JPH04187311A - High chromium cast iron roll with excellent crack resistance - Google Patents

Abstract

PURPOSE:To reduce the roll consumption unit by welding the outer shell layer and the inner layer to one body, crystallizing the graphite in the composition of outer shell layer and making the specific property to the outer shell layer. CONSTITUTION:The outer shell layer 2 and the inner layer of the high chromium cast iron roll 1 are welded to one body. And also, the graphite is crystallized in the composition of outer shell layer 2, and the outer shell layer is made to the inequality I. On the inequality I, da/dN; the fatigue crack propagating speed (mm/cycle), K; stress enlarging coefficient (kg/mm<2/3>). The chemical component of the outer shell layer is, by weight, 2-3.5% C, 1-2.2% Si, 0.5-1% Mn, 4-10% Ni, 5-10% Cr, 0.1-5% Mo, the balance Fe practically. Therefore, a small crack can be prevented from growing to a deep crack.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a roll for hot rolling, and in particular, in a roll having a structure in which an outer shell layer and an inner layer are welded and integrated, the outer shell layer This invention relates to high chromium cast iron rolls with low fatigue crack propagation speed and excellent crack growth resistance.

[Conventional technology]

Conventionally, rolls for hot rolling have been used in which an outer shell layer made of high chromium cast iron and an inner layer made of high-grade cast iron, ductile cast iron, or graphite cast copper are welded together. Metallurgically, the high chromium cast iron constituting the outer shell layer of such a roll contains fine chromium carbide in its matrix structure, so it has a feature of excellent wear resistance. However, due to its low thermal conductivity, when used for hot rolling, the temperature of the roll surface that comes into contact with the rolled material increases significantly, resulting in insufficient roughening and cracking resistance. There is.

In order to solve the above-mentioned drawbacks, a composite roll in which graphite is crystallized in the structure of the outer shell layer made of high chromium cast iron has been proposed (for example, Japanese Patent Publication No. 16415/1983).

(See Japanese Patent Application Laid-Open No. 2-232339).

[Problem to be solved by the invention]

However, in high chromium cast iron as mentioned above, in order to crystallize graphite in one structure, St, which is a graphitizing element, is used.
It is necessary to contain a large amount of Ni, especially Si. On the other hand, S
When the content of i increases.

There is a problem that crack resistance deteriorates because the toughness of the base in the structure decreases. In other words, when rolls made of such materials are used for hot rolling involving severe rolling conditions,
When cracks occur on the roll surface, the cracks propagate quickly, causing problems such as large peeling of the roll body surface during rolling.

In addition, in conventional rolls, most of the methods for improving crack resistance involve analysis and research on the balance of chemical components and control of the particle size of crystallized graphite through heat treatment, which inevitably occurs on the roll surface. There has been no analysis or research on the manner in which microcracks develop into cracks that cause accidents.

It is an object of the present invention to solve the problems existing in the above-mentioned prior art and to provide a high chromium cast iron roll that has excellent crack growth resistance without reducing wear resistance.

[Means to solve the problem]

In order to achieve the above object, one aspect of the present invention is as follows.

The outer shell layer and the inner layer are welded and integrated, graphite is crystallized in the structure of the outer shell layer, and the outer shell layer has a da/d N≦5.
．． 1xlO” ・(ΔK)9.6 However, da/dN
: Fatigue crack propagation speed (m/cycle) ΔK : Stress intensity factor fluctuation range (kg/ml).

Generally, during actual hot rolling operations, the rolls are intermittently subjected to thermal stress due to repeated heating and cooling and contact loads from the reinforcing rolls or intermediate rolls and the rolled material during each rotation. Although it is difficult to experimentally reproduce these rolling conditions, environments, etc., in actual rolling operations, fatigue rolls were investigated for the accidental rolls whose roll body surfaces were largely peeled off and the healthy rolls that continued rolling without any problems. By measuring the crack propagation speed da/dN, it is possible to quantitatively understand the difference in characteristics of the roll outer shell layer.

As a result, da/d N≦5.1X1G-
”.

(ΔK)! - It has become clear that by setting it to 0, crack propagation resistance can be significantly improved.

In the present invention, the chemical components of the outer shell layer are C2,0 in weight ratio.
~3.5%, Si 1.0~2.2%, Mn 0.
5-1.0%, P 0.08% or less, S 0.06
% or less, Ni4.0~10.0%, Cr5.0~
10.0%, Me 0.1 to 5.0%, and the balance is preferably substantially Fe.

Next, the reasons for limiting the above chemical components will be described.

C:2. O ~ 3.5% C is an essential element for crystallizing graphite in the structure of the outer shell layer and for crystallizing hard chromium carbide, and 2.0%
If it is less than this, crystallization of graphite cannot be obtained, which is disadvantageous. On the other hand, if 3.5% is reduced, Si.

The combined action with Cr results in a hypereutectic composition, and coarse primary carbides crystallize, making the material rough and causing rough skin, which is undesirable.

St: 1.0 to 2.2% St is a deoxidizing element and an effective graphitization promoting element, so it must be contained in an amount of 1.0% or more.

However, if 2.2% is added, Fe3Si is formed between Fe and Fe, resulting in deterioration of toughness and increase in da/dN, which is undesirable. It is necessary to add 0.15% or more of this by inoculation.

Mn-: 0.5 to 1.0% Mn is necessary for deoxidizing action and for fixing S as MnS, and must be contained in an amount of 0.5% or more. However, if it exceeds 1.5%, the toughness deteriorates, which is not preferable.

P: 0.08% or less Since P makes the material brittle, a smaller amount is preferable, but in consideration of practicality, it is set to 0.08% or less.

S: 0.06% or less S also has the effect of making the material brittle, so the less it is, the better.
It should be 0.06% or less.

Ni: 4.0~IO0θ% Ni is necessary for crystallizing graphite and improving hardenability.
It is necessary to contain 4.0% or more. But 10.0%
If this occurs, austenite becomes too stable, making it difficult to transform into martensite or bainite. This results in a prolonged heat treatment cycle during manufacturing and cracking during heat treatment.

This is not preferable because it increases the amount of retained austenite in the product.

Cr: 5.0-10.0% Cr is Cr-based MyC, which is a feature of high chromium cast iron.

It is an essential element for crystallizing carbides, and if it is less than 5%, M, C, and carbides will not crystallize, which is disadvantageous. On the other hand 1
If it exceeds 0.0%, graphite will not crystallize due to the strong whitening effect of Cr, which is not preferable.

Mo: 0.1 to 5.0% Mo improves hardenability and tempering resistance, so it must be contained in an amount of 0.1% or more. But 5.0%
Exceeding this is not preferable because it inhibits crystallization of graphite due to the whitening effect.

(Example) First, a graphite crystallized high chromium composite roll having a trunk diameter of 760 cm, a trunk length of 1851 cm, and a total length of 4279 cm was manufactured as follows.

(1) Using molten metal with the chemical components shown in the table as the outer shell layer,
This was inoculated using a Fe-Si based inoculant containing 0.4% by weight of Si in two ladles.

(2) The molten metal was cast into a rotating centrifugal casting mold.

(3) After the molten metal forming the outer shell layer is completely solidified,
The rotation of the centrifugal casting mold was stopped, the mold was stood upright, molten ductile cast iron was poured into the mold to form the inner layer, and the outer shell layer and the inner layer were welded and integrated.

(4) After the outer shell layer and inner layer were cooled, they were taken out from the mold and heated to undergo heat treatment of quenching and tempering.

As a comparative example, the chemical composition of a roll that caused a peeling accident on the surface of the roll body during hot rolling is also listed in the table, and the manufacturing process is approximately the same as that of the above example.

FIGS. 1(a) and 1(b) are photographs showing the metal structure of the outer shell layer of a roll observed from the surface side in an example of the present invention, and FIG. 1(6) is a state without corrosion, and FIG. (bl indicates a corroded state. As is clear from Fig. 1(a), relatively fine and medium crystallized graphite is observed due to the heat treatment, and these crystallized graphites form !111 of the outer shell layer. It can be seen that it is uniformly dispersed in the @.

Moreover, as is clear from FIG. 1(b), the matrix structure of the outer shell layer is mainly composed of bainite, and it is recognized that the transformation from austenite to bainite was carried out smoothly.

The roll manufactured as described above was installed as the fourth stage work roll of a hot finishing rolling mill, and as a result of hot rolling work, the rolling time was 2.5 hours, and the weight of the rolled material was 1400.
It was confirmed that the steel sheet was sound even after the rolling operation of t.

Figure 2(a) (bl is a front view and right side view showing the CT test piece, respectively, Figure 3 is the CT specimen shown in Figures 2(a) and (b).
FIG. 3 is an explanatory diagram showing the sampling position of a test piece. That is, for a roll having the chemical composition shown in the above table, as shown in FIG. 3, from a position near the surface of the outer shell layer 2 of the roll 1, but 200 meters away from the end face, where there is no influence of cooling from the end face,
CT test piece 3 was taken. The CT test piece 3 is collected in such a way that the propagation direction of the crack 4 shown in FIG. 2(a) is in the circumferential direction of the roll 1 shown in FIG. It was taken as close as possible.

Each of the CT specimens collected as described above was attached to an electro-hydraulic fatigue testing machine, a load was applied repeatedly, and after a predetermined number of repetitions, the length of the crack that appeared in the CT specimen was measured using a microscope. did.

Figure 4 shows the stress intensity factor width ΔK and the crack propagation speed da/d.
It is a diagram showing the relationship with N, and both ΔK and da/dN are shown on a logarithmic scale. The stone in Figure 4 is.

This corresponds to the floors shown in the table above. Note that the regression linear equation for each equation is expressed as 9th order.

N11l da/dN=6-11XIO-”-(Δ
K) Drop/0hN [L2 da/dN=d=9.35X
10-” ・(ΔK)1.411h3 da/dN=
1.11X10-” ・(ΔK)1.l*l!14
da/dN=1.39X10-”-(ΔK)lL?3
As is clear from FIG. 4, NCL4. In other words, for the roll that caused the peeling accident, the crack propagation speed d
It can be seen that a/d N is extremely large. - In general, the value of the stress intensity factor width ΔK is 20 to 20 during normal hot finish rolling.
Because it is considered to be 40kg・Kaku-1.

For example, when ΔK=40 kg·-1, da/d N is 3.4×10−′ m/cycle in the case of Nl14. On the other hand, in the case of N11l to floor 3 in the example, the da/dNO value is (2 to 5) Xl0-'m
/cycle, which is 1/7 to 1/17 of the above t4. Therefore, it is recognized that the reciprocal of the crack propagation speed, that is, the crack growth resistance, is 7 to 17 times higher.

On the other hand, as a range that can prevent peeling accidents, ΔK is 40k.
In g·m-4, da/dN≦1. OX10
-It is considered to be 'kaku/cycle. Therefore, in correspondence with the regression linear equation corresponding to the broken line shown in FIG.

da/dN=5.10×10−”−(ΔK)9·0 is considered to be the boundary.

〔Effect of the invention〕

Since the present invention has the structure and operation as described above,
In addition to the inherent wear resistance of high chromium cast iron rolls, crack propagation resistance can be significantly improved. That is, even if a minute crack occurs on the roll surface, the propagation speed of this crack is extremely low. Therefore, in the unlikely event that microcracks on the roll surface that occur during rolling cannot be detected by, for example, ultrasonic flaw detection, even if rolling is performed with microcracks existing on the surface, the cracks will not be detected. Because the development is extremely slow, cracks that are deep enough to cause a peeling accident will not grow within the number of repeated load applications during one rolling process.This can greatly improve production efficiency and reduce roll consumption. The effect is that it can be significantly reduced.

[Brief explanation of drawings]

FIGS. 1(a) and 1(b) are photographs showing the metal structure of the outer shell layer of the roll observed from the surface side in the examples of the present invention, respectively;
2(a) and 2(b) are a front view and a right side view showing the CT test piece, respectively, and FIG. 3 is an explanatory diagram showing the sampling position of the CT test piece shown in FIG. FIG. 4 is a diagram showing the relationship between the stress intensity factor width ΔK and the crack propagation speed da/dN. 1st mouth (O-) (b) Cat = 2nd concave tail 3 m,,,,, -+--~ 4th concave tail

Claims (2)

[Claims] (1) Weld and integrate the outer shell layer and the inner layer, crystallize graphite in the structure of the outer shell layer, and reduce the outer shell layer to da/dN.
≦5.1×10^-^1^9・(ΔK)^9^. ^8 However, da/dN: Fatigue crack propagation speed (mm/cycle
)ΔK: Stress intensity factor fluctuation range (kg/mm^3^/^2
) High chromium cast iron roll with excellent crack growth resistance. (2) The chemical composition of the outer shell layer is C2.0 to 3.5% by weight.
, Si1.0-2.2%, Mn0.5-1.0%, P0
．． 0.08% or less, S0.06% or less, Ni4.0-10.
0%, Cr5.0-10.0%, Mo0.1-5.0%
The high chromium cast iron roll having excellent crack growth resistance according to claim 1, wherein the remainder is substantially Fe.