GB2258469A

GB2258469A - Stainless steel for a razor

Info

Publication number: GB2258469A
Application number: GB9202084A
Authority: GB
Inventors: Atushi Kumagai; Kunio Taira
Original assignee: Hitachi Metals Ltd
Current assignee: Proterial Ltd
Priority date: 1991-08-05
Filing date: 1992-01-31
Publication date: 1993-02-10
Anticipated expiration: 2012-01-31
Also published as: JPH0539547A; SE514878C2; GB9202084D0; GB2258469B; SE9200290D0; JP3354163B2; SE9200290L

Abstract

Stainless steel for a razor consisting, by weight, of more than 0.55% but not more than 0.73% C, not more than 1.0% Si, not more than 1.0% Mn, not less than 12% but not more than 14% Cr and the balance Fe and incidental impurities, a carbide density in an annealed state being 140 to 600 pieces/100 mu m<2>. The above stainless steel may contain least one of not more than 1.0% Ni and not more than 2% Mo. There is also disclosed a process for producing stainless steel for a razor, comprising the steps of introducing a strip of the above-mentioned steel into a continuous furnace set to a temperature higher than a transformation temperature AC1 of the steel, prior to or during cold rolling, and annealing the steel sheet so that a carbide density of the steel strip is 140 to 600 pieces/100 mu m<2>.

Description

Stainless Steel for a Razor This invention relates to stainless steel for a razor (razor blade) and also to a method of producing such stainless steel.

As steel for a razor, two kinds of materials have been commercially used. One is high carbon steel corresponding to SK1 (JIS), and the other is martensitic stainless steel containing 12 to 13% Cr. The former can have a high quenching hardness, and is excellent in sharpness (cutting quality). On the other hand, the martensitic stainless steel, when subjected to quenching and tempering (heat treatment), can have a hardness of HV620 to HV650 which is the level required for a razor blade. Also, the martensitic stainless steel is more excellent in rust resistance and corrosion resistance than high carbon steel, and therefore has been extensively used.

Usually, the above martensitic stainless steel for a razor is first formed into a strip by a combination of hot rolling, cold rolling and annealing, and then is supplied to the next step of the production process. In the next step, the strip is stamped, and then is subjected to a heat treatment (quenching and tempering), a blade formation, and a surface treatment (e.g. a Teflon coating, or sputtering), thereby providing a final product (razor). With respect to the micro structure of the above martensitic stainless steel, chromium carbide is dispersed in the matrix, and the particle size of this carbide and the condition of distribution of the carbide greatly influence the processability of the steel and the characteristics of the razor blade.

A method of making carbide fine mainly for the purpose of enhancing a cold workability after hot working is disclosed in U.S. Patent No. 4,021,272. In this method, a strip of steel is formed by hot rolling, and is wound into a coil, and then this coil is loosened (this is commonly referred to as "open coil11), and in this condition the steel strip is annealed at a constant temperature in a salt bath.

U.S. patent No. 418,420 discloses a razor blade in which, in order to achieve a corrosion resistance and a high hardness among the characteristics of a razor blade, the density of carbide in high chromium steel having a relatively low carbon content of 0.30 to 0.15% is made to have a value of 200 to 500 pieces/100 ;ism2.

Japanese Patent Unexamined Publication No. 54121218 discloses steel for a razor and a method of producing the same, in which in order to enhance a corrosion resistance and a cutting quality (sharpness), the steel has a relatively low carbon content of 0.30 to 0.5%, and the average particle size of carbide in an annealed state is not more than 0.5 zm.

There have now been posed two new problems to be solved with resect to steel for a razor. One is to achieve a higher hardness in order to further enhance a cutting quality of a razor blade. The other is to improve the productivity during the manufacturing and particularly to increase the production line speed at the quenching and tempering steps and the production line speed at the coating treatment step. Generally, steel for a razor (razor steel) is formed by hot rolling into a hot coil having a thickness of around 2 mm, and this strip is sufficiently annealed, and then is formed by cold rolling into a coil of strip having a thickness of about 0.1 mm. In the manufacture of a razor using this razor steel, this razor steel is stamped into a predetermined shape, and then the thus stamped steel is subjected to annealing and tempering using a continuous furnace and to a blade formation.Also, the razor blade is subjected to a Teflon coating treatment at 3500C to 4000C so as to give a good slide relative to the skin of a man. The required hardness of the razor blade and the productivity are determined almost by the heat treatment in this continuous furnace. Therefore, in order to solve the above two problems, there has been needed the type of razor steel which can achieve a high hardness even at the quenching and tempering treatment or at the Teflon coating treatment in the continuous furnace, and also can make the furnace passage speed as high as possible, that is, makes the time of residence (maintaining) of the material in the furnace as short as possible.

In the proposals made in the above Japanese Patent Unexamined Publication No. 54-121218 and the above U.S. Patent No. 418,420, it is intended to obtain a sufficient heat treatment hardness by making the particle size of the carbide small or by increasing the density of the carbide; however, with respect to the chemical composition, since the content of C is small, it is difficult to obtain a sufficient hardness, and therefore these materials are not suited for practical use The carbon (0.65%)-chromium (13%) steel currently used is not sufficient in carbide density, and its carbide density is as small as 100 pieces/100 zmz at best, and therefore this steel is not suited for a short-time annealing.

Although U.S.. Patent No. 4,021,272 proposes a method of making the carbide of the razor steel fine, it does not give any indication of how the carbide density should be adjusted in order to achieve a short-time annealing which is one of main objects of the present invention.

Further, since U.S. Patent No.' 4,021,272 uses the salt bath, the handling of the salt bath is not easy, and also the treatment time is long, and therefore it has been desired to improve these points. Namely, the ccnventionally known techniques are directed merely to the type of stainless steel having a low carbon content mainly in order to improve the corrosion resistance, and in order to ensure the heat treatment hardness liable to be deteriorated because of the low carbon content, the carbide density is limited to a specified range, or the complicated method is used for effectively making the carbide fine. Thus, there have not yet been known steel for a razor suited for a short-time annealing with the carbon content of more than 0.55% (which is the new problem to be solved), and a novel and simple method of providing such razor steel.

It is an object of this invention to provide steel for a razor which enables a short-time heat treatment by optimally selecting its chemical composition and its carbide density, and also can achieve a high hardness.

Another object of the invention is to provide a method of producing such razor steel, in which the density of the carbide can be made high more easily than the conventional methods.

With respect to steel containing, as main components, more than 0.55% but not more than 0.73% C, and not less than 12% but not more than 14% Cr, the conditions of the carbide density for enabling a shorttime annealing (which is a new requirement for razor steel in order to increase the production line speed) and for achieving a sufficient heat treatment hardness and a corrosion resistance, as well as the conditions of the production for achieving this carbide density, have been studied, and as a result the present invention has been made. Particularly, the steel of the present invention, unlike the conventional steel, has a relatively high carbide density with a relatively high carbon content.

Specifically, according to one aspect of the present invention, there is provided stainless steel for a razor consisting essentially, by weight, of more than 0Z55% but not more than 0.73% C, not more than 1.0% Si, not more than 1.0% Mn, not less than 12% but not more than 14% Cr, and the balance Fe and incidental impurities, a carbide density in an annealing state being 140 to 600 pieces/l00 Clam2.

According to another aspect of the invention, there is provided stainless steel for a razor consisting essentially, by weight, of more than 0.55% but not more than 0.73% C, not more than 1.0% Si, not more than 1.0% Mn, not less than 12% but not more than 14% Cr, not more than 1.0% Ni, optionally at least one kind selected from the group consisting of not more than 1.0% Ni and not more than 2% Mo, and the balance Fe and incidental impurities, a carbide density in an annealing state being 140 to 600 pieces/100 pom2.

According to a further aspect of the invention, there is provided a process for producing stainless steel for a razor comprising the steps of introducing a strip of steel, consisting essentially, by weight, of more than 0.55% but not more than 0.73% C, not more than 1.0% Si, not more than 1.0% Mn, not less than 12% but not more than 14% Cr, optionally at least one kind selected from the group consisting of not more than 1.0% Ni and not more than 2% Mo, and the balance Fe and incidental impurities, into a continuous furnace set to a temperature higher than a transformation temperature Acl of the steel, prior to or during cold rolling, and annealing the steel strip in the furnace so that a carbide density of the steel sheet is 140 to 600 pieces/100 clam2.

The reasons for the above specified chemical composition of the steel will be explained in the following.

Carbon (C) is an important element which becomes a solid-solution state from carbide at an austenitizing temperature during the quenching, and determines the hardness of the martensite produced by the quenching.

The content of C should be at least more than 0.55% in order to achieve a sufficient hardness of the razor steel and also to stably bring the carbide density to 140 to 600 pieces/100 jim2. Also, in the martensitic stainless steel, depending on the balance between the C content and the Cr content, large eutectic carbide is crystallized at the time of solidification. Such large carbide must be absolutely avoided for the razor blade material having a thickness of about 0.1 mm and having a sharp cutting edge, because this will cause an edge tear-out of blade edge. Therefore, the upper limit of the C content has been decided to be 0.73% in view of the balance with the Cr content.

Si is usually used as a deoxidizer when refining steel, and is known as an element which exists in a solid-solution state in steel and restrains the softening of the steel at the time of a low-temperature tempering. The lower limit of Si is preferably not less than 0.420/o However, it is quite likely that Si can reside or remain as a hard non-metallic inclusion (SiO2) in the steel, and causes an edge tear-out of blade edge and spots or rust. Therefore, the upper limit of the Si content has been decided to be 1%.

Like Sit Mn also performs the funetion of a deoxidizer at the time of refining, but if this content exceeds 1%, the hot workability is lowered in the chemical composition of the present invention.

Therefore, the upper limit of the Mn content has been decided to be 1%. The lower limit of Mn is preferably not less than 0.3%.

As is well known, Cr is an indispensable element for stainless steel, because it enhances a corrosion resistance. In order to sufficiently achieve this corrosion resistance and also to combine Cr with carbon in such a manner that the fine chromium carbide is dispersed at the carbide density of the present invention, the Cr content should be at least not less than 12%. In the Cr content exceeds 14%, M7C3 carbide (M = Cr, Fe) is crystallized, and this becomes out of the condition of the carbide density of the present invention because of its balance with the C content. Therefore, the upper limit of the Cr content has been decided to be 14%.

Ni is an element effective for improving the corrosion resistance against a non-oxidizing acid such as H2SO4. However, if the Ni content exceeds 1%, Ni lowers the martensite transformation starting temperature (Ms point), and causes an excessive amount of residual austenite at the time of the quenching, thereby lowering the quenching hardness. Therefore, the Ni content should be limited to not more than 1%. The lower limit of Ni is preferably nnt 1 less than 0.3%.

Mo is excellent in corrosion resistance against halogen elements, such as chlorine, tending to induce pitting. However, like Ni, if Mo is added in an excessive amount, Mo lowers the Ms point, and causes an excessive amount of residual austenite at the time of the quenching, thereby lowering the quenching hardness.

Therefore, the Mo content should be limited to not more than 2%. The lower limit of Mo is preferably not less than 0.3%.

Next, reference is now made to the condition of the carbide density which is an important feature of the razor steel of the present invention.

In order to obtain a high hardness with a short quenching maintaining time, it is necessary that at the austenitizing temperature, the carbide should be rapidly and sufficiently brought into a solid solution state to increase the carbon amount in the matrix. To achieve this, in the annealing condition, the fine carbide need to be dispersed at a high density. The inventors of the present invention have found that in order to obtain the effect of the short-time quenching, the density of at least 140 pieces/100 zm2 is necessary, as compared with the density of 100 pieces/100 zm2 of the currently-used material. The higher the carbide density is, the more efficiently the high hardness is obtained with a shorttime quenching.On the other hand, the higher the density is, the higher the annealing hardness iso This adversely affects the characteristics of the cold rolling of the material. In view of this, if the density exceeds 600 pieces/100 g2, much manhour is required for the cold rolling, and besides the possibility of rupture of the steel strip at the time of the cold rolling increases.

Therefore, this value has been decided to be the upper limit.

If the carbide is made fine in such a range, this is effective in keeping the corrosion resistance.

At the austenitizing temperature, the corrosion resistance of high carbon stainless steel is influenced by the content of Cr solid-solutioned in the matrix.

Under the same quenching condition, the interface between the matrix and the carbide is increased with the increase of the carbide density, and therefore the amount of solid solution of Cr is also increased. Therefore, the corrosion resistance can be further improved.

Next, a method of obtaining the carbide density of the present invention will be explained. In a heat treatment in a process for the production of conventional 0.65C-13Cr steel for a razor, a steel strip having been subjected to hot rolling is annealed in a batch-type annealing furnace set to temperatures of 800 to 8400C, and then the steel strip is repeatedly cold rolled and annealed to be finished into a predetermined size. On the other hand, in an annealing method of obtaining the razor steel of the present invention, the steel is passed continuously at least more than once through a heating region heated to a temperature of about 8500C higher than the transformation point Acl of the razor blade of the present invention, and then the steel is repeatedly subjected to cold rolling and annealing, thereby finishing the steel into a predetermined size.The conventional batch-type annealing furnace requires between ten and several hours because of the heating and the cooling, and it is difficult to control the carbide density. On the other hand, in the method of the present invention, the material is passed through the heating region in a short time, and therefore the high carbide density can be achieved.

The invention will now be described with reference to the following drawi.ngs in. which: Fig. 1 is a photograph showing a metallic structure of stainless steel for a razor according to the present invention; Fig. 2 is a photograph showing a metallic structure of conventional stainless steel for a razor; Fig. 3 is a graph showing the relation between the austenitizing maintaining time and the hardness of the steels of the present invention and the conventional steel; and Fig. 4 is a graph showing the relation between the austenitizing maintaining temperature and the hardness of the steels of the present invention and the conventional steel.

Examples Each of steels A to G of the present invention having respective compositions shown in Table 1 was hot rolled into a strip having a thickness of 1.7 mm Then, the steel strip was introduced into a continuous furnace having a heating region set to 8500C x 20 min., and was annealed therein. Then, the steel sheet was subjected sequentially to cold r.olling, annealing (7800C x 5 min.), cold rolling, annealing (7800C x 5 min.), and cold rolling, thereby finishing the steel strip into a thickness of 0.1 mm.

Table 1

Chemical composition (wt.%) Carbide density Note C Si Mu Cr MO Ni Fe (pieces/100 m2) A 0.56 0.42 0.63 12.05 -- -- balance 145 B 0.65 0.45 0.71 13.20 -- -- balance 212 C 0.66 0.63 0.75 13.59 -- -- balance 560 D 0.65 0.62 0.75 13.14 1.43 -- balance 148 Steels of the E 0.69 0.51 0.62 12.94 -- 0.72 balance 253 present invention F 0.71 0.65 0.72 13.05 0.62 0.51 balance 405 G 0.73 0.58 0.73 12.55 1.50 0.55 balance 328 H 0.66 0.40 0.71 13.13 -- -- balance 97 conventional steel On the other hand, steel H (the conventional steel) having the composition shown in Table 1 was hot rolled into a strip having a thickness of 2.0 mm, and then this steel strip was annealed in a batch-type annealing furnace set to 8400C x 5 hours (This is a typical conventional step). Then, the steel strip was subjected sequentially to cold rolling, annealing (7800C x 5 min.), cold rolling, annealing (7800C x 5 min.), and cold rolling, thereby finishing the steel strip into a thickness of 0.1 mm.

The carbide density shown in Table was determined by taking photographs from five fields of view by an electronic microscope of 8000 magnifications, and then by processing these photographs by an image analyzer.

Although the conventional steel H falls within the range of the chemical composition of the steels of the present invention, its carbide density is 97 pieces/100 jim2, because the production method thereof is different from that of the steels of the present invention, as described above.

As representative examples, the metallic structures of the steel C of the present invention and the conventional steel H are shown in Figs. 1 and 2, respectimely. Figs. l and 2 are photographs of 4000 magnifications showing the metallic structures.

M23C6 (M = Cr, Fe) carbide 1 is present in both of the steels of Figs. 1 and 2, and has a good spheroidal shape. In the steel of the present invention shown in Fig. 1, the particle size of the carbide 1 is small, and is distributed uniformly at a high density of 560 pieces/100 jim2. On the other hand, in the conventional steel shown in Fig. 2, the particle size varies greatly, and the void of the matrix is large, thus indicating a low density of 97 pieces/100 jim2.

In order to confirm the characteristics of the heat treatment in the production of the razor blades of the present steels A to G and the conventional steel H, each steel was maintained at the austenitizing temperature in vacuum, and then was quenched, and was subjected to a deep freezing treatment (-750C x 15 min.) as in an actual production of a razor blade, and the maintaining time required to obtain a hardness of HV750 was measured.

Results obtained are shown in Table 2. Further, from the hardness of each product obtained as a result of tempering at 3500C for one hour as in a Teflon coating treatment in an actual production process, and also from the polarization characteristics in this condition, a corrosion potential indicative of one index of the corrosion resistance of the razor blade was measured, and results thereof are also shown in Table 2.

Table 2

Hardness of razor Austenitizing maintaining Hardness of Corrosion steel having a time at 1100 C for razor blade potential thickness of 0.1 mm obtaining HV800 by after quenching Ecorr quenching and deep and tempering 5% H2SO4 freezing (-75 C) (HV) (sec) (HV) mV(vsSCE) A 243 31 660 -428 B 261 23 690 -412 C 357 12 720 -408 D 248 30 675 -401 E 273 20 685 -405 F 315 15 713 -392 G 293 18 692 -385 HV800 was not obtained.

H 230 HV795 was obtained 640 -432 with 30 sec.

As is clear from Table 2, with respect to the conventional steel H, HV795 can be only obtained with the austenitizing maintaining time of 30 sec., whereas with respect to the steels A to G of the present invention, HV800 can be obtained with the austenitizing maintaining time of 12 to 31 sec. And, besides, when the carbide density is not less than 400 pieces/100 jim2, the steels of the present invention are much higher in hardness than the conventional steel, with 1/2 to 1/3 of the maintaining time for the conventional steel. Also, with respect to the hardness of the razor blades subjected to tempering, any of the steels A to G of the present invention are harder than the conventional steel H, and the effect of the present invention achieved by the high carbide density is clearly indicated.

Further, it will be appreciated from Table 2 that the corrosion potential of the steels A to G of the present invention is electrochemically shifted to the noble side than that of the conventional steel H, and the amount of solid solution of Cr in the matrix is increased, thereby enhancing the corrosion resistance.

And besides, the corrosion potential of the steels D, E, F and G of the present invention containing Mo and/or Ni is further shifted to the noble side, and therefore the corrosion resistance of these steels is excellent.

Fig. 3 shows the relation between the austenitizing maintaining time and the hardness of the steels B and C of the present invention and the conventional steel H (which had generally the same chemical composition) subjected to the deep freezing treatment (-750C x 15 min.) after the quenching, when the austenitizing maintaining temperature for the quenching was 11000C.

Fig. 4 shows the relation between the austenitizing maintaining temperature and the hardness of the steels B and C of the present invention and the conventional steel H subjected to the deep freezing treatment (-750C x 15 min.), when the austenitizing maintaining time for the quenching was 30 sec., and also shows such relation when the tempering was further effected at 3500C for one hour.

As can be seen from Fig. 3, the steels B and C of the present invention can have a greater hardness than the conventional steel H with the same austenitizing maintaining time, and also can have the same hardness with a shorter austenitizing maintaining time than the conventional steel H.

Further, it can be appreciated from Fig. 4, that the steels B and C can have a greater hardness than the conventional steel H at any austenitizing maintaining temperature.

Thus, it has been confirmed that the steels of the present invention can have a greater hardness than the conventional steel with a shorter time, and can have a greater hardness than the conventional steel on the same quenching condition.

The razor steels of the present invention can have a greater hardness than the conventional steel with a shorter quenching time, and therefore the speed of quenching at the process for the production of the razor blades can be increased 2 to 3 times higher. And besides, a higher heat treatment hardness, can be obtained with the steels of the present invention than with the conventional steel on the same quenching condition, and the improved corrosion resistance can be obtained with the steels of the present invention.

Therefore, in the present invention, the razor blades of a high performance and a high productivity can be produced.

Claims

1. Stainless steel for a razor, said steel comprising, by weight, more than 0.55% but not more than 0.73% C, not more than 1.0% Si, not more than 1.0% Mn, not less than 12% but not more than 14% Cr, and the balance Fe and incidental impurities, the carbide density thereof in an annealing state being 140 to 600 pieces/100 pom2*

2. Stainless steel as claimed in claim 1 further comprising not more than 100% Ni.

3. Stainless steel as claimed in claim 1 further comprising not more than 2% Mo.

4. Stainless steel as claimed in any one of the preceding claims further comprising not more than 1.0% Ni and not more than 2% Mo.

5. Stainless steel as claimed in claim 1 substantially as herein described with reference to the Examples.

6. A razor blade manufactured from a stainless steel as claimed in any one of the preceding claims.

7. A process for producing steel for a razor, said process comprising the steps of introducing a strip of steel comprising, by weight, more than 0.55% but not more than 0.73t C, not more than 1.0% Si, not more than 1.0% Mn, not less than 12% but not more than 14% Cr, optionally at least one selected from the group consisting of not more than 1.0% Ni and not more than 2% Mo, and the balance Fe and incidental impurities, into a continuous furnace set to a temperature higher than the transformation temperature Ac1 of said steel, prior to or during cold rolling, and annealing said strip of steel so that the carbide density of said strip of steel is 140 to 600 pieces/100 pom2.

8. A process for producing steel for a razor as claimed in claim 7 substantially as herein described with reference to the Examples.