JPS5852442B2 - Method for suppressing surface cracking of steel billet 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 processed as shown below ■ Ingot making → ffi → same → blooming rolling + U → zone → hot rolling ■ Continuous i making →! →It went through the process of mouth hot rolling ■ or ■.

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, a process has been developed in which the finishing process and heating process marked [ ] are omitted 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 causes the fracture of the cast structure and the grain size during solidification. In addition to reducing segregation and precipitation in boundaries and refining austenite grains, s, p, etc. have a detrimental effect on hot engineering properties.

Elements such as sulfides, phosphides, and oxides are fixed within the grains.

Therefore, even in carbon steel containing Si)Mn as a main component, or Nb and V steel containing Nb, which is used for general structural steels such as aluminum killed and aluminum silicon killed steel, hot working during conventional reheating and rolling processes is difficult. 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, which causes tensile stress caused by hot working. When stress is applied, intergranular cracking occurs and surface flaws occur on the steel billet.

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, the P content is 0.02% or less, the S content is 0.01% or less, and even 0 ([
Although hot workability can be greatly improved by suppressing the content of 0.001% or less, the introduction of desulfurization and dephosphorization processes in inexpensive steel types will lead 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.

Figure 1 schematically shows the temperature history of continuously cast slabs or ingots during direct rolling (Figure 1 ■) and hot charge rolling (Figure 1 ■). .

Direct rolling and hot charge rolling differ from the traditional reheating/rolling process (Fig. 1 ■) in that the billets are 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°C, transverse cracks or cracks on the edges of the billet occur on the surface of the billet in the 1st to 5th passes of rolling, and subsequent continuous rolling Cracks expand inside the product and remain as surface flaws, and if these flaws are severe, products often become unusable, resulting in a decrease in yield.

Especially P. S, 0. In steel containing more than a certain amount of N, AA, etc., surface cracking of the steel billet becomes noticeable when the first few passes of rolling are carried out in the temperature range of 1140 to 990°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 simulation experiment methods described below, the following points have become clear.

That is, melting-solidification-cooling coma (Fe.Mn)S,
(Fe, Mn)0. If AlN or the like is precipitated 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.

A simulation experiment method for evaluating the hot workability of steel slabs during direct rolling or hot charge rolling (we will explain the trick).

Using a horizontal tensile tester using electrical heating, a test piece with a cross section of 10 mm diameter was melted and then solidified.
Hot rolling during cooling (corresponding deformation speed (50mm/5
ec) to perform uniaxial tension and measure the cross-sectional shrinkage rate at each temperature 1i.

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% or more in the temperature range of 0°C, the frequency of surface cracking during direct rolling or hot charge rolling is extremely low.

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

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

Furthermore, 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-0.017%S (all percentages by weight), after remelting -2
Solidification and cooling at a cooling rate of 0°C/sec, quenching at each temperature, and metastable (Fe,
This figure shows the precipitation onset temperature of Mn)S and (Fe,Mn)0 precipitates, and the temperature/time curve at which these metastable precipitates start to become spheroidal and coarse.

Here, (Fe, Mn)S means a composite precipitate consisting of Fe, Mn, and S.

The precipitation behavior of these precipitates and hot workability are very closely related. When processed, the hot workability was extremely poor (high susceptibility to surface cracking), and furthermore, 1.
The value of cross-sectional shrinkage in the temperature range of 140 to 900°C is also 60% or less, and in severe cases it is less than 10% (sometimes less than 10%).

On the other hand, under the thermal history along the ■curve in Figure 3,
Even when directly processed in the 140-900°C temperature range, hot workability is very good (susceptibility to surface cracking is small), and the cross-sectional shrinkage rate in the 1140-900°C temperature range in simulation experiments is over 60%. shows the value of

Therefore, one method for improving hot workability is melting.
Spheroidizing and coarsening metastable sulfides or oxides, or composite folds thereof, that precipitate at dendrite or austenite grain boundaries during solidification and cooling.

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

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

However, the precipitation temperature range of AlN is usually lower than the precipitation temperature range of the above-mentioned sulfides, oxides, or their composite precipitates, and is in the temperature range of 1000 to Ar5 (Ar3 is the austenite-riferite transformation start temperature).

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

In this method, sufficient retention heat treatment is performed in the temperature range where metastable precipitates precipitate, and the precipitates become spheroidal and coarse, thereby improving workability.

For this purpose, there is a method in which hot rolling is carried out after holding in a temperature range of 1150 to 900° C. for 10 minutes or more.

As shown in Figure 3, when rolling the first pass at 1150°C, wait at least 3 minutes before rolling the first pass again.
When performing at 050°C (in this case, hold for 8 minutes, then hold for 1 more time)
When the pass is performed at 1000°C, the shape of the precipitates changes by holding the roll for 20 minutes, thereby suppressing the formation of surface flaws during hot rolling.

When considering the surface temperature control of the steel billet in actual operation, it is desirable to hold the temperature in the 1150-950°C temperature range for 10 minutes or more before starting rolling.

Therefore, retention for 10 minutes or more is preferred here.

Although this effect can be maintained for a long time, from a practical standpoint, the upper limit is about 60 minutes.

The present invention can be applied to carbon steels whose main component is 51-Mn, especially low-cost steel types that do not undergo refining treatments such as desulfurization and dephosphorization.
n/S ratio is 40 or less, or in addition l' is 0
．． This method is very effective for direct rolling and hot charge rolling of steel types containing 0.4% or more.

The example shown in Figure 3 is a metastable precipitate that is harmful to hot workability.
(Fe2Mn)S, (Fe+Mn)0,
Alternatively, it shows the precipitation curve of the composite precipitate and the spheroidization and coarsening initiation curve of those precipitates. Although this curve may deviate somewhat depending on the basic component system, the basic trend is It doesn't change.

It should be noted that grain boundary precipitation of AlN 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.

The content of the present invention will be explained below based on examples. Example I C0.12%, Si0.01%, Mn0.32%.

Po, 021%, 80.017%, A70.05%.

A simulation experiment was conducted using a small sample (10 mmφ) prepared from a continuously cast piece having a composition of NO. 004% (all weight percentages).

After melting a small test piece, 20°C/5e (5)
Cooling following solidification is performed at a cooling rate of 1200 to 9
After holding in the 00°C temperature range for a certain period of time, uniaxial tension was applied at a strain rate equivalent to hot rolling (5/5ec), and changes in holding conditions and cross-sectional shrinkage were measured.

The results are shown in Figure 4.

3 minutes or more at 1150℃, and 10 minutes at 950% temperature range.
Since the cross-sectional shrinkage ratio shows a value of 60% or more by holding the steel for more than 10 minutes, the occurrence of cracks during hot rolling can be suppressed by applying this treatment even during hot charge rolling or direct rolling.

Example 2 C0, 42%, SiO, 24%, Mn 1.2%, PO,
A simulation experiment was conducted using a small specimen (10 mmφ) prepared from a steel ingot having a composition of 0.022%, 80.018%, and 0.022% Al (all weight percentages).

20℃/s after melting a small specimen at 1425℃
Cooling following solidification was performed at a cooling rate of 120 ec.
After holding in the temperature range of 0 to 900°C for a certain period of time, axial tension was applied at a strain rate equivalent to hot rolling (5/5ec), and changes in holding conditions and cross-sectional shrinkage were measured.

The results are shown in Figure 5. By performing a holding treatment in a temperature range of 1200 to 1000'C for 10 minutes or more, the cross-sectional shrinkage rate shows good hot ductility of 60% or more.

Example 3 C0.13%) Si0.2%, Mn0.4%, PO,0
21%, 80.016%, AA? Steel having a composition of 0.06% (all weight percentages) was melted in a vacuum melting furnace, weighing 25 kg), and a mold (upper part 135 mm square, lower part 145 mm square).
Immediately after casting to a diameter of 180 mm in height, the mold was taken out into the atmosphere, removed from the mold, and hot rolled.

** Figure 6 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. 7 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 rolling is performed after performing a holding treatment during cooling, surface cracking of the steel piece is significantly suppressed.

Example 4 C0.06%t S t 0.25%, Mn 0.78
%.

Po, 025%, 80.02%, AlO, 067%.

Steel having a composition of NbO, 0.45% and Vo, 0.4% (both weight percentages) is melted in a vacuum melting furnace. 25
kg), and hot rolling was performed in the same manner as in Example 3.

Table 2 shows the results.

1200-100 directly during the cooling process following melting-solidification.
In the case of a comparative example in which hot rolling was performed in a temperature range of 0'C, cracks on the surface of the steel piece were significant.

On the other hand, if rolling is carried out after carrying out holding treatment for a predetermined period of time during cooling, surface cracking of the steel billet is significantly suppressed.

[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 precipitate phases and spherical precipitates, Figure 4 is a chart showing the relationship between cross-sectional shrinkage rate and holding time in Example 1, Figure 5 is a chart showing the relationship between cross-sectional shrinkage rate and holding time in Example 2, and Figure 6 is a graph showing the change in steel ingot surface temperature. FIG. 7 is a schematic diagram of various processing heat treatments.

Claims (1)

[Claims] 1 1150-950 during the cooling process following melting and solidification in direct rolling or hot charge rolling of steel billets
A method for suppressing surface cracking of a steel billet during hot rolling, which comprises rolling after holding in a temperature range of 10 minutes or more.