JPH075963B2 - Manufacturing method of hot rolled steel sheet with excellent mechanical descaling property - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention can be easily descaled by mechanical means such as wire brush work, and is suitable as a conduit tube (wiring protection tube) base material and the like. The present invention relates to a method for manufacturing a soft hot-rolled steel sheet.

<Prior art and its problems> Steel pipes used as wire protection pipes for electric wires, etc. are required to be soft and have good bendability because they need to be bent during piping work. Corresponding softness is also required for the hot rolled steel sheet, which is the material (base material).

On the other hand, in order to impart resistance to corrosion, the above-mentioned pipe is usually manufactured by using a hot-rolled steel sheet that has been descaled by pickling as a raw material, pipe-working this, and then performing plating treatment. It was

However, in recent years, for the sake of rationalization, it has come to be considered to carry out scale removal of the hot rolled steel sheet in the pipe manufacturing process by mechanical descaling (mechanical descaling) instead of pickling.

In this case, the key to adoption or non-adoption of mechanical descaling depends on the mechanical descaling property of the hot rolled steel sheet, but it cannot be said that generally manufactured hot rolled steel sheet has good mechanical descaling property. There wasn't.

However, as a measure to improve the mechanical descaling property of hot-rolled steel sheet, there is also a method of cooling the coil after high-temperature winding at a specific temperature or higher with water as it is, or water-cooling while rewinding the coil that has been high-temperature winding at a specific temperature or higher. Although it has been proposed (Japanese Patent Laid-Open No. 57-9504), this method requires special water cooling equipment for cooling the coil with water and requires a secondary treatment called "coil water cooling". Therefore, it was not put to practical use.

Moreover, when considering the use as a conduit, etc., in the above method, the hot-rolled plate is hardened by the water-cooling treatment so that a soft material cannot be obtained and, for example, the mechanical descaling property is improved. However, since there is a problem that the essential characteristics required for the product deteriorate, it was considered to be more difficult to put it into practical use.

<Means for Solving the Problems> From the viewpoints described above, the present inventor changed the existing rolling equipment and did not require large equipment investment, and the mechanical descaling of the scale generated in the hot rolled coil was performed. As a result of various studies to provide a manufacturing method of hot-rolled steel sheet that can improve the scaling property and sufficiently secure the softness required for the product, the following findings have been obtained. .

(A) Needless to say, the mechanical descaling property of the hot-rolled steel sheet depends on the properties of the formed scale, but in particular, the scale thickness, the constituent components of the scale, and the soundness of the scale layer have a great influence. If methods such as)) reducing the scale thickness, ii) increasing the FeO ratio in the scale to 60% or more, and iii) introducing cracks into the scale are established, mechanical descaling properties will be improved stably and significantly. What you can do.

(B) However, if a very simple means of immediately starting cooling the steel sheet immediately after the hot rolling is finished and winding it when cooled to a specific low temperature region lower than the conventional one is carried out, the Oxidation time is shortened and the thickness of the produced scale becomes thin, and the rapid cooling increases the FeO ratio in the scale.In addition, this rapid cooling increases the thermal expansion difference with the base steel and causes microcracks in the scale. A favorable effect for improving mechanical descaling, which is called introduction, can be obtained.

That is, the scale of the steel sheet produced at high temperature is FeO,
When the steel sheet is cooled, it is usually heated at a temperature of 570 ° C [FeO → Fe ₂ O ₃
+ FeO] transformation occurs. However, if the steel sheet after hot rolling is rapidly cooled to increase the passing speed of the transformation point, the transformation is suppressed and FeO remains,
The mechanical strength of the scale is reduced. Moreover,
If water cooling in a cooling bed, which is usually performed immediately after the end of rolling, is performed up to a specific low temperature range, many microcracks will occur at the interface of the base iron due to the difference in thermal expansion with the base iron.

(C) However, if the rolled steel sheet is immediately cooled to a low temperature range, it may cause hardening of the material, which may make it unsuitable as a steel sheet for a conduit tube base material that is required to be soft and have good workability. However, adding a small amount of B to the material steel eliminates the above-mentioned concern, and it is possible to maintain the softness of the steel sheet to the extent that it does not cause any particular inconvenience even if it is rapidly cooled to a low temperature range to some extent. To become.

The present invention has been made based on the above findings. "C: 0.02 to 0.10% (hereinafter,% representing a component ratio is a weight ratio), Mn: 0.05 to 0.35%, Al: 0.010 to 0.030% , B: 0.0010 to 0.0050% of steel, the balance of which is substantially Fe, is hot-rolled in a temperature range of ₃ or more points of Ar, and then wound at 350 to 500 ° C.
It is characterized in that it is possible to stably manufacture a hot-rolled steel sheet that is excellent in mechanical descaling property and exhibits sufficient softness at low cost. "

Here, the reasons why the composition and the rolling / winding temperature of the steel targeted by the present invention are limited as described above will be explained.

(A) Steel composition C: C is an element that controls the strength of the steel sheet, and its content is 0.
If it exceeds 10%, it becomes impossible to obtain a soft plate required for conduit pipe materials and the like. As described above, in the case of the present invention, the lower the C content is, the more preferable. However, in consideration of the steelmaking cost, it is practical to limit the lower limit of the C content to 0.02%. Therefore, the C content is set to 0.02 to 0.10%, but preferably adjusted to 0.03 to 0.07%.

Mn: Mn is also an element that governs the strength of the steel sheet like C, and the lower the content, the better. And the Mn content is especially 0.
If it exceeds 35%, it will not be possible to obtain the soft plate required for conduit pipe materials, etc. On the other hand, if the content is 0.05% or less, it will lead to a disadvantage in steelmaking cost, so the Mn content is 0.05 to 0.35%. However, preferably 0.08 to 0.15
It is good to adjust to%.

Al: Al is a necessary component as a deoxidizer for steel, but if its content is less than 0.010%, a sufficient deoxidizing effect cannot be obtained, while
Even if the content exceeds 0.030%, the deoxidizing effect will be saturated and the cost will increase, so the Al content is 0.010 to 0.0
It was set at 30%.

B: B has a hardening at the time of low-temperature winding, especially a remarkable hardening that suppresses an increase in the yield point. Therefore, even if the treatment according to the present invention is applied, it is an essential component for ensuring the desired flexibility. However, if the content is less than 0.0010%, it is not possible to prevent the yield point from rising during low-temperature winding. On the other hand, if the content exceeds 0.0050%, the above effect is saturated, and B
Precipitates such as carbides increase and ductility decreases. Therefore, although the B content was set to 0.0010 to 0.0050%,
It is preferably adjusted to 0.0020 to 0.0030%.

Fig. 1 shows C: 0.04%, Si: 0.01%, Mn: 0.12%, P: 0.018%,
S: 0.008%, Al: 0.012%, N: 0.0021%, the balance is essentially Fe
Steel as a basic component, with and without addition of 0.0041% B to the slab (25
(0 mm thickness) is hot-rolled, finish-rolled at 890 ° C to make a 1.6 mm-thick steel strip, and the results are shown in which the yield strengths of those rolled at various temperatures are compared. From FIG. 1 also, it is clear that the yield stress of the hot-rolled steel sheet is remarkably reduced by adding a small amount of B.

Since B is an element having a strong affinity with N, B
In order to maximize the effect of addition, it is desirable to keep the N content in steel as low as possible.

(B) Rolling / winding conditions Rolling temperature: Rolling at a temperature lower than the Ar ₃ point is a state in which pro-eutectoid ferrite is produced, so ferrite is processed and strain remains, resulting in workability of steel sheet. Will cause a decline.
Therefore, hot rolling must be carried out in a temperature range of Ar ₃ points or higher.

Winding temperature: Mechanical descaling is not sufficiently improved when winding is carried out in the temperature range above the eutectoid transformation temperature above 500 ° C, while winding at a temperature range below 350 ° C results in low temperature winding. Since the steel hardens, the coiling temperature was set to 350-500 ° C.

As mentioned above, the present invention limits the content of C and Mn,
After hot rolling a steel containing a small amount of B, it is simply cooled (water-cooled) to a specific low temperature range in an ordinary cooling bed, and has sufficient softness as a base material for conduit pipes. Moreover, it is intended to obtain a hot-rolled steel sheet whose surface scale has excellent mechanical descaling properties that are practically satisfactory and which can be obtained stably and at low cost. Hereinafter, the present invention will be compared with Comparative Examples. However, a specific description will be given with reference to examples.

<Example> Example 1 First, C: 0.03 to 0.06%, Si: 0.01% by a normal method.
Or less, Mn: 0.08 to 0.14%, P: 0.028% or less, S: 0.016% or less, A
l: 0.012-0.026%, N: 0.0013-0.0038%, B: 0.0025-0.00
A steel containing 41% and the balance essentially Fe was melted.

Then, this molten steel is continuously cast to obtain a slab with a thickness of 250 mm, which is heated to 1200 ° C and then hot-rolled at a finishing temperature of 860 ° C (temperature higher than Ar _{3 of} any steel) to a thickness of 4 mm. The hot rolled sheet of No. 1 was wound at various temperatures.

The relationship between the winding temperature and the mechanical descaling property of the hot-rolled steel sheet thus obtained was investigated, and the results are shown in FIG.

The "mechanical descaling" was evaluated by grinding the surface of the obtained hot-rolled steel sheet with a rotating wire brush, and ○: most of the scale was peeled off, △: only part of the scale was peeled off, ×: scale was It was displayed in three stages without peeling.

As is clear from the results shown in FIG. 2, the hot-rolled steel sheet manufactured according to the conditions of the present invention exhibits excellent mechanical descaling properties, while the winding temperature falls within the range defined by the present invention. It can be seen that the mechanical descaling property drops sharply when the value becomes higher than the above value.

Example 2 A continuous cast slab (250 m
(thickness m) was heated to 1200 ° C., and then hot-rolled at a finishing temperature of 860 ° C. to obtain a 4 mm-thick hot rolled sheet, which was then wound at various temperatures shown in Table 1.

For the hot-rolled steel sheet thus obtained, its mechanical properties, mechanical descaling properties, and Fe
The O ratio was investigated, and the results are also shown in Table 1.

The results shown in Table 1 also show that the hot-rolled steel sheet produced under the conditions specified in the present invention has sufficiently low strength and good ductility, and the FeO ratio in the scale is 60% or more. While exhibiting excellent mechanical descaling properties, when the winding temperature was higher than the range specified in the present invention (test numbers 1, 5 and 6), the FeO ratio in the scale was low, which was sufficient. Mechanical descaling is not ensured, while the amount of C and Mn in steel is It can be seen that when the content is higher than the specified range, the strength becomes too high and the ductility is inferior.

<Summary of Effects> As described above, according to the present invention, it is possible to stably mass-produce a hot-rolled steel sheet that is soft and has good workability and also has excellent mechanical descaling. When applied to materials and the like, it is possible to further improve the production cost and product performance of the product, resulting in extremely useful effects in industry.

[Brief description of drawings]

FIG. 1 is a graph comparing the relationship between the coiling temperature after hot rolling and the yield stress for the B-added hot rolled steel sheet and the B-free hot rolled steel sheet. FIG. 2 is a graph showing the relationship between the winding temperature and the mechanical descaling property of the hot-rolled steel sheet investigated in the examples.

Claims (1)

[Claims] 1. A steel containing C: 0.02 to 0.10%, Mn: 0.05 to 0.35%, Al: 0.010 to 0.030%, B: 0.0010 to 0.0050%, and the balance being substantially Fe, by weight. _A method for producing a hot-rolled steel sheet excellent in mechanical descaling property, which comprises rolling at 350 to 500 ° C after hot rolling in a temperature range of ₃ points or more.