JPS63235491A - Manufacture of tool for hot extrusion - Google Patents

Abstract

PURPOSE:To obtain a tool for hot extrusion having a scale layer excellent in denseness characteristic, having long service life, and also having high strength with superior warkability, by subjecting a tool for hot extrusion made of low-alloy steel to heating and holding in an inert mixed-gas atmosphere of prescribed composition, to slow cooling down to a temp. right above the A2 transformation point, and then the quenching hardening. CONSTITUTION:A tool for hot extrusion made of 3Cr-1Ni or 1Cr-3Ni low-alloy steel is heated and held at <=1,000 deg.C, e.g., 950 deg.C in an inert-gas atmosphere which contains >=5.0vol.% steam and 0.5-5.0vol.% O2 and in which he volume ratio of O2 to steam is regulated to <=1/3. Subsequently, the above tool for extrusion heated and held as mentioned above is cooled slowly (e.g., furnace cooling, about 50 deg.C/h cooling rate) down to a temp. right above the A2 transformation point (e.g., 795 deg.C) in order to prevent cracking and then subjected to quenching hardening. As a result, the tool for hot extrusion having a dense scale having high resistance to wear, erosion, seizure, etc., and excellent in adhesive strength on the surface, also having high strength, and remarkably improved in service life can be obtained surely and stably with superior workability.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a heat treatment method for extending the life of a hot extrusion tool such as a core metal for a seamless steel pipe.

<Background technology> Generally, when manufacturing seamless steel pipes using the Mannesmann method or Eugene-Séjournet method, for example, the core metal used for pipe manufacturing is heated to high temperatures during the pipe manufacturing process, and is also subjected to high pressure. Therefore, it is no exaggeration to say that the product life is mainly affected by local wear, melt damage, and seizure, which have a negative impact on product quality. It was said that there was no such thing.

For this reason, various measures have been considered to suppress wear, erosion, seizure, etc. of the core metal for seamless steel pipes, but usually JISSKD61 or 5KD61A
Hardening (
After quenching (oil-cooling or air-cooling) and tempering, the surface was plated with chrome.

However, in recent years, when a scale layer is formed on the surface of the core metal for seamless steel pipes by heat treatment, the wear of the base metal,
Reports that the occurrence of melting damage, seizure, etc. can be effectively suppressed and the life of the core metal can be extended have attracted attention, and since then, Japanese Patent Publication No. 58-19363 and Japanese Patent Application Laid-open No. 5
No. 6-62922 and the like propose measures for extending the life of seamless steel pipe cores based on the same purpose.

The former is made of ``seamless steel pipe core metal (3Cr-1Ni) made of 9
The method relates to a method of blowing steam into the furnace to form scales that deeply bite into the core metal base during heat treatment at 00 to 940 degrees Celsius. The idea was to create a co-co-system atmosphere inside the furnace.

Indeed, it is known that the method of forming scale on the surface of the core metal for seamless steel pipes is a relatively inexpensive and easy-to-work method for improving the life of the core metal. Formation of "scale with good peeling resistance that deeply bites into the substrate" as described in Japanese Patent Application Laid-Open No. 19363 or JP-A-56-62922 improves the peeling resistance of the scale and shortens the life of the core metal. Further improvement was seen, but subsequent studies revealed that even if the scale forming means shown in the above publications were adopted as they were, a core metal with scales with good peeling resistance could be reproduced. It has become clear that it is difficult to manufacture reliably with good performance, and that the characteristics of the scale formed vary widely, making it impossible to stably extend the life of the core metal. In order to solve the problem, the applicant first proposed that ``water vapor: 5.0% by volume or more and 0□: 0.5 to 5% by volume.
3Cr-IXi low alloy steel in a mixed gas atmosphere containing 0% by volume and the remainder substantially consisting of inert gas, and in which the volume ratio of 0 to water vapor is less than 1/3. We proposed a method to extend the service life of seamless steel tube cores by heating and maintaining them at temperatures below 1000°C to form a dense scale with excellent adhesion on the core metal surface. (Japanese Patent Application Laid-Open No. 60-86262)
.

However, in the course of studies through long-term practical work that continued thereafter, the present inventors discovered that ``a seamless steel pipe produced by the method of JP-A No. 60-86262 related to the earlier proposal. Although the core metal has high resistance to wear, erosion, seizure, etc. and has a superior lifespan compared to conventional products, there remains some dissatisfaction in terms of strength. When applied to mandrels used for hot extrusion pipe manufacturing, even relatively small diameter mandrels, elongation occurs due to lack of strength, and as a result, the diameter of the mandrel becomes smaller and the life of the mandrel tends to end. This led to the realization that

<Means for Solving the Problems> From the above-mentioned viewpoint, the present inventors have developed a material that has a dense scale with good adhesion on the surface and exhibits high resistance to wear, erosion, seizure, etc. In order to find a method to stably manufacture a low-cost hot extrusion tool that has sufficient strength, low deformation resistance at high temperatures, and has a further improved service life, we have carried out research to find a method to stably manufacture it with good workability. As a result, it was found that ``the cause of dissatisfaction with the strength of the pipe core manufactured by the method proposed in JP-A No. 60-86262 is the slow cooling process after the scale-forming heat treatment.'' Find out,
Furthermore, it applies to hot extrusion tools made of r3Cr-1Ni low alloy steel.
Even if a dense scale is formed using the above-mentioned method, if rapid cooling/quenching treatment such as oil quenching or air quenching is performed from a predetermined temperature during the slow cooling process after the scale formation heat treatment is completed, , a tool with high strength can be obtained without problems such as cracking, and the formed scale will not peel off or have any negative effect on the scale properties, so it has high resistance to wear, erosion, seizure, etc. It has high strength that can sufficiently suppress deformation during use.
A hot extrusion tool with a long service life can be realized. and,
This is the same even when the material is 1cr-3Ni low alloy steel. ” We came to the conclusion that

The present invention has been made based on the above findings, and includes a hot extrusion tool made of aCr 1Ni or 1Cr-3Ni low alloy steel, water vapor: 5.0% by volume or more.

oz: 0.5 to 5.0% by volume, and the remainder substantially consists of inert gas,
In a mixed gas atmosphere in which the volume ratio of 0□ to water vapor is less than 1/3, the mixture is heated and maintained at a temperature of 1000°C or less, for example, as shown in the heat treatment pattern shown in Figure 1, and then heated to just above the At transformation point. By slowly cooling and then rapidly cooling and quenching, it is possible to manufacture a high-strength hot extrusion tool with a long service life and a dense scale layer with excellent adhesion on the surface with good workability. It has characteristics.

In addition, in the method of this invention, the tool material is 3Cr-1N.
The i-series low-alloy steel or 1Cr-3Ni-series low-alloy steel is used because these steels are considered suitable for hot extrusion tools such as core metal materials in terms of durability. be. In addition,
3Cr-1Ni low alloy steel usually contains: C: 0.25-0.35% by weight, Si: 0.4-0.8% by weight, Mn: 0.4-0.8% by weight, Cr: 2 .50 to 3.50% by weight, Ni: 1.
00 to 2.50% by weight, and 1Cr-3Ni low alloy steel has C: 0.13 to 0.20% by weight, Si: 0.15 to 0.35. Weight %, Mn: 0.
80-1.20% by weight, Cr: 1.40-1.80
Weight%, Ni: 2.8Q to 3.20% by weight, Mo
: 0.40 to 0.60% by weight.

In addition, examples of inert gas include N2 gas, Ar gas, He
In addition to gases, a mixture of these gases can also be used.

Next, in the method of this invention, the reason why each ratio of the amount of water vapor and the amount of Ox in the heating atmosphere, the volume ratio of O1 to water vapor, the heating holding temperature, etc. are limited as described above will be explained.

(a) Oxygen amount In order to extend the tool life by forming scale on the surface of the tool, it is important that the scale formed is dense and has good adhesion. Injecting water vapor into the heat treatment furnace atmosphere is effective in generating scale, but this alone is not sufficient; in addition to this, it is also necessary to properly control the 02 content in the heating atmosphere to form the desired shape. This is essential for stably forming scales.

That is, if the oxidizing gas in the heating atmosphere is water vapor alone, the oxidation rate is low and the desired scale cannot be formed, so mixing a predetermined amount of 02 is an important condition in the method of the present invention.

If the 02 ratio in the heating atmosphere is less than 0.5% by volume, scales that bite into the base metal will be formed, but their growth will be extremely slow, and it will take a long time to form the scales to a predetermined thickness. On the other hand, it is time consuming and impractical;
If the 0□ ratio exceeds 5.0% by volume, even if the amount of water vapor is increased, the scale will become porous and the mechanical strength will deteriorate, and the scale that has penetrated into the base metal will not develop (i.e., Since the grain boundary oxidation becomes insufficient and it becomes easy to separate, the amount of 0 is reduced to 0.5 to 5.0% by volume.
It was determined that

In other words, by allowing water vapor and 0.5 to 5.0 volume % of 0□ to coexist in the heated atmosphere, scale that has sufficiently penetrated into the base metal material quickly develops.

(b) Volume ratio of 02 to water vapor The formation of scales that are likely to peel off is also influenced by the ratio of the amount of water vapor in the atmosphere to the amount of 0□, that is, the value k expressed in water vapor (volume %). As a result of heat treatment experiments in various atmospheres, when the 0□ ratio is within 5.0% by volume and k is a value smaller than 173, a dense scale that penetrates sufficiently into the base metal is formed. It was confirmed that when the above value was 173 or more, a good scale could not be stably formed. Therefore, against water vapor. The capacity ratio k of 2 was set to be less than 1/3.

(C1 Water vapor amount The main oxidizing gas in the heating atmosphere applied in the method of this invention is water vapor, but in order to grow scale uniformly, the amount of water vapor should be 5.0% by volume or more. is necessary.

In other words, when the water vapor is reduced while maintaining the value below 1/3 of the above value, it is recognized that the thickness of the oxide scale becomes uneven.This is due to the influence of the gas flow in the furnace. This is because oxidation progresses better on the surface that is often exposed to gas than on other parts.This phenomenon is noticeable when the amount of water vapor is less than 5.0% by volume; It becomes inconspicuous when the amount is more than % capacity.

For this reason, the amount of water vapor was determined to be 5.0% by volume or more.

Note that the upper limit of the amount of water vapor is 0. It is determined only in relation to quantity and is not limited by other factors.

(dl Heating and holding temperature It is necessary to keep the heating and holding temperature in the heating atmosphere below 1000°C. This is because if the temperature exceeds 1000°C, grain boundary oxidation becomes difficult to proceed and scales to the base metal are formed. This is because there is no more digging in. This phenomenon is also well known as a high-temperature oxidation phenomenon of metals.

The penetration of scale into the base metal due to grain boundary oxidation is greatest when heated and held at approximately 850°C, but in order to ensure the thickness of the Fe-Cr-0 oxide layer in contact with the base metal, A high temperature is preferable for this purpose, and therefore approximately 950° C. is suitable as the heating holding temperature.

Note that the lower limit of the heating holding temperature is determined by the quenching start temperature for quenching.

Further, the heating and holding temperature may be determined depending on the desired scale thickness. Generally, when the heating temperature (oxidation temperature) is constant, the scale thickness is X, and the heating holding temperature (oxidation time) is t.

Of the scales, it is the direct oxidation layer that adheres to the substrate that helps extend the life of the tool.
When heated and held at 950° C. in an atmosphere with water vapor of 5.0% by volume or more, it grows according to the formula %. From this equation, it is possible to easily derive the heating holding time to obtain a scale of desired thickness.

(8) Rapid cooling start temperature for quenching The upper limit of the quenching start temperature for quenching is not particularly limited, but if scale formation is performed at a relatively high temperature to improve efficiency, cracks etc. From the viewpoint of prevention, it is better to start rapid cooling after slow cooling (e.g., furnace cooling: cooling rate of about 50°C/h) to just above the A2 transformation point (note that it goes without saying that scale formation will proceed even during the slow cooling described above). ).

If the quenching start temperature at this time is lower than the At transformation point, a sufficient hardening effect cannot be obtained. Also,
The quenching may be carried out by oil quenching or air quenching, which are commonly employed for quenching tool steel.

By the way, if the quenching treatment is performed completely separately after the scale-forming heat treatment, the scale generated during the scale-forming heat treatment (in direct contact with the tool base) will be removed during heating and soaking during quenching. ) New scale with poor adhesion is generated and peeled off during the cooling process, and the scale generated during the scale-forming heat treatment also peels off, making it impossible to obtain the effect of improving tool life.

As described above, the present invention includes water vapor: 5.0% by volume or more, Ol: 0.5 to 5.0% by volume, the remainder being substantially an inert gas, and a capacity of 0□ for water vapor. In a mixed gas atmosphere with a ratio smaller than 173, 3
A Cr-1Ni or 1Cr-3Ni low-alloy steel seamless pipe core is heated and maintained at a temperature of 1000°C or less, then slowly cooled to just above the A2 transformation point (e.g. 795°C), and then rapidly cooled.・It is hardened, but in this case,
First, after inserting the material tool into the furnace, gas replacement is performed so that the atmosphere in the furnace becomes the above-mentioned mixed gas atmosphere before or during heating. In particular, it is desirable that the atmosphere in the furnace be the above-mentioned atmosphere when the temperature of the core metal is about 600° C. or higher.

Next, this invention will be specifically explained with examples.
This is just one example of the present invention, and it goes without saying that this invention should not be construed as being unduly limited by this.

<Example> Example 1 JISSNCM6 used for Eugene Sejournet type pipe making machine
16 nickel chrome molybdenum steel man FL.

20% water vapor-N by volume.

97 in an electric furnace where the atmosphere was adjusted by mixing 02 at a ratio of 3% by volume into a mixed gas (water vapor amount: 10% by volume).
After being held at 5°C for 3 hours, it was furnace cooled to 820°C (cooling rate: 25°C/hr) and oil quenched from this temperature.

On the other hand, for comparison, a) A similar JIS SNCM616 nickel chrome molybdenum steel mandrel was subjected to quenching and tempering treatment according to Fig. 4 (maintained at 1020°C for 45 minutes, then oil quenched,
A JIS S K D61A mandrel was hardened and tempered according to Fig. 4 (1020 A sample (conventional example) which was subjected to two tempering treatments (holding at 590°C for 45 minutes, oil quenching, holding at 590°C for 2 hours, and air cooling) and then chrome plating was also prepared.

Next, the hardness of each part of each of these mandrels was measured, and they were tested on an actual machine to investigate their lifespan.

The results are shown in FIGS. 2 and 3.

In addition to the above two comparative examples, FIG. 2 showing the hardness of each part shows a JISSNCM616 nickel chrome molybdenum steel mandrel heated with a 20 volume % water vapor-N2 mixed gas (water vapor amount: 10 volume %). ) in 3% by volume
After holding at 975°C for 3 hours in an electric furnace in which the atmosphere was adjusted by mixing 02 at a ratio of .

In addition, Figure 3 shows the mandrel life based on an actual machine test.
．． The standard was ``If it becomes smaller than 7, it will be discarded.''

As is clear from the results shown in Figs. 2 and 3, the mandrel obtained according to the conditions of the present invention has sufficient strength of the base material, and also has a good strength due to the action of the dense scale layer on the surface. It can be seen that the tool life is about 2 to 3 times better than that of conventional tools.

In this example, only the ICr aNi-based low alloy steel mandrel was explained, but the 3Cr-1
It goes without saying that equally excellent results can be obtained with tools for hot extrusion, whether they are made of Ni-based low alloy steel or other hot extrusion tools.

<Summary of Effects> As explained above, according to the present invention, there is no wear and tear on the surface.
We stably and reliably produce hot extrusion tools that have a dense scale with good adhesion that is highly resistant to melting damage and seizure, and have high strength and a significantly improved service life. Moreover, it can be manufactured with good workability, and has a significant effect on reducing tool costs and tool replacement time in hot extrusion work, making it an extremely significant contribution to industry.

[Brief explanation of the drawing]

FIG. 1 shows one of the heat treatment patterns of the tool material according to the present invention.
example. FIG. 2 is a graph comparing the hardness of various parts of the mandrels obtained in Examples. FIG. 3 is a graph comparing the lifespan of mandrels obtained in Examples. FIG. 4 shows a conventional heat treatment pattern for a tool material.

Claims (1)

[Claims] A hot extrusion tool made of 3Cr-1Ni or 1Cr-3Ni low-alloy steel, containing water vapor: 5.0% by volume or more, and O_2: 0.5-5.0% by volume. with the remainder consisting essentially of an inert gas,
The material is heated and maintained at a temperature of 1000°C or less in a mixed gas atmosphere in which the volume ratio of O_2 to water vapor is less than 1/3, and then slowly cooled to just above the A_2 transformation point, and then rapidly cooled and quenched. A method for manufacturing a high-strength hot extrusion tool having a dense scale layer with excellent adhesion on the surface.