WO2016105067A1 - Ferritic stainless steel material having superb machinability and method for producing same - Google Patents

Abstract

The present invention relates to ferritic stainless steel material having superb machinability and a method for producing same, and more specifically to a novel ferritic stainless steel material having improved machinability compared to the ferritic stainless steel material which was difficult to use when high machinability was required due to poor machinability compared to the austenite stainless steel material, and a method for producing the novel ferritic stainless steel. The ferritic stainless steel material according to one aspect of the present invention can have a ratio of 20% or more of the crystal grain for which the surface orientation parallel to the rolling surface is {111}, and a ratio of 10% or more of the crystal grain for which the surface orientation parallel to the rolling surface is {112}. Here, each ratio of the crystal grain indicates a value measured by means of the electron back scattering pattern (EBSP) method.

Description

Ferritic stainless steel with excellent workability and manufacturing method

The present invention relates to a ferritic stainless steel having excellent workability and a method of manufacturing the same, and more particularly, to improve the processability of ferritic stainless steel, which is difficult to be used in applications requiring poor workability due to poor workability compared to austenitic stainless steel. It relates to a novel ferritic stainless steel material and a manufacturing method thereof.

Ferritic stainless steel is a steel having a high price competitiveness compared to the austenitic stainless steel because it is excellent in corrosion resistance even though less expensive alloying elements are added. Ferritic stainless steels are used for construction materials, transportation equipment, kitchen appliances, etc., but are inferior in workability and cannot replace austenitic stainless steels in many fields. Accordingly, studies are being actively conducted to improve the workability and to expand the use thereof. These studies are intended to improve deep drawing or anti-ridging of ferritic stainless steels, and as a result, their use is expanding in applications such as kitchen appliances and automotive parts. However, there is a limit to expect further expansion of application of ferritic stainless steels only by improving deep drawing resistance and ridging resistance. Therefore, in order to expand the use of ferritic stainless steels, it is necessary to satisfy properties such as bulging properties required in various processing fields.

One aspect of the present invention provides a ferritic stainless steel having excellent deep drawing and bulging properties.

Another aspect of the present invention provides a method for producing a ferritic stainless steel having excellent deep drawing and bulging properties.

The subject of this invention is not limited to what was mentioned above. Additional objects of the present invention are described throughout the present specification, and those skilled in the art will have no difficulty understanding the additional objects of the present invention.

In the ferritic stainless steel according to the aspect of the present invention, the ratio of grains having a plane orientation of {111} parallel to the rolled surface is 20% or more, and the ratio of grains having a plane orientation of {112} parallel to the rolled surface is 10. It may be more than%.

However, the ratio of each grain here means the value measured by the EBSP (Electron Back Scattering Pattern) method.

At this time, by weight%, C: 0.02% or less, Si: 1.0% or less, Mn: 1.5% or less, P: 0.035% or less, S: 0.03% or less, Al: 0.08% or less, N: 0.015% or less, Cr: 10-12%, Ti: 0.3% or less, the balance may include a composition containing Fe and unavoidable impurities.

In addition, the ferritic stainless steel of the present invention may further include one or two kinds selected from weight%, Ni: 0.2% or less and Cu: 0.3 to 0.7%.

Another aspect of the present invention is a method for producing a ferritic stainless steels comprising the steps of preparing a slab of ferritic stainless steels; Hot rolling the slab to obtain a hot rolled plate; Performing a hot-rolled sheet annealing to maintain the hot-rolled sheet in a temperature range of 700 to 850 ° C. for 180 seconds or less; Cold rolling the hot rolled plate to obtain a cold rolled plate; The cold rolled sheet may include the step of performing cold rolled sheet annealing at a temperature range of 850 ℃ or more.

At this time, the maximum rolling reduction per pass during the hot rolling can be controlled in the range of 20 ~ 60%.

In addition, the slab is by weight, C: 0.02% or less, Si: 1.0% or less, Mn: 1.5% or less, P: 0.035% or less, S: 0.03% or less, Al: 0.08% or less, N: 0.015% or less , Cr: 10 to 12%, Ti: 0.3% or less, the balance may include a composition containing Fe and unavoidable impurities.

In addition, the slab may further include one or two kinds selected from weight%, Ni: 0.2% or less, and Cu: 0.3 to 0.7%.

The manufacturing method of the present invention may further include the step of skin pass rolling at an elongation of 0.5 to 1.5% after the cold rolled sheet annealing.

As described above, the present invention can provide a ferritic stainless steel excellent in not only the conventional deep drawing property but also the bulging property by appropriately controlling the orientation of the internal structure of the ferritic stainless steel. Such ferritic stainless steels can replace expensive austenitic stainless steels in various fields, and may have a cost reduction effect.

1 is an EBSP picture of Comparative Example 1, and

2 is an EBSP photograph of Inventive Example 1. FIG.

Hereinafter, the present invention will be described in detail.

The bulging property is a characteristic expressed by the bulging height as an index indicating how far it can be bulged without tearing when the center portion is bulged by a press while the plate end of the metal plate is restrained. On the other hand, the deep drawing property is usually indicated by the r value as an index used when pressing without restraining the plate end of the metal plate.

The inventors of the present invention have examined ways to improve the defdrawing and bulging properties of ferritic stainless steels at the same time. As a result, it is necessary to control the orientation of the steel internal texture in addition to controlling the composition of the steel in an appropriate range. It was found that the present invention was reached.

That is, in one aspect of the present invention, grains parallel to the rolled surface are {111} (hereinafter, simply {111} grains) and grains parallel to the rolled surface of {112} (hereinafter, simply {112) } The deep drawing and bulging properties are improved by limiting the ratio of grains. Here, {111} grains do not mean only grains with a plane orientation of {111}, but also include grains that are {111} ± 10 ° (ie, a bearing deviating within 10 ° from the {111} direction). Similarly, it should be noted that {112} grains also mean grains with {112} ± 10 ° parallel to the rolling surface. In the present invention, the crystal grain having each orientation is etched with aqua regia after mirror-polishing the thickness direction of the steel sheet, and the crystal orientation analysis is performed by the EBSP (Electron Back Scattering Pattern) method on the area of 1/4 of the total thickness from the outermost layer portion. Calculate.

The more {111} grains, the higher r value is obtained. As a result, the deep drawing property is improved. Therefore, in one aspect of the present invention, the ratio of the above-mentioned {111} grains is preferably 20% or more. However, if the ratio of the {111} grains is too high, the balance between the bulging properties is not balanced, so the upper limit of the ratio of the {111} grains is set to 90%, more preferably 80% or less, and more specifically 70% or less. Can be determined as {111} grains in the present invention means ferrite grains.

Also, the more {112} grains, the better the bulging properties. In the present invention, it is preferable that the ratio of {112} grains is 10% or more. However, when the ratio of {112} grains is too high, the balance of deep drawing performance is not balanced, so the upper limit of the ratio of {112} grains is set to 80% or less, more preferably 70% or less, more specifically 60 It can be set below%. As used herein, the term {112} grains refers to ferrite grains.

In addition, in order to combine the corrosion resistance and workability of the ferritic stainless steel, it is preferable to control the composition of the ferritic stainless steel according to one aspect of the present invention as follows. In the following, it should be noted that the content of each component element is based on the weight% unless otherwise indicated.

C: 0.02% or less

C is an element that combines with Ti to form Ti carbonitrides, and increases the strength. When the C content is high, the unbound solid solution C content is high, the strength is high, and the elongation is decreased. Therefore, the content is 0.02% or less. .

Si: 1.0% or less

Si is a ferrite forming element which increases the stability of ferrite phase and improves the oxidation resistance. However, if it is added more than 1.0%, Si is disadvantageous for improving elongation because it increases hardness, yield strength, tensile strength and lowers elongation. .

Mn: 1.5% or less

Mn is used as a deoxidizer in the dissolution step of the steel, but if the amount exceeds 1.5%, MnS is eluted to limit the pitting resistance, so it is limited to 1.5% or less.

P: 0.035% or less, S: 0.03% or less

Since P and S form inclusions such as MnS to inhibit corrosion resistance and hot workability, P and S should be kept as low as possible, so they are limited to P: 0.035% or less and S: 0.03% or less.

Al: 0.08% or less

Al is used as a deoxidizer in the dissolution step of the steel, but if the amount exceeds 0.08%, surface defects are caused by an increase in nonmetallic inclusions, and the workability is deteriorated. Therefore, Al amount is made into 0.08% or less.

N: 0.015% or less

When the amount of N exceeds 0.015%, the steel material becomes hard and the ductility decreases. Therefore, the amount of N is made 0.015% or less.

Cr: 10-12%

Cr content is less than 10%, the corrosion resistance is lowered, and if the content is higher, the corrosion resistance is improved, but if the content is more than 12%, the strength is increased and the moldability is reduced due to the decrease in elongation, the content is 10% ≤Cr ≤ 12%.

Ti: 0.3% or less

Ti is a passivating film stabilizing element, so if the Ti content is too high, the elongation and formability are reduced, so the content is preferably limited to 0.3% or less. In addition, the lower limit of the Ti content is not necessarily limited, but in one aspect of the present invention, if the amount of Ti added is too low, grain boundary corrosion occurs in the weld zone, so that the lower limit may be set to 10C ≦ Ti in order to prevent weld boundary grain corrosion. Where C and Ti represent the content of each element in weight percent.

As described above, the ferritic stainless steel of the present invention is a weight%, C: 0.02% or less, Si: 1.0% or less, Mn: 1.5% or less, P: 0.035% or less, S: 0.03% or less, Al: 0.08% or less, N: 0.015% or less, Cr: 10-12%, Ti: 0.3% or less, residual Fe and inevitable impurities.

In addition, the ferritic stainless steel of the present invention may further include one or two selected from Ni and Cu, in addition to the above components, in the following content ranges.

Ni: 0.2% or less

Ni is an element which is effective in securing corrosion resistance. In the austenitic stainless steel, Ni is an element that is added in a large amount to secure corrosion resistance, but in ferritic stainless steel, the Ni content may be greatly reduced for cost reduction. Therefore, in one aspect of the present invention, Ni is added at 0.2% or less.

Cu: 0.3 ~ 0.7%

Cu has an action of reducing dissolution of ferrous iron by the anode reaction, thereby helping to grow the {112} orientation, but when the amount is less than 0.3%, such an effect cannot be obtained. On the other hand, when Cu amount exceeds 0.7%, CuS will precipitate and deteriorate workability. Therefore, Cu amount shall be 0.3 to 0.7%.

As described above, the balance is Fe and unavoidable impurities, but the amount of unavoidable impurities is preferably reduced as much as possible. For example, it is preferable to set it as B: 0.001% or less, Mo: 0.1% or less, V: 0.05% or less, Mg: 0.01% or less, and Ca: 0.01% or less.

The method for producing the ferritic stainless steel of the present invention is not particularly limited. However, the following is an example of a manufacturing method derived by the inventor of the present invention.

The ferritic stainless steel sheet of the present invention can be produced by a process including hot rolling-hot rolled sheet annealing-cold rolled-cold rolled sheet annealing. More specifically, after hot rolling, the slab having a composition within the scope of the present invention is subjected to a hot rolled sheet annealing to be maintained at a temperature range of 700 to 850 ° C for 180 seconds or less, followed by cold rolling, at a temperature range of 850 ° C or higher. It can manufacture by the method of cold-rolled sheet annealing. Hereinafter, each process is demonstrated in detail.

Hot Rolled Annealing: 180 seconds or less at temperature of 700 ~ 850 ℃

Since the steel sheet having the composition of the present invention is hard after hot rolling, there is a problem that the rolling load increases during cold rolling. In order to solve this problem, it is necessary to perform hot-rolled sheet annealing at a temperature of 700 ° C or more. However, if the annealing temperature exceeds 850 ° C or the annealing time exceeds 180 seconds, the ferrite grains after the hot-rolled sheet annealing coarsen, and the ratio of {111} grains does not become more than 20% after the cold-rolled sheet annealing, or {112 } Since the ratio of a crystal grain becomes less than 10%, the deep drawing property and the dripping property after cold-rolled sheet annealing tend to deteriorate. Therefore, it is necessary to perform hot-rolled sheet annealing on conditions of 180 second or less in the temperature range of 700-850 degreeC. The lower limit of the hot rolled sheet annealing time does not need to be specifically determined, but in order to obtain a sufficient effect, the hot rolled sheet annealing time is preferably performed for 15 seconds or more.

Cold Rolled Annealing: Above 850 ℃

When the annealing temperature during cold rolling annealing is less than 850 ° C., recrystallization may be inhibited or ferrite in the form of being pressed in the rolling direction may be present as it is, thereby deteriorating characteristics such as ductility. Therefore, cold rolling annealing needs to be performed in the temperature range of 850 degreeC or more. In addition, although the upper limit temperature of a cold rolled sheet annealing temperature does not need to be specifically determined, if the aggregate structure adjusted by the process to cold rolling is maintained in a preferable range, and energy efficiency etc. are considered, the upper limit of a cold rolled sheet annealing temperature shall be 1000 degreeC. You can also decide. The retention time during cold annealing may be set to 10 to 200 seconds.

In addition to the above conditions, in order to further improve the physical properties of the steel, the conditions described below may be further added.

Hot Rolled: 20 to 60% maximum rolling reduction per pass

In order to secure an area ratio of {111} grains at a quarter of the plate thickness by 20% or more, it is possible to achieve at least a maximum reduction ratio of 20% or more per pass in the rough rolling process, but {112} grains For growth, the maximum rolling reduction per pass can be limited to less than 60% in the rough rolling process.

In addition, in one aspect of the present invention, in order to make the {112} grain more than 10%, the difference between the plate center temperature and the plate surface temperature during rolling may be 150 ° C or less.

In addition, after cold-rolled sheet annealing, in order to remove shape correction and yield point elongation (YPE), skin pass rolling may be further performed at the elongation rate of 0.5 to 1.5%.

When manufacturing conditions are controlled as mentioned above, other conditions which are not specifically limited can be performed according to the manufacturing conditions of a normal ferritic stainless steel plate.

Hereinafter, the present invention will be described in detail through examples. However, it should be noted that the following examples are only intended to illustrate the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is achieved by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

The ferritic stainless steel slab of Table 1 was prepared, and the hot rolled sheet, hot rolled sheet annealing, cold rolling, and cold rolled sheet annealing were performed on the conditions of Table 2, and the cold rolled sheet of 0.8 mm was obtained. The content of each element shown in Table 1 means weight%.

Table 1 division C Si Mn P S Al N Cr Cu Ti Ni A 0.005 0.35 0.4 0.025 0.01 0.035 0.0105 10.4 - 0.2 - B 0.012 0.25 0.16 0.031 0.005 0.040 0.0095 11.5 0.45 0.15 - C 0.015 0.62 0.8 0.025 0.012 0.032 0.0087 11.9 - 0.21 0.13

TABLE 2 division Maximum rolling reduction per 1 pass of hot rolled rough rolling Hot Rolled Annealing Temperature (℃) Hot Rolled Annealing Time (sec) Cold Rolled Annealing Temperature (℃) Skin Pass Roll Elongation (%) Remarks A 35 800 150 850 - Inventive Example 1 A 20 780 120 870 - Inventive Example 2 A 46 870 90 900 - Comparative Example 1 A 50 730 200 880 - Comparative Example 2 B 28 780 120 800 - Comparative Example 3 B 45 830 120 880 One Inventive Example 3 C 18 750 120 880 - Comparative Example 4 C 35 820 120 880 - Inventive Example 4 C 65 790 120 880 - Comparative Example 5

About the steel plate manufactured by the above-mentioned process, the ratio of the {111} and {112} grains of the quarter thickness point from the outermost layer part was measured by the EBSP method. Deep drawing property and quality property of each steel plate were measured in the following manner.

Deep Drawing Castle

Using a Japanese Industrial Standard (JIS) No. 13 test piece (3 pieces of 200 mm from the width of the plate width direction and the judgment end every 50 m in the rolling direction), a preliminary 15% shortening bulging preliminary strain was given. R values (r _L , r _D , and r _c ) in each direction were obtained, and the r values of the respective sampling sites were calculated by the following equation, and their average values were obtained.

r = (r _L + 2r _D + r _c ) / 4

Where r _L and r _D And r _c Denotes the r value in the rolling direction, the direction of 45 ° to the rolling direction, and the direction of 90 ° to the rolling direction, respectively.

Bulging

A bulging height was obtained by performing a hydraulic bulging test with a clamping pressure of 980 kN using a 100 mm ø (100 mm diameter) circular dice.

The values measured in each test method are shown in Table 3 below.

TABLE 3 division % Of {111} % {112} r Bulging Height (mm) Inventive Example 1 25 27 1.45 35 Inventive Example 2 32 18 1.67 31 Comparative Example 1 15 21 0.81 33 Comparative Example 2 35 5 1.72 19 Comparative Example 3 26 32 1.39 37 Inventive Example 3 35 27 1.68 30 Comparative Example 4 19 28 1.11 33 Inventive Example 4 31 26 1.53 37 Comparative Example 5 25 8 1.38 28

As can be seen in Table 3, in the case of the invention example in which the {111} ratio is 20% or more and the {112} ratio is 10% or more, the r value is 1.3 or more and the bulging height is 30 mm or more. It was found to be humble. However, Comparative Example 1 and Comparative Example 4 in contrast to this, the {111} ratio was somewhat low, the r value had a low value of less than 1.3. In Comparative Example 2 and Comparative Example 4, the {112} ratio was less than 10%, and the bulging height was less than 30 mm. In Comparative Example 3, the r value and the bulging height were sufficient, but there was a problem in that the rolling load increased during cold rolling.

1 and 2 show an EBSP photograph of Comparative Example 1 and an EBSP photograph of Inventive Example 1, respectively. As can be seen in the drawing, Comparative Example 1 shows that the ratio of {111} grains (parts indicated by ① in the drawing) and {112} grains (parts indicated by ② in the drawing) does not satisfy the numerical range defined in the present invention. Inventive Example 1, the ratio of {111} crystal grain (part indicated by ① in the drawing) and {112} crystal grain (part indicated by ② in the drawing) satisfied the conditions of the present invention. As a result, an r value of 1.3 or more and 30 mm were obtained. The above bulging height was shown.

Thus, the advantageous effects of the present invention could be confirmed.

Claims (8)

A ferritic stainless steel having excellent workability, wherein the ratio of grains having a plane orientation {111} parallel to the rolled surface is 20% or more, and the ratio of grains having a plane orientation of {112} parallel to the rolled surface is 10% or more. However, the ratio of each grain here means the value measured by the EBSP (Electron Back Scattering Pattern) method. The method of claim 1, wherein, in weight%, C: 0.02% or less, Si: 1.0% or less, Mn: 1.5% or less, P: 0.035% or less, S: 0.03% or less, Al: 0.08% or less, N: 0.015% Hereinafter, a ferritic stainless steel having excellent workability having a composition containing Cr: 10 to 12%, Ti: 0.3% or less, balance Fe, and unavoidable impurities. The ferritic stainless steel having excellent workability according to claim 2, further comprising one or two selected from the group: by weight, Ni: 0.2% or less and Cu: 0.3 to 0.7%. Preparing a slab of ferritic stainless steel; Hot rolling the slab to obtain a hot rolled plate; Performing a hot-rolled sheet annealing to maintain the hot-rolled sheet in a temperature range of 700 to 850 ° C. for 180 seconds or less; Cold rolling the hot rolled plate to obtain a cold rolled plate; Performing an annealing of the cold rolled sheet at a temperature range of 850 ° C. or higher. Method for producing a ferritic stainless steel having excellent workability comprising a. The method of manufacturing a ferritic stainless steel having excellent workability according to claim 4, wherein the maximum rolling reduction per pass during the hot rolling is 20 to 60%. The slab according to claim 4 or 5, wherein the slab is by weight, C: 0.02% or less, Si: 1.0% or less, Mn: 1.5% or less, P: 0.035% or less, S: 0.03% or less, Al: 0.08. A method for producing a ferritic stainless steel having excellent workability having a composition containing% or less, N: 0.015% or less, Cr: 10-12%, Ti: 0.3% or less, balance Fe and unavoidable impurities. The method of manufacturing a ferritic stainless steel having excellent workability according to claim 6, wherein the slab further comprises one or two selected from the group by weight of Ni: 0.2% or less and Cu: 0.3 to 0.7%. The method of manufacturing a ferritic stainless steel having excellent workability according to claim 4 or 5, further comprising skin pass rolling at an elongation rate of 0.5 to 1.5% after the cold rolled sheet annealing.

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