JPS6111310B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a cold rolling work roll with excellent wear resistance and spalling resistance. The main material properties required for cold rolling work rolls include wear resistance and spalling resistance. If the wear resistance is poor, scratches and dulls on the surface of work rolls will disappear quickly and the surface will become rough, resulting in short roll replacement cycles and problems such as reduced rolling efficiency. On the other hand, spalling is caused by a thermal shock in which the roll surface is rapidly heated and cooled due to slips between rolls, rolling plates, and rolls, and the clutch progresses due to the subsequent rolling load.
Or, if a roll is ground after being subjected to thermal shock and cracks are generated, and the remaining cracks are overlooked and reused, the remaining cracks may be caused by rolling load and residual stress in the roll. This progresses and leads to large-scale peeling damage of the surface layer. When a spalling accident occurs, a large number of peeled off pieces are scattered, which is not only dangerous but also leads to the roll being discarded, resulting in a large economic loss. Because work rolls for cold rolling have high hardness, they tend to generate cracks due to thermal shock, and the cracks that occur tend to propagate, making them susceptible to spalling, and there is a demand for improved spalling resistance. Improved wear resistance of conventional cold rolling work rolls〓〓〓〓〓
As a means to do this, measures such as increasing the hardness and increasing the amount of component elements such as C have been taken. However, simply increasing the hardness, for example, increases the susceptibility to thermal shock that occurs during accidents such as slips and drawing during rolling, making deep cracks more likely to occur, and eventually surface peeling accidents more likely to occur. Therefore, measures taken to improve wear resistance must also improve spalling resistance, but at least not degrade it. However, until now, it has not been clarified how wear resistance depends on material factors. Furthermore, it has not been clarified by what mechanism the occurrence of cracks due to thermal shock and spalling accidents due to their propagation occur, and how they depend on material factors. Therefore, attempts to develop cold work rolls with excellent wear resistance and spalling resistance have not been successful so far. The present invention overcomes the above problems, provides wear resistance,
The object of the present invention is to provide a work roll for cold rolling that is excellent in all properties including spalling resistance. In order to achieve the above object, the present inventors conducted a detailed investigation into the material factors governing the wear resistance of roll materials and the spalling process. As a result, we discovered that wear resistance is mainly controlled by the hardness and the volume fraction of carbides contained in the material, and by clarifying the relationship between the volume fraction of carbides and the component composition, we It was possible to predict wear resistance from the combination of hardness and hardness. In addition, the inventors have determined that the thermal shock resistance, particularly the depth of cracks that occur when subjected to thermal shock, and the resistance to the process of crack propagation and spalling are determined by the fracture of the material of the outer layer of the roll. It was found that the toughness value K _IC is controlled. Furthermore, in addition to clarifying how K _IC depends on hardness and component composition, it is necessary to adjust the hardness and C and Cr contents within appropriate ranges in order to improve K _IC and improve spalling resistance. It has been found that it is effective to add appropriate amounts of Mn and V while limiting the amount of Mn and V. The present inventors completed the present invention based on the above knowledge. That is, the present invention provides C0.7 to 1.6
%, Si0.15~1.6%, Mn0.15~1.6%, Cr3.5~12
%, the relationship between C and Cr satisfies the following simultaneously: 16Cr%+15×C%27 Cr%21×C%-9, and Mo0.4~3
%, V0.2~2%, or two types, if necessary, Ni1% or less, the balance consisting of iron and impurities, and the surface hardness is adjusted to Hv720~800 for wear resistance and spalling resistance. This is a work roll for cold rolling with excellent properties. The present invention will be explained in more detail below. The present inventors have discovered that C0.6-1.4%, Si0.4-2.0%,
Mn0.4~1.0%, Cr3.0~13%, Mo0.2~3.0%,
Test pieces with various composition combinations in the range of V0.1-3.0% were quenched and tempered to increase the hardness of Hv0.4-1.0%, Cr3.0-13%, Mo0.2-3.0%,
Wear tests were performed on test pieces with various combinations of component compositions in the range of V0.1 to 3.0% by quenching and tempering to adjust the hardness to a range of Hv560 to 860. As a result, it was found that hardness and carbide volume fraction mainly control wear resistance. That is, for example, when the relationship between hardness and wear amount was investigated for a 5% Cr material by varying the amount of C, the results shown in FIG. 1 were obtained. As can be seen from FIG. 1, the higher the hardness, or for the same hardness, the greater the amount of C, the less the amount of wear. Next, when the Cr content is changed for the 0.85% C material, the results shown in Figure 2 are obtained, and as the Cr content increases, the amount of wear decreases. In this way, even if the hardness is constant, the wear resistance changes greatly depending on the combination of component compositions, but the inventors believe that the wear resistance is determined not only by the hardness of the matrix but also by the carbide particles contained in the material. Thinking that it plays a role in resisting wear, I rearranged the wear test results in terms of hardness and carbide volume percentage. As a result, as shown in FIG. 3, it was found that given the hardness and carbide volume fraction, a constant amount of wear was exhibited regardless of the component composition of the material. According to Figure 3, at any hardness level, when the carbide volume fraction reaches 12%, the wear resistance improves with the amount of carbide, but once it exceeds 12%, the effect becomes saturated even if the amount of carbide is increased further. has become clear. Regarding the lower limit of carbide volume fraction, the amount of wear increases as the amount of carbide decreases;
Especially for those with Hv700 or higher, the carbide volume fraction is 3.
% or less, the amount of wear tends to increase rapidly〓〓〓〓〓
Therefore, it is desirable that the lower limit of the carbide volume fraction be 3%. Therefore, in order to investigate the influence of C and Cr on the carbide volume fraction, the relationship between the carbide volume fraction and C and Cr was investigated for each sample material, and the results shown in FIG. 4 were obtained. In Figure 4, an equal carbide volume fraction line can be drawn in relation to C and Cr, and according to this, the contour line corresponding to a carbide volume fraction of 12% is Cr% + 15 × C% = 27, and The contour line corresponding to a carbide volume fraction of 3% is Cr%+15×C%=16. Next, regarding spalling resistance, we investigated both susceptibility to thermal shock and resistance to the progression of cracks caused by thermal shock due to rolling load and residual stress possessed by the rolls. In this study, susceptibility to thermal shock was represented by the depth of cracks caused by thermal shock, and resistance to the progression of cracks was represented by the critical depth of cracks at which cracks rapidly progress, and these representative values were In order to clarify which material-related factors govern the fracture mechanics, we carried out fracture mechanics analysis and experiments. As a result, both the depth of cracks caused by thermal shock and the critical crack depth at which rapid crack propagation occurs are both the material's fracture toughness value K.
I discovered a new fact that it is controlled by _IC . In other words, when a relative slip occurs between the roll surface and the rolled plate or the mating roll, the roll surface is heated due to frictional heat, and then cooled by contact with the lubricant, but during this cooling process, This tensile thermal stress causes cracks. This crack has a stress intensity factor value K that corresponds to its depth and the magnitude of the thermal stress occurring around it. Initially, the value of K increases as the crack gets deeper, but from the surface When the depth of K exceeds a certain limit, the stress caused by thermal shock changes from tensile to compressive, so the value of K decreases. In general, if the value of K becomes lower than K _IC , which is the fracture characteristic value of the material, the crack will stop progressing, so the higher the K _IC of the material, the shallower the crack, the more K will fall below K _IC , and the crack will stop progressing. Therefore, cracks that occur when subjected to thermal shock become shallower. The present inventors have quantitatively proven this through analytical calculations of thermal stress and stress intensity factors, as well as experiments in which thermal shock is applied to roll materials and the depth of cracks generated is measured. Furthermore, if these cracks caused by thermal shock are not completely removed by subsequent re-grinding and are overlooked and remain and are reused, the cracks will be affected by the rolling load and residual rolls. We analyzed the fracture mechanics to see how it progresses due to stress. As a result, both the contact surface pressure and residual stress due to the rolling load are compressive, but cracks develop due to the force that tries to bite the crack surfaces against each other due to shear stress in the compressive stress field, and the conditions for this development are Under such compressive stress, the fracture toughness value K _IC of the material is the same as under normal tensile stress. Saha, K
It was found that the value increases approximately in proportion to the square of _IC . As mentioned above, K _IC plays an important role in both the generation process of thermal shock cracks leading to spalling and the subsequent propagation process. Therefore, the present inventors conducted a fracture toughness test using the same test materials as those used in the above-mentioned study of wear resistance, and investigated the effects of component composition and hardness on K _IC . In Fig. 5, the solid line shows K _I when the hardness is set to Hv750 and the combination of C and Cr amounts is changed.
It shows how _C changes. It can be seen from this that reducing the amount of C and increasing the amount of Cr are both effective in increasing K _IC and improving spalling resistance. However, there is a limit to increasing K _IC only by adjusting the amounts of C and Cr.
On the other hand, when Mo and V are added, as shown by the broken line in FIG. 5, it becomes possible to significantly improve K _IC . However, K _IC due to the addition of Mo and V
The improvement effect of Cr does not appear below 3.5%. Figure 6 shows the results of a detailed investigation of the KIC improvement effect of Mo and V. Figure 6 shows 0.85%C-5
2 is a diagram showing how K _IC changes with hardness when Mo0.2 to 3% and V0.1 to 2% are added singly or in combination to a Cr material. As can be seen from Figure 6, the 0.2% Mo + 0.1% V additive material has no improvement effect on K _IC compared to the Mo and V non-additive material.
stomach. On the other hand, when it comes to 0.4% Mo or 0.2% V additive material, K _I
The improvement effect of _C will appear. If the amounts of Mo and V are further increased, K _IC will show a maximum value at a certain hardness value, but this effect is only 3% in the case of Mo.
In the case of V, it saturates at about 2%. From these facts, in order to improve K _IC with Mo and V,
0.4-3% Mo and 0.2-2% V should be added alone or in combination. Next, we will examine the relationship between wear resistance, spalling resistance, and hardness. As can be seen from Figure 6,
Since the improvement effect of K _IC by Mo and V cannot be expected when the HV is 800 or more, the HV should be 800 or less from the viewpoint of spalling resistance. In addition, from the viewpoint of wear resistance, if the hardness is HV720 or less, indentation flaws will occur due to foreign matter mixed during cold rolling, so the hardness should be HV720 or higher. As mentioned above, in the present invention, the hardness is regulated to Hv720 to 800, but along with this hardness regulation, adjustment in terms of component composition is required. i.e. maximum hardness Hv800
In order to obtain this, the hardness during quenching must be Hv800 or higher. This quenching hardness and C, Cr
When the relationship between the contents was investigated, it was as shown by the dashed dotted line in Fig. 4, and it was found that in the region to the right of this dashed dotted line, the quenching hardness was Hv800 or higher. This one-dot chain line is expressed by the relational expression Cr%=21×C%−9, so in order to have a quenching hardness of Hv800 or higher, it is necessary that Cr%21×C%−9. From the viewpoint of wear resistance and spalling resistance,
We have explained the composition and hardness regulations, but to summarize the amount of C and Cr, it is the area surrounded by the solid line and the dashed-dotted line in Figure 4, that is, Cr3.5% Cr% + 15 × C%16 This is the area surrounded by Cr%+15×C%27 Cr%21×C%−9. From this relational expression, the ranges of C and Cr are C0.7~1.6% and Cr3.5~12, respectively.
%. As for Mo and V, as mentioned above, Mo0.4~3
%, V0.1-2%. Furthermore, the hardness is Hv720~800
It is. In the present invention, Si and Mn are contained as deoxidizing agents and as hardenability improving elements, but if excessively added, deoxidizing products increase and have a negative effect on the cleanliness of steel, so Si0.15 ~1.6%,
Mn should be 0.15 to 1.6%. Ni is an element effective in improving hardenability, and is added as necessary in the present invention, but if added excessively, it inhibits the spheroidization of carbides, so
% or less. Next, it will be shown together with examples of the present invention. Work roll materials for cold rolling having four types of component compositions were melted and subjected to quenching and tempering treatments to obtain test materials. Table 1 shows the composition, heat treatment conditions, and hardness of the test materials. In Table 1, No. 1 is a conventional work roll material for cold rolling, and No. 2 to 4 are work roll materials for cold rolling of the present invention. For these four types of roll materials, wear amount, fracture toughness value, and if deep cracks caused by thermal shock remain,
Table 2 shows the measurement results of the critical crack depth that rapidly propagates and causes a peeling accident. However, the wear amount, impact crack depth, and critical crack depth are each expressed as relative values, with the conventional roll material No. 1 being 1.0. Among the roll materials of the present invention, No. 2 mainly focuses on improving spalling resistance, and has improved thermal shock crack depth by about 1/5 and critical crack depth by about 2 times. Improvements were also seen in wear resistance. No. 3 mainly focused on improving wear resistance, and the amount of wear was reduced by nearly 1/3, and the effect of improvement in spalling resistance was also seen. No. 4 is No.
2. Further enriched in C, Cr, Mo, and V than No. 3,
The aim is to improve both wear resistance and spalling resistance by appropriately adjusting the hardness.
The amount of wear is reduced to 1/3, and the depth of thermal shock cracks is approximately
1/7 limit〓〓〓〓〓

【table】

[Table] The crack depth has been improved by 1.8 times. As described above, according to the present invention, the wear resistance and spalling resistance of work roll materials for cold rolling can be improved at the same time, and furthermore, the wear resistance can be improved depending on the usage conditions of the roll. It is possible to obtain a work roll for cold rolling, which allows both properties of spalling resistance to be freely manipulated, and which guarantees dramatically superior roll performance compared to conventional roll materials.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between hardness and wear amount depending on the amount of C for 5% Cr material, and Figure 2 is 0.85.
%C is a graph showing the relationship between hardness and wear amount depending on the amount of Cr. Figure 3 is a graph showing the relationship between carbide volume fraction, hardness, and wear amount. Figure 4 is a graph showing the relationship between the amount of C and Cr and the amount of wear. A graph showing the relationship between the carbide volume fraction and quenching hardness. Figure 5 is a graph showing the relationship between the amounts of C, Cr, Mo, and V and the fracture toughness value K _IC . Figure 6 is a graph showing the relationship between the amount of C, Cr, Mo, and V and the fracture toughness value K IC. It is a graph showing the relationship between hardness and fracture toughness value K _IC when Mo and V are added to the material. 〓〓〓〓〓

Claims (1)

[Claims] 1 C0.7-1.6%, Si0.15-1.6%, Mn0.15-1.6
%, Cr3.5~12%, and the relationship between C and Cr simultaneously satisfies 16Cr%+15×C%27 Cr%21×C%−9, and Mo0.4~3
%, V0.2-2%, the balance is iron and impurities, and the surface hardness is adjusted to Hv720-800, and has excellent wear resistance and spalling resistance. . 2 C0.7~1.6%, Si0.15~1.6%, Mn0.15~1.6
%, Ni1% or less, Cr3.5-12%, the relationship between C and Cr is such that it simultaneously satisfies 16Cr%+15×C%27 Cr%21×C%-9, and Mo0.4 ~3
%, V0.2-2%, the balance is iron and impurities, and the surface hardness is adjusted to Hv720-800, and has excellent wear resistance and spalling resistance. .