JPH0241575B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) This invention is applicable to rolls used in rolling where the rolling load is large and the contact time between the roll and the rolled material per roll rotation is long, such as rolling steel pipes with a mandrel mill in a seamless steel pipe manufacturing process. Regarding. (Conventional technique) In hot rolling of long steel, steel plates, steel pipes, etc., when the rolling load is large and the contact time on the rolling surface between the roll and the rolled material per roll rotation becomes long, the thermal load that the roll receives increases. growing. In such a case,
Not only does the temperature of the roll surface rise, but the heat input into the roll also increases, and the temperature several millimeters below the surface becomes very high. This rise in the temperature of the surface layer of the roll is caused by the fact that the roll is water-cooled as soon as it leaves the arc of contact with the rolled material, so the roll is given a thermal cycle of increasing and decreasing temperature each time it rotates.
This is the main cause of early thermal cracking on the roll surface. Moreover, this temperature rise on the surface layer of the roll accelerates the development of cracks inside the roll, often leading to breakage of the roll. Furthermore, the development of thermal cracks on the roll surface
This results in early surface roughening due to roll seizure and thermal fatigue wave breakage of the roll surface layer. The above-mentioned increase in heat input to the roll surface layer is caused by an increase in the contact arc surface between the roll and the rolled material due to an increase in the reduction ratio during rolling, as well as when the rolling speed is slow, or when the groove side surface or the groove side surface in groove rolling increases. This is caused by an increase in the contact time between the roll and the rolled material when rolling with large diameter rolls. In particular, in continuous rolling of seamless steel pipes using a mandrel mill with 6 to 7 stands, the amount of reduction in stands 1 to 3 in the previous stage is large, and especially in rolling with a semi-floating mandrel mill, the hole roll load on the No. 2 stand is large. There is a lot of wear and tear on the rolls. In normal rolling using rolls with a large rolling reduction and a bottom diameter of about 300 mm as described above, the contact time with the rolled material is often 0.02 seconds or more per rotation of the roll. In general, when the roll is in contact with the hot rolled material for more than 0.02 seconds per roll rotation, the development of thermal cracks on the surface of normal rolls increases, and the cracks even bulge. Causes early skin irritation. When low carbon, low alloy cast steel rolls with excellent heat cracking resistance are used for rolling under such high heat loads,
Although thermal cracks and bulges in the rolls are eliminated, in the case of groove rolling, the relative sliding friction is large, so seizing is likely to occur, and this seizing causes early surface roughness, making it unusable. Therefore, the inventors tested spheroidal graphite cast iron as disclosed in Japanese Patent Publication No. 55-31177, but with such spheroidal graphite cast iron, the seizure phenomenon disappeared and the roll skin remained in good condition. Although it is kept, the wear and tear is very high. Roll surface hardness is required to improve wear resistance. Therefore, in order to improve the wear resistance of the rolls, we conducted tests using high-alloy cast iron rolls with a reduced amount of spheroidal graphite and increased eutectic carbides, and as a result, we found that the wear resistance was improved to some extent. However, thermal cracks occurred and progressed rapidly, and the rolls had to be replaced due to rough skin after several hundred rolling operations, that is, approximately 8,000 rolls. (Problems to be Solved by the Invention) The present invention aims to solve the above-mentioned problems in the prior art and provide a hot rolling roll with excellent heat load resistance, wear resistance, and seizure resistance. It was done for a purpose. (Means for Solving the Problems) The gist of the present invention is to provide a roll for hot rolling of steel, which has a contact time with the rolled material for 0.05 seconds or more per rotation of the roll, and has a C: 3.2 by weight. ~3.8%, Si≦2.8
%, Mn≦1.0%, Cr≦1.0%, Ni≦4.0%, Mo:
0.5-3.0%, to further make graphite spheroidal
Contains Mg or Ca, and further contains V, Ti, Nb and W
The total amount of one or more of the following is 1.0% or less, the balance is Fe and unavoidable impurities, and the area ratio of black grain is 10 to 35% and the area ratio of MC type eutectic carbide is 2 to 10%. This is a high heat load resistant, abrasion resistant, and seizure resistant roll. Below, the process and experiments to arrive at this invention will be explained in detail. First, the inventors investigated in detail the roughening phenomenon of various roll materials, and found that the contact time of the roll with the pressure edge material greatly affects the amount of heat input to the roll surface layer.
As shown by line A in Figure 2, it was found that thermal cracks on the roll surface significantly progressed when the contact time was 0.1 seconds or more. Note that line A in Figure 2 shows the roll at 550° with a contact time (high temperature holding time) of 0.05 to 0.3 seconds.
It shows the depth of thermal cracks after heating up to 100℃ and cooling down 2000 times. As a result of research, the inventors have found that adjusting the amount of graphite is extremely useful in order to improve the heat cracking resistance of the roll and to obtain a roll that does not impair seizure resistance against sliding friction. I found out. Therefore, in order to improve the heat cracking resistance of the roll, the inventors first added it to the basic spheroidal graphite cast iron.
Ten types of rolls shown in Table 1 were manufactured by adding Mo and subjected to thermal cracking tests.

【table】

[Table] As a result, the heat crack resistance properties were significantly improved, and the crack index η (η = crack initiation depth of conventional ductile cast iron/crack of ductile cast iron with Mo added depth)
That is, the ratio of the crack depth of the spheroidal graphite cast iron containing Mo according to the present invention to the thermal crack depth of the alloy cast iron that does not contain Mo is as shown in Fig. 1, which is 2%.
In the case of roll material containing Mo, approximately η=
It became 4. As can be seen from Figure 1, the heat cracking resistance characteristics of Mo
The effect is seen from the addition of 0.5% of Mo, reaching the maximum at 2% Mo, almost saturated, and a stable state can be obtained even when Mo is added up to about 3%. However, if more than that is added, the component balance will be disrupted, it will be difficult to produce graphite, and the amount of Mo-based composite carbide will increase, so 3%
is the limit. Next, the inventors found that increasing the amount of carbides is effective in improving hot wear resistance, but that large crystals such as Fe ₃ C, M ₇ C ₃ and M ₂₃ C ₆ If the amount of carbide is increased, the large crystallized carbides are likely to become cracked and the heat cracking resistance will be reduced. is an area ratio of 2 to 10
I found that it is good to keep it within the range of %. Therefore, the inventors focused on this point, and in order to disperse and crystallize carbides in the cast iron base, the inventors
As in Test Nos. 11 to 16 of 2, one of V, W, Nd, and Ti was added.

[Table] Figure 2 shows several examples of improvements in crack resistance due to the addition of these MC-type carbide-forming elements. B line, C line, and D line are each 1% Mo with W added, 0.5% V added, Ti+
The Nb addition example, E line, and F line are each examples with 2% Mo added, and all have shorter contact times than the conventional A line.
The crack depth decreases significantly between 0.05 seconds and 0.3 seconds, especially over 0.1 seconds. However, if too many of these MC-type carbide-forming elements are added, the toughness of the base will be reduced, so the amount should be limited to around 1%, and even when two or more MC-forming elements are added, the total amount is It was found that it is better to keep it below 1%, and since this also consumes the C contained, it also reduces the amount of C that graphitizes, so 1% is the upper limit. . Therefore, the total content of these elements that cause the formation of MC type carbides was limited to 1.0% or less, and the amount of crystallized graphite and the amount of crystallized carbides were kept within the range of appropriate values described below. As mentioned above, the crystallization of MC type carbides increases the hardness of the base and improves the wear resistance. (Function) Next, there are added elements in the high heat load resistant and wear resistant roll of the present invention. The roles of Si, Ni, Cr, etc. will be explained. Since Mo has already been mentioned, it will be omitted here. First, Si is an element that adjusts the amount of graphite crystallization in the Mo-containing graphite cast iron of this invention, and when added in a large amount, the amount of graphite crystallization increases, and in the absence of carbide-forming elements such as Cr, Si is an element that adjusts the amount of graphite crystallization. When added in excess of 100%, the solidification rate in normal sand mold casting results in almost no carbide crystallization, graphite crystallization, and a material containing a large amount of ferrite. Such a material is unsuitable for use as a roll material in terms of hardness and wear resistance, so the Si content should be 2.8% or less.
It is necessary to adjust the amount of Si added in accordance with the amount of Cr and MC-generating elements added. Furthermore, it is easy to make adjustments by considering the addition of Si inoculation along with Mg and Ca inoculated for the purpose of graphite spheroidization. Ni is an element that increases the toughness of the matrix, and like Si, it is an element that promotes graphitization and crystallization of C.
When Ni is added from 3% to 4%, the hardenability of the base of Mo cast iron according to the present invention is greatly increased, making it possible to make it martensitic, and it also improves solidification cooling.
By subsequent heat treatment, it can be turned into bainite or fine lamellar pearlite. Ni is added to adjust the strength and hardness of the base, and the upper limit is about 4%; if more than this is added, the characteristics of Ni will be too pronounced, making it difficult to adjust the overall structure.
The upper limit was %. Cr is a carbide-forming element, and V, Ti, Nb,
Together with MC-type carbide forming elements such as W, 1%
If the adjustment is not made below, the amount of carbide produced will increase,
It becomes difficult to adjust the area ratio of MC type carbide to 2 to 10%. The Mn content was regulated to 1% or less.
If the content is higher than this, it inhibits the graphitization and spheroidization of C and expands the austenite region, resulting in a hardened state during heat treatment, which may cause cracks to occur. Mn is normally adjusted to 0.3 to 0.5%, but it is an element that is advantageous in forming the base into fine lamellar pearlite, and is particularly effective when Ni is small, but for the reasons stated above, the upper limit was set at 1%. Next, regarding the amount of graphite that affects the heat cracking resistance of the alloy-based spheroidal graphite cast iron material of the present invention, as shown in Figure 3, when the amount of crystallization is 10% or more, the heat cracking resistance is greatly improved. However, if it is less than 10%, the amount of eutectic carbides increases and cracks are likely to occur. Line G in Figure 3 shows the relationship between the area ratio of graphite and the thermal crack depth at a contact time of 0.2 to 0.3 seconds and Mo2%.
Line H shows an example that does not contain Mo. As you can see, the amount of graphite is approximately 10% in terms of the area ratio seen in the cross section.
If the cooling rate at the casting stage is controlled and adjusted to ~35%, the progression of thermal cracks can be largely inhibited. On the other hand, when the crystallization rate of graphite is increased to 35% or more, there are almost no crystallized carbides, so in that state,
Wear resistance decreases. However, the seizure resistance is significantly improved. Preferably, the crystallization rate of graphite is 20 to 25%,
When about 5% of MC type carbides are present, good seizure resistance is maintained, the base is strengthened, wear resistance is improved, and heat cracking resistance is also within an extremely good range.
Therefore, it is better to control the solidification of cast iron so that it falls within this composition range. In addition, in order to fall within this range, the amount of C should be
It should be within the range of 3.2% to 3.8%. In addition, even with the material created in this way and having the above-mentioned structure, if the contact time of the roll with the rolled material during the rolling process is 0.05 seconds or less per roll rotation, there is almost no difference from the conventional material. , its characteristics have not yet been realized. The roll material of the present invention is effective when the contact time of the roll with the rolled material during rolling is 0.05 seconds or more per roll rotation, so in the present invention, the contact time is 0.05 seconds or more. limited to the above. It should be emphasized that the characteristics of the present invention are particularly evident in hole shapes with a high slip ratio. (Example) Next, results of an example of Mo spheroidal graphite cast iron according to the present invention will be shown. Table 3 shows the chemical components of the rolls in Examples. In the example, this roll was installed in the No. 2 stand of a semi-floating mandrel mill in a seamless steel pipe manufacturing process. This roll had a lifespan 3 to 4 times longer than the conventional Adamite roll. In addition, in this example, the roll is
The contact time with the rolled material per rotation was approximately 0.2 seconds.

[Table] (Effects of the invention) Since this invention is constructed as described above,
Problems caused by roll wear and seizure, as well as the frequency of roll replacement, can be reduced.Therefore, the decline in equipment operating rate can be suppressed, productivity can be maintained at a high level, and the roll consumption rate can be lowered. play.

[Brief explanation of the drawing]

Figure 1 shows the thermal fatigue of the roll according to this invention.
Figure 2 is a diagram showing the relationship between the crack improvement index and Mo content, Figure 2 is a diagram showing the relationship between the depth of thermal cracks and the contact time between the roll and the rolled material, and Figure 3 is the relationship between the depth of thermal cracks and It is a figure showing the relationship with the space factor of graphite.

Claims (1)

[Claims] 1. Contact time with rolled material per roll rotation is 0.05
A roll for hot rolling of steel, which
By weight, C: 3.2-3.8%, Si≦2.8%, Mn≦1.0%, Cr≦1.0%, Ni≦4.0%, Mo: 0.5-3.0%, and Mg or Ca to make graphite spheroidal.
and one or more of V, Ti, Nb, and W in a total amount of 1.0% or less, the balance being Fe and unavoidable impurities, and the area ratio of graphite is 10 to 1.0%.
35%, and the area ratio of MC type eutectic carbide is 2 to 10%. A roll with high heat load resistance and wear resistance.