JP5320084B2 - Manufacturing method of high Cr content steel with good descalability - Google Patents

Description

  The present invention relates to a method for producing a high-Cr steel material for cold pressure from a steel billet heated in a combustion atmosphere heating furnace, in particular, the strip steel material, and suppresses the scale generated on the surface of the steel material to improve the production yield, The present invention relates to an improvement in heating furnace control conditions for obtaining a product having a good surface quality by increasing the peelability of the scale.

  Cold-rolled steel bars frequently used as automotive parts materials, and especially high Cr-containing steels manufactured for the purpose of ensuring high strength, have the following problems due to the inevitably generated scale. Accompanying.

When hot-rolling a billet of Cr-containing steel that has been heat-treated, as shown in the schematic diagram shown on the left side of FIG. 1, chromite (FeCr _{2) is formed} as an inner oxide layer at the interface between the steel and the scale. A so-called subscale mainly composed of O ₄ ) is generated. This subscale formation unfortunately increases the scale adhesion to the steel and worsens the scale peelability required during descaling after hot rolling. As a result, the remaining scale is pushed into the steel material during rolling, and a so-called indentation ridge is imprinted on the steel material surface.

  The product steel with the indentation flaws brought in deteriorates not only the appearance but also becomes the starting point of destruction and embrittlement and becomes defective. As countermeasures, conventionally, a solution has been devised that increases the temperature of the billet heating furnace to increase the peelability of the scale.

  For example, in the following Patent Document 1, a conventional heating condition of 1000 to 1200 ° C. and several tens of minutes to several hours is set to a high temperature treatment of 1100 ° C. or higher, 10 minutes or higher, or 1200 ° C. or higher and 5 minutes or higher, and at the same time in the furnace Shows how to supply water. This method tries to increase the peelability of the scale by high-temperature heating, but high-temperature heating at 1100 to 1200 ° C. or even 1200 ° C. or more has a fatal practical problem that increases scale loss. .

  Moreover, the following patent document 2 attaches importance to the fact that the residual of the lower scale is unavoidable due to pores in the descaling by high-pressure water injection after hot rolling, which is normally performed, and has a temperature of 1200 to 1350 ° C and a water vapor concentration of 20 to We propose a method for producing hot rolled steel with good surface properties by primary heating of 35% or more and secondary heating at 1000 to 1200 ° C. and a water vapor concentration of less than 20%. However, in this way, the method of lowering the temperature from the primary heating at the high temperature to the secondary heating obviously decreases the production efficiency, but may increase the scale loss.

  Although not intended for Cr-containing steels, some proposals have been made to control the atmosphere of the heating furnace in order to produce hot-rolled steel with good surface properties.

  For example, Patent Document 3 below focuses on the point that defects at the time of continuous casting remain in the steel and move to the surface of the product when the so-called direct feed rolling is speeded up to reduce scale loss, and the atmosphere of the heating furnace Is controlled as follows. That is, the dew point W ° C. of the heating furnace is calculated by the following equation (1) as a function of the temperature T ° C. of the heating furnace and the heating time Hhr, and the operation is performed at or above the dew point. By controlling the dew point of the heating furnace under these conditions, it is assumed that casting defects on the slab surface layer generated by the continuous casting machine can be removed.

W = 5.85 × 10 ⁻⁵ /(T+273)−1.5H+10 ⁻³ H ² −294
... (Formula 1)
However, from the viewpoint of implementation, the following problems are inferred. First, the atmosphere of the heating furnace differs due to the difference in the steel type that depends on the amount of the concentrated element that is a predisposing factor for the formation of subscales such as Si and Cr contained in this seed steel. Therefore, it is considered difficult to guarantee the improvement of descaling performance even if the atmosphere is manipulated in general.

Moreover, normally, the steel sheet for cold pressure is manufactured by heating at a temperature of 1000 to 1350 ° C. for 30 to 120 minutes. For example, by setting the conditions at 1100 ° C. and 120 minutes, the above equation (1) When the dew point is obtained, W = 129.1. Then, by substituting this value into the following formula (2) for obtaining the water vapor concentration X _H2O in the present invention together with the conditions of 1100 ° C. and 120 minutes, a water vapor concentration of X = 6.035% is calculated.

_{X H2O, 1 = {7.02 ×} (W + 273.15) 0.105 -12.08} 9.52
... (Formula 2)
Although the target value of the water vapor concentration X _H2O % calculated by this (Equation 2) lacks clarity, if it is reasonably considered to be volume%, this water vapor concentration is the scale of the Cr-containing steel that is the steel type to be treated. As a result, the releasability is desperately deteriorated, and there is no practicality.

  Patent Document 4 below proposes control conditions in a combustion heating furnace as follows in order to obtain a steel material with good surface quality while minimizing the scale-off. This is a method in which the extraction temperature of the heating furnace is 1100 ° C. or less in terms of the average temperature of the steel, the water vapor concentration of the atmosphere gas in the furnace is 5% or more, and at the same time the furnace temperature and oxygen partial pressure are optimized.

  However, even if this method is to be applied to Cr-containing steel, depending on the precondition that the extraction temperature of the heating furnace is 1100 ° C. or less of the average temperature of the Cr-containing steel, it depends on the Cr content of the target Cr-containing steel. Therefore, it is difficult to optimize the water vapor concentration and the ratio between the water vapor concentration and the oxygen partial pressure, and improvement of the scale peelability of the Cr-containing steel cannot be expected.

  In addition, Patent Document 5 below discloses an oxygen atmosphere in a heating furnace in order to prevent nonuniform generation of scale due to reheating during hot rolling of this seed steel or surface defects due to local deep wedge-shaped grain boundary oxidation. It is assumed that fuel gas and air may be supplied to the combustion burner so that the concentration is 0.1 to 1.0% and the water vapor atmosphere concentration is 25% or more. However, since oxygen and water vapor concentrations in the furnace atmosphere under heating conditions below 1100 ° C. are not suggested here, there is no guarantee that a product with good surface properties can be obtained when applied to high Cr content steel.

  As described above, in the known methods, it is actually impossible to manufacture a high Cr content steel material for cold pressure with a very small surface wrinkle by reliably and sufficiently reducing the scale indentation wrinkles due to the primary scale.

JP 2002-316207 A JP 2003-119517 A JP-A-6-184627 JP 11-217629 A JP-A-11-286718

  The present invention is directed to a steel material for cold pressure containing a large amount of Cr exceeding the extent that generation of surface flaws is not substantially concerned, and suppresses the generation of primary scale generated in a heating furnace and reduces scale loss. It is an object of the present invention to provide a steel material for cold pressure containing high Cr that has excellent surface properties by improving the descalability and reducing the inevitable scale indentation flaws and reducing the surface flaws.

  The present invention is directed to a steel material for cold pressure containing a large amount of Cr more than the extent that generation of surface flaws is not substantially concerned, and is characterized by heating control under the following conditions and hot rolling By this, it is a method which can manufacture the steel material for cold pressure containing high Cr which was excellent in the surface property with few surface flaws.

  (1) In a method in which a billet of steel containing 0.2% to 4.0% by mass of Cr is heated in a combustion atmosphere heating furnace and kept soaked, and then extracted outside the heating furnace and hot rolled. And the Cr concentration in the heating furnace is (4.47X + 17.1) vol. In an atmosphere in which the ratio of the water vapor concentration and the oxygen concentration (vol.%) In the heating furnace is adjusted to 10 or more, and soaking is maintained for 10 minutes or more at a temperature of less than 1100 ° C. A method for producing a high Cr-containing steel material for cold pressure with good descaling characteristics.

  (2) The steel further contains Si: 0.1 to 0.5% by mass, and after the soaking is maintained, the soaking temperature is + 70 ° C. or higher and less than 1180 ° C. The method for producing a steel material for cold pressure containing high Cr with good descaling property according to the above (1), characterized in that rapid heating is performed at a temperature rising rate of min.

  (3) The steel is further C: 0.02-0.6 mass%, Si: 0.1-0.5 mass%, Mn: 0.02-3.0 mass%, S <0.03 mass. %, Al <0.05 mass%, and Ca <0.003 mass%, The manufacturing method of the high Cr content cold-pressure steel material with the good descaling property as described in said (1) or (2) characterized by the above-mentioned.

  The present invention targets high Cr-containing steel, and controls the water vapor and oxygen concentration in the furnace to a specific numerical range as described above under the conditions when the billet is heated and soaked in the heating furnace. Thus, the treatment can be performed at a temperature of 1100 ° C. or lower. As a result, the amount of scale generated is suppressed to an extent that it does not become a problem, the yield due to scale loss reduction is improved, and the scale properties have good peelability, and the scale-derived indentations left behind by descaling in the subsequent process Thus, a steel material for cold pressure having a good surface property that is substantially free of can be easily produced.

The schematic diagram of the production | generation of the inner oxide layer and outer oxide layer on the steel surface. The correlation graph which shows the appropriate relationship of the water vapor | steam density | concentration with respect to Cr content of steel. The graph which shows the influence of the temperature increase rate with respect to the scale peelability of steel. The graph which shows the influence of the ultimate temperature with respect to the scale peelability of steel. Schematic of firelight destruction by rapid heating of steel surface.

  In the present invention, a steel billet containing Cr: 0.2 to 4.0% by mass (the content of each steel material component is simply described as “%” in the following) is heat-treated in a heating furnace and hot-rolled. When manufacturing a steel for cold pressure, the Cr concentration in the steel is X%, and the water vapor concentration is (4.47X + 17.1) vol. %, And in a heating furnace having an atmosphere in which the ratio of water vapor concentration to oxygen is adjusted to 10 or more, soaking is maintained at a temperature of less than 1100 ° C. for 10 minutes or more. This is a method for producing a steel material for cold pressure containing high Cr.

  The subject of the present invention is a steel for cold pressure containing Cr: 0.2-4.0%, in order to give the mechanical strength required for the product steel material, containing 0.2% or more of Cr. This is because, of course, if the Cr content is less than this level, there is substantially no risk of surface flaws occurring in the steel by the mechanism described below. On the other hand, even if the Cr content is increased to 4% or more, the effect of increasing the inner oxide layer as described below by steam in the furnace cannot be expected within the normal furnace time, so the present invention is applied. There is no positive profit.

  Here, before explaining the processing conditions of the present invention, steel components other than Cr will be explained. In addition to Cr: 0.2-4.0%, the steel that is the subject of the present invention is C: 0.02-0.6%, Si: 0.1-0.5%, Mn: 0.02 -3.0%, S <0.03%, Al <0.05% and Ca <0.003%. In addition to these required elements, it goes without saying that various alloy elements such as Mo <0.3% can be blended depending on the purpose of use.

  The reason for determining the range of each element content is as follows.

  C needs to contain 0.02% or more in order to ensure the required strength of the steel material, but if it exceeds 0.6%, the cold workability of the steel material will be reduced.

  Si has a lower limit of 0.1% in order to ensure the required strength, but excessive addition of 0.5% or more is not preferable because it deteriorates the ductility of the steel material and at the same time inhibits the effect of Cr in the present invention.

  Mn is required to be 0.02% or more in order to ensure the strength and toughness of the steel material, but if it is 3.0% or more, the toughness and weldability are hindered.

  S forms sulfide inclusion MnS and segregates during hot working of steel, embrittles the steel material. Therefore, it is necessary to suppress it to 0.03% or less in order to prevent cracking.

  Al is a deoxidizing material added in order to coarsen austenite crystal grains during normalizing heating of the steel material, but an excessive addition of 0.05% or more is not preferable because the crystal grains become unstable.

  Ca is effective in suppressing an increase in the hydrogen ion concentration in the interface atmosphere accompanying corrosion of the steel material surface and improving the corrosion resistance of the steel material, but excessive addition degrades the ductility.

Mo may be added as necessary for the purpose of increasing the strength of the steel material, 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, it should be 0.3% or less.

  Now, the present invention is characterized in that both the temperature when the billet of the Cr-containing steel described above is heat-treated and the water vapor concentration and the oxygen concentration in the furnace atmosphere are regulated as described above. The technical background described below is the background behind the conditions.

When a high Cr content steel as the object of the present invention is oxidized during heating, it is composed of wustite (FeO), magnetite (Fe ₃ O ₄ ) and hematite (Fe ₂ O ₃ ) as schematically shown in FIG. An outer oxide layer and an inner oxide layer called a subscale mainly composed of chromite (FeCr ₂ O ₄ ) are generated. The outer oxide layer is a scale generated outward from the surface of the steel material before oxidation, and grows by diffusion of Cr from the steel material. Further, the subscale of the inner oxide layer grows as oxidation proceeds inward from the steel surface before oxidation.

  Although the mechanism of such a phenomenon remains unexplained, when the inside of the heating furnace is in a steam atmosphere, hydrogen ions formed by the dissociation of steam at the scale surface layer reach the steel surface through the outer oxide layer, A phenomenon occurs in which the oxide in the same layer is reduced to moisture, which is considered as a kind of corrosion reaction that grows an oxide layer toward the inside of the steel.

  From these considerations, the following understanding can be obtained regarding the functional aspects of these generation scales.

That is, the chromite forming the main subscale is a composite oxide of Fe and Cr (FeCr ₂ O ₄ ) having a spinel structure, and the generated oxygen pressure is lower than that of wustite (FeO). It has the property of concentrating to a very strong adhesion between the two. And this adhesiveness tends to be strengthened as the chromite concentration in the subscale increases, which means deterioration of descaling property.

  Based on this understanding, it was found that the concentration of chromite can be reduced if the subscale growth is promoted by adjusting the water vapor atmosphere in the heating furnace. This is the solution principle of the present invention, and is the reason why the concentration of water vapor and oxygen in the furnace atmosphere during heating is controlled within an appropriate range.

  That is, referring to FIG. 1 again, the left half of the figure shows a schematic diagram of the conditions in which the concentration of water vapor and oxygen in the furnace atmosphere deviates from the appropriate range, but compared to the right half of the figure. Thus, it should be understood that the subscale area is narrow and the chromite concentration is high. In other words, in the left half of the figure, the effect of inward oxidation by water vapor is not exhibited, and the growth rate of the subscale is slow, so chromite can be gradually concentrated in its narrow area, resulting in a thickness increase. Is a subscale property with a small chromite concentration.

  On the other hand, when the concentration of water vapor and oxygen in the heating furnace atmosphere is controlled within an appropriate range regulated by the present invention, the effect of inward oxidation by water vapor is promoted and the growth rate of the subscale is promoted. In the scale region, chromite can be generated and grown quickly. As a result, as shown in the right half of the figure, the chromite concentration is thin and the subscale property is thick, thereby improving the peelability. In addition, the effect of increasing the amount of FeO with good peelability can be confirmed as the amount of chromite decreases.

  The present invention controls the concentration of water vapor and oxygen in the furnace atmosphere in the proper range as described above, and further shows that there is a correlation that the chromite concentration increases as the Cr content in the steel increases. There are facts found. This means that steel with a higher Cr content exhibits a scale property with poor peelability due to heating.

  Based on the above understanding and knowledge, the present invention succeeded in improving the peelability of the scale by increasing the water vapor concentration in the atmosphere in the heating furnace. Specifically, assuming that the Cr content in the steel is X%, the water vapor concentration is (4.47X + 17.1) vol. The characteristic is that the adjustment is made to be at least%, and this relationship is shown in FIG. This correlation is a result obtained by discriminating whether the scale peelability is good or not from a large number of experimental results, as disclosed in part in the examples below. That is, the above formula (4.47X + 17.1) vol. 0.02 mm or more for rolled steel that is appropriately heated under the Cr content and water vapor concentration corresponding to the conditions of the black squares distributed above the primary line No surface defects remain. However, it goes without saying that the oxygen concentration at this time is appropriately adjusted under the conditions described in detail below.

  That is, when the present invention increases the water vapor concentration in the furnace atmosphere as described above, if the coexisting furnace oxygen is high, the amount of water dissociation on the scale surface decreases and It is confirmed that the amount of diffusion also decreases, and the remarkable increase effect of the inner oxide layer due to water vapor is not recognized.

  Therefore, in the present invention, the oxygen concentration is controlled to be as low as possible and the water vapor concentration is increased as much as possible. Specifically, the ratio between the water vapor concentration and the oxygen concentration is set to 10 or more.

  When a steel billet heated under these conditions is rolled by a conventional method, the product steel cannot be wrinkled by 0.02 mm or more, and when the ratio is 20 or more, it is 0.015 mm or more. It can be confirmed that wrinkles of 01 mm or more cannot be formed, and it is clear that the peelability of the scale is surely improved.

  Further, according to the present invention, when heating the steel billet in the above-mentioned atmosphere, it is necessary to keep soaking for 10 minutes or more, and in a short time shorter than this, of course, insufficient heating causes a rolling failure. Moreover, heating is insufficient to generate the scale and the thickness of the scale remains thin, and the residual stress in the scale is insufficient, which causes a descaling failure. As described above, the heating temperature at this time, that is, the soaking temperature, is set to less than 1100 ° C. in order to reduce the scale loss as much as possible. On the other hand, at a low temperature of less than 950 ° C., the load on the rolling mill during hot rolling is reduced. Since it becomes so large that normal rolling becomes difficult, the temperature is preferably set to 950 ° C. or higher.

  In addition, it is necessary to supplement explanation about the relationship between the Cr content of steel and the heating conditions. The reason why the present invention limited the Cr content to 0.2 to 4.0% has been explained above. This is because the heating conditions of the present invention are assumed. That is, in a heating furnace temperature of 1090 ° C., a water vapor concentration of 15%, an oxygen concentration of 1%, and a soaking time of 15 minutes, three types of Cr content of 0.18%, 3.5%, 4.05% Each of the steels was heated and rolled, and a descaling test was conducted. Table 1 shows the state of surface defects on each rolled material, and Table 1 shows the result of determining pass / fail by presence or absence of defects of 0.002 mm or more.

From the table, even if the heat treatment satisfies the conditions in accordance with the instructions of the present invention, a material having a Cr content exceeding 4% is rejected, and as an applicable steel type, the range of 0.2 to 4.0% is naturally. It is understood that it is limited to.
(Example 1 group)
As shown in Table 2, 6 billet steel billets were melted. All of these are within the range defined by the present invention, and the Cr content is changed little by little, and the processing conditions are changed to be used in common with the present invention examples and comparative examples. All other contained elements were also within the range defined by the present invention.

  As shown in Table 3 (Comparative Example) and Table 4 (Example), these steel billets were heated for 15 minutes each under different conditions of heating temperature, water vapor concentration and oxygen concentration, and a high pressure of 150 MPa. After primary descaling with water, rough rolling and finish rolling were performed, and a wire rod having a diameter of 10 mm was prepared by winding with a winding machine. These hot-rolling conditions are not exceptional because of conventional methods.

  Next, a total of 12 wires are cut out from each of the wire coils having a length of 1 m from each of the three positions of the front end portion, the center portion, and the rear end portion of each wire rod coil, and the cross sections at arbitrary four points of each wire rod can be observed. Each resin-filled sample was created.

  And the wrinkles scattered on the cross section of each resin embedding sample were observed with the optical microscope, and the presence or absence of wrinkles of 0.02 mm or more, wrinkles of 0.015 mm or more, and wrinkles of 0.01 mm or more was investigated.

  The evaluation was evaluated as rejected as unsatisfactory when any single wrinkle of 0.02 mm or more was found. Moreover, since the thing without a wrinkle of 0.015 mm or more is preferable as a wire, and the thing without a wrinkle of 0.01 mm or more is more preferable, this was also considered from the viewpoint of evaluation. In addition, about the descaling effect, what can count that the mass ratio of the final product wire with respect to the original billet is less than 95% was set as evaluation with many scale losses.

  These results are as shown in Table 5 (Comparative Example) and Table 6 (Example). That is, in the comparative example of Table 5, No. 1-6 and no. No. 21 is that the water vapor and oxygen concentrations in the furnace atmosphere are both within the specified range of the present invention, but only the heating temperature is over 1100 ° C., so the scale loss is large and it is rejected.

  No. of the comparative example. 7-13 and no. No. 22 is rejected because wrinkles of 0.02 mm or more occur because the water vapor concentration is less than the specified value of the present invention relative to the Cr concentration of the steel material.

  Similarly, the comparative example No. 2-20 and no. 23, since the ratio of the water vapor concentration to the oxygen concentration is less than 10 which is the specified value of the present invention, the action of increasing the inner oxide layer by water vapor is insufficient, and soot of 0.02 mm or more is generated. Fail.

  On the other hand, No. of the Example of Table 6 is shown. Nos. 24-37 satisfy the specified range of the present invention in all of the heating temperature, water vapor concentration and oxygen concentration, and those having a water vapor concentration to oxygen concentration ratio of 10 or more are 0.02 mm or more. No spider has been found. Further, no wrinkles of 0.015 mm or more were found for those having this ratio of 20 or more, and no wrinkles of 0.01 mm or more were found for those having this ratio of 30 or more.

As described above, when high-Cr steel containing 0.2% to 4.0% by mass of Cr is heated in a combustion atmosphere heating furnace and maintained soaking, the Cr concentration in the steel is adjusted. As X%, the water vapor concentration in the heating furnace is (4.47X + 17.1) vol. %, And in the atmosphere in which the ratio of water vapor concentration and oxygen in the heating furnace is adjusted to 10 or more, soaking is maintained at a temperature of less than 1100 ° C. for 10 minutes or more. By extracting and hot rolling outside the heating furnace, it is possible to improve the peelability of the subscale mainly composed of chromite (FeCr ₂ O ₄ ), which has very strong adhesion to the steel material generated in the heating furnace. This makes it possible to produce a steel material for cold-pressure with excellent surface properties that has substantially no indentation flaws derived from residual scale, that is, specifically no surface flaws of 0.02 mm or more. It became.

By the way, in the case of a steel material containing Si in addition to Cr as a component of steel, a complex oxide called firelite (Fe ₂ SiO ₄ ) is formed in the subscale separately from the chromite, and this is formed on the surface of the steel material. Because it has the property of being concentrated in layers and firmly adhering to the steel material, this firelite is a detachable scale when it is used for high Cr steels with Si content of 0.1% by mass or more, especially 0.3% by mass or more. Will be disturbed.

  By controlling the soaking atmosphere in the heating furnace of the present invention, even when high Cr steel containing 0.1 mass% or more of Si is targeted, the growth rate of the subscale forming this firelight is increased, and the peelability is increased. It is clear from the above-described embodiment that the surface roughness of 0.02 mm or more can be prevented.

  However, since the formation of the chromite layer is relatively slow, the application of the soaking atmosphere control of the present invention produces wustite (FeO) with good releasability in the subscale even when heated at a low temperature of less than 1100 ° C. However, since the formation of the firelite layer is faster than that of the chromite layer, there is a problem that it is not always possible to sufficiently ensure the same subscale peelability as that of chromite by this atmosphere control. In other words, the chromite layer is effectively taken into the inner oxide layer mainly composed of wustite by steam oxidation, whereas the firelite layer is difficult to be taken into the inner oxide layer because of its high generation rate, and is concentrated on the steel surface. There is a tendency to become.

  Therefore, when high Cr steel containing 0.1% by mass or more, especially 0.3% by mass or more of Si is targeted, with this firelite generation, the peelability of the scale is relatively lowered. Although the produced steel sheet for cold pressure does not generate a surface flaw of 0.02 mm or more, a finer surface flaw, particularly a surface flaw of 0.01 mm or more and less than 0.020 mm, is generated in a considerable amount.

  In view of such a situation, the present inventors have conducted various experiments in order to further reduce fine surface defects of 0.01 mm or more and less than 0.020 mm even when a high Cr steel material containing a large amount of Si is used. As a result of repeated studies, it has been found that it is preferable to perform rapid heating control in addition to the soaking atmosphere control in the heating furnace.

  Specifically, after the steel billet is soaked by the soaking atmosphere control in a heating furnace, it is further maintained at a temperature of 20 ° C./until it reaches a predetermined temperature of the soaking temperature + 70 ° C. or more and less than 1180 ° C. We propose a method characterized by rapid heating at a heating rate of min.

  According to this method, even in the case of a high Cr steel material containing 0.1% by mass or more of Si, as described later, the releasability of the scale is dramatically improved, and the fine surface resulting from the residual scale. It is possible to greatly reduce wrinkles, and thereby it is possible to produce a cold-pressing steel material with extremely excellent surface properties.

  Below, the specified reason is demonstrated about the conditions in the rapid heating control of this invention. FIG. 3 is directed to an Fe-0.45% Si standard steel, treated for 10 minutes in an atmosphere containing a water vapor concentration of 20% at 1050 ° C. and an oxygen concentration of 1% (corresponding to the soaking atmosphere control of the present invention), The relationship between each heating rate and the residual scale area ratio is illustrated for samples (shape: φ10 mm × 12 mm) in which this steel is rapidly heated at various heating rates up to 1140 ° C. The residual scale area ratio on the vertical axis in the figure was calculated by compressing the sample after the heat treatment at a compression ratio of 50% and binarizing the scale adhered to the side surface into black and white by image processing. The evaluation of the peelability of the scale was accepted when the residual scale area ratio was less than 50% based on the correlation between the residual scale area ratio in the laboratory experiment and the actual scale peelability in the actual machine. As is apparent from FIG. 3, the rate of temperature increase in the rapid heating control is 20 ° C. min. When it becomes above, the residual scale area rate is rapidly decreasing beyond the pass line, and it can be seen that the peelability of the scale is dramatically improved even with a steel component having a very high Si content of 0.45%.

Next, FIG. 4 shows the same Fe-0.45% Si standard steel, and is similarly treated for 10 minutes in an atmosphere containing a water vapor concentration of 20% and an oxygen concentration of 1% at 1050 ° C. (soaking atmosphere control of the present invention). This time, the temperature increase rate is 25 ° C. min. The relationship between each ultimate temperature and the residual scale area ratio is also illustrated for the samples having the same shape rapidly heated to various ultimate temperatures. From FIG. 4, it is clear that the residual scale area ratio decreases beyond the pass line when the temperature reached in the rapid heating control is 1120 ° C. (soaking temperature 1050 ° C. + 70 ° C.) or more and less than 1180 ° C. It is known that the peelability of the scale is dramatically improved even with a steel component having a very high Si content of 0.45%.

  By performing such rapid heating control in which the rate of temperature rise and the temperature reached are within the above specific range, the scale peelability can be achieved even when a sub-scale is formed on the steel surface that tends to concentrate and adhere closely. The mechanism that can be remarkably improved is not necessarily clear, but will be described with reference to the schematic diagram of FIG. The leftmost figure in FIG. 5 shows the surface condition of the steel when extracted from the heating furnace at less than 1100 ° C. (1050 ° C.) by the above-mentioned soaking atmosphere control, but it is outside on the inner side (steel side) of the grown subscale. A firelight layer concentrated without being taken into the oxidization layer remains in close contact with the steel. On the other hand, after controlling the soaking atmosphere at less than 1100 ° C. (1050 ° C.), rapid heating control is performed under conditions that satisfy the range of the temperature increase rate (20 ° C. min. Or more) and the ultimate temperature (1120 ° C. to less than 1180 ° C.). When extracted from the heating furnace, the remaining firelite layer is broken and divided as shown in the center of FIG. 5 and is easily peeled off from the steel surface. As a result, the peelability of the scale is greatly improved. However, when the rate of temperature rise and the temperature reached during heating are lower than the conditions of the present invention, the firelight layer is not broken or divided, and the scale peelability is deteriorated. In addition, when the temperature reached is high, that is, when heated to a high temperature of 1180 ° C. or higher, the firelight layer once broken and divided as shown in the rightmost diagram of FIG. The phenomenon of forming a deformed layer and closely adhering to the steel surface occurs, and the peelability of the scale deteriorates.

  Therefore, on the basis of these experimental results and considerations thereof, the temperature is increased in the rapid heating control that is carried out following the soaking atmosphere control in the heating furnace according to the present invention especially for high Cr steel materials having a high Si content. The speed was selected to be 20 ° C./min. Or higher, and the ultimate temperature was selected to be a soaking temperature of + 70 ° C. or higher and less than 1180 ° C. Needless to say, rapid heating control can be used in combination with a high Cr steel material having a Si content of less than 0.1%, and the present method is not limited to a high Si content. In addition, the upper limit of the heating rate is not particularly specified, but considering that the temperature distribution in the steel material becomes non-uniform if it is too rapid (low temperature inside the steel material and high temperature on the steel surface), it is 100 ° C / min. It can be said that it is desirable.

  In addition, it is important that the steel billet after the rapid heating control in the heating furnace is quickly extracted outside the furnace. Preferably, it should be extracted within 15 seconds after the end of the rapid heating control. This is because a new subscale is formed in the lower layer of the subscale that has been destroyed by maintaining the high temperature state after rapid heating for 15 seconds or more.

(Example 2 group)
Example α
F steel (Fe-3.94% Cr-0.02% C-0.50% Si-3.005Mn-0.003% S-0.022Al shown in Table 2 described in Example 1 group) % -0.003% Ca steel), the billet of Example No. Soaking atmosphere treatment in a heating furnace under the conditions of 34 (soaking temperature: 1005 ° C., steam concentration in the furnace: 35 vol.%, Oxygen partial pressure in the furnace: 2.9, steam concentration / oxygen concentration ratio: 12) For 15 minutes, and the sample (A) extracted outside the furnace and the same F steel were subjected to the same soaking atmosphere treatment for the billet for 15 minutes, and then the heating rate at the outlet side in the heating furnace in the same atmosphere 30 ° C./min. The surface flaws of the steel material of the sample (B) immediately subjected to rapid heating treatment for 4 minutes until reaching 1125 ° C. and immediately extracted (within 15 seconds) were investigated. Both samples are extracted after heating furnace until they become wire rods with a diameter of 10 mm by conventional method (heating furnace extraction → high pressure water descaling → rough rolling → finish rolling → winding machine → cooling on stealmore conveyor → tabbed) Five pieces of wire were sampled from the upper, middle and lower portions of the rolled and wound coil. In investigating surface wrinkles, each of these wire pieces was resin-embedded, the surface of the wire cross section was observed with an optical microscope, and indentation wrinkles of 0.01 mm or more and less than 0.020 mm were counted. As a result of this investigation, the total number of scissors of 0.01 mm or more and less than 0.020 mm in a total of 15 cross sections was 57 for sample (A) and 16 for sample (B). In addition, no wrinkles of 0.020 mm or more were found in both samples (A) and (B).

Example β
Steel I shown in Table 2 (Fe-1.03% Cr-0.37% C-0.21% Si-0.08Mn-0.2Mo-0.020% S-0.020Al% -0.002% Example No. 5 shown in Table 4 for billets of (Ca steel). Soaking atmosphere treatment in heating furnace under 36 conditions (soaking temperature: 1071 ° C., steam concentration in furnace: 30 vol.%, Oxygen partial pressure in furnace: 2.7, steam concentration / oxygen concentration ratio: 11) Is applied to the sample (C) extracted outside the furnace and the same G steel billet is subjected to the same soaking atmosphere treatment for 15 minutes, and the temperature rise rate is further increased at the outlet side in the heating furnace in the same atmosphere. 20 ° C./min. Then, a rapid heating treatment was performed for 3 minutes until reaching 1152 ° C., and the surface flaw of the steel material of the sample (D) immediately extracted (within 15 seconds) was investigated. Manufacturing conditions such as rolling after extraction from the heating furnace, sampling conditions, evaluation conditions for soot, and the like were the same as in the previous example α. As a result of this investigation, the total number of ridges of 0.01 mm or more in a total of 15 sections was 26 for sample (C) and 3 for sample (D). In addition, no wrinkles of 0.02 mm or more were found in both samples (C) and (D).

  As demonstrated from the results of Example α and Example β, Si is 0.1% by mass or more by combining the soaking atmosphere control and the rapid heating control in the heating furnace according to the present invention. Compared with the single implementation of soaking atmosphere control, the scale peelability is remarkably improved in the high Cr steel material with a high content, and the fine surface flaws of less than 0.02 mm are greatly reduced. High Cr steel products can be provided.

Claims (3)

  In a method in which a billet of steel containing Cr: 0.2 to 4.0% by mass is heated in a combustion atmosphere heating furnace and kept soaked, and then extracted outside the heating furnace and hot-rolled, the Cr concentration in the steel Is X mass%, and the water vapor concentration in the heating furnace is (4.47X + 17.1) vol. In an atmosphere in which the ratio of the water vapor concentration and the oxygen concentration (vol.%) In the heating furnace is adjusted to 10 or more, and soaking is maintained for 10 minutes or more at a temperature of less than 1100 ° C. A method for producing a high Cr-containing steel material for cold pressure with good descaling characteristics.   The steel further contains Si: 0.1 to 0.5% by mass, and after the soaking, the soaking temperature is + 70 ° C. or higher and reaches a temperature of less than 1180 ° C. at 20 ° C./min. The method for producing a steel material for high Cr content cold pressure with good descaling property according to claim 1, wherein rapid heating is performed at the above temperature rising rate.   The steel is further C: 0.02-0.6% by mass, Si: 0.1-0.5% by mass, Mn: 0.02-3.0% by mass, S <0.03% by mass, Al < It contains 0.05 mass% and Ca <0.003 mass%, The manufacturing method of the steel material for high Cr content cold pressure with good descaling property of Claim 1 or 2 characterized by the above-mentioned.

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