JPH0688062B2 - How to prevent scale defects in hot-rolled steel sheets - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for preventing scale flaws in a hot-rolled steel sheet.

[Prior Art] Generally, when a slab is charged into a heating furnace and heated, silicon, which is an additional element of the slab, is enriched on the surface of the slab, and when this diffuses from the oxidizing atmosphere of the heating furnace to the surface of the base steel, First react with oxygen to form SiO ₂ .

This SiO ₂ then reacts with iron oxide produced on the slab surface in a high temperature oxidizing atmosphere to produce 2FeO, SiO ₂ (firelite), etc., which are low melting point substances. This low-melting substance forms a scale that is deeply bitten into the austenite grain boundaries of the base steel, so even if the scale is removed with high-pressure water after extracting the slab from the heating furnace, surface scale flaws remain. It was

As a method for preventing this scale flaw, Japanese Patent Publication No. 58-11
Methods such as Japanese Patent No. 67 are disclosed. However, these methods are not sufficient, and the steel component S for satisfying the mechanical properties required for the recent hot-rolled steel sheet as well as P are lowered, and the degree of scale flaw of the hot-rolled steel sheet is deteriorated accordingly.
We are trying to strengthen descaling by high-pressure water spraying, but no solution has been reached.

[Problems to be Solved by the Invention] The conventional scale flaw prevention method for hot-rolled steel sheet has the following problems.

(1) Sufficient scale removal cannot be performed only by high-pressure water injection, and the amount of water used is large, which is not preferable from the viewpoint of energy saving.

(2) Generally, the influence of steel composition is a strong factor influencing special steel scale flaws. Until now, measures against Si and S eutectic compounds have included restrictions on heating conditions and composition ranges. In order to satisfy the mechanical properties required for hot rolled steel sheet at the
The scale flaw was worse.

That is, as shown in FIG. 3, the relationship between the P level contained in the slab and the special steel scale flaw is such that as the P level decreases, the occurrence rate of scale flaw increases, and in particular, the low P
In the component region (P ≦ 5 × 10 ⁻³ %), it is 10 times or more that in the high P component region (P ≧ 10 × 10 ⁻³ %). Also, as shown in FIGS. 4 and 5, the scale flaw is formed in the state of holes B and scale C with respect to the base metal A, and the surface of the slab is shown in FIG. Formed as shown.

The present invention has been made to solve the above problems, and particularly in a state where the P component in steel is at a low level,
Moreover, it is an object of the present invention to provide a method for preventing scale flaws in a hot-rolled steel sheet for reducing the occurrence of scale flaws.

[Means for Solving the Problems] The scale flaw prevention method for a hot-rolled steel sheet according to the present invention is a slab having a P (phosphorus) content of 0.01% or less, charged into a heating furnace, heated, extracted, and then used in a rough rolling mill. Rough rolling, in the method for producing a hot-rolled steel sheet to obtain a coil by finish rolling in a finishing mill, a phosphorus-based treating agent was applied to the slab surface before charging the slab into a heating furnace. Later, when performing heating and rolling, the heating condition of the heating furnace is that the extraction temperature is 11
This is a method in which the temperature at the outlet of the rough rolling mill is set to 1000 to 1150 ° C. at a heating time of 2 to 4 hours at 00 to 1300 ° C.

[Operation] In the scale flaw prevention method for hot-rolled steel sheet according to the present invention,
After removing the phosphorus-based treatment agent directly on the slab surface or after removing the oxide film with a grinder, etc.
It is applied at a concentration of 5.0 g / m ² , charged into a heating furnace, and the heat is extracted for rolling. The operating conditions at this time vary depending on the steel type, but the heating time is from 2:00 to 4:00 to 4:00, and the extraction temperature is 1100 to 1300 ° C. Is set to 1000 to 1150 ° C. When a phosphoric acid ester is used as the phosphorus-based treating agent applied to the surface of the slab, the following reaction proceeds in the heating furnace under the above heating conditions.

That is, by this reaction, the peelability of the primary scale in the heating furnace is improved, and the scale is sufficiently removed by the rough descaling operation before the rough rolling.

Therefore, the scale flaw occurrence rate of the hot-rolled steel sheet having a low P component slab is improved by about 40% as compared with the case where the phosphorus-based treatment agent is not applied, and the P component does not remain on the surface of the rolled material after rolling. Further, it is possible to obtain a rolled material having a low P component content and excellent mechanical properties, and having very few scale defects, without adversely affecting the base material.

[Examples] Hereinafter, preferred examples of a method for preventing scale flaws of a hot-rolled steel sheet according to the present invention will be described with reference to the drawings.

First, in FIG. 1, first and second heating furnaces 1 and 2 are shown.
The slab 6 extracted from the slab 6 is roughly rolled by the first and second rough rolling mills 3 and 4, and then passes through a finishing rolling mill 5 including six-stage finishing rolling mill stands F _{1 to} F ₆ and then cooling means. After being cooled by the coil 7, the coil 9 is wound by the winder 8.
Is finished.

The slab used in the present invention is a steel material having a composition range as shown in Table 1 below, and is composed of a low P component in order to have mechanical properties of a predetermined strength.

In order to prevent the scale flaw of the low P component slab 6 and obtain a good quality product, a phosphorus-based treating agent is directly applied to the surface of the slab 6 so as to affect only the surface layer portion that becomes iron oxide in the scale formation process, or Immediately after removing the oxide film with a grinder or the like, it is applied so as to have a P component concentration of 0.1 to 5.0 g / m ² depending on the steel type. If it is less than 0.1 g / m ² , the effect of coating is thin, and if it exceeds 5.0 g / m ² , the cost is high and it is economically disadvantageous.

As described above, the slab 6 coated with the predetermined phosphorus processing agent is charged into the heating furnaces 1 and 2 and heated. The heating time and the heating temperature differ depending on the steel type, but heating is performed for 2 to 4 hours, and the heating extraction temperature is preferably 1100 ° C or higher in order to perform the next step rough rolling suitably by extracting in the range of 1100 ° C to 1300 ° C. The slab extracted at the extraction temperature was roughly rolled at a surface temperature on the outlet side of the rough rolling mills 3 and 4 in the range of 1000 to 1150 ° C.

When the slab 6 is heated under the above-described heating conditions, the phosphorus-based treating agent containing the phosphorus-based ester is heated in the heating furnaces 1 and 2 by
The above reaction proceeds.

As a result of applying the method of the present invention to the steel types as shown in Table 1, the occurrence of scale flaws was reduced by about 40% in terms of area ratio as compared with the case where no phosphorus-based treating agent was applied. Further, in order to compare the method of the present invention and the conventional method, the slab is divided into left and right from the center in the surface width direction, divided into a portion coated with a phosphorus-based treating agent and a portion not coated, and charged into a heating furnace and heated. 2 at a temperature of 1260 ℃
After heating for 20 minutes, it was extracted and rolled. An example of the distribution of scale defects on the surface is shown in FIG. As is clear from FIG. 2, only slight scale flaws were observed on the surface of the steel sheet on which the phosphorus-based treating agent was applied.

[Advantages of the Invention] Since the method for preventing scale flaws of the hot rolled steel sheet according to the present invention is configured as described above, the following effects can be obtained.

That is, by coating the slab surface with the phosphorus-based treating agent and heating and rolling, the peelability of the primary scale in the heating furnace is improved, and the scale of the slab before rough rolling is sufficiently removed.

Therefore, originally, a product having the contradictory characteristics that the mechanical strength of the rolled material is high, the P component is low, and the scale flaw is low can be obtained very easily, and the result is tremendous.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a hot rolling facility showing a method for removing scale flaws of a hot rolled steel sheet according to the present invention, and FIG. 2 is a scale flaw of a portion to which a phosphorus-based treating agent is applied and a portion to which a phosphorus-based treating agent is not applied. Sketch drawing showing the steel plate surface comparing the occurrence state of
3 to 5 are for showing the state of the conventional scale flaw, FIG. 3 is a characteristic diagram showing the P component and the state of the scale flaw, and FIG. 4 is a diagram showing the state of the scale flaw of the steel material. 5 is a perspective view showing a scale flaw of a steel material. 1 is a first heating furnace, 2 is a second heating furnace, 3 is a first rough rolling mill,
4 is a second rough rolling mill, 5 is a finish rolling mill, and 6 is a slab.

 ─────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-54-116308 (JP, A) JP-A-57-165115 (JP, A) JP-B-51-41009 (JP, B2)

Claims (2)

[Claims] 1. A method for producing a hot-rolled steel sheet, comprising heating a slab having a P (phosphorus) content of 0.01% or less in a heating furnace, rough-rolling it in a rough rolling mill, and finish rolling it in a finishing rolling mill. After applying a phosphorus-based treating agent on the slab surface before charging into the furnace, charging and heating the slab into a heating furnace, when performing rolling, the heating conditions of the heating furnace, the extraction temperature is 1100 ~ 1300 ℃. A method for preventing scale flaws of a hot rolled steel sheet, characterized in that the heating time is 2 to 4 hours and the temperature at the rough rolling mill exit side is 1000 to 1150 ° C. 2. The phosphorus concentration of the phosphorus treatment agent is 0.1 to 5.0 g / m ^2.
The method for preventing scale flaws of a hot-rolled steel sheet according to claim 1, which is applied so that