JPH079027B2 - Forming method of low alloy steel for high temperature - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for forming a high-alloy low-alloy steel, and more particularly to a high-temperature low-alloy steel containing V or V and Nb containing no normalizing treatment. The present invention relates to a method of forming low alloy steel.

(Prior art) High-temperature low-alloy steels containing Cr and Mo as the main alloying elements are superior in high-temperature strength and corrosion resistance to carbon steels, so they are used in fields such as boilers, chemical industry, and nuclear power in austenitic steels and high-grade steels. It is widely used for heat resistant members that do not require Cr ferritic steel, such as heat exchange organs, pipes, structural steel plates and the like.
G 3462 STBA established in JIS as a representative
22 steel, STBA24 steel, STBA25 steel.

On the other hand, a low alloy steel having high strength at high temperature in which V or Nb as a precipitation strengthening element is further added to a low alloy steel containing Cr and Mo as main alloying elements is also used as a practical material. Typical examples are 1% Cr-1% Mo-0.25% V steel used as a turbine rotor and casing material, and 2.25% Cr-1% Mo- used as a structural material for fast breeder reactors. There is 0.3% Nb steel.

For low alloy steels containing the latter precipitation strengthening element,
In general, the molding process was performed by the process shown in FIG. That is, after the final hot forming process such as hot rolling or hot extrusion, it is allowed to cool to room temperature and then Ac ₃
This is a forming method in which normalizing treatment at a temperature of the transformation point or higher and tempering treatment at a temperature of the Ac ₁ transformation point or higher are performed.

When hot bending, cold working, and welding are performed after the above tempering treatment, post heat treatment such as stress relief annealing is often performed after these treatments.

By the way, in this conventional forming method, heat treatment after hot working is an essential step in order to increase creep strength which is particularly important as a high temperature material and to obtain stable strength.
This point is different from the so-called solid solution strengthened high Cr ferritic steel to which V and Nb are not added.

That is, in the conventional manufacturing method, in the normalizing and tempering treatments after forming, in order to ensure good creep strength and high-temperature strength, in the normalizing treatment, the material after hot working is made to have an Ac ₃ transformation point or higher. It is necessary to heat to the temperature range of 1 to dissolve precipitates such as coarsened carbides and to eliminate segregation of various alloy components, and rapidly cool after heating to make the structure martensite or bainite. Further, in the tempering treatment, it is necessary to precipitate fine precipitates to form a stable structure at high temperature.

However, in the conventional forming method, since the heat treatment step and the hot working step are independent, there is a drawback that the energy cost for heating increases. Especially in the normalizing process, since the material after hot working has to be reheated to a high temperature of Ac ₃ transformation point or higher, a large amount of thermal energy is inevitably used, and the heat treatment causes the material to remarkably increase. After heat treatment, it had to be straightened and maintained due to its deformation and oxidation. Therefore, the product cost has been further increased.

As described above, the normalizing process has many problems and is not preferable for the manufacturer. However, if this normalizing treatment is simply omitted, the strength and toughness of the product will be impaired, and especially, the decrease in high temperature creep strength will become remarkable, and the product will become worthless.
The reason for this is that if normalizing treatment is omitted, undissolved carbonitrides during hot working become coarse and fine precipitates cannot be obtained, and the structure does not become stable martensite or bainite. The high temperature creep strength is lowered due to the fact that the segregation of the steel remains segregated.

The DIN standard of West Germany is based on 14MoV63 steel (0.5% Cr-0.5% M
For o-0.3% V steel), it is possible to omit the normalizing process after hot finishing, but no detailed hot working conditions have been established. See also Sho 63-3
No. 928 discloses a solid solution strengthened low alloy steel, 2.1 / 4Cr-1.
Although a manufacturing method is disclosed in which normalizing treatment is omitted for Mo steel, the present invention differs from the present invention in the object of the object alloy and heat treatment.

(Problems to be Solved by the Invention) An object of the present invention is to provide a method of forming a precipitation-strengthened low alloy steel containing at least one of V and Nb, even if the normalizing treatment is omitted, the same as or better than the conventional one. An object of the present invention is to provide a molding processing method capable of obtaining a product having the above characteristics.

(Means for Solving the Problems) The inventors of the present invention, when forming a precipitation-strengthened low alloy steel containing at least one of V and Nb, perform hot working conditions such as final hot rolling and hot extrusion. It was found that by limiting the value to a specific range, it is possible to obtain a product having characteristics equal to or higher than those of the conventional products without performing a normalizing treatment after hot working.

In other words, the final hot working is performed by the structure heat treatment method
During the final hot working, the carbonitride is uniformly solid-dissolved and the alloy components are homogenized, and a sound martensite or bainite structure is generated during the hot working and cooling, and the V and / or Nb fine Carbide is dispersed and precipitated, and then heated to a temperature below the Ac ₁ transformation point to further precipitate fine carbide and soften the structure hardened by processing, and if the structure is stable at high temperature, it is possible to perform normalization after hot working. High strength and excellent toughness can be obtained without any treatment.

Here, the gist of the present invention is "by weight, C: 0.02 to 0.3%, Cr:
0.1 to 5%, one or two of Mo and W in a total amount of 0.2 to 4
%, And one or two of V and Nb in a total amount of 0.01
In molding method of a low alloy steel containing 1%, the final hot-molding Ac ₃ transformation point + 50 ℃ or higher, the temperature range of heating than 1 minute to 1150 ° C. or less of the temperature range to 700 ° C. to 1150 ° C. At a processing rate of 20% or more, and then 500
After cooling at a cooling rate of ℃ / h or more, it is heated to a temperature range below the Ac ₁ transformation point and tempered, and it is described in “Formation processing method for low alloy steel for high temperature”.

Further, the present invention is "in weight%, C: 0.02 to 0.3%, N: 0.1% or less, Cr: 0.1 to 5%, and one or two kinds of Mo and W in a total amount.
A low alloy steel containing 0.2 to 4% and further containing one or two kinds of V and Nb in a total amount of 0.01 to 1% is processed under the same conditions as the above final hot forming, cooling and tempering, Forming method of high temperature low alloy steel characterized by cooling and heat treatment "," Temperature of Ac ₁ transformation point or less in the final step of the above method is 500 ° C or more, temperature of Ac ₁ transformation point or less Forming method of low alloy steel for high temperature, characterized by performing warm working after heating to the temperature range "and" after the warm working, further heating to a temperature range below the Ac ₁ transformation point to perform stress relief annealing "The method for forming and processing low-alloy steel for high temperature, characterized in that".

(Operation) Hereinafter, the method of forming the low alloy steel for high temperature according to the present invention will be described in detail.

First, the reason why the contents of the low alloy steel contained in the present invention are limited as described above will be explained together with the function and effect.

C: C is an element that combines with V and Nb described later to precipitate carbides and give strength. However, if its content is less than 0.02%, the strength is not sufficient, stable carbides are not formed, and a sound martensite or bainite structure is not obtained. Therefore, sufficient creep strength and toughness cannot be secured. On the other hand, if the content exceeds 0.3%, the carbides are remarkably coarsened, and not only the strength and toughness are lowered, but also the steel is hardened and the workability and weldability are impaired.

Cr: Cr is a main alloying element in high temperature materials, and if its content is less than 0.1%, the oxidation resistance required for high temperature steel cannot be obtained. As the content of Cr increases, the oxidation resistance, hydrogen corrosion resistance, Na corrosion resistance, etc. improve. However, if the content exceeds 5%, a bainite structure is not formed, and sufficient creep strength and toughness cannot be obtained.

Mo and W: Both of these elements are major solid solution strengthening elements in high temperature materials, and have the action of increasing the high temperature creep strength when added alone or in combination. However, if the total content of 1 type or 2 types is less than 0.2%, sufficient strength cannot be obtained, and if the total amount exceeds 4%, the steel is significantly hardened and the workability and weldability are impaired. Not only that, it also reduces strength and toughness.

V and Nb: All of these elements combine with C or N to form V (C, N)
Alternatively, it is an important element that forms fine precipitates of Nb (C, N) and improves creep strength. However, when these carbonitrides are coarsely deposited, not only the effect disappears and the creep strength is remarkably lowered, but also the workability, weldability and toughness are adversely affected. Therefore, in the present invention, the content of these elements is limited to 0.01 to 1% in total of one or two kinds. If the total content is less than 0.01%, the carbonitride will not be sufficiently precipitated and the creep strength will not be improved. Also, if the total content exceeds 1%, V
Precipitates of (C, N) or Nb (C, N) become coarse, which causes deterioration of strength, toughness, workability and weldability.

N: N stabilizes austenite and has a function of precipitating a nitride. Although this effect is obtained even at a normal impurity level, it may be positively contained above the impurity level. However, if its content exceeds 0.1%, the workability and weldability are significantly impaired, and the toughness and strength also decrease. Therefore, even when N is positively added, its content is set to 0.1% or less.

The precipitation-strengthened low alloy steel targeted by the method of the present invention can achieve the object if the content of at least each of the above elements is within the above range, but contains the elements as shown below. Good.

Si: 0.5% or less, Mn: 1.5% or less, Al: 0.04% or less, Ni: 1%
Less than. Or, if necessary, Ti, B, Zr, Ca, La
The total amount of one or more of the above may be 0.3% or less. These elements have the effect of improving strength, toughness, workability and weldability.

In addition, P, S, Ab, Sn, etc. become impurities,
Each of these impurities should be as low as possible within the range of 0.03% or less.

Next, the molding method of the present invention will be described with reference to the accompanying drawings, and the reasons for limiting the molding conditions will be described.

FIG. 1 is a schematic view showing the steps involved in the molding method of the first aspect of the invention.

(I) Heating step: The heating step in the final hot working is a carbonitride coarsened in the preceding process such as casting, hot working to form billets or slabs and hot working to form pipes, plates, etc. The purpose is to solidify precipitates such as the above, to homogenize segregation of additional alloying elements, and to homogenize non-uniform processed structure. In particular, this heating has the purpose of sufficiently dissolving V and / or Nb carbonitrides. Therefore, in this heating, the low alloy steel having the above composition is subjected to Ac ₃ transformation point + 50 ° C. or higher (usually, in the target steel of the present invention, the Ac ₃ transformation point is about 850 ° C. to 950 ° C.) and a temperature range of 1150 ° C. or lower. It is necessary to carry out the heating under the condition of heating for 1 minute or more. If the heating temperature is lower than the Ac ₃ transformation point + 50 ° C or higher, the solid solution of the above precipitates will be insufficient, and if the conventional normalizing process is omitted, the high temperature creep strength will be significantly impaired, and the non-uniformity of the previous process will occur. The structure and segregation remain, resulting in a decrease in strength and toughness. On the other hand, the higher the heating temperature is, the more uniform the solid solution of precipitates is promoted. However, if the heating temperature exceeds 1150 ° C., grain growth occurs and the toughness and strength are reduced.

In addition, it is desirable that the material containing a large amount of V and / or Nb is heated on the high temperature side of the above temperature range to promote the solid solution of carbonitride.

If the heating / holding time is less than 1 minute, there are many undissolved precipitates, and because of the segregation of alloying elements, fine carbonitrides of V and Nb are not sufficiently precipitated, so that the creep strength cannot be improved.

The upper limit of the heating and holding time is not particularly specified in the present invention. Although it does not deteriorate in characteristics even if it is heated for a long time, it is desirable to limit the thickness to about 1h / 25mm in actual production. Further, if the material temperature after heating is a predetermined processing start temperature, the material is immediately subjected to processing.
It is better to heat and hold for more than a minute.

(II) Final hot forming process: In the final hot forming process such as hot rolling and hot extrusion, the workability of the heated material is 20% at 700 to 1150 ° C.
It is performed under the above conditions. The processing may be carried out in any temperature range of ferrite single phase, two phases of ferrite and austenite, and austenite single phase, but 20% or more of processing is applied to the material to give sufficient processing strain, segregation of alloying elements It is important to control the precipitation of fine precipitates such as carbonitrides of V, Nb, Cr, etc.

If the processing temperature is less than 700 ° C., not only the deformation resistance is high and sufficient forming processing cannot be performed, but also the creep strain, toughness and weldability are impaired due to the residual processing strain.
Further, when processed at a temperature higher than 1150 ° C, the recovery of the working strain is fast, and it becomes impossible to control the fine precipitates and the structure due to the working strain, and the strength and toughness are impaired.

If the workability in this step is less than 20%, the precipitation of carbonitrides and the like will be insufficient and stable fine precipitates will not be formed, so that the creep strength will be impaired.

(III) Cooling step after hot working: The cooling step after hot working is also one of the important steps of the method of the present invention. Its purpose is to control the precipitation morphology of fine precipitates by processing strain during cooling after hot working. That is, by quenching at a cooling rate of 500 ° C./h or more after hot working, V and Nb carbonitrides are finely precipitated in a dot sequence and sound martensite due to processing strain below the Ar ₁ transformation point. A structure or a bainite structure (a bainite structure partially containing ferrite) is formed.

However, if the cooling rate is less than 500 ° C / h, the carbonitrides of V and Nb are coarsened and the conformity is lost, the creep strength is impaired, and the carbides of Fe and Cr are precipitated and the structure is pearlite or ferrite. + It becomes pearlite, which impairs strength and weldability.

Here, the consistency means a state in which carbonitrides of V and Nb are finely dispersed and deposited while maintaining the crystal orientation relationship in the matrix.

Generally, if the size of the precipitate is 500 Å or less, it will be in a state with lattice distortion that matches the matrix, and this will impede dislocation movement due to creep and increase strength, but if the consistency of the precipitate disappears, That is, coarse precipitates of 0.1 μm or more no longer contribute to the improvement of creep strength. In the present invention, by defining the conditions of the final hot forming, cooling and subsequent tempering,
The precipitate can be brought into the state as described above.

In addition, cooling is performed at 500 ° C /
Since it is difficult to secure a cooling rate of h or more, it is preferable to rapidly cool such a material by an accelerated cooling method such as water cooling or mist cooling.

The molding processing method according to the present invention includes the above (I) to (II
The material after the step I) may be tempered to obtain a final product, or as shown in the attached FIG.
The material after the step (III) may be reheated and warm-processed to obtain the final product. Further, as shown in FIG. 3 of the attached drawings, stress-relief annealing may be carried out after the warm working to obtain a final product.

The purpose of performing these processing steps after hot forming is as follows.

(IV) Tempering treatment step: The tempering treatment is to secure a stable structure at high temperature and further precipitate fine carbonitrides by treating it at the Ac ₁ transformation point or lower as in the conventional case. At temperatures above the Ac ₁ transformation point, strength and toughness are impaired.

This tempering is 72 in terms of removing hot working strain.
It is preferable to perform the treatment under a treatment condition of heating in a temperature range of 0 to 750 ° C. for 1 to 2 hours.

(V) Reheating step, (VI) Warm working step: The purpose of the reheating step and the warm working step is to fine V and / or Nb during heating and warm working (for example, bending).
The purpose of this is to further precipitate the carbonitride of to increase the creep strength. Further, it is to further stabilize the martensite structure and bainite structure formed by the thermo-mechanical treatment of the previous steps (I) to (III) to enhance the strength and toughness.
For that purpose, it is preferable that the material after hot working is heated in a temperature range of 500 ° C. or higher and the Ac ₁ transformation point or lower, and then subjected to warm working with a working degree of preferably 20% or less.

When the heating temperature is lower than 500 ° C, the structure becomes hard martensite or bainite, and the toughness, high temperature creep strength and ductility are impaired. On the other hand, when heated at a temperature higher than the Ac ₁ transformation point, re-austenization occurs due to partial transformation, and strength and toughness are significantly impaired.

(VII) Stress relief annealing step after warm working: This step removes internal strain accumulated by warm working by heating the material after warm working to a temperature below the Ac ₁ transformation point. Is the purpose.

The product obtained by the above-described forming method may be further cold worked, or may be warm worked by welding and heating at the Ac ₁ point or lower.

The present invention will be further described below with reference to examples.

(Example) A low alloy steel having the chemical composition shown in Table 1 was melted in a 150 kg vacuum furnace and cast into a mold to produce an ingot.
After heating to a temperature of 900 ℃, hot forging 40mm thick × 80mm
A block having a width of 200 mm and a length of 200 mm was prepared. Using this as a raw material, molding was performed under the hot working and heat treatment conditions shown in Table 2.

Hot working is performed by roll rolling in the working temperature range shown in Table 2.
Processing was performed at a wall thickness reduction rate of 40%. The warm working was also performed by roll rolling at a wall thickness reduction rate of 10%.

In addition, "conventional method" shown in the column of the forming method in Table 2 means the step shown in FIG. 4 attached, "(a)" means the step shown in FIG. 1, and "(b)" means the step shown in FIG. In the process shown in FIG. 2, “(c)” means that the molding process was performed in the process shown in FIG. Further, the “(a) comparative method” means that the molding process was performed in the step shown in FIG. 1, but the molding process was performed under the condition that the heating temperature or the processing temperature was out of the range specified by the present invention. Means that.

A JIS No. 4 Charpy impact test piece and a tensile test piece with a diameter of 6 mm and a rating distance (GL) of 30 mm were taken from the center portion of the wall thickness in the rolling direction of the processed plate material thus obtained, and subjected to a room temperature tensile test, a Charpy impact test and A 550 ° C creep rupture test was performed. The results are shown in Table 3. Further, FIG. 5 shows a graph of creep rupture strength with respect to the conventional example.

As is clear from Table 3, the products formed by the method of the present invention have characteristics such as tensile strength, toughness and creep rupture strength as compared with those formed by the conventional method of normalizing. Equal or better. On the other hand, although the molding process was performed by the same steps as the method of the present invention, the processing temperature is low in Comparative Examples A3, B3, and C3, while the tensile strength is remarkably high, but the ductility, toughness, and creep rupture strength are low. . Moreover, the creep rupture strength of A4, B4, and C4, which are comparative examples with low heating temperatures, are extremely low because the structure and material are unstable.

As is clear from FIG. 5, the creep rupture strength ratio of the inventive example is equal to or higher than that of the conventional example, whereas the comparative example is extremely low. Whether the history of the previous step of hot working remains and whether fine V and Nb carbonitrides are not precipitated (A3, B3, C3), high working strain and a sound bainite structure (A4, B4, C4), due to either cause.

The processed plate material obtained by the method of the present invention was extracted and observed with an electron microscope for plica. As a result, fine V, Nb, and Cr carbonitrides were deposited in a point sequence and were sound. It was an organization.

(Effects of the Invention) As described above, according to the method of the present invention, the normalizing process in the forming process of the low alloy steel can be omitted, so that the process is rationalized and the cost is significantly reduced. Further, it is possible to obtain a product having characteristics equal to or higher than those of the conventional products without performing the normalizing treatment.

Therefore, the method of the present invention has a great effect as a method for manufacturing plate materials, pipe materials, and forged products for boilers, nuclear power, chemical industries, and the like.

[Brief description of drawings]

FIG. 1 is a heat pattern diagram showing the steps of the molding method of the present invention. FIG. 2 is a heat pattern diagram showing the steps of another molding method of the present invention. FIG. 3 is another figure of the present invention. FIG. 4 is a heat pattern diagram showing the steps of the molding method, FIG. 4 is a heat pattern diagram showing the steps of the conventional molding method, and FIG.
5 is a graph showing the creep rupture strength at 10 ° C. × 10 ⁴ h as a strength ratio with respect to the conventional example.

Claims (4)

[Claims] 1. By weight%, C: 0.02 to 0.3%, Cr: 0.1 to 5
%, 1 or 2 kinds of Mo and W in a total amount of 0.2 to 4%, and 1 or 2 kinds of V and Nb in a total amount of 0.01 to 1%
In the forming method of the low alloy steel containing, the final hot forming process is heated to a temperature range of Ac ₃ transformation point + 50 ° C or more and 1150 ° C or less for 1 minute or more and the working degree in the temperature range of 700 ° C to 1150 ° C Of 20% or more, then cooled at a cooling rate of 500 ° C./h or more, and then heated to a temperature range below the Ac ₁ transformation point and tempered. Molding method. 2. By weight%, C: 0.02-0.3%, N: 0.1% or less,
Cr: 0.1-5%, 1 or 2 kinds of Mo and W in a total amount of 0.2
~ 4%, and 1 or 2 of V and Nb in total
A low-alloy steel for high temperature, characterized in that the low-alloy steel containing 0.01 to 1% is subjected to final hot forming, cooling and tempering under the conditions of claim 1, cooling, and heat treatment. Molding processing method. 3. In place of the tempering below the Ac ₁ transformation point in the method according to claim 1 or 2, instead of heating to a temperature range of 500 ° C. or more and below the Ac ₁ transformation point, warm A method of forming a low-alloy steel for high temperature, which comprises processing. 4. A low-alloy steel for high temperature, which is characterized in that after the warm working in the method according to claim 3, the alloy is further heated to a temperature range below the Ac ₁ transformation point and stress-relieved and annealed. Molding method.