JPH02185915A - Manufacture of tight scale steel plate - Google Patents

Abstract

PURPOSE:To manufacture a steel plate having good and stable tight scale property at high yield by executing the specific soaking, rolling and coiling after casting and solidifying a steel for hot rolled steel plate containing the specific composition of Si and Cr. CONSTITUTION:After casting and solidifying the steel for hot rolled steel containing 0.02-0.2wt.% Si and 0.02-0.2% Cr, the soaking is executed at >=1150 deg.C to develop the first scale having good detachability. After that, this steel is rolled under condition of >=90% draft, <=1000 deg.C roll-starting temp. and <=860 deg.C roll-finishing temp. At the time of roll-starting, the above first scale is detached and after that, easy-to-detaching secondary scale developed with roll is detached until roll-finishing, too. After that, the rolled plate is coiled at <=500 deg.C and then, desirably under low oxygen atmosphere and/or at >=0.5 deg.C/min cooling velocity, this is cooled to the room temp. By this method a third scale having small thickness of <= about 6mum and excellent adhesive strength is formed and, the tight scale steel plate having <=3 of both 90 deg. bending scores in length direction and width direction, is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for manufacturing a tight scale steel sheet suitable for use as a material for manufacturing steel frame building materials, iron furniture, or automobile parts.

<Prior art> In the above-mentioned applications, with the goal of expanding product sales, we are considering changing the material from cold-rolled steel sheets to hot-rolled steel sheets at the expense of reducing manufacturing costs, and Studies are continuing on how to prevent contamination caused by scale peeling during the press processing process.

A proposal to meet this requirement is disclosed in Japanese Patent Laid-Open No. 194112/1983. This proposal, in weight%, C: 0.03-0.25% P≦0.025%Si
: 0.04~0.5% S≦0.01%Mn:
0.3-2. After casting and solidifying steel containing %A+≦0.08%, hot rolling at a temperature of Ar 3 point temperature + 50℃ or less, water injection cooling at a cooling rate of 40℃/second or more is started within 5 seconds, and the temperature is 500℃ or less. This is a method for producing a hot rolled steel sheet having a scale of 8 μm or less in winding thickness at a temperature of 300″C or higher.

<Problems to be Solved by the Invention> The above proposal strictly limits the hot rolling conditions and cooling conditions as described above, so the workability is poor, the scale thickness is 8μ or less, and the 90° bending score ( Radius R = plate thickness x 1
．． Tape was pasted on the bent outer surface of a steel plate piece bent to 90 degrees by pressing a punch rated 5, and then peeled off to determine the scale peeling area ratio (%), which was divided into 9 categories and ranked as a score. ) is 5 or less, that is, Gr5 or less (referring to the 90° bending score of 5 or less). The actual situation is that a 90" bending score of 3 or less has not been obtained.

The present invention also provides 90° in the length direction and width direction of the steel plate.
It is an object of the present invention to provide a method for producing a steel plate with stable tight scaling, which always has a bending score of 3 or less, at a high yield.

Means for Solving the Problems> In order to solve the above problems, the present invention provides (1) weight %
So, st: 0.02-0.2% Cr: 0.02
After casting and solidifying hot-rolled steel containing ~0.2%, 1150
The basic method is to start rolling at a reduction rate of 90% or more after soaking at a temperature of at least 1000°C, finish at a temperature of 860°C or less, and wind up at a temperature of 500"C or less. (2) In weight %, Si: 0.02-0.2% Cr: 0.
After casting and solidifying hot-rolled steel plate steel containing 0.02 to 0.2%, 1
After soaking at 150℃ or higher, rolling with a rolling reduction of 90% or higher is carried out at 100℃.
Start below 0℃, end below 860℃, 500℃
In the following, the second method is to lower the temperature to room temperature in a low oxygen atmosphere after winding. (3) In weight%, Si: 0.02 to 0.2% Cr: 0.02
After casting and solidifying hot-rolled steel containing ~0.2%, 1150
After soaking at temperatures above ℃, rolling with a reduction rate of 90% or more is started at 1000℃ or below, finished at 860℃ or below, and after winding at 500℃ or below, the temperature is lowered to room temperature at a rate of 0.5℃/win or above. as the third means, (4) in weight%, Si: 0.02 to 0.2% Cr: 0.02
After casting and solidifying hot rolled H plate steel containing ~0.2%, 1150
After soaking at temperatures above ℃, rolling with a reduction rate of 90% or more is started at 1000℃ or below, finished at 860℃ or below, and after winding at 500℃ or below, rolling is performed at a speed of 0.5℃/■in or above in a low oxygen atmosphere. The fourth method is to lower the temperature to room temperature.

Generally, the steel for hot-rolled steel sheets used in the industrial application field of the present invention includes, for example, the above-mentioned Japanese Patent Application Laid-Open No. 61194112.
The steel for hot-rolled steel sheets disclosed in the publication, namely C, has a lower limit to maintain the strength of the hot-rolled steel sheet and an upper limit to maintain good peelability of the secondary scale, and Si has a weldability limit. A lower limit is set for maintenance, and an upper limit is set for generating a good product scale. Similar to C, a lower limit is set for Mn to maintain the strong collapse necessary for hot rolled steel sheet applications, and good economic efficiency is maintained. An upper limit is set for both P and S to prevent deterioration of the adhesion of the product scale, and an upper limit is set for AI to maintain the adhesion of the product scale. Upper limits are set to obtain the scale with good economy, and in weight %, C: 0.03-0.25% Mn: 0.2-2
．． %Si: 0.04-0.5% P≦0.025
%S≦0.015% Cr:≦0.1%A1≦0
．． There is a steel for hot-rolled steel sheets containing 0.08%.

Cough steel can also be used in the present invention, and furthermore, in the present invention, Si up to 0.02% and Cr up to 0.2% can be used for the same reason as above. Further, there is no restriction on hot rolling for shape and dimension adjustment before hot rolling, which is limited by the present invention.

<Function> In order to solve the above-described problems, the present inventors repeatedly conducted various experiments and studies using test steel having the chemical components shown in Table 1, and obtained the following knowledge.

Table 1 (% by weight) The present inventors used the 90° bending score, which is used in this industry and technical field to evaluate whether the material has good tight scaling properties, to calculate the We evaluated various hot rolled steel materials.

As a result, steel materials that have achieved a 90° bending rating of 3 or less are
It has been confirmed that this product satisfies the needs of all applications in this field.

As a result of further study based on this fact, the present inventors found that a steel plate that achieves a <90" bending score of 3 or less, as shown in Figure 1, has a product scale thickness of 6 μ- or less, and that As shown in FIG. 2, one condition was found to be that the steel material had a reduction ratio of 90% or more in the final hot rolling (finish hot rolling).

Regarding this point, the present inventors clarified the cause-and-effect relationship and found that when the reduction rate is 90% or more, the surface area of the steel billet where secondary scale is generated decreases, and the encroachment of primary scale during hot rolling decreases. The conditions for adhesion of tertiary scales are improved; ■ Since the amount of reduction for the same sheet thickness increases, the separation of secondary scales during hot rolling is sufficiently performed, and the conditions for the formation of tertiary scales are improved in the longitudinal direction of the steel sheet. It was found that the width direction was improved, and one condition for achieving the object of the present invention was found to be the reduction ratio of hot rolling.

Here, the present inventors have found that although the scale that occurs during the heating stage of steel and the product scale are completely different on the surface, the interface between the base steel and scale seems to have a deep relationship with the scale that occurs during the heating stage of the steel. Focusing on the following three scales,
We organized the knowledge obtained by classifying the species.

The classification of scale is that the scale that occurs during heating is called primary scale, and the scale that is generated and peeled off after the completion of hot rolling after the primary scale peels off after heating is called secondary scale. The scale that appeared between the completion of hot rolling after peeling and the time of winding was defined as tertiary scale.

In this experiment, the inventors tested a steel piece at room temperature at a temperature of 1150°C at the stage where the above-mentioned primary scale was generated, as shown in Figure 3.
We discovered that when heated at temperatures below 1150°C, the 90° bending score, which indicates the peelability of the tertiary scale, worsened to over 3, and when heated above 1150°C, the 90" bending score became 3 or less, achieving the objective. It has been discovered that another condition for achieving the object of the invention lies in the heating temperature.

The present inventors have discovered the relationship shown in Figure 3 above, and the relationship between the holding temperature of the slab following casting and the Cr enrichment shown in Figure 4, as shown by the amount of Cr (wt%) at the interface between the primary scale and the base steel. We noticed that they matched well, and as a result of further investigation, we found that Figures 1, 3,
From the findings in Figure 4, the heating temperature or holding temperature, that is, the soaking temperature at which the surface temperature of the strip before rolling is 1150"C is 11
When the temperature exceeds 50℃, Cr at the interface between the primary scale and the base metal
1L becomes 1% or less, concentration decreases, and the releasability of the primary scale becomes good, which leads to the formation of secondary scale with good releasability, resulting in the formation of a good surface condition of the steel plate. The above-mentioned soaking temperature is still another condition of the present invention because it produces a thin but stable tertiary scale with a thickness of 6 μm or less and a good 90° bending score of 3 or less. I discovered something.

The relationship shown in FIG. 5 was obtained from the results of rolling under such conditions.

Figure 5 shows the relationship between hot rolling start temperature and scale thickness, and as seen in Figure 5, steel sheets with good tertiary scale formation were found to be stable even after taking into consideration the variations in each process capacity. The starting temperature of hot rolling does not exceed 1000°C and the finishing temperature is 86°C.
It was discovered that the steel sheet was manufactured at a temperature not exceeding 0°C.

Further, when the winding temperature exceeds 500° C. as disclosed in JP-A-61-194112, the thickness of the tertiary scale easily becomes 6μ or more, and the 90° bending score exceeds 3, which meets the purpose of the present invention. I found that it became impossible to achieve this.

Also, similar to the disclosure of JP-A No. 61-194112, 0. This is desirable because tertiary scale does not grow after cooling, and the same effect can be obtained by reducing atmospheric oxygen instead of this cooling.If the forced cooling and reducing atmospheric oxygen are used together,
It was found that the effect of preventing the growth of the next scale is even higher, which is preferable.

The present invention has been made based on the above findings.

<Example> Examples of the present invention are shown below along with comparative examples.

(1) Test wJ (shown in Table 2) (2) Manufacturing conditions and obtained results (shown in Table 2) Table 2
As is clear from the above, the fragrances 1 to 6 of the present invention have good releasability of the primary and secondary scales, and are thin but uniform in thickness and are covered with good tertiary scales that adhere to the base metal of the steel plate. was gotten.

On the other hand, in Comparative Example Perfume 7, the soaking temperature (heating temperature) was low, so the peelability of the formed primary scale was poor, the thickness of the tertiary scale exceeded 6 μ■, and the rating was 3 or less. Furthermore, in Perfume 8, the hot rolling reduction ratio was low, the peelability of secondary scale was poor, and tertiary scale with poor adhesion was formed.

In addition, in perfume 9, the starting temperature of hot rolling was too high, and like perfume 8, tertiary scale with poor adhesion was formed, and in perfume 10, the finishing temperature of hot rolling was too high, and perfume 11 The winding temperature was too high, the scale thickness exceeded 6 μm, and the 90° bending score exceeded 3.

<Effects of the Invention> In the present invention, first, the soaking temperature of the steel slab is limited to generate primary scale with good peelability, so that the primary scale is easily peeled off at the start of rolling.

Next, since the start and end temperatures of hot rolling and the rolling reduction rate of hot rolling are limited, the surface on which secondary scale is formed is smoothed and the secondary scale has good peelability, making it thin and stable. At the same time, conditions on the surface of the steel plate are prepared so that a good tertiary scale that adheres to the base steel is formed.

The hot-rolled steel strip is then wound in a temperature range that suppresses and prevents the formation of tertiary scales, so thin and homogeneous tertiary scales with excellent adhesion are formed on the resulting steel sheet, which solves the above-mentioned problems. All resolved.

The steel plate thus obtained always exhibits a 90° bending score of 3 or less in both the length and width directions, which satisfies the needs of the industry and brings great benefits to the industry.

[Brief explanation of the drawing]

Figure 1 shows the relationship between the cubic scale and the 90° bending score.
Figure 2 shows the relationship between hot rolling reduction ratio and tertiary scale thickness, Figure 3 shows the relationship between the 90° bending score of a steel billet and heating temperature, and Figure 4 shows the relationship between the holding temperature of a steel billet and The relationship between the concentration of Cr at the interface between the base iron and the primary scale of the steel slab to be secured is shown, and the fifth
The figure shows the relationship between the start temperature and end temperature of hot rolling and the thickness of the tertiary scale. Patent applicant: Nippon Steel Corporation Agent: Masu Tegori (and 2 others)

Claims (4)

[Claims] (1) After casting and solidifying hot-rolled steel containing Si: 0.02 to 0.2% Cr: 0.02 to 0.2% in weight%, and soaking at 1150°C or higher, the rolling reduction rate is 90%. A method for producing a tight scale steel sheet, characterized in that the above rolling is started at 1000°C or lower, finished at 860°C or lower, and wound at 500°C or lower. (2) After casting and solidifying hot-rolled steel containing Si: 0.02-0.2% Cr: 0.02-0.2% in weight%, and soaking at 1150°C or higher, the reduction rate is 90%. A method for producing a tight scale steel sheet, characterized in that the above rolling is started at 1000°C or lower, finished at 860°C or lower, rolled up at 500°C or lower, and then cooled down to room temperature in a low oxygen atmosphere. (3) After casting and solidifying hot-rolled steel containing Si: 0.02 to 0.2% Cr: 0.02 to 0.2% in weight%, and soaking at 1150°C or higher, the rolling reduction rate is 90%. The above rolling was started at 1000°C or lower, finished at 860°C or lower, and rolled up at 500°C or lower.
A method for producing a tight scale steel sheet, characterized in that the temperature is lowered to room temperature at a cooling rate of 5° C./min or more. (4) After casting and solidifying hot-rolled steel containing Si: 0.02-0.2% Cr: 0.02-0.2% in weight%, and soaking at 1150°C or higher, the reduction rate is 90%. The above rolling was started at 1000°C or lower, finished at 860°C or lower, and rolled up at 500°C or lower.
A method for producing a tight scale steel sheet, characterized in that the temperature is lowered to room temperature in a low oxygen atmosphere at a cooling rate of 5° C./min or more.