JPS5852443B2 - Method for suppressing steel billet surface cracking during hot rolling - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to carbon steels containing Si and Mn as main components, such as aluminum killed steel, aluminum semi-killed steel, and aluminum/silicon killed steel, which are used for general building materials, shipbuilding, machine structural steel, wire rods, etc. For steel types that contain Nb or V, such as materials for line pipes or oil country tubular goods, hot rolling is carried out immediately after ingot formation or continuous casting (hereinafter referred to as direct rolling), or ingot formation or continuous casting. The present invention relates to a method of suppressing cracking of a steel billet during hot rolling in a process in which the billet is charged into a heat retention furnace as it is after casting and then hot rolled (hereinafter referred to as hot charge rolling).

Conventional steel material manufacturing processes employ the following methods.

In other words, the molten metal melted in a converter or electric furnace is subjected to the following processes: ■ Ingot making - S - Blooming rolling → Mouth □ 0 9 yaw" r - Hot rolling ■ Continuous casting - Giant ff1-[EEl-Hot rolling process ① to ② was passed.

However, in recent years, with the aim of improving productivity by shortening the manufacturing process and saving energy by reducing the unit heat energy consumption, the process of omitting the finishing process and heating process, which are marked with the old mark, has been adopted from the process marked with ■ or ■. The development of the above-mentioned direct rolling or hot charge rolling process technology has been attracting attention.

In order to transition to such a new process, it is natural to guarantee the quality of product materials, but it is also essential to develop measures to prevent the formation of cracks on the surface of steel billets.

In order to develop this process, the inventors of the present invention have spent many years studying methods for manufacturing defect-free steel ingots or slabs, and researching and developing measures to prevent cracking during hot rolling. revealed.

In other words, in the conventional process of reheating and rolling a cold piece, making it into a rough cold piece, removing defects, and then reheating and rolling, the repeated heat treatment of cooling and heating breaks up the cast structure and breaks down the grains during solidification. In addition to reducing segregation and precipitation at boundaries and refining austenite grains, elements such as S, P, and 0, which are harmful to hot workability, are contained within the grains as sulfides, phosphides, and oxides. Fixed.

Therefore, carbon steels containing Si and Mn as main components, or Nb-containing steels, which are used as general structural steels such as aluminum killed and aluminum silicon killed steels, cannot be heated during hot working in conventional reheating and rolling processes. The occurrence of cracks was so slight that they were hardly considered a problem.

On the other hand, in the direct rolling or hot charge rolling process, the above-mentioned segregation and precipitation of various elements occur on dendrite interfaces and austenite grain interfaces during the melting-solidification-cooling process.

For this reason, when tensile stress is applied due to hot working, intergranular cracking occurs and surface flaws occur on the steel piece.

Therefore, in such a new process, it is important to limit the amount of elements harmful to hot workability to the amounts described below, or to control the precipitation characteristics of precipitates.

In the 51-Mn steel targeted by the present invention, P
Hot workability can be greatly improved by suppressing S to 0.02φ or less, S to 0.01% or less, and O (oxygen) to o, ooiφ or less, but desulfurization and dephosphorization are The introduction of processes leads to an increase in production costs;
From an industrial perspective, this may not always be the best solution.

Therefore, the present inventors investigated P and S, which are harmful to hot workability.

Focusing on the fact that segregation and precipitation of elements such as 0 and N occur in a certain temperature range, we developed a method for suppressing 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.

The first method is direct rolling of continuously cast slabs or ingots (Fig. 1 ■), and hot charge rolling (first step
The temperature history and processing history of the steel slab in Figure ■) are schematically shown.

Direct rolling and hot charge rolling are characterized by the fact that, unlike the traditional reheating/rolling process (Fig. 1 ■), the billet is not cooled to room temperature, but is charged into a hot rolling or heating furnace for post-rolling. There is.

In such hot rolling, the passing rolling temperature range is 120
When continuous multi-pass rolling is performed in the temperature range of 0 to 900℃, transverse cracks or edge cracks occur on the surface of the billet during the 1st to 5th passes, and during the subsequent continuous rolling. The cracks expand and remain in the product stream, and if the cracks are severe, the product is often unusable, resulting in a decrease in yield.

Especially p, s, o. In a steel containing N, A6, etc. in a certain mass market, surface cracking of the steel billet becomes noticeable when the first few passes of rolling are carried out in a temperature range of 1140 to 900° C. during direct rolling or hot charge rolling.

As a result of repeated research on the cracking mechanism during hot rolling in this new process using the simulation experiment method described below, the following points have become clear.

That is, (FeMn)S, (F
e, Mn) 0. If #N etc. precipitate at the dendrite interface or austenite grain interface, cracks will occur if a tensile stress of a certain value or more is applied due to hot rolling or the like.

This article describes a simulation experiment method for evaluating the hot workability of steel billets during direct rolling or hot charge rolling.

Using a horizontal tensile tester using electrical heating, a test piece with a cross section of 10 mm was melted and then solidified.
During cooling, the deformation speed is equivalent to hot rolling (50 min/
Uniaxial tension was performed at 5 ec), and the cross-sectional shrinkage rate at each temperature was measured.

When we examined the correlation between the cross-sectional shrinkage rate 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 results are as shown in Figure 2. 1300-90 by simulation experiment method
In steel exhibiting a cross-sectional shrinkage rate of 60 mm or more in a temperature range of 0° C., the frequency of surface cracking during direct rolling or hot charge rolling becomes extremely low.

Conversely, if the cross-sectional shrinkage rate is less than 60φ, surface cracks tend to occur frequently.

Therefore, hot workability was determined to be 1 in simulation experiments.
The minimum cross-sectional shrinkage rate is 60φ in the 300-900℃ temperature range.
It will be difficult to judge whether it exceeds the above.

Using the simulation method described here, melting
An example of the results of investigating grain boundary precipitation characteristics during solidification and cooling is shown in Part 3.
As shown in the figure.

This figure shows 0.13%C-0,2%5i-0 produced by atmospheric furnace melting.
, 4%Mn-0,021%P-0.017φS (all weight φ) was remelted at 20℃/
The metastable (Fe, Mn)S that precipitated at the austenite grain boundaries was solidified and cooled at a cooling rate of
It also shows the temperature at which (Fe, Mn)O precipitates start to precipitate, and the temperature/time curve at which these metastable precipitates start to become spheroidal and coarse.

Note that (FeyMn)S here means a composite precipitate consisting of Fe, Mn, and S.

The precipitation behavior of these precipitates and hot workability are very closely related. In the case of
The value of cross-sectional shrinkage in the temperature range of 0 to 900°C is also 60% or less, and in severe cases it may be less than 10%.

On the other hand, even when directly processed in the temperature range of 1140 to 900°C with a thermal history that follows the ■ curve in Figure 3, the hot workability is very good (susceptibility to surface cracking is small). Further, the cross-sectional shrinkage ratio in the temperature range of 1140 to 900° C. in the simulation experiment also shows a value of 60φ or more.

Therefore, one method to improve hot workability is to spheroidize and coarsen metastable sulfides or oxides or their composite precipitates that precipitate at the dendrite or austenite grain interface during melting-solidification and cooling. .

Furthermore, the presence of phosphorus is often observed at the interface between these harmful metastable substances and the matrix.

Further, other precipitates that inhibit hot workability include nitrides such as AeN.

However, the precipitation temperature range of AeN is usually lower than the above-mentioned precipitation temperature range of sulfides, oxides, or their composites;
The temperature range is 00 to Ar3 (Ar3 is the temperature at which austenite → ferrite transformation starts).

The following describes a method for suppressing steel billet cracking by rendering harmful precipitates harmless in direct rolling and hot charge rolling processes.

The method uses a melting-solidification process followed by a cooling process in which a
This is a method in which the material is cooled to just below one point (austenite→ferrite+cementite reaction initiation temperature), then reheated and rolled.

In this case, the effect is expected even if the Ar point is not cut by 1 point, but from the viewpoint of ensuring hot workability, the Ar
By lowering the temperature to 1 point or less, the transformation is completely completed, and by subsequent reheating, the austenite grains can be made finer, so this is a very preferable method.

The method of the present invention can be applied to carbon steel whose main component is SiMn, especially to low-cost steel types that do not undergo refining treatments such as desulfurization and dephosphorization. This is a very effective method for direct rolling and hot charge rolling of steel grades containing Ae of 0.40% or less or 0.04% or more in addition to these.

The example shown in Figure 3 is a metastable precipitate that is harmful to hot workability.
(Fe, Mn)S.

It shows the precipitation curve of (Fe, Mn)O, or its composite precipitates, and the spheroidization and coarsening initiation curves of those precipitates. Although this curve may deviate somewhat depending on the basic component system, Basically, this trend remains unchanged.

Incidentally, grain boundary precipitation of A and 6N also has a significant adverse effect on hot workability, but it has been found that by hot rolling according to the method of the present invention, this harmful effect can be minimized to a minimum.

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

Example 1 CO, 16hiroSi 0.03%, MnO, 81%,
PO, 024%, 80.020%, Aeo, 06% (
A smaller specimen (lQmtnφ) was prepared from the continuous casting piece having a composition of (all weight percent), and the simulation experiment described above was conducted.

After melting the specimen at 1450°C, 20°C/sec
After cooling to 550°C, reheating to 1200°C, and tensile deformation (strain rate 5/sec) at a predetermined temperature during cooling at 20°C/sec. This is the 0 curve in FIG.

In addition, curve 2 in FIG. 4 shows the result of cooling the sample to 750 DEG C., reheating it to 1200 DEG C., and then subjecting it to tensile deformation at a predetermined temperature.

Note that the Ar1 transformation temperature of this steel is 640°C.

As is clear from these results, hot workability is significantly improved by cooling to below the Arl transformation temperature and then reheating, and there is no fear of cracking during direct rolling or hot charge rolling.

However, the upper limit temperature of cooling after melting and solidification is Ar,
If the temperature is not lower than the above, the result is that no improvement in hot workability can be expected.

In addition, the temperature of the shallow layer of the continuous cast slab of one bottom steel is 1000℃.
If hot rolling is performed directly at the point where the
There was significant side cracking, and hot rolling in the subsequent process was discontinued.

Example 2 CO, 13%, Si 0.2%, Mn 0.4%,
Po, 021 Hiroshi S0.016 Hiroshi Affl 0.06 (
Steel having a composition of Hot rolled.

FIG. 5 shows the transition of the steel ingot surface temperature at that time.

It took six minutes to complete the mold removal, and it was separately confirmed that solidification had been completed to the center of the steel ingot.

FIG. 6 schematically shows the processing and heat history of the method of the present invention and a comparative example.

Table 1 also shows the results. In the case of the comparative example in which hot rolling was directly performed in the temperature range of 1150 to 1000°C during the cooling process following melting and solidification, cracks on the surface of the steel piece were significant.

On the other hand, when the steel piece is cooled down to just below the Ar point during cooling and then reheated and rolled, cracking on the surface of the steel billet is significantly suppressed.

Example 3 CO, 0521%, SiO, 15%, Mn 0.8
5%.

Po, 024 Section, So, 018 Man, AlO, 04 Hiroshi Nb
A steel having a composition of 0.04 mm and 0.03 mm (all weight percent) is melted in a vacuum melting furnace (25 mm), and a mold (upper part 135 mm square, lower bottom 145 mm square, height 180 mm)
Immediately after casting, the mold was taken out into the atmosphere and the mold was removed, followed by hot rolling.

The steel ingot surface temperature transition at that time is the same as that shown in FIG.

Under the thermal history shown in Figure 6 ■, 550℃ (Ar1
= 640°C), then reheated to 1150°C and immediately hot rolled (continuous 4-pass rolling, rolling reduction of 15 mm for each pass).
However, almost no cracks occurred on the surface of the steel piece.

Example 4 C0, 25%, SiO, 2'%, Mn 0.9%,
Po, 017%, S O, 018%, Ag o, o
3%, TiO, 0.15% (both weight percent) was melted in a vacuum melting furnace and weighed 25 kg).
, mold (upper part 1357 mm flow angle, lower part 145 mm angle, height 1
Immediately after casting to a diameter of 80 mm), it was taken out into the atmosphere, removed from the mold, and hot rolled.

The steel ingot surface temperature transition at that time is the same as that shown in FIG.

Under the thermal history shown in Figure 6 ■, 450'C (Ar
, = 520°C), then reheated to 1100°C and immediately hot rolled (continuous 4-pass rolling, 15% reduction in each pass).
However, almost no cracks occurred on the surface of the steel piece.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of various manufacturing processes, Figure 2 is a diagram showing the correlation between simulation experiment results and surface cracking of a steel billet during direct rolling, Figure 3 is a precipitation curve of metastable and spherical precipitates, and Figure 4 The figure is a chart showing the relationship between cross-sectional shrinkage rate and deformation temperature, FIG. 5 is a chart showing changes in steel ingot surface temperature, and FIG. 6 is a schematic diagram of various processing heat treatments.

Claims (1)

[Claims] 1 In direct rolling or hot charge rolling of steel billets, the steel billet is once heated to Ar1 in the cooling process following melting and solidification.
A method for suppressing surface cracking of a steel billet during hot rolling, which comprises cooling to just below the point, then reheating and rolling.