JPS5852441B2 - Method for preventing surface cracking of steel slabs during hot rolling - Google Patents

Description

Detailed Description of the Invention The present invention involves hot rolling a steel billet immediately after ingot making or continuous casting (hereinafter referred to as direct rolling), or loading the steel billet into a heat retention furnace as it is after ingot making or continuous casting. The present invention relates to a method for preventing cracking of a steel billet during hot rolling in a process in which hot rolling is performed after charging (hereinafter referred to as hot charge rolling).

Conventional steel material processes have adopted the following methods.

In other words, the molten metal melted in a converter or electric furnace undergoes the following process: ■ Ingot-making → Mouth □□□e → m → Blooming → Hiragaki Kenkime → ■ Cover → Hot rolling■ The process was continuous casting - M → long-lasting → hot rolling (■ or ■).

However, in recent years, with the aim of increasing production volume by simplifying processes and saving energy by reducing heat energy consumption, the finishing process and heating process marked with [)El]t have been omitted from the process of ■ or ■. As a process, the development of the above-mentioned direct rolling or hot charge rolling process technology has been attracting attention.

In order to transfer such a process, it goes without saying that the quality of the product material must be guaranteed, but in addition, it is essential to prevent the occurrence of scratches on the surface of the steel billet.

In order to establish this process, the inventors of the present invention have made efforts for many years to study methods for producing defect-free steel ingots or defect-free cast slabs and to study measures to prevent hot cracking, and as a result, the following points have been clarified. did.

In other words, in the conventional process of cooling a slab, then reheating and rolling it, making it into a cold slab, removing defects, and then reheating and rolling it, the austenite grain size becomes finer and at the same time due to the repeated heat treatment of cooling and heating. ,
Elements such as s, p, o, etc. that have a harmful effect on hot workability are
It is fixed within the grain as sulfides, phosphides, and oxides.

Therefore, the hot workability was also extremely excellent. On the other hand, in the direct rolling or hot charge rolling process, the above-mentioned segregation and precipitation of elements occur on dendrite interfaces and austenite grain interfaces during the melting-solidification-cooling process, which adds tensile stress due to hot working. This causes intergranular cracking and causes surface flaws on the steel piece.

Therefore, in such a new process, it is sufficient to remove elements harmful to hot workability in advance, but for example, the introduction of desulfurization or dephosphorization processes increases production costs and is not always the best option from an industrial perspective. In some cases, this may not be the case.

Therefore, the inventors proposed that P, S, 0. Focusing on the fact that segregation and precipitation of elements such as N occur in a certain temperature range.
We developed a method to prevent hot cracking in steel slabs by controlling the precipitation form of these elements.

The content of the present invention will be explained in detail below.

FIG. 1 schematically shows the temperature and working history of continuously cast slabs during direct rolling (shown in FIG. 1) and hot charge rolling (■ in FIG. 1).

Direct rolling and hot charge rolling are characterized in that, unlike the conventional process (Fig. 1 ■), the billet is hot rolled or charged into a heating furnace and then rolled without being cooled to room temperature.

120, which is the normal rolling temperature range in such hot rolling.
When continuous multi-pass rolling is performed in the temperature range of 0 to 900℃, horizontal cracks, vertical cracks, or edge cracks occur on the surface of the steel billet during the 1st to 5th rolling passes, and cracks occur during the subsequent continuous rolling. This spreads and remains as surface flaws, and if these flaws are severe, some products become unusable, resulting in a decrease in yield.

In particular, P, S, 0. When steel containing more than a certain amount of N, A7, etc. is subjected to hot rolling in a temperature range of 1140 to 900°C during direct rolling and hot charge rolling, surface cracking of the steel piece becomes noticeable.

The reason for this is that (Fe, Mn) S-0, which has a peculiar shape during melting, solidification, and cooling, is shown in Figure 2.
-P.

It was found that this is because A7N etc. precipitate at the austenite grain boundaries in the temperature range of 1150 to 900°C.

However, in the present invention, even in direct rolling and hot charge rolling, temperatures above the precipitation start temperature range of elements harmful to hot workability, that is, 1300
By performing processing at least twice with a reduction rate of 15% or more per pass in the temperature range of ~1150℃, the precipitation morphology changes, hot workability is significantly improved, and the surface cracking rate of the steel piece is reduced to 5.
We were able to keep it below %.

The optimum hot working temperature range varies somewhat depending on the component system.
Since hot rolling at 1300°C or higher may cause brittleness due to grain boundary melting, so-called burning, the upper limit temperature for hot rolling was set at 1300°C.

Moreover, the lower limit temperature of rolling is (Fe, Mn) S-0-
P.

However, in the present invention, 1150°C is set as the lower limit because precipitation starts and the frequency of cracking actually increases when the temperature falls below 1150°C.

Regarding the rolling reduction ratio, it is necessary that the austenite grains be refined by annealing and recrystallization in the rolling temperature range.

For this purpose, in the temperature range of 1,300 to 11,500 G, continuous reduction is required at least twice at a reduction rate of 15% or more per pass.

When the rolling reduction is less than 15%, recrystallization does not occur sufficiently, and susceptibility to billet cracking cannot be reduced.

Furthermore, in the case of one-pass reduction, the deformation becomes uneven.
After all, it is difficult to obtain the desired effect.

The important thing is 1 pass 1 in the temperature range of 1300 to 1150℃
Continuous rolling at least twice at a reduction rate of 5% or more is expected to have the effect of crushing the solidified structure, controlling segregation during solidification, and controlling the morphology of precipitates, as well as recrystallizing the austenite grains. This can lead to changes in the morphology of grain boundary precipitates.

What is most desirable is a temperature range of 1250 to 1170°C, with a reduction of 15% or more per pass and a total reduction of 30%.
The above primary rolling is performed.

By performing the second rolling in a temperature range of less than 1150° C. after the above-mentioned first rolling, it is possible to almost eliminate surface flaws on the steel piece.

Furthermore, the steel types targeted by the present invention include Al-killed steel for hot-rolled thin sheets, At-8i killed steel, and 5 for general structural steel.
S41 to 5M50 class, so-called low-grade, inexpensive steel types, especially those that do not undergo desulfurization or dephosphorization (phosphorus) treatment, etc.
This method is particularly effective for steel with an n/S ratio of 50 or less.

Next, a simulation experiment method for evaluating the hot workability of a steel billet during direct rolling or hot charge rolling will be explained.

A specimen with a cross section of 10mm diameter was melted using a horizontal tensile testing machine using electrical heating, and during subsequent solidification and cooling, the deformation rate equivalent to hot rolling (= 50 plates/Se) was applied.
c) Uniaxial tension is performed and the cross-sectional shrinkage rate at each temperature is measured.

After sorting out the correlation between the cross-sectional shrinkage rate value obtained by this experimental method and the surface cracking during direct rolling or hot charge rolling using an actual large-sized hot rolling mill, we found that the simulation results are as shown in Figure 3. 1300-700℃ depending on experimental method
No cracks were observed during direct rolling or hot charge rolling in steels exhibiting cross-sectional shrinkage of 60% or more in the temperature range of .

The cross-sectional shrinkage rate measured by simulation experiment method is 6
It was found that in steel slabs exhibiting a value of 0% or less, surface cracking was actually significant, and there was a good correlation between the evaluation by the simulation experiment method and the actual occurrence of surface cracking.

The contents of the present invention will be explained below based on examples.

Example I C0.12%, 5i (0.01%, Mn 0.35%
, PO, 015%, So, 015%, 00.002%,
A small specimen (101n111φ) was prepared from steel having a composition of A70.06% (all weight percentages), and a simulation experiment was conducted.

A small specimen is melted and then solidified and cooled to 1150 ml.
5”/s after reaching the specified temperature in the temperature range of ~750℃
Uniaxial tension was performed until fracture at a deformation rate of ec.

(The uniform deformation band length of the sample in this test was 1Qii, and therefore the strain rate was 5/sec, which corresponds to the strain rate during normal hot rolling.

) The relationship between deformation temperature and cross-sectional shrinkage rate is shown in the closing part of Figure 3.

In this way, when steel with a high content of P, S, At, etc. is actually subjected to direct rolling or hot charge rolling, surface cracking occurs significantly, and even in simulation experiments, the cross-sectional shrinkage rate shows a poor value of 60% or less. ing.

On the other hand, a small specimen of the same composition was melted and then subjected to tensile-compressive deformation four times in the temperature range of 1,250 to 1,200°C during the solidification-cooling process, with each deformation being equivalent to 15%. After reaching a predetermined temperature in the temperature range of 1150 to 750°C, uniaxial tension was applied at a deformation rate of 507 nm/Sec until breakage.

The results are shown in Figure 3 (■), and it was found that when such treatment was applied, the cross-sectional shrinkage rate was 60% or more at any deformation temperature, and hot workability was significantly improved. .

Example 2 C0.19%, Si0.20%, Mn 0.51%,
An air furnace melted flat steel ingot (55 x 280 x 200 mm) with a composition (product analysis) of P0021%, 80.016%, AtO, 05% (all weight percent) was heated in a heating furnace at 1320°C without being made into a gold ingot. After charging, hot rolling was performed according to the following pass schedule.

The surface temperature during hot rolling was measured using an infrared thermometer.

55mm-38, 5mm-27mm-20mm-16m
yn-13mm30% 30% 25%20% 20% The relationship between the surface temperature after two-pass rolling and the surface crack frequency is
Shown in the figure.

As is clear from FIG. 4, when the surface temperature after two-pass rolling is 1150° C. or higher, almost no surface cracking occurs on the surface of the steel billet.

Example 3 A slab with a thickness of 250 mm was produced by continuous casting of a molten metal containing steel A shown in Table 1 as its main component and impurity elements that inevitably entered the fine particles. is 1
After charging from a state of 000°C to a heat retention furnace at 1320°C, hot rolling was performed.

The results are shown in Table 2. When primary rolling was performed for at least two passes at 1,300 to 1,150°C with a reduction of 15% or more in each pass, almost no surface cracking occurred; If the temperature is outside the range, 1200~
The frequency of cracking increases when continuous rolling is performed in the 800°C temperature range.

Example 4 Steel ingots of steel B and C shown in Table 1 (20 to 30 kg)
was melted using a vacuum melting furnace, poured into a flat mold prepared in advance in the atmosphere, and hot-rolled under various heat histories as shown in Figure 5 during the cooling process following solidification.

That is, in FIG. 5, a is a schematic diagram of direct rolling, b is a hot charge rolling method, C is a schematic diagram of a pseudo hot charge rolling method, and d is a schematic diagram of a normal reheat rolling method.

The steel ingot dimensions before rolling are 55 (thickness) x 280 (width)
x200m7M and 100 (thickness) x200 (width) x15
Two types are used: 0 (length) 7n1rL.

The results are shown in Table 3.

When the conditions of the present invention are met, the frequency of cracking on the surface of the steel piece is very low ([phase] to [phase]), but in other cases, surface cracking occurs frequently (Table 3@- @).

As described above, according to the present invention, surface cracking of steel slabs can be prevented during direct rolling or hot charge rolling, and the effect is extremely significant.

[Brief explanation of drawings]

FIG. 1 is a conceptual diagram of the temperature for one hour in direct rolling, hot charge rolling (2), and reheat rolling (2). Figure 2 shows metastable ((Fe, Mn)S, 0.P) and A
, 5N temperature-hour curve showing grain boundary precipitation characteristics. Figure 3 is a chart showing the relationship between deformation temperature and cross-sectional shrinkage rate based on simulation experiments. FIG. 4 is a chart showing the relationship between the surface temperature after two-pass rolling and the number of cracks on the surface of a steel slab. Figure 5 is a schematic diagram showing various thermal histories, a is direct rolling, b
is hot charge rolling, C is pseudo hot charge rolling,
d is the heat received by the material in each case of normal reheat rolling.
Indicates processing history.

Claims (1)

[Claims] 1 In the temperature range of 1300 to 1150°C as the first rolling in direct rolling or hot charge rolling.
A method for preventing surface cracking of a steel billet during hot rolling, which comprises rolling at least twice at a pass reduction rate of 15% or more, and then performing a second rolling in a temperature range of less than 1150°C.