JP3032273B2 - Manufacturing method of high strength steel belt - Google Patents

Abstract

A high strength steel belt (1) having an excellent flatness and a duplex structure of austenite and martensite has been prepared by a process which comprises providing a cold rolled or cold rolled and annealed strip of a martensitic structure from low carbon martensitic stainless steel containing from 10 to 17 % by weight of Cr and having a carbon content of not exceeding 0.15 % by weight, connecting ends of the strip or ends of a plate cut from said strip to provide an endless belt, causing the endless belt to circularly move between rolls (2,3) under tension and to repeatedly pass through a heating furnace (4) where the belt is heated to a temperature within the range from (the As point of the steel + 30 DEG C.) to the Af point of the steel and not higher than 900 DEG C. so that a part of the martensitic phase may be changed to a reversed austenitic phase, and cooling the heated belt to ambient temperature.

Description

The present invention relates to a method for producing a high-strength stainless steel belt having a fine two-phase structure of a martensite phase and an austenite phase and having excellent flatness. .

[Conventional technology]

Stainless steel belts are widely used in various fields such as continuous cookie baking at high temperature, food freezing at low temperature, continuous pressing of plywood, etc., and transport of machine parts. Due to the need for high strength and excellent fatigue strength, work-hardening austenitic stainless steel, low-carbon martensitic stainless steel,
Alternatively, precipitation hardening stainless steel or the like is applied.
Japanese Patent Publication No. Sho 51-31085 and Japanese Unexamined Patent Publication No.
No. 9903.

When manufacturing such a stainless steel belt, a flattened stainless steel strip is made by shape modification, and then both ends are welded into an endless shape.
Usually, a method of applying tension again to flatten at room temperature is adopted.

[Problems to be solved by the invention]

In the conventional steel belt manufacturing method, in order to manufacture a steel belt having an excellent shape, it is first necessary to obtain a steel belt material plate having an excellent flatness. In order to obtain a material plate having excellent flatness, a rolling process for correcting the shape is required in the final process of the steel belt material. In this shape-correcting rolling, for both work-hardening austenitic stainless steel and martensitic stainless steel, reduction ratio, roll diameter, rolling speed, sheet thickness,
It is necessary to perform the process properly in consideration of the history of the previous process, and the success or failure is an extremely advanced technology that greatly affects the product yield. Therefore, if possible, it is possible to obtain a steel strip having excellent flatness without this shape correction rolling, but the technology has not been completed in this type of steel strip.

Even if a material plate with excellent flatness is obtained by shape correction rolling, when the both ends of the material plate are welded into an endless shape, the heat input during welding causes the material plate to expand and contract, and the welding It is inevitable that the vicinity of the part is distorted. For this reason,
After the endless belt is formed, it is necessary to apply tension to the endless steel belt again to perform a flattening operation.

For these reasons, precise and complicated operations were required to produce a high-strength stainless steel belt at a high yield. In particular, when manufacturing a wide steel belt, it is necessary to weld a steel belt material plate by longitudinal joint (welding in the longitudinal direction) to form a wide steel strip, and then weld both ends of the steel belt to an endless shape. However, in this case, even if the tension is applied to the endless belt to correct the distortion of the welded portion and the flattening work is performed, the entire flattening operation becomes more complicated and requires much time. , A very problematic manufacturing method.

The present invention is directed to solving such a problem.

[Means for Solving the Problems] According to the present invention, the steel contains 10 to 17% of Cr and has a C content of 0.1 to 0.1%.
A cold-rolled or annealed steel strip having a martensitic structure is manufactured from a low-carbon martensitic stainless steel of 5% or less, and this steel strip is used as a material plate and its ends are connected to form an endless belt. Continuously pass through a heating furnace while rotating it under the required tension between the rolls, and in this heating furnace, a temperature range of (As point + 30 ° C) or more and Af point or less (however, a range of 900 ° C or less) ) To convert a part of the martensite phase into a reverse transformed austenite phase, and then cool to room temperature to form a dual-phase structure of austenite and martensite phases. provide. The As point means the temperature at which the transformation starts from the martensite phase to the austenite phase during the temperature rise process, and the Af point means the temperature at which the transformation from the martensite phase to the austenite phase ends during the temperature rise process.

[Action]

An endless belt with a martensitic structure, even if the shape of the steel strip, which was not subjected to shape correction rolling in the final manufacturing process of the steel strip, was further deteriorated when the end was formed into an endless belt by welding. Is passed through a heating furnace under the required tension between the rolls and passed through a heating furnace, and heated to a temperature above the As point. In the furnace, martensite reversely transforms into austenite while tension is applied in the longitudinal direction of the belt. The material is planarized as the reverse transformation proceeds.

Temperature not exceeding 900 ° C (As point + 30 ° C) or more, Af
In the temperature range below the point, a part of martensite can be reverse transformed into austenite, and the austenite phase formed by this reverse transformation can be made fine and stable. Does not transform again into quenched martensite. For this reason, a steel belt having a fine two-phase structure of a martensite phase and a reverse transformation austenite phase can be obtained.

In addition, the absence of transformation to quenched martensite during cooling from the heating temperature means that no quenching strain is generated, and thus good flatness is maintained up to room temperature.

As described above, according to the present invention, even if the steel strip from which the shape correction rolling is omitted is used as the material (although there is no particular problem even if the steel strip subjected to the shape correction rolling is used as the material), the endless wire after the welding connection is used. By continuous heat treatment of the belt, flattening and formation of a dual phase structure are simultaneously achieved, and a stainless steel belt having excellent shape and high strength can be manufactured with high productivity.

In addition, such an effect is sufficient if the steel strip before being passed through the heating furnace has a substantially martensitic structure.
For example, even if it has a ferrite phase or an austenite phase of 20% by volume or less, it is not so much damaged.

As described above, the feature of the present invention is that a fine austenite phase is formed from a martensite phase by a reverse transformation during heat treatment to form a fine two-phase structure, and by maintaining this fine two-phase structure, a high flatness excellent in flatness is obtained. I have obtained a strong stainless steel belt. In other words, as is known from the Hall-Petch equation, strength is obtained by refining the structure, and the reverse transformation proceeds while relaxing the tension in the state where the tension is relaxed while passing through the furnace. Thus, a tension annealing state is obtained, and an excellent shape can be obtained. Therefore, obtaining a stable and fine two-phase structure is key.
If the As point + 30 ° C or lower, the amount of reverse transformed austenite generated is too small, and if the Af point or higher and 900 ° C or higher, the amount of martensite does not remain or conversely becomes too small. A two-phase structure cannot be obtained.

(Preferred embodiment of the invention)

FIG. 1 shows an example of a heat treatment furnace used in the present invention. An endless belt 1 made of a material plate having a martensite structure is stretched over rolls 2 and 3, and the belt 1 is rotated using one of these rolls as a drive roll. A heat treatment furnace 4 is provided so that the rotating belt 1 is continuously passed. An air injection nozzle 5 may be provided on the belt exit side of the heat treatment furnace 4 to cool the belt heat-treated by the nozzle 5. It is convenient to heat the heat treatment furnace 4 by an electric resistance heating element heating method, but it may be a fuel combustion method. To prevent belt oxidation, use a reducing gas or an inert gas.
There is no particular problem even in the air atmosphere.

The endless belt 1 is made by cutting out a material plate of a required length from a steel strip having a martensitic structure, and welding both ends thereof. In the case of a wide material, in addition to welding at both ends, the side edges of a plurality of material plates can be longitudinally welded in a longitudinal direction. In some cases, they can be joined by billet striking. In any case, a steel strip having a martensitic structure that does not undergo shape correction rolling can be used, and even if the flatness is deteriorated by joining to a belt, it can be flattened by the heat treatment described above. .

The belt 1 is stretched under the required tension between the rolls 1 and 2, and the degree of the tension is 0.5 kgf / mm ² in a low temperature region near the As point.
The above high tension is desirable, and 0.5 kg in the high temperature region near the Af point
A low tension of less than f / mm ² is desirable. It is preferable that the heat treatment time is adjusted to be long in a low temperature range and short in a high temperature range. Such tension and holding time are related to the composition of the steel and the heat treatment temperature. However, a part of the martensite phase is reversely transformed into an austenite phase, and a fine two-phase structure is achieved and flattening is achieved. It is essential that the steel be adjusted in such a way that this can be seen by several tests of the steel.

In this way, heat treatment under tension results in a fine two-phase structure, and by maintaining this fine two-phase structure, a steel belt with excellent shape and high strength can be obtained, and the welded portion is substantially the same as the base metal. Have the same texture and flatness. Therefore, the key to heat treatment of this belt is to obtain a stable and fine two-phase structure. As point +
The amount of reverse transformed austenite generated by heat treatment at a temperature lower than 30 ° C is too small, and the amount of reverse transformed austenite increases and the amount of martensite increases by heat treatment at a temperature exceeding the Af point or a temperature exceeding 900 ° C. No more remains or the residual amount becomes too small, so that a stable fine two-phase structure cannot be obtained. Therefore, the heat treatment should be performed at a temperature range of (As point + 30 ° C) or higher and Af point or lower (900
℃ or lower).

The gist of the stainless steel to which the present invention is applied is a martensitic stainless steel exhibiting a martensite structure in an annealed state. Before passing the belt through the heat treatment furnace, the structure of the belt must be substantially martensitic, and this martensitic structure is related to the components in the steel. An annealed steel strip having a martensite structure, and a cold-rolled steel strip obtained by finish cold-rolling this annealed steel strip (in some cases, a cold-rolled steel strip in which work-induced martensite is formed by cold rolling) is used as a material plate. I do. However, it is not always necessary to have a 100% martensite phase, and a ferrite phase or an austenite phase up to 20% by volume may be present. Anyway, which state a tensile strength of dual phase obtained by heat-treating after making the steel belt is to one of the pillars of the present invention to obtain a high strength of at least 100 kg / mm ² class The present invention encompasses steel components and microstructure ratios in a range satisfying this requirement.

Regarding the composition of steel, 10-17% Cr and 0.15% or less C
Mainly of low carbon martensitic stainless steel containing. Ni can also be one of the main components, and it is of course possible to add ordinary alloying elements to be contained in this type of steel as long as the above requirements are satisfied.

The typical chemical components and their contents are as follows.

C: 0.15% or less (excluding 0), Si: 6.0% or less (excluding 0), Mn: 10.0% or less (excluding 0), Ni: 8.0% or less (excluding 0), Cr: 10.0 to 17.0%, N: 0.3% or less (excluding 0), Mo: 4.0% or less (including 0), Cu: 4.0% or less (including 0), Co: 4.0% or less (including 0), Further, Ti, Al, Nb, V, Zr, B, and rare earth elements can contain 1.0% or less in total and unavoidable impurities.

However, when the composition does not include Ti, Al, Nb, V, Zr, B and rare earth elements, Ni _eq = Ni + Mn + Cu + Mo + 0.2Co + 0.5Cr + 0.3Si + 20 (C + N) Ti, Al, Nb, V, Zr, B, Rare earth In the case of a composition containing an element, the amount of each component is adjusted so that the value of Ni _eq (Ni equivalent) defined by Ni _eq = Ni + Mn + Cu + Mo + 0.2Co + 0.5Cr + 0.3Si is in the range of 8.0 to 17.5. The content range of these main elements is regulated in this way for the following reasons.

C is an austenite-forming element and effectively acts to stabilize the reverse-transformed austenite phase formed by heat treatment at a temperature range between the As point + 30 ° C and the Af point, and strengthens the reverse-transformed austenite phase and martensite phase. Works effectively. However, if it is contained in a large amount, Cr carbide is generated during the reverse transformation treatment and the corrosion resistance deteriorates, so the upper limit is set to 0.15%.

Cr is a basic component of stainless steel, and must be contained at 10.0% or more to obtain good corrosion resistance. However, Cr is a ferrite-forming element. If it is contained in a large amount, a large amount of δ-ferrite phase is formed and it becomes difficult to obtain a martensitic single phase structure at room temperature after annealing.
And

Ni is an austenite-forming element and effectively acts to stabilize the reverse-transformed austenite phase formed by heat treatment at a temperature range of As point + 30 ° C or higher and Af point or lower. However, if it is contained in a large amount, it becomes difficult to obtain a martensitic single-phase structure at room temperature after annealing during the production of a steel strip.

Si widens the temperature range between the As and Af points and thus has an advantageous effect on obtaining a stable two-phase structure of austenite and martensite phases, and also strengthens the reverse transformed austenite and martensite phases after heat treatment. It is an effective element. However, if it is contained in a large amount, the productivity is deteriorated.

Mn is an austenite forming element and has As point + 30 ° C or higher, A
Effectively stabilizes the reverse transformed austenite phase generated by heat treatment in the temperature range below point f. However, if it is contained in a large amount, Mn fumes are generated during melting of the steel, and the productivity of the steel deteriorates. Therefore, the content is preferably set to 10.0% or less.

N is the same austenite forming element as C and As point + 30 ℃
As described above, it is an element that effectively acts to stabilize the reverse transformed austenite phase generated by heat treatment in the temperature range below the Af point, and is also effective in strengthening the reverse transformed austenite phase and the martensite phase. However, if it is contained in a large amount, blowholes are generated at the time of smelting of steel, and a sound steel ingot cannot be obtained. Therefore, the content is preferably set to 0.30% or less.

Mo is an element that improves corrosion resistance and is effective in strengthening the reverse transformed austenite phase and martensite phase after heat treatment. However, Mo is a ferrite-forming element.
After annealing during steel strip production, it is difficult to obtain a martensitic single phase structure at room temperature.

Cu, like Ni, is an austenite-forming element and is effective in forming an austenite phase after heat treatment. However, if it is contained in a large amount, hot workability during the production of steel strips decreases.
% Or less.

Co, like Ni, is an austenite-forming element and is effective in forming an austenite phase after heat treatment. However, if it is contained in a large amount, the steel becomes expensive, so it is better to make it 4.0% or less.

Ti, Al, Nb, V, and Zr are all effective in maintaining the austenite-martensite two-phase structure formed by the reverse transformation process in a stable, fine and uniform structure, and also suppress the formation of Cr carbide. , Is also an effective element for maintaining corrosion resistance. However, if it is contained in a large amount, it becomes difficult to produce steel. Therefore, it is preferable that the content of each is not more than 1.0% and the total amount thereof is also not more than 1%.

Ni _eq (Ni equivalent value) is as follows. In the present invention, a high-strength steel belt having excellent fatigue characteristics is obtained by maintaining a fine two-phase structure by reverse transformation of a fine austenite phase from a martensite phase during belt heat treatment to maintain a fine two-phase structure. Is obtained. Therefore, the key to the present invention is to obtain a stable and fine two-phase structure. When Ni _eq is less than 8.0, the amount of reverse transformed austenite decreases even when low-temperature heat treatment is performed at a temperature range of As point + 30 ° C or higher and Af point or less, and when Ni _eq exceeds 17.5, the amount of reverse transformed austenite is too large In each case, it is difficult to obtain a stable fine two-phase structure. Therefore, Ni _eq is 8.0-1
It is preferable to adjust the amount of each component so as to be 7.5.

〔Example〕

A steel having the composition shown in Table 1 was melted by the usual method.
It was forged and hot rolled to a thickness of 6 mm, subjected to solution treatment and pickling, cold rolled and annealed, and then finish cold rolled at a predetermined rolling ratio to obtain a cold rolled material of 1 mm. In the case of cold rolling, conditions for intentionally deteriorating the rolling shape were set in order to confirm the shape correction by heat treatment. Some finished cold rolled materials are 10
Annealed at 30 ° C and pickled. Material plates were cut from these steel strips, and both ends were TIG-welded to form endless belts with a width of 300 mm and a length of 10.5 m, which were subjected to the heat treatment described below. Table 1 also shows the As and Af points, and these transformation points are the changes in the temperature-electric resistance relationship curve obtained by raising the temperature at a heating rate of 1 ° C / min using an electric resistance measuring device. It was determined from the point of the curve.

Each belt was heat-treated by the heat treatment equipment shown in FIG. 1 under various conditions shown in Table 2. Table 2 also shows the shape of the steel belt after the heat treatment and the results of the tensile tests of the tensile test pieces taken out of the steel belt. In addition,
The shape inspection was performed before and after the heat treatment.
As shown in the figure, the swell height h (mm) is the distance l in the rolling direction.
(Mm) was defined as the shape value in the L direction, and as shown in FIG. 3, the value obtained by dividing the undulation height h (mm) by the steel strip width l (300 mm) was defined as the shape value in the T direction.

As is clear from the results in Table 2, according to the method of the present invention, each of the test materials has a high strength of not less than 80 kgf / mm ² , and the shape is 2.5 / 1000 or less in the L direction and the T direction. Shows an excellent shape of 1/300 or less.

On the other hand, in the method of comparing the heat treatment conditions out of the range specified by the present invention, which is indicated by * in Table 2, the shape is poor or the strength is low, which causes inconvenience. Not suitable.

[Effects of the Invention] As described above, according to the present invention, both characteristics of flatness and high strength required for a stainless steel belt are simultaneously achieved only by heat treatment after welding. This means that a steel strip from which shape-correction rolling has been omitted can be used as a material plate, and that a flattening process by tension before and after welding need not be separately performed. Therefore, it can greatly contribute to the saving of the process and the high yield in the steel belt production. In addition, it is possible to easily manufacture wide steel belts with longitudinal joints.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view showing a belt heat treatment facility according to the method of the present invention, FIG. 2 is a perspective view showing a L-direction (rolling direction) measurement method of a steel strip shape measurement method before and after heat treatment, and FIG. FIG. 3 is a perspective view showing a measuring method in the T direction (perpendicular to the rolling direction).

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-62-4831 (JP, A) JP-A-63-26345 (JP, A) JP-B-51-31085 (JP, B2) (58) Field (Int. Cl. ⁷ , DB name) C21D 9 / 46,9 / 00 C22C 38/00-38/58

Claims (3)

(57) [Claims] 1. A cold-rolled or annealed steel strip having a martensitic structure is produced from a low-carbon martensitic stainless steel containing 10 to 17% of Cr and having a C content of 0.15% or less,
The steel strip is used as a material plate, and the ends are connected to form an endless belt. The endless belt is continuously stretched between rolls under a required tension and continuously passed through a heating furnace while being rotated. (As point + 30 ° C) or higher and below the Af point (900 ° C or lower) to convert a part of the martensite phase into an inverse transformed austenite phase, and then cool to room temperature to cool the austenite phase and the martensite phase. A method for producing a high-strength steel belt characterized by forming a dual-phase structure of a site phase, where the As point is the temperature at which the transformation from the martensite phase to the austenite phase starts during the temperature rise process, and the Af point is the temperature rise. This is the temperature at which the transformation from the martensite phase to the austenite phase ends in the process. 2. The steel strip before passing through the heating furnace is 20% by volume.
The method for producing a high-strength steel belt according to claim 1, which has the following ferrite phase or austenite phase. 3. The stainless steel is 8.0% in addition to Cr and C.
The method for producing a high-strength steel belt according to claim 1 or 2, which is a martensitic stainless steel containing the following Ni, 6.0% or less Si, 10.0% or less Mn, and 0.3% or less N.