming → (tempering) → welding Assembly → Stress relief annealing →
It is possible to omit the normalizing process after hot forming, which not only has the economical effect of reducing heat treatment costs, but also eliminates distortion of hot formed products due to normalizing. It has now become possible to provide hot-formed products that are stable and have excellent mechanical properties without having to worry about such concerns. In the above, the process of tempering before welding and assembly in the conventional process and the process of the present invention is shown in parentheses to indicate that it can be omitted in some cases. In other words, both the hot-formed and normalized state in the conventional process and the as-hot-formed state in the inventive process have the same fairly high strength and low toughness, so there is a risk of cracking during welding assembly. Therefore, it is desirable to perform a tempering treatment before welding and assembly, but in some cases, it may be omitted if careful attention is paid to the welding and assembly work and a certain degree of cracking can be tolerated. In addition, the materials used in the process of the present invention are steel materials that have undergone a normal rolling or forging process, and are not subjected to any heat treatment, those that have been subjected to ordinary normalizing/tempering treatment, and those that have been subjected to only tempering treatment. ,
Applicable to products that have undergone any processing and heat treatment, including those that have been completely annealed at an Ac of ₃ or more points or those that have been annealed at an Ac _{of 1} or less.
Appropriate austenitization is achieved at a temperature of Ac ₃ or higher and 950°C or lower during molding, so it is possible to obtain the effects of the invention with materials of any coating history.
However, due to the homogeneity of the material and the transportation before heating during molding,
In order to avoid problems such as cracking during gas cutting and cold working, it is desirable to perform at least an annealing treatment, preferably a normalizing-tempering treatment. The gist of the present invention is as follows.
That is, C: 0.15% or less, Si: 0.50% by weight
Below, Mn: 0.27 to 0.63%, P: 0.035% or less,
S: 0.035% or less, Cr: 1.88-2.62%, Mo: 0.85
~1.15%, with the remainder consisting of Fe and unavoidable impurities, in a method for producing a hot-formed product of Cr-Mo steel, in which the Cr-Mo steel material is heated to a temperature above the Ac ₃ transformation point and below 950°C. Then, it is molded in a temperature range below the Ar ₃ transformation point and above the Bs point, and then heated to a temperature above 500℃ and below the Ac ₁ transformation point, and then cooled.
This is a method for manufacturing Cr-Mo steel hot-formed products. Next, the reason for limiting the range of components in the present invention will be explained. When considering the combination of strength and toughness as well as weldability as a welded structural steel, it is necessary to include C at an upper limit of 0.15%. Si needs to be added for the purpose of deoxidizing during refining, imparting strength, and oxidation resistance during high-temperature use in Cr-Mo steel. On the other hand, an increase in Si causes embrittlement during use and a decrease in creep rupture strength due to the tempering embrittlement phenomenon during high-temperature use of 400 to 550°C, so the upper limit is set to 0.50%. Like C, Mn is used to impart strength and toughness.
Although 0.27% or more of Mn is added, increasing the amount of Mn will promote embrittlement during use at high temperatures especially in the case of Cr-Mo steel, so the value is significantly lower than that of general welded structural steel, that is, 0.63 % must be the upper limit. Mo is an effective element for imparting creep strength at 400 to 550°C and high-temperature short-time tensile strength, and must be added at a minimum of 0.85% to enable application to oil refining pressure vessels. , and the high-temperature strength properties improve with further increase, but
If the content is 1.15%, the effect will be saturated, leading to a decrease in toughness and also being disadvantageous from an economic point of view, so the upper limit is set at 1.15%. Cr is an essential component for imparting high-temperature strength properties.
Furthermore, the steel of the present invention is useful for improving oxidation resistance at high temperatures and for imparting hydrogen corrosion resistance, which is an unavoidable characteristic for steels for pressure vessels for petroleum refining such as deoxidation reactors. A minimum content of 1.88% is required to meet the purpose, and as the Cr content increases, the oxidation resistance and hydrogen attack resistance will improve, but if it exceeds 2.62%, the creep strength will decrease. The upper limit is set at 2.62%. The upper limits of P and S are set at 0.035% in consideration of weldability, hot workability, and embrittlement during use. In the present invention, by combining the above steel composition and the heat treatment described below, it is possible to omit the normalizing treatment after hot forming, and the hot forming of 2 1/4Cr-1Mo steel with excellent mechanical properties is possible. Molded products can be obtained. Moreover, the hot forming conditions of the present invention are as follows. Heating temperature for hot forming is above Ac ₃ transformation point
The temperature shall be 950℃ or less, and the processing temperature shall be limited to below the Ar ₃ transformation point and above the Bs point. Here, the Ac ₃ transformation point refers to the A ₃ transformation point during heating, that is, the temperature above which the entire steel structure becomes austenitic, and the Ar ₃ transformation point refers to the A ₃ transformation point during cooling from the austenitizing temperature. That is, it means the temperature at which pro-eutectoid ferrite begins to precipitate, and the Bs point means the temperature at which benite transformation occurs after completion of precipitation of pro-eutectoid ferrite during cooling from the austenitizing temperature. The reason why the hot forming conditions were limited as described above is as follows. First, determine the heating temperature for hot forming.
The reason for limiting the temperature to 950°C or lower is that heating above 950°C will cause significant scale formation and deteriorate the surface quality of the hot-formed product, and will only increase the cost and time required for heating without providing any benefit. That's because there isn't. In addition, the heating temperature was limited to the Ac ₃ transformation point or higher.
If the temperature is below Ac ₃ transformation point or above Ac ₁ transformation point, the temperature range becomes a two-phase coexistence region of ferrite and austenite, which leads to an increase in the space factor of ferrite in the structure after hot forming. If only heat treatment is performed by heating to a temperature below the post-Ac ₁ transformation point and then cooling, the strength will decrease significantly, and the normalizing treatment after hot forming cannot be omitted. Furthermore, if the heating temperature is lower than the Ac ₁ transformation point, the deformation resistance during processing increases significantly, making molding virtually impossible. For the above reasons, the lower limit of the heating temperature was set as the Ac ₃ transformation point. The reason why the processing temperature during hot forming was set below the Ar ₃ transformation point is as follows. In other words, when processing in a temperature range above the Ar ₃ transformation point, the ferrite space factor in the structure of the hot-formed product increases due to the accelerating effect of ferrite transformation during processing, and the temperature below the Ac ₁ transformation point after hot forming increases. This is because the strength, especially the yield strength, decreases if only heat treatment is performed by heating to and cooling, and the normalizing treatment after hot forming cannot be omitted. Therefore, the upper limit of the processing temperature was set at the Ar ₃ transformation point. Furthermore, if the processing temperature is lower than the Bs point, the difficulty of processing and forming will increase due to an increase in deformation resistance, and after hot forming, the processing temperature will be lowered to the Bs point after being heated to a temperature below the Ac ₁ transformation point and then cooled. More than Ar ₃
The lower limit of the processing temperature was set as the Bs point because the toughness was inferior to that obtained when the temperature was higher than the transformation point. Therefore, the processing temperature should be limited to above the Bs point and below the _Ar3 transformation point. Although the amount of processing is not particularly limited in the present invention,
Generally, hot forming is carried out within the range of 3% to 30%, and the effects of the present invention are fully exhibited at any processing amount within this range. Here, the amount of machining is defined _as (l
−l ₀ )/l ₀ ×100 (unit: %) means the absolute value of the amount of distortion. In the present invention, the heating temperature during hot forming is
By performing hot forming that satisfies two conditions: Ac ₃ transformation point or higher and 950℃ or lower, and processing temperature lower than Ar ₃ transformation point or higher than Bs point, the normalizing treatment after hot forming can be omitted. It is possible to provide hot-formed products with excellent strength and toughness only by heating and cooling treatments below the Ac ₁ transformation point. Among these hot forming conditions, the most important point is that the hot forming processing temperature is limited. In other words, in conventional hot forming, the material is heated above the Ac ₃ transformation point and then formed during the cooling process.The higher the temperature, the lower the deformation resistance, making the forming process easier, and the overall structure is uniform austenite. It is assumed that the material temperature is higher than _{the Ar 3 transformation} point shown above, which is better for the material of the molded product than when the material temperature reaches the end of the Ar ₃ transformation point and forms a two-phase mixed structure of ferrite and austenite. While it has been common knowledge to carry out the forming process before the temperature drops to Ar3, the present invention breaks this common sense and waits for the material temperature to drop below _{the Ar3} transformation point. Hot forming is performed at certain points between the points. Therefore, in the present invention, the time required for hot forming is slightly longer than before due to the waiting time for the material temperature to drop below the Ar ₃ transformation point, and since the processing temperature is lower than conventionally, the deformation resistance of the material is slightly higher. Although there are some disadvantages such as a slight increase in the energy required for processing, this disadvantage can be said to be more than compensated for by the great effect that normalizing after forming can be omitted. Next, as a heat treatment after hot forming, the present invention requires heat treatment of heating to 500° C. or higher and lower than Ac ₁ transformation point, and then cooling. In other words, even if hot forming is performed under the conditions according to the method of the present invention, the hot forming as it is will show high strength and low toughness, and after hot forming, it will not exceed 500℃ Ac ₁
A hot-formed product with excellent strength and toughness can only be obtained by applying a heat treatment that involves heating the material below its transformation point and cooling it. Heat treatment that involves heating to a temperature above 500°C and below the Ac ₁ transformation point and cooling is an tempering process in which air is evacuated from the heating temperature, or an Ac ₁ transformation that is performed after welding assembly to relieve stress in the weld and soften the weld. This refers to stress-relief annealing, which involves slow heating to a temperature below a point, and then slow cooling. Further, in the present invention, the heat treatment required after hot forming, which involves heating to a temperature of 500° C. or more and below the Ac ₁ transformation point and then cooling, may be performed only once or may be repeated two or more times. That is, there are cases in which tempering after hot forming is omitted and welding assembly is performed and only stress relief annealing is applied after that, and cases where tempering is performed after hot forming, welding assembly is performed, and stress relief annealing is then applied. In either case, it is possible to obtain a hot-formed product with excellent strength and toughness. Next, the chemical composition of the steel limited in the present invention is:
The subject of this invention is ASTM standard A387.
-22 steel, which is G4105 SCMV4 steel according to the JIS standard, has a chemical composition specified by the standard, that is, in weight%, C: 0.15% or less, Si: 0.50% or less,
Mn: 0.27-0.63%, P: 0.035% or less, S: 0.035
% or less, Cr: 1.88 to 2.62%, Mo: 0.85 to 1.15%, and the rest is Cr consisting of Fe and inevitable impurities.
It is made of Mo steel. In this case, unavoidable impurities refer to alloying elements or non-metallic inclusions that are unavoidably mixed in during Cr-Mo steel melting.
An amount that does not impair the basic properties of Mo steel is permissible. For example, 0.20% or less Ni and Cu,
Incorporation of impurity elements such as V and Nb in an amount of 0.05% or less is allowed. Furthermore, although Al is not specified in the standards, it is generally allowed to add 0.060% or less. The main focus of the present invention is on the production of hot-formed products of 2 1/4Cr-1Mo steel used as members of pressure vessels;
It goes without saying that the present invention can also be applied to the production of hot-formed products made of 1/4Cr-1Mo steel. Embodiments of the present invention will be described with reference to the accompanying drawings. The steel with the chemical composition shown in Table 1 is 2 1/4Cr-1Mo
It has a typical composition of steel, and is melted in an electric furnace-ladle refining furnace process and hot-rolled to a thickness of 45 mm. Table 2 shows the results of measuring the transformation point of this steel. For Ac ₁ and Ac ₃ transformation points, the heating rate is 5℃/
The Ar ₃ and Bs points were measured at a cooling rate of 12° C./mm, which is equivalent to air cooling a plate thickness of 45 mm. Table 3 shows 2 1/4Cr-1Mo steel, i.e.
ASTM standard A387-22 steel and JIS standard G4105
The standard values of mechanical properties required for normalized and tempered SCMV4 steel are shown.

【table】

【table】

[Table] Table 4 shows that the steel with the chemical composition shown in Table 1 and having a thickness of 45 mm has been normalized and tempered.
No. 2 steel is obtained by further stress-relieving annealing at 690°C for 15 hours on steel No. 1, and when hot-formed products are manufactured using No. 1 steel by various methods including the present invention. The results of measuring the mechanical properties of No. 3 to No.
8. For example, in the description in Table 4, “925℃×45
mm heating → 750℃ 10% processing → 690℃×15hrF.C."
Heat to 925℃ for 45 minutes and 10% at a temperature of 750℃ in air cooling
Processing (In this example, all processing was performed by rolling down with a rolling roll, and the processing amount was expressed as the plate thickness reduction rate.)

【table】

[Table], followed by air cooling, followed by stress relief annealing by heating to 690°C for 15 hours and furnace cooling. In Table 4, No. 3, No. 4, and No. 5 steels are all hot-formed products manufactured according to the method of the present invention that omit the normalizing treatment after forming, and No. 1 steel is used as the raw material. Comparison with steel plate No. 2, which was not hot-formed and only subjected to stress relief annealing, and hot-formed product No. 8, which was made using a conventional method that included a new normalizing process after hot forming. However, the strength is almost the same and the toughness is actually superior, and even when compared with the standard values in Table 3, it can be seen that the hot-formed products are fully satisfactory as 2 1/4Cr-1Mo steel. Steels No. 6 and No. 7 obtained by the comparative method do not meet any of the conditions of the present invention, and No. 6 steel is obtained when the heating temperature during hot forming is below the range of the present invention, and the processing temperature is lower than the range of the present invention. Also, it is outside the Ar ₃ -Bs point of the present invention, and in this case, the strength, especially YS (yield strength), decreases greatly and shows a value near the lower limit of the standard value, which is not preferable. No. 7 is a case where the processing temperature exceeds the upper limit of the limited range of the present invention, but when the processing temperature exceeds the upper limit, the strength, especially YS, decreases greatly, which is not preferable. Figures 1 and 2 show the hot forming process using No. 1 steel in Table 4 as a material, with the heating temperature during hot forming being 925°C, and the processing temperature being varied in various ways → 690°C
This figure shows the mechanical properties of YS, TS, and vTrs of a hot-formed product that was subjected to stress relief annealing for 15 hours → FC. The numbers in the figure correspond to the numbers in Table 4. As is clear from Figures 1 and 2, when the processing temperature is higher than the Ar ₃ transformation point, the strength, particularly the yield strength, is lower than when processing between Ar ₃ and Bs points, and the toughness shown by vTrs is also inferior. Recognize. Furthermore, when the processing temperature is below the Bs point, the strength is almost the same compared to processing between Ar ₃ and Bs points, but the toughness tends to gradually deteriorate. As is clear from the above examples, the method of the present invention has a great economical effect of omitting the normalizing treatment after hot forming, and also produces 2 1/4Cr-1Mo steel with excellent mechanical properties. We were able to achieve the effect of being able to obtain hot-formed products.

[Brief explanation of the drawing]

Figures 1 and 2 are made of No. 1 steel in Table 4, with a heating temperature of 925°C during hot forming and a working degree of 5 to 5.
690℃ after changing the processing temperature to 10% and variously changing the processing temperature.
The relationship between the mechanical properties and processing temperature of a hot-formed product subjected to stress-relieving annealing (furnace cooling after heating for ×15 hours) is shown. Line A in Figure 1 shows the processing temperature and yield strength (YS).
Line B is a diagram showing the relationship between processing temperature and tensile strength (TS), and line C in FIG. 2 is a diagram showing the relationship between processing temperature and transition temperature (vTrs).

Claims (1)

[Claims] 1% by weight: C: 0.15% or less, Si: 0.50% or less,
Mn: 0.27-0.63%, P: 0.035% or less, S: 0.035
% or less, Cr: 1.88 to 2.62%, Mo: 0.85 to 1.15%, and the remainder is Fe and unavoidable impurities. is heated to a temperature above the Ac ₃ transformation point and below 950°C, then subjected to molding at a temperature range below the Ar ₃ transformation point and above the Bs point, and then heated to a temperature above 500°C and below the Ac ₁ transformation point and cooled. A method for producing a hot-formed Cr-Mo steel product with excellent mechanical properties.