JPS63210257A - Heat resisting steel and its production - Google Patents

Abstract

PURPOSE:To continuously cast a heat resisting steel having excellent hot workability without a breakout by drawing out a molten steel having the specific compsn. consisting of C, Si, Mn, Ni, Cr, N, Ca, Al and Fe under appropriate conditions. CONSTITUTION:The molten metal of the heat resisting steel is charged to a cooling cast mold 3 via a dipping nozzle 2 from a tundish 1 to continuously draw out solidified ingot 5. The steel consisting of by weight, 0.20-0.60% C, <=0.50% Si, 6.70-16.00% Mn, 1.50-4.50% Ni, 16.50-22.00% Cr, 0.30-0.50% N, 0.001-0.020% Ca, 0.005-0.100% Al and furthermore contg. at need 0.0005-0.002% B with the balance Fe consisting substantially of Fe is used as the heat resisting steel in the production of the above-mentioned heat resisting steel. The temp. in the above-mentioned tundish 1 is regulated to that higher than the melting point of said steel in a range of <=35 deg.C. The rate of the drawing is further regulated to <=1.6m/min. Cast mold powder 8 having >=2.0 poise viscosity is as well charged to an oil face 5C of the molten metal.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to an austenitic heat-resistant steel used as a material for vehicle engine exhaust valves, etc., and a method for producing the heat-resistant steel using a continuous casting method. .

(Prior art and problems to be solved by the invention) Conventionally,
For example, the material for engine exhaust valves etc. is 2l-
Austenitic heat-resistant steels such as 4N and 212N are heavily used. When processing various parts from such heat-resistant steel, it is common to first cast an ingot of the heat-resistant steel, and then roll this ingot to form a wire rod with a predetermined outer diameter.

However, the above-mentioned austenitic heat-resistant steel has low hot workability, such as a reduction of area, and it is difficult to obtain a large reduction rate during rolling, so a multi-stage rolling process is required, which causes a decrease in manufacturing yield. was.

Therefore, it would be extremely advantageous if the above-mentioned austenitic heat-resistant steel could be directly produced as a small-section billet by, for example, a continuous casting method. As this continuous casting method, one using a continuous casting apparatus that combines a submerged nozzle and casting powder as shown in FIG. 1 is known. That is, in the figure, a immersion nozzle 2 is attached to the lower surface of a tundish l filled with molten steel, and molten steel is injected into a mold 3 from this immersion nozzle 2. Since the mold 3 is cooled with cooling water, the injected molten steel is cooled on the wall of the mold 3 and partially solidifies to form a solidified layer (shell) 5a, and this solidified layer 5a and the liquid phase 5b inside it are separated. The cast slab 5 is continuously pulled out by pinch rolls (not shown) while being held by the guide rolls 4. The slab 5 pulled out from the mold 3 is cooled by being sprayed with cooling water by the spray 6, and solidification is completed.

In addition, in FIG. 1, a stopper 7 is inserted into the molten steel 5 of the tundish 1.
The injection rate of the molten metal from the immersion nozzle 2 into the mold 3 is adjusted. On the other hand, the immersion nozzle 2 has its lower opening immersed below the surface 5C of the molten metal injected into the mold 3, and when the molten steel is injected from the tundish 1 into the mold 3, it is oxidized by the atmosphere, and as a result, inclusions are generated. It prevents you from doing so. In addition, casting powder 8 is administered to the molten metal surface 5C, which functions as a lubricant between the mold 3 and the slab 5.
Suppresses the occurrence of surface defects such as minute cracks and pinholes on the surface.

However, the austenitic heat-resistant steel described above has the following problems. In other words, when trying to apply the above-mentioned continuous casting method to this heat-resistant steel, the strength of the solidified layer 5a of the slab 5 released from the mold 3, that is, the shell strength, is weak, and the internal liquid phase 5 is weak.
It cannot withstand the static pressure of b and breaks out (the coagulated layer ruptures,
(The molten steel inside flows out). The inventors
To prevent such breakouts from occurring,
It has been found that the tundish temperature, drawing speed, and viscosity of the casting powder can be controlled.

By the way, it has been known that in austenitic stainless steel, hot workability deteriorates because S, an unavoidable impurity in the steel, precipitates at grain boundaries. add Ca and change S to Ca
Generally, it was fixed as S in grains. On the other hand, the present inventors have discovered that the cause of deterioration in hot workability in austenitic heat-resistant steel is the formation of carbides, unlike in the case of the austenitic stainless steel described above. Furthermore, we have found that adding Ca changes the morphology of carbides (granulation), thereby significantly improving the hot workability of austenitic heat-resistant steel. Also, in order to stabilize the effect of Ca, Aj2
It was recognized that it was necessary to perform deoxidation by adding .

However, A7! The following new problems arise due to the addition of . That is, the cluster-like A7! generated when Al was added! As shown in FIG. 2, zOa9 gradually accumulates around the opening 1a of the immersion nozzle 2 at the bottom of the tundish 1, and after a certain amount of accumulation, a part of it peels off. As described above, the injection speed of molten steel from the tundish 1 through the immersion nozzle 2 into the mold 3 is determined by
Since it is controlled by adjusting the gap between a and the stopper 7, it becomes difficult to accurately control the injection rate when Ah03 is deposited in clusters as described above. This injection speed naturally affects the drawing speed, and as a result, it becomes difficult to control the drawing speed, resulting in the problem that the shell strength during continuous casting cannot be increased as described above.

Furthermore, even if a billet could be produced by continuous casting, the properties of the austenitic heat-resistant steel itself have not yet been resolved, and its hot workability is low, making it difficult to roll the resulting billet. However, there is the disadvantage that cracks may occur in the wire.

The present invention has been made in view of the above-mentioned conventional problems, and is concerned with the occurrence of breakouts when manufacturing an austenitic heat-resistant steel with excellent hot workability such as rolling, and such heat-resistant steel by a continuous casting method. It is an object of the present invention to provide a method for manufacturing heat-resistant steel in which the injection rate of molten steel can be accurately controlled.

(Means and effects for solving the problems) The present invention has the following features:
First, the hot workability of heat-resistant steel is improved by adding both elements Ca and Al, and the shell strength is improved by controlling the kundish temperature, drawing speed, and viscosity of the casting powder to predetermined values during continuous casting. This was determined from A1203M, which increases the above-mentioned Ca1l, and improves the controllability of the drawing speed by preventing the cluster-like /1203 from being generated.

That is, the heat-resistant steel of the first invention has C:
0.20-0.60%, Si: 0.50% or less, Mn:
6.70-16.00%, Nr: 1.50-4.50%
, Cr: 16.50-22.00%, N:
0.30-0.50%, Ca: 0.001-0
．． 020%, Aj! : 0.005~o,io
The second invention for producing this heat-resistant steel is to inject the molten heat-resistant steel from a tundish into a cooling mold, and continuously pour the solidified slab into a cooling mold. In the method for producing a heat-resistant steel billet, the heat-resistant steel is C: 0.20 to 0.6 in weight percentage.
0%, 3i: 0.50% or less, Mn: 6.70-1
6.00%, Ni: 1.50-4.50%, Cr
: 16.50~22.00%, N: 0.30~
0.50%, Ca: 0.001-0.020%, A1
: 0.005 to 0.100%, and the remainder substantially Fe, the temperature in the tundish is set to 35°C or less higher than the melting point of the heat-resistant steel, and the drawing speed is set to 1. Drawing is carried out at a speed of 6 m/min or less, and casting powder having a viscosity of 2.0 poise or more is charged onto the surface of the molten metal.

First, the reason for limiting the range of each component (% by weight) in the heat-resistant steel fin 7), which is the first aspect of the present invention, will be described.

c: o, 2o~0.60% C is a component element that contributes to improving the strength of the steel of the present invention, and in order to exhibit a sufficient strength improving effect, it should be at least 0.2%.
0% is necessary, but excessive increase causes intergranular corrosion, so the upper limit was set at 0.60%.

Si: 0.50% or less Si is an effective component element as a deoxidizing agent, but if added excessively, it deteriorates toughness, so the upper limit was set to 0.50%.

Mn: 6.70 to 16.00% Mn is an element that contributes to stabilizing the austenite structure, and is added to supplement expensive Ni.

The amount of Mn added must be at least 6.70%.

However, if it is too large, work hardening will occur during cold working, so the upper limit was set at 16.00%.

Ni: 1.50 to 4.50% Like Mn, Nr is a component that contributes to stabilizing the austenitic structure and is also effective in improving corrosion resistance. Therefore, at least 1.50% is required, but Ni
Since is an expensive element, the upper limit was set at 4.50%.

Cr: 16.50-22.00% Cr is an element particularly effective in improving corrosion resistance and oxidation resistance. In order to fully exhibit such characteristics, at least 16.50% is required. However, if the amount of Cr is increased unnecessarily, it becomes difficult to maintain the austenite structure, and -
Since the amount of additives such as i increases, leading to higher prices, the upper limit is set to 2.
It was set at 2.00%.

N1.30-0.50% N is an element that stabilizes the austenite structure, and for this purpose at least 0.30% is required, but if the content increases too much, hot workability will decrease, so the upper limit is set. was set at 0.50%.

Ca: 0.001 to 0.02% Ca is an additive element for making carbides in steel granular and fragmented to improve hot workability. From the viewpoint of improving hot workability, at least 0.001% or more is necessary. However, when the amount of C and a increases, during continuous casting,
Since there is a possibility that the precipitated CaO may clog the immersion nozzle opening in the tundish, the upper limit was set to 0.02%.

A7! : 0.005 to 0.100% All is an element with strong deoxidizing power, and for example, in order to deoxidize 5 inH, at least 0.005% is required. However, if there are too many AI! The upper limit was set at 0.100% because there is a problem that zO3 is generated and blocks the immersion nozzle.

In addition to the above-mentioned component elements, the heat-resistant steel of the present invention has 0.
0005 to 0.002% of B may be contained. B
is a component that contributes to improving hot workability, and is required at least 0.0005% to obtain a sufficient effect, but if it is too large, boron nitride will be formed, resulting in poor performance after solution treatment. The upper limit should be set to 0, as this will cause deterioration of the micro-Mi weave.
．． 002%.

Furthermore, the heat-resistant steel of the present invention contains 0.0 as an impurity element.
4% or less P, 0.01% or less S, 0.70% or less Cu, 0.60% or less Mo, 0.0067% or less
．． 0.001% or less pb, o, so% or less W and 0
．． Each element such as V may be contained in an amount of 50% or less.

In the heat-resistant steel of the present invention, the concept of "effective Ca content" described below was introduced to determine the Ca content. That is, the following formula: Effective Ca amount - Total Ca amount - 0.9 x Total 01
(1) When this effective Ca amount is zero, Ca is 12Ca 0-7A120.
Therefore, there is no shortage or excess. 12Ca□−7Aρ20
Since Al1203 has a low melting point and melts sufficiently at the temperature inside the tundish, it is prevented from clogging the immersion nozzle as would be the case when Al1203 is produced alone. Therefore, when the effective Ca amount≧0, there is Ca that causes the carbide to become granular.

Next, a method for manufacturing heat-resistant steel, which is the second aspect of the present invention, will be explained.

This manufacturing method is a method of manufacturing a heat-resistant steel billet by a continuous casting method using a continuous casting apparatus as shown in Fig. 1, and the conditions at this time are: tundish temperature,
This specifies the drawing speed and viscosity of the casting powder.

First, the tandice temperature is 35° below the melting point of the heat-resistant steel.
The temperature should be high within the range below ℃. Then, the drawing speed of the slab is set to 1.6 m/min or less.

Further, as the casting powder, one having a viscosity of 2.0 poise or more is used. Each of these conditions is essential in order to increase the shell strength when manufacturing the above-mentioned heat-resistant steel billet by the continuous casting method, and if any one of them is out of the above range, there is a risk of breakout occurring. There is.

Since the heat-resistant steel of the present invention has a small solidification shrinkage rate, it is preferable to use a straight mold as shown in FIG. 1 rather than a tapered mold in the continuous casting apparatus.

(Example) Heat-resistant steels A to ■ having the component compositions shown in Table 1 and the effective Ca amount calculated from the above formula (1) were prepared, and the continuous casting equipment shown in Fig. 1 was used to cast them. Continuous casting was carried out under the conditions shown in Table 2 to produce a billet of 1451 cm x 1451 weight. Table 2 shows whether or not breakout occurred during this continuous casting process.

Next, each of the obtained billets was made into 95 seals in 6 passes.
The workability was evaluated by hot rolling the wire to a thickness of 95 mm and observing scratches on the surface of the wire. Specifically, no scratches...◎, scratch depth 1 mm or less...○, scratch depth 1 to 2 fl...Δ, scratch depth 3 mm or more...X, and the results are summarized as follows. 2 Shown in Table 2.

(The following is a blank space) As is clear from Table 2, when a heat-resistant steel billet is manufactured by the manufacturing method of the second invention using the heat-resistant steel having the composition of the first invention, during manufacturing, that is, continuous casting. It was confirmed that the occurrence of breakout during the process was effectively prevented, and the hot workability of the obtained billet was also significantly improved.

(Effect of the invention) As explained above, according to the surface 1 heat steel of the first invention,
In weight%, c: 0.20-0.60%, Si20.
50% or less, Mn: 6.70-16.00%, Ni
;1.50~4.50%, Cr: 16.50~22
．． 00%, N: 0.30-0.50%, Ca:
0.001-0.020%, A7! :0.005~0.
Since 100% and the remainder are substantially made of Fe, hot workability such as hot rolling is improved, and in addition, cluster-like AI is formed during continuous casting to produce this billet. Since deterioration in the controllability of the injection rate due to the formation of 03 is prevented, it is possible to accurately control the withdrawal rate, and the shell strength is increased. It became possible to apply the casting method.

Furthermore, according to the second method of manufacturing heat-resistant steel of the present invention, in the method of manufacturing a heat-resistant steel billet, in which molten heat-resistant steel is injected into a cooling mold from a tundish and a solidified slab is continuously pulled out, the heat-resistant steel as, weight%, c:o, 2o
~0.60%, Si: 0.50% or less, Mn: 6.
70-16.00%, Ni: 1.50-4.50%, C
r: l 6.50-22.00%, N: 0.3
0-0.50%, Ca: 0.001-0.020%
, AI: 0.005 to 0.100%, and the balance substantially made of Fe, the temperature in the tundish is set to a temperature higher than the melting point of the heat-resistant steel by 35°C or less, and the drawing speed is set to 1. .6 m/min or less, and casting powder with a viscosity of 2.0 poise or more was added to the surface of the molten metal, increasing the shell strength and preventing breakouts during continuous casting. This has the advantage that the continuous casting method can be applied, and the manufacturing yield is improved.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram showing the configuration of a general continuous casting apparatus, and FIG. 2 is a partially enlarged sectional view showing a conventional problem in the apparatus of FIG. 1. 1... tanditshu, 2... immersion nozzle, 3...
・Mold, 5... Slab, 5a... Solidified layer (shell),
5b...liquid phase, 5C...molten metal surface, 7...stopper,
8...Casting powder.

Claims (3)

[Claims] (1) In weight%, C: 0.20-0.60%, Si: 0
．． 50% or less, Mn: 6.70-16.00%, Ni:
1.50-4.50%, Cr: 16.50-22.00
%, N: 0.30-0.50%, Ca: 0.001-0
．． 020%, Al: 0.005 to 0.100%, and the balance substantially consisting of Fe. (2) The heat-resistant steel according to claim 1, further containing 0.0005 to 0.002% of B. (3) In a method for producing heat-resistant steel in which molten heat-resistant steel is poured into a cooling mold from a tundish and solidified slabs are continuously pulled out, the heat-resistant steel has C: 0.2% by weight.
0 to 0.60%, Si: 0.50% or less, Mn: 6.7
0-16.00%, Ni: 1.50-4.50%, Cr
:16.50~22.00%, N:0.30~0.50
%, Ca: 0.001-0.020%, Al: 0.00
5 to 0.100%, and the remainder substantially made of Fe, the temperature in the tundish is set to a temperature higher than the melting point of the heat-resistant steel by 35°C or less, and the drawing speed is 1.6 m/min. A method for producing heat-resistant steel, which comprises performing drawing as follows, and adding casting powder having a viscosity of 2.0 poise or more to the surface of the molten metal.