JPH07150244A - Production of ferritic stainless steel for cold working - Google Patents

Abstract

PURPOSE:To obtain a material having uniform deformability against high speed and high-degree working by hot rolling a ferritic stainless steel of specific composition under specific conditions, applying cold working strain, and then performing annealing treatment at a temp. in a specific region. CONSTITUTION:The steel has a composition consisting of, by weight, <=0.0100% C, <=0.40% Si, <=0.50% Mn, <0.20% N, 11.0-18.0% Cr, <=0.0120% N, 0-0.10% Nb, 0-0.10% Ti, 0-0.10% Al, 0-0.50% Mo, 0-0.50% Cu, and the balance Fe with inevitable impurities. This steel is heated up to 700-950 deg.C, hot rolled at a finishing temp. controlled to 700-850 deg.C, and finished to frrite grain size No.3 or above. Then, cold working is done at =25% reduction of area. Further, the steel is reheated to 750-850 deg.C and tempered. By this method, the ferritic stainless steel for cold working, withstanding high speed and heavy draft and having uniform deformability, can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing ferritic stainless steel having excellent cold workability.

[0002]

2. Description of the Related Art In recent years, the use of ferritic stainless steel containing 11 to 28% Cr has been expanding as a highly economical stainless steel. These Cr-based ferritic stainless steels have the advantages that they are not only inexpensive but also have high strength and that they have excellent corrosion resistance in environments containing chloride ions.

However, it is known that these Cr-based ferritic stainless steels are generally extremely brittle and have poor cold workability. Further, in general, when various cold workings (forging, wire drawing, rolling, upsetting, shearing, extrusion, etc.) are performed on the material after hot rolling, work cracks may occur, which results in complicated When manufacturing shaped products, cold cutting is often employed at the expense of yield. However, the cutting yield is reduced and the machining process becomes long, resulting in an increase in cost as a whole. For example SUS
410L (JIS G4303-1981, typical component: 0.02C-
0.5Si-0.6Mn-13Cr) is known as a ferritic stainless steel with excellent workability, but the problem that it cannot withstand cold working with an upsetting ratio exceeding 60% in the working process. Have a point.

This is considered to be due to the fact that the ferritic stainless steel is inferior in high-speed deformability in addition to the factors of hardness and strength. That is, ferritic stainless steel generally has poor toughness and has the drawback that it cannot follow the processing accompanied by high-speed deformation.

As a remedy for this, a small amount of Ni is added to ferritic stainless steel to facilitate the occurrence of cross flicker in the crystal grains during the deformation process (for example, Japanese Patent Laid-Open No. 63-219551) and low temperature rolling. A method of fine-graining the structure by controlled rolling such as (for example, JP-A-63-219527 and JP-A-63-235450) can be considered. However, in this case, although the toughness is improved by facilitating the generation of cross grain of the crystal grains and making the grains finer, the aggregation of the structure (deterioration of anisotropy) progresses due to the low temperature rolling, and it becomes uniform during cold working. Deformation performance cannot be obtained.

[0006]

The ferritic stainless steels, including the steels shown in the above composition ranges, are characterized in that the ferrite single phase and the ferrite phase occupy most of them during hot rolling and at room temperature. Even if a small amount of Ni is added or low temperature rolling is performed to improve the toughness (improvement of high-speed deformability) of these steels, the problem of anisotropy is not solved and practically sufficient cold workability cannot be obtained. As a solution to this problem, the present inventors have discovered that there is an optimum combination condition between cold working and heat treatment, and the details will be described below.

[0007]

In order to solve such problems, the present invention defines the chemical composition of ferritic stainless steel, and further defines the heating temperature and finish rolling temperature during hot rolling. To reduce the initial particle size of the material. After that, by applying a certain amount of cold working strain and then annealing in a regulated temperature range, cold working with high toughness and excellent anisotropy that could not be obtained with conventional steel For obtaining ferritic stainless steel for use. That is, the present invention provides the following means for solving the problems.

By weight, C: 0.0100% or less, Si:
0.40% or less, Mn: 0.50% or less, Ni: 0.20
%, Cr: 11.0 to 18.0%, N: 0.0120
% Or less, Nb: 0 to 0.10%, Ti: 0 to 0.10%,
Al: 0 ~ 0.10%, Mo: 0 ~ 0.50%, Cu: 0
~ 0.50%, the balance Fe and unavoidable impurities steel is heated to a temperature of 950 ℃ or less 700 ℃ or more, finish temperature 850 ℃ or less, controlled to 700 ℃ or more, hot rolling is performed, ferrite grain size. After finishing to No. 3 or more and then cold working with a surface reduction rate of 25% or more, 750 to 750
A method for producing a ferritic stainless steel for cold working, which can withstand high-speed and high-working degree and has uniform deformability by reheating to a temperature range of 850 ° C. and annealing.

[Action]

First, the reason why the components are limited as described above in the present invention is shown below. C: C is an element necessary for imparting a predetermined strength to steel, but it is also an element that deteriorates corrosion resistance, toughness, and workability. In order to realize the improvement of the cold workability which is the object of the present invention, it is an essential requirement to control to 0.0100% or less.

Si: Si is an element necessary for deoxidizing the steel and also for imparting a predetermined strength to the steel, but if its content exceeds 0.40%, corrosion resistance, Since weldability and coarsening are induced, and cold workability deteriorates, it is necessary to set the content to 0.40% or less.

Mn: Mn, like Si, is an element necessary for deoxidizing steel, but if its content exceeds 0.50%, some austenite phase is mixed in the ferrite and cold workability deteriorates. Therefore, the upper limit is set to 0.5%.

Ni: Ni is an element that generally improves the toughness of the ferrite phase, but it is the object of the present invention C
In r-type ferritic stainless steel, it is necessary to control to less than 0.2%. If this is added to 0.20% or more, N
Since i is an austenite-stabilizing element, austenite is mixed in the ferrite and impairs the cold workability. Therefore, in the present invention, Ni is set to less than 0.20%.

Cr: Cr is an element necessary for stabilizing the ferrite phase and imparting a predetermined corrosion resistance, but if it is less than 11.0%, the above effect is not sufficiently exhibited, and the ferrite single phase is However, since the austenite phase is mixed in part, the cold workability deteriorates. On the other hand, 18
If it exceeds 0.1%, the strength is remarkably increased, and the toughness and cold workability are deteriorated, so the upper limit is 18%.

N: Like C, N is an element necessary for imparting a predetermined strength to steel, but on the other hand, it is an element that deteriorates corrosion resistance and cold workability. Conventionally, Cr is 11.0
The effect of N content in ferritic stainless steel containing 18.0% has not been clarified, and JIS G-4303-198
There is no stipulation regarding the N content in 11.0 to 18.0% Cr ferritic stainless steel. However, in the present invention, the following findings were obtained as a result of detailed and systematic investigation of the influence of the N content on the plastic deformation behavior of the ferrite phase. That is, when the N content exceeds 0.0120%, cross-slip is significantly suppressed during plastic deformation at room temperature, the plastic deformation is locally concentrated, and only the concentrated portion of the plastic deformation is work-hardened. Cracks occur. Therefore, in order to improve the cold workability, the N content needs to be 0.0120% or less.

Nb: Nb precipitates fine carbonitrides in the steel, substantially reduces the C and N contents, and also has the function of refining ferrite grains. It is effective when you want to improve the quality. In order to fully exert the effect, it is preferable to set it to 0.01% or more, and if it exceeds 0.10%, the carbonitride of Nb is coarsened to embrittle the ferrite, and as a result,
The cold workability rather deteriorates, so the content is 0.10.
%, And the preferable content is 0.01 to 0.10%.

Ti: Ti, like Nb, has the function of precipitating fine carbonitrides in the steel to substantially reduce the C and N contents, and also has the function of refining ferrite grains. In particular, it is an effective element when it is desired to improve toughness and cold workability. In order to fully exert the effect, addition of 0.01% or more is preferable. On the other hand, if it exceeds 0.10%, the carbonitride of Ti becomes coarse, and the coarse precipitates become the starting point to cause cracking during cold working. Therefore, the content is set to 0.10% or less, and the preferable content is 0%. 0.01-0.1
0%

Al: Al is an effective element because it improves cold workability through the deoxidizing effect of steel. Addition of 0.01% or more is preferable in order to sufficiently bring out the effect.
On the other hand, if added in excess of 0.10%, coarse Al oxide in the steel is formed, and cracks occur from it during cold working, so the content should be 0.10% or less, and the preferred content is Is set to 0.01 to 0.10%.

Mo: Mo significantly improves the corrosion resistance of ferritic stainless steel, so it can be added depending on the required level of corrosion resistance. In order to fully exert the effect of improving the corrosion resistance, addition of 0.01% or more is preferable, but if it exceeds 0.50%, the economical efficiency is impaired.
The upper limit is 0%. Therefore, the content is set to 0.50% or less, and the preferable content is set to 0.01 to 0.50%.

Cu: Cu, like Mo, is effective for improving the corrosion resistance of ferritic stainless steel,
It can be added according to the required level of corrosion resistance, but in order to fully exhibit the above effects, 0.01%
The above additions are preferred. On the other hand, if added over 0.50%, cracks are likely to occur during hot rolling.
% Is the upper limit. Therefore, the content is set to 0.50% or less, and the preferable content is set to 0.01 to 0.50%.

In the present invention, the above component limitation is an indispensable requirement, but the improvement of toughness, which is the object of the present invention (energy transition temperature ( _{v TrE} ) at 0 ° C. in the Charpy impact test), which is the object of the present invention, is limited only by this component limitation. The following conditions), improvement in anisotropy and cold workability (a crack compression limit compression ratio of 60% or more in cold upsetting cannot be ensured), and various conditions during hot rolling described below. It is necessary to specify the cold working strain and the subsequent annealing temperature range.

Conventionally, in the hot rolling of ferritic stainless steel, in consideration of the hot deformation resistance and the resulting load on the hot rolling mill, is the hot rolling of the steel billet performed at 1000 ° C. or higher? It was common. However, in the present invention, at the time of hot rolling the steel based on the above-mentioned compositional regulations, at 950 ° C.
Hereinafter, it is characterized in that hot rolling is performed after soaking at a heating temperature of 700 ° C. or higher. Heating above 950 ° C not only causes deterioration of cold workability and toughness due to coarsening of ferrite crystal grains, but also fine Cr carbonitrides are strain-induced precipitated in the ferrite matrix during hot rolling. It has been found that the ferrite matrix itself also becomes brittle, which further promotes the deterioration of cold workability. Further, if the heating temperature is 700 ° C. or lower, the hot deformation resistance becomes large, which increases the load on the hot rolling mill, which is not preferable. In addition, the hot deformability is lowered and the surface properties of the rolled material are markedly impaired. Therefore, the heating temperature during hot rolling needs to be 950 ° C. or lower and 700 ° C. or higher.

On the other hand, to obtain a stable toughness at room temperature, energy transition temperature Charpy impact test _(v T _rE) 0
As a result of research, it has been clarified that it is necessary to control the temperature to be equal to or lower than 0 ° C., and for that purpose, it is necessary to control the ferrite grain size number of the steel material to 3 or higher. In order to secure the ferrite grain size number 3 or more, it is needless to say that the heating temperature in hot rolling is set to 950 ° C. or less as described above and the coarsening of the initial grain size is controlled, but the finishing temperature is regulated. That is also important.

When the finishing temperature is 850 ° C. or higher, the crystal grains become coarse during the standing cooling after the rolling, and the ferrite grain size number 3
It will be the number below. On the other hand, when the finishing temperature is 700 ° C. or less, grain growth during cooling after completion of rolling is suppressed, but the texture develops by rolling and anisotropy occurs. The anisotropy due to this texture is not preferable because it causes nonuniform deformation during cold working. Therefore, the finish rolling temperature is 850 ℃
Hereinafter, it is necessary to regulate the temperature to 700 ° C. or higher.

The rolled material obtained under the above rolling conditions has a ferrite grain size number of 3 or more, but since the rolling temperature is relatively low, the anisotropy due to the texture cannot be said to be nonexistent, and there is some influence. Is recognized. Therefore, it has been found that in order to completely eliminate this anisotropy, an optimum combination of cold working and heat treatment is realized. That is, the reduction ratio of the rolled material is 25% when cold.
As described above, processing of 40% or less (for example, drawing, cold rolling) is performed to introduce strain into the matrix. At this time, the reduction rate is 2
If it is less than 5%, the amount of strain introduced is small, and the nuclei of recrystallization are small in the subsequent heat treatment, so that it is difficult to cause fine and uniform recrystallization, and rather, coarsening and mixing of grains are caused. Larger area reduction is generally more effective for fine recrystallization of grains by heat treatment,
Even if the processing is performed over 40%, the grain refining effect is saturated. In addition, in order to add large processing, equipment size increase,
Not economical due to repeated cold working. Therefore, the optimal condition for the reduction rate of cold working is 25% or more and 40% or less.

Next, regarding the heat treatment temperature, the heat treatment temperature is 7
If the temperature is lower than 50 ° C., the release of cold work strain mainly proceeds, and the nonuniform crystal (mixed grain structure) is generated due to the movement of grain boundaries. Further, when heat treatment is performed at a temperature higher than 850 ° C., useful fine carbonitrides that have been the crystal nuclei up to that point cause solid solution in the mother phase and coarsening of agglomerates, resulting in coarsening of crystal grains. There was found.

As described above, 750 ° C. or higher, 850
When the heat treatment is performed at a temperature of not higher than 0 ° C., a fine and uniform recrystallized structure with the cold-working strained portion as a new starting point is obtained, and the anisotropy disappears completely. The features of the present invention will be clarified by showing examples below.

[0027]

【Example】

[Example 1] Steels having the chemical components shown in Tables 1 and 2 were vacuum-melted into 1-ton ingots. 1 by slab rolling of this steel ingot
Hot rolled to a 60 mm square billet. This steel slab was heated at 900 ° C. for 1 hour, continuously hot-rolled into a 25 mmφ steel bar, and air-cooled to room temperature. The finish rolling temperature at this time was adjusted to 800 ° C.

[0028]

[Table 1]

[0029]

[Table 2]

Using the 25 mmφ steel bar thus obtained, a tensile test was conducted to measure 0.2% proof stress, tensile strength and drawing value. The ferrite grain size is JIS G0552.
The ferrite grain size was measured according to the ferrite grain size test method.

The toughness and cold workability, which are the main objects of the present invention, were evaluated by the following evaluation methods. First, the toughness is JI in the axial direction from the center of the 25 mmφ steel bar.
S Z2202 A No. 4 test piece (2 mmV notch Charpy test piece) defined in “Metallic material impact test piece” was sampled, and the energy transition temperature ( _{v TrE} ) was obtained. The evaluation of the transition temperature is _{v Tr E} 0 from the viewpoint of ensuring stable toughness at room temperature.
The point at which the temperature was lower than or equal to ℃ was used as a branch point for evaluation.

Regarding cold workability, an upsetting compression test piece of 20 mmφ × 30 mm (ho) was prepared by cutting from a 25 mmφ steel bar (see FIG. 1 (a)), and this test piece was kept at room temperature. The compression ratio (φc) when statically compressed (see FIG. 1B) and cracks were generated on the circumferential side surface was calculated by the following equation.

At this time, the compression rate at which cracking occurred was set as the limit compression rate (φc), and from the viewpoint of dividing the practical workability, the limit compression rate of 60% was set as the evaluation branch point. In addition, the deformation ratio is calculated from the maximum overhang length (Dmax) and the minimum overhang length (Dmin) of the most overhanging portion (height direction) of the test piece after the compression test based on the following formula, and an index for anisotropy evaluation. And 2C, 2D and 2F show the initial shape and the overhanging shape of the test piece.

When the deformation ratio is 100%, it means that the film is uniformly projected (deformed) during the compression test.

In the compression test, the limit compression rate was 6
The evaluation of the deformation ratio was omitted because those that did not reach 0% basically had no cold deformability.

[0036]

## EQU1 ## Deformation ratio = [1− (Dmax / Dmin)] × 100 [where Dmax is the maximum overhang diameter and Dmin is the minimum overhang diameter. ]

[0037]

## EQU2 ## φc = [(ho-hf) / ho] × 100 (%) [where ho is the height (30 mm) before compression of the upsetting compression test piece, and hf is the height after compression. is there. ]

In Tables 1 and 2, the tensile properties, ferrite grain size, energy transition temperature, cracking limit compression rate (hereinafter abbreviated as φc) and deformation rate of steel are also shown.

All of the steels 1 to 22 of the present invention having the chemical composition defined in the present invention have values of _{v TrE of} 0 ° C. or less and φc of 60% or more, and have high toughness and excellent cold workability. There is. On the other hand, in the comparative steel having a composition outside the range of the chemical composition defined in the present invention, the ferrite grain size is #
If 3.0 or more, _v T _rE ductility values 0 ℃ below that is obtained (aperture value) is poor overall, .phi.c has become less than 60%.

This is because if C, N, Nb, and Ti are excessively added, they become brittle due to the formation of relatively large carbonitrides. (Steel No. 23, 29, 30, 31, 3
(3, 36 and 37) Further, excessive addition of Mn and Ni impairs workability due to austenite formation (two-phase steel formation). (Steel No. 25, 26 and 34)
Others that have coarsened due to excessive addition of Si (steel number N
o.24), those whose strength increased and ductility decreased due to excessive addition of Cr and Mn (steel No. 28, 35) and A
Alumina-based inclusions are generated in the steel by the excessive addition of 1
The workability is deteriorated due to the deterioration of the workability (Steel No. 32, 37).

As a result, even if the heating temperature (950 ° C. or less) of the steel specified in the present invention is satisfied, the toughness and cold workability of the comparative steels out of the range of the chemical composition specified in the present invention It is clear that a material having both properties cannot be obtained.

Example 2 Next, using Steel 1 of the present invention (Table 1), an example relating to the effect of hot rolling conditions will be shown. A 160 mm square steel piece of the invention steel 1 is 800, 850, 900, 9
The steel was soaked under 6 conditions of 50, 960, and 1000 ° C. for 1 hour, and was hot-rolled in the same manner as in Example 1 to obtain a 25 mmφ steel bar. The finish rolling temperature was controlled to 750, 800, 850, 860, 900 ° C by cooling in the middle and adjusting the rolling speed. The characteristics of the steel bar thus obtained were investigated in the same manner as in Example 1. The results are shown in Table 3.

[0043]

[Table 3]

According to Table 3, when the heating temperature and finish rolling temperature ranges specified in the present invention are controlled, the ferrite grain size becomes 3 or less, _v T _rE is 0 ° C. or less, and _φc is 6 or less.
Values of 0% and above are obtained. However, as shown in the comparative method, when the heating temperature and the finish rolling temperature are out of the ranges specified in the present invention, it is understood that _v T _rE and _φc do not reach the predetermined values.

The lower limit of the heating temperature is not particularly specified, but from the capability of the rolling mill, about 700 ° C. is considered to be a practical standard. Further, similarly, the finish rolling temperature is not particularly limited, but it is considered that about 700 ° C. is a practical standard in view of the hot workability and the ability of the rolling mill.

[Embodiment 3] Above Embodiments 1 and 2
The optimum chemical composition and hot rolling conditions were clarified. However, since the hot rolling condition is performed at a relatively low temperature, it was found that a texture is likely to be formed during rolling and the uniform deformability is poor (the deformation ratio is low) as shown in Tables 1 and 3. Therefore, an example of improving the deformation ratio by a combination of cold drawing (drawing) and annealing temperature will be shown below.

Using the steel 1 of the present invention (Table 1), a 25 mmφ steel bar rolled under hot rolling conditions (Category 1 in Table 3) in which anisotropy is most likely to occur is used as a material, and the area reduction ratio is 0, 20, 25. Three
After cold drawing to 8 levels of 0, 35, 40, 45 and 50%, each drawing material was 550, 600, 65
It was annealed by holding it at 8 levels of 0, 700, 750, 800, 850 and 900 ° C for 1 hour. Cylindrical test pieces of [height / diameter] = 1.5 were prepared [see FIGS. 2 (c) and (e)] from each of the annealed materials, and the temperature was set to 7 at room temperature.
A 0% static compression test was performed and the deformation ratio was determined according to the previous example. If this deformation ratio is less than 95%, shape defects will occur due to lack of wall or insufficient overhang during cold heading, and it will not be put to practical use. Therefore, 95% was used as the evaluation branch point. That is, the condition that a deformation ratio of 95% or more can be secured was evaluated as “good”.

[0048]

[Table 4]

Considering practical problems, the above deformation ratio is 9
The condition that 5% or more can be secured was evaluated as “good”. In the case of annealing at 650 ° C. or lower, which is relatively low temperature as shown in Table 4, since the hardening temperature by the drawing process is insufficient, it is not sufficiently softened and cracks occur at a compressibility of 70% or less. On the other hand, work cracking occurred at 900 ° C., but as a result of microscopic observation, it was observed that coarse (ferrite grain size 3 or less) crystal grains coexisted and coagulated carbonitride was precipitated.

Therefore, in order to ensure uniform cold workability, 700 to 850 ° C. after cold working with a surface reduction rate of 25% or more.
Annealing in is a prerequisite. It should be noted that if the degree of cold working is too large in actual production activities, it is not economical, and even if the area reduction rate is increased, uniform cold working is possible.
The upper limit is not particularly limited, but about 25 to 40% can be said to be efficient processing.

[0051]

As described above, by rolling the steel having the chemical composition satisfying the requirements of the present invention under the hot rolling conditions specified in the present invention, the dynamic and static cold workability can be improved. A very good ferritic stainless steel is obtained. However, since the rolling conditions specified in the present invention are controlled at a relatively low temperature, anisotropy may occur, which is also caused by the processing such as cold drawing specified in the present invention and the subsequent annealing. It turned out that it was improved to the extent that there was no problem in practical use. As a result, it has become possible to provide a material having excellent uniform deformability for high-speed cold-working applications of high workability, which has hitherto been impossible with ferritic stainless steel.

[Brief description of drawings]

FIG. 1A is a diagram showing a state of an upsetting compression test piece before a compression test used in an evaluation method of cold workability, and FIG. 1B is a diagram showing a state of the test piece after the compression test.

FIG. 2 (c) is a sketch of the test piece before the compression test performed to obtain the deformation ratio of the compression test piece, (d) is a sketch of the test piece after the compression test, and (e) is the test of the compression test. Piece (c)
A-A 'cross section, and (f) shows a BB' cross section of the test piece (d) after the compression test.

Claims (1)

[Claims] 1. C: 0.0100% or less by weight, S
i: 0.40% or less, Mn: 0.50% or less, Ni: 0.2
Less than 0%, Cr: 11.0 to 18.0%, N: 0.012
0% or less, Nb: 0 to 0.10%, Ti: 0 to 0.10
%, Al: 0 to 0.10%, Mo: 0 to 0.50%, C
u: 0 to 0.50%, the steel consisting of the balance Fe and inevitable impurities is heated to a temperature of 950 ° C or lower and 700 ° C or higher,
The finishing temperature is controlled to 850 ° C or lower and 700 ° C or higher, and hot rolling is performed to finish the ferrite grain size number 3 or higher.
Then, after cold working with a surface reduction rate of 25% or more, 7
A method for producing a ferritic stainless steel for cold working, which has high toughness and is excellent in anisotropy, which comprises reheating to a temperature range of 50 to 850 ° C and annealing.