JPH0696752B2 - Low Cr heat resistant steel with excellent toughness for chromizing - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a low Cr heat-resistant steel used after being subjected to a chromizing treatment as a heat resistant material for boilers, chemical industry, nuclear power, etc., and more specifically, The present invention relates to a chromizing steel used as a heat exchanger, a piping material, and a forged product in the above fields, and relates to a low Cr heat-resistant steel having improved toughness and corrosion resistance.

(Prior Art) As a high temperature heat resistant pressure resistant heat exchanger tube for boiler, chemical industry, nuclear power, etc., austenitic stainless steel, 9-12Cr
The high Cr ferritic steels are classified into low Cr steels and carbon steels having a Cr content of 3.5% or less (in this specification, all% of alloy component contents mean% by weight).

Among the above materials, the low alloy steel with a Cr content of 3.5% or less is characterized by containing Cr and being superior in oxidation resistance, high temperature corrosion resistance and high temperature strength to carbon steel, while it is austenitic stainless steel. It is cheaper than steel, has a small coefficient of thermal expansion and does not cause stress corrosion cracking, and is cheaper and has better thermal conductivity and weldability than high Cr ferritic steel. Therefore, this type of low-alloy steel has been widely used since ancient times.
・ 1 / 4Cr-1Mo steel (STBA24), STBA22, STBA20, etc. are standardized. In addition, with emphasis on strength, Nb, V, Ti
There is a steel described in Japanese Patent Laid-Open No. 63-62848 utilizing the precipitation strengthening by the above, and a steel described in Japanese Patent Publication No. 64-2185 improved in strength and toughness. However, these low alloys are far inferior to austenitic stainless steels in terms of high temperature corrosion resistance.

For example, in a thermal power boiler heat exchanger, the outer surface of the pipe is exposed to combustion gases such as heavy oil, coal, and LNG, which causes significant corrosion thinning due to Na, K, S, and V in these fuels.
As a countermeasure against this, so-called chromizing treatment has been considered effective as a countermeasure for improving the corrosion resistance by forming a coating having a high Cr concentration on the surface of a low alloy steel. However, when the existing low alloy steel is subjected to chromizing treatment, the solid solution C of the base material reacts with Cr during the cooling after the treatment to generate Cr carbide, resulting in a Cr diffusion layer (usually 50 μm from the surface). It was found that a Cr-deficient layer was formed at the grain boundaries within a depth of several hundreds of μm, and that pitting was significantly caused depending on the corrosive environment. As such a severe environment, there is a coal gasification boiler which has been recently developed. Previous reports in the fuel Cl ^- by HCl
It has been found that existing materials subjected to chromizing treatment cannot be used because of pitting corrosion because of insufficient corrosion resistance.

As a material that does not generate Cr carbide in the chromizing layer, so-called stabilized low Cr steel and C containing a large amount of Nb and Ti added.
Ferrite single-phase type low Cr steels with a low content are considered, but these materials have the following problems. That is, in the chromizing treatment, since heat treatment is performed at 1000 ° C. or longer for a long time, crystal grains are significantly coarsened in these stabilized steels and low C ferritic steels, and strength and toughness are impaired, and design criteria cannot be satisfied. . In particular, the low C ferritic single phase steel cannot be put to practical use at all because of insufficient strength and toughness.

(Problems to be Solved by the Invention) Among existing heat-resistant steels, the problems with the chromizing material of low Cr steel, which is inexpensive, and has excellent thermal conductivity and strength, are as follows.

(I) A Cr-deficient layer is formed due to the precipitation of Cr carbide during the chromizing treatment, and the corrosion resistance is significantly deteriorated. (Ii) In the existing ferritic single-phase steel with a low C content, the crystal grains become coarse due to the chromizing treatment, which causes remarkable deterioration in toughness, and is not suitable for practical use due to insufficient strength.

(Iii) In the chromizing treatment of the existing steel, in order to suppress grain growth from the viewpoint of toughness, when a low temperature treatment of 1000 ° C or less is required, it takes not only a long time to form the Cr diffusion layer but also 50μ.
A diffusion layer having a sufficient thickness of m or more cannot be obtained and cannot be practically used.

(Iv) Even as a so-called stabilized steel containing a large amount of Nb and Ti added, it is not suitable for practical use due to remarkable deterioration of toughness due to chromizing treatment, deterioration of strength and weldability, and high cost.

The object of the present invention is to utilize the characteristics of conventional low Cr steel,
It is a new low Cr heat resistant steel that does not cause deterioration of corrosion resistance and deterioration of strength and toughness even when subjected to chromizing treatment.
An object of the present invention is to provide a low Cr heat resistant steel which has improved HCl pitting property and can be used even in a severe corrosive environment which has been difficult to apply due to its material.

(Means for Solving the Problem) The present inventors take the means of 1) and 2) described below for the deterioration of toughness and the reduction of strength due to the formation of Cr carbide by the chromizing treatment of low Cr steel and the coarsening of crystal grains. I found that can be solved by.

1) An appropriate amount of C that imparts strength and toughness to the steel is added, and an appropriate amount of Ti and Nb are added in combination to form an α + γ2 phase structure at the chromizing treatment temperature (1000 to 1200 ° C). As a result, coarsening of crystal grains is suppressed, and a fine ferrite + carbide structure is formed after cooling, and deterioration of strength and toughness can be prevented.

2) Obtain the limit amount of solute C that does not form Cr carbide, and adjust the amount of Nb, Ti, N, and C added to adjust the amount of solute C during the chromizing treatment. These can be adjusted by an empirical formula obtained from many experimental results.

The present invention based on the above findings is summarized by the following heat resistant steels for chromizing.

In weight%, C: 0.02 to 0.15%, Si: 0.7% or less,
P: 0.025% or less, S: 0.015% or less, Mn: 0.1 to 1.5%, Ni: 0.
8% or less, Cr: 1.5 to 3.5%, Mo: 0.01 to 2.2%, W: 0.01 to 3.
0%, Ti: 0.005 to 0.6%, Nb: 0.005 to 0.9%, N: 0.001 to 0.
05%, Al: 0.001-0.05%, Nb, Ti,
A low Cr heat-resisting steel for chromizing, which has an N and C content satisfying the formula (a) described later, and the balance is iron and inevitable impurities and has excellent toughness.

In addition, V: 0.01-0.3 wt% containing the above low Cr
Series heat resistant steel.

Further, the above or the low Cr heat resistant steel containing 0.0001 to 0.02% by weight of B.

Furthermore, 0.01 to 0.2 wt% of La, Ce, Y, and C, respectively
A low Cr heat resistant steel as described above or containing at least one of a, Zr and Ta.

The expression (a) is the following empirical expression.

However, the element symbol in the formula (a) means the content (% by weight) of the element.

(Operation) Hereinafter, the operation of the alloying elements constituting the heat-resistant steel of the present invention, the reason for limiting the content thereof, and the technical meaning of the formula (a) will be described in detail.

C: C combines with Nb, Ti, Cr, Mo, etc. in the steel to form carbides and give strength, but since it is an austenite stabilizing element itself, it is a phase that forms a γ phase at high temperatures. It plays an important role in adjusting balance. If it is less than 0.02%, it completely dissolves in the base material to form a single ferrite phase, and γ-transformation does not occur at high temperature, so the ferrite grain growth becomes so great that the toughness and strength of the steel are lowered and it cannot be put to practical use. On the other hand, C
If the content exceeds 0.15%, the steel will be significantly hardened and the toughness will be reduced.
Since a Cr-deficient layer is generated, the corrosion resistance also deteriorates. Therefore, C
The proper content of is 0.02-0.15%.

Cr: Cr is an indispensable element that imparts corrosion resistance, oxidation resistance and high temperature strength to the base material of low Cr steel, and if it is less than 1.5%, the predetermined characteristics of the base material cannot be obtained. On the other hand, when it exceeds 3.5%, it is no longer low Cr
It loses the advantages of steel, deteriorates toughness and weldability, and deteriorates thermal conductivity. Therefore, the proper range of Cr content is 1.5 to 3.5.
%.

Si: Si is an element that is added as a deoxidizer and enhances the steam oxidation resistance performance, but if it exceeds 0.7%, the toughness and workability are significantly deteriorated and it is also harmful to the strength. Especially in low Cr steel, since it promotes temper embrittlement due to segregation of P and S at grain boundaries, it is set to 0.7% or less.

Mn: Mn is an element that improves hot workability and is effective in stabilizing the structure, but if it is less than 0.1%, it will not have a sufficient effect, and if it exceeds 1.5%, it hardens the steel and causes workability and weldability. Along with damaging
Increases susceptibility to temper embrittlement like Si. Therefore, the amount of Mn is set to 0.1 to 1.5%.

P and S: P and S are unavoidable impurities of steel, and both are harmful to toughness, workability, and weldability, and particularly promote the temper embrittlement. Therefore, P is 0.025% or less and S is 0.0
It was decided to limit it to 15% or less.

Ni: Ni is an austenite stabilizing element and contributes to the improvement of toughness, but if it is added in an amount exceeding 0.8%, the high temperature strength will be impaired and the transformation temperature will decrease, making it impractical.
Therefore, it is set to 0.8% or less.

Mo: Mo is a solid solution strengthening element, and when added together with W, it contributes to the strength improvement from a small amount and is an important element that improves the corrosion resistance. If it is less than 0.01%, sufficient strength as a heat resistant steel cannot be obtained, and the effect of improving corrosion resistance is poor. On the other hand, 2.2
%, The steel is significantly hardened and impairs toughness, workability and weldability. Therefore, the proper content of Mo is 0.01 to 2.2%.

W: W is also a solid solution strengthening element, and when it is added in combination with Mo, the solid solution strengthening action is exhibited more than the single addition, and the strength is further improved. In particular, the high temperature creep strength is improved. If it is less than 0.01%, the above effect is not obtained, and if it exceeds 3.0%, the steel is significantly hardened and the toughness, workability and weldability are impaired. Therefore, the proper content of W is 0.01 to 3.0%. In addition, it is 0.8 with the amount of Mo + 1 / 2W
It is desirable to adjust the W content within the above-mentioned appropriate range so as to be ~ 1.5%.

Ti: Ti is an important element in the steel of the present invention together with Nb. That is, it combines with N and C in the base material to form carbonitrides, and prevents the formation of Cr carbides during the high temperature chromizing treatment. 0.00
If it is less than 5%, carbonitride formation is insufficient and the intended properties cannot be obtained. On the other hand, if it exceeds 0.6%, not only the workability and weldability are impaired, but also all C is stabilized as a carbide, and the steel structure becomes a ferrite single phase, impairing toughness and strength. Therefore, the proper content of Ti is 0.005-0.6%
Is.

Nb: Nb, together with Ti, mainly combines with C to prevent Cr carbide formation. If it is less than 0.005%, the effect is not sufficient, and during the high temperature chromizing treatment, it is significantly embrittled by grain growth. On the other hand, if it exceeds 0.9%, not only the workability and weldability are impaired, but also the structure becomes a ferrite single phase and impairs toughness and strength. Therefore, the Nb content is properly 0.005 to 0.9%.

N (nitrogen): N is mixed in from the raw material and atmosphere during melting, but forms carbonitrides of Ti and Nb and contributes to strength improvement by refining the crystal grains of the steel. If less than 0.001%, the above effect does not occur, and 0.05%
If it exceeds 1.0, a large amount of Ti nitride is precipitated and the toughness is impaired.
Therefore, the content of N is set to 0.001 to 0.05%. A preferred range is 0.005 to 0.015%.

In addition to the above, alloy components that can be contained as necessary are as follows.

V: V is a carbonitride forming element, but its action is smaller than that of Nb and Ti. However, by adding a trace amount, toughness,
Workability is improved. This property cannot be obtained if it is less than 0.01%, while if it exceeds 0.3%, the strength and toughness are rather deteriorated. Therefore, when V is added, its content is set to 0.01 to 0.3%.

B: B disperses and stabilizes carbides by adding an extremely small amount, and also contributes to grain boundary strengthening and grain refinement. 0.00
If it is less than 01%, there is no effect, and if it exceeds 0.02%, the weldability and workability are impaired. Therefore, when B is added, its content should be 0.0001 to 0.02%.

Cu, La, Ce, Y, Ca, Zr, Ta: When one or more of these elements are contained, they are combined with the impurity elements P, S, O (oxygen) and the toughness and working of steel are performed. Improves sex and strength. If the content is less than 0.01%, the above effect is not clear, and if it exceeds 0.2%, nitrides and oxides increase as inclusions, and the toughness and strength are impaired. It is preferable to set each in the range of 0.01 to 0.2%.

Furthermore, it is a major feature of the present invention that the contents of Nb, Ti, N, and C are adjusted within the above range and satisfy the condition of the following formula (a).

This equation (a) is obtained from many experimental results of the inventor, Represents a calculation formula for the amount of dissolved C. Hereinafter, the value calculated by this formula is referred to as the P value. This P value is -0.020 (%) to 0.030
It is a characteristic of the steel of the present invention that it is in the range of (%).

C in the first term of the above P value formula is the content of C, and the second term is Ti.
The formula for the amount of C bound as C, and the third term for the formula for the amount of C bound as NbC. Therefore, P expressed by the above equation
The value is an index showing the amount of solid solution C obtained by subtracting the amount of C bound as a carbide from the total C content.

As will be shown in later Examples, the heat resistant steel for chromizing having excellent toughness and sufficient corrosion resistance can be obtained only by setting the amount of solute C in an appropriate range. If the P value in the above equation is −
When it is smaller than 0.020 (%), that is, C, N
On the other hand, in a so-called stabilized ferritic steel to which a large amount of Ti and Nb are added, the structure becomes a ferrite single phase, and crystal grain growth remarkably occurs during the chromizing treatment to deteriorate the toughness.

On the other hand, when the P value exceeds 0.030 (%), that is, C,
When not sufficiently stabilized with Nb and Ti with respect to N, a large amount of Cr carbide is precipitated at the grain boundaries of the Cr diffusion layer during the chromizing treatment, and a Cr deficient layer is formed, which significantly deteriorates the corrosion resistance.

The heat treatment conditions for the steel of the present invention are not particularly limited, but usually by normalizing at 950 to 1050 ° C and tempering at 720 to 800 ° C, or by heating at 950 to 1050 ° C and slow cooling. Annealing or isothermal annealing treatment of holding at 720 to 750 ° C during cooling after heating at 950 to 1050 ° C can be applied. Also, omit heat treatment or 650-850
The residual strain removal annealing at about ℃ may be used for chromizing treatment.

The chromizing treatment is usually performed at 1000 to 1200 ° C. for 1 hour or more to obtain a Cr diffusion layer having a thickness of several tens of μm or more. Preferred conditions
It is about 10 hours at 1100 ℃.

The post heat treatment may be any of the above normalization + tempering, annealing, isothermal annealing, and residual strain removal annealing, but 750 to 850 ° C to keep the chromizing layer stable.
The softening treatment is preferable.

(Example) Steels each having a chemical composition shown in Table 1 were melted in a 50 kg vacuum melting furnace, and ingots were forged at 1150 to 950 ° C to form a plate having a thickness of 15 mm. Steels A to S are steels of the present invention. T steel and U steel are comparative steels in which a small amount of Nb and Ti are added to the conventional 2.1 / 4Cr-1Mo steel (STBA24), and Y steel is a small amount of N in the same Mo and W additive steel.
Comparative steels containing b and Ti, V steels and W steels are comparative steels containing excess Nb and Ti, and X steels are comparative steels having a P value satisfying the conditions of the present invention but having a low C content.

The heat treatment was omitted, and each corrosion test piece and the mechanical test piece having the dimensions described later were cut out and subjected to chromizing treatment. The treatment is a powder pack method which is usually used, and chromium powder (particle size 8 to 32 mesh) Al ₂ O ₃ and NH _{4 is} put in a steel container.
Cl was blended, each of the test pieces was embedded in the mixture, and diffusion treatment was performed at 1100 ° C. for 10 hours while passing H ₂ gas. As a result, a Cr diffusion layer having a thickness of about 100 μm was obtained. The post heat treatment was a softening treatment at 800 ° C. for 10 minutes.

The conditions of the mechanical test and the high temperature corrosion test are shown below.

(1) Charpy impact test Specimen: 10 × 10 × l55 (mm), 2mmV notch (JIS4) Test temperature: 0 ℃ (2) Normal temperature tensile test Specimen: φ6 × GL30 (mm) Test temperature: Normal temperature (3 ) Salt spray corrosion test (JIS Z2371) Specimen: φ10 × l50 (mm) Conditions: 5% NaCl (35 ° C) for 24 hours (4) Ferric chloride solution corrosion test Specimen: 30 × 30 × t3mm Solution: FeCl ₃ · 6H ₂ O (50g / l) + 1 / 20HCl Conditions: 35 ° C, 50 ° C, 65 ° C for 24 hours (5) Sulfuric acid immersion corrosion test specimen: 30 × 30 × t3 (mm) Conditions: 24 with 0.1%, 1%, and 5% H ₂ SO ₄ (40 ℃)
Time (6) Hydrochloric acid immersion corrosion test specimen: 30 × 30 × t3 (mm) Condition: 1%, 5% HCl (60 ℃) for 24 hours (7) High temperature corrosion test specimen: 30 × 30 × 3t (mm) Gas composition: 30vol.% H ₂ −44vol.% CO−10vol.% CO ₂ −14vo
l.% H ₂ O-0.6vol.% H ₂ S-0.2vol.% HCl-1.2vol.% N ₂ condition: 600 ℃ × 100 hours Table 2 shows the tensile properties at room temperature and the Charpy impact value at 0 ℃. Show. Each of the steels of the present invention has a tensile strength of 42 kgf / mm ² or more and a proof stress of 2
Good at 1 kgf / mm ² or more and 0 ° C impact value of 10 kgf-m / cm ² or more.

On the other hand, in the comparative steel, the strength and toughness of the V steel, the W steel, and the Z steel, which have become ferrite single-phase by adding excessive amounts of Nb and Ti, are low. Further, X steel having a low C content also has a single-phase ferrite structure, and has low strength and toughness. None of these are suitable for use.

FIG. 1 shows the relationship between the 0 ° C. impact value and P value in Table 1. As can be seen from this figure, the P value is -0.030.
If it is (%) or more, good toughness of 10 kgf-m / cm ² or more is obtained.

This is a result of the structure becoming two phases of α + γ during the chromizing treatment, and the remarkable coarsening of crystal grains was prevented.

Next, various corrosion test results are shown in Table 3, and graphs showing the corrosion test results in relation to the P value are shown in FIGS. 2 to 5. FIG. 2 is a graph of the results of the ferric chloride test, FIG. 3 is a graph of the results of the sulfuric acid immersion test, and FIG. 4 is a graph of the results of the hydrochloric acid immersion test. FIG. 5 is a graph showing the results of the high temperature corrosion test.

As is clear from these tables and figures, in any corrosion test, when the P value exceeds 0.030 (%), the weight loss due to corrosion and the erosion depth increase and the corrosion resistance deteriorates. As a result of detailed investigation of T steel, U steel, and Y steel with low Nb and Ti contents, a large amount of Cr carbide was generated in the Cr diffusion layer and
It was found that a deficient layer was formed and the corrosion resistance was deteriorated.

In the steel of the present invention, the generation of Cr carbide that causes such deterioration of corrosion resistance is extremely small, and there is no Cr-deficient layer. This is the main reason why the steel of the present invention exhibits excellent corrosion resistance.

From these test results, the P value is -0.020 (%) to 0.030.
It is necessary to be in the range of (%), and it has been revealed that a chromizing steel having excellent strength and toughness and good corrosion resistance can be obtained.

(Effects of the Invention) According to the present invention, a new material having excellent toughness and corrosion resistance can be provided as a low Cr steel for chromizing treatment.

INDUSTRIAL APPLICABILITY The steel of the present invention is industrially extremely useful as a chromizing material used in places where corrosion is severe, such as in boilers, chemical industries, and nuclear power.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the value (P value) of the empirical formula indicating the amount of solid solution C and the 0 ° C. Charpy impact value. FIG. 2 is the same P value and the corrosion weight loss by the ferric chloride corrosion test. FIG. 3 is a diagram showing the relationship between the P value and the corrosion weight loss by the sulfuric acid immersion corrosion test, and FIG. 4 is a diagram showing the relationship between the P value and the corrosion weight loss by the hydrochloric acid immersion corrosion test. FIG. 5 is a diagram showing the relationship between the P value and the internal erosion depth in the high temperature corrosion test.

Claims (4)

[Claims] 1. By weight%, C: 0.02 to 0.15%, Si: 0.7% or less, P: 0.025% or less, S: 0.015% or less, Mn: 0.1 to 1.5%, N
i: 0.8% or less, Cr: 1.5 to 3.5%, Mo: 0.01 to 2.2%, W: 0.01
~ 3.0%, Ti: 0.005-0.6%, Nb: 0.005-0.9%, N: 0.001
~ 0.05%, Al: 0.001-0.05%, further Nb,
A low Cr heat-resistant steel for chromizing which has excellent toughness and whose contents of Ti, N and C satisfy the following formula (a) and whose balance is Fe and unavoidable impurities. However, the element symbol in the formula (a) means the content (% by weight) of the element. 2. The low Cr heat-resistant steel according to claim 1, further comprising V: 0.01 to 0.3% by weight. 3. The low Cr heat-resistant steel according to claim 1, further comprising B: 0.0001 to 0.02% by weight. 4. Further, 0.01 to 0.2% by weight of Cu and L, respectively.
The low Cr heat-resisting steel according to claim (1), (2) or (3), containing at least one of a, Ce, Y, Ca, Zr and Ta.