Patent Application
20160361828
The present invention relates to stainless steel or steel strips used for razor blades and in particular for blades of the bent type.
Razor blades are typically formed of a suitable metallic sheet material such as stainless steel, which is slit to a desired width and heat-treated to harden the metal. The hardening operation utilizes a high temperature furnace, where the metal may be exposed to temperatures greater than 1000° C., followed by quenching. After hardening, a cutting edge is formed on an elongated edge of the blade. The cutting edge typically has a wedge-shaped configuration with an ultimate tip having a radius less than about 1000 angstroms, e.g., about 200-300 angstroms.
The razor blades are generally mounted on a plastic housing (e.g., a cartridge for a shaving razor) or on a bent metal support that is attached to a housing. The razor blade assembly may include a planar blade attached (e.g., welded) to a bent metal support. The blade may include a tapered region that terminates in a sharpened cutting edge. This type of assembly is secured to shaving razors (e.g., to cartridges for shaving razors) to enable users to cut hair (e.g., facial hair) with the cutting edge. The bent metal support may provide the relatively delicate blade with sufficient support to withstand forces applied to blade during the shaving process. Examples of razor cartridges having supported blades are shown in U.S. Pat. No. 4,378,634 and in U.S. Pat. No. 7,131,202, which are incorporated by reference herein.
The performance and commercial success of a razor cartridge is a balance of many factors and characteristics that include rinse-ability (i.e., the ability of the user to be able to easily rinse cut hair and skin particles and other shaving debris from the razor cartridge and especially from between adjacent razor blades or razor blade structures). The distance between consecutive cutting edges or so-called “span” is theorized to affect the shaving process in several ways. The span between cutting edges may control the degree to which skin will bulge between blades, with smaller spans resulting in less skin bulge and more skin comfort during shaving, but may also increase opportunities for double engagement. Larger spans may reduce opportunities for double engagements, but may result in more skin bulge between cutting edges and less skin comfort. The span between cutting edges and, thus between blades, may affect rinsing of shave preparations and shave debris after a shaving stroke, with larger spans easing or quickening rinsing and smaller spans slowing or making rinsing more difficult. A razor cartridge including a razor blade having a bent portion can have certain advantages, such as decreased manufacturing costs and improved rinsability.
The manufacture of commercially acceptable razor cartridges, having one or more bent blades, presents issues such as failure of the blade during manufacturing or even during shaving. Various bent blade designs have been suggested in the literature; however, these designs often result in failure in certain types of steel (e.g., the blades crack or fracture during bending). WO 2012/006043, incorporated herein by reference, discloses a bending process applied to a razor blade for a razor cartridge but describes a problem that the blade is cracked or fractured during the bending process.
A martensitic stainless steel has been widely used for cutlery, surgical knives, and razor blade applications because it has high hardness and good corrosion resistance. Particularly, a high-carbon martensitic stainless steel strip material containing Cr in an amount of about 13% by mass is most commonly used as a material for razor blades. One example is found in JP-A-5-117805 which discloses a steel alloy containing, in weight percent, 0.45 to 0.55% of C, 0.4 to 1.0% of Si, 0.5 to 1.0% of Mn, 12 to 14% of Cr, and 1.0 to 1.6% of Mo, with the balance made up of Fe and unavoidable impurities. This martensitic stainless steel alloy for a razor blade exhibits both high corrosion resistance and high hardness. However, inevitably the resultant high brittleness in this steel results in cracking and fracturing in shapes other than flat blades.
Accordingly, one solution is to alter the geometry of the bent blade, but this compromise to prevent failure (e.g., with a bent portion having a larger radius) may result in decreased rinsability in multi-bladed razor systems. Alternatively, a softer steel may be used to achieve a desired bend radius; however, this also has drawbacks. Blades manufactured from softer steels often do not have the necessary edge strength for a durable cutting edge for a close and comfortable shave.
Thus, a stainless steel (e.g., martensitic) for a razor blade is desired that exhibits high hardness and resistance to corrosion, but with decreased cracking so as to not compromise the robustness of the razor blade and shaving attributes.
The present invention relates to a razor blade formed of a substrate, the substrate comprising an amount of Molybdenum (Mo) ranging from about 1.6% to about 5% by weight of composition. The razor blade further comprises a bent portion in a bend zone. The bent portion of the razor blade comprises substantially no cracks, substantially no tempered carbides (M3C), or tempered carbides of about 0.1 μm or smaller in diameter.
The razor blade further comprises an amount of Carbon (C) ranging from about 0.45 to about 0.55% by weight percent of composition, an amount of Chromium (Cr) ranging from about 12 to about 14% by weight percent of composition, an amount of Silicon (Si) ranging from about 0.4 to about 1.0%, an amount of Manganese (Mn) ranging from about 0.5 to about 1.0%, with the balance in weight percent of composition made up of an amount of Iron (Fe) and unavoidable impurities, or any combination thereof.
The present invention relates to an amount of Molybenum (Mo) from about 2.1% to about 2.8% by weight of composition. The substrate of the present invention is a martensitic stainless steel.
A further aspect of the present invention is a peak breaking angle ranging from about degrees to about 130 degrees, a ductility test breaking angle ranging from about 77 degrees to about 81 degrees, and a blade breaking energy is about 6 millijoules.
Still further, the razor blade of the present invention has an inner radius in said bend zone ranging from about 0.20 mm to about 0.50 mm, a bend angle formed in said bend zone ranging from about 35 degrees to about 75 degrees, a thickness of said razor blade ranging from about 0.05 mm to about 0.15 mm, and a ratio of said inner radius to a thickness of said razor blade ranges from about 1 to about 10.
The present invention relates to a razor cartridge comprising a plurality of razor blades, wherein at least one of said plurality of razor blades is formed of a substrate comprising an amount of Molybdenum ranging from about 1.6% to about 5% by weight of composition.
The present invention relates to a method of manufacturing a razor blade comprising the steps of: providing at least one strip of a steel substrate, said substrate comprising an amount of Mo ranging from about 1.6% to about 5% by weight of composition, heat treating the at least one steel strip, tempering the at least one steel strip, and bending a portion of the at least one steel strip forming a bend zone in the portion. The method comprises a razor blade steel strip with substantially no tempered carbides (M3C) present after the heat treating step. The method comprises a razor blade steel strip with substantially no cracks generated in the bent portion after the bending step. The method comprises a razor blade steel with an amount of Carbon (C) ranging from about 0.45 to about 0.55% by weight percent of composition, an amount of Chromium (Cr) ranging from about 12 to about 14% by weight percent of composition, an amount of Silicon (Si) ranging from about 0.4 to about 1.0%, an amount of Manganese (Mn) ranging from about 0.5 to about 1.0%, with the balance in weight percent made up of Iron (Fe) and unavoidable impurities or any combination thereof.
While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as forming the present invention, it is believed that the invention will be better understood from the following description which is taken in conjunction with the accompanying drawings in which like designations are used to designate substantially identical elements, and in which:
The novel stainless steel of a razor blade substrate of the present invention has a higher Molybdenum (Mo) content, of up to 5%, over conventional steel.
While it is generally known that the presence of Molybdenum (Mo) in steel substrates significantly increases the resistance to both uniform and localized corrosion and assists with increasing hardness, the present invention steel composition for a razor blade, with its increased Mo content, also surprisingly provides for improved ductility in the steel which in turn has a unexpected effect of suppressing the formation of cracks in the steel, a benefit for bent blades.
Increasing ductility or softness, as mentioned above in the Background of the Invention section, is generally not desired in the prior art since softer steel compositions often do not have the necessary edge strength for a close and comfortable shave.
Just as it was not known in the prior art that the ductility of a steel could be improved by increasing Mo, it was also not known in the prior art that increasing Mo has the affect of decreasing tempered carbide (M3C) formation. In the application to the razor blade bending process, as will be explained below, an increased amount of Mo in the steel reduces cracks in the razor blade when forming razor blades of the bent type.
The present invention was realized by focusing on the relationship between the state of the cracks formed on the surface of the steel and the metal structure of the blade steel substrate itself after heat treatment (e.g., quenching and tempering).
After heat treatment of razor blades, it was realized that the amount of formed M3C (tempered carbide) deposited on a crystal grain boundary, as shown below in
The bending workability or ductility of the steel material, after quenching and tempering, it was determined, can be improved by modifying the steel composition so as to decrease the amount of M3C formed at the crystal grain boundary.
It was found that increasing Mo in turn decreased the carbide precipitation (M3C) and surprisingly improved the ductility of the steel without compromising its high hardness and mechanical strength. In particular, with Mo content larger than 1.6%, and preferably with Mo larger than 2.1%, the Mo desirably suppresses tempered carbide (M3C formation) and reduces the size of the tempered carbide to 0.1 μm or smaller during heat treatment processes. It was realized that Molybdenum (Mo), being an element that is capable of forming carbide on its own, is hardly dissolved in M3C, where M is a metal element such as Fe, Cr or Mo.
The present invention is directed to a strip of a steel substrate for razor blades, which has a composition containing, in weight percent, of Mo in an amount between about 1.6% to about 5.0%.
In one specific embodiment, the present invention has a stainless steel composition in weight percent of 0.45% to 0.55% of C, 0.4% to 1.0% of Si, 0.5% to 1.0% of Mn, and 12% to 14% of Cr, and further contains Mo, with the balance made up of Fe and unavoidable impurities, or any combination thereof, wherein Mo is contained in an amount between about 1.6% to about 5.0% and more preferably, in an amount between about 2.1% to about 2.8%.
While the embodiments of the present invention focus on compositions with the above elements for practical purposes, the present invention contemplates that the elements, with the exception of the Mo, may be modified in amount, type, and in weight percent. For instance, the substrate may comprise substantially only C, Cr, and Si, in addition to the Mo within the novel range of 1.6% to 5%.
The term “ductility” or “ductile” as used herein signifies the ability of a material to deform plastically before fracturing or cracking. Ductile materials may be malleable or easily molded or shaped. A bending process with a bend-to-fail type instrument can generally be used to assess the ductility of razor blade steel by measuring values for the peak breaking angle and the amount of energy it takes to break or bend the steel blade.
The term “crack” as used herein can be understood as signifying a “macro crack” or a “micro crack.” While a “macro” crack generally refers to a type of crack that is visible with the naked eye or with low magnification, usually about 50× but not to exceed 100×, a “micro” crack generally refers to a crack that can only be seen under a high magnification, generally greater than 100× or 200×. A macro crack may also tend to be longer and extend deeper into a substrate when compared to a micro crack.
The peak breaking angle and further description of a blade of the bent type is shown in
While no crack is generally seen at the macro scale (e.g., “macro” crack) during formation of the razor blade 10, one or more cracks or fractures 17 (e.g., “micro” cracks) would likely be visible when the tensile surface is examined using SEM at high magnification. These cracks 17, which are sometimes referred to as fractures, are shown illustratively in
The razor blade 10 is formed to have a bend angle 18, desirably ranging from about 35 to about 75 degrees, preferably about 70 degrees, to provide a close and comfortable shave. The ductility of the razor blade is determined with a breaking angle or a peak breaking angle 19. The peak breaking angle 19 of the present invention may range from 0 degrees to 130 degrees, generally between about 60 degrees to about 130 degrees, preferably about 90 degrees, and more preferably about 68.5 to 80 degrees. It should be noted that the peak breaking angle 19 is generally larger than the bend angle 18 since it represents the angle at which a test razor blade would break.
An effective thickness T of the razor blade of the present invention including a razor blade of the bent type shown in
The bent razor blade has a length L of about 2.7 mm to about 3.2 mm and preferably about 2.84 mm.
Desirably, the ratio of the inner radius 16 to the thickness T of the blade of the present invention ranges from about 1 to about 10. For instance, a razor blade of the present invention having an inner radius of 0.33 mm and a thickness T of 0.074 mm has a ratio of 4.46.
Table 1 lists the chemical compositions of prior art martensitic stainless steel and an example martensitic stainless steel of the present invention. As noted below in Table 1, the novel Mo content of the present invention is between about 1.6 and about 5.0% by weight percentage of the composition.
The rational for the various elements shown above and their ranges in the present invention are as follows:
Content of Molybdenum (Mo): about 1.6% to about 5.0%
The content of Mo is desired to be 1.6% or more in weight percent so as to decrease the formation of tempered carbides (M3C) and also to obtain an effect of miniaturizing the size of the tempered carbide. This is because Mo is one of the elements capable of forming a carbide of its own, and has properties that it is hardly dissolved in M3C. In a tempering temperature range, M3C is generated due to the diffusion of only Carbon (C). However, it is considered that when a specific amount of Mo is present in a base, Mo prevents M3C from aggregating or increasing its size (e.g., Mo miniaturizes M3C).
When the lower limit content of Mo of the present invention is about 1.6% or greater, (e.g., about 1.8%, about 2.1%, about 2.3%), almost no M3C having a size of 0.1 μm or greater is observed on the tensile surface 14 of the razor blade. For instance, this is shown clearly in
This M3C deposited by tempering has a higher hardness than a martensite matrix, and therefore, when bending stress is applied to a razor blade, due to a difference in hardness between M3C and the martensite matrix, a crack is liable to occur at the boundary between M3C and a martensite matrix. M3C continues to be deposited in a grain or along a crystal grain boundary. Such M3c formed at the boundary is liable to be an origin from which the cracks formed during the bending process may extend. A decrease in the content of M3C at the boundary is thus advantageous to the suppression of crack formation.
Depending on the other elements present in the substrate composition and their respective weight percents, if the content of Mo is increased beyond an upper limit, deformation resistance may also be increased which may deteriorate the bending workability of the steel. Thus, an upper limit for Mo may be set at about 5%, preferably at about 3.5%, and most preferably about 2.8%.
Content of Carbon (C): about 0.45% to about 0.55%
With a content of C in the range from about 0.45 to about 0.55% a sufficient hardness for razor blades is achieved while also suppressing the crystallization of eutectic carbides during casting or solidification to the minimum. If the content of C is less than 0.45%, a sufficient hardness for a razor blade generally cannot be obtained. On the other hand, if the content of C exceeds 0.55%, the amount of crystallized eutectic carbides is increased depending on the balance with the amount of Cr which may cause a chip in the razor blade during sharpening processes. For this reason, the content of C preferably ranges from about 0.45% to about 0.55%. For achieving the above-described effect of C, a preferred lower limit of the content of C is 0.48% and the preferred upper limit of the content of C is 0.52%.
Content of Silicon (Si): about 0.2% to about 1.0%
Si is added to a steel substrate as a deoxidizing agent during refinement. In order to obtain a sufficient deoxidizing effect, the residual amount of Si is generally 0.2% or more. On the other hand, if the content of Si exceeds 1.0%, the amount of inclusions increases which may undesirably cause one or more chips in the razor blade during sharpening. Accordingly, the content of Si ranges desirably from about 0.2% to 1.0%. A preferred lower limit of the content of Si is 0.40% and the preferred upper limit of the content of Si is 0.60%.
Content of Manganese (Mn): about 0.2% to about 1.0%
Mn is also added as a deoxidizing agent during refinement in the same manner as Si. In order to obtain a sufficient deoxidizing effect, the residual amount of Mn is about 0.2% or more. On the other hand, if the content of Mn exceeds 1.0%, the hot workability of the razor blade substrate may begin deteriorating. Accordingly, the content of Mn ranges desirably from about 0.2% to about 1.0%. A preferred lower limit of the content of Mn is 0.60% and the preferred upper limit of the content of Mn is 0.90%.
Chromium (Cr): about 12% to about 14%
The reason why the content of Cr is desirably set from about 12% to about 14% is to achieve sufficient corrosion resistance and also to suppress the crystallization of eutectic carbides during casting or solidification to the minimum. If the content of Cr is less than 12%, sufficient corrosion resistance in stainless steel cannot be obtained. On the other hand, if the content of Cr exceeds 14%, the amount of crystallized eutectic carbides is increased to cause a chip in the razor blade when sharpening the razor blade. For this reason, the content of Cr is set to 12% to 14%. For achieving the above-described effect of Cr, the preferred lower limit of the content of Cr is 13.2% and the preferred upper limit of the content of Cr is 14%.
The balance of a specific composition of the present invention, other than the elements described above, may be made up of Iron (Fe) and other impurities. Examples of representative impurity elements include Phosphorus (P), Sulfur (S), Nickel (Ni), Vanadium (V), Copper (Cu), Aluminum (Al), Titanium (Ti), Nitrogen (N), and Oxygen (O). These elements may generally be unavoidably mixed therein, however, it is desirable to regulate these impurities within the following ranges so as to not impair the effects of the present invention: P≦0.03%, S≦0.005%, Ni≦0.15%, V≦0.2%, Cu≦0.1%, Al≦0.01%, Ti≦0.01%, N≦0.05%, and O≦0.05%.
A martensitic stainless steel of the present invention was tested for razor blade applications, and in particular razor blades of the bent type were formed and tested. Table 2 below lists the composition of a prior art razor blade steel substrate (A) and two novel razor blades having steel substrates (B) and (C) of the present invention, both within the novel Mo content range. Embodiment #1 (steel B) comprises a Mo content of about 2.31% and Embodiment #2 (steel C) comprises a Mo content of 2.61% in weight percent.
Each of the types of steel substrates used for the razor blades in Table 2 undergoes heat treatment and blade bending processes.
The heat treatment of the blade strip comprises hardening in an inline furnace, going through many steps such as austenization, quenching and tempering processes. Thus, high hardness is achieved for each of razor blade steel substrate types A, B, and C. Heat treatment generally may include quenching to 1100° C. for 40 seconds, quenching to room temperature, a cryogenic treatment at −75° C. for 30 minutes, and tempering at 350° C. for 30 minutes.
Heat treatment conditions may be specially selected for ductility evaluations. For example, U.S. Patent Publication No. 2007/0124939 and U.S. Pat. No. 8,011,104 disclose methods of locally heat treating a portion of a hardened razor blade body to enhance ductility for facilitating formation of a bent portion. A localized heat treatment or scoring processes can be used with the present invention method if desired.
As can be seen from Table 3, the hardness for each of the razor blades steel substrate types, A, B, and C formed is generally within the same range.
After the blades are heated, hardened, and tempered, the blade bending process formed the blades with a bend having about a 70 degree bending angle and an inner radius of 0.33 mm. While generally no cracks can be seen in any steel blades A, B, or C within macro scale during the forming of the bend, the tensile surface of the bend zone of each is examined using a scanning electron microscope at high magnification as will be shown and described below.
Referring now to
A carbide having a spherical shape or a size exceeding 0.2 μm seen in
Additionally, as shown in
Subsequently, a bending test at about 90 degrees was performed on razor blade with steel substrate A having an amount of Mo in the prior art range of about 1.30%. Using a scanning electron microscope, the presence or absence of any type of crack can generally be observed on a tensile surface from directly above the bent portion.
Referring now to
As can be seen, several micro cracks 26, some of which are deep, are present. M3C carbides 27, as best can be seen, are generally dispersed along a crystal grain boundary 28 of crystal grains 29 forming a network 27a in steel A and their presence is reduced after the bending process is performed.
Referring now to
A carbide having a spherical shape or a size exceeding 0.2 μm seen in
Subsequently, a bending test at about 90 degrees was performed on razor blade with steel substrate B having an amount of Mo of about 2.31%. Using a scanning electron microscope, the presence or absence of a crack can generally be observed on a tensile surface from directly above the bent portion.
Referring now to
As can be seen, there are substantially no cracks (or a negligible amount of cracks) visible in steel B of
While M3C carbides are generally dispersed along a crystal grain boundary of crystal grains forming a network in steel B, there are substantially no M3C carbides readily found in the tensile surface 35 shown. This may be attributed to the bend angle of the steel B being lower than that of steel A shown in
Subsequently, a bending test at about 90 degrees was performed on razor blade with steel substrate C having an amount of Mo of about 2.61%. As noted, using a scanning electron microscope, the presence or absence of a crack can generally be observed on a tensile surface from directly above the bent portion. Where no M3C carbides were observed after heat treatment as shown in
It is apparent that, as the amount of Mo was increased, the cracks in a bent portion in a bend zone of a razor blade became shallower or begin to disappear. From this testing, it was found that cracks were preferentially formed from M3C deposited along the grain boundary during the bending process. When the amount of Mo was increased, M3C at the grain boundary was decreased, thereby suppressing the formation of cracks.
Unlike prior art steel A (
The graph shown in
The graph shown in
For instance, steel blade A is shown in
As shown in
At second step 84 and third step 85 a heat treating and tempering of the at least one steel strip with conditions described above occurs, respectively. A fourth step 86 is a step of bending a portion of the at least one steel strip forming a bend zone in that portion.
There are substantially no tempered carbides (M3C) present after step 84. There are substantially no cracks generated in said bend zone after step 86.
The razor blade steel substrate further includes an amount of Carbon (C) ranging from about 0.45% to about 0.55% by weight percent of composition, an amount of Chromium (Cr) ranging from about 12% to about 14% by weight percent of composition, an amount of Silicon (Si) ranging from about 0.4% to about 1.0%, an amount of Manganese (Mn) ranging from about 0.5% to about 1.0%, with the balance in weight percent made up of Iron (Fe) and unavoidable impurities or any combination thereof.
The dimensions and values disclosed herein are not to be understood as being strictly limited to the exact numerical values recited. Instead, unless otherwise specified, each such dimension is intended to mean both the recited value and a functionally equivalent range surrounding that value. For example, a dimension disclosed as “40 mm” is intended to mean “about 40 mm.”
Every document cited herein, including any cross referenced or related patent or application and any patent application or patent to which this application claims priority or benefit thereof, is hereby incorporated herein by reference in its entirety unless expressly excluded or otherwise limited. The citation of any document is not an admission that it is prior art with respect to any invention disclosed or claimed herein or that it alone, or in any combination with any other reference or references, teaches, suggests or discloses any such invention. Further, to the extent that any meaning or definition of a term in this document conflicts with any meaning or definition of the same term in a document incorporated by reference, the meaning or definition assigned to that term in this document shall govern.
While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.
