JPH01167396A - Lubricant for hot-rolling stainless steel - Google Patents

Abstract

PURPOSE:To obtain the title lubricant which improves seizing to milling rolls and prevent the graphite dust from sedimenting in the conduit, by dispersing a specific amount of graphite dust in a viscous aqueous solution. CONSTITUTION:The subject lubricant is obtained by dispersing (A) 1-30wt.% of graphite dust of less than 10mum particle size (preferably average particle size of less than 1mum) to (B) a viscous aqueous solution such as an aqueous solution of water-soluble polymer such as acrylic acid polymer. The lubricant has preferably a viscosity of 1X10<3>-5X10<5>CP. The component B is preferably natural graphite or flaky graphite in Kisch graphite.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a lubricant designed to prevent seizing on rolling rolls during hot rolling of stainless steel.

[Background of the Invention and Problems with the Prior Art] Since stainless steel products are required to have a beautiful surface texture, it is necessary to prevent the occurrence of surface flaws as much as possible in the manufacturing process. However, in the past, during hot rolling of stainless steel in a tandem mill, surface flaws due to seizure on the rolling rolls often occurred. If such surface flaws occur, even if they are minor, not only will the product be unsuitable for mirror finishing, which will limit its use, but it will also require surface care by putting it into a polishing process. It's coming. Moreover, if the degree of flaw is significant, the product will be scrapped and the economic loss will be large.

Regarding the problem of surface flaws on stainless steel caused by seizure on the rolls, many studies have been made from the viewpoint of reducing the load on the rolls, rolling conditions, etc. However, at present, a sufficient solution from this aspect has not been achieved.

Further, studies have been made from the aspect of roll material, and it has been proposed that grain rolls are effective in preventing seizure. However, the use of grain rolls in the pre-finishing stages (1st to 3rd stands of the finishing rolling mill group), which have a large rolling load, causes a large amount of roll wear, and the freedom of width reversal rolling, which is essential in modern rolling technology, is lost. There is a problem. Even if the roll seizure problem is solved, mixed rolling of stainless steel and ordinary steel still has the disadvantage that the surface texture grade of ordinary steel products is slightly lowered. Therefore, the adoption of grain rolls is not a sufficient countermeasure in recent rolling technologies that aim at schedule-free rolling and rolling of different steel types.

On the other hand, lubricating oil has also been considered as another countermeasure. Lubricating oil is present between the rolling rolls and the rolled material and has the effect of lowering the coefficient of friction, so it is quite popular in rolling nine-pass steel. A similar effect was expected in rolling stainless steel and was thought to be effective in preventing seizure, but existing animal and vegetable oils, as well as mineral and synthetic lubricants do not have sufficient anti-seizure effects. The reality is that we are not getting it.

[Purpose of the invention]

If mixed rolling of different steel types, schedule-free rolling, etc. are used, it is desirable to use high chromium rolls as work rolls in the first stage of finishing where the rolling load is large. However, high chromium rolls are made of stainless steel and Therefore, the problem arises that seizure tends to occur during rolling of stainless steel.

The present invention is suitable for rolling stainless steel even when using work rolls with high chromium rolls and when rolling at high temperatures (e.g. 900 to 1200°C) and high surface pressures (e.g. lθ to 50 kg/m"). The purpose of this invention is to provide a lubricant that focuses on preventing steel roll seizure.

The main cause of surface flaws during stainless steel rolling is that the rolled material seizes on the rolls and is transferred to the rolled material, which makes it impossible to mix different steel types and reduce production efficiency. ing. The present invention also aims to solve the above-mentioned problems such as an increase in roll unit consumption and an increase in rolling down time, since rolls that have been seized must be replaced.

[Structure of the invention]

The present invention uses Graffito powder in a viscous aqueous solution of 1 to 30%
The present invention provides a lubricant for hot rolling of stainless steel, which is dispersed in an amount of % by weight.

The lubricant of the present invention is a suspension composed of graphite powder and a viscous aqueous solution, and its viscosity is lX 103~5
It is preferable that the graphite powder be adjusted to X1 x 103 cP (centipoise), and 90% by weight or more of the graphite powder is fine powder with a particle size of 10 pm or less.

The viscous aqueous solution is preferably a solution in which a water-soluble polymer thickener is dissolved in water.

The graphite used in the lubricant of the present invention may be natural graphite, Kish graphite, etc.
From the viewpoint of lubricity, natural graphite and scaly graphite belonging to Quiche graphite are preferred. In the present invention, water is used as a dispersion medium for such graphite powder, and appropriate viscosity is imparted to this water. The thickener may be one that thickens when dissolved in water, and its type is not particularly limited, but high-temperature thickeners such as acrylic acid polymers and carboxyvinyl polymers can be added in small amounts to increase viscosity. It is suitable because it is stable in terms of points. In addition, considering the corrosivity to rolling equipment if the lubricant scatters.

It is desirable that the thickener is neutral to weakly alkaline, and a thickener whose viscosity remains stable even after such pH adjustment is selected.

[Details of the Invention The presenters have been conducting research on roll seizure in stainless steel rolling for many years. Meanwhile, the cause of roll seizure is the high pressure and high speed rolling associated with larger slabs, and the reason why roll seizure is a phenomenon unique to stainless steel is that stainless steel has excellent oxidation resistance. I learned that the scale formation rate is slow (i.e., there is not enough time to form a scale layer sufficient to protect the new surface when a new surface is formed during short, multi-stage rolling). The fact that problems often occur in Lindem mills with high rolling speeds, and fewer cases in Stetskell mills, etc., also shows the importance of the protective effect of the scale mentioned above.

Based on such knowledge, the present inventors were able to develop the lubricant of the present invention based on the idea of supplying graphite from the outside as a substitute for the insufficient scale.

The basic concept of the present invention is to prevent metal contact between the roll and the stainless steel material to be rolled by replenishing graphite, and to eliminate the cause of seizure by preventing this metal contact. Graphite has a hexagonal crystal structure, and therefore has the function of causing sliding deformation on the cleavage plane to lower the coefficient of friction.

The lubricant of the present invention can also utilize this property to lower the frictional resistance between the rolled material and the roll, thereby lowering the surface pressure, and this also prevents seizure of the roll. That is, the present invention includes 1 to 30% by weight of graphite powder in water, and adjusts the viscosity of the water to ensure proper adhesion to the rolling rolls and to prevent the graphite powder from depositing in the conduit. We have developed a lubricant for stainless steel hot rolling.

In the composition of the lubricant of the present invention, graphite powder is an important substance in order to obtain the effect of preventing roll seizure. A suitable content of graphite powder in the lubricant is 1-30% by weight. If it is less than 1% by weight, the anti-seizure effect of graphite powder addition will be small, and if it exceeds 30% by weight, the discharge energy applied to the rolling rolls will become significantly large, which will not only impair economic efficiency but also cause graphite powder to be added in the conduit. There are concerns about problems such as sediment buildup and clogging.

Oils and fats are also conceivable as the liquid with which graphite powder is mixed, but since graphite powder does not necessarily have lipophilic properties, uniform dispersion and mixing is difficult. Furthermore, considering the metallurgical phenomenon of seizure, in which metal adheres to metal, lowering the roll temperature, that is, making the roll cooling water sufficiently effective, is meaningful in preventing seizure. However, the use of oils and fats tends to create a heat insulating film on the roll surface, which is disadvantageous in terms of roll cooling.

On the other hand, it is also possible to suspend graphite powder in simple water and supply it to the rolling rolls, but if simple water is used, not only does it have poor adhesion to the rolls, but also significant precipitation builds up in the conduit. Not suitable for actual use.

The present inventors conducted detailed research on prevention of clogging in conduits and improvement of adhesion to rolls, which are important issues when actually adopting a solid lubricating material called graphite. Hot rolling lines are shut down for about a week for repairs, etc., but during this period, the relationship between the viscosity of the aqueous solution and the particle size of the graphite powder to prevent the graphite powder in the lubricant that has accumulated in the conduit from settling, as well as roll adhesion. The present invention was achieved by clarifying the relationship between the amount and the viscosity of an aqueous solution.

First, we investigated the stability of dispersion of graphite powder, which acts as a solid lubricant, in a mixed aqueous solution, or in other words, the effects of the viscosity of the aqueous solution and the particle size of the graphite powder on the sedimentation of the graphite powder. The test was carried out by preparing a mixed aqueous solution with varying viscosity of the aqueous solution and particle size of the graphite powder, and after uniformly mixing this solution, it was allowed to stand and the degree of sedimentation of graphite was measured. The graphite powder used was obtained by pulverizing natural graphite, especially scaly graphite, into various particle sizes using a pulverizer. The viscosity of water was varied by changing the amount of thickener added. After the aqueous solution containing the graphite powder is mixed uniformly, it is placed in a 1ffi (liter) beaker and allowed to stand for one day and one week.Then, the liquid is collected from the upper layer, lower layer, and the middle part of the beaker and contained. The graphite content was measured.

The test results are shown in Table 1. As is clear from the table, if graphite powder with an appropriate particle size is used and the viscosity of the mixed liquid is properly adjusted, almost no sedimentation of the graphite powder will occur even if the graphite powder is left standing for one week. figure.

A product with excellent dispersion stability can be obtained. Namely.

More than 90% by weight of graphite powder has a particle size of 10/7
It can be seen that if the fine powder is 11 or less, uniform dispersion is maintained even in an aqueous solution with a low viscosity of 1.3×1×10 3 cP, and it can be left for one week. In addition, it is best to use as fine particles as possible in order to prevent the graphite powder from having a negative effect on the surface quality of the stainless steel that is the material to be rolled. It is preferable to use graphite fine particles having a value of lμ or less.

Next, we will discuss the results of an investigation into the influence of the viscosity of an aqueous solution containing graphite powder (referred to as a mixed solution) on the adhesion to the roll. The test was conducted by spraying the mixed solution onto a rotating rolling roll and measuring the amount of adhesion as shown in FIG.

In Figure 1, I is a diameter of 350s+s and a barrel length of 30s.
It is a 0-1 rolling roll and rotates at 30 rpm.

Reference numeral 2 denotes a mixed liquid supply device, which sprays the dispersion liquid from a nozzle 3 onto the surface of the roll 1 using a pump with a pressure of 100 kg/ct. 4 is a water supply device.

Water is sprayed from a nozzle 5 onto the surface of the roll 2 using a pump with a pressure of 3 kg 7 cm. This water supply corresponds to roll cooling water in hot rolling.

The graphite powder in the mixed liquid has an average particle size of 1.1 μm and a maximum particle size of 6 μm.The viscosity of the aqueous solution is adjusted by changing the amount of a water-soluble thickener made of acrylic acid polymer. We used a variation of . The mixed solution was sprayed onto a roll for 10 seconds using a mixed solution supply bag rI12 while simultaneously spraying water under the above conditions. After stopping the mixed liquid and water, collect the mixed liquid adhering to the roll.

The weight of the graphite powder contained therein was measured. The test results are shown in Table 2.

As can be seen from the results in Table 2, it can be seen that the amount of graphite attached to the roll is greatly affected by the viscosity of the mixed liquid. If the viscosity of the mixed liquid is too low, the graphite powder will be diluted by the roll cooling water and washed away, resulting in a poor adhesion rate. If the viscosity is too high, the discharge energy applied to the rolling rolls will increase, making it impossible to spray properly. It disappears.

A graphite powder-containing lubricant according to the invention.

The weight% of graphite powder in the mixed liquid is 1 to 30% by weight
The viscosity of the mixture was adjusted to within the range of lXl by adding an appropriate amount of thickener.
Table 2 shows that adjustment to x103 to 5x105 cP is necessary for actual operation.

There is no need to limit the thickener used to create such a viscous aqueous solution, but polymeric thickeners such as acrylic acid polymers and carboxyvinyl polymers exhibit stable thickening properties when added in small amounts. So it is suitable. Furthermore, depending on the mode of use, it is necessary to consider the corrosivity of the rolling equipment when the lubricant is scattered. For this purpose, it is best to adjust the pH of the lubricant to a neutral or weakly alkaline pH of 311. It is also important to select a thickening agent whose viscosity remains stable even when such pH adjustment is performed.

According to the lubricant of the present invention constructed in this way, the problem of seizure during hot rolling of stainless steel can be avoided, as shown in Examples below. The lubricant of the present invention is composed of water, graphite powder, and a very small amount of a water-soluble polymer thickener, so there is no need to take precautions like oil when disposing of waste liquid, and although it is a viscous aqueous solution, it can be When diluted with water, the viscosity decreases rapidly, so there are no problems such as clogging of drain pipes, and the above-mentioned objects of the present invention can be effectively achieved.

In addition, it can be suitably used in, for example, a rough rolling mill during hot rolling of stainless steel and three stands in the pre-finishing stage, and can be supplied in a spray form to the roll bit on the inlet side of the rolling mill. As a result, graphite powder contained in the lubricant is introduced into the roll bite and coats the roll surface, suppressing direct metal-to-metal contact between the roll and the rolled material, and preventing roll seizure.

In alternating rolling of different steel types, the lubricant of the present invention acts effectively during rolling of stainless steel.

At that time, whether or not the lubricating oil used during normal rolling is acting, there is no significant change in the anti-seizure effect.

EXAMPLES The effects of the lubricant of the present invention will be specifically illustrated below with reference to Examples.

Example 1 5US430LX (7) stainless steel (chemical composition value:
0.02χC, 0.51χSi, 0.28χMn.

0.017XP, 0.004! S, 17.23χC,
0.11χN1.0.41XNb) (7) The material to be rolled was rolled using a roll made of hot work tool steel (SKD61) with a diameter of 150
It was hot rolled with a roll of mm. Rolling temperature is 900°C9
One-pass rolling was carried out with a reduction ratio of 70%, and the material to be rolled was heated in a short time in a furnace containing inert gas to minimize the amount of scale generated on one surface (scale thickness approximately 1 μm). )s During hot rolling, 100kg
A lubricant was injected onto the roll using a pump with a pressure of /c+*''. At the same time, roll cooling water was injected onto the roll using a pump with a pressure of 3 kg and 7 cmg.

The lubricant used had an average particle size of 1.1μ and a maximum particle size of 5μ.
- The following graphite powder was mixed into a viscous aqueous solution using an acrylic acid polymer as a thickener, and the content of graphite powder and the viscosity of the mixed liquid were varied. By observing the morphology of the roll surface and the surface of the rolled material after rolling, the presence or absence of seizure was investigated under each lubricant condition. The results are shown in Table 3.

As is clear from the results shown in Table 3, it can be seen that the lubricant according to the present invention acts effectively in preventing seizure of the rolls during hot rolling of stainless steel.

To explain more specifically, under the condition of Comparative Example 1 where no lubricant was supplied, the surface of the rolled material was in a peeled state due to seizure of the rolls, and the exposed metal surface was observed. It was done.

On the other hand, no seizure occurred when the lubricant according to the present invention was used, and this can be considered to be because the surface of the rolled material was covered with graphite powder. Even when a mixture of graphite powder and a viscous aqueous solution is used as a lubricant, as shown in Comparative Examples 2 and 3, seizure can be prevented if the content of graphite powder is low or the viscosity of the mixture is low. has not yet been reached. In addition, in the case of Comparative Examples 6 and 7, where the viscosity of the mixed liquid became excessive due to a large content of graphite powder or a large amount of added viscosity increasing side,
Due to reasons such as nozzle clogging, the mixed liquid cannot be sufficiently sprayed onto the roll, and as a result, seizure cannot be prevented.

Example 2 Thickness 200mm, width 1030-1120mm,
5US430 stainless steel with a unit weight of approximately 13 tons (chemical composition values: 0.03~0.07χC, 0.41~0.5
4χS t, 0.21-0.23%Ni, 0.01
5~0.025%P, 0.002~0.016%S,
16.40-16.56% Cr, 0.09-0.
12%N +), and 5US430LX stainless steel (chemical composition value: 0.003~0.04%C,0,
45-0.56%Si, 0.22-0.30%N
i, 0.016~0.024%P, 0.003~
0.014%S, 16.51S-17,22%Cr,
o, os~0.11%Ni, 0.35~0.52
%Nb) for each slab.

Each material was heated to 1200° C. and roughly rolled into a rough bar with a thickness of 251I11, and then rolled into a hot coil with a thickness of 3.2 to 3.8 m using a finishing mill group consisting of 6 stands.

High chromium rolls were used as work rolls in the first to third stands of the finish rolling mill group. These stands were also provided with a system for spraying and supplying the lubricant according to the present invention to the rolls, separate from the conventional rolling lubricating oil supply system that supplies rolling lubricating oil by walk injection. In this way, the occurrence of surface flaws (flaws due to seizure) on rolled materials was investigated when rolling was performed using the conventional rolling lubricant and when the lubricant of the present invention was used. The number of rolled slabs was 21 in the case of 5US430 and 7 in the case of 511S4, 30LX for each lubricant condition.

The results are shown in Table 4, and it can be seen that the lubricant of the present invention is more effective in preventing roll seizure than conventional rolling lubricants. In other words, in the case of conventional rolling lubricating oil, roll seizure occurred mainly near the coil edges where the surface pressure was high during rolling, and indentation-like flaws were generated on the surface of the rolled material. When used, 5US430
Now, for all 21 slabs, 5IIS430LX has 7 out of 8 slabs.

No surface flaws caused by seizure occurred in any of the coils.

As explained above, the lubricant according to the second invention is effective in preventing roll seizure in stainless steel rolling, and it goes without saying that it improves product quality.

It can make a great contribution to the production of stainless steel, such as reducing roll consumption and improving hot rolling efficiency. Although the description has focused on the plastic working method of hot rolling, the lubricant according to the present invention can also be effectively used in other plastic working methods such as extrusion.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a testing machine used to examine the adhesion of lubricant to rolls. l...Rolling roll, 2...Lubricant supply device. 3...Lubricant spray nozzle,...Water supply device. 5. Water spray nozzle.

Claims (4)

[Claims] (1) A lubricant for hot rolling stainless steel comprising graphite powder dispersed in a viscous aqueous solution in an amount of 1 to 30% by weight. (2) The lubricant according to claim 1, wherein the viscous aqueous solution is a liquid obtained by dissolving a water-soluble polymer thickener in water. (3) The lubricant according to claim 1 or 2, wherein 90% by weight or more of the graphite powder is fine powder with a particle size of 10 μm or less. (4) The viscosity of the lubricant is 1 x 10^3 to 5 x 10^5c
Claims 1 and 2 which are P (centipoise)
The lubricant according to item 1 or 3.