JP2672392B2 - Method for manufacturing thin-scale hot-rolled steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention eliminates scale defects that adversely affect surface quality, productivity, etc., and reduces the scale thickness to enhance the productivity of descaling by pickling or the like. Therefore, the present invention relates to a method for producing a thin-scale hot-rolled steel sheet in which the difficult-to-peel scale before finish rolling is removed by a predetermined high-pressure water jet.

[Prior Art] Generally, when manufacturing a hot-rolled steel sheet, the raw material slab is inserted into a heating furnace in an oxidizing atmosphere, and is manufactured through each process of rough rolling, finish rolling, cooling, and winding. If the primary scale on the slab that is sometimes formed or the secondary scale that is generated after the primary scale is removed is rolled without being sufficiently peeled off, the surface of the product is hard to leave as a scale flaw. In addition, the scale thickness after hot rolling becomes thicker. When such scale flaws occur, the surface quality is significantly impaired, and it acts as a starting point of cracks when bending and descending, which causes a serious adverse effect on product quality. Although the scale after hot rolling is removed by pickling, if the scale becomes thicker, pickling time becomes longer and productivity is significantly impaired.

Usually, in order to prevent the occurrence of such scale flaws, a water jet descaling device with a supply pressure of about 100 to 150 Kgf / cm ² is installed on the rolling line, which allows the scale on the steel plate surface to be peeled and removed while rolling. It is carried out. However, the quality of scale releasability is strongly influenced by the properties of the scale in addition to the operating conditions of the descaling device, that is, the composition and structure, etc.
In particular, it is known that the primary scale of steel containing a large amount of Si is extremely difficult to peel off. This is due to the fact that Si contained in steel undergoes selective oxidation during high-temperature oxidation during heating to form 2FeO · SiO ₂ (fairlite), which has a large thermoplasticity, and the interface between the scale and metal is complicated and peculiar. This is due to the formation of subscales of the structure. For example, when heat-treating steel containing 0.1% or more of Si, the above-mentioned subscale formation remarkably develops, and as a result, numerous scale flaws remain on the product surface, which can significantly reduce the commercial value of the product. is there.

For example, the following means have been disclosed and proposed as means for avoiding such a problem.

(1) Japanese Patent Publication No. 60-1085 discloses "Si0.10-
During hot rolling of a slab made of 4.00% steel to produce hot rolled steel sheet, the cumulative rolling reduction calculated from the rolling start time is 65% or more, and the billet temperature is 1000 ° C or more within the rolling period. , The descaling by a high-pressure water jet with a supply pressure of 80 to 250 Kg / cm ² is performed for 0.04 seconds or more in the cumulative time, the descaling method during the hot rolling of Si-containing steel. Has been proposed and its gist lies in the following two points. In other words, the descaling property of the eutectic of fairite and wustite has a remarkable temperature dependence, and there are temperature ranges that are easy to descale and temperatures that are not so [iron and steel lecture summary 1982
The details are described in S439. The FET (finishing rolling side temperature) has a worst scale peeling property at about 900 ° C], and the FET is set to 1000 ° C or more to avoid this difficult peeling region.
Fairlites generated during slab heating penetrate into the ground iron in a mosquito-like form and have a complicated interfacial structure. Therefore, this is not the case in descaling with high-pressure water, especially when the rolling reduction is small (for example, the initial stage of rough rolling). Although it is impossible to remove by peeling off, the erosion region floats on the surface of the steel sheet as the rolling progresses, so that the descaling property improves at a cumulative rolling reduction of 65% or more. That is, it is most effective to perform descaling with high-pressure water immediately before finish rolling.

(2) As a method for reducing the scale thickness, some methods for suppressing the scale growth after hot rolling have been proposed, and in Japanese Patent Laid-Open No. 56-93821, a steel sheet is rapidly cooled immediately after finish rolling. It is intended to reduce the scale thickness on the surface of the hot rolled steel sheet.

(3) In addition, "a method of covering the space between the finish rolling mill and the winding machine with a tunnel type compartment of non-oxidizing gas (Japanese Patent Laid-Open No. 53-4366).
No. 1) "or" A cooling bath is installed in the section from the finishing mill to the winding machine, and the steel sheet after hot rolling is immersed in it for cooling, and the entire section including the cooling bath is a tunnel type. Method of covering with a compartment to create a non-oxidizing gas atmosphere (JP-A-54-56)
No. 060) ".

[Problems to be solved by the invention]

However, in various conventional examples as described above,
In the case of (1), in order to perform rolling under operating conditions that satisfy the above-mentioned points regarding the scale flaw prevention method, a high temperature FET is required.
Therefore, the slab heating temperature becomes high, and the productivity deteriorates due to an increase in fuel consumption rate and an increase in scale loss. Further, there are problems that the rolling work becomes complicated because various restrictions are applied to the rolling reduction and the descaling time. Further, the publication does not mention the influence of the descaling state before finish rolling on the scale thickness of the steel sheet surface after hot rolling. When the hot-rolled steel sheet is not used as it is, it is necessary to remove the surface scale mainly by pickling. In this case, if the scale thickness is large, there is a problem that the pickling time becomes long, and a decrease in work efficiency cannot be avoided. Further, in the method (2), since there is a measuring instrument such as a thermometer and a plate thickness gauge on the exit side of the finishing rolling mill, rapid cooling cannot be performed for a certain period of time, and therefore a sufficient effect cannot be obtained.

Further, in the method shown in (3), all of them require a large amount of equipment and also have technical problems in the gas sealing method, etc., so that the method cannot be practically used.

The present invention has been made in view of the above conventional problems, and an object of the present invention is to solve the above problems by subjecting a steel sheet to high pressure water spraying under predetermined conditions prior to finish rolling. .

[Means for solving the problem]

This invention, in order to achieve the above object, when producing a hot-rolled steel sheet by hot rolling steel containing 0.1% by weight or more of Si, prior to finish rolling, collision pressure per unit spray area is 20 g / mm ² or more and 40 g / mm ² or less, and the flow density per unit time was 0.1 / min.mm ² or more and 0.2 / min.mm ² or less. It is a thing.

[Action]

By adopting the above method, the present invention is a simple method that does not suffer from various restrictions such as temperature, reduction rate, descaling time, etc. and does not require special equipment after hot rolling. The scale thickness can be reduced by completely removing the conductive scale before finish rolling. Therefore, as to the reason why the present invention is established, the inventors of the present invention have made it possible to reduce the descaling property of the primary scale before finish rolling and the scale thickness after hot rolling during hot rolling of Si-containing steel that is difficult to scale. Detailed experiments were conducted on the relationship.

The experiment is based on the actual hot strip mill FSB (Finishe
r Scale Braker-High-pressure water descaling device just before the finishing rolling mill), by changing the supplied high-pressure water pressure, water amount, spray spread angle and thickness, and the distance between the nozzle and the steel plate surface, per unit area of high-pressure water spray. The collision force Pt and the flow rate density V per unit time were changed, and the scale thickness at that time was investigated. As a result, the steel plate collision of the high-pressure water spray is not affected by various restrictions such as the high-pressure water supply pressure, the descaling time, the steel plate temperature at the time of descaling, and the cumulative rolling reduction ratio of the steel plate up to the time of descaling, which are based on the conventional knowledge. The scale peeling property can be evaluated by the collision pressure Pt per unit spraying area and the flow rate density V per unit time, and the scale is almost peeled off when Pt is 20 g / mm ² or more and V is 1.0 / min.mm ² or more. In such a case, it was found that the scale after hot rolling can be thinned without suppressing the scale growth by a special device after finish rolling.

The present invention was created based on the above findings. The reasons for limiting the constituent features of the present invention will be described in detail below.

The effect of the present invention is to peel and remove the scale of the Si-containing steel that is difficult to peel. If Si is less than 0.1%, a particularly difficult-to-peeling scale is not formed. Therefore, the lower limit of the Si content is 0.1%.

High pressure water spray collision pressure and flow rate 20g / mm respectively
^The reason for restricting to ² or more and 0.1 / min.mm ² or more is described. The slabs with the composition shown in Table 1 were used as the test steel, and the high-pressure water supply pressure to the descaling header of the FSB immediately before the finishing mill, the amount of water supplied, and the header height (distance between the nozzle and the steel plate) were changed The product was rolled under the conditions shown in the table, and the scale thickness of the product after hot rolling was examined (scale thickness was measured by observing the cross section of the plate with an optical microscope).

The sample steel A is a steel that contains a large amount of Si, the primary scale is difficult to peel off, the finish rolling outlet temperature is high, the winding temperature is high, and the scale thickness tends to be thick.

The detailed survey results are shown in FIG. The numbers in the figure indicate the scale thickness of hot rolled products. As you can see, Pt is 20
It is clear that the scale thickness sharply decreases when g / mm ² or more and V is 0.1 / min.mm ² or more.

Fig. 2 shows steel sheets for hot rolled products with V set to 0.15 / min.mm ² and Pt descaled to 10g / mm ² (Fig. (A)) and 30g / mm ² (Fig. (B)). The XMA analysis result of the surface section is shown. When Pt is 10 g / mm ² , it can be seen that the subscale layer is formed between the base steel and the scale, and the scale is not completely peeled and removed by descaling. On the other hand, when Pt was 30 g / mm ^2, no subscale monk was observed, and it is presumed that the subscale layer was completely removed by descaling.

FIG. 3 shows the influence of the high-pressure water flow rate density V on the scale thickness of the hot rolled product. As is clear from the figure, when the collision pressure is 30 g / mm ² and V becomes 0.1 / min.mm ² or more, the scale thickness is remarkably reduced.

That is, the action for thinning in the present invention is
It is to completely remove scale by descaling before finish rolling. Furthermore, the higher the collision pressure and flow rate of the high-pressure water spray, the more effective it is for thinning the scale. However, the increase in collision pressure and flow rate causes a huge equipment cost in terms of pump capacity, piping, and pressure resistance of pipe connections. Therefore, Pt
And V are 40 g / mm ² or less and 0.2 / min.mm ² or less, respectively.

〔Example〕

 Hereinafter, the present invention will be described with reference to examples.

When hot rolling a slab made of steel having the chemical composition shown in Table 3, descaling was performed under the conditions shown in Table 4. The scale thickness and scale flaw occurrence rate after hot rolling are shown.

As can be seen from Nos. 1 to 13 relating to Steel B in Table 4, when Pt and V are within the range of the present invention, there is no scale flaw and the scale thickness is as thin as 10 μm or less. If it is out of the range of the present invention, the scale thickness becomes as thick as 20 μm or more. Therefore, the effect of the present invention is clear. Further, it is clear that the reduction of the scale thickness depends only on the conditions of Pt and V, and not on other factors such as the high pressure water supply pressure, the descaling temperature, the descaling time and the magnitude of the cumulative reduction. The scale flaw occurrence rate indicates the rate of scale flaw occurrence area on the surface of the steel sheet.

As is clear from Nos. 14 to 17, the above effects are the same for steels having a Si content of 0.1% or more even when the steel components are different.

In addition, when the Si content is less than 0.1%, as is clear from Nos. 18 and 19, the scale of difficult peeling is not formed, and the effect of the present invention is not recognized.

In the case of the hot-rolled steel sheet thinned by the present invention, the pickling time is about half that in the case where the present invention is not carried out, and the productivity is remarkably improved due to the increase in pickling efficiency.

〔The invention's effect〕

As described above, according to the present invention, Si is 0.1 wt%.
In the steel containing above, among the descaling conditions at the time of hot rolling, if the collision pressure and the flow rate density are within the scope of the present invention, the scale that was conventionally difficult to peel off is easily removed, and the scale A hot-rolled steel sheet having a remarkably thin thickness can be produced, and industrially useful effects such as an increase in productivity without any particular hindrance to pickling can be obtained.

[Brief description of the drawings]

FIG. 1 is a table showing the results of investigating the effects of the collision pressure and the flow rate density of the high-pressure water spray during descaling on the scale thickness, and FIG. FIG. 3A is a diagram showing the distribution of oxygen and Si in the steel plate surface cross section, and FIG. 3 is a diagram showing the effect of the flow density of the high-pressure water spray on the scale thickness.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor, Hishinuma Toshi, Mizushima Kawasaki-dori, 1-chome, Kurashiki City, Okayama Prefecture (no address) Inside the Mizushima Works, Kawasaki Steel Co., Ltd. (72) Tadaro Tanomura, 1 Kawasaki-cho, Chiba City, Chiba Prefecture Kawasaki Steel Co., Ltd. Chiba Steel Works

Claims (1)

(57) [Claims] 1. A upon a steel containing more than 0.1 wt% of Si to produce a hot rolled steel sheet by hot rolling, finish prior to rolling, a unit spraying the collision pressure per area of 20 g / mm ² or more 40 g / mm ² Below, the flow density per unit time is 0.1 / min.mm ² or more 0.2
A method for producing a thin-scale hot-rolled steel sheet, which comprises applying a high-pressure water spray of /min.mm ² or less to the steel sheet surface.