JP5128401B2 - Method for producing Cr-containing strip with excellent scale peelability - Google Patents

Description

  The present invention relates to a method for producing a Cr-containing strip having excellent scale peelability.

In steel products such as cold rolling steel and bearing steel used in automobiles, Cr is generally added to ensure strength. Such a Cr-containing strip steel material is produced by heating a billet and the like, performing scale removal (hereinafter, sometimes referred to as “descaling”), and then hot rolling, in the same manner as a normal steel material. The requirements for the surface quality of products manufactured by hot rolling are becoming stricter year by year. However, when steel is heated at a high temperature, primary scales (such as Fe-based single oxides such as wustite (FeO), magnetite (Fe ₃ O ₄ ), hematite (Fe ₂ O ₃ ), etc.) are formed on the surface, and Cr is added. In the steel type to be contained, iron oxide (subscale) containing Cr is formed at the primary scale / base metal interface (hereinafter, the primary scale and the subscale may be collectively referred to as “scale”). . Since this subscale has high adhesion to steel, even if scale peeling with high-pressure water is performed after heating, the primary scale is almost removed, but the subscale remains. When the remaining subscale is pushed into the steel surface during hot rolling, surface defects such as fine surface defects and rough skin often occur.

  In view of such circumstances, various methods for suppressing wrinkles due to scale and improving the surface quality of products have been proposed by improving the scale peelability of steel materials.

  For example, Patent Document 1 discloses a technique in which heating is performed at a relatively low temperature in a heating furnace, and after that, induction heating is separately performed in an air atmosphere using an induction heating device (induction heater), and then the scale on the steel material surface portion is removed. It is disclosed.

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, in the heating prior to hot rolling, water is directly supplied to the steel bar and kept at a certain temperature for a certain period of time. It is prescribed to do. Also in Patent Document 3, 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 2007-330984 A JP 2002-316207 A JP 2003-119517 A

  However, when the method described in Patent Document 1 is applied to a Cr-containing strip steel, induction heating is performed in the atmosphere, so that the chromite concentration as described above is very close between the steel and the primary scale. Therefore, it is considered that it is difficult to sufficiently improve the scale peelability.

  In Patent Documents 2 and 3, the temperature and time and the water vapor concentration are controlled in the heating furnace to improve the scale peelability. However, it is difficult to sufficiently improve the scale peelability in this method. Further, Patent Document 2 and Patent Document 3 described above increase the pores generated in the subscale in order to improve the scale peelability, that is, improve from the physical viewpoint (shape) of the subscale. In order to further improve the properties, it is important to improve from the chemical viewpoint (component composition) of the subscale.

  The present invention has been made by paying attention to the above-described circumstances, and the purpose thereof is excellent in scale peelability even if it is a strip steel material containing Cr that adversely affects scale peelability, and the surface by the scale. An object of the present invention is to provide a method for producing a Cr-containing strip having good surface properties with no wrinkles.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have controlled the heating temperature in the heating furnace and extracted it from the heating furnace and before descaling even for the Cr-containing strip steel material. In addition, it has been found that if the steel slab is rapidly heated under predetermined conditions, the scale can be sufficiently peeled off in the subsequent descaling step, and the surface properties after hot rolling can be improved, thereby completing the present invention.

That is, in the method for producing a Cr-containing strip according to the present invention, a steel piece containing 0.10 to 2.00% of Cr (meaning of mass%; the same shall apply hereinafter in the chemical composition of steel) is taken out from the heating furnace, A method for producing a Cr-containing strip steel material to be hot-rolled after descaling,
In the heating furnace, after heating for 15 minutes or more in a temperature range where the surface temperature of the steel slab is 1000 ° C. or higher and 1150 ° C. or lower, the steel slab whose surface temperature (extraction temperature) is in the above temperature range is converted into the heating furnace. And then
Until the surface temperature of the steel slab becomes a temperature range of 1200 ° C. or higher and 1350 ° C. or lower from the extraction temperature in an atmosphere where the O ₂ concentration is 10% by volume or more and the H ₂ O concentration is 5% by volume or more and 35% by volume or less. The steel slab is rapidly heated at a rate of temperature increase of 20 ° C./min or more and then descaled.

  As the steel slab, the component composition further satisfies Si: 0.10 to 0.40%, C: 0.10 to 1.50%, and Mn: 0.01 to 1.00%, and the balance Is composed of iron and inevitable impurities. The steel slab may further contain Mo: 0.01 to 0.40%.

  According to the present invention, as the subscale, a brittle FeO-based oxide layer having poor adhesion to the ground iron can be formed, so that the scale peelability can be sufficiently improved, and the Cr-containing steel strip having excellent surface properties is hot. It can be manufactured by rolling.

  The present inventors have already proposed a method capable of improving the scale peelability even in the case of a steel strip containing Cr that adversely affects the scale peelability (for example, Japanese Patent Application No. 2007-214025). Intensive research was conducted to remove scale more easily in scaling. As a result, the idea that the subscale composition should be controlled based on the following idea was obtained. That is, the subscale is an inner oxide layer formed by oxidation progressing (inward oxidation progresses) inside the steel material. When this rate of inward oxidation is slow, Cr is gradually concentrated to form a subscale with a high Cr concentration. This subscale with a high Cr concentration is not preferable because it has high adhesion to the steel as described above.

  On the other hand, when the rate of inward oxidation is high, a subscale is formed before Cr concentration progresses, so that a subscale having a low Cr concentration mainly composed of FeO (wustite) is considered to be formed. It is done. This sub-scale mainly composed of FeO is brittle and easily peels off from the base iron. Therefore, in order to increase the rate of inward oxidation under the idea that if the speed of inward oxidation is increased to form a subscale mainly composed of FeO (wustite), the scale peelability will improve. Further study was conducted on the appropriate conditions.

As a result, it is found that the rate of inward oxidation increases as the O ₂ concentration and H ₂ O concentration in the atmosphere increase, and if rapid heating effective for subscale destruction is performed together, the scale peelability will be dramatically improved. Found to improve.

And when the present inventors examined further in order to materialize the said means, in the process of performing (extraction after heating a steel slab with a heating furnace)->(descaling)-> (hot rolling) in the said heating furnace. The heating temperature is controlled rapidly, and after heating is removed from the heating furnace and before descaling, it is rapidly heated in an atmosphere having a higher O ₂ concentration than in the heating furnace and a higher H ₂ O (water vapor) concentration than in the atmosphere. Then, it was found that the scale peelability improved dramatically, and a rolled material with excellent surface properties was obtained by subsequent hot rolling, and the present invention was conceived.

  Hereinafter, the reason why the heating temperature and the rapid heating condition in the heating furnace are specified will be described in detail.

<Heating in heating furnace: Heating for 15 minutes or more in a temperature range where the surface temperature of the steel slab is 1000 ° C. or higher and 1150 ° C. or lower>
When the surface temperature of the steel slab in the heating furnace exceeds 1150 ° C., the primary scale and the steel material are in close contact with each other through the subscale, so that the effect is exhibited even when rapid heating described later is performed. Hard to do. Therefore, the surface temperature of the steel slab in the heating furnace needs to be 1150 ° C. or less. Considering also the point which improves productivity and energy efficiency, it is preferable to set it as 1100 degrees C or less.

  On the other hand, if the surface temperature of the steel slab is too low, the primary scale does not grow much in the heating furnace, the internal stress contributing to the scale peeling becomes small, and it is difficult to improve the scale peeling property. Therefore, in the heating furnace, the surface temperature of the steel slab needs to be heated at 1000 ° C. or more for 15 minutes or more. Preferably it is 1050 degreeC or more. However, if the heating time in the above temperature range (1000 ° C. or higher and 1150 ° C. or lower) is too long, the scale loss becomes excessive. Therefore, the heating time is preferably within 180 minutes.

  There are no particular restrictions on conditions other than those described above in the heating furnace. For example, as a heat pattern, the temperature may be gradually raised from the start of heating to extraction, and as shown in the examples described later, the temperature range (1000 ° C. or higher) After raising the temperature to 1150 ° C. or lower, a soaking zone may be provided in which the temperature is maintained for 15 minutes or longer.

<Rate of temperature rise of steel slab: 20 ° C / min or more>
After heating at the above temperature, the steel slab whose surface temperature is in the above temperature range (1000 ° C. or higher and 1150 ° C. or lower) is taken out from the heating furnace (the surface temperature of the steel slab at the time of taking out (extraction) from this heating furnace is “extracted”. The temperature of the steel slab surface is raised at a rate of 20 ° C./min or higher from the extraction temperature (temperature range of 1000 ° C. or higher and 1150 ° C. or lower) (heating at such a rate, Hereinafter sometimes referred to as “rapid heating”). By applying rapid heating in this way, the growth stress of the subscale generated in the heating furnace can be rapidly increased, and the subscale can be easily broken. If the rate of temperature rise on the surface of the steel slab is less than 20 ° C./min, the growth stress does not increase abruptly and the subscale is not sufficiently broken, which is not preferable. Preferably, the rate of temperature increase on the surface of the steel slab is 30 ° C./min or more. The upper limit of the rate of temperature rise is not particularly limited as long as the temperature reached by rapid heating does not exceed 1350 ° C.

<Achieved temperature range by rapid heating: 1200 to 1350 ° C.>
The higher the temperature achieved by rapid heating, the more the growth of the scale is promoted, and the growth stress is increased accordingly, and the scale peelability is improved. In order to express such an effect, it is necessary to set the temperature reached by rapid heating to 1200 ° C. or higher. On the other hand, the upper limit of the ultimate temperature is 1350 ° C. from the viewpoint of reducing scale loss.

<O ₂ concentration in a rapid heating atmosphere: 10% by volume or more>
By setting the O ₂ concentration in the rapid heating atmosphere to a higher oxygen concentration than in the heating furnace, Fe diffusion from the steel material is promoted, and brittle FeO is easily generated in the subscale. As a result, Cr in the subscale is formed. Since the concentration can be reduced, the scale peelability is improved. In order to exhibit such an effect, the O ₂ concentration in the rapid heating atmosphere needs to be 10% by volume or more. Preferably, the O ₂ concentration is 20% by volume or more. On the other hand, the upper limit of the O ₂ concentration is preferably about 30% by volume from the viewpoint of equipment.

<H ₂ O concentration in a rapid heating atmosphere: 5% to 35% by volume>
When the steel material is oxidized in an H ₂ O-containing atmosphere, steam oxidation proceeds toward the steel material side. This is because water molecules dissociate on the surface of the scale and release protons, and the protons deprive oxygen in the scale at the scale / steel interface and become water again to promote oxidation (inward oxidation) into the steel. It is said. Thus, by combining the effects of rapid heating and steam oxidation, it becomes easy to form a sub-scale mainly composed of brittle FeO and easily broken by growth stress, and the scale peelability is dramatically improved. To sufficiently exhibit the effect, it is necessary to be of H _{2 O} concentration: 5% by volume or more in the rapid heating atmosphere. Preferably it is 10 volume% or more. On the other hand, if the H ₂ O concentration is too high, the oxygen partial pressure is relatively lowered. Since H ₂ O has a smaller oxidizing action than O ₂ , if the H ₂ O concentration is excessively increased, the oxidizing action is reduced as a whole atmosphere, and scale generation is not promoted. Therefore, in the present invention, the H ₂ O concentration in the rapid heating atmosphere is set to 35% by volume or less. Preferably it is 30 volume% or less.

The H ₂ O concentration can be adjusted by spraying water from the nozzle or bringing it into contact with humidified air. The rapid heating treatment can be performed using a high-frequency induction heating device on a steel slab conveyor immediately after the steel slab is extracted from the heating furnace. In order to bring the steel piece into contact with the atmosphere satisfying the prescribed O ₂ concentration and H ₂ O concentration, for example, flowing humidified air satisfying the prescribed O ₂ concentration and H ₂ O concentration in a high-frequency induction heating device. Can be implemented.

  After the rapid heat treatment, descaling may be performed immediately (within about 20 seconds). As descaling, high-pressure water descaling is common, but mechanical descaling may be performed.

  Other manufacturing conditions are not particularly limited. For example, when a billet is obtained by continuous casting, the billet is introduced into a heating furnace, passed through a pre-tropical zone, a heating zone, a soaking zone, and the billet is brought to a specified extraction temperature. The method of raising temperature is mentioned. Then, the billet is taken out from the heating furnace at the extraction temperature and subjected to the above rapid heating treatment, and then the scale is removed with a high-pressure water descaler, and then the steel wire rod is sequentially subjected to rough rolling, finish rolling, product water cooling, and winding. Can be obtained.

  In addition, as above-mentioned, this invention improves the scale peelability of the strip steel containing Cr which has a bad influence on scale peelability. Therefore, the present invention is directed to a Cr-containing strip steel material containing 0.10% or more (preferably 0.80% 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 is, for example, 1.00% or more, preferably 1.10% or more, more preferably 1 20% or more. When Cr is added in this way, the strength can be improved. However, if the amount of Cr is excessive, it becomes difficult to ensure ductility. Therefore, the amount of Cr is 2.00% or less, preferably 1.90% or less. More preferably, it is 1.80% or less.

  Since the present invention controls the subscale caused by Cr, steel components other than Cr are not particularly limited as long as they can be used as hot-rolled material (strip steel), but the elements other than Cr and their amounts For example, steel materials satisfying Si: 0.10 to 0.40%, C: 0.10 to 1.50%, and Mn: 0.01 to 1.00%, with the balance being iron and inevitable impurities can be mentioned. The steel material may further contain Mo: 0.01 to 0.40%. Hereinafter, each of the above elements will be described.

[Si: 0.10 to 0.40%]
Si is an important element for ensuring strength, and the lower limit is preferably set to 0.10% as the minimum amount of Si necessary for cold rolling steel (CH steel). On the other hand, from the viewpoint of ensuring ductility, the Si content is preferably 0.40% or less.

[C: 0.10 to 1.50%]
C is also an important element for ensuring strength, and is preferably contained in an amount of 0.10% or more. On the other hand, in order to ensure excellent cold workability, the Cr content is preferably 1.50% or less.

[Mn: 0.01 to 1.00%]
Mn is an element useful for securing the strength and toughness of the steel material. To that end, it is preferable to contain Mn in an amount of 0.01% or more. On the other hand, from the viewpoint of ensuring the toughness and weldability of the steel material, the Mn content is preferably 1.00% or less.

  The balance other than Cr, Si, C, and Mn may be iron. When the balance is iron, it is naturally allowed that inevitable impurities (for example, impurities brought in depending on the situation of raw materials, materials, manufacturing equipment, etc.) are contained in the steel. About P, S, Cu, and Ni contained as impurities, as described in detail below, the surface properties and characteristics of the steel material are adversely affected.

  That is, since brittleness increases when P is excessively contained, the P content is preferably suppressed to 0.05% or less. S reacts with Mn to form sulfide inclusion MnS. This MnS segregates during hot working, embrittles the steel material, and causes steel material cracking. Therefore, it is recommended to reduce the amount of S. The amount of S is preferably suppressed to 0.05% or less.

  Cu is an element inevitably mixed in. It becomes a liquid phase at 1356K and enters the austenite grain boundary during deformation during hot rolling, causing surface cracks. Therefore, it is preferable to suppress the amount of Cu to 0.30% or less (including 0%). Ni is also an element that is inevitably mixed in, and is unevenly concentrated on the surface of the steel material, increasing the irregularities on the surface of the scale and degrading the scale peelability. In order to suppress such an adverse effect, the Ni content is preferably suppressed to 0.30% or less (including 0%).

  The Cr-containing strip steel material of the present invention may further contain Mo as shown below as required.

[Mo: 0.01-0.40%]
Mo is an element effective for increasing the strength of the steel material, and in order to exhibit this effect, it is preferable to contain 0.01% or more. On the other hand, when the amount of Mo becomes excessive, the ductility of the steel material decreases, so the amount of Mo is preferably set to 0.40% or less.

  Specifically, the method of the present invention can be applied to a strip steel material satisfying the above component composition. The strip steel is a general term for rod-shaped or linear steel materials, and can be used for suspension springs, valve springs, bearings, etc. for automobiles, for example.

  Hereinafter, the present invention will be described in more detail with reference to experimental examples, but the present invention is not limited by the following experimental examples, and appropriate modifications are made within a range that can meet the above and the following purposes. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

[Example 1]
A steel material having an alloy composition shown in Table 1 was melted and cut into a cylindrical shape of φ10 mm × 12 mmt to prepare a sample. This sample was subjected to the treatment shown in Example 1A or 1B below using a high-temperature compression tester heat treatment furnace capable of controlling the atmosphere, and then the sample was compressed by simulating descaling at each ultimate temperature, Scale removal was performed to evaluate scale peelability.

  The compression conditions were an Ar atmosphere with a compression strain rate of 50% and a strain rate of 10 mm / sec. After compression, the appearance of the side surface of the sample was captured with a scanner, the image was binarized, and the area ratio (residual scale area ratio) of the scale remaining portion in the sample after compression was calculated. And when the residual scale area ratio was 20% or less, it was judged that the scale peelability was excellent.

Example 1A
In Example 1A, it heated with the heat pattern shown in FIG. 1, compression (descaling) was performed, and scale peelability was evaluated.

Specifically, the heating furnace atmosphere was N ₂ −1 volume% O ₂ -10 volume% CO ₂ -23 volume% H ₂ O and heated at 1100 ° C. for 30 minutes to generate a primary scale. The heating atmosphere is air + 10% by volume H ₂ O (that is, O ₂ concentration in the rapid heating atmosphere: 21% by volume, H ₂ O concentration: 10% by volume), and the heating rate is x ° C./min. For y minutes ( However, the values of x and y were as shown in Table 2).

  In the actual operation, from the start of the temperature rise of the steel slab to the extraction corresponds to the heating furnace atmosphere, but in Example 1A, in order to verify the improvement effect of the scale peelability by rapid heating in a limited manner, up to 1100 ° C. The heating atmosphere was an Ar atmosphere (atmosphere that does not oxidize), and the experiment was performed with the soaking time at 1100 ° C. as described above (the same applies to Example 1B described later).

  As comparative examples, No. 13 and No. 1 in Table 2 were used. In No. 14, compression was performed without performing the rapid heating treatment. The compression test was performed under the above conditions in an Ar atmosphere at a temperature reached by rapid heating.

  The results of obtaining the residual scale area ratio are shown in Table 2.

  From the results in Table 2, the following can be understood. First, no. Examples 1, 2 and 9 are examples in which steel materials having different Cr contents are used, and the scale peelability is compared with constant rapid heating conditions and the like. From the comparison of these results, it can be seen that when the Cr content increases, the scale peelability tends to deteriorate, but excellent scale peelability can be ensured by rapid heating as defined in the present invention.

  Nos. 3 to 6 are examples in which the temperature rise rate is different, but the ultimate temperature is 1280 ° C., and the scale is removed by compressing at 1280 ° C. From the comparison of these results, it can be seen that even when the ultimate temperature is the same, the faster the rate of temperature rise, the better the scale peelability.

  In Nos. 7 to 9, the heating rate was 20 ° C./min. In this example, the temperature reached is changed by changing the rapid heat treatment time. From these results, it can be seen that even when the rate of temperature increase is the same, the higher the temperature reached, the better the scale peelability.

  Nos. 10 to 12 have a temperature rising rate of 30 ° C./min. However, the reached temperature is changed by changing the rapid heat treatment time. In this case as well, as in No. 7 to No. 9, it can be seen that even if the rate of temperature increase is the same, a higher ultimate temperature tends to be superior in scale peelability.

  Nos. 13 and 14 are examples in which heating (oxidation treatment) is performed at 1100 ° C. and 1250 ° C. for 30 minutes in a heating furnace atmosphere, and then compression (descaling) is performed without performing rapid heating treatment. From these results, it can be seen that when the rapid heat treatment is not performed, the residual scale area ratio is high and the scale peelability is poor.

(Example 1B)
In Example 1B, it heated with the heat pattern shown in FIG. 2, compression (descaling) was performed, and scale peelability was evaluated.

Specifically, after the heating furnace atmosphere is N ₂ -0.5 volume% O ₂ -10 volume% CO ₂ -23 volume% H ₂ O and heated at 1000 ° C. for 90 minutes to generate a primary scale. The temperature rising atmosphere is N ₂ -x volume% O ₂ -y volume% H ₂ O (that is, the concentration of O ₂ and H ₂ O is changed), and rapid heat treatment (at 25 ° C./min. For 10 minutes, The final temperature was 1250 ° C.). The O ₂ concentration x (volume%) and the H ₂ O (water vapor) concentration y (volume%) are as shown in Table 3. The compression test was performed under the above conditions in an Ar atmosphere at a temperature reached by rapid heating.

  Table 3 shows the results of the residual scale area ratio.

From the results in Table 3, the following can be understood. First, no. 15-19 is an example in which the _{O 2} concentration is changed of _{H 2} O concentration as constant at 10 vol%. From these results, it can be seen that the residual scale area ratio increases if the H ₂ O concentration is too low or too high.

On the other hand, no. 16 and no. 20 to 22 are examples in which the O ₂ concentration was changed with the H ₂ O concentration kept constant at 10% by volume. From these results, it can be seen that the residual scale area ratio tends to decrease as the O ₂ concentration in the temperature rising atmosphere increases.

No. No. 23 is a comparative example in which the H ₂ O concentration satisfies the conditions of the present invention, but the O ₂ concentration is low. In this case, it can be seen that the residual scale area ratio is high and the scale peelability is poor.

No. In No. 24, since both the O ₂ concentration and the H ₂ O concentration are within the specified range of the present invention, the residual scale is small and the scale peelability is excellent.

[Example 2]
Heating and scale peeling were performed simulating actual operation, and the surface properties of the steel material after scale peeling were examined.

First, steel materials shown in Table 4 below were melted and cast to obtain 150 mm square billets. And after heating up to the temperature shown in Table 5 with a heating furnace and performing the heating which hold | maintains for 30 minutes at this temperature, this billet was taken out out of the heating furnace at the temperature (extraction temperature) shown in Table 5. Incidentally, the combustion gas in the furnace using the LNG gas, the furnace atmosphere were all adjusted to _N 2 -1 _vol% O 2 -10 _vol% CO 2 -23 vol% _{H 2} O. Immediately after extraction outside the heating furnace, rapid heating treatment was performed in an induction heater booth adjacent to the heating furnace. The rapid heat treatment conditions are as shown in Table 5. Specifically, the temperature was increased from (extraction temperature from the heating furnace) to (attainment temperature) at a temperature increase rate shown in Table 5. Table 5 shows the O ₂ concentration and the H ₂ O concentration in this rapid heating atmosphere (atmosphere in the induction heater). The atmosphere inside the induction heater was adjusted by introducing oxygen gas, nitrogen gas and water from a nozzle installed on the inner wall of the induction heater.

  Then, after reaching the ultimate temperature, it was immediately extracted from the induction heater, immediately subjected to high pressure water scale peeling of 150 MPa in an air atmosphere, and then hot rolled to produce a steel wire having a diameter of 8.0 mm.

  In order to evaluate the surface defects due to the scale of each manufactured steel wire, the cross section of the steel wire was observed with an optical microscope at a magnification of 100 times, and the presence and the number of surface defects were counted.

In addition, the surface wrinkle targeted in the present invention means “a surface wrinkle caused by a scale in which the wrinkle depth reaches 10 μm or more”. A wrinkle having a wrinkle depth of less than 10 μm is not recognized as a surface wrinkle and does not substantially cause a problem such as cracking during processing. Therefore, only wrinkles reaching 10 μm or more are targeted. In addition, the following method was used to confirm whether the surface wrinkles were caused by scale. That is, the cross-section of the entire surface defect is analyzed by EPMA mapping at a magnification of 500 times, and the surface defect in the region where the Cr concentration is more than twice the average Cr concentration of the entire steel wire rod is the subscale (FeCr ₂ O ₄ ). It was judged that it was a surface defect caused by the pressed scale.

The evaluation of the surface properties is performed by measuring the number of surface defects observed at 10 or more cross sections perpendicular to the longitudinal direction (steel rolling direction) of the steel wire, and calculating the average value (average value of the number of surface defects). Was calculated by the following formula (1), and was classified and evaluated in the following five stages. And the case of rank 1 or less was evaluated as having no problem as a product with respect to surface defects caused by scale.
Total number of surface defects / total number of measured cross sections = number of surface defects per measured cross section (1)
(Rank of surface flaw)
-Rank 0: The average number of surface defects is 0 (no defects)-Rank 1: The average number of surface defects is more than 0 and less than 10-Rank 2: The average value of surface defects is 10 More than 20 and less than 20 ・ Rank 3: The average number of surface defects is 20 or more and less than 30 ・ Rank 4: The average number of surface defects is 30 or more

Furthermore, more than the weight of the billet before the heat treatment in the heating furnace _{W A,} when the weight _{W B} of the steel wire can be from the billet, the 10% value of _{(W A -W B) × 100} / W A In the case, the scale loss was remarkable, and it was rejected from the viewpoint of productivity. These results are shown in Table 5.

  From the results in Table 5, the following can be understood. First, no. Nos. 25 to 28 are examples examined by using SCr steel and changing the extraction temperature from the heating furnace, and all the conditions of the rapid heating treatment satisfy the prescribed conditions. From these results, no. In No. 25, since the holding temperature in the heating furnace (the extraction temperature from the heating furnace) was lower than the temperature specified in the present invention, the scale peelability was not sufficiently improved, and the surface flaw was at a reject level. No. 28, because the holding temperature (extraction temperature from the heating furnace) in the heating furnace is too high, the primary scale and the steel material are in close contact with each other through the subscale generated in the heating furnace, and even by rapid heating treatment Scale peelability did not improve. No. In Nos. 26 and 27, the holding temperature in the heating furnace (the extraction temperature from the heating furnace) was also within the predetermined range, and both the soot levels were acceptable levels.

  No. using SCM steel. No. 29 has a low temperature rise rate in rapid heating, so that the effect of rapid heating is insufficient, and the surface flaw becomes a rejected level. On the other hand, no. Since No. 30 satisfies the conditions specified in the present invention, the surface flaw was an acceptable level.

  No. Nos. 31 to 34 are examples in which SUJ2 steel is used and examined by changing the ultimate temperature. From these results, no. Since 31 reached too low temperature, the effect of rapid heating became insufficient. No. No. 34 was rejected because the reached temperature was too high and the scale loss increased. No. Since 32 and 33 satisfy | filled the conditions prescribed | regulated by this invention, the surface flaw was a pass level.

  No. 35 and 36 are comparative examples in which the prescribed rapid heat treatment is not performed, and both have a high wrinkle level and fail.

FIG. 1 is a diagram showing a heat pattern in Example 1A. FIG. 2 is a diagram showing a heat pattern in Example 1B.

Claims (3)

Manufacture of Cr-containing strips that are hot-rolled after taking out a steel piece containing Cr from 0.10 to 2.00% (meaning mass%, the same applies to the chemical composition of steel) from the heating furnace and descaling A method,
After heating for 15 minutes or more in the temperature range where the surface temperature of the steel slab is 1000 ° C. or higher and 1150 ° C. or lower in the heating furnace, the steel slab whose surface temperature (extraction temperature) is in the temperature range is the heating furnace. And then
Until the surface temperature of the steel slab becomes a temperature range of 1200 ° C. or higher and 1350 ° C. or lower from the extraction temperature in an atmosphere where the O ₂ concentration is 10% by volume or more and the H ₂ O concentration is 5% by volume or more and 35% by volume or less. A method for producing a Cr-containing strip steel material excellent in scale peelability, wherein the steel slab is rapidly heated at a rate of temperature increase of 20 ° C./min or more and then descaled. The billet further comprises:
Si: 0.10 to 0.40%,
C: 0.10 to 1.50%, and Mn: 0.01 to 1.00%
The manufacturing method according to claim 1, wherein the balance is made of iron and inevitable impurities.   The manufacturing method according to claim 2, wherein the steel piece further contains Mo: 0.01 to 0.40%.

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