JPH0617505B2 - Hot working method for Mo-containing, N-austenitic stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a hot working technique for Mo- and N-austenitic stainless steels.

[Conventional technology]

It is known that austenitic stainless steel generally has a large deformation resistance during hot rolling, and that sulfides and the like are precipitated at the austenite grain boundaries to easily cause hot cracking. In the case of Mo-containing N austenitic stainless steel, the deformation resistance becomes higher than that of ordinary austenitic stainless steel, so that hot cracking is promoted. The following methods are known as methods for preventing high temperature embrittlement of ordinary austenitic stainless steel.

(1) Reduce P, S, O and N in the chemical composition. For example, Al, Ca, RE, which have a high affinity for these impurity elements
Add M or the like to precipitate it as a stable compound on the γ base in steel, or efficiently remove impurities by decarburizing, deoxidizing, and desulfurizing during refining. In some cases, the raw material itself may be of high quality.

(2) In the solidification process, the impurity element is dissolved in δ ferrite. For example, in the process of determining the steel composition, the balance between Ni equivalent and Cr equivalent is considered, and the composition is designed so that a small amount of δ-ferrite occurs in the solidification process.

(3) Improving the casting structure. For example, forging is a typical example, but in the case of continuous casting, casting is performed with the casting temperature close to the melting point, or columnar crystals are made as equiaxed as possible using an electromagnetic stirring device to improve the cast structure. .

[Problems to be solved by the invention]

In the case of Mo-containing and N-austenitic stainless steels, as described above, the hot deformation resistance becomes higher than that of ordinary austenitic stainless steels, and the problem of hot cracking becomes serious. Therefore, it is desired to adopt the above methods (1), (2) to (3) alone or in combination.

However, the method (1) cannot completely remove the impurity element, and the amount of removal is limited within the range of the current steelmaking technology, which is naturally limited. In addition, if raw materials containing few impurities are used in advance, the cost will increase.

The method (2) is effective in suppressing the grain boundary precipitation of impurity elements, but when the finished product is welded, δ ferrite may precipitate and the corrosion resistance may deteriorate.
It is also a problem of this method that the range of adding alloy elements such as Mo and Ni is limited. Furthermore, even if the method of (3) is used, there is a limit to improving hot workability due to constraints such as slab thickness and equipment capacity.

From a comprehensive judgment in terms of hot yield and manufacturing cost, it is best to improve the casting structure of the continuous cast slab by the method of (3) and perform direct rolling. However, normal hot rolling conditions tend to cause hot cracking. Therefore, although slabs have been downsized or steel slab casting has been switched to improve the structure by slab rolling, it is inevitable to reduce hot yield and increase manufacturing cost. Generally, the slumping conditions are low speed and light pressure, but the number of passes per heat increases and the temperature drop increases accordingly. Therefore, in the case of large steel ingots, the number of heat increases and the manufacturing cost increases. Will rise.

Thus, it can be said that Mo-containing N-N austenitic stainless steel is a material system in which it is difficult to prevent hot cracking even if the impurity elements are reduced compositionally and the cast structure is improved.

The present invention is intended to reduce and prevent such hot cracking of Mo-containing and N-austenitic stainless steels, especially slab surface cracking during rough rolling and edge cracking of hot rolled coils.

[Means for solving problems]

The present invention relates to hot working of a continuous cast slab or ingot having columnar crystals of Mo-containing and N-austenitic stainless steel containing 4% by weight or more of Mo and 0.10% by weight or more of N,
The continuous cast slab or steel ingot is soaked in the temperature range of 1200 to 1260 ° C, and the reduction amount per operation is 8 mm in the temperature range of 1250 to 1150 ° C.
The slab is characterized by performing slab rolling with the above passes to manufacture a slab, and heating this slab to 1200 ° C or higher and hot rolling.

Here, slab-rolling generally means forming a steel ingot obtained by the ingot-making method into a shape (slab) that can be hot-rolled. Not only the steel ingot obtained by the method, but also the continuous cast slab obtained by the continuous casting method is called rolling to make the microstructure of these steel pieces with columnar crystals hot-rollable. The "hot rolling" referred to in the present specification is to be distinguished from the slabbing rolling, and means passing through a normal continuous hot rolling line having a rough rolling mill and a finish rolling mill to carry out hot rolling. is doing. In the present invention, the slabbing for continuous cast slabs can be carried out using a rough rolling mill.

In the present invention, "containing Mo, N austenitic stainless steel" means that Mo is 4% by weight or more and N is 0.10% by weight.
The austenitic stainless steels contained above are meant. Typical examples are Cr: 18-25%, Ni: 20-30
%, Cu: 0.3 to 3.0%, Mn ≦ 2.0%, Mo: 4 to 8
%, N: 0.1-0.30% steel. Such a steel is a full austenitic steel containing no α phase.

The details of the present invention will be described below.

Figure 1 shows in-situ melted 20Cr-24Ni-6Mo-0.5Cu-0.2N
Round bar specimens were cut from steel continuous cast slabs, agglomerates and forged materials, and hot tensile was performed at each temperature in the figure, and the maximum breaking stress (hatched area in Figure 1 (a)) and cross section at that time Shrinkage rate (Fig. 1
(b)) is plotted. The maximum breaking stress is shown in Fig. 1 (a) in comparison with that of SUS304. Mo- and N-austenitic stainless steels with high deformation resistance are about twice as high as those of SUS304, especially at low temperatures. It will be noticeable. While the first
As shown in Fig. (B), the embrittlement area associated with liquid film embrittlement at high temperature of 1270 ℃ or higher in any of the specimens of columnar crystal material, slab material and forged material of Mo-containing, N-austenitic stainless steel. Exists. Therefore, during hot rolling of Mo-containing and N-austenitic stainless steels, Fig. 1 (a)
As shown in Fig. 1, the deformation resistance is high at low temperature, so it is more convenient for rolling in the high temperature region. However, from Fig. 1 (b), it is not preferable because too high temperature causes embrittlement. Recognize. In other words, during hot rolling, the temperature that should not exceed 1260 ° C, but is as high as possible within this temperature range, should be the extraction temperature of the billet from the heating furnace.

On the other hand, focusing on the columnar crystal specimen (marked with ○) in Fig. 1 (b),
In addition to the embrittlement on the high temperature side above 1270 ° C, there is a region where embrittlement occurs near 1100 ° C. Therefore, in the case of hot rolling using a steel piece with columnar crystals, it is necessary to avoid this region near 1100 ° C as much as possible and roll at, for example, 1150 ° C or higher. For example, when a continuous cast slab having columnar crystals is hot-rolled, it should be hot-rolled mainly in the temperature range of 1150 to 1250 ° C. At that time, the strain rates of the columnar crystal materials of Fig. 1 (b) were 1 / S (○) and 10 / S (●).
Compared with the one marked with 10), the 10 / S type has a wider ductility region, so high speed rolling is more advantageous.

Fig. 2 shows the recrystallization characteristics of the same steel as in Fig. 1 in the hot rolling of columnar crystals.
After heating at the heating temperature shown in Fig. (A), the reduction rate when reduced in one pass was changed variously as shown in Fig. 2 (a), and immediately after the reduction, water cooling was performed and a metallurgical microscope by acid etching was used. Check the recrystallized state in the photograph (magnification 81.5 times),
A representative example is shown in FIG. 2 (b). As can be seen from the results in FIG. 2, the higher the temperature (1250 ° C.) and the larger the reduction amount, the easier the recrystallization is. Recrystallization generally acts to improve hot workability.

Therefore, in the billet having columnar crystals of Mo-containing and N-austenitic stainless steel, the billet is uniformly heated and extracted at a heating temperature of 1200 to 1260 ° C, and is extracted once in the temperature range of 1150 to 1250 ° C. It can be seen that it is better to roll by increasing the reduction amount and increasing the strain rate, and by reducing the reduction amount at 1150 ° C or lower.

Next, regarding the agglomerated material and the forged material, as shown in Fig. 1 (b), unlike the columnar crystal specimen, the embrittlement around 1100 ° C is improved. Therefore, the above heating temperature and working temperature should be adopted when agglomerating the steel pieces having columnar crystals, but if the obtained agglomerated material is further hot-rolled, the rolling temperature of 1150 ° C or less is required. Can also be adopted.
However, even during this hot rolling, it is better to increase the amount of reduction in the temperature range of 1150 to 1250 ° C. Therefore, in the case of producing a billet for hot rolling from a billet having a columnar crystal through the slabbing rolling, according to the present invention, the higher the temperature is in the temperature range of 1150 to 1250 ° C, the larger the reduction amount per pass is. When the lump is rolled at a temperature of 1150 ° C or lower, and the rolling amount is reduced at a temperature of 1150 ° C or lower to make a lump, the obtained lump is 11
Hot rolling can be performed even at a temperature of 50 ° C or lower. At that time, if the thickness of the steel ingot or slab is large, the temperature falls until it is reduced to the thickness for hot rolling, and the necessary reduction amount in the temperature range of 1150 to 1250 ° C (in the present invention, 8 mm or more per pass). In some cases, the slabbing may be stopped when the temperature drops, and the slabbing may be carried out by charging the reheating furnace again and heating it at 1200 to 1260 ° C. That is, heating at 1200 to 1260 ° C and 11
By repeatedly rolling at 50 to 1250 ° C and lumping to a thickness required for normal hot rolling, rolling can be performed while avoiding the embrittlement region near 1100 ° C. As shown in Table 1 below, the reduction amount of the lumps at 1150 to 1250 ° C is preferably 8 mm / pass or more, and the reduction rate should be 4% or more.

In hot rolling, even if the embrittlement around 1100 ° C is improved, if the temperature becomes too low, ductility decreases and deformation resistance increases, causing ear cracks, etc.
It is important to heat above and hot roll to finish at 950 ° C or higher.

As described above, according to the method of the present invention, even with Mo-containing N austenitic stainless steel, hot rolling can be performed with significantly reduced cracking.

Examples Table 1 below shows 20Cr-24 according to the present invention method and the conventional method.
2 shows the cracking situation and yield when a hot rolled coil is manufactured from Ni-6Mo-0.5Cu-0.2N steel.

In addition, (1) in the method of the present invention in Table 1 is a continuous cast slab
Extraction at ℃ ± 10 ℃, increase the amount of reduction per pass as the temperature rises from 1150 to 1250 ℃, and decrease the amount of reduction per pass as the temperature decreases from 1150 ℃, and then agglomerate.
This agglomerated slab is heated again to 1250 ℃ ± 10 ℃ and extracted,
Hot rolled coil finished at 950 ℃ or higher by passing through rough rolling mill and finish rolling mill. The same (2) extracts steel ingot at 1250 ℃ ± 10 ℃, and 1150 ～ 1250 ℃ per pass Increase the amount of reduction, decrease the amount of reduction per pass as the temperature decreases from 1150 ℃, perform slabbing, heat again to 1250 ℃ ± 10 ℃, and extract,
After forming a slab of slab by increasing the reduction amount per pass at 50 ° C and performing slab rolling, it is heated to 1250 ° C ± 10 ° C for extraction and passed through a rough rolling mill and finish rolling mill. hand
It is a hot rolled coil finished at 950 ° C or higher.

As can be seen from the results in Table 1, in the conventional method in which the continuous cast material of Mo-containing and N-austenitic stainless steel is hot-rolled as it is, the yield is high even if the extraction temperature in the heating furnace is the same as the method of the present invention. 75%, and even if slabbing was performed, a high reduction amount was not taken on the high temperature side, and the yield was 70% in the comparative method that took a high reduction amount in the latter half of the temperature drop.
% Or less, the slabs rolled according to the method of the present invention (method of the present invention (1)) and the slabs subjected to slab rolling according to the method of the present invention (method of the present invention (2)) are both Yield 9
It can be seen that a good hot rolled coil was obtained at 0% or more.

[Brief description of drawings]

Figure 1 shows in-situ melted 20Cr-24Ni-6Mo-0.5Cu-0.2N
Round bar specimens were cut from steel continuous cast slabs, agglomerates and forged materials, and hot tensile was performed at each temperature in the figure, and the maximum breaking stress (hatched area in Figure 1 (a)) and cross section at that time Shrinkage rate (Fig. 1
Fig. 2 (b)) is a plot of Fig. 2 and Fig. 2 shows the recrystallization characteristics of the same steel as in Fig. 1 during hot rolling of columnar crystals. Fig. 2 (a) shows a 30 mm thick slab. The condition table when the amount of reduction is varied in one pass after heating at the indicated heating temperature. Fig. 2 (b) shows the condition of Fig. 2 (a) immediately after water cooling. In the table, a, b, c,
Fig. 3 shows a metallurgical micrograph (magnification: 81.5 times) of each sample under the conditions of d, he, n, mosquito, and ta.

Claims (2)

[Claims] 1. Hot working of a continuous cast slab having columnar crystals of Mo-containing austenitic stainless steel containing 4% by weight or more of Mo and 0.10% by weight or more of N or a steel ingot made of a steel ingot. , The steel slab is soaked in the temperature range of 1200 to 1260 ℃, and the lump slab is manufactured by performing slab rolling with a pass of a rolling reduction of 8 mm or more in the temperature range of 1250 to 1150 ℃. A hot working method for Mo-containing N austenitic stainless steel, characterized in that this slab is heated to 1200 ℃ or higher and hot-rolled. 2. A Mo-containing N austenitic stainless steel containing Cr: 18-25%, Ni: 20-30%, Cu: 0.3-3.0.
%, Mn≤2.0%, Mo: 4-8%, N: 0.1-0.30%, the balance being iron and unavoidable impurities, The hot working method according to claim 1.