JPH0733543B2 - Method for producing hot rolled steel sheet with excellent scale adhesion - Google Patents

Description

TECHNICAL FIELD The present invention relates to a general method for producing a hot-rolled steel sheet for working containing 0.20% by weight or less of C, more specifically, working without descaling treatment. -It relates to a method for producing a hot-rolled steel sheet that can be painted and has excellent scale adhesion.

<Prior art> In recent years, due to the progress of rolling technology, it has become possible to manufacture a thin hot-rolled steel sheet in place of the cold-rolled thin sheet, but there is a disadvantage that the pickling cost increases as the sheet thickness decreases, There has been a demand for a hot-rolled steel sheet having excellent scale adhesion, which can be processed → welded → painted without hot picking only by hot rolling.

As an attempt to produce such a hot rolled steel sheet having excellent scale adhesion, a method of cooling a coil wound at high temperature in a non-oxidizing atmosphere after hot rolling (Japanese Patent Laid-Open No. 59-222533, JP JP-A-59-229420 and JP-A-60-77921), quenching a hot-rolled steel sheet and winding it at a relatively low temperature, or quenching a hot-rolled steel sheet to a relatively low temperature. A method has been proposed in which the material is wound up at a temperature and then cooled further in a non-oxidizing atmosphere (JP-A-60-63319).

<Problems to be Solved by the Invention> As described above, a method for manufacturing a hot-rolled steel sheet having excellent scale adhesion has been proposed.

However, in the former manufacturing method, air enters from the edge part of the coil during winding at high temperature and between the winding and the insertion of the coil into a box in a non-oxidizing atmosphere, and oxidation progresses. , The scale becomes thicker on the whole surface,
Scale adhesion may deteriorate. Even in the latter method where the coiling temperature is low, the scale thickness of the edge part becomes thicker for the same reason during coiling or after coiling and before inserting the coil into a box in a non-oxidizing atmosphere. However, the scale adhesion at the edges may deteriorate.

An object of the present invention is to provide a method for manufacturing a hot-rolled steel sheet which is excellent in scale adhesion over the entire surface of the coil, by further improving the above-mentioned conventional technique.

<Means for Solving the Problems> As a result of studying a method for producing a hot-rolled steel sheet having excellent scale adhesion that can withstand bending and pressing, the present inventors have found that the profile and the steel sheet during hot rolling are used. By controlling the coiling temperature and the re-heating of the hot-rolled steel sheet, the manufacturing conditions of the hot-rolled steel sheet satisfying the above characteristics were found.

That is, in the present invention, in producing a hot-rolled steel sheet containing 0.20% by weight or less of C, there is a reduction in sheet thickness (edge / drop) from a point 125 mm to a point 25 mm from the edge in the sheet width direction.
Hot rolling is performed to 15 μm or less, the hot rolled steel sheet is wound at a temperature of 600 ° C. or less, and then reheated to a temperature of 600 ° C. or more and 700 ° C. or less in a non-oxidizing atmosphere to It is intended to provide a method for producing a hot-rolled steel sheet having excellent scale adhesion, which comprises soaking for 10 hours and then cooling at a cooling rate of 50 ° C./hour or less.

The present invention will be described in detail below.

The present invention can be applied to steel types containing 0.20% by weight or less of C.

If the C content exceeds 0.20% by weight, most of the C in the steel becomes Fe ₃ C, which deteriorates the scale adhesion, which is not preferable.

There are no particular restrictions on the other components in the steel.

However, the following ranges are preferable for the main components.

Si: 0.10 wt% or less Mn: 0.05 to 2.0 wt% P: 0.03 wt% or less S: 0.02 wt% or less Al: 0.005 to 0.10 wt% N: 0.01 wt% or less Reinforcing elements such as Ti, Nb, V: In total 0.1% by weight or less If Si content exceeds 0.1% by weight, the surface properties deteriorate.

If the content of Mn exceeds 2.0% by weight, the workability deteriorates. On the other hand, if it is less than 0.05% by weight, crack defects are likely to occur during hot rolling.

If the content of P exceeds 0.03% by weight, workability deteriorates.

S is detoxified as MnS by Mn, but the content of S is
If it exceeds 0.02% by weight, a large amount of corresponding Mn is required,
Workability deteriorates.

Al plays an important role in fixing O and N,
The effect is saturated when its content is about 0.10% by weight. Further, if the content is less than 0.005% by weight, O cannot be sufficiently fixed during casting, blow holes frequently occur, and cause cracking during processing.

When the content of N exceeds 0.01% by weight, it becomes difficult to secure workability.

In addition to these, there are Ti, Nb, V, etc. as strengthening elements, but the effect is saturated when the total amount of them is 0.1% by weight.

Further, in the present invention, the above-mentioned steel types are used, and hot rolling is performed by controlling so that the reduction in plate thickness (hereinafter referred to as edge drop) from 125 mm to 25 mm from the end in the plate width direction is 15 μm or less. Then, after winding the steel sheet into a coil at a temperature of 600 ° C. or lower, it is reheated to a temperature of 600 ° C. or higher and 700 ° C. or lower in a non-oxidizing atmosphere and soaked for 2 to 10 hours,
Then, it is cooled at a cooling rate of 50 ° C./hour or less.

Controlling the edge drop has a remarkable effect on improving the scale adhesion of the coil edge portion.

The fact that the edge drop is more than 15 μm means that the plate thickness of the coil edge portion is thin. The edge portion is susceptible to air oxidation. When the edge part is oxidized, the thickness of the base iron layer (Fe layer) further decreases and the scale layer (iron oxide layer) becomes thicker, but if the proportion of Fe ₂ O _{3 in} iron oxide increases, the post-process In the reheating, soaking, and cooling processes, the reverse transformation and transformation (Fe ₂ O ₃ → FeO → Fe ₃ O ₄ ) of the oxidation state of iron are not sufficiently performed, and Fe ₂ O ₃ remains. Therefore, the scale adhesion of the steel sheet is deteriorated. Therefore, the edge drop from the point 125 mm to the point 25 mm from the edge in the plate width direction is controlled to be 15 μm or less.

In addition, as a method of controlling edge drop, trapezoidal crown work using the geometric effect of roll
Use of rolls, use of stepped back-up rolls in which the length of the back-up roll is shorter than that of the work roll to improve the work roll bender effect, or the work roll is used The use of a work roll shift that shifts the direction is mentioned as a simple thing,
By combining these, the trapezoidal roll may be controlled so that the effective taper length is about 150 mm or more, or the roll bending force is about 100 tons / chock or more.

The coiling temperature of the steel sheet shall be 600 ° C or less.

If the winding temperature is higher than 600 ° C, the oxidation of iron will proceed excessively, and the proportion of Fe ₂ O ₃ with poor scale adhesion will increase,
As described above, since the reverse transformation and transformation of the iron oxidation state in the subsequent step are not sufficiently performed, the scale adhesion becomes poor. Further, abnormally large grains are generated on the surface of the steel sheet, and the material is deteriorated such as roughening of the surface during bending.

In addition, Table 1 shows the edges for scale adhesion of steel sheets.
The effects of drop and winding temperature are shown. Also from Table 1, edge drop 15μm or less, winding temperature 600 ℃
The necessity of doing the following is clear.

The process after the reheating is performed in a non-oxidizing atmosphere, which is also for preventing excessive oxidation of iron and suppressing the formation of a Fe ₂ O ₃ layer having poor scale adhesion.

To create a non-oxidizing atmosphere, an inert gas such as nitrogen, argon or helium may be filled in the apparatus.

The reheating temperature, soaking time and cooling rate greatly contribute to the reverse transformation of the oxidation state of iron in the scale, transformation and scale adhesion.

That is, the scale of the hot-rolled steel sheet wound in the usual 500 to 600 ° C. is, FeO from base steel side in order, Fe ₃ O _4, and consists of three layers of Fe ₂ O _3, in the proportion coiling temperature, etc. Dependent. By reheating and soaking the hot rolled steel sheet in a non-oxidizing atmosphere, Fe ₂ O ₃ and Fe ₃ O ₄ in the scale are reversely transformed into FeO. Then, by gradually cooling the hot rolled steel sheet,
Steel sheet surface and scale - to a base steel interface to FeO Fe ₃ O ₄ is transformed. Of these, Fe ₃ O ₄ generated at the interface is considered to contribute to the improvement of scale adhesion.

The reheating temperature is 600 ° C or higher and 700 ° C or lower.

If the reheating temperature is lower than 600 ° C., the reverse transformation of iron in the scale is not sufficiently carried out, so that the scale adhesion becomes extremely poor.

On the other hand, if the reheating temperature exceeds 700 ° C., abnormally coarse particles are generated on the surface of the steel sheet, and the material deteriorates such as roughening of the surface during bending.

The soaking time is 2 to 10 hours.

If the soaking time is less than 2 hours, the reverse transformation of iron in the scale is not sufficiently carried out, resulting in poor scale adhesion.

On the other hand, when it is kept for 10 hours or more, not only the effect of reheating is saturated, but also abnormally large grains are generated on the surface of the steel sheet, and the material is deteriorated such as roughening during bending.

The cooling rate shall be 50 ° C / hour or slower cooling.

If the average cooling rate after soaking is higher than 50 ° C./hour, the iron in the scale is not sufficiently transformed, so that the scale adhesion becomes poor.

FIG. 1 shows the influence of the reheating temperature and the average cooling rate after reheating on the scale adhesion.

Further, FIG. 2 shows the influence of soaking time on the scale adhesion.

From Fig. 1 and Fig. 2, the reheating temperature is 600 ° C or higher, 7
It is clear that it is necessary to set the soaking time to 00 ° C or less, the soaking time to 2 hours or more, and the cooling rate to 50 ° C / hour or less.

Incidentally, as one of the specific means for carrying out the reheating, soaking and gradual cooling treatments referred to in the present invention, a box annealing furnace used for recrystallization and refining of a cold rolled steel sheet can be mentioned, which is advantageous. Fits.

<Examples> The present invention will be specifically described with reference to Examples.

(Example 1) Hot rolled steel sheet of JIS standard SPHC class manufactured under conditions of an effective taper length of 0 to 200 mm and a roll bending force of 50 to 150 tons / chock so as to obtain the edge drop shown in Table 1. SRT = 1250 ° C, FDT = 890 ° C, plate thickness 2.0mm) After winding at the temperature shown in Table 1, the width of the hot rolled steel sheet is 20mm from the end of the widthwise direction and 50mm from the widthwise center. , Length 200m
A steel plate of m was cut out.

This was placed in a nitrogen atmosphere, heated at a rate of 40 ° C./hour, reheated at a temperature of 630 ° C. for a holding time of 2 hours, soaked, and then cooled in a furnace at a rate of 20 ° C./hour. .

A test piece with a width of 20 mm and a length of 100 mm is cut out from the hot-rolled steel sheet, a transparent adhesive tape is attached to one surface of this test piece, and the attachment surface is placed outside, and a 3-point bending test with a bending radius of 4 mm and 180 ° is performed. It was

After the test, the tape was peeled off, the amount of scale attached to the tape was measured with a color difference meter, and the value was expressed as brightness to evaluate the scale adhesion of the hot-rolled steel sheet.

The results are shown in Table 1.

(Example 2) JIS standard SPHC class hot rolled steel sheet (SRT = 1250 ° C, FDT = 890
℃, CT = 520 ℃, thickness 2.0mm, edge drop 12μm)
A steel plate with a width of 50 mm and a length of 200 mm cut out from the widthwise central part of the is heated in a nitrogen atmosphere at a rate of 40 ° C./hour, soaked at a predetermined temperature for 2 hours, and then cooled in a furnace for 20 hours. Cooling was performed at a rate of ° C / hour (a), 50 ° C / hour (b) and 70 ° C / hour.

Then, a test piece having a width of 20 mm and a length of 100 mm was cut out from the hot rolled steel sheet, and the scale adhesion was evaluated by the same method as in Example 1.

The results are shown in Fig. 1.

(Example 3) JIS standard SPHC class hot rolled steel sheet (SRT = 1250 ° C, FDT = 890)
℃, CT = 520 ℃, thickness 2.0mm, edge drop 12μm)
A steel plate with a width of 50 mm and a length of 200 mm cut out from the center in the width direction of was heated in a nitrogen atmosphere at a rate of 40 ° C./hour to 60
The temperature was soaked at 0 ° C. (d) or 630 ° C. (e) for a predetermined time, and subsequently, it was cooled by furnace cooling at a rate of 20 ° C./hour.

Then, a test piece having a width of 20 mm and a length of 100 mm was cut out from the hot rolled steel sheet, and the scale adhesion was evaluated by the same method as in Example 1.

The results are shown in Fig. 2.

(Example 4) Low carbon aluminum killed steel having the composition shown in Table 2 was melted in a converter and continuously cast to have a thickness of 240 mm and a width of 9 mm.
Cast a 00mm slab.

Next, this slab is trapezoidal roll, work roll shift,
Use work roll bender, effective taper length 0-25
After hot rolling under conditions of 0 mm, roll bending force of 50 to 150 tons / chock, thickness 2.0 mm, width 910 mm, edge-drop hot rolled steel sheet shown in Table 3 are finished and then water cooled.
It is cooled at an average cooling rate of 40 ° C / sec, wound at the temperature shown in Table 3, and then reheated, soaked and cooled in a nitrogen atmosphere under various conditions shown in Table 3 as well. went.

Next, a test piece having a width of 20 mm and a length of 100 mm was cut out from a point 20 mm from the widthwise end of the obtained hot-rolled steel sheet and from the widthwise center, and the scale adhesion was evaluated by the same method as in Example 1. did.

The results are shown in Table 3.

In addition, in the test pieces of Table 3, for No. 12, 18, and 24, the scale adhesion at the widthwise end portion and the central portion was evaluated at a plurality of locations in the longitudinal direction, and the results are shown in FIG. It is shown in FIG.

Example 5 Medium carbon aluminum killed steel having the composition shown in Table 4 was melted in a converter and then continuously cast to have a thickness of 220 mm and a width of 1 mm.
A 200 mm slab was cast.

Next, this slab is trapezoidal roll, work roll shift,
Use work roll bender, effective taper length 0-25
After hot-rolling under conditions of 0 mm, roll bending force of 50 to 150 tons / chock, thickness 2.3 mm, width 1210 mm, edge-drop hot-rolled steel sheet shown in Table 5, water-cooled, average cooling rate It was cooled at 35 ° C./sec, wound at the temperature shown in Table 5, and subsequently reheated, soaked and cooled in a nitrogen atmosphere under the various conditions shown in Table 5. Next, a test piece having a width of 20 mm and a length of 100 mm was cut out from a point 20 mm from the widthwise end of the obtained hot-rolled steel sheet and from the widthwise center, and the scale adhesion was evaluated by the same method as in Example 1. did.

The results are shown in Table 5.

In addition, for the No. 26, 33, and 39 of the test pieces in Table 5, the scale adhesion at the widthwise end portion and the central portion was evaluated at a plurality of points in the longitudinal direction, and the results are shown in FIG. It is shown in FIG.

(Example 6) An extremely low carbon aluminum killed steel having the composition shown in Table 6 was melted in a converter and then continuously cast to a thickness of 200 mm,
A slab with a width of 1200 mm was cast.

Next, this slab is trapezoidal roll, work roll shift,
Use work roll bender, effective taper length 0-25
After hot rolling under conditions of 0 mm, roll bending force of 50 to 150 tons / chock, thickness 1.4 mm, width 1210 mm, edge-drop hot rolled steel sheet shown in Table 5, water-cooled, average cooling rate It was cooled at 35 ° C./sec, wound at the temperature shown in Table 7, and then reheated, soaked and cooled in a nitrogen atmosphere under the various conditions shown in Table 7.

Next, a test piece having a width of 20 mm and a length of 100 mm was cut out from a point 20 mm from the widthwise end of the obtained hot-rolled steel sheet and from the widthwise center, and the scale adhesion was evaluated by the same method as in Example 1. did.

The results are shown in Table 7.

In addition, in the test pieces of Table 7, for No. 42, 48, and 54, the scale adhesion at the widthwise end portion and the central portion was evaluated at a plurality of points in the longitudinal direction, and the results are shown in FIG. It is shown in FIG.

Example 1 examined the influence of edge drop and winding temperature on the scale adhesion of a steel sheet. As is clear from Table 1, when the edge drop was 15 μm or less and the winding temperature was 600 ° C. or less, the scale adhesion was good, but when the edge drop was more than 15 μm, the coil end When the winding temperature is higher than 600 ° C, the scale adhesion on the entire surface of the coil was poor, and in addition, the surface condition was deteriorated during the bending process (rough skin).

Example 2 examined the influence of the reheating temperature and the average cooling rate after the reheating on the scale adhesion of the central portion of the steel sheet. As is clear from Fig. 1, the reheating temperature is 60
Above 0 ℃ and below 700 ℃, the average cooling rate after reheating is 50 ℃
If the reheating temperature was less than 600 ° C, the scale adhesion was extremely poor, and if the reheating temperature was more than 700 ° C, the average cooling rate was Even at 50 ° C./hour or less, the scale adhesion was slightly deteriorated, and in addition, abnormally coarse grains were generated on the surface of the steel sheet, and the surface condition was deteriorated during bending (rough skin).

Example 3 examined the influence of soaking time on the scale adhesion of the central portion of the steel sheet. As is clear from FIG. 2, when the soaking time was 2 hours or more, the scale adhesion was good, but when the soaking time was less than 2 hours, the scale adhesion was poor. If it is over time,
Not only was the effect of reheating saturated, abnormally coarse grains were generated on the surface of the steel sheet, and the surface condition deteriorated during bending (rough skin).

In Example 4, a low carbon aluminum killed steel was used to manufacture a hot rolled steel sheet under various conditions, and its scale adhesion was examined. As is apparent from Table 3, the hot-rolled steel sheet obtained by applying the present invention was significantly superior in scale adhesion to the comparative example. Further, as apparent from FIGS. 3 and 4, the hot rolled steel sheet obtained by applying the present invention is
The scale adhesion was good over the entire area not only in the width direction but also in the longitudinal direction.

In Example 5, a medium-carbon aluminum killed steel was used to produce a hot-rolled steel sheet under various conditions, and its scale adhesion was examined. As is clear from Table 5, the hot-rolled steel sheet obtained by applying the present invention was significantly superior in scale adhesion to the comparative example. Further, as is clear from FIG. 5 and FIG. 6, the hot-rolled steel sheet obtained by applying the present invention is
The scale adhesion was good over the entire area not only in the width direction but also in the longitudinal direction.

In Example 6, a hot rolled steel sheet was manufactured under various conditions using ultra low carbon aluminum killed steel, and its scale adhesion was examined. As is apparent from Table 7, the hot-rolled steel sheet obtained by applying the present invention was significantly superior in scale adhesion to the comparative example. Further, as is clear from FIGS. 7 and 8, the hot-rolled steel sheet obtained by applying the present invention had good scale adhesion over the entire region not only in the width direction but also in the longitudinal direction.

<Effects of the Invention> According to the present invention, it is possible to obtain a hot-rolled steel sheet having excellent scale adhesion over the entire width and length directions.

Since the hot-rolled steel sheet produced by the method of the present invention has excellent scale adhesion, it can be used in place of the conventional cold-rolled steel sheet even if the pickling step is omitted, and has extremely high practical value.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between scale adhesion, reheating temperature, and average cooling rate after reheating. FIG. 2 is a graph showing the relationship between scale adhesion and soaking time. FIG. 3 is a graph showing the scale adhesion in the coil longitudinal direction of the test piece cut from the coil width direction end portion in Example 4. FIG. 4 is a graph showing the scale adhesion in the coil longitudinal direction of the test piece cut out from the central portion in the coil width direction in Example 4. FIG. 5 is a graph showing the scale adhesion in the coil longitudinal direction of the test piece cut from the coil width direction end portion in Example 5. FIG. 6 is a graph showing the scale adhesion in the coil longitudinal direction of the test piece cut out from the central portion in the coil width direction in Example 5. FIG. 7 is a graph showing the scale adhesion in the coil longitudinal direction of the test piece cut from the coil width direction end portion in Example 6. FIG. 8 is a graph showing the scale adhesion in the coil longitudinal direction of the test piece cut out from the central portion in the coil width direction in Example 6.

Front page continuation (72) Inventor Kozo Kadoyama 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Technical Research Division

Claims (1)

[Claims] 1. When manufacturing a hot-rolled steel sheet containing 0.20% by weight or less of C, the thickness reduction (edge drop) from a point 125 mm to a point 25 mm from the edge in the sheet width direction is 15 μm or less. Hot-rolled steel sheet to 600 ℃
After winding at the following temperature, 600 ℃ in non-oxidizing atmosphere
As described above, the method for producing a hot-rolled steel sheet having excellent scale adhesion, which comprises reheating to a temperature of 700 ° C. or less, soaking for 2 to 10 hours, and then cooling at a cooling rate of 50 ° C./hour or less.