JP2005314797A - Ferritic stainless steel for combustor generating black scale having excellent adhesion - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide ferritic stainless steel generating black scale having excellent adhesion and used, e.g., for the combustion tube of a combustor. <P>SOLUTION: The ferritic stainless steel for a combustor generating black scale having excellent adhesion has a composition comprising, by mass, ≤0.020% C, ≤0.20% Si, ≤0.20% Mn, ≤0.040% P, ≤0.020% S, ≤0.020% N, 16.0 to 18.0% Cr and ≤0.1% Mo, and in which C and N also satisfy the relation of C+N≤0.025%, and further, comprising Ti satisfying the relations of 10×(C+N)≤Ti≤0.3, and Ti≤4×(C+N)-Mn+0.3, and the balance Fe with inevitable impurities. Regarding the ferritic stainless steel, in a combustion atmosphere composed of, by volume, 0.1 to 10% CO, 5 to 20% CO<SB>2</SB>, 1 to 10% O<SB>2</SB>, 0.1 to 2% H<SB>2</SB>, ≤1% CH<SB>4</SB>, ≤0.2% SO<SB>2</SB>, ≤10% H<SB>2</SB>O, and the balance N<SB>2</SB>with inevitable impurity gas, the quantity of scale to be peeled after an oxidation test at 800°C for continuous 200 hr is ≤0.5 mg/cm<SP>2</SP>, and also, the color of the scale at this time satisfies L*≤32, a*=-2 to +2, and b*=-2 to +2 by a color specification method specified by an L*a*b* color system in JIS Z 8729. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a ferritic stainless steel for combustion equipment that produces a black scale with excellent adhesion, which is used in a combustion cylinder of a combustion equipment.

  In order to improve the combustion efficiency of oil stoves, especially small stoves, a method of making the combustion tube red hot and directly using the radiant heat has been adopted. Ferritic stainless steel represented by SUS430 is used for this combustion cylinder. However, in SUS430 steel, reddish brown powdery oxide (so-called red rust) is generated after a relatively short time of use, and not only the combustion efficiency and red heat property are lowered, but also the appearance is impaired.

As a conventional technique for solving such a problem, Patent Document 1 discloses an invention of ferritic stainless steel containing Si suitable for a combustion cylinder application. Further, as disclosed in Patent Document 2, a steel type to which Zr is added has been developed in order to improve oxidation resistance.
Japanese Patent Laid-Open No. 55-161049 JP 50-146512 A

However, the invention of the ferritic stainless steel containing Si of the above prior art has a problem that the scale is easily peeled off, and the invention of the ferritic stainless steel containing Zr has a difficulty in manufacturability in steelmaking. Previous materials have been insufficient.
Accordingly, an object of the present invention is to provide a ferritic stainless steel for combustion equipment that produces a black scale with excellent adhesion, which is used in a combustion cylinder of a combustion equipment.

The gist of the present invention is as follows.
(1) In mass%,
C: 0.020% or less, Si: 0.20% or less,
Mn: 0.20% or less, P: 0.040% or less,
S: 0.020% or less, N: 0.020% or less,
Cr: 16.0 to 18.0%, Mo: 0.1% or less, and C and N are C + N ≦ 0.025%
Satisfy the relationship, and
10 × (C + N) ≦ Ti ≦ 0.3
Ti ≦ 4 × (C + N) −Mn + 0.3
Containing Ti satisfying the relationship, characterized in that the balance Fe and unavoidable impurities, in _{volume%, CO: 0.1~10%, CO} 2: 5~20%, O 2: 1~10 %, H ₂ : 0. Continuous 200 hours at 800 ° C. in a combustion atmosphere consisting of 1 to 2%, CH ₄ : 1% or less, SO ₂ : 0.2% or less, H ₂ O: 10% or less, balance N ₂ and unavoidable impurity gas The amount of scale peeling after the oxidation test is 0.5 mg / cm ² or less, and the color of the scale at that time is represented by the L * a * b * color system specified in JIS Z 8729 Ferritic stainless steel for combustion equipment that produces a black scale with excellent adhesion, L * ≦ 32, a * = − 2 to +2, and b * = − 2 to +2.
(2) A ferritic stainless steel for combustion equipment that produces a black scale with excellent adhesion as described in (1) above, wherein a work hardened layer remains on the surface.

  According to the present invention, it is possible to provide a ferritic stainless steel that produces a black scale with excellent adhesion, which is used for a combustion cylinder of a combustion device, which is great not only for the manufacturer but also for those who use the steel of the present invention. Profits can be obtained and the industrial value is extremely high.

The best mode and limiting conditions for carrying out the present invention will be described in detail below.
The present inventors have investigated the optimum stainless steel used in a combustion cylinder in a system in which the combustion cylinder is red-hot with a petroleum stove, particularly a small stove, and the radiant heat is directly used.
The characteristics required for such a combustion cylinder are:
1) Oxidized scale is generated in the combustion cylinder during use, but the scale does not cause abnormal oxidation phenomenon and scale peeling phenomenon.
2) A black scale that ensures combustion efficiency and redness.
3) Having processability to the extent that it can be processed into a combustion cylinder,
4) Be inexpensive
It turned out that. In particular, it is important to produce a black scale having excellent adhesion as in 1) and 2).

As a result of conducting detailed examinations in a combustion atmosphere and a simulated atmosphere, the present inventors have found that, with regard to adhesion, Cr is added by 16% or more and Si is reduced as much as possible, so that there is no abnormal oxidation, It was clarified that a scale that does not peel easily is formed.
As for the color of the scale, it can be seen that the blacker the color, the higher the combustion efficiency and the redness, and the degree of black is expressed in the L * a * b * color system defined in JIS Z 8729. L * was 32 or less, and a * and b * were −2 or more and +2 or less were found to be excellent ranges.
Furthermore, regarding the blackness of the scale, it has been clarified that the blackness is reduced as Cr decreases, the blackness is lowered when Mo is added, and the blackness is improved when Ti and Mn are limited to a certain amount or less.
It was also clarified that a scale with higher blackness was generated by leaving a work hardened layer on the surface of the steel sheet.
Based on the above knowledge, the present inventors proceeded with further detailed studies and completed the present invention.

Next, the limiting conditions regarding each component will be described.
C is an inevitable impurity contained in the steel, and it is preferable that it is as small as possible because it deteriorates workability and corrosion resistance. Therefore, although it is fixed as carbonitride to remove harmful effects, the upper limit of its content is set to 0.020% or less in order to reduce the addition amount of Ti, which is a fixed element for that purpose, as much as possible. In addition, although the minimum of content is not specifically limited, Since refining cost will become large to make it less than 0.002%, it is preferable to set it as 0.002% or more.

Si is an element that enhances the oxidation resistance, but is a cause of hindering the adhesion between the Cr ₂ O ₃ film and the parent phase, and the scale easily peels off. As a result of detailed studies, the inventors have found that when the Cr content is 16.0% or more, the Si addition amount is 0.20% or less, the oxidation resistance can be sufficiently maintained, and the scale peeling is hardly caused. Clarified that there is no. Therefore, the upper limit of Si is 0.20%. In order to improve workability, Si should be low, more preferably 0.15% or less. Moreover, since it becomes difficult to deoxidize in a refining process to make Si amount less than 0.03%, 0.03% or more is more preferable.

  Mn is a component inevitably contained in the steel, but when contained in a large amount, it tends to segregate on the oxide scale surface. The inventors have found that when Mn segregates on the oxide scale surface, the blackness decreases. Therefore, it is necessary to limit the addition of Mn, and the upper limit of Mn is 0.20% or less. On the other hand, if the amount of Mn is less than 0.05%, the cost increase in refining increases, so 0.05% or more is more preferable.

  P is a component inevitably contained in the steel, but if it exceeds 0.040%, weldability decreases, so 0.040% was made the upper limit.

  S is a component inevitably contained in the steel, but if it exceeds 0.020%, the corrosion resistance is lowered by the element forming MnS, so 0.020% was made the upper limit.

  N is an unavoidable impurity contained in the steel, but it is preferably as small as possible because deterioration of workability and weldability are reduced in the same manner as C. Therefore, it was made 0.02% or less. Also. If it is less than 0.005%, the cost increases in refining, so 0.005% or more is more preferable.

Cr is an element that forms a protective Cr ₂ O ₃ film and improves oxidation resistance. When Cr is less than 16.0%, a sound Cr ₂ O ₃ film is difficult to form and abnormal oxidation tends to occur. Therefore, in the present invention, the lower limit is made 16.0%. Also, when the content of Cr exceeds 18.0%, _{the Cr} 2 _{O 3} film is thinner oxide scale becomes strong, blackness decreases. Therefore, the upper limit of Cr is made 18.0%.

Although Mo is an element that improves oxidation resistance and corrosion resistance, the present inventors have found that the blackness of the oxide scale decreases when Mo is added. Therefore, Mo is made 0.1% or less. Since the present invention contains almost no expensive Mo, the steel of the present invention is relatively inexpensive.
Furthermore, if the amount of C + N exceeds 0.025%, workability deteriorates, so this value was made the upper limit. In the case where the overhanging process is severe, C + N is preferably as low as possible, more preferably 0.015% or less.

  Ti is added to fix C and N as carbonitrides. If the amount added is less than 10 × (C + N), the intergranular corrosion resistance is inferior, which is not preferable. If it exceeds 0.3%, the solid solution Ti increases, the workability deteriorates, and the blackness of the scale to be generated is low. In order to decrease, it is set to 10 × (C + N) or more and 0.3% or less.

The inventors have found that Mn and Ti are elements that reduce the blackness. These elements are elements that easily diffuse and segregate on the oxide scale surface. The reason for lowering the blackness is considered to be due to solid solution in the oxide scale. Therefore, in order to obtain the required blackness of the oxide scale, the amount must be limited. Regarding Ti, since the amount consumed for fixing C and N does not adversely affect the oxide scale, it may be limited by Ti-4 (C + N) + Mn. If this value is 0.3% or less, it is necessary. Blackness is obtained.
With the above component design, it is possible to obtain a ferritic stainless steel that produces a black scale with excellent adhesion.

Further, when a work hardened layer is formed on the surface of the steel plate by polishing finish or the like, the blackness is also improved. This is because many defects are introduced in the work hardened layer, and the diffusion of Fe and Cr is accelerated, so that oxidation of Mn and the like is suppressed in the initial oxidation, and a scale with high blackness is generated. ing.
The method for forming the work hardened layer is not particularly limited, but for example, a method using shot blasting or a skin pass is also effective. The simplest and preferred method is polishing finish. As the number of polishing, # 200 to # 1000 are preferable. If it is rougher than # 200, the undulation on the surface becomes too large, which is not preferable, and if it is smaller than # 1000, an effective hardened layer is hardly obtained, which is not preferable.

As a result of investigating the combustion atmosphere of a petroleum stove or a gas stove, the present inventors have found that in volume%, CO: 0.1 to 10%, CO ₂ : 5 to 20%, O ₂ : 1 to 10%, H ₂ : 0.1 to 2%, C H ₄ : 1% or less, SO ₂ : 0.2% or less, H ₂ O: 10% or less, found that the range is composed of the balance N ₂ and unavoidable impurity gas. . Its feature is that it contains water vapor at a low oxygen partial pressure. This is a harsher environment than in the atmosphere.

The present inventors conducted an oxidation test in a combustion atmosphere gas and a simulated atmosphere within the above ranges, and investigated the oxidation scale. As a result, in a combustion gas atmosphere in the above range, in a continuous oxidation test at 800 ° C. for 200 hours, the metal surface is not exposed when the scale peel-off amount is 0.5 mg / cm ² or less. no problem. The case where there is no scale peeling is most preferable. The measuring method of the scale peeling amount is as follows.

The shape of the test piece was a 20 mm square, and the surface was as received or polished from # 200 to # 1000, and all sides were # 400 polished. The evaluation method of the amount of increase in oxidation and the amount of peeling was performed as follows.
Insert the test piece whose mass was measured before the test into a furnace heated to 950 ° C., remove it from the furnace after 200 hours, and immediately store it in a metal container with a lid whose mass was measured in advance in an empty state. Air-cool. First, the whole metal container was subjected to mass measurement, and then the test piece was taken out of the metal container, and only the test piece was subjected to mass measurement.

The amount of increase in oxidation and the amount of scale peeling were calculated from the mass measurement results as follows.
The increase in oxidation was evaluated by the value obtained by subtracting the pre-oxidation test piece mass and the empty container mass from the mass of the oxidation test piece contained in the container and subtracting it by the surface area of the test piece.
The scale peeling amount was evaluated by a value obtained by subtracting the post-oxidation test piece mass and the empty container mass from the post-oxidation post-oxidation test piece mass and dividing by the test piece surface area.

  Further, as a result of investigating the blackness of the scale, the detailed reason is unknown, but it is understood that the blackness is stronger, that is, the black is more excellent in radiation and red heat, and JIS Z 8729 is an index of blackness. In the color display method represented by the L * a * b * color system defined in the above, L * is a positive number representing brightness, and the smaller the number, the lower the brightness and the darker the tendency have. a * and b * are positive and negative numbers representing chromaticity, and the closer to 0, the blacker the color becomes. In other words, the blackness becomes blacker as L * = 0, a * = 0, and b * = 0. The measurement by this L * a * b * color system can be easily performed since a simple measuring instrument is commercially available.

  As a result of measuring the blackness of the black scale generated in the combustion atmosphere using the L * a * b * color system, L * is 32 to ensure sufficient red heat. Hereinafter, it is desirable that a * is from −1 to +1 and b * is from −1 to +1. When L * exceeds 32, it becomes whitish, when a * is less than -1, it becomes greenish, when it exceeds -1, it becomes reddish, when b * is less than -1, it becomes bluish, and when it exceeds +1, it is yellow It is unpreferable because it becomes lustrous and both reduce redness.

  The method of producing the stainless steel of the present invention usually involves melting steel ingots such as ingots and slabs, and through hot rolling, hot-rolled sheet annealing / pickling, pickling, cold rolling, annealing / pickling. Product. A method of omitting hot-rolled sheet annealing may be used. Further, cold rolling and annealing / pickling may be repeated. Since steel containing Zr is likely to cause nozzle clogging, there is difficulty in manufacturability at the steel making stage, but since the steel of the present invention does not contain Zr, it can be said that such problems can be avoided and the manufacturability is excellent.

Hereinafter, it will be described in more detail in accordance with an embodiment.
A steel ingot having a thickness of 200 mm having chemical components shown in Table 1 was melted and heated to 1200 ° C. to perform hot rolling to obtain a hot rolled plate having a thickness of 5 mm. At this time, the hot rolling start temperature was 1150 ° C. to 1200 ° C., and the hot rolling end temperature was 800 ° C. to 880 ° C. Then, the hot-rolled sheet annealing which heated a hot-rolled sheet to 900 degreeC and hold | maintained for 60 s was performed. Furthermore, after cold-rolling to a cold-rolled sheet having a thickness of 2 mm, the steel plate obtained by heating to 950 ° C. and holding for 60 s and pickling with hydrofluoric acid was used as a test steel. .

The tensile test at room temperature was performed according to JIS Z 2241. The direction of the measured specimen is the rolling direction (L direction). All the test pieces used were No. 13B test pieces defined in JIS Z 2201.
The oxidation test was performed in the atmosphere shown in Table 2. The test conditions are 800 ° C. and 200 hours continuous. The shape of the test piece was a 20 mm square, the surface was as received or # 400 polished, and all sides were # 400 polished. All test pieces were degreased with acetone before the test. The evaluation method of the oxidation increase amount and the peeling amount was performed as follows.
Insert the test piece whose mass was measured before the test into a furnace heated to 950 ° C., remove it from the furnace after a certain period of time (200 hours in this case), and immediately measure the mass in an empty state with a lid. Store in a metal container and cool in air. First, the whole metal container was subjected to mass measurement, and then the test piece was taken out of the metal container, and only the test piece was measured for mass.

The amount of increase in oxidation and the amount of scale peeling were calculated from the mass measurement results as follows.
The increase in oxidation was evaluated by the value obtained by subtracting the pre-oxidation test piece mass and the empty container mass from the mass of the oxidation test piece contained in the container and subtracting it by the surface area of the test piece. The scale peeling amount was evaluated by a value obtained by subtracting the post-oxidation test piece mass and the empty container mass from the post-oxidation post-oxidation test piece mass and dividing by the test piece surface area.
Furthermore, the blackness after the oxidation test was measured using a color difference meter. This is a device for measuring the color difference using an illumination optical system designed in accordance with JIS Z 8722 and displaying in an L * a * b * color system in accordance with JIS Z 8729.

The results are shown in Table 3. It can be seen that B steel and C steel, which are examples of the present invention, produce a black scale with excellent adhesion, and are excellent as combustion cylinder materials. In addition, in the polishing finish as received, the polishing finish is closer to L * = 0, a * = 0, and b * = 0, indicating that the blackness is higher.
On the other hand, the comparative steel A with less Cr is not preferable because it has low oxidation resistance and abnormal oxidation. On the other hand, steel D with a large amount of Cr is not preferable because L * representing brightness is large and the scale is slightly whitish. E steel with a lot of Mo is not preferable because L * is large and the scale is slightly whitish. F steel with a large amount of Si is not preferable because of large scale peeling. Steel G with more Ti-4 (C + N) + Mn than the range of the present invention is not preferable because of insufficient blackness. Steel H is equivalent to SUS430 steel, but red rust is generated under these test conditions, which is not preferable.

Using the C steel and H steel of Table 1, a continuous oxidation test was conducted at 800 ° C. for 200 hours in the atmosphere shown in Table 4 in the same manner as in Example 1. However, the surface finish is only # 600 polishing. The method for measuring the amount of scale peeling and the blackness of the scale is the same as in Example 1. The results are shown in Table 5.
It is clear that the steel C as an example of the present invention produces a scale having excellent adhesion not only in the atmosphere (C3) but also in an atmosphere (C2) that simulates the combustion atmosphere of a gas stove. It turns out that it is excellent as a material.
On the other hand, H steel, which is SUS430 equivalent steel, produces a black scale with excellent adhesion in the atmosphere (H3), but red rust is generated in the combustion atmosphere (H2), which is not preferable. . Therefore, when selecting the material for the combustion cylinder, it is necessary to conduct a test in a situation close to the actual atmosphere.
From the above examples, it is apparent that the steel of the present invention produces a black scale with excellent adhesion in a combustion atmosphere and is excellent as a material for combustion equipment.

Claims (2)

% By mass
C: 0.020% or less,
Si: 0.20% or less,
Mn: 0.20% or less,
P: 0.040% or less,
S: 0.020% or less,
N: 0.020% or less,
Cr: 16.0 to 18.0%,
Mo: 0.1% or less, and C and N are further C + N ≦ 0.025%
Satisfy the relationship, and
10 × (C + N) ≦ Ti ≦ 0.3
Ti ≦ 4 × (C + N) −Mn + 0.3
Containing Ti satisfying the relationship, characterized in that the balance Fe and unavoidable impurities, in _{volume%, CO: 0.1~10%, CO} 2: 5~20%, O 2: 1~10 %, H ₂ : 0. Continuous 200 hours at 800 ° C. in a combustion atmosphere consisting of 1 to 2%, CH ₄ : 1% or less, SO ₂ : 0.2% or less, H ₂ O: 10% or less, balance N ₂ and unavoidable impurity gas The scale peel-off amount after the oxidation test is 0.5 mg / cm ² or less, and the color of the scale at that time is represented by the L * a * b * color system defined in JIS Z 8729 Ferritic stainless steel for combustion equipment that produces a black scale with excellent adhesion, L * ≦ 32, a * = − 2 to +2, and b * = − 2 to +2. The ferritic stainless steel for combustion equipment that produces a black scale with excellent adhesion according to claim 1, wherein a work hardened layer remains on the surface.