JPH0342150A - Production of cr-ni stainless steel sheet having excellent surface quality - Google Patents

Abstract

PURPOSE:To form a finer-grained casting structure by adjusting the overheating degree at the time of casting according to the delta Fecal calculated from the component values of a molten steel prior to casting, then casting the molten steel. CONSTITUTION:The Cr-Ni stainless steel having <= 10 delta Fecal value defined by deltaFecal%=3(Cr+1.5Si+Mo+2Ti+Nb)-2.8(Ni+0.5Mn+0.5Cu)-84(C+N)-19.8wt.% is cast by a continuous casting machine in which the wall surfaces of a casting mold move in synchronization with the ingot. The delta Fecal is calculated by measuring the molten steel components prior to the casting. The molten steel is then continuously cast to the thin strip-like ingot of <=10mm thickness at <=100 deg.C/sec cooling rate at the time of solidification at the casting temp. TC determined by specifying the DELTAT defined by the casting temp. TC = the solidification initiation temp. TS of the molten steel + overheating degree DELTAT to DELTAT<=30 deg.C when delta Fecal <- 2 and to 30 deg.C <DELTAT<=100 deg.C when -2<= deltaFecal <=10. The resulted ingot is cooled from the highest possible temp.from below the solidification temp. at >=100 deg.C/sec cooling rate down to 1,200 deg.C and at >=10 deg.C/sec down to 1,200 to 550 deg.C, by which the gamma grain size of the ingot is fined to <=50mum in average.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention casts a slab of a size close to the product thickness by a so-called synchronous continuous casting process in which there is no relative speed difference between the slab and the inner wall surface of the mold. The present invention relates to a method of manufacturing a CrNi stainless steel thin plate.

[Conventional technology]

Conventionally, in order to manufacture a thin stainless steel plate using the continuous casting method, the mold was vibrated in the casting direction and the thickness was 100 mm.
After casting the slab into a slab with a diameter of + or above, treating the surface of the slab obtained, and heating it to 1000°C or higher in a heating furnace, QH rolling is performed using a hot strip mill consisting of a rough rolling mill and a finishing rolling mill row, It was a hot strip with a thickness of several mm.

When cold rolling the hot strip obtained in this way, the shape (flatness), material, and
In order to secure the surface quality, hot-rolled sheets were annealed to soften the hot strips that had undergone intense hot working, and scales and the like on the surface were removed by grinding after the pickling process.

In the conventional process, a large amount of energy is required to heat and process the material using a long hot rolling facility, and it is difficult to say that the manufacturing process is excellent in terms of productivity. In addition, the final product had a developed texture, and the user had to take into account its anisotropy when applying press processing, etc., and there were many restrictions on use.

Therefore, in order to solve the problem that rolling slabs with a thickness of 100 mm or more into hot strips requires a long hot rolling equipment and a large amount of energy and rolling power, we have recently developed a continuous casting process. Is it the same as a hot strip?
Research is progressing on a process for obtaining slabs (thin strips) of thicknesses that are or are close to this thickness.

For example, "Tetsu to Hagane J'85. A197~"^256
The article featured in No. 0-1705 discloses a process for obtaining hot strip directly by continuous casting.

In such a continuous casting process, the twin drum method is used when the gauge of the strip to be obtained is 1 to 10 mm, and the twin drum method is used when the gauge of the strip to be obtained is 20 mm.
At the level of ~50 mm, a twin belt system is being considered.

[Problem to be solved by the invention]

In this type of continuous casting process, slabs are produced in a shape close to the final shape, and intermediate steps such as hot rolling and heat treatment are omitted or reduced. Therefore, it is known that the structure, surface condition, etc. of the slab have a great influence on the material quality and surface quality of the product.

As a result of detailed research by the present inventors on the manufacturing process of Cr-Ni stainless steel thin sheets by continuous strip casting, we found that surface defects called rovings and uneven gloss occur in the products as specifically shown below. There was found.

(1) Roving: WJ and fine irregularities are produced on the surface during cold rolling.

(2) Uneven gloss: Uneven gloss appears on the surface after cold rolling, annealing, and pickling.

Problems regarding the surface properties of these products are unique to the production of thin slabs of nearly final shape from molten austenitic stainless steel and cold rolling.

As a result of a detailed study of the causes of these surface quality problems, the inventors found that the γ grains of the material before cold rolling were 50 μm.
It has been found that these surface defects occur when the thickness of In is larger than In or when thin slabs are insufficiently cooled in the temperature range where Cr-based carbon particles precipitate.

In order to prevent these surface defects, we improved the molten steel composition and cooling conditions during the process of solidifying and cooling the molten steel, reducing the average γ grain size before cold rolling to 50 μm or less, and precipitating Cr-based carbides. The inventors have invented a method for producing thin Cr-Ni stainless steel plates that can provide products with good surface properties without causing any problems.

For example, δFeca
A method of setting i to 12% to 10% (Patent application No. 63-22147)
No. 1).

However, depending on the wI type of Cr-Ni stainless steel, it may not contain any δ ferrite due to its composition (for example, JIS 5US305, 5US310, 5US31).
6 etc.) Or, there are cases where the composition includes a composition where δFecal≦-2% (for example, JIS 5tlS304 etc.),
Since the above method of setting Fecaz to 12% to 10% cannot be applied, for example, in a slab with a thickness of 3ms+, the γ grain size is 3%.
00 μm or more, and significant roving (roving height of 1.2 μm or more) occurred in some cases.

An object of the present invention is to provide a method for producing a thin plate for Cr--Ni stainless steel with excellent surface quality, which can effectively reduce grain size and significantly reduce roving even when δFecal≦-2%.

[Means to solve the problem]

The above object, according to the invention, is achieved by: δFecaz (%)
= 3 (Cr+1.5Si+Mo+27i+Nb)
-2,8 (Ni+0.5Mn+0.5Cu) -84
δ defined as (C+N)-19,8 (each component is 1%)
Cr-Ni having a composition with a Fecad value of 10 or less
When casting stainless steel using a continuous casting machine in which the mold wall surface moves in synchronization with the slab, the molten steel composition is measured before casting, δFeca is calculated using this measured value, and the casting temperature (Tc )=solidification start temperature (Ts) of molten steel;
The casting temperature (T
In c), the cooling rate during solidification is set to 100 ° C / sec or more to continuously form a thin strip slab with a thickness of 10 mm + or less, and cooling of the obtained slab is started from the highest possible temperature below the solidification temperature. , 100℃/sec or more up to 1200℃ and 1200℃
℃ to 550℃ at a cooling rate of 10℃/sec or more to refine the γ grain size of the slab to an average of 50μm or less, and then coiling, descaling, cold rolling, final annealing,
This is achieved by a method for manufacturing a Cr--Ni stainless steel thin plate with excellent surface quality, which is characterized by carrying out temper rolling. Note that the above casting temperature (Tc) is the temperature of molten steel in the tundish during casting.

In the present invention, δFec calculated from the molten steel composition values before casting
By adjusting the degree of superheating (ΔT) during casting according to al, the cast structure is effectively refined over the entire range of δFecai≦10%.

Conventionally, especially in the case of continuous casting of stainless steel, the degree of superheating (ΔT) was set quite large in consideration of problems such as the formation of skin in molten steel, and the casting temperature was set at a high temperature such that ΔT was 40 to 50°C or higher. I was doing casting.

The present inventor has discovered that by performing low-temperature casting while reducing ΔT within a range that does not cause the above-mentioned problems, the cast structure can be changed from the coarse columnar crystals formed at conventional casting temperatures to fine equicyclic islands. The present invention was completed by discovering that refinement is particularly effective in the case of less than -2% δFe6al, and that this refinement can significantly reduce roving in product thin plates.

That is, when δFecaz < 2%, ΔT
By setting the temperature to 30°C or less to generate equicyclic islands, and cooling after casting at a cooling rate of the specified i range,
The gamma grain size of the slab can be reduced to 50 μm or less.

In the above-mentioned prior application, by setting δpecat to -2 to 10%, the γ grains are divided by using the δ ferrite phase as primary crystals, and the γ grains are made finer by dispersing V&fine ferrite in the γ grain boundaries. do. In the present invention, the above δFe
This effect is utilized for the Ca1 range. In this case, the γ grain size of the slab can be reduced to 50 μm or less by refining the coarse columnar crystals by fragmenting them and performing cooling after casting at a cooling rate within the specified range. However, in this range of δFeCa1, ΔT is 30
℃ or less, a larger refining effect can be obtained by dividing the columnar crystals than by making the columnar crystals into equicyclic islands and refining them.

Therefore, when δFeeal is between 12 and 10?5, it is necessary to set ΔT larger than 30°C to maintain the cast structure as columnar crystals.

With reference to FIGS. 1 and 2, δpecat, ΔT,
The relationship between the γ grain sizes of the slab will be specifically explained using 5US304 steel as an example.

Cr-Ni stainless steels A and B having the compositions shown in Table 1 were cast using a twin-roll continuous casting machine at various casting temperatures (that is, by changing ΔT), and the cooling rate during solidification was set to 5.
Cast at 60℃/sec into a thin strip slab with a thickness of 2mm, and after solidification at 350℃/s from 1400 to 1200℃.
Cooling at a cooling rate of EC, 20°C/s from 1200°C
It was cooled to room temperature at a cooling rate of EC.

The obtained slab was pickled, cold rolled at a rolling reduction of 50%, and then final annealed, pickled, and temper rolled to obtain a thin plate product.

Figure 1 in the margin below shows the change in slab γ grain size due to ΔT, δFeCa
1 = 2.59% Aim and δFecal = 6
．． δFecal shown in comparison for 06% ill
When < -2%, the γ grain size becomes extremely coarse when ΔT>30°C, but becomes sharply finer when ΔT≦30°C.

When δI”ecaz≧−2%, the γ grain size does not change significantly depending on ΔT, but it tends to become coarser when ΔT≦30°C than when ΔT>30°C.Second. As shown in the figure, the change in roving height after cold rolling due to ΔT corresponds very well to the change in γ grain size in Figure 1. That is, δFecaz < -2
%, the roving height can be reduced to an acceptable level by setting ΔT to 30° C. or less. On the other hand, when δF'eCal≧-223, the roving height is good and below the allowable level over the entire range of ΔT, but the roving level is better when ΔT is greater than 30°C.

Therefore, δFt2cI. Since there is a risk that the solidified shell of the piece may break or that the solidified structure becomes coarse due to the heat of the molten steel, ΔT needs to be 80° C. or less.

In order not to deteriorate the workability of the product, it is necessary to limit the amount of deformation-induced martensite generated by setting δFeeal to 10% or less.

The relationship in Figures 1 and 2 is shown as the effect of γ grain size on roving height, as shown in Figure 3. According to the present invention, the roving height can be controlled within an allowable level if the γ grain size is set to 50 μm or less.

After solidification, the slab is cooled to 1200°C at a rate of 100°C/sec or more in order to prevent coarsening of the γ grains due to tX=. From 1200°C to 550°C, the temperature is set at 10°C 7/sec to prevent the slab from becoming sensitized due to the precipitation of Cr-based carbides.
Cool at the cooling rate above.

〔Example〕

Molten Cr-Ni stainless steel having the composition shown in Table 2 was cast at various casting temperatures (
In other words, by changing ΔT), the cooling rate during solidification was set to 56
Cast into a thin strip slab with a thickness of 2 mm at 0°C sec,
After solidification, the temperature range from 1400 to 1200℃ is reduced to 100℃.
/sea cooling rate and then continue to cool at 20℃/sea.
The mixture was cooled to room temperature at a cooling rate of sec. After pickling the obtained slabs, they were cold rolled at two rolling reductions of 50% and 85%, followed by final annealing, pickling, and temper rolling. The roving of the obtained sheet product was evaluated. The results are shown in Table 3.

Margin below From the above results, if ΔT is set appropriately according to the value of δFeca according to the method of the present invention, the γ grain size will be 50 μm.
The roving height is within the permissible range (0,
2 μm or less). On the other hand, in the case of the comparative method in which the combination of δ)"ecat and ΔT was inappropriate even for molten steel of the same composition, the γ grain size became coarse to 160 to 180 μm, and the roving height exceeded the allowable range. It was beyond.

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to refine the structure of the slab and prevent roving, including the composition of low δFecal, which was previously impossible.

[Brief explanation of drawings]

Figure 1 shows the relationship between ΔT and slab γ grain size at two levels of δl'.
The graph shown in Fig. 2 for comparison of ecaz is ΔT
FIG. 3 is a graph showing a comparison of the relationship between δI'ecaz and roving height for two levels of δI'ecaz, and FIG. 3 is a graph showing the relationship between slab γ grain size and roving height.

Claims (1)

[Claims] 1, δFe_c_a_l(%)=3(Cr+1.5Si
+Mo+2Ti+Nb)-2.8(Ni+0.5Mn+
0.5Cu)-84(C+N)-19.8(Each component is w
When casting Cr-Ni stainless steel having a composition with a δFe_c_a_l value of 10 or less, which is defined as , calculate δFe_c_a_l using this measured value, and calculate ΔT defined by casting temperature (Tc) = solidification start temperature of molten steel (Ts) + degree of superheat (ΔT), and when δFe_c_a_l<-2, ΔT≦
3 when 30℃, -2≦δFe_c_a_l≦10
A thickness of 10 mm with a casting temperature (Tc) of 0°C<ΔT≦100°C and a cooling rate of 100°C/sec or more during solidification.
The following thin strip-shaped slabs are continuously cast, and the obtained slabs are cooled from as high a temperature as possible below the solidification temperature, at a rate of 100℃/sec or more up to 1200℃, and from 1200℃ to 550℃.
The cast slab is cooled at a cooling rate of 10°C/sec or more to refine the gamma grain size of the slab to an average of 50 μm or less, and then subjected to winding, descaling, cold rolling, final annealing, and temper rolling. A method for manufacturing a Cr-Ni stainless steel thin plate with excellent surface quality.