JPS592738B2 - Baked roll material with excellent spalling resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the improvement of a forged steel quenched roll material used as a backup roll for hot rolling or cold rolling, and particularly to improving its spalling resistance.

The main material properties required for backup rolls for hot rolling or cold rolling include spalling resistance and wear resistance.

Backup roll spalling is caused by minute cracks caused by thermal shock or transfer fatigue that propagate during roll transfer, and can reach tens of millimeters in depth during one roll installation. Depending on the situation, surface peeling may occur.

As a backup roll material, medium carbon low chromium steel that has been quenched and tempered to have a surface hardness (Hv) of 340 to 450 has been widely used.

Its chemical composition is generally C014-0.5, S
iO, 15-1.6%, Mn 0.15-1.6%, N
i O, 99 or below, Cr 2. C) -5,0%, Mo
0.4'% or less, small amount of VO12O12 and other impurities,
The remainder consists of iron.

However, in recent years, due to the demand for improved wear resistance, there is a tendency to increase the surface hardness of rolls by increasing the amount of C and lowering the tempering temperature.
It is expected that demand for high hardness rolls with increased front and back hardness will increase.

However, when increasing the C content and lowering the tempering temperature to increase surface hardness, there is a problem that at the same time, the other main property, spalling resistance, deteriorates.

Improvements in this spalling resistance have conventionally been achieved mainly by increasing transfer fatigue strength, that is, the strength limit under repeated rotation, and by increasing toughness, such as impact value, in order to prevent brittle fractures that lead to peeling. It has been found that rolls with good rolling fatigue strength and impact values are also excellent in spalling resistance.

However, even with such roll materials, the reality is that the frequency of spalling in actual use has hardly been improved.

The present invention aims to overcome the above-mentioned problems regarding backup roll materials and to develop a new roll material that is excellent in both spalling resistance and abrasion resistance.

In order to achieve the above object, the present inventors conducted a detailed investigation into the spalling process in the backup roll, and found that the process that controls the spalling life is what is generally said to be the We discovered a new fact that fatigue crack propagation characteristics are completely different from transfer fatigue strength and toughness values, but are completely different from these characteristics.

Note that this fatigue crack propagation characteristic is a characteristic value based on the relationship between the crack propagation speed da/dn (mm/cycle) and the stress intensity factor ΔK (kg·rnm−3/2), as described later.

In other words, according to detailed research by the present inventors, microcracks on the surface when a backup roll is used occur relatively early from inclusions and grinding scratches existing on the roll surface, and even when transferred due to fatigue. Even if the strength is increased, the timing of crack initiation can only be slightly delayed, and it does not contribute much to the overall spalling life. For surface delamination caused by surface delamination, even if the toughness value is increased, the critical length of the crack can only be slightly increased, and therefore, if the crack growth rate is fast, there will not be much difference in the spalling life. It turns out there isn't.

However, unlike such transfer fatigue strength and toughness values, if the fatigue crack propagation characteristics are excellent, that is, if the propagation speed is sufficiently slow, even if a crack occurs on the surface, it will hardly grow, so it will be slow. It is unlikely that the crack will develop into poling, and even if it does begin to grow, the propagation speed will be sufficiently slow that it will be time to replace the rolls before the crack reaches the critical length that causes brittle fracture, and grinding will occur at that time. Fatigue crack propagation characteristics are the most important factor governing spalling, and improving this characteristic can improve spalling resistance. It has become clear that polling performance can be effectively improved.

Until now, there has been no attempt to improve spalling resistance by focusing on fatigue crack propagation characteristics, but based on the above findings, the present inventors have attempted to improve spalling resistance by improving the propagation characteristics. In order to achieve this by modifying the chemical composition of the roll material, we conducted more detailed research on the effects of various alloying elements on these properties.

As a result, it was found that Mo and (1) have a remarkable effect on improving these properties, and in particular, by adding a certain amount of both elements in combination, it was found that spalling resistance can be effectively improved. .

The present invention was completed based on the above findings.

That is, in the present invention, C0.6% or less and 0.95% or less, S i O, 15 to 1.6%, M n O, 1,5
-1.6 ratio, Ni 0.99% or less, Cr 3.0-5.
0%, M.

To provide a new backup roll material which is composed of 0.5 to 1.2 ratio, Vo, 2 to 0.8 ratio, balance iron and unavoidable impurities, satisfies wear resistance and also has excellent spalling resistance. It is something.

Both elements Mo and (2) have almost no effect on improving transfer fatigue strength or toughness, and there have been no examples of them being used as main components of backup roll materials.

In contrast, the present invention is characterized in that it has a unique composition in which both of these elements are enriched in specific amounts as spalling resistance improving elements.

The reasons for limiting the component composition of the backup roll material according to the present invention will be explained below.

C is an element useful for increasing the hardness of the roll material and imparting wear resistance, but as shown in Figure 6, the amount of C is 0.6
%, the amount of wear decreases significantly, while at about 0
．． When it exceeds 95%, uniform dispersion of carbides cannot be obtained, and toughness and other mechanical properties deteriorate.

Therefore, the amount of C is set to more than 0.6% and less than 0.95%.

The test conditions are as follows.

Test material: 0.3 to 0.85 C-0,7S i -0
,5Mn-3,5Cr-0,I Ni-0,6M
The hardness of o-0,3V material was adjusted to Hv approximately 500.

Mating material: 0.85 C-3,5Cr (Hv 880
) Material test conditions: Pressing force Pmax 140 ky/71
-71 Slip rate: 9% Under lubricated conditions, Si and Mn are used as deoxidizers in the steelmaking process, and are also effective elements for improving the hardenability of roll materials.

For this reason, it should be added in an amount of about 0.15% or more, but if added excessively, deoxidation products will increase and the cleanliness of the steel will be impaired.

Therefore, Si and Mn are each 0.15 to 1.6%
can be added within the range of

Ni is an element useful for improving hardenability and refining crystal grains.

However, if it is added excessively, it will inhibit the spheroidization of the carbide.

Therefore, the upper limit of the amount added is 0.99%.

Cr is an important element that not only imparts toughness but also forms carbides to increase hardness and wear resistance.

Regardless of whether C or Cr is increased, the volume fraction of carbides increases, and wear resistance improves with this increase.However, if only the amount of C is increased without changing the amount of Cr, fine Cr7C3 type carbides are formed. In addition, coarse cementite-type (Cr3C2) carbides are formed, which significantly reduces toughness, so it is necessary to increase the amount of Cr as well as the amount of C.

In this case, if the Cr content is less than about 3.0%, toughness will be insufficient, while if it is added in excess of about 5.0%, problems will arise in steelmaking technology, making it difficult to produce a sound steel ingot. .

Therefore, it is added in a range of 360 to 5.0%.

As mentioned above, Mo and (2) are effective elements for improving fatigue crack propagation characteristics and increasing spalling resistance.

The effect of adding Mo and V will be explained using steel having the chemical composition shown in Table 1 as an example.

Figure 1 shows roll materials made from each of the steels listed in Table 1, which are heat-treated to achieve a hardness of Hv 500.
/61~4: Quenching temperature 930℃, tempering temperature 540℃
℃, steel A65-7: quenching temperature 930℃, tempering temperature 575℃), respective crack propagation speed d a / d
It is a graph in which n (am/cycIe) and stress intensity factor range ΔK (kg·mm−3/2) are measured and plotted.

The fatigue crack propagation characteristics are determined as a function of the propagation speed da/dn and the stress intensity factor range Δ corresponding to the stress load amplitude. Therefore, the closer the curve is to the right in the figure, the more quickly the fatigue crack propagation characteristics are determined. It can be evaluated as follows.

In the figure, curve numbers 1 to 5 represent the steel numbers in Table 1, respectively.

From this, it is recognized that the fatigue crack propagation characteristics are improved as the amount of Mo added increases.

Furthermore, as shown in the fracture toughness and transfer fatigue strength (repetition number I x 106 times) listed in Table 1, Mo and ■ do not have a substantial effect on these properties, and only affect fatigue crack propagation. It exhibits a unique effect regarding its characteristics.

Furthermore, Fig. 2 shows the fatigue crack growth lower limit stress intensity factor ΔKth (kg・mm a/2) for each steel listed in Table 1 above.
) is a graph showing the relationship between the value of Mo and the amount of Mo added, and the numbers in the figure represent the respective steel numbers in Table 1.

This stress intensity factor ΔKth is determined by the crack propagation speed d a /
d n becomes 0 (i.e. crack growth slows down)
This indicates the limit value of the stress intensity factor range Δ under repeated loading, and is a characteristic value indicating the quality of fatigue crack propagation characteristics.

The higher the value, the better the fatigue crack propagation characteristics.

As shown in FIGS. 1 and 2 above, in order to fully exhibit the effect of MO addition and obtain good fatigue crack propagation characteristics, it is desirable to add about 0.5 modulus or more of MO.

However, even if it is added in a large amount exceeding about 1.2 parts, it is hardly expected that the effect will increase.

Therefore, it is preferably added in a range of 0.5 to 1.2%.

FIG. 3 is a graph showing the fatigue crack propagation characteristics when the addition amount is changed, and the curve number in the figure represents the steel number listed in the first display.

As is clear from the comparison between the same figure and the above-mentioned FIG. 1, it is recognized that the fatigue crack propagation characteristics are significantly improved by the combined addition of Mo and ■.

It is also clear that the improvement effect further increases as the amount added increases.

However, ■ it is difficult to obtain a sufficient effect when the ratio is less than about 0.2%, and on the other hand, even if it is added in excess of about o, s%, no significant increase in effect can be expected, resulting in the economic disadvantage of using a large amount of expensive 7a'V. accompany.

Therefore, ■ is added within the range of 0.2 to 0.8%.

In addition to the above component elements, P is inevitably mixed in during the steelmaking process.
, S and other impurities may be present within the range normally tolerated for this type of steel, for example P up to about 0.035%;
As long as S is about 0.040 or less, the spirit of the present invention is not impaired in any way.

ε Next, the material characteristics of the roll material of the present invention will be specifically explained with reference to Examples.

EXAMPLE The characteristics of each roll material having the composition shown in Table 2 were measured and compared.

In the same table, test materials AI, II, and ■ are comparative materials, and among these, al and ■ have typical compositions of conventionally commonly used roll materials.

Test materials ① to ② are roll materials having component compositions according to the present invention.

In addition, each sample material was subjected to the heat treatment shown in Table 3 in order to impart a high hardness of H■500.

Figure 4 shows the fatigue crack propagation characteristics of each of the above-mentioned test materials.

The vertical axis represents the crack propagation velocity (am/cyc Ie), and the horizontal axis represents the stress intensity factor range (kg·m712-3/2).

From the figure, it can be seen that the roll materials according to the present invention (Ita ■ to ■) have better fatigue crack propagation characteristics than any of the comparative materials (AI, II, 111).

Figure 5 shows an actual roll (roll diameter: 1200rILmφ) based on the fatigue crack propagation characteristics shown in Figure 4.
, Work roll diameter: 600m1φ, contact crab 140
kg/rItm, number of roll rotations in one installation: 1.4X 106 times), the number of roll rotations until a micro crack with a depth of 0.2 mm grows into a crack with a depth of 2.0 mm is shown. It is a graph.

From the figure, the number of transfers for the comparison materials (AI, II, and ■) is about 4×105 or less, while that of the roll material of the present invention (//6.1V to V) is about 1.8XI.
It is recognized that it has spalling resistance ranging from O6 to 1×107, and its life span is approximately 5 to 25 times longer than that of the former.

As described above, the present invention improves spalling resistance by increasing fatigue crack propagation characteristics, and also imparts a high degree of hardness to enhance characteristics as a backup roll material.
This guarantees dramatically superior durability compared to conventional materials.

[Brief explanation of drawings]

Figures 1, 3 and 4 are graphs showing fatigue crack propagation characteristics, and Figure 2 is the lower limit stress intensity factor and M. A graph showing the relationship between the addition amount and FIG. 5 is a graph showing the relationship between the number of roll transfers and the crack depth. FIG. 6 is a graph showing the relationship between the amount of C and the amount of wear.

Claims (1)

[Claims] I C0.6, over 0.95%, SiO, 15
~1.6 ratio, Mn 0.15-1.6%, NiO, 99
% or less, Cr 3.0-5.0%, Mo 0.5-1.
A backup roll material with excellent spalling resistance consisting of 2% Vo, 2 to 0.8%, the balance iron and unavoidable impurities.