JP5144390B2 - Method for producing Cr-containing steel with excellent scale peelability - Google Patents

Description

  The present invention relates to a method for producing a Cr-containing steel excellent in scale peelability, and more specifically, a steel bar made of Cr-containing steel having good surface properties with little surface flaw due to excellent scale peelability. Regarding the method.

  The surface quality required for steel strips for mechanical structures such as SCR420 and SCM435 belonging to JIS G4053 and high-carbon chromium bearing steels such as SUJ2 belonging to JIS G4805 is becoming stricter year by year. In such steel bars, surface defects such as fine surface wrinkles and rough surfaces may occur after hot rolling in the manufacturing process. Especially in steel types that contain a lot of easily oxidizable alloy elements such as Si and Cr, iron oxide containing alloy elements in the heating and rolling processes (hereinafter referred to as “subscale”, subscale is a part of the scale) Generated in the lowest layer of the scale (near the railway interface). Since this subscale has an effect of improving the adhesion between the scale and the ground iron, it is difficult to remove the scale by descaling using high pressure water (hereinafter referred to as “high pressure water descaling”). When the scale is rolled in a state where the scale remains on the surface of the ground iron, the scale is pushed into the ground iron to form a scale rod, which deteriorates the quality of the steel bar material.

In particular, since the subscale generated in the heat treatment using a heating furnace with the furnace temperature being less than 1200 ° C. is dense and thin and has high adhesion to the ground iron, in normal high-pressure water descaling, Removal is extremely difficult. Therefore, as one method for improving the scale peelability by high-pressure water descaling, the steel bar is heat-treated at a furnace temperature of 1200 ° C or higher while water is supplied directly to the steel bar, and then the high-pressure water descaling is performed. And a rolling process are known (see, for example, Patent Document 1). Further, as another method for improving the scale peelability, a heat treatment method is known in which the heat treatment temperature and the water vapor concentration are changed in two stages in the method of heat treating the strip steel while supplying water vapor to the heating furnace. (For example, refer to Patent Document 2).
JP 2002-316207 A JP 2003-119517 A

  However, as disclosed in Patent Document 1, when heat treatment is performed at 1200 ° C. or higher, there is a problem that scale loss increases and energy intensity increases. In addition, since the heat treatment of the strip steel is generally performed at around 1100 ° C., it is necessary to perform the heat treatment separately from other steel materials, so that the production management becomes complicated, and the temperature inside the furnace is changed. During the time required, production is interrupted, resulting in a problem that productivity decreases. Furthermore, there is a problem that the equipment cost increases, such as the need for a dedicated heating furnace. On the other hand, in the method disclosed in Patent Document 2, the scale peelability of the steel strip made of SCM435 is improved, but the scale peelability of the steel strip made of SUJ2 and SCR420 is not improved, and the applicable range is limited. There is.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a method for producing a Cr-containing steel excellent in scale peelability, and has good surface properties with less surface flaws caused by scale. The Cr-containing steel provided is provided.

The method for producing a Cr-containing steel having excellent scale peelability according to the present invention contains Si: 0.05 to 0.4 mass%, Cr: 0.1 to 2.0 mass%, with the balance being Fe and inevitable. When the steel piece made of Cr-containing steel made of impurities is heat-treated immediately before descaling, the heat treatment temperature is 1000 to 1150 ° C., the holding time at the heat treatment temperature is 1 to 30 minutes, and heating The treatment atmosphere includes H ₂ O [water vapor]: 25 to 30% by volume, O ₂ : 1 to 10% by volume, and the balance is N ₂ and inevitable gas components.

Thus, scale loss can be suppressed small by setting the heat treatment temperature to 1000 to 1150 ° C. (condition a). Moreover, since this heat processing temperature is in the range of the heat processing temperature of general steel materials, since a dedicated installation is not required, manufacturing cost can be maintained low. Further, by setting the holding time to 1 to 30 minutes (condition b), it is possible to avoid an increase in manufacturing cost and an increase in scale loss due to an increase in energy intensity while ensuring necessary scale peelability. Further, _{N 2 H 2} O [steam]: 25-30 vol% and _O 2: With heating atmosphere was added a 1 to 10 vol% (condition c), excellent peeling property from a steel matrix Since a FeO layer can be formed, favorable scale peelability can be obtained for various steel materials.

  It should be noted that the heat treatment of the steel slab does not have to be performed under the conditions a to c, but at least the second half of the heat treatment of the steel slab is performed under the conditions a to c, thereby removing the scale. Can be produced. Further, after the heat treatment under a predetermined condition, the scale peelability can be improved by further performing the heat treatment under the conditions a to c.

  In the method for producing a Cr-containing steel according to the present invention, the Cr-containing steel includes, as optional components, C: 0.02 to 1.0 mass%, Mn: 0.2 mass% or more and less than 3.0 mass%, Mo It is preferable to contain at least one element selected from: 0.3% by mass or less.

  Strength can be increased by containing the specified amount of C, strength and toughness can be increased by containing the specified amount of Mn, and strength can be increased by containing the specified amount of Mo. Can do.

  According to the present invention, by obtaining excellent scale peelability, it is possible to produce a strip material having excellent surface properties with few surface defects such as fine surface wrinkles and rough skin, and in this case, scale loss is reduced. It is possible to reduce the production unit and the equipment cost because the energy intensity can be kept low and the dedicated heat treatment equipment is not required. Furthermore, the present invention has an advantage that it can be applied to various steel materials.

Hereinafter, embodiments of the present invention will be described in detail. In the method for producing a Cr-containing steel according to the present invention, Si: 0.05 to 0.4% by mass, Cr: 0.1 to 2.0% by mass, with the balance comprising Fe and unavoidable impurities When heat-treating a steel slab (steel bar, steel plate) made of steel, the following conditions a to c are satisfied.
(Condition a) Heat treatment temperature shall be 1000-1150 degreeC.
(Condition b) The holding time at the heat treatment temperature is 1 to 30 minutes.
(Condition c) The heat treatment atmosphere contains H ₂ O [water vapor]: 25 to 30% by volume, O ₂ : 1 to 10% by volume, and the balance is N ₂ and inevitable gas components.

  A specific example of the heat treatment satisfying the conditions a to c is soaking treatment performed immediately before the descaling treatment performed before hot rolling of the steel slab. In this case, the steel slab is processed in the order of soaking treatment → descaling treatment (for example, high pressure water descaling) → hot rolling. The requirements will be described below.

<< Cr-containing steel >>
The Cr-containing steel to which the present invention is applied includes the above-described predetermined amounts of Si and Cr. These Si and Cr are components constituting the scale by oxidation by heat treatment, and greatly affect the scale peelability. Moreover, this Cr-containing steel is selected from C: 0.02 to 1.0% by mass, Mn: 0.2% by mass or more and less than 3.0% by mass, Mo: 0.3% by mass or less as optional components. It is preferable to contain at least one kind of element. These affect the strength and toughness of the Cr-containing steel.

Specific examples of Cr-containing steel include alloy steels for machine structural use such as SCM415 to 445 and SCR415 to 445 belonging to JIS G4053, and H steel having the same composition as these (SCM415H to 445H, SCR415H to 445H (JIS G4052)). ), High carbon chromium bearing steel such as SUJ2 belonging to JIS G4805.
Table 1 shows steel types and compositions of typical Cr-containing steels. The balance of these various Cr-containing steels contains inevitable impurities. Hereinafter, each component will be described.

<Si: 0.05 to 0.4% by mass>
In order to ensure the required strength, Si has a lower limit of 0.05% by mass required for Cr content, but excessive addition of 0.4% by mass or more deteriorates the ductility of Cr-containing steel. This is not preferable because it inhibits the effect of Cr in the present invention (that is, improvement in scale peelability). Therefore, Si content shall be 0.05-0.4 mass%.

<Cr: 0.1 to 2.0% by mass>
Cr contributes to strength improvement by improving hardenability. A steel material generally called Cr-containing steel contains 0.1% by mass or more of Cr. In such a Cr-containing steel, Cr is concentrated at the scale-metal interface, and FeCr ₂ O ₄ is generated as a subscale. When the Cr content is excessive, the thickness of the FeCr ₂ O ₄ layer is increased and the adhesion of the scale is increased, so that the scale peelability is lowered. Further, when Cr is added excessively, martensite is likely to be generated. Therefore, Cr content shall be 0.1-2.0 mass%.

<C: 0.02 to 1.0% by mass>
C needs 0.02 mass% or more of content in order to ensure the required intensity | strength of Cr containing steel. However, when C is added exceeding 1.0 mass%, the cold workability of the Cr-containing steel material is lowered. Therefore, C content shall be 0.02-1.0 mass%. Preferably, the upper limit of the C content is 0.6% by mass.

<Mn: 0.2% by mass or more and less than 3.0% by mass>
Mn needs to be contained in an amount of 0.2% by mass or more in order to ensure the strength and toughness of the Cr-containing steel. However, when the Mn content is 3.0% by mass or more, toughness and weldability are hindered. Therefore, the Mn content is set to 0.2% by mass or more and less than 3.0% by mass.

<Mo: 0.3 mass% or less>
Mo may be added as necessary for the purpose of increasing the strength of the Cr-containing steel, but it promotes the infiltration of Si oxide firelite (Fe ₂ SiO ₄ ) into the steel by segregation at the grain boundaries. In order to prevent this, the content is preferably 0.3% by mass or less.

<Other ingredients>
P, S, Al, and Ca are mentioned as an element which it is preferable to control content in Cr containing steel. P segregates at the grain boundary as an impurity, but forms a segregation zone when it is stretched in the forging and rolling direction by forging or rolling. Α-Fe is formed in this segregation zone, and C is excluded from this segregation zone. As a result, α-Fe is formed in a band shape in the segregation band of P, and pearlite is formed in a band shape in other portions. Usually, this is called a ferrite band, and when the ferrite band is formed, the ductility in the direction perpendicular to the band is lowered. In order to prevent this decrease in ductility, it is necessary to suppress the P content to 0.003% by mass or less. Further, S forms MnS, which is a sulfide-based inclusion, and segregates during hot working of the Cr-containing steel, causing the Cr-containing steel to become brittle. It is necessary to suppress to 005 mass% or less.

  Al is a deoxidizing material that is added to coarsen austenite crystal grains during normalizing heating of Cr-containing steel, but excessive addition of 0.05% by mass or more destabilizes the crystal grains. Therefore, it is not preferable. Therefore, it is preferable to suppress the Al content to less than 0.05% by mass. Ca is effective in suppressing the increase of the hydrogen ion concentration in the interface atmosphere accompanying the surface corrosion of the Cr-containing steel and improving the corrosion resistance of the Cr-containing steel, but it should be noted that excessive addition deteriorates the ductility. Added.

<Heat treatment conditions>
In the case where the conditions a to c are soaking performed immediately before the descaling process, it is preferable that the conditions a to c are satisfied in the entire range from the start to the end of the soaking. However, the soaking is not necessarily limited to such a form, and the above-described conditions a to c only have to be satisfied at least in the latter half of the soaking. For example, the first half of the soaking may be performed in an N ₂ atmosphere that does not contain H ₂ O [water vapor] and O ₂ , and the second half may be processed so as to satisfy the conditions a to c. Moreover, scale peelability can also be improved by performing the heat processing which satisfy | filled conditions ac further to the steel piece heat-heat-treated on another condition.

<Condition a-heat treatment temperature: 1000 to 1150 ° C.>
The heat treatment temperature of the steel slab (in-furnace temperature when a heating furnace is used) is 1000 to 1150 ° C. Since this temperature is within the range of the normal slab heat treatment temperature, there is no need to modify the heating furnace so that heating at a higher temperature is possible, and therefore an increase in equipment cost should be avoided. Can do. Moreover, by processing at the temperature of 1000-1150 degreeC, a scale loss can be restrained small and, thereby, an energy basic unit can be restrained low and the manufacturing cost per unit mass can be lowered | hung. Furthermore, when the heat treatment temperature is different from other steel materials, a dedicated heating furnace is required. However, by setting the heat treatment temperature to 1000 to 1150 ° C., it is not necessary to manufacture it by distinguishing from various steel types. . Therefore, an increase in equipment cost can be avoided.

  In addition, when the heat processing temperature of a steel slab is less than 1000 degreeC, the inner oxide layer (it demonstrates in detail later) which has sufficient thickness is difficult to be formed, Therefore, favorable scale peelability is not obtained. On the other hand, when the heat treatment temperature of the steel slab is higher than 1150 ° C., a modification of the heating furnace or a dedicated heating furnace is required, an increase in manufacturing cost due to an increase in energy intensity, an increase in scale loss, etc. The disadvantages are large.

<Condition b-Holding time at heat treatment temperature: 1 to 30 minutes>
When the steel slab is heat-treated at the above heat treatment temperature, if the holding time is less than 1 minute, scale growth is insufficient and sufficient scale peelability cannot be obtained. Therefore, it becomes difficult to improve the surface properties of the steel slab. If the holding time is lengthened, the scale grows and the scale peelability improves, but if the heat treatment is performed longer than necessary, the manufacturing cost increases due to the increase in energy consumption rate, and the scale loss increases. Inviting it is not preferable. Therefore, the upper limit of the holding time is set within 30 minutes. Therefore, the holding time at the heat treatment temperature is 1 to 30 minutes.

<Conditions c- heat treatment _{atmosphere: H} 2 O [steam]: 25-30 _vol%, O 2: 1 to 10 vol%, the remainder being _{N 2} and unavoidable gas component>
Control of the heat treatment atmosphere of the billet is extremely important. Generally, as the thickness of the scale increases, the stress value (absolute value of stress) at the interface between the scale and the ground iron increases, so that the scale peelability is improved and the scale is easily peeled off. However, the scale peelability of Cr-containing steel is not determined solely by the thickness of the entire scale, but is greatly influenced by the inner oxide layer that is a constituent element of the scale.

  The surface of the steel slab is oxidized by heat-treating the steel slab under the conditions a to c described above. The scale generated by such high-temperature oxidation is roughly classified into an “outer oxide layer” and an “inner oxide layer”, and the difference in the oxidation mechanism is largely involved in these generations. For example, when a steel material (Fe alloy) is oxidized at a high temperature, the scale grows because Fe diffuses outward in the scale and O (oxygen) diffuses inward. At this time, the scale layer generated by the outward diffusion of Fe playing a dominant role is the “outer oxide layer”, and the scale layer generated by the inward diffusion of O being “inner oxidation layer” It is a “oxidized layer”.

In high temperature oxidation using H ₂ O [water vapor], an inner oxide layer is likely to be generated. The inner oxide layer is an oxide layer generated at the bottom of the subscale (the side of the ground iron), and is formed by oxidizing the ground iron at the bottom of the subscale after the subscale is formed. In the case of Cr-containing steel, the subscale generated by high-temperature oxidation usually has FeCr ₂ O ₄ as a main component. After this subscale is formed, a layer deficient in Cr, which is an alloy element, is generated in the base iron at the bottom of the subscale. When O (oxygen) is diffused in this depletion layer portion and oxidation occurs, a subscale mainly composed of FeCr ₂ O ₄ is not generated because Cr is depleted. A oxidization layer is formed.

FeCr ₂ O ₄ which is the main component of the subscale has high adhesion to the ground iron, while FeO which is the main component of the inner oxide layer has low adhesion to the ground iron. Therefore, scale peelability can be improved by generating an inner oxide layer mainly composed of FeO.

In the case of SCM435 steel, as shown in a reference example described later, when the content of H ₂ O [water vapor] (hereinafter referred to as “H ₂ O concentration”) in the heat treatment atmosphere is up to 30% by volume, _{As the 2} O concentration increases, the thickness of the outer oxide layer and the inner oxide layer to be generated both increases, thereby improving the scale peelability. However, when the H ₂ O concentration exceeds 30% by volume, the thickness of the outer oxide layer becomes thicker, but the thickness of the inner oxide layer starts to decrease, so the scale loss increases and the scale peelability also decreases. To do. Therefore, for the SCM435 steel, “H ₂ O concentration: 25 to 30% by volume”, which is a condition for obtaining a particularly excellent scale peelability with little scale loss, is set as the H ₂ O concentration condition.

At this time, even if a part of H ₂ O [water vapor] is replaced with O ₂ and supplied to the heat treatment atmosphere, the same effect can be obtained. In that case, the total oxygen concentration (O concentration) contained in the heat treatment atmosphere may be 25 to 30% by volume when converted to the H ₂ O concentration. For example, of _{H 2} O concentration of 25 vol%, and the total oxygen concentration of when the content of _{O 2} (hereinafter referred to as _{"O 2} concentration") and 2% by volume, and 29% by volume of _{H 2} O concentration It will be the same as

On the other hand, in both SUJ2 steel and SCR420 steel, the scale peelability is not improved only by adding 25 to 30% by volume of H ₂ O [water vapor] to the N ₂ atmosphere. This is considered to be due to the fact that, in the case of SUJ2 steel and SCR420 steel, a dense subscale layer with a large Cr content is generated as compared with SCM435 steel.

  That is, since SUJ2 steel has a higher Cr content in the base iron than SCM435 steel (see Table 1), a dense subscale layer with a higher Cr content is generated. On the other hand, SCR420 steel and SCM435 steel have the same Cr content in the ground iron, but SCR420 steel does not contain Mo, whereas SCM435 steel contains 0.15 to 0.35 mass of Mo. % (See Table 1). The equilibrium oxygen pressure of Mo is slightly higher than the equilibrium oxygen pressure of Cr, but there is no significant difference. Therefore, in SCM435 steel, a subscale containing Cr and Mo is generated by oxidation of Cr and oxidation of Mo. On the other hand, in SCR420 steel, a subscale with a large Cr content is generated.

  The sub-scale of SCM435 steel has low density and barrier properties, and does not suppress the inward diffusion of O (oxygen). Therefore, SCM435 steel has an inner oxide layer mainly composed of FeO, which has excellent peelability from the steel. Produces. On the other hand, the sub-scale of SCR420 steel has a high density and a high barrier property, and since the inward diffusion of O is suppressed, the formation of an inner oxide layer that is excellent in peelability from the ground iron is inhibited. It is considered that such a difference in the formation form of the inner oxide layer appears as a difference in scale peelability between SCR420 steel and SCM435 steel.

Therefore, even when a dense sub-scale with high barrier properties such as SUJ2 steel and SCR420 steel is formed, in order to generate an inner oxide layer with excellent peelability, SCM435 steel has good scale peelability. Based on the obtained condition of “H ₂ O concentration: 25 to 30% by volume”, an atmosphere in which 1 to 10% by volume of O ₂ is further contained is used.

When the O ₂ concentration in the heat treatment atmosphere is increased, only the oxidizing action can be strengthened without increasing the reducing action by H ₂ constituting H ₂ O, so that the growth rate of the scale is increased and the subscale is increased. The thickness of becomes thicker. As a result, the Cr concentration in the subscale is lowered, the subscale density and barrier properties are lowered, and inward diffusion of O (oxygen) is likely to occur. Is generated, and the scale peelability is improved.

As described above, by increasing the O ₂ concentration in the heat treatment atmosphere, the growth of the inner oxide layer is promoted, thereby improving the scale peelability. However, it is possible to grow the inner oxide layer more than necessary. In addition, an increase in energy intensity causes an increase in manufacturing cost and an increase in scale loss. From such a viewpoint, the O ₂ concentration is set within 10% by volume. On the other hand, in the case of SUJ2 steel or SCR420 steel, if the O ₂ concentration is less than 1% by volume, the effect of improving the scale peelability is not substantially obtained. Therefore, the O ₂ concentration is set to 1% by volume or more.

  As described above, the thickness of the inner oxide layer has a large effect on the scale peelability. However, the scale peelability is not determined only by the thickness of the inner oxide layer, and the interface between the scale and the ground iron is smooth. Affected by properties and Cr concentration. For this reason, it is preferable to control the thickness of the inner oxide layer in consideration of these effects so that good scale peelability can be obtained.

  Examples of the present invention will be described below. It should be noted that the present invention is not limited to the following examples, and can be implemented with modifications within a range that can be adapted to the gist of the present invention, all of which fall within the technical scope of the present invention. included.

<< Reference Examples, Comparative Examples 1 and 2 >>
<Sample preparation>
Steel materials having the components and compositions shown in Table 2 were produced by melting. Here, steel type A corresponds to SCM435, steel type B corresponds to SCR420, and steel type C corresponds to SUJ2. Steel type A was used for the reference example, steel type B was used for comparative example 1, and steel type C was used for comparative example 2.

A cylindrical steel piece of φ8.0 mm × 12 mm was collected from various molten steel materials by machining, and used as a sample (test piece). At that time, the side surface of the sample was polished so that the surface roughness was Rmax ≦ 6 μm. The sample thus prepared was placed in an atmosphere-controlled heat treatment apparatus and held at 1100 ° C. for 30 minutes in an atmosphere in which H ₂ O was changed to 0 to 40% by volume in N ₂ . In this heat treatment (oxidation treatment), the rate of temperature increase and the rate of temperature decrease were set to 10 ° C./min. Further, the atmosphere of the temperature raising and lowering process was an N ₂ flow, and only a holding process at 1100 ° C. was an N ₂ + H ₂ O [water vapor] mixed gas atmosphere.

<Evaluation method of scale thickness>
The heat-treated sample was embedded in resin and cut to expose the sample cross section, and the sample cross section was polished. The scale cross section that appeared on the polished surface was observed by SEM, and the thicknesses of the outer oxide layer and the inner oxide layer constituting the scale were determined as the scale thickness. In the SEM observation of the scale cross section, in order to clearly distinguish the outer oxide layer and the inner oxide layer, a Pt thin film (film thickness: 1 μm) as a marker is formed on a part of the side surface of the sample before the heat treatment. An outer oxide layer was formed on the outer side and an inner oxide layer was formed on the inner side from the Pt thin film (marker) formed by sputtering and recognized in the scale cross section.

[Scale thickness of reference example (steel type A: SCM435)]
FIG. 1 shows the relationship between the H ₂ O concentration in the processing atmosphere, the outer oxide layer thickness, and the inner oxide layer thickness for the reference example. In the reference example, the outer oxide layer thickness showed the maximum value when the H ₂ O concentration was 35% by volume, but the inner oxide layer thickness showed the maximum value when the H ₂ O concentration was 30% by volume. It can be seen that the change in the thickness of the inner oxide layer in the range where the H ₂ O concentration is 25 to 30% by volume is small and is about 42 μm.

[Scale thickness of comparative example 1 (steel type B: SCR420)]
For Comparative Example 1, the relationship between the H ₂ O concentration in the processing atmosphere and the scale thickness is shown in FIG. In Comparative Example 1, the outer oxide layer thickness has the maximum value when the H ₂ O concentration is 25% by volume, but the inner oxide layer thickness is hardly affected by the H ₂ O concentration, and is about 15 μm. It can be seen that it is thin and constant.

[Scale thickness of comparative example 2 (steel type C: SUJ2)]
Comparative Example 2 shows the relationship between the H _{2 O} concentration and the scale thickness of the treatment atmosphere in FIG. In Comparative Example 2, the outer oxide layer thickness takes the maximum value when the H ₂ O concentration is 25% by volume, but the inner oxide layer thickness is less affected by the H ₂ O concentration, and is about 15 μm. It can be seen that it is thin and constant. Thus, in steel type B: SCR420 and steel type C: SUJ2, the relationship between the H ₂ O concentration in the processing atmosphere and the thickness of the inner oxide layer was almost the same.

<Method for evaluating scale peelability>
About each sample which concerns on a reference example and the comparative examples 1 and 2, the scale peelability (in other words, high temperature adhesiveness) was evaluated as follows. That is, first, the sample was set on the cacao for master, heated in vacuum, and after reaching 1000 ° C., the sample was compressed to peel off the scale. The sample compression conditions at this time were compression strain rate: 50%, strain rate: 10 mm / sec, and the sample after compression was rapidly cooled to room temperature in Ar to suppress scale formation after peeling (secondary scale generation). Next, the area of the scale remaining in the sample thus obtained was determined, and the ratio of this area to the total area of the sample (hereinafter referred to as “residual scale area ratio”) was quantified. The smaller the value of the residual scale area ratio, the better the scale peelability and the lower the high temperature adhesion of the scale.

[Reference Example (Steel Type A: SCM435) Scale Peelability]
FIG. 4 shows the relationship between the H ₂ O concentration in the processing atmosphere and the residual scale area ratio for the reference example. In the reference example, when the H ₂ O concentration was in the range of 25 to 30% by volume, the residual scale area ratio was 35% or less, and it was confirmed that excellent scale peelability was obtained. The residual scale area ratio took the minimum value when the H ₂ O concentration was 30% by volume, and at this time, the scale peelability was maximized.

[Scale peelability of Comparative Example 1 (Steel Type B: SCR420)]
Comparative Example 1 illustrates the relationship between the H _{2 O} concentration in the treatment atmosphere and the residual scale area rate in Fig. Residual scale area ratio of Comparative Example 1 hardly depends in H _{2 O} concentration, H 2 _O concentration was not obtained satisfactory scale peelability in the range of 0 to 40% by volume.

[Scale peelability of Comparative Example 2 (steel type C: SUJ2)]
For Comparative Example 2, the relationship between the H ₂ O concentration in the processing atmosphere and the residual scale area ratio is shown in FIG. In the case of Comparative Example 2, as in the case of Comparative Example 1, the residual scale area ratio hardly dependent in H _{2 O} concentration, good scale peelability range H 2 _O concentration of 0 to 40% by volume It was not obtained.

<< Examples 1 and 2 >>
<Sample preparation>
Each sample (steel piece) made of steel types B and C (see Table 2) was added with 25% by volume of H ₂ O [water vapor] to N ₂ , and O ₂ was changed in the range of 0 to 20% by volume. Hold at 1100 ° C. for 1 minute in atmosphere. The sample made of steel type B obtained by this heat treatment was set as Example 1, the sample made of steel type C was set as Example 2, and the thickness of the inner oxide layer was determined in the same manner as in the above-described reference example. Evaluated.

<Inner oxide layer thickness>
For Examples 1 and ₂ , the relationship between the O ₂ concentration in the processing atmosphere and the thickness of the inner oxide layer is shown in FIG. In both Examples 1 and 2, when the O ₂ concentration is less than 1% by volume, the inner oxide layer is hardly formed. However, when the O ₂ concentration is 1% by volume or more, the inner oxide layer has its thickness. It has been confirmed that the thickness of the inner oxide layer continuously increases as the O ₂ concentration increases. Such growth of the inner oxide layer suggests an improvement (improvement) in scale peelability.

<Scale peelability>
For Examples 1 and ₂ , the relationship between the O ₂ concentration in the processing atmosphere and the residual scale area ratio is shown in FIG. In both Examples 1 and 2, when the O ₂ concentration was less than 1% by volume, the residual scale area ratio was as high as 70% or more, and the scale peelability was judged to be poor. However, when the O ₂ concentration was 1% by volume or more, the residual scale area ratio was less than 60%, indicating good scale peelability. In particular, when the O ₂ concentration was 10% by volume or more, the residual scale area ratio was 35% or less, and it was confirmed that better scale peelability was exhibited.

<< Examples 3 and 4 >>
<Sample preparation>
Each sample (steel piece) made of steel types B and C (see Table 2) was added with 25% by volume of H ₂ O [water vapor] to N ₂ , and O ₂ was changed in the range of 0 to 20% by volume. The atmosphere was held at 1100 ° C. for 30 minutes. The sample made of steel type B obtained by this heat treatment was set as Example 3, the sample made of steel type C was set as Example 4, and the thickness of the inner oxide layer was determined in the same manner as in the above-described reference example. Evaluated.

<Inner oxide layer thickness>
For Examples 3 and 4, the relationship between the O ₂ concentration in the processing atmosphere and the thickness of the inner oxide layer is shown in FIG. In both Examples 3 and 4, when the O ₂ concentration is less than 1% by volume, the inner oxide layer is hardly formed. However, when the O ₂ concentration is 1% by volume or more, the inner oxide layer has a thickness thereof. It was confirmed that the thickness of the inner oxide layer continuously increased as the O ₂ concentration increased. Such growth of the inner oxide layer suggests an improvement (improvement) in scale peelability.

<Scale peelability>
For Examples 3 and 4, the relationship between the O ₂ concentration in the processing atmosphere and the residual scale area ratio is shown in FIG. In both Examples 3 and 4, when the O ₂ concentration was less than 1% by volume, the residual scale area ratio was 60% or more, and the scale peelability was judged to be poor. However, when the O ₂ concentration was 1% by volume or more, the residual scale area ratio was 45% or less, indicating good scale peelability. In particular, when the O ₂ concentration was 10% by volume or more, the residual scale area ratio was 35% or less, and it was confirmed that better scale peelability was exhibited.

<Effect of holding time at heat treatment temperature on inner oxide layer thickness and scale peelability>
<Sample preparation>
Each sample (steel piece) made of steel types B (SCR420) and C (SUJ2) (see Table 2) was added to an atmosphere in which 25% by volume of H ₂ O [water vapor] and 10% by volume of O ₂ were added to N _2. And held at 1100 ° C. for 0-60 minutes. This holding time is hereinafter referred to as “isothermal holding time”. For each of the sample made of steel type B and the sample made of steel type C obtained by this heat treatment, the inner oxide layer thickness was determined in the same manner as in the above-described reference example, and the scale peelability was evaluated.

<Inner oxide layer thickness>
FIG. 11 shows the relationship between the isothermal holding time and the inner oxide layer thickness. In both steel types B (SCR420) and C (SUJ2), the growth of the inner oxide layer was observed by setting the isothermal holding time to 1 minute or longer, and the inner oxide layer thickness was held isothermally as the isothermal holding time increased. It was confirmed that it increased almost linearly with time. As the isothermal holding time increases, the energy intensity increases and the scale loss also increases. Therefore, it is desirable that the isothermal holding time is short as long as good scale peelability is obtained. Therefore, the isothermal holding time is determined based on the scale peelability as will be described later.

<Scale peelability>
FIG. 12 shows the relationship between the isothermal holding time and the residual scale area ratio. For both steel types B (SCR420) and C (SUJ2), the residual scale area ratio was 35% or less when the isothermal holding time was set to 1 minute or more, and it was confirmed that good scale peelability was exhibited. It can be seen that as the isothermal holding time becomes longer, the residual scale area ratio decreases and the scale peelability is improved, but the reduction rate of the residual scale area ratio in the isothermal holding time longer than 30 minutes (gradient of the graph) Is not big. For this reason, it was determined that the treatment with a balance of scale peelability, scale loss, and energy intensity can be performed by setting the isothermal holding time within 30 minutes.

<Effect of heat treatment temperature on inner oxide layer thickness and scale peelability>
<Sample preparation>
Each sample (steel piece) made of steel types B (SCR420) and C (SUJ2) (see Table 2) was added to an atmosphere in which 25% by volume of H ₂ O [water vapor] and 10% by volume of O ₂ were added to N _2. And kept at 900 to 1200 ° C. for 30 minutes. For each of the sample made of steel type B and the sample made of steel type C obtained by this heat treatment, the inner oxide layer thickness was determined in the same manner as in the above-described reference example, and the scale peelability was evaluated.

<Inner oxide layer thickness>
FIG. 13 shows the relationship between the heat treatment temperature and the inner oxide layer thickness. It can be seen that, for both steel types B (SCR420) and C (SUJ2), when the heat treatment temperature is less than 1000 ° C., a sufficient inner oxide layer is not formed. When the heat treatment temperature was set to 1000 ° C. or higher, growth of the inner oxide layer was observed, and it was confirmed that the inner oxide layer thickness increased as the heat treatment temperature was increased. From this result, it can be seen that even when the heat treatment temperature is higher than 1150 ° C., good scale peelability can be obtained. However, when the heat treatment temperature is higher than 1150 ° C., the energy intensity increases and the scale loss is increased. In consideration of the demerits such as increase in the temperature, the heat treatment temperature is set to 1000 to 1150 ° C. in the present invention.

<Scale peelability>
FIG. 14 shows the relationship between the heat treatment temperature and the residual scale area ratio. The residual scale area ratio of steel type B (SCR420) when heated at 1000 ° C. is about 37% (value indicated by the graph (line)), and the residual scale area ratio of steel type C (SUJ2) is about 42% (graph ( The value indicated by the line)), and both exhibited good scale peelability. And it was confirmed that the residual scale area ratio decreased as the heat treatment temperature increased, and the scale peelability improved. The reason why the heat treatment temperature is set to 1000 to 1150 ° C. is as already described in the description of the relationship between the heat treatment temperature and the thickness of the inner oxide layer.

Reference example, is a graph showing the relationship between the H _{2 O} concentration and the scale thickness of the treatment atmosphere. Comparative Example 1 is a diagram showing the relationship between the H _{2 O} concentration and the scale thickness of the treatment atmosphere. Comparative Example 2 is a diagram showing the relationship between the H _{2 O} concentration and the scale thickness of the treatment atmosphere. Reference example, is a graph showing the relationship between the H _{2 O} concentration and a residual scale area ratio in the treatment atmosphere. Comparative Example 1 is a diagram showing the relationship between the H _{2 O} concentration and a residual scale area ratio in the treatment atmosphere. Comparative Example 2 is a diagram showing the relationship between the H _{2 O} concentration and a residual scale area ratio in the treatment atmosphere. For Examples 1 and 2 is a diagram showing the relationship between the O ₂ concentration and the inner oxide layer thickness of the treatment atmosphere. For Examples 1 and 2 is a diagram showing the relationship between the O ₂ concentration and the residual scale area ratio in the treatment atmosphere. For Examples 3 and 4 is a diagram showing the relationship between the O ₂ concentration and the inner oxide layer thickness of the treatment atmosphere. For Examples 3 and 4 is a diagram showing the relationship between the O ₂ concentration and the residual scale area ratio in the treatment atmosphere. It is a figure which shows the relationship between isothermal holding time and an inner oxide layer thickness. It is a figure which shows the relationship between isothermal holding time and a residual scale area rate. It is a figure which shows the relationship between high temperature heat processing temperature and an inner oxide layer thickness. It is a figure which shows the relationship between high temperature heat processing temperature and a residual scale area rate.

Claims (2)

Immediately before descaling a steel slab made of Cr-containing steel containing Si: 0.05 to 0.4% by mass, Cr: 0.1 to 2.0% by mass, the balance being Fe and inevitable impurities When heat-treating
The heat treatment temperature is 1000 to 1150 ° C., the holding time at the heat treatment temperature is 1 to 30 minutes, and the heat treatment atmosphere is H ₂ O [water vapor]: 25 to 30% by volume, O ₂ : 1 to 10% by volume. A method for producing Cr-containing steel excellent in scale peelability, characterized in that the balance is N ₂ and inevitable gas components.   The Cr-containing steel is selected as an optional component from C: 0.02 to 1.0 mass%, Mn: 0.2 mass% to less than 3.0 mass%, Mo: 0.3 mass% or less. The method for producing Cr-containing steel having excellent scale peelability according to claim 1, comprising at least one element.

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