JP7009312B2 - Steel products with excellent oxidation resistance - Google Patents

Description

The present invention relates to a steel product such as a hearth roll, a kiln furnace, and a radiant tube in which a centrifugal casting portion and a static casting portion are welded and joined.

A hearth roll that transports steel strips in an annealing furnace that continuously heat-treats steel strips, and a kiln furnace that is used for externally heated rotary kilns that heat the object to be processed inside a cylindrical core tube from the outside. Members for heating furnaces used in high temperature environments such as industrial heat treatment furnaces, especially steel products such as radiant tubes, are used in steel mills and the like.

Since these steel products are used in a high temperature environment in the atmosphere, the metal in the base metal is oxidized, and Cr oxide mainly composed of Cr (including Fe and Ni) is formed on the surface. The Cr oxide is easily peeled off due to a temperature change or contact with the object to be treated, and the peeling may damage the steel material or cause foreign matter to be mixed in the object to be treated. Then, peeling of the Cr oxide and polishing for removing the Cr oxide in order to suppress the peeling may accelerate the thinning of the steel product itself.

Therefore, a product in which a sprayed layer obtained by spraying a CoCrAlY alloy or the like on a sleeve constituting the outer periphery of the hearth roll is also proposed (see, for example, Patent Document 1).

On the other hand, the hearth roll is configured to be rotatable via a shaft portion by joining an axle and a tubular sleeve. Further, for example, Patent Document 2 discloses a radiant tube in which a portion to be a straight tube portion is centrifugally cast, a curved tube portion is manufactured by static casting, and welded to each other.

Japanese Unexamined Patent Publication No. 2008-240072 International Publication No. WO2012 / 133539

However, even if the sprayed layer is formed on the sleeve, the sprayed layer is also oxidized, so that the oxide film formed on the sprayed layer is peeled off, the oxidation resistance is lowered, and the peeled oxide film causes the steel material. May be damaged. Further, since the amount of Al added to the sprayed layer is large (about 10% by mass), mechanical properties such as tensile ductility of the sprayed layer may be deteriorated, and further, weldability may be deteriorated.

Therefore, it is desired to improve the oxidation resistance by adding Al to the sleeve itself to facilitate the formation of the Al oxide layer directly on the outer periphery of the sleeve.

However, when Al is added to the sleeve, there is a concern that the weldability of the sleeve may be deteriorated due to Al.

An object of the present invention is to provide a steel product having excellent oxidation resistance and excellent weldability.

The steel product according to the present invention is
It is a steel product with excellent oxidation resistance.
The centrifugal casting part manufactured by centrifugal casting is attached to the static casting part formed by static casting by welding.
The centrifugal force casting portion is based on mass%.
C: 0.2% -0.7%,
Si: More than 0% and 2.0% or less,
Mn: More than 0% and less than 3.0%,
Cr: 15.0% -40.0%,
Ni: 18.0% to 55.0%,
Al: 1.0% to 5.5%, and
It contains at least one selected from the group consisting of Ti: 0.01% to 0.6% and Nb: 0.1% to 1.8%.
It consists of the balance Fe and unavoidable impurities.
The static casting portion is based on mass%.
Si: More than 0% and less than 1.0%,
Mn: More than 0% and 1.0% or less, and
Al: Contains 0.01% to 0.5%.

The centrifugal force casting portion is based on mass%.
Rare earth element (REM): can contain more than 0% and 0.4%.

The centrifugal force casting portion is based on mass%.
It can contain at least one selected from the group consisting of W: more than 0% and 5.0% or less, and Mo: more than 0% and 2.0% or less.

The centrifugal force casting part is
Pa = -11.1 + 28.1 x C + 29.2 x Si-0.25 x Ni-45.6 x Ti + 18.0 x REM-16.6 x Nb
Ya = -13.75 x Al + 63.75
When
Pa <Ya
Is desirable.

The static casting portion is based on mass%.
Cr: It is desirable to contain 24.0% to 50.0%.

The centrifugal force casting portion can be formed on a surface that comes into contact with a steel material.

A ceramic sprayed layer can be provided on the surface of the centrifugal casting portion.

The steel product can be a hearth roll, a kiln furnace, or a radiant tube.

According to the present invention, by setting the centrifugally cast portion formed in the steel product to the above composition, Al has priority over Cr to form Al oxide, and the formation of Cr oxide can be suppressed. , Problems such as peeling of Cr oxide can be suppressed. Further, since the amount of Al added is as low as 1.0% to 5.5%, deterioration of mechanical properties can be suppressed and weldability can be ensured.

Further, according to the present invention, when the static casting portion contains Al in the above range, oxides or nitrides are generated in the molten metal, and the oxygen content and the nitrogen content in the molten metal are lowered. As a result, the soundness of the static casting portion can be improved, and the heat resistance and oxidation resistance can be improved. On the other hand, by containing Al in the static casting portion, crack sensitivity tends to increase and weldability tends to decrease. However, Al is set to the above concentration range and Si and Mn are set to 1.0% or less. Thereby, it is possible to suppress the crack sensitivity and the deterioration of the weldability.

In the present invention, since the centrifugal casting part and the static casting part each contain Al and the weldability can be ensured, the steel having excellent mechanical bonding strength by directly welding the centrifugal casting part to the static casting part. You can get the product.

Since the centrifugal casting section and the static casting section each contain Al as described above, Al oxide is likely to be formed on the surface, and excellent oxidation resistance can be ensured. In particular, according to the present invention, in the centrifugal casting portion and the static casting portion, an oxide film is formed on the welded portion and the heat-affected zone thereof by the welding heat. Then, with this oxide film as a starting point, oxidation proceeds to the centrifugal casting part and / or the static casting part in the used state, Al oxide is preferably formed on the surface of the steel product, and good oxidation resistance can be provided. ..

FIG. 1 is a cross-sectional view of a steel product (hearth roll) according to an embodiment of the present invention. FIG. 2 is a graph showing the results of regression analysis of the test centrifugal cast portion based on weldability, with the Pa value on the vertical axis and the Al content on the horizontal axis. FIG. 3 is an explanatory diagram showing the criteria for determining the bead by the method A (filler metal (welding rod): none) and the method B (filler metal: yes).

Hereinafter, embodiments of the present invention will be described in detail. Unless otherwise specified, "%" means mass%.

In the steel product of the present invention, a hearth roll used in direct contact with a steel material such as a steel piece or a steel plate in a high temperature environment in the atmosphere, or a material to be treated charged inside a cylindrical core tube is externally placed. It is suitable for application to heating furnace members used in high temperature environments such as kiln furnaces used for heating externally heated rotary kilns and industrial heat treatment furnaces, especially products such as radiant tubes. As shown in FIG. 1, the steel product 10 of the present invention has a groove of a tubular centrifugal casting section 20 manufactured by centrifugal casting and a hemispherical static casting section 30 manufactured by static casting. By welding after processing, it can be integrally joined and formed by the welded portion 40. When the steel product 10 is a hearth roll, the centrifugal casting section 20 is a sleeve and the static casting section 30 is an axle.

Centrifugal force casting part
C: 0.2% -0.7%,
Si: More than 0% and 2.0% or less,
Mn: More than 0% and less than 3.0%,
Cr: 15.0% -40.0%,
Ni: 18.0% to 55.0%,
Al: 1.0% to 5.5%, and
It contains at least one selected from the group consisting of Ti: 0.01% to 0.6% and Nb: 0.1% to 1.8%.
It can be composed of the balance Fe and unavoidable impurities.

The reasons for limiting the components of the centrifugal casting section are as follows.

C: 0.2% -0.7%
C has an effect of improving castability and increasing high temperature creep rupture strength. In addition, it has the effect of increasing high-temperature strength by combining with Ti, Nb, Cr and the like to form carbides. Therefore, it contains at least 0.2%. However, if the content is too large, the primary carbide of Cr ₇ C ₃ is likely to be widely formed, the transfer of Al to the surface of the centrifugal casting portion is hindered, the supply of Al is insufficient, and Al ₂ O ₃ is generated. The formation of Al oxide such as is suppressed. In addition, excessive precipitation of secondary carbides causes a decrease in ductility and toughness. Therefore, the upper limit is 0.7%. The C content is more preferably 0.35% to 0.6%.

Si: More than 0% and 2.0% or less Si is contained as a deoxidizing agent for the molten alloy, and also for increasing the fluidity of the molten alloy and improving the oxidation resistance. However, excessive addition of Si causes a decrease in ductility, a decrease in high temperature creep rupture strength, a deterioration in surface quality after casting, and a decrease in weldability. Therefore, the upper limit of the Si content is 2.0%. The Si content is preferably 1.5% or less, more preferably 1.0% or less.

Mn: More than 0% and 3.0% or less Mn serves as a deoxidizing agent for the molten alloy, and S in the molten metal is fixed to improve weldability and ductility. However, excessive addition of Mn causes a decrease in high temperature creep rupture strength and a decrease in oxidation resistance, so the upper limit is set to 3.0%. The Mn content is more preferably 1.0% or less.

Cr: 15.0% -40.0%
Cr contributes to the improvement of high temperature strength and repeated oxidation resistance. Further, Cr exhibits excellent heat resistance in a high temperature range exceeding 1000 ° C. together with Ni and Fe, and forms primary carbides with C and N to improve the high temperature creep rupture strength. Then, an oxide layer is formed together with Al, and the centrifugally cast portion is provided with excellent properties of oxidation resistance and corrosion resistance. Therefore, it is contained at least 15.0% or more. On the other hand, excessive formation of Cr carbides and Cr nitrides causes a decrease in ductility, so the upper limit of the content is set to 40.0%. The Cr content is more preferably 22.0% to 35.0%.

Ni: 18.0% -55.0%
Ni is an element necessary for ensuring repeated oxidation resistance and stability of the metal structure, ensuring high-temperature creep strength, and stabilizing austenitization of the centrifugal casting part. Further, together with Cr, it contributes to the improvement of high temperature strength and oxidation resistance. Further, when the Ni content is low, the Fe content is relatively high, which inhibits the formation of Al oxide. Therefore, it is contained at least 18.0% or more. On the other hand, even if Ni is added excessively, the effect is saturated and it is economically disadvantageous, so the upper limit is set to 55.0%. The Ni content is more preferably 29.0% to 46.0%.

Al: 1.0% to 5.5%
Al is an indispensable element for forming an Al oxide in the centrifugal casting part. The formation of Al oxide improves the oxidation resistance and carburizing resistance of the centrifugal casting portion together with Cr oxide. Further, Al forms a γ'phase together with Ni to strengthen the austenite phase of the centrifugal casting portion. Therefore, Al is contained in an amount of 1.5% or more. However, excessive addition of Al leads to a decrease in ductility, and the γ'phase becomes unstable, leading to the formation of an embrittled phase. Further, excessive addition of Al causes deterioration of castability and lowers the cleanliness of the centrifugal casting portion. Therefore, the upper limit is 5.5%. The Al content is more preferably 2.0% to 4.5%.

At least one selected from the group consisting of Ti: 0.01% to 0.6% and Nb: 0.1% to 1.8% Ti and Nb are elements that easily form carbides and creep. Contributes to improvement of breaking strength and high temperature tensile strength. Nb also contributes to the improvement of aging ductility. Therefore, at least one of Ti: 0.01% or more and Nb: 0.1% or more is contained. On the other hand, excessive addition of these elements leads to a decrease in ductility. Then, Nb causes a decrease in the peeling resistance of the Al oxide layer and a decrease in the oxidation resistance. In addition, excessive addition of Ti promotes the formation of Ti oxide and lowers the cleanliness of the centrifugal casting portion. Therefore, the upper limit is Ti: 0.6% and Nb: 1.8%. The Ti content is more preferably 0.05% to 0.30%, and the Nb is more preferably 0.1% to 1.3%.

In addition, the centrifugal force casting portion can contain the following elements.

Rare earth element (REM): More than 0% and 0.4%
REM means 18 kinds of elements by adding Y, Hf and Sc to 15 kinds of lanthanum series from La to Lu in the periodic table, and REM contained in the centrifugal casting part is Ce, La, Nd is the main component, and the total amount of these three elements is preferably about 80% or more, more preferably about 90% or more of the total rare earth elements. Since REM contributes to the stabilization of the Al oxide layer and is an active metal, the adhesion of the oxide film can be enhanced. Further, it is desirable that REM is contained because it prevents the spokeling fracture of the oxide layer due to the temperature change in the furnace and further dissolves in the base material to contribute to the improvement of oxidation resistance. On the other hand, REM preferentially forms an oxide and causes deterioration of cleanliness and ductility of the base material, so the upper limit is set to 0.4%. The REM content is more preferably 0.01% to 0.30%.

W: At least one selected from the group consisting of more than 0% and 5.0% or less, and Mo: more than 0% and 2.0% or less W and Mo are solid-solved in the base material and are mothers. In order to strengthen the austenite phase of the material and improve the creep rupture strength, it is desirable to contain either one or both. However, excessive content of W and Mo causes deterioration of ductility and carburizing resistance, and inhibits the formation of Al oxide particularly when Al oxide is formed at a temperature of 1050 ° C. or lower. Further, excessive content of W and Mo causes a decrease in the oxidation resistance of the base material. Therefore, the upper limit of W is 5.0%, and the upper limit of Mo is 2.0%. The upper limit of the W content is 3.0%, and the Mo content is more preferably 1.0%.

The centrifugal force casting portion can be composed of the balance Fe and unavoidable impurities. The upper limit of unavoidable impurities is preferably 1.0%.

In addition, each element contained in the centrifugal casting part is
Pa = -11.1 + 28.1 x C + 29.2 x Si-0.25 x Ni-45.6 x Ti + 18.0 x REM-16.6 x Nb,
When Ya = -13.75 x Al + 63.75,
It is desirable that Pa <Ya. If the element indicated above is not included in the Pa value, the value of the element is treated as zero.
When the Pa value and the Ya value satisfy the above equations, the weldability and oxidation resistance (formation of the Al oxide layer) of the centrifugally cast portion can be ensured.

The Pa value is related to the content of each element of C, Si, Ni, Ti, REM, and Nb, and a test centrifugal force casting section in which the content of each element and the content of Al are variously changed is produced. did. Then, the bead placement test in the examples described later was carried out for each test centrifugal force casting section, and data on the weldability of the test centrifugal force casting section was obtained. From the obtained data, the influence coefficient of the element that affects the weldability was obtained by regression analysis.

When referring to the influence coefficient of the Pa value, positive C, Si, and REM are elements that adversely affect weldability, respectively, and the larger the numerical value (absolute value), the greater the degree of the adverse effect. Further, Ni, Ti, and Nb having a negative influence coefficient are elements that improve weldability, and the larger the numerical value (absolute value) is, the more favorable the influence is.

FIG. 2 is a plot in which the Pa value of the test centrifugal force casting portion is plotted on the vertical axis and the Al content is plotted on the horizontal axis. Things are plotted in squares.

In order for the Al oxide layer to be well formed in the test centrifugal force casting part and to have oxidation resistance, the Al content range (Al: 1.0% to 5.5%) is set. There is a need. With reference to FIG. 2, regarding the Pa value and the content of Al in which the Al oxide layer is formed well, the group having excellent weldability and the group having insufficient weldability are clearly divided into regions. I understand. From this graph, it can be seen that the correlation could be clearly analyzed for the Ya value including the Al content based on the weldability.

Then, the linear Ya value based on the content of Al that divides these populations could be determined as Ya = -13.75 × Al + 63.75. That is, it can be seen that by satisfying Pa <Ya in the range of Al: 1.0% to 5.0%, a centrifugally cast portion having excellent oxidation resistance and weldability can be obtained.

The centrifugal casting portion is formed into a cylindrical shape by centrifugal casting, and a steel product can be manufactured by welding with a static casting portion such as an axle.

The static casting part is based on mass%.
Si: More than 0% and less than 1.0%,
Mn: More than 0% and 1.0% or less, and
Al: It can be composed of a material containing 0.01% to 0.5%.

Si: More than 0% and 1.0% or less Si is contained as a deoxidizing agent for the molten alloy, and also for increasing the fluidity of the molten alloy and improving the oxidation resistance. In the present invention, by defining the amount of Si in the static casting portion as described above, the crack sensitivity of the welded portion can be suppressed even if Al is contained, and the deterioration of the weldability can be suppressed. The Si content is preferably 0.3% or more and 0.8% or less.

Mn: More than 0% and 1.0% or less Mn serves as a deoxidizing agent for the molten alloy, and S in the molten metal is fixed to improve weldability and ductility. In the present invention, by defining the Mn amount of the static casting portion as described above, the crack sensitivity of the welded portion can be suppressed even if Al is contained, and the deterioration of the weldability can be suppressed. The upper limit of the Mn content is preferably 0.8% or less.

Al: 0.01% or more and 0.5% or less Al is an indispensable element for forming an Al oxide even in a static casting portion. The formation of Al oxide improves oxidation resistance and carburizing resistance together with Cr oxide. On the other hand, Al is generally an element that increases crack sensitivity during welding and lowers weldability. However, in the present invention, by containing Si and Mn in the static casting portion in the above range as described above, it is possible to suppress a decrease in crack sensitivity of the welded portion even if Al is contained, and a decrease in weldability can be achieved. It can be suppressed. It is desirable that the upper limit of the Al content is 0.3%. By setting the Al concentration of the static casting section to be lower than the Al concentration of the centrifugal casting section, it is possible to suppress a decrease in weldability. Further, the static casting portion is often cast by using a sand casting method, and the cooling rate of the cast product is generally slow, so that the metal structure tends to be rough and the strength and ductility tend to be low. Therefore, the Al concentration of the static casting section is preferably 1/5 or less of that of the centrifugal casting section, and more preferably 1/10.

The static casting section can contain the following elements.

Cr: 24.0% to 50.0%
Cr contributes to the improvement of high temperature strength and repeated oxidation resistance. Further, Cr exhibits excellent heat resistance in a high temperature range exceeding 1000 ° C. together with Ni and Fe, and forms primary carbides with C and N to improve the high temperature creep rupture strength. Then, an oxide layer is formed together with Al, and the static casting portion is provided with excellent properties of oxidation resistance and corrosion resistance. Therefore, it is contained at least 24.0% or more. On the other hand, excessive formation of Cr carbides and Cr nitrides causes a decrease in ductility, so the upper limit of the content is set to 50.0%. The Cr content is more preferably 25.0% to 40.0%.

C: 0.2% -0.6%
C is contained in an amount of 0.2% or more in order to ensure the weldability of the static casting portion and reduce the crack sensitivity. On the other hand, in order to ensure ductility (prescription aging), the upper limit is set to 0.5%. The C content is more preferably 0.35% to 0.5%.

Ni: 10.0% -50.0%
Ni is added to ensure the heat resistance of the static casting portion in the temperature range used. Further, Ni, together with Cr, contributes to the improvement of high temperature strength and oxidation resistance. Further, when the Ni content is low, the Fe content is relatively high, which inhibits the formation of Al oxide. Therefore, it is contained at least 10.0% or more. On the other hand, even if Ni is added excessively, these effects are saturated and economically disadvantageous, so the upper limit is set to 50.0%. The Ni content is more preferably 13.0% to 45.0%.

W: At least one selected from the group consisting of more than 0% and 5.0% or less, and Mo: more than 0% and 2.0% or less W and Mo are included in the matrix of the static casting section. It distorts the crystal structure and is added to improve tensile strength and creep strength.

Nb: More than 0% and 2.0% or less Nb is added to contribute to the improvement and recovery of the aging ductility of the static casting portion and to improve the weldability. Excessive addition of Nb causes a decrease in the peeling resistance of the Al oxide layer and also reduces the oxidation resistance, so the upper limit is set to 2.0%.

N: More than 0% and 0.2% or less N can be added in the above range because it contributes to the improvement of the altitude and the tensile strength of the static casting portion.

The static casting portion can be composed of the balance Fe and unavoidable impurities. The upper limit of unavoidable impurities is preferably 1.0%.

Although the centrifugal casting section and the static casting section of the present invention contain Al, they have excellent weldability to each other and can suppress crack susceptibility. Therefore, the steel product obtained by welding these is sufficiently mechanical. Welding strength can be secured. Steel products are integrated by fitting a cylindrical centrifugal casting part into a shaft-shaped static casting part by shrink fitting, etc., and welding the end edge of the centrifugal casting part to the static casting part. Can be transformed into. For welding, it is preferable to use austenitic steel containing high Cr and Ni containing Al as the welding material. The weld material generally does not contain Al in order to suppress deterioration of the workability of the welding rod, but by containing Al, Al oxide can be formed in the welded portion and its heat-affected zone. .. It is desirable that the welding material is a component of the centrifugal casting part and the static casting part that matches the material having a high content of Ni and Cr.

The steel product of the present invention can form an Al oxide on the surface by containing Al in the centrifugal casting section and the static casting section, respectively, and can have oxidation resistance and carburizing resistance. In particular, since the centrifugal casting part and the static casting part contain Al, an oxide film is formed on the welded part and the heat-affected zone thereof at the time of welding. Then, with this oxide film as a starting point, oxidation proceeds in the centrifugal casting section and the static casting section in the used state, Al oxide is preferably formed on the surface of the steel product, and good oxidation resistance can be provided.

The steel product of the present invention can form an Al oxide on the surface when used in a high temperature environment, but when the steel product is positively formed with an Al oxide, it is an independent process. It can be carried out by heat-treating the steel product in an oxidizing atmosphere, or it can be carried out by installing the steel product in a heating furnace.

When the Al oxide formation treatment is positively performed, the temperature of the steel product is 900 ° C., preferably 1000 ° C., more preferably in an oxidizing atmosphere in which an oxidizing gas containing 1% by volume or more of oxygen, steam and CO ₂ are mixed. Is preferably heat-treated at a temperature of 1050 ° C. or higher. The heat treatment time is preferably 1 hour or more.

The centrifugal casting part and the static casting part each come into contact with oxygen, and Al, Cr, Ni, Si, Fe and the like diffused on the surface of the base metal are oxidized to form an oxide layer. At this time, by performing the heat treatment in the above temperature range, Al forms an oxide in preference to Cr, Ni, Si, and Fe. Further, Al in the base metal also transfers to the surface to form an oxide, and an oxide layer mainly composed of Al ₂ O ₃ is formed.

The obtained steel products have excellent weldability in the centrifugal casting part and the static casting part, so that they can be firmly joined to each other without causing weld cracks and the like, and have excellent mechanical properties. Further, not only the centrifugal casting part but also the static casting part has excellent mechanical properties, and the Al oxide formed on the surface can exhibit excellent oxidation resistance when used in a high temperature environment. Therefore, steel products can be satisfactorily used for hearth rolls, kiln furnaces, and radiant tubes.

If necessary, ceramic spraying may be performed on the surface of the centrifugal casting portion to form a ceramic spraying layer on the surface of the centrifugal casting portion.

A test piece for the centrifugal casting section and a test piece for the static casting section were prepared in the following manner. A part of the test piece of the centrifugal casting part was welded to the test piece of the static casting part to prepare a test piece of a steel product. The centrifugal casting part was tested for oxidation resistance and cracking susceptibility, and the static casting part was tested for casting property, oxidation resistance and cracking susceptibility with the centrifugal casting part.

<Centrifugal force casting part>
Test piece of centrifugal casting part with alloy composition (unit: mass%, balance Fe and unavoidable impurities) listed in Table 1 by centrifugal force casting (test piece for oxidation resistance test: thickness 25 mm x 3 pieces, welding Specimens for bead placement test for confirming the properties (invention examples only): thicknesses of 25 mm or less and 25 mm or more were prepared, respectively. An example of the invention is Test No. 101-110, the comparative example is the test No. It is 201 to 206. In Table 1, REM shows the total amount of Ce, La, and Y. Although all of the invention examples are within the range of the component composition of the present invention, the test No. of the comparative example. 201 to 204 are Al zero, test No. 205 and 206 are comparative examples of Al shortage, and "*" is added to the corresponding part.

In addition, Pa and Ya were calculated for each test piece in Table 1 and their magnitude relations were compared. In Table 1, a check mark is entered in the “Pa <Ya” column for the test piece satisfying Pa <Ya. With reference to Table 1, the test No. It can be seen that 103, 109, 110, 205 and 206 are test pieces that do not satisfy Pa <Ya.

<Standing casting section>
Specimens of static casting parts with alloy composition (unit: mass%, balance Fe and unavoidable impurities) listed in Table 2 by static casting (test specimens for oxidation resistance test: thickness 25 mm x 3, welding Specimens for bead placement test to confirm the properties: 25 mm or less in thickness and 25 mm or more in thickness) were prepared respectively. An example of the invention is Test No. 301-308, the comparative example is the test No. It is 401 to 405. Although all of the invention examples are within the range of the component composition of the present invention, the test No. of the comparative example. 401 is an excess of Al, and the test No. 402 is an excessive amount of Si, and the test No. 403 is an excess of Al, and the test No. 404 is a comparative example in which Al is too small, and Comparative Example 405 is a comparative example in which Mn is excessive, and "*" is added to the corresponding portion.

<Castability>
Prior to the following tests, the castability of the static casting section was first compared. Castability means the ease of casting in static casting. The castability was evaluated on a three-point scale, with excellent castability being evaluated as "A", generally good ones being evaluated as "B", and inferior ones being evaluated as "C". The results are shown in Table 2. Referring to Table 2, the test No. of the comparative example. The castability of any of the test pieces except 401 is "A" or "B", and the test No. Only 401 was "C". Test No. of the comparative example. It is considered that the reason why 401 is inferior in castability is that Al is excessive.

<Oxidation resistance test>
Subsequently, after measuring the weight of each specimen of the centrifugal casting part and the static casting part, the test piece was held in a heating furnace (atmospheric atmosphere) at 1000 ° C. for 100 hours, and the scale of the surface of the test piece after heating was performed. Was removed with an acid solution and weighed again. Then, the oxidative weight loss was calculated from the amount of change in weight of each specimen before and after heating, and the average of the oxidative weight loss per hour (mg / cm ² · h) was obtained. The results of the centrifugal casting section are shown in "Oxidation resistance" and "evaluation" in Table 1, and the results of the static casting section are shown in "Oxidation resistance (evaluation)" in Table 2. In the oxidation resistance evaluation, the oxidative weight loss was evaluated as "A" for 0.1 mg / cm ² · h or less, "B" for 0.3 mg / cm ² · h or less, and 1 mg / cm ² · h or less. Those exceeding "C" and 1 mg / cm ² · h were evaluated as "D".

Regarding the centrifugal force casting section, referring to Table 1, all of the invention examples have an evaluation of "A" or "B", and the oxidative weight loss is smaller than that of the comparative example having an evaluation of "C" or "D", and the oxidation resistance is small. You can see that it is excellent in sex. This is because Al oxide was generated on the surface of the test piece of the centrifugal casting part of the invention example, and further oxidation was suppressed. On the other hand, in the sample piece of the centrifugal casting part of the comparative example, Cr oxide and Si oxide are generated on the surface, and these are less dense than Al oxide, and the oxygen intrusion prevention function is not sufficient. This is because the oxidation could not be suppressed.

Comparing the invention examples of the centrifugal casting section, the test No. 102, 103, 106, 108 to 110 are evaluated as "A", and it can be seen that they are particularly excellent in oxidation resistance. It is presumed that this is because the Al concentration was high and the Cr concentration at which the oxide layer was easily formed was relatively low, so that a particularly excellent Al oxide layer was formed.

As for the static casting section, as shown in Table 2, the invention examples are No. Except for 308, all of them have an evaluation of "A" and are excellent in oxidation resistance. Test No. The 308 also has an oxidation resistance evaluation of "B", which is slightly inferior to the other specimens of the invention, but has sufficient oxidation resistance. Test No. It is considered that the reason why 308 is inferior in oxidation resistance as compared with other invention examples is that the Cr content is 19%, which is less than that of other invention examples. On the other hand, the static casting section of the comparative example is the test No. 402, 403, and 405 have an oxidation resistance evaluation of "A" and have excellent oxidation resistance. 401 has an evaluation of "B" and 404 has an evaluation of "C". This is the test No. It is considered that the cause of 401 is that Al is excessive but Cr is small, and sufficient oxidation resistance cannot be ensured. In addition, the test No. It is considered that 404 is caused by the fact that Al is too small.

<Bead placement test>
The bead placement test was carried out in the following manner for the test pieces (thickness: 25 mm or less and 25 mm or more) of the centrifugal casting part of the invention example shown in Table 1, and the crack susceptibility due to welding was determined.

Prior to the bead placement test, the test surface of the test piece was machined with a grinder to smooth the surface. The test surface is a portion to be a weld groove and a portion affected by heat due to welding.

In addition, a liquid penetrant inspection was conducted on the test surface of each specimen, and it was confirmed that there was no crack on the test surface.

Inventive example of centrifugal casting part whose test surface was confirmed to be sound The test piece was subjected to the condition bead placement test shown in Table 3 by TIG welding. The bead is a straight bead, and the bead length is 50 to 100 mm.

As for the construction sequence of this test, after the test by the method A, the test by the method B was carried out when a defect was found in the liquid penetrant inspection test.

FIGS. 3 and 4 show the criteria for determining the bead by the method A (filler metal (welding rod): none) and the method B (filler metal: yes). In the B method, the determination is "OUT" even for a minute crack. In the figure, reference numeral 50 indicates a bead, 52 indicates a crater, 54 indicates a crack, and 56 indicates a punctate defect.

As a result of the above test, no defect was found in any of the specimens having a thickness of 25 mm or less and a thickness of 25 mm or more by the A method. However, the test piece in which no defect was found by the B method was evaluated as a crack susceptibility evaluation "B", and the test piece in which a defect was found by the B method was evaluated as a crack susceptibility evaluation "C". The results are shown in "Crackability" in Table 1.

With reference to Table 1, the test piece of the centrifugal casting section, which is an example of the invention, has a test No. 101, 102, 104 to 108 were evaluated as "A", and the test No. 103, 109 and 110 were rated "B".

Examining the invention examples, it can be seen that all the specimens having a crack susceptibility evaluation of "A" had a larger Ya than Pa and Pa <Ya.

Subsequently, with respect to the test piece of the static casting part, the test No. of the typical composition (intermediate component) of the centrifugal casting part. 101 and No. Welding with 107 was actually performed, and crack susceptibility was evaluated in the same manner. The evaluation was made by welding the test pieces of the static casting part to the test pieces of the two centrifugal casting parts, respectively. The specimen in which the defect was found in the method B but the defect was not found in the method B is "B", and the specimen in which the defect was found in the method B is "C". The results are shown in Table 2.

With reference to Table 2, examples of the invention are described in Test No. The evaluation of crack susceptibility was "A" except for 307, indicating that the crack susceptibility could be suppressed. This is because Si and Mn are within the predetermined ranges even if Al is contained. Test No. 307 is slightly inferior in the evaluation of crack susceptibility, which is considered to be due to an excessive amount of Cr. On the other hand, the example of the invention is the test No. Except for 404, all were rated "C". This is the test No. 401, No. 403 is Al excess, test No. 402, No. 405 is due to the fact that the Al content is within the scope of the present invention, but is excessive in Si or Mn.

<Comprehensive judgment>
First, for each specimen of the centrifugal force casting part, the comprehensive judgment "A" is given if the evaluation of the oxidation resistance test and the bead placement test is "A", one is "A", and the other is "B". Those containing the comprehensive judgment "B" and the evaluation "C" or "D" were judged as the comprehensive judgment "C". The results are shown in "Comprehensive Judgment" in Table 1. Referring to Table 1, the test pieces of the invention examples were all judged as "A" or "B", and the test pieces of the comparative example were all judged as "C". That is, it can be seen that the test piece of the invention example is less likely to crack during welding and has excellent oxidation resistance as compared with the test piece of the comparative example. Therefore, the centrifugal casting section of the present invention is extremely suitable for application to hearth rolls, kiln furnaces, radiant tubes, and the like.

Similarly, for each specimen of the static casting section, those having a castability, oxidation resistance, and crack susceptibility evaluation of "A" are comprehensively judged as "A", and those containing "B" in the evaluation are comprehensively evaluated. Those including the determinations "B" and "C" were determined as the comprehensive determination "C". The results are shown in "Comprehensive Judgment" in Table 2. Referring to Table 2, all of the invention examples had a comprehensive judgment of "A" or "C", and all of the comparative examples had a judgment of "C". That is, it can be seen that the static casting portion of the invention example has excellent castability and excellent oxidation resistance, and can suppress crack sensitivity to the centrifugal casting portion of the invention example. Therefore, the static casting portion of the present invention has excellent mechanical bonding strength and oxidation resistance by being directly welded to the above-mentioned centrifugal casting portion after groove processing, and thus has a hearth. It can be seen that it is extremely suitable for application to steel products such as rolls, kiln furnaces, and radiant tubes.

The above description is for the purpose of explaining the present invention, and should not be construed as limiting or limiting the scope of the invention described in the claims. In addition, the configuration of each part of the present invention is not limited to the above embodiment, and it goes without saying that various modifications can be made within the technical scope described in the claims.

10 Steel products (hearth roll)
20 Centrifugal force casting part (sleeve)
30 Static casting part (axle)
40 Welded part

Claims (10)

It is a steel product with excellent oxidation resistance.
The centrifugal casting part manufactured by centrifugal casting is attached to the static casting part formed by static casting by welding.
The centrifugal force casting portion is based on mass%.
C: 0.2% -0.7%,
Si: More than 0% and 2.0% or less,
Mn: More than 0% and less than 3.0%,
Cr: 15.0% -40.0%,
Ni: 18.0% to 55.0%,
Al: 1.0% to 5.5%, and
It contains at least one selected from the group consisting of Ti: 0.01% to 0.6% and Nb: 0.1% to 1.8%.
It consists of the balance Fe and unavoidable impurities.
The static casting portion is based on mass%.
C: 0.2% -0.6%,
Si: More than 0% and less than 1.0%,
Mn: More than 0% and 1.0% or less ,
Cr: 19.0% to 55.0%,
Ni: 10.0% -50.0%,
Al: Contains 0.01% to 0.5% ,
Remaining Fe and unavoidable impurities,
Steel products. The centrifugal force casting portion is based on mass%.
Rare earth element (REM): contains more than 0% and 0.4%,
The steel product according to claim 1. The centrifugal force casting portion is based on mass%.
W: contains at least one selected from the group consisting of more than 0% and 5.0% or less, and Mo: more than 0% and 2.0% or less.
The steel product according to claim 1 or 2. The centrifugal force casting part is
Pa = -11.1 + 28.1 x C + 29.2 x Si-0.25 x Ni-45.6 x Ti + 18.0 x REM-16.6 x Nb
Ya = -13.75 x Al + 63.75
When
Pa <Ya
Is,
The steel product according to any one of claims 1 to 3. The static casting portion is based on mass%.
W: contains at least one selected from the group consisting of more than 0% and 13.2% or less, and Mo: more than 0% and 5.2% or less .
The steel product according to any one of claims 1 to 4. The static casting portion is based on mass%.
Nb: Contains more than 0% and 2.0% or less,
The steel product according to any one of claims 1 to 5. The static casting portion is based on mass%.
N: Contains more than 0% and 0.2% or less,
The steel product according to any one of claims 1 to 6. The centrifugal force casting portion is formed on a surface that comes into contact with a steel material.
The steel product according to any one of claims 1 to 7 . A ceramic sprayed layer is provided on the surface of the centrifugal casting portion.
The steel product according to any one of claims 1 to 8 . The steel product is a hearth roll, a kiln furnace, or a radiant tube.
The steel product according to any one of claims 1 to 9 .

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