JPS63115659A - Roll for continuous casting - Google Patents

Abstract

PURPOSE:To obtain a roll for continuous casting provided with both of corrosion resistance and heat-cracking resistance by coating a material having excellent thermal conductivity as core material and a material having the coefficient of thermal expansion larger than that of the core material by the specific ratio and large corrosion resistance at the specific thickness on the surface of the roll. CONSTITUTION:One kind of steel selected among ferritic stainless, martensitic stainless, low alloy steel or carbon steel having excellent thermal conductivity is used as the core material 11 and on the other hand, one kind of material is selected between nickel-base alloy or austenitic stainless having the coefficient of thermal expansion larger than that of core material 11 by 5-30% and large corrosion resistance as the surface material 12 and coated by 0.1-5 mm thickness. Therefore, the heatcracking resistance is improved by using the difference of thermal expansions between both materials and at the same time, the corrosion resistance is satisfied.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) This paper relates to a single casting roll used directly below the mold of a continuous casting machine in a severely corrosive environment.

(Prior art and its problems) In the continuous casting method, in order to maintain quality by preventing oxidation of molten steel, and to provide lubricity to the mold during casting, flano gas containing highly corrosive substances is used. Furthermore, water or steam is sprayed to cool the slab. As a result, the continuous casting equipment is severely corroded, resulting in lost opportunities due to shutdown of operations for parts replacement, and high maintenance costs. Particularly severe corrosion occurs in the equipment under the mold.

On the other hand, the cause of the short life of continuous casting equipment is not only corrosion of the roll surface but also heat cracks caused by contact with hot slabs and water cooling.

By the way, when a roll of φ380 x 2140 comes into contact with a slab at 1000°C without spraying, as shown in Figure 2 (a)
The temperature distribution inside the roll is as shown in the figure, and the maximum temperature is 850°C. Further, when a roll of φ160×310 comes into contact with a slab at 1220°C without spraying, the temperature distribution inside the roll is shown in FIG. 2(b), and the maximum temperature is 900°C.

On the other hand, when a roll of diameter 250 x 2140 comes into contact with a slab at a temperature of 120°C, when sprayed, the temperature distribution inside the roll is shown in Fig. 2(c), and the maximum temperature is 550°C. Therefore, it is not always possible to extend the service life simply by improving corrosion resistance alone; it is also necessary to improve heat crack resistance at the same time.

By the way, it is said that a heat-resistant alloy higher than stainless steel is effective for improving heat crack resistance, but in order to have corrosion resistance as well, it is necessary to use a fairly high alloy. However, because its thermal conductivity is lower than that of low-alloy steel, the effectiveness of cooling the slab is reduced, and the surface temperature increases, making it more likely to cause heat cracks due to temperature changes due to water cooling, so its life can only be slightly extended. We can only expect the effects of

(Objectives of the Invention) An object of the present invention is to elucidate the corrosion phenomenon of continuous casting equipment, and to provide a roll for continuous casting that has excellent corrosion resistance and improved heat crack resistance.

(Means for Solving the Problems) The present invention has been developed in view of the fact that ferrite or martensite integral structure rolls lack corrosion resistance and heat crack resistance, while austenite integral structure rolls lack material strength and heat crack resistance. In order to simultaneously satisfy the corrosion resistance and heat crack resistance required for continuous casting rolls, we used a ferritic material as the core material and coated the surface with an austenitic material, taking advantage of the difference in thermal expansion between the materials due to temperature changes. It was developed based on the idea that it would be a good idea to create compressive stress in the austenite layer to improve its resistance to heat cracking caused by water or mist cooling. A type selected from ferritic or martensitic stainless steel, low alloy steel, or carbon steel is used, and a nickel-based alloy or austenitic stainless steel with a thermal expansion coefficient 5 to 30% larger than that of the core material is selected as the surface material. Used, 0゜1mm~
``A roll for continuous casting with excellent heat crack resistance and corrosion resistance, characterized by being coated with a thickness of 5 mm.''

In the present invention, the roll core material must be a ferritic material with excellent thermal conductivity due to the mechanical properties, thermal properties, and economic efficiency required for the roll, and ferritic stainless steel, martensitic stainless steel for rolls are , low alloy steel or carbon steel.

On the other hand, as the roll surface material, it is preferable to use one type of austenitic material, nickel-based alloy, or austenitic stainless steel, which are highly corrosion resistant materials. However, because it is necessary to utilize the difference in thermal expansion between the core material and the surface material, nickel-based alloys are used when the roll core material is ferrite or martensitic stainless steel, and austenitic alloys are used when the roll core material is low alloy steel or carbon steel. It is preferable to coat it with stainless steel, such as 316 stainless steel or 304 stainless steel. This is because there is almost no difference in thermal expansion coefficient between two units even if they are coated with a nickel-based alloy, so the effect of modified 4g on heat cracking is small. The reason why it was decided to give a difference of 5 to 30% in the coefficient of thermal expansion between the sheathing material and the core is to generate pressure i11 stress in the sheathing material that is effective for heat crack resistance.

In addition, the present invention utilizes the appropriate difference in thermal expansion coefficient between materials to prevent heat cracks that occur due to rapid temperature changes in the coating layer at high temperatures. Occasionally, the degree of generating necessary and sufficient compressive stress is 0.1 to 5.
A range of mm is preferable. If the thickness is 5 mm or more, it will be too far away from the roll core material, and the difference in thermal expansion coefficient will not work effectively, and the cost will increase. On the other hand, 0.1mm
This is because the coating layer may be torn due to small scratches.

(Example 1) A 160 mm roll for continuous casting shown in FIG. 1 was manufactured under the following conditions, and the number of operating yarns and the cause of damage thereof were investigated.

In the drawings, (+) is a continuous casting roll according to the present invention, which has a core material 11 covered with a surface material 12 of a predetermined coating thickness, and both ends of which are supported by bearings (2), (2). There is. Further, cooling water is supplied from the rotary joint (3) to the pipe I3 at the center of the roll (1).

Core material: 13Cr stainless steel Coating material: Nickel-based alloy (equivalent to Inconel 625) Coating method: Welding + grinding Coating thickness: 4 mm Ratio of coefficient of thermal expansion: 1. +62 (Example 2) A 160 mm roll for continuous casting was manufactured and tested in the same manner as in Example I except that the core material was low alloy steel and the coating material was 316 stainless steel.

(Example 3) A 160 mm roll for continuous casting was manufactured and tested in the same manner as in Example 1 except that the core material was carbon steel and the coating material was 304 stainless steel.

(Comparative Examples 1 to 4) A 160 mm integral tank forming roll was manufactured using 13cr stainless steel, austenitic stainless steel, carbon steel, and nickel-based alloy, and tested in the same manner as in the examples.

The results are as follows.

Based on the results of Example 1, the thermal strain of the roll according to the present invention is estimated as follows.

(In the case of the roll of the present invention) Preconditions: Linear expansion coefficient of core material -9,9x 10-'/coating material linear expansion coefficient -11,5xlO-'/core material thermal conductivity -2
5,2W/(m・10 Thermal conductivity of coating material = 15,
I W/(m-K) Roll diameter - 160 mm Roll surface temperature during normal operation υj = 50 to 560°C Instantaneous temperature change on roll surface due to spray water = 210°C Consider thermal distortion of the roll surface, that is, the limb covering surface and as below.

Thermal strain = 0.0000115x210 = 0.002415
That is, since the strain exceeds 0.2%, plastic deformation will occur.

However, at the joint with the core material, if the coating thickness is 3 mm, the temperature change at that part is 1.
00 to 500°C, and the thermal strain of the covering material at that time is: Thermal strain of the covering material = 0.0000115X 210
(500-100)/(56G-50)=0.0018
941 On the other hand, the thermal strain of the core material is: Thermal strain of the core material = 0.0000099 x 210 (50
0-100)/(560-50)=0.0016305
Both are within the elastic limit.

Also, between them, the change in circumference due to volume expansion of the core material = (160/2-3) X 3.14 X O,00
00099X 2 X (500-100) = 1.91
488 Change in circumference on inner side of coating material = (160/2-3)X3.14X0.00001+5
X2X(500-100) = 2.22432 Therefore, when it reaches 500℃, (483,56+2.22)/(483,56+1
．． 91) = 1.000640.064% distortion.

This acts as a compressive stress on the covering material, so
The distortion of the first value 0.002415 is 0.0017
It will be reduced to 75. This is a feature of the present invention, resulting in a roll that has both corrosion resistance and heat crack resistance.

In reality, this effect decreases as the distance from the joint increases due to the thickness of the covering material, but it does have the effect of suppressing the propagation of cracks that occur due to the presence of compressive stress on the inner side of the pad.

On the other hand, the case in the comparative example is as follows.

(Comparative example 1) 13Cr diameter stainless steel [1-ru case Prerequisites.

Coefficient of linear expansion -9,9X I O-'/Coefficient of thermal expansion -25
, 2W/ (m-K) Roll diameter - 160mm Roll surface temperature during normal operation = 50-550'C Instantaneous temperature change on roll surface due to spray water = 200°C Trial calculation of thermal distortion Roll with no residual distortion at room temperature When the above-mentioned operating condition is reached, compressive stress is generated on the surface because the temperature rise on the surface side, which is in direct contact with the hot slab, is faster than on the inner surface side. The maximum strain in that case is calculated as follows as an instantaneous temperature change (200°C) when there is no heat conduction.

Thermal strain = 0.0000099x200 = 0.001913
In other words, compressive stress is generated on the surface side and relative tensile stress is generated on the inner surface side of the roll surface and its inner surface that are in contact with the hot slab at the closest portion thereof.

However, the distortion at that time is 0.198% or 0.2
% proof stress, and no plastic deformation is considered to occur.

Also, during operation, a similar thing occurs when spray water is cooled, and tensile stress is generated on the surface side or compressive stress is generated on the inner surface side, but the instantaneous temperature change is thought to be up to 200 degrees Celsius, so this occurs at that time. It is expected that the strain will be less than 0.2%, or less than 4%, and will not lead to plastic deformation. Actually, 13c
R-series stainless steel is a practical material with excellent heat crack resistance and is widely used. However, in reality, there is a durability limit due to the problem of fatigue caused by repeated stress.

(Comparative Example 2) In the case of austenitic stainless steel roll Prerequisites: Coefficient of linear expansion -17.3X I O-''/Thermal conductivity -1
6,4W/(m-K) Roll diameter = 160mm Roll surface temperature during normal operation -50 to 570°C Instantaneous temperature change on roll surface due to spray water = 220°C Trial calculation of thermal distortion Based on the same concept as Comparative Example 1 The calculation will be as follows. However, it was assumed that the roll surface temperature and instantaneous temperature change were 20°C larger because the thermal conductivity was small.

Thermal strain - 0,0000173x220=0.003806
That is, the generated shear strain is 0.38%, and plastic deformation occurs during heating and cooling, resulting in heat cracking.

(Comparative example 3) For carbon steel rolls Prerequisites.

Linear expansion coefficient -] 1.6xl O-6/ thermal conductivity = 4
2.8W/(m-K) Roll diameter = 160mm Roll surface temperature during normal operation -50 to 530°C Instantaneous temperature change on roll surface due to spray water = 180°C Trial calculation of thermal strain Same as Comparative Example 1 Based on this idea, the calculation is as follows. However, since the thermal conductivity is high, it was assumed that the roll surface temperature and instantaneous temperature change were 20°C smaller.

Thermal strain - 0,0000116x180=0.002088
That is, the shear strain generated is 0.2%, which can be said to be within the elastic limit.

However, in reality, cracking occurs due to fatigue.

(Comparative example 1) In case of nickel-based alloy roll! 17j Prerequisites: Linear expansion coefficient -11.5X10-'/thermal conductivity -15, 1W/(m-K) Roll diameter = 160mm Roll surface temperature during normal operation -50~570℃ Roll surface with spray water Instantaneous temperature change = 220°C Trial calculation of thermal strain Using the same concept as Comparative Example 1, the trial calculation is as follows. However, it was assumed that the roll surface temperature and instantaneous temperature change were 20° C. higher due to the lower thermal conductivity.

Thermal strain=0.0000115x220=0.00253, that is, the generated strain is 0.125%, and a small amount of plastic deformation occurs during heating and cooling, and heat cracks gradually occur.

(Operations and Effects of the Invention) As is clear from the above explanation, according to the present invention, as a roll for continuous casting, a ferritic material with excellent thermal conductivity is used as the core material of the roll, while the surface is made from the core material and Since the coating is constructed using an austenitic material with a high coefficient of expansion and high corrosion resistance, it is possible to effectively utilize the characteristics of both materials and improve heat crack resistance by utilizing the difference in thermal expansion between the two materials. can. Therefore, it can simultaneously satisfy the corrosion resistance and heat crack resistance required for continuous casting rolls, making it an extremely useful roll.

[Brief explanation of the drawing]

FIG. 1 is a partially cutaway front view of a continuous casting roll according to the present invention;
FIGS. 2(a) to 2(c) show the internal temperature distribution of a conventional roll. (+)...Roll, (11)...Heartwood, (12)...
...Surface material.

Claims (1)

[Claims] (1) As the roll core material, one selected from ferritic stainless steel, martensitic stainless steel, low alloy steel, or carbon steel with excellent thermal conductivity is used, while the surface material has a thermal expansion coefficient of 5 to 50% higher than that of the core material. A roll for continuous casting, characterized in that it is coated with a type of nickel-based alloy or austenitic stainless steel that is 30% larger and has higher corrosion resistance and has a thickness of 0.1 mm to 5 mm.