JP4295554B2 - Hot rolled steel sheet with excellent scale adhesion - Google Patents

Description

【００６１】
【表２】

【００６２】
【表３】

【００６３】
【発明の効果】
本発明によれば、スケールの密着性を大幅に向上させた熱延鋼板を提供することができる。このため、実際の熱延鋼板の酸洗や冷間圧延時にも、また実際の熱延鋼板の曲げ加工やプレス成形時にも、スケール剥離が少ないかほとんど起こらず、剥離したスケールの押し込みや噛み込みにより、表面品質を損なうと言った問題が生じない。したがって、これら処理や加工時の熱延鋼板表面品質を著しく向上させることが出来る。
【図面の簡単な説明】
【図１】熱延鋼板スケール層中のMnFe₂O₄ 比率とスケールの残留応力との関係を示す説明図である。
【図２】熱延鋼板スケール層中の（Fe,Mn)O の比率とスケールのクラック個数との関係を示す説明図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a hot-rolled steel sheet that is used for pickling in particular, and has excellent adhesion of a scale layer (hereinafter also simply referred to as scale), and heat with less scale peeling that causes indentation flaws in the pickling process. It relates to rolled steel sheets.
[0002]
[Prior art]
Normally, a hot-rolled steel sheet is formed with a secondary scale layer having a thickness of about 10 μm on the surface after hot rolling. If the scale on the surface of the hot-rolled steel sheet has poor adhesion to steel (base metal, steel base), a part of the scale tends to peel off before the pickling process. When this scale peeling occurs, the peeled scale is pushed into the surface of the steel sheet in the pickling process, so that it tends to become an indentation flaw on the surface of the steel sheet. This indentation flaw becomes a surface defect of the hot-rolled steel sheet in an application for pickling. For this reason, a hot-rolled steel sheet having a tight scale that has excellent adhesion to steel and little scale peeling is desired for hot-rolled steel sheets used for pickling.
[0003]
Normally, the secondary scale layer has a three-layer structure. From the bottom, (1) magnetite (Fe_ThreeO_Four, Magnetite seam), (2) mixed structure of magnetite and iron with eutectoid transformation of wustite (FeO), (3) magnetite grown from the surface layer, and hematite (Fe₂O_Three). In general, it is said that the adhesiveness of the scale improves as the amount of magnetite in the scale layer increases, such as the magnetite seam (1) and the eutectoid structure (2).
[0004]
On the other hand, depending on the manufacturing conditions, wustite (FeO) may remain untransformed. In this case, since the breaking strength of wustite is low, the larger the amount of untransformed wustite in the scale layer, the more the scale is cracked and the scale is more easily peeled off, and the adhesion is significantly impaired.
[0005]
Therefore, conventionally, by making the scale as magnetite as possible, cracks are hardly formed in the scale and the adhesion of the scale is improved. Wustite (FeO) to magnetite (Fe_ThreeO_FourThe transformation to) occurs around 560 ° C. Therefore, in the prior art, the transformation rate from wustite to magnetite is promoted by slowing down the cooling rate after winding the steel sheet at the end of hot rolling and sufficiently proceeding with the transformation, thereby improving the adhesion of the scale. I am trying. In addition, the scale layer is made thinner to reduce the thickness of the scale layer (see, for example, Patent Documents 1 to 4).
[0006]
[Patent Document 1]
Japanese Patent Laid-Open No. 11-277105
[Patent Document 2]
JP-A-9-272918
[Patent Document 3]
JP-A-9-271806
[Patent Document 4]
Japanese Patent Laid-Open No. 9-272917
[0007]
[Problems to be solved by the invention]
However, according to the knowledge of the present inventors, especially when the Mn content (Mn concentration) in the steel material is high, the magnetization of the scale is promoted by controlling the cooling rate after winding as described above. In some cases, adhesion is not improved and scale peeling occurs.
[0008]
As a result of detailed investigation of the correlation between the scale composition and the scale adhesion, the present inventors have found that MnFe in the scale layer₂O_FourIt has been found that the adhesion of the scale decreases when and (Fe, Mn) O 2 increase. That is, as in the prior art, even if the scale is magnetized as much as possible and thinned, the MnFe in the scale layer₂O_FourIt has been found that the adhesion of the scale is remarkably reduced when both (Fe, Mn) O and C are increased.
[0009]
In other words, the said prior art had no recognition about the influence on the scale adhesiveness of these Mn oxides in the scale layer of the hot-rolled steel plate surface. For this reason, especially when the Mn content (Mn concentration) in the steel material is high, the adhesion of the scale could not be improved with good reproducibility.
[0010]
The present invention has been made by paying attention to such circumstances, and the purpose of the present invention is to achieve good reproducibility and reliable scale even when the Mn content (Mn concentration) in the steel material is particularly high. An object of the present invention is to provide a hot-rolled steel sheet that can improve adhesion.
[0011]
[Means for Solving the Problems]
  In order to achieve this object, the gist of the hot-rolled steel sheet of the present invention is as follows:In mass%, C: 0.01 to 0.30%, Si: 0.01 to 2.0%, Mn: 0.2 to 2.0%, balance Fe and inevitable impuritiesMnFe contained in the scale layer composed of magnetite on the surface of hot-rolled steel sheet₂O_FourAnd the ratio of (Fe, Mn) O 2 is 1.0% or less by volume ratio, and the residual compressive stress of this scale layer is 400 Mpa or less.
[0012]
In the present invention, the scale layer on the surface of the hot-rolled steel sheet is composed of magnetite, as long as the adhesion of the scale layer is not hindered, in addition to magnetite, hematite (Fe₂O_Three) Etc. are allowed to be contained. The scale layer of hot-rolled steel sheet, especially when containing Mn, is magnetite (Fe_ThreeO_Four) As the main component and the rest is hematite (Fe₂O_Three), MnFe₂O_FourAnd (Fe, Mn) O, which is wustite in which Mn is dissolved in untransformed wustite (FeO). In the present invention, the MnFe in the scale layer₂O_FourAnd (Fe, Mn) O are both reduced as much as possible to increase the MnFe to scale adhesion.₂O_FourAnd (Fe, Mn) O 2 are excluded.
[0013]
Here, each ratio of magnetite and hematite constituting the scale layer on the surface of the hot-rolled steel sheet, and MnFe contained in the scale layer on the surface of the hot-rolled steel sheet₂O_FourEach ratio of or (Fe, Mn) O 2 can be quantified by X-ray diffraction under normal conditions as a volume ratio (vol%) to the scale layer.
[0014]
As described later, when the Mn content in the steel material is high, depending on the production conditions, Mn in the steel material diffuses into the scale on the steel surface, and MnFe₂O_FourForm. Similarly, (Fe, Mn) O, which is a wustite in which Mn is dissolved in untransformed wustite, is increased. When Mn in the steel material diffuses into the scale on the steel material surface, the existence form of the Mn oxide in the scale layer is almost specified by these two Mn oxide forms.
[0015]
Mn in steel or these Mn oxides in the scale layer hinders the eutectoid transformation of wustite, increases the residual stress of the scale, and easily introduces cracks into the scale. For this reason, when these Mn oxide contents in a scale layer increase, the adhesiveness of a scale will be reduced and as above-mentioned, a scale will peel easily before a pickling process.
[0016]
MnFe₂O_FourThe following can be considered as a cause of the influence of the ratio of Mn oxides such as (Fe, Mn) O 2 on the adhesion of the scale. A compressive stress is generated in the scale during cooling after the rolling is completed. When the fracture strength of the scale is low and the adhesion between the scale and the interface is weak, the scale is easily peeled off from the steel due to cracks generated in the scale due to compressive stress. MnFe produced more when the Mn content of steel is high₂O_FourHas a very low fracture strength and a very large difference in thermal expansion coefficient from iron. For this purpose, in addition to the compressive stress generated by the temperature difference during cooling after hot rolling, MnFe₂O_FourSince the compressive stress resulting from is generated, an extremely large compressive stress is generated in the scale. And many cracks generate | occur | produce in a scale due to this compressive stress, and it will peel easily from steel. In addition, Mn diffused in the scale hinders the eutectoid transformation of FeO, and (Fe, Mn) O in which Mn is dissolved in untransformed FeO remains in the scale layer, but this is because the fracture strength is extremely low. Cracks easily and the scale is easily peeled off.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
For this reason, in the present invention, MnFe contained in the scale layer composed of magnetite on the surface of the hot-rolled steel sheet.₂O_FourThe volume ratio is 0.3% or less, and the ratio of (Fe, Mn) O 2 is 1.0% or less. MnFe₂O_FourWhen the volume ratio exceeds 0.3% by volume ratio, the residual compressive stress of the scale layer does not decrease below 400 Mpa, which is the upper limit necessary to ensure adhesion. If the ratio of (Fe, Mn) O exceeds 1.0% by volume, the number of cracks in the scale increases remarkably, and adhesion cannot be guaranteed. Thus, MnFe₂O_FourBoth the ratio of (Fe, Mn) O need to be controlled. For example, if the ratio of (Fe, Mn) O is reduced to reduce the number of cracks in the scale, but the ratio of (Fe, Mn) O is large and the residual compressive stress of the scale layer exceeds 400 MPa, The adhesion of the scale layer cannot be improved.
[0018]
Thus, MnFe in the scale layer₂O_FourThe ratio of the scale is reduced to 0.3% or less by volume ratio, and the production of (Fe, Mn) O in the scale layer is suppressed to the ratio of 1.0% or less by volume ratio. Residual stress can be reduced to 400 MPa or less, and a tight scale having high adhesion without cracks can be obtained. As a result, it is possible to obtain a hot-rolled steel sheet having good scale adhesion when passing through the pickling process and little scale peeling. However, even if the amount of Mn oxide is regulated, the residual stress increases and the adhesion of the scale decreases depending on the manufacturing conditions in the actual hot rolling line such as the water cooling condition after hot rolling. It can happen. For this reason, in the present invention, in order to achieve reproducibility and guarantee of improved adhesion of the scale, the residual stress of the scale is independently defined as 400 MPa or less.
[0019]
Figure 1 shows the MnFe in the scale layer made of magnetite in a hot-rolled steel sheet.₂O_FourThe relationship between the ratio, the residual stress of the scale, and the scale adhesion is shown. FIG. 2 shows the relationship between the ratio of (Fe, Mn) O in the scale layer made of magnetite in the hot-rolled steel sheet, the number of cracks generated in the scale, and the scale adhesion. Figures 1 and 2 use the test steels of A (Mn: 0.22%), C (Mn: 0.95%), and D (Mn: 1.42%), which have different Mn contents, as shown in Table 1 of Examples below. For each hot-rolled steel sheet obtained through a normal hot-rolling process, the ratio of Mn oxide in the scale layer on the surface, the residual stress of the scale was measured, and the adhesion of the scale was further evaluated. .
[0020]
As can be seen from Fig. 1, the residual stress of the scale is the MnFe in the scale layer.₂O_FourThe higher the content, the greater. As can be seen from FIG. 2, the number of scale cracks increases as the (Fe, Mn) O content in the scale layer increases. And the adhesion of the scale is common. These MnFe₂O_FourThe higher the content and the (Fe, Mn) O content, the lower the value. This is, as mentioned earlier, MnFe₂O_FourHas a large difference in thermal expansion coefficient from that of iron.₂O_FourThe reason is that the fracture toughness of the steel is small and brittle, and the cause is not clear, but Mn prevents the eutectoid transformation of FeO and increases the amount of brittle (Fe, Mn) O.
[0021]
Further, from the results of FIGS. 1 and 2, MnFe₂O_FourWhen the ratio of the volume ratio exceeds 0.3% by volume, or when the ratio of (Fe, Mn) O exceeds 1.0% by volume ratio, the scale peeling rate in the scale adhesion evaluation method exceeds level 3 described later. It turns out that it falls. Therefore, in the present invention, MnFe contained in the scale layer made of magnetite on the surface of the hot-rolled steel sheet₂O_FourThe critical significance of the provisions of the present invention can be understood that the ratio of is made 0.3% or less by volume ratio and the ratio of (Fe, Mn) O is 1.0% or less by volume ratio.
[0022]
Here, the ratio of the Mn oxide in the scale layer was quantified by X-ray diffraction. At the same time, the ratio of magnetite and hematite in the scale layer was also measured by X-ray diffraction. As a result, the ratio of magnetite in each of the test steels A, C and D was 85 to 95% by volume (the hematite ratio was 3 to 15%).
[0023]
The residual stress of the scale was calculated by measuring the lattice strain in the vicinity of the steel sheet surface with an X-ray diffraction stress measuring device. In the stress measurement, the scale composition is magnetite (Fe_ThreeO_Four) And Fe with CrK α rays to obtain high-angle diffraction lines_ThreeO_FourThe (511) plane was selected.
[0024]
The number of cracks generated was determined by observing microscopically the cross section in the rolling direction of the scale, measuring the number of cracks generated per 25 μm length in the rolling direction of the scale, and taking this measurement as an average value obtained from 10 fields of view.
[0025]
In this scale adhesion evaluation method, if the evaluation is the above level 2 or less, the scale peeling hardly occurs even by bending or press forming of an actual steel sheet, and the peeled scale is also removed during pickling or cold rolling. The problem of biting and losing surface quality rarely occurs. On the other hand, if the evaluation exceeds the above level 3, scale peeling occurs in the above various cases in an actual steel sheet, and there is a high possibility that a problem of impairing the surface quality of the steel sheet due to biting of the peeled scale occurs.
[0026]
The scale adhesion was determined by applying a tensile strain of 10% to the hot-rolled steel sheet with a tensile tester, then attaching an adhesive tape to the surface of the steel sheet to peel off the scale, and measuring the amount of scale attached to the tape. The scale peeling amount is measured by attaching the tape with the peeled scale to a transparent sheet, capturing the image with a scanner, reversing the brightness of the image, and specifying a range of 300 x 300 pixels. The area was calculated by binarization, and the scale peeling rate = (number of pixels on the peeling scale) / (total number of pixels). For scale adhesion, scale peeling rate of less than 1% is Level 1, 1% to less than 3% is Level 2, 3% to less than 5% is Level 3, and 5% or more Was rated as level 4.
[0027]
This scale adhesion evaluation method is characterized in that it corresponds well with the actual evaluation or situation of scale adhesion (scale peeling) of hot-rolled steel sheets. If the strain in advance of tension is less than 10%, the conditions are too loose, and it becomes difficult to cope with the scale adhesion (peeling) behavior of the actual hot-rolled steel sheet. In addition, even when the tensile strain exceeds 10% in advance, the conditions are too severe and it becomes difficult to cope with the scale adhesion behavior of the actual hot rolled steel sheet. Furthermore, by applying a method in which the scale is peeled off by applying an adhesive tape after pre-straining the tension, it corresponds well with the scale adhesion behavior of an actual hot-rolled steel sheet.
[0028]
These MnFe in the scale layer₂O_FourIn order to reduce Mn oxides such as (Fe, Mn) O 2 as much as possible, it is effective to prevent Mn in the steel from diffusing into the scale layer. The longer the steel is exposed to high temperatures, the more Mn diffusion into the scale layer is promoted, and MnFe₂O_FourIs easily formed. Therefore, it is more fragile to cool in the heating furnace and in the hot rolling in the heating process, during water cooling after hot rolling, etc.₂O_FourIt is difficult to do.
[0029]
For this purpose, in the heating step before hot rolling, it is preferable that the heating temperature is as low as possible and the heating time is as short as possible within a range that can be rolled. The preferred heating temperature range for this is 950-1100 ° C. A normal hot-rolled steel sheet has a heating temperature range of 950 to 1300 ° C. However, when the heating temperature exceeds 1100 ° C., Mn in the steel material tends to diffuse into the scale layer. On the other hand, when the heating temperature is less than 950 ° C., the hot rolling itself becomes difficult.
[0030]
  In addition, if the coiling temperature after hot rolling is too low or the cooling rate after coiling is too fast, the eutectoid transformation of (Fe, Mn) O, which is a wustite in which Mn is dissolved, hardly progresses. , Brittle (Fe, Mn) O tends to remain. In addition, before and after the coiling temperature, MnFe₂O_FourIs not generated, so the coiling temperature should be high so that the eutectoid transformation proceeds as much as possible. However, if it is too high, hematite will be generated and distortion will occur in the scale, making it easy to peel off.The
[0031]
Considering these manufacturing conditions, MnFe₂O_FourThe production conditions for appropriately controlling the production ratio of (Fe, Mn) O 2 and the residual stress in the scale may be determined.
[0032]
Of course, in addition to these conditions at the time of production, it is also effective to keep Mn content in steel low to prevent Mn in steel from diffusing into the scale layer. There are significant limits and limits in terms of component composition (component specifications) and required properties. In other words, the Mn content in the steel can be prevented from diffusing into the scale layer by controlling the various conditions at the time of manufacture, so the Mn content in the steel can be reduced until the above-mentioned changes in the component specifications and the required characteristics are reduced. There is no need to suppress it. That is, in the present invention, even in high Mn steel, Mn in the steel material can be prevented from diffusing into the scale layer, Mn oxide in the scale layer can be reduced, and the adhesion of the scale layer can be improved. There is also.
[0033]
In the present invention, it is assumed that the scale layer on the surface of the hot rolled steel sheet is made of magnetite. More specifically, the scale layer (secondary scale layer generated by hot rolling) on the surface of the hot rolled steel sheet containing Mn, which is the subject of the present invention, is magnetite (Fe_ThreeO_Four) And the rest should be regulated in the present invention.₂O_Three), MnFe₂O_FourAnd (Fe, Mn) O, which is wustite in which Mn is dissolved in untransformed wustite (FeO).
[0034]
Therefore, in the present invention, the fact that the scale layer on the surface of the hot-rolled steel sheet is made of magnetite allows hematite other than magnetite to be included within a range that does not hinder the adhesion of the scale layer. As a practical problem, it is difficult and impractical to make the scale layer only from magnetite. In this regard, it is preferable that the ratio of magnetite in the scale layer is 80% or more by volume ratio. The allowable range of hematite other than magnetite is preferably 20% or less in terms of the total volume ratio of hematite other than magnetite contained in the scale layer. If the ratio of magnetite in the scale layer falls below 80% and the ratio of hematite other than magnetite increases beyond 20%, the adhesion of the original scale layer is significantly reduced, and There is a high possibility that the meaning of reducing the Mn oxide is lost.
[0035]
In order to magnetize the scale layer on the surface of the hot-rolled steel sheet, in the same manner as in the prior art described above, by slowing the cooling rate after winding the steel sheet at the end of hot-rolling and sufficiently proceeding with the above transformation, It is effective to promote the transformation to magnetite. For example, immediately after the end of hot rolling, even if it is rapidly cooled by means such as ordinary accelerated cooling, wustite (FeO) to magnetite (Fe_ThreeO_FourIt is preferable to temporarily hold at a temperature in the vicinity of 560 ° C. where the transformation to) occurs, or to slow down the cooling rate and gradually cool to promote the transformation. In addition, after the scale layer is magnetized, it may be rapidly cooled.
[0036]
Furthermore, the thickness of the magnetite scale layer on the surface of the hot-rolled steel sheet is preferably 6 μm or less. When the thickness of the scale layer exceeds 6 μm, even if the scale layer is magnetite, the scale adhesion is reduced, and the scale is likely to peel off during pickling, cold rolling, or molding. Become.
[0037]
Next, the preferable chemical component and manufacturing method of the hot-rolled steel sheet in the present invention will be described below. The chemical composition range of a normal hot-rolled steel sheet can be applied to the hot-rolled steel sheet in the present invention. The manufacturing method also includes normal manufacturing processes and manufacturing, except for the heating temperature condition that prevents Mn in the steel material from diffusing into the scale layer and the cooling condition after hot rolling that promotes magnetization of the scale layer. Conditions are applicable. This point is also an advantage of the present invention.
[0038]
  Basic chemical composition in the hot-rolled steel sheet of the present inventionRange ofThe surroundings are C: 0.01 to 0.30%, Si: 0.01 to 2.0%, Mn: 0.2 to 2.0%The restPart Fe and inevitable impurities. If necessary, one or more elements of Nb, V, Ti, Cu, Ni, Cr, Mo, B, etc., or one or two elements of Ca, rare earth elements, etc. The above may be contained as appropriate.
[0039]
The basic elements of the content of each element will be described below. In addition, the display of the following element content is all the mass%.
C is an element that ensures the required strength of steel. If the content is less than 0.01%, this effect is insufficient, but if the content exceeds 0.30%, CO gas is generated at the interface between the scale and the steel, so the scale peels off during hot rolling. Is likely to cause scale flaws. Therefore, the C content is preferably in the range of 0.01 to 0.30%.
[0040]
Si is an element for deoxidizing and strengthening solid solution of molten steel. These effects are insufficient when the Si content is less than 0.01%. In addition, when the content of Si is relatively high, such as 0.1% or more, even if the Mn content in the steel material is as high as 1% or more, the firelite generated at the interface is in the Mn scale layer. Demonstrates the effect of suppressing diffusion to MnFe₂O_FourIn addition to the reduction in the amount of formation, the effect of stress relaxation is exhibited, the residual stress of the scale is reduced, and a scale with good adhesion can be obtained. However, if Si is contained in excess of 2.0%, the descaling property at the time of hot rolling is remarkably deteriorated, and scale defects such as red scale are likely to occur. Therefore, the Si content is preferably in the range of 0.01 to 2.0%.
[0041]
Mn is an element for securing the strength of steel. If the content is less than 0.2%, this effect is insufficient, but if the content exceeds 2.0%, the toughness may be reduced. In the present invention, a high Mn hot-rolled steel sheet containing 0.2% or more of Mn, which has a great influence on Mn scale adhesion deterioration, is particularly applicable. Therefore, the Mn content is preferably in the range of 0.2 to 2.0%.
[0042]
Al is an effective element for improving the toughness of steel by capturing solid solution oxygen as a deoxidizing element of molten steel and preventing the occurrence of blowholes. If the Al content is less than 0.002%, these sufficient effects cannot be obtained.On the other hand, if the Al content exceeds 0.10%, the toughness of the steel is deteriorated due to the deterioration of weldability or the increase of alumina inclusions. Deteriorate. Therefore, the Al content is preferably in the range of 0.002 to 0.10%.
[0043]
The following explains other elements other than the basic elements.
Elements such as Nb, V, Ti, Cu, Ni, Cr, Mo, Co, W, and B are contained in a certain amount of one or more, contributing to the strengthening and toughness of steel. To do. For this purpose, it is selectively contained as required.
[0044]
Nb, V and Ti precipitate as carbonitrides in the steel to increase the strength, and the microstructure of the steel is refined to improve the strength and toughness. However, when it is contained in a large amount, the toughness of the weld heat-affected zone is greatly reduced. Therefore, each content in the case of containing these is made into each range of Nb: 0.001-0.10%, V: 0.001-0.1%, Ti: 0.001-0.2%.
[0045]
Cu, Ni, Cr, Mo, Co, W, and B improve the hardenability of the steel and increase the strength. However, when contained in large amounts, the toughness of the steel is greatly reduced. Therefore, each content in the case of containing these, Cu: 0.001-0.3%, Ni: 0.001-0.3%, Cr: 0.001-0.3%, Mo: 0.001-0.3%, Co: 0.001-0.3%, W: The range is 0.001 to 0.3% and B: 0.0003 to 0.003%.
[0046]
Element groups such as Ca, La, Ce, and other rare earth elements (REM) are contained in a certain amount of one kind or two or more kinds and contribute to refinement of steel structure and toughness, respectively. It also has the effect of fixing S, which is harmful to corrosion resistance, etc., and cleaning the steel matrix. For this reason, it is selectively contained as necessary. However, when it is contained in a large amount, the toughness of the steel is greatly reduced. Therefore, the content in the case where it is contained is in the range of Ca: 0.0003 to 0.010% and rare earth element: 0.001 to 0.10%.
[0047]
In addition, S, P, N, O, etc. are impurities, and their content is preferably as low as possible, but these elements may be included in the manufacturing process of steel materials such as steel making, and various properties of the steel sheet of the present invention It is permissible to be contained at a normal impurity level as long as it does not inhibit the above.
[0048]
Next, the manufacturing method of this invention steel plate is demonstrated below. The steel sheet of the present invention can be applied to normal production processes and production conditions except for the heating temperature condition and the cooling condition after hot rolling. That is, a steel slab having the above-mentioned composition range melted by continuous casting or ingot forming is heated after or without appropriate hot working such as ingot rolling, hot forging and thick plate rolling. After that, it is hot-rolled to obtain a hot-rolled steel sheet having a predetermined thickness. And it descals by a pickling process after that and is set as this invention hot-rolled steel plate. The hot-rolled steel sheet is subjected to surface treatment as it is, or is further subjected to cold rolling and surface treatment as necessary to obtain a product steel sheet. Note that the hot working conditions for hot rolling and the conditions for the heat treatment after hot working are appropriately determined according to required characteristics such as required strength and specifications of the steel sheet. Therefore, in addition to normal hot rolling, forced cooling such as online accelerated cooling after hot rolling or controlled rolling may be performed.
[0049]
【Example】
In Table 1, steels of the components indicated by A to E are melted and made into slabs by continuous casting. Then, this is heated in a heating furnace at various temperatures and times shown in Table 2, and normal descaling is performed. Further, in normal hot rolling, as shown in Table 2, the finish rolling temperature was changed and the sheet was rolled to a thickness of 1 to 2 mm. At that time, as shown in Table 2, the cooling rate after finishing rolling is changed to water cooling, and the coil is wound around the coil at various winding temperatures (CT), and the cooling rate after winding is changed to the atmosphere. Cooled in.
[0050]
A sample was taken from the central part in the plate width direction of the hot-rolled steel sheet produced in this way, and the MnFe in the scale layer was measured using the measurement method described above.₂O_FourAnd the ratio of (Fe, Mn) O 2, the residual stress of the scale, and the adhesion of the scale. Here, the ratio of the Mn oxide in the scale layer was quantified by X-ray diffraction. The results obtained are shown in Table 3. The ratio of magnetite and wustite in the scale layer was also measured, and the ratio of magnetite in each of the test steels A to E was 85 to 95% by volume (the hematite ratio was 3 to 15%). .
[0051]
By controlling the heating conditions, heating time, winding temperature, and cooling rate after winding as shown in Table 2, even if the Mn content in the steel is high, as shown in Table 3, MnFe₂O_FourAnd the ratio of (Fe, Mn) O 2 can be controlled, and a scale with good adhesion can be obtained. More specifically, Invention Examples 1, 6, 10, 13, 17, and 18 in which the heating temperature in the hot rolling process is low or the heating time is short, Mn diffusion is suppressed, and MnFe₂O_FourAs a result, the scale residual stress was reduced and the scale adhesion was good.
[0052]
On the other hand, when the heating temperature is high or the heating time is long even when the heating temperature is low, in the case of Comparative Examples 2, 4, 7, 9, 11, 12, 14, 15, MnFe₂O_FourA large residual stress was generated due to an increase in the amount of produced, and the scale adhesion deteriorated.
[0053]
MnFe₂O_FourIn the case of Comparative Examples 3, 8 and 16 where the cooling rate after winding is high and the eutectoid transformation to magnetite does not proceed even when the amount of produced is suppressed, brittle (Fe, Mn) O remains. Cracks occurred and scale adhesion decreased.
[0054]
On the other hand, even in the case where the heating temperature was low and the heating time was short, in the case of Comparative Example 5 where the coiling temperature was too low, the eutectoid transformation did not proceed, cracks were generated, and the scale adhesion decreased.
[0055]
In the case of Invention Examples 17, 18, and 20, where the Mn content in the steel material is 1.45%, which is the highest, the Si content is also as high as 0.5%. Demonstrates the effect of suppressing diffusion into MnFe₂O_FourAs a result, the stress relaxation effect was exhibited, the residual stress of the scale was reduced, and a scale with good adhesion was obtained. However, even in the case of Comparative Example 19 where the cooling rate after winding was too fast even if the Si content was high, untransformed FeO remained and cracks were easily formed in the scale, resulting in a decrease in scale adhesion.
[0056]
Furthermore, even if it is a scale made of magnetite on the surface of a hot-rolled steel sheet, and even if the ratio of (Fe, Mn) O contained in the scale layer is 1.0% or less by volume, it is contained in the scale layer. MnFe₂O_FourIn Comparative Examples 2, 4, 7, 9, 12, 12, and 15 where the volume ratio exceeds 0.3% in the volume ratio, the residual compressive stress of the scale layer exceeds 400 MPa, and the scale adhesion is significantly higher than level 4. Low.
[0057]
And even if it is a scale made of magnetite on the surface of a hot-rolled steel sheet, MnFe contained in the scale layer₂O_FourComparative Examples 3, 5, 8, and 16 in which the ratio of (Fe, Mn) O contained in the scale layer exceeds 1.0% by volume even though the ratio of the volume ratio is 0.3% or less is the number of cracks in the scale. The scale adhesion is significantly lower than level 4.
[0058]
In addition, the comparative example and the invention example having a scale thickness exceeding 6 μm are relatively inferior to the scale in comparison with the comparative example and the invention example having a scale thickness of 6 μm or less.
[0059]
Therefore, from the above results, it can be seen that the critical significance of the present invention regulation and the significance of the preferred regulation for regulating the amount of both Mn oxides contained in the scale layer.
[0060]
[Table 1]

[0061]
[Table 2]

[0062]
[Table 3]

[0063]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the hot-rolled steel plate which improved the adhesiveness of the scale significantly can be provided. Therefore, there is little or almost no scale peeling during pickling or cold rolling of the actual hot-rolled steel sheet, or during bending or press forming of the actual hot-rolled steel sheet. Thus, the problem of impairing the surface quality does not occur. Therefore, the surface quality of the hot-rolled steel sheet during these treatments and processing can be remarkably improved.
[Brief description of the drawings]
[Fig. 1] MnFe in a hot-rolled steel plate scale layer₂O_FourIt is explanatory drawing which shows the relationship between a ratio and the residual stress of a scale.
FIG. 2 is an explanatory diagram showing the relationship between the ratio of (Fe, Mn) O 2 in the hot-rolled steel plate scale layer and the number of cracks in the scale.

Claims (6)

MnFe ₂ contained in a scale layer composed of magnetite on the surface of a hot-rolled steel sheet composed of C: 0.01 to 0.30%, Si: 0.01 to 2.0%, Mn: 0.2 to 2.0%, the balance Fe and inevitable impurities. The scale is characterized in that the ratio of O ₄ is 0.3% or less in volume ratio and the ratio of (Fe, Mn) O is 1.0% or less in volume ratio, and the residual compressive stress of this scale layer is 400 Mpa or less. Hot rolled steel sheet with excellent adhesion. The steel plate according to claim 1, wherein the scale layer on the surface of the hot-rolled steel plate has a thickness of 6 µm or less . The hot-rolled steel sheet according to claim 1 or 2, wherein the ratio of magnetite contained in the scale layer on the surface of the hot-rolled steel sheet is 80% or more by volume ratio . The hot-rolled steel sheet according to any one of claims 1 to 3, wherein the hot-rolled steel sheet further contains Al: 0.002 to 0.10% by mass% . The hot-rolled steel sheet further contains one or more elements of Nb, V, Ti, Cu, Ni, Cr, Mo, Co, W, and B elements in mass%, Nb: 0.001 to 0.10% , V: 0.001 to 0.1%, Ti: 0.001 to 0.2%, Cu: 0.001 to 0.3%, Ni: 0.001 to 0.3%, Cr: 0.001 to 0.3%, Mo: 0.001 to 0.3%, Co: 0.001 to 0.3%, The hot-rolled steel sheet according to any one of claims 1 to 4, which is contained in a range of W: 0.001 to 0.3% and B: 0.0003 to 0.003%. The hot-rolled steel sheet further contains, in mass%, one or more elements of Ca and rare earth elements in a range of Ca: 0.0003 to 0.010% and rare earth elements: 0.001 to 0.10%. The hot-rolled steel sheet according to any one of 1 to 5.

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