JP5324964B2 - Method for producing Cr-containing strip steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To manufacture Cr-containing bar-steel material capable of enhancing the scale peelability and having excellent surface property even when the Cr-containing bar-steel material contains Cr exerting adverse effect on the scale peelability. <P>SOLUTION: In a method for manufacturing Cr-containing bar-steel material including a step of: heating and descaling a steel slab containing &ge;0.1% Cr (by mass%, hereinafter, referred to the chemical component of steel); and hot-rolling the descaled steel slab, the heating is executed under the following condition. (Heating condition) Heating temperature: 1,150-1,250&deg;C, temperature-raising rate from the room temperature to the heating temperature: 5-30&deg;C/min, holding time at the heating temperature: 3-60 seconds, holding atmosphere at the heating temperature: atmosphere in which O<SB>2</SB>concentration is 10-30 vol.%. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a method for producing a Cr-containing strip material, and in particular, a Cr-containing strip material having excellent surface properties because the scale is peeled off favorably by scale removal (hereinafter sometimes referred to as “descaling”). It relates to a method for manufacturing.

In order to secure strength, Cr and Si are generally added to steel products (SCR420, SCM435, SUJ2, etc.) such as steel for hot rolling, steel for hot forging, and bearing steel used for automobiles (hereinafter referred to as “rolling steel”). Thus, the steel strip containing Cr and Si is collectively referred to as “Cr-containing steel strip”). Similarly to general steel materials, Cr-containing strip steel materials are manufactured by descaling after heating billets and the like, and then hot rolling. Generally, when steel is heated at a high temperature, a primary scale (scale composed of Fe-based oxides such as wustite (FeO), magnetite (Fe ₃ O ₄ ), hematite (Fe ₂ O ₃ )) is formed on the surface. . Further, in a steel type containing a large amount of an easily oxidizable alloy element such as Cr, an iron oxide containing the alloy element (hereinafter referred to as “subscale”, in particular, chromite (FeCr ₂ O ₄ )) in the heating process or the like, It is generated in the lowermost layer of the scale (near the iron-iron interface) and has the effect of enhancing the adhesion between the steel and the scale (hereinafter, the primary scale and the subscale may be collectively referred to as “scale”). Therefore, even when descaling with high-pressure water (high-pressure water descaling) is performed, the primary scale is almost removed, but the subscale tends to remain. When hot rolling is performed in a state where this subscale remains, the subscale is pushed into the steel, and surface defects such as fine surface flaws and rough skin often occur in the hot rolled finish. The surface quality of the deteriorated.

  In order to meet the demands of the surface quality (surface properties) of steel products that are becoming stricter year by year, the scale is peeled off well by descaling (hereinafter referred to as “excellent scale peelability”). There have been proposed various methods for suppressing surface defects due to scales and improving the surface quality of steel products. For example, in Patent Document 1, in order to promote the growth of pores in the scale and form a scale that is easy to peel off, in the heating before hot rolling, water is directly supplied to the steel bar material, and at a certain temperature for a certain time or more. It is prescribed to hold. In Patent Document 2, in order to promote the growth of pores in the scale and form a scale that is easy to peel off, two-stage heating is performed in the heating before hot rolling. Defines time and water vapor concentration.

JP 2002-316207 A JP 2003-119517 A

  In Patent Document 1 and Patent Document 2 described above, it is shown that a scale that easily peels off during descaling is formed by increasing the water vapor concentration in the heating atmosphere or supplying water. However, it was found that the effect differs depending on the target steel type. For example, when SCM steel is targeted, the addition of water vapor to the heating atmosphere is effective. However, for example, when SCR steel represented by SCR420 is targeted, there is not much effect and the scale is good when descaling. There is a problem that it is not peeled off.

  The present invention has been made paying attention to the above-mentioned circumstances, and its purpose is that even if the target is SCR steel, the scale can be peeled off satisfactorily by descaling (excellent in scale peelability). ), To provide a method for producing a Cr-containing strip steel material having excellent surface properties.

  As a result of intensive studies to solve the above problems, the inventors of the present invention have the following descaling process if the heating conditions before descaling are appropriately controlled even when the SCR steel is targeted. It has been found that the scale can be sufficiently peeled off and can be manufactured by hot rolling a Cr-containing strip steel material having excellent surface properties.

That is, the method for producing a Cr-containing strip according to the present invention includes a step of descaling after heating a steel slab containing 0.1% or more of Cr (meaning mass%, the same shall apply hereinafter in the chemical composition of steel). Then, a method for producing a Cr-containing strip steel material including a step of hot rolling the descaled steel slab, characterized in that the heating is performed under the following conditions.
(Heating conditions)
Heating temperature: 1150-1250 ° C
Rate of temperature increase from room temperature to the heating temperature: 5 to 30 ° C./min.
Holding time at the above heating temperature: 3 to 60 seconds Atmosphere when holding at the above heating temperature: An atmosphere in which the O ₂ concentration is 10 to 30% by volume

As the steel slab, its component composition is further
C: 0.1 to 0.5%
Examples include those satisfying Si: 0.05% or more and Mn: 0.1% or more, with the balance being iron and inevitable impurities.

  In particular, if the steel piece satisfying the chemical composition of SCR415, SCR420, SCR430, SCR435, SCR440 or SCR445 (SCR steel) defined in JIS G 4104 (1997) is used, the effect of the present invention can be obtained. To be demonstrated.

  According to the manufacturing method of the present invention, in the manufacturing process of the strip steel product, the scale peelability at the time of descaling performed after heating the steel slab is improved, and after hot rolling, such as fine surface wrinkles and rough skin due to the scale It is possible to obtain a steel bar product (especially a steel bar product made of SCR steel) that has sufficiently suppressed surface defects and excellent surface properties.

FIG. 1 is a graph showing the relationship between the subscale thickness and the residual scale area ratio. FIG. 2 is a graph showing the relationship between the average Cr concentration in the subscale and the residual scale area ratio. FIG. 3 is a graph showing the relationship between the reheating temperature and the subscale thickness. FIG. 4 is a graph showing the relationship between the reheating temperature and the average Cr concentration in the subscale. FIG. 5 is a graph showing the relationship between the reheating temperature and the residual scale area ratio. FIG. 6 is a graph showing the relationship between the reheating holding time and the subscale thickness. FIG. 7 is a graph showing the relationship between the reheating holding time and the average Cr concentration in the subscale. FIG. 8 is a graph showing the relationship between the reheating holding time and the residual scale area ratio. FIG. 9 is a graph showing the relationship between the O ₂ concentration in the reheating atmosphere and the subscale thickness. FIG. 10 is a graph showing the relationship between the O ₂ concentration in the reheating atmosphere and the average Cr concentration in the subscale. FIG. 11 is a graph showing the relationship between the O ₂ concentration in the reheating atmosphere and the residual scale area ratio.

As described above, there is a steel type (Cr-containing strip steel material) that can be exfoliated well at the time of subsequent descaling by heating in a steam atmosphere shown in Patent Document 1 or the like. For example, when the target is SCM435, the scale peelability at the time of descaling is improved by heating in an atmosphere of water vapor (H ₂ O concentration: 25 to 30% by volume is an optimal range). However, when SCR steel typified by SCR420 is used as a target, there is a problem that heating in the water vapor atmosphere is not very effective, and scale peelability at the time of descaling is not improved.

  In the case where the SCR steel is particularly targeted among the Cr-containing strip steel materials, the present inventors have first ascertained that the above problem exists, and even when the SCR steel is targeted. As a result of diligent research to improve scale peelability during descaling, first, as a subscale, (a) thickness: 40 μm or more, and (b) an average Cr concentration in the subscale (shown in the examples described later) It is understood that a value satisfying 10% by mass or less may be formed.

Furthermore, when a specific means for forming a subscale that satisfies the above conditions was examined, in particular, if the O ₂ concentration in the atmosphere of heating performed immediately before descaling is increased, and other conditions for the heating are specified. I found a good thing.

First, the O ₂ concentration in the heating atmosphere will be described. The sub-scale is formed by the internal oxide layer mainly composed of Cr ₂ O ₃ being transferred to the surface to become an external oxide layer, and the FeO layer formed on the surface and the external oxide layer (mainly composed of Cr ₂ O ₃ ) being combined. Is done. In general scale, is formed by the inward diffusion of out-diffusion and O ₂ of Fe ions, a lower O ₂ concentration in the atmosphere, i.e., the inward diffusion of O ₂ is slow, the equilibrium oxygen pressure is lower than the Fe Cr is preferentially oxidized, and as a result, a high Cr concentration is formed as the subscale. On the other hand, if the O ₂ concentration in the atmosphere is increased, the inward diffusion of O ₂ becomes faster, and the oxidation of Cr and the oxidation of Fe occur in an antagonistic manner. It is considered that a subscale having a low Cr concentration is formed.

In order to sufficiently exhibit the above-described effects, the O ₂ concentration in the atmosphere (heating atmosphere) at the time of holding the heating performed immediately before descaling at the heating temperature described later is set to 10 vol% or more. By increasing the O ₂ concentration in the heating atmosphere, it is considered that the inward diffusion of O ₂ is accelerated and a subscale that is easy to peel off is formed as described above, but the effect is saturated. Further, heating (oxidation) more than necessary causes an increase in cost due to an increase in energy intensity, and a decrease in yield due to excessive scale generation (material loss). Therefore, in the present invention, the O ₂ concentration in the heating atmosphere is 30 vol% or less.

In addition, it is desirable that an oxidizing gas such as CO, CO ₂ , H ₂ O or the like other than the above O ₂ is included in the heated atmosphere because scale growth is further promoted. However, in the present invention, the O ₂ concentration is increased as described above, and the scale growth rate is governed by the O ₂ concentration. Therefore, the oxidizing gas other than O ₂ may or may not be included. Good.

In the heating atmosphere, the gas other than O ₂ is not particularly limited, and includes, for example, N ₂ , Ar, H _{2 in} addition to the oxidizing gas: CO, CO ₂ , H ₂ O that may be further included as necessary. It can be done.

  Below, the reason for having specified other conditions of heating performed just before descaling is explained.

[Heating temperature: 1150 to 1250 ° C.]
By setting the heating temperature to 1150 ° C. or higher, the subscale thickness is increased, and the average Cr concentration in the subscale is reduced, so that a subscale that is easily peeled off during descaling can be formed. On the other hand, heating (oxidation) more than necessary causes an increase in cost due to an increase in energy intensity, and a decrease in yield due to excessive scale generation (material loss). From such a viewpoint, the upper limit of the heating temperature in the present invention is 1250 ° C.

  In the present invention, the heating temperature, the heating rate described later, and the cooling rate are the surface temperature of the steel slab, the heating rate of the surface temperature of the steel slab, and the cooling rate, respectively.

[Holding time at the above heating temperature: 3 to 60 seconds]
In order to increase the subscale thickness and reduce the average Cr concentration in the subscale by heating at the heating temperature, it is necessary to keep the time at the heating temperature for 3 seconds or more. Preferably it is 5 seconds or more, more preferably 10 seconds or more.

Normally, combustion gas is used for heating the steel slab, and the atmosphere in the heating furnace is generally N ₂ -1% (volume%) O ₂ -10% CO ₂ -20% H ₂ O. However, since this atmosphere has a low O ₂ concentration of 1%, as described above, the inward diffusion of O ₂ is slow, and a subscale with a high Cr concentration is likely to be formed. In contrast, in the present invention, as described above, since heating is performed in an atmosphere in which the heating temperature is increased and the O ₂ concentration is increased, a thick subscale in which Cr concentration is suppressed can be formed in a short time. Peelability can be improved.

  By further increasing the holding time, the scale can grow more and the scale peelability can be improved, but the effect is saturated. Further, heating (oxidation) more than necessary causes an increase in cost due to an increase in energy intensity, and a decrease in yield due to excessive scale generation (material loss). Therefore, in the present invention, the holding time is set to 60 seconds or less. Preferably it is 20 seconds or less.

[Raising rate from room temperature to the above heating temperature: 5 to 30 ° C./min. ]
In the present invention, the rate of temperature increase from room temperature to the heating temperature is set to 5 to 30 ° C./min. And The heating rate is 5 ° C./min. If it is less than this, it takes a long time to heat the steel slab, which leads to a decrease in the productivity of the steel bar material, which is not preferable. If it is in the range of the said temperature increase rate, it can heat up using a general heating furnace, without using special apparatuses, such as a high frequency heating furnace. The atmosphere at the time of raising the temperature is not particularly limited.

In the present invention, the H ₂ O concentration in the heating atmosphere is not particularly limited, and H ₂ O may or may not exist in the heating atmosphere. The actual operation level is approximately in the range of 10 to 25% by volume. Further, the temperature lowering rate and atmosphere when the temperature is lowered from the heating temperature to room temperature are not particularly limited.

The following (A) and (B) are mentioned as a manufacturing process (part) including the process of descaling after heating on the said conditions, and then the process of hot-rolling the descaled steel slab, for example.
(A) (Extract after heating the steel slab under the above conditions in a heating furnace) → (Descaling) → (Hot rolling)
(B) (Extract after heating the steel slab in a heating furnace) → (Heating under the above conditions (heating in this case may be referred to as “reheating”)) → (Descaling) → (Hot rolling)

  The steel slab is usually heated in a heating furnace at a temperature around 1100 ° C. for 30 to 60 minutes. Thereby, the temperature equalization of the steel piece optimal for the hot rolling of a post process can be achieved. On the other hand, as described in the above step (A), the scale peelability of the steel slab is improved when heated to around 1200 ° C. in a heating furnace, but it is technical to realize such a high-temperature heating equipment (heating furnace). It is difficult in cost. Moreover, since it is common to heat steel types other than target steel types (for example, SUJ2, SCR420, SCM435, etc.) at around 1100 ° C., heating in a heating furnace is divided into target steel types and steel types other than the target steel types. This must be done, increasing the manufacturing cost.

  From this point of view, as shown in (B) above, after heating normally performed in a heating furnace (heating around 1100 ° C.), for example, an electric heating device or gas heating in a booth provided on the conveyor In the apparatus (which may be a high-frequency heating apparatus), heating is performed under the above conditions (the heating temperature at this time is different from the heating temperature in this case by the reheating temperature, the heating temperature). It is preferable that the holding time is referred to as a reheating holding time and the heating atmosphere is referred to as a reheating atmosphere.

  When the step (B) is carried out, the conditions for heating the steel slab in the heating furnace are not particularly limited, and as usual, for example, maintained at a temperature of 1000 ° C. to 1150 ° C. for 30 to 60 minutes ( The heating atmosphere is not particularly limited, and may be an atmosphere formed using, for example, combustion gas), and heating (reheating) under specified conditions after extraction.

  Other production conditions are not particularly limited. For example, after obtaining a slab or billet by continuous casting, for example, heating as shown in (A) or (B) above, immediately (for example, within 20 seconds), the scale Is removed with a high-pressure water descaler, and then, for example, rough rolling, finish rolling, product water cooling, and winding are sequentially performed to obtain a bar steel (linear steel material).

  In addition, as above-mentioned, this invention improves the scale peelability of the strip steel material which especially has a bad influence on scale peelability and contains Cr. Therefore, the present invention is directed to a Cr-containing strip steel material containing 0.1% or more (preferably 0.8% or more) of Cr. Furthermore, according to the present invention, even if a large amount of Cr is contained, the scale peelability can be improved, and the Cr content may be, for example, 1.0% or more, preferably 1.1% or more. . In addition, since it becomes difficult to ensure ductility when the amount of Cr becomes excessive, the amount of Cr is preferably 2.0% or less, and more preferably 1.5% or less.

  Since the present invention controls the subscale caused by Cr in particular, the steel component other than Cr is not particularly limited as long as it can be used as a hot rolled material (strip steel), but the elements other than Cr and their amounts Are, for example, those containing C: 0.1 to 0.5%, Si: 0.05% or more, and Mn: 0.1% or more.

  The balance other than Cr, C, Si and Mn may be iron and inevitable impurities. As an inevitable impurity, for example, it is naturally allowed that impurities brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, and the like are included in the steel.

  In particular, when the slab steel satisfying the chemical composition of SCR415, SCR420, SCR430, SCR435, SCR440, or SCR445 defined in JIS G 4104 (1997) shown in Table 1 below is used as the steel slab, the effect of the present invention. Even when the SCR steel is used as a target, the scale peelability during descaling can be sufficiently enhanced.

  The strip steel material in the present invention is a general term for rod-shaped or linear steel materials, and can be used for, for example, suspension springs, valve springs, and bearings for automobiles.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

[Example 1]
First, the effect of the subscale thickness and the average Cr concentration in the subscale on the scale peelability was examined.

The following four types of steel slabs (all of which are iron and inevitable impurities other than the alloy elements (C, Cr)) were obtained by melting.
-Fe-0.1% C alloy containing 0.1% by mass of carbon,
An Fe-0.1% C-0.5% Cr alloy containing 0.1% by weight carbon and 0.5% by weight Cr,
An Fe-0.1% C-1.0% Cr alloy containing 0.1% by weight of carbon and 1.0% by weight of Cr,
-Fe-0.1% C-1.5% Cr alloy containing 0.1% by mass of carbon and 1.5% by mass of Cr

  A sample cut out from the above steel piece into a cylindrical shape of φ8.0 mm × 12 mm by machining was prepared. During the processing, the side surface of the cylindrical sample was polished to a surface roughness of Rmax ≦ 6 μm.

The sample was placed in an atmosphere-controlled heat treatment apparatus and heated at various temperatures and times in an atmosphere of N ₂ -1% (volume%) O ₂ -10% CO ₂ -18% H ₂ O. The temperature is changed in the range of 1050 to 1300 ° C., and the holding time at the temperature is 30 to 120 min. The thickness and the average Cr concentration were varied in the range, and various subscales were formed. The rate of temperature increase from room temperature to the heating temperature was 10 ° C./min. The cooling rate from the heating temperature to room temperature is 10 ° C./min. It was.

(Evaluation of scale peelability)
Set the heated sample on the processing for master, raise the temperature in vacuum and reach 1000 ° C, simulate descaling, compress the sample, peel off the scale, and scale from the remaining scale area ratio The peelability was evaluated.

  The above compression is performed in a vacuum atmosphere under conditions of a compression strain rate of 50% and a strain rate of 10 mm / sec. After the compression test, the sample is rapidly cooled to room temperature in Ar, and scale generation after peeling (secondary scale generation) is performed. Suppressed. After compression, the appearance of the side surface of the sample was captured by a scanner, the image was binarized, and the area ratio of the scale remaining portion (residual scale area ratio) in the sample after compression was calculated.

(Measurement of subscale thickness and average Cr concentration in subscale)
In addition, after embedding the heated sample (simultaneously heated sample separate from the sample for evaluation of scale peelability) in the resin, cross section polishing is performed, and the sample cross section (cross section passing through the central axis of the cylindrical sample) ) Was exposed.

  This cross-section exposed sample is observed with an SEM, the subscale thickness is measured from the scale cross section observation, the subscale is quantitatively analyzed with EPMA, and the average Cr concentration in the subfield of the three fields of view in the SEM-EPMA photograph. (Mass%) was determined.

  Note that the subscale can be specified from the difference in contrast if the reflected electron image of the SEM is taken. In addition, the sub-scale layer can be reliably identified by taking a mapping image with respect to the Cr element by EPMA.

  The relationship between the subscale thickness and the residual scale area ratio is shown in FIG. It can be seen that as the subscale thickness increases, the residual scale area ratio decreases (scale peelability improves). In order to ensure good scale peelability, it is desirable to set the residual scale area ratio to 50% or less. For this purpose, FIG. 1 shows that the subscale thickness should be 40 μm or more.

  FIG. 2 shows the relationship between the average Cr concentration in the subscale and the residual scale area ratio. As can be seen from FIG. 2, when the average Cr concentration in the subscale decreases, the residual scale area ratio decreases (scale peelability improves). In order to ensure good scale peelability, it is desirable to set the residual scale area ratio to 50% or less. To that end, from FIG. 2, the average Cr concentration in the subscale should be 10% by mass or less. Recognize.

[Example 2]
Next, as a condition for obtaining a subscale having a thickness of 40 μm or more and an average Cr concentration of 10% by mass or less, the reheating temperature in reheating was examined.

  First, including C: 0.22%, Si: 0.31%, Cr: 0.95%, Mn: 0.72%, P: 0.002%, and S: 0.003%, the balance iron and A steel piece which is an inevitable impurity was produced by melting. This steel type corresponds to SCR420.

  A sample processed in the same manner as in Example 1 was prepared using the steel piece.

The sample was placed in an atmosphere-controlled heat treatment apparatus, and was heated for 60 minutes at 1100 ° C. in an atmosphere of N ₂ −1% O ₂ -10% CO ₂ -18% H ₂ O. At this time, the rate of temperature increase from room temperature to 1100 ° C. and the rate of temperature decrease from 1100 ° C. to room temperature were both 10 ° C./min. It was. The atmosphere in the temperature raising and lowering process was an N ₂ flow, and the atmosphere was used only when held at 1100 ° C.

Subsequently, reheating is performed for 10 seconds at the heating temperature (reheating temperature) described below in an atmosphere of N ₂ -20% O ₂ -10% CO ₂ -18% H ₂ O using the atmosphere control type heat treatment apparatus. It was. At this time, the rate of temperature increase from room temperature to the reheating temperature and the rate of temperature decrease from the reheating temperature to room temperature were both 10 ° C./min. It was. The reheating temperature was changed in the range of 1100 ° C to 1300 ° C.

  The reheated sample was subjected to evaluation of scale peelability (measurement of residual scale area ratio), subscale thickness and average Cr concentration in the subscale in the same manner as in Example 1.

  The relationship between the reheating temperature and the subscale thickness is shown in FIG. FIG. 3 shows that the subscale tends to be thicker when the reheating temperature is higher, and that the subscale thickness is 40 μm or more when the reheating temperature is 1150 ° C. or higher.

  FIG. 4 shows the relationship between the reheating temperature and the average Cr concentration in the subscale. As shown in FIG. 4, the average Cr concentration in the subscale tends to decrease as the reheating temperature increases. If the reheating temperature is 1150 ° C. or higher, the average Cr concentration in the subscale is 5% by mass or less. I understand.

  Further, FIG. 5 shows the relationship between the reheating temperature and the residual scale area ratio. FIG. 5 shows that the residual scale area ratio tends to decrease as the reheating temperature increases, and that the residual scale area ratio can be reduced to 50% or less when the reheating temperature is 1150 ° C. or higher.

  The results of FIGS. 3 to 5 show that, in FIGS. 1 and 2, when the subscale thickness is 40 μm or more and the average Cr concentration in the subscale is 10 mass% or less, the residual scale area ratio is 50% or less. This is almost consistent with the reduction.

Example 3
Next, as a condition for obtaining a subscale having a thickness of 40 μm or more and an average Cr concentration of 10% by mass or less, the reheating holding time in reheating was examined.

  First, in the same manner as in Example 2, a steel piece corresponding to SCR420 was melted, and a processed sample was prepared.

The sample was placed in an atmosphere-controlled heat treatment apparatus, and was heated for 60 minutes at 1100 ° C. in an atmosphere of N ₂ −1% O ₂ -10% CO ₂ -18% H ₂ O. At this time, the rate of temperature increase from room temperature to 1100 ° C. and the rate of temperature decrease from 1100 ° C. to room temperature were both 10 ° C./min. It was. The atmosphere in the temperature raising and lowering process was an N ₂ flow, and the atmosphere was used only when held at 1100 ° C.

Subsequently, reheating was performed by maintaining the temperature at 1200 ° C. in an atmosphere of N ₂ -20% O ₂ -10% CO ₂ -18% H ₂ O using the atmosphere control type heat treatment apparatus. At this time, the rate of temperature increase from room temperature to 1200 ° C. and the rate of temperature decrease from 1200 ° C. to room temperature were both 10 ° C./min. It was. Moreover, the time (reheat holding time) held at 1200 ° C. was changed in the range of 0 to 60 seconds.

  The reheated sample was subjected to evaluation of scale peelability (measurement of residual scale area ratio), subscale thickness and average Cr concentration in the subscale in the same manner as in Example 1.

  FIG. 6 shows the relationship between the reheating holding time and the subscale thickness. From FIG. 6, it can be seen that the subscale tends to become thicker as the reheat holding time becomes longer, and the subscale thickness becomes 40 μm or longer if the reheat holding time is 3 seconds or longer.

  FIG. 7 shows the relationship between the reheating holding time and the average Cr concentration in the subscale. From FIG. 7, the average Cr concentration in the subscale tends to decrease as the reheating holding time becomes longer. If the reheating holding time is 3 seconds or more, the average Cr concentration in the subscale is 10% by mass or less. I understand that

  Furthermore, the relationship between the reheating holding time and the residual scale area ratio is shown in FIG. From FIG. 8, it can be seen that the residual scale area ratio tends to decrease as the reheat holding time increases, and that the residual scale area ratio can be suppressed to 50% or less when the reheat holding time is 3 seconds or longer.

  The results of FIGS. 6 to 8 show that, in FIGS. 1 and 2, when the subscale thickness is 40 μm or more and the average Cr concentration in the subscale is 10 mass% or less, the residual scale area ratio is 50% or less. This is almost consistent with the reduction.

Example 4
Next, the reheating atmosphere was examined as a condition for obtaining a subscale having a thickness of 40 μm or more and an average Cr concentration of 10% by mass or less.

  First, in the same manner as in Example 2, a steel piece corresponding to SCR420 was melted, and a processed sample was prepared.

The sample was placed in an atmosphere-controlled heat treatment apparatus, and was heated for 60 minutes at 1100 ° C. in an atmosphere of N ₂ −1% O ₂ -10% CO ₂ -18% H ₂ O. At this time, the rate of temperature increase from room temperature to 1100 ° C. and the rate of temperature decrease from 1100 ° C. to room temperature were both 10 ° C./min. It was. The atmosphere in the temperature raising and lowering process was an N ₂ flow, and the atmosphere was used only when held at 1100 ° C.

Subsequently, reheating was carried out by holding at 1200 ° C. for 10 seconds under the following atmosphere (reheating atmosphere) in the atmosphere control type heat treatment apparatus. At this time, the rate of temperature increase from room temperature to 1200 ° C. and the rate of temperature decrease from 1200 ° C. to room temperature were both 10 ° C./min. It was. The reheating atmosphere is a mixed gas atmosphere of N ₂ , O ₂ , CO ₂ and H ₂ O. The CO ₂ concentration is fixed to 10% by volume and the H ₂ O concentration is set to 18% by volume, and the O ₂ concentration is set to 0% by volume. To 40% by volume with the balance being N ₂ .

  The reheated sample was subjected to evaluation of scale peelability (measurement of residual scale area ratio), subscale thickness and average Cr concentration in the subscale in the same manner as in Example 1.

FIG. 9 shows the relationship between the O ₂ concentration in the reheating atmosphere and the subscale thickness. FIG. 9 shows that the subscale tends to be thick when the O ₂ concentration in the reheating atmosphere is high, and the subscale thickness is 40 μm or more when the O ₂ concentration is 10% by volume or more.

FIG. 10 shows the relationship between the O ₂ concentration in the reheating atmosphere and the average Cr concentration in the subscale. From FIG. 10, when the O ₂ concentration in the reheating atmosphere increases, the average Cr concentration in the subscale tends to decrease. When the O ₂ concentration is 10% by volume or more, the average Cr concentration in the subscale is It turns out that it becomes 10 mass% or less.

Further, FIG. 11 shows the relationship between the O ₂ concentration in the reheating atmosphere and the residual scale area ratio. From FIG. 11, the residual scale area ratio tends to decrease as the O ₂ concentration in the reheating atmosphere increases. If the O ₂ concentration is 10% by volume or more, the residual scale area ratio can be suppressed to 50% or less. I understand that.

  The results of FIGS. 9 to 11 show that, in FIGS. 1 and 2, if the subscale thickness is 40 μm or more and the average Cr concentration in the subscale is 10 mass% or less, the residual scale area ratio is 50% or less. This is almost consistent with the reduction.

Claims (3)

A step of heating and descaling a slab containing 0.1% or more of Cr (meaning mass%; the same applies in the chemical composition of steel), and then hot rolling the descaled slab. A method for producing a Cr-containing strip steel material,
The said heating is performed on the following conditions, The manufacturing method of Cr containing strip steel material characterized by the above-mentioned.
(Heating conditions)
Heating temperature: 1150-1250 ° C
Rate of temperature increase from room temperature to the heating temperature: 5 to 30 ° C./min.
Holding time at the above heating temperature: 3 to 60 seconds Atmosphere when holding at the above heating temperature: An atmosphere in which the O ₂ concentration is 10 to 30% by volume The billet further comprises:
C: 0.1 to 0.5%
The manufacturing method according to claim 1, wherein Si: 0.05% or more and Mn: 0.1% or more are satisfied, and the balance consists of iron and inevitable impurities.   The manufacturing method according to claim 2, wherein the billet satisfies a chemical component of SCR415, SCR420, SCR430, SCR435, SCR440, or SCR445 defined by JIS G 4104 (1997).

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