Patent Application
20180127858
The invention relates to a martensitic stainless steel. This steel is primarily intended to manufacture cutting tools, in particular pieces of cutlery, such as scalpels, scissor blades, or knife blades or household food processors.
Steels intended for cutlery must have a high corrosion resistance, ability to be polished and hardness.
The martensitic stainless steels currently used to produce the blades of cutting tools, such as steels of type EN 1.4021, EN 1.4028 and EN 1.4034, have Cr content levels of less than or equal to 14 or 14.5 wt % and variable C content levels, i.e., 0.16%-0.25% for EN 1.4021, 0.26-0.35% for EN 1.4028 and 2.43-0.50% for EN 1.4034. The hardness level of the steel depends primarily on this C content level.
When an even better corrosion resistance is sought, grade EN 1.4419 with 0.36-0.42% C, 13.0-14.5% Cr and 0.60-1.00% Mo can be used.
During manufacturing, these steels are typically melted in an AOD or VOD converter, then poured continuously in the form of slabs, blooms or billets, then hot-rolled so as to obtain a coil, a rolled bar or a wire rod. They next undergo annealing to obtain a ferritic structure containing carbides, which is soft enough for it to be possible to perform cold rolling for the flat products, or to facilitate sawing before forging the hot-rolled semi-finished product for long products.
The product next undergoes a recrystallization annealing. In this softened state of re-crystallized ferrite containing carbides, the product is cut to give it its final shape, for example that of a knife blade, before undergoing a thermal treatment comprising high-temperature austenitizing, typically between 950° C. and 1150° C., followed by quenching to ambient temperature, which leads to a primarily martensitic structure.
In this martensitic state, the product has a high hardness, which is higher when the carbon content is high, but is also very fragile. An annealing treatment, typically between 100° C. and 300° C., is then done to reduce the fragility without lowering the hardness too much. The blade next undergoes various operations, including sharpening and polishing to give it its cutting quality and aesthetic appearance.
None of the four cited grades simultaneously allows good corrosion resistance, good surface condition and high hardness, for a reasonable cost.
Grade EN 1.4419 has good corrosion resistance and a high hardness, but it is cost prohibitive due to the addition of a large quantity of Mo.
Grade EN 1.4034 has a high hardness, but also has a mediocre surface appearance after polishing, due to the presence of a large number of carbides not dissolved during austenitizing, due to the high C content level of this grade. The corrosion resistance is insufficient, since the Cr content level is not high enough in the matrix, particularly given that part of the Cr is trapped in the non-dissolved carbides. Furthermore, the cutting edge of the blade is often subjected to crevice corrosion, coming from the cleavage of large primary carbides that appear at the end of solidification during continuous casting.
Grades EN 1.4021 and 1.4028, which contain less C, have lower hardnesses, but without having sufficient corrosion resistance due to the excessively low Cr content levels.
The present invention aims to resolve the aforementioned problems. It in particular seeks to propose a martensitic stainless steel for a cutting tool that is as cost-effective as possible, which nevertheless has good corrosion resistance, good polishing ability and a high hardness.
To that end, the invention relates to a martensitic stainless steel, characterized in that its composition consists of, in weight percentages:
the rest being iron and impurities resulting from the melting.
Its microstructure preferably includes at least 75% martensite. The invention also relates to a method for producing a semi-finished product made from martensitic stainless steel, characterized in that:
Said semi-finished product can be a sheet, and said shaping operation can be a cold rolling.
Said semi-finished product can be a bar or a wire rod, and said shaping operation can be a forging.
Said shaped semi-finished product, if its Cr content level is comprised between 15 and 17%, can next be austenitized between 950 and 1150° C., then cooled at a speed of at least 15° C./s to a temperature of less than or equal to 20° C., then undergoes annealing at a temperature comprised between 100 and 300° C.
Said shaped semi-finished product can next be austenitized between 950 and 1150° C., then cooled at a speed of at least 15° C./s to a temperature of less than or equal to 20° C., then undergoes a cryogenic treatment at a temperature from −220 to −50° C., then an annealing at a temperature comprised between 100 and 300° C.
The invention also relates to a cutting tool, characterized in that it has been made from a semi-finished product prepared according to the preceding method.
The cutting tool can be a cutlery item such as a knife blade, a food processor blade, a scalpel, or a scissor blade.
As one will have understood, the invention consists of using, to produce the cutting tool, a martensitic stainless steel with a specific composition, free of costly elements with high content levels, but containing relatively large quantities of nitrogen situated in a well-defined range. Particular balancing of the Cr, C and N content levels is also necessary.
Other features and advantages of the invention will appear upon reading the description below, provided as an example and done in reference to the appended
Regarding the chemical composition of the steel according to the invention, the following support is submitted. It must be clear that the content level ranges of the various elements considered to be preferential are independent of one another, and that any combination of the ranges defined in the description that follows can be considered in the context of the invention, as long as the individual content levels of C, N and Cr that they allow at the same time can respect the relationships that must exist between them according to the invention.
C increases the hardness in the martensitic state after austenitizing, quenching and annealing. However, it also favors the precipitation of M7C3 primary carbides during solidification, which can be stripped during polishing or sharpening of the blade, which deteriorates the surface appearance of the product. The sites where they were found before polishing may also become the seat of crevice corrosion. An excessive C content level also leads, depending on the austenitizing temperature, either to an overly high C content level in the austenitic matrix that no longer makes it possible to obtain a sufficient martensite fraction after annealing, or to the persistence of undissolved M23C6 carbides that deplete the Cr in the austenitic matrix. They thus reduce the corrosion resistance and are detrimental to polishability.
The C content level must therefore be at least 0.10% to obtain a sufficient hardness, and no more than 0.45% to obtain good corrosion resistance and a satisfactory surface appearance after polishing. Depending on the casting and solidification method used, it may, however, prove useful to limit the maximum C content level a bit more, for the case where this method risks not guaranteeing sufficient homogeneity of the steel during solidification to avoid M7C3 primary carbides precipitation. In this case, it is advisable to limit the C content level to 0.38%, preferably 0.20%≤C≤0.38%; better 0.20%≤C≤0.35%; optimally 0.30%≤C≤0.35%.
The optimal range in particular makes it possible to avoid a high hardness while limiting carbide formation to within acceptable proportions, the possible loss of hardness due to the decrease in the maximum C content level relative to the more general range being able to be compensated by a sufficient nitrogen presence to that end, as will be seen later.
Furthermore, the C content level must satisfy formulas linking it with the N content level and with the N and Cr content levels, as will be explained later.
Mn is a so-called gammagenous element, since it stabilizes the austenitic structure. An excessive Mn content level leads to an insufficient martensite level after austenitizing and quenching treatment, which leads to decreased hardness. For this reason, the Mn content level must be comprised between traces resulting from melting and 1.0%. Preferably, its content level is limited to 0.6% to help obtain an optimally low Ms temperature.
Si is a useful element during the steelmaking process. It is highly reducing, and therefore makes it possible to reduce the Cr oxides in the reduction phase of the steel that follows the decarburization phase in the AOD or VOD converter. However, the Si content level in the final steel must be comprised between traces and 1.0%, since this element has a hot hardening effect that limits the possibilities for hot deformation during hot rolling or during forging. Preferably, its content level is limited to 0.6% to help obtain an optimally low Ms temperature.
S and P are impurities that decrease the hot ductility. P segregates easily at the grain boundaries and facilitates cleavage thereof. Furthermore, S reduces the resistance to corrosion caused by pitting, by forming compounds with the Mn that serve as initiating sites for this type of corrosion. To that end, the S and P content levels must respectively be comprised between traces and, respectively, 0.01 wt % and 0.04 wt %. Preferably, the S content level does not exceed 0.005% to still better ensure sufficient corrosion resistance.
Cr is an essential element for corrosion resistance. However, its content level must be limited, since a high content level risks lowering the temperature Mf (the temperature at the end of martensitic transformation) below the ambient temperature. This would lead, after austenitizing and quenching to ambient temperature, to an excessively incomplete martensitic transformation and an insufficient hardness. For these various reasons, the Cr content level must be comprised between 15.0 wt % and 18.0 wt %. It is, however, advisable to limit the Cr content level to 15.0-17.0%, better 15.2-17.0%, still better 15.5-16.0%, above all when a cryogenic treatment of the steel is not done, so as not to have an excessively high temperature Ms at the beginning of martensitic transformation, and therefore not to leave too much residual austenite, which would limit the hardness, therefore the tensile strength Rm, which is not desirable in a martensitic steel. If necessary, the decreased corrosion resistance caused by the decrease in the maximum Cr content level may be compensated by a high N content level, within the limits stipulated elsewhere.
However, the solubility of N in the liquid metal decreases when the Cr content level decreases, such that it is no longer possible below 15% Cr to retain, in the liquid metal, enough dissolved N at the solidification temperature of the steel, which leads to the formation of N2 bubbles during solidification, and no longer allows N to compensate the decrease in Cr with respect to the corrosion resistance. This lower Cr limit for the solubility of N also increases when the ferrostatic pressure at solidification decreases. It may be preferable to increase the minimum Cr content level from 15.0% to 15.2% or 15.5% depending on the type of casting method and the casting conditions used in order to protect against any risk of N2 bubble formation.
The Cr content level must also satisfy a formula linking it to the N and C content levels, as will be explained below.
The elements Ni, Cu, Mo and V are expensive, and also decrease the temperature Mf. The content level of each of these elements must therefore be limited, between traces and 0.50 wt %, preferably no more than 0.10% for Mo. It is therefore not necessary to add any after melting the raw materials. It is still more favorable for the Mo content level not to exceed 0.05%, to help obtain an optimally low temperature Ms. For the same reason, it is preferable for the Cu content level not to exceed 0.3%, and for the V content level not to exceed 0.2%.
Nb, Ti and Zr are so-called “stabilizing” elements, which means that they form, in the presence of N and C and at high temperatures, carbides and nitrides more stable than the carbides and nitrides of Cr. These elements are, however, undesirable, since their respective carbides and nitrides, once formed during the production process, can no longer easily be dissolved during austenitizing, which limits the content levels of C and N in the austenite, and therefore the corresponding hardness of the martensite after quenching. The content level of each of these elements must therefore be comprised between traces and 0.03%.
The Al content level must likewise be comprised between traces and 0.010% to avoid the formation of Al nitrides, the dissolution temperature of which would be too high and would decrease the N content level of the austenite, therefore the hardness of the martensite after quenching.
The O content level results from the production method of the steel and its composition. It must be comprised between traces and 0.0080% (80 ppm) maximum, so as to avoid forming too many and/or overly large oxide inclusions, which could constitute favored initiation sites for corrosion by pitting, and also be stripped during polishing, such that the surface appearance of the product would not be satisfactory. The O content level also influences the mechanical properties of the steel, and it may optionally be possible, traditionally, to set a limit lower than 80 ppm that may not be exceeded, depending on the requirements of the users of the end product.
The Pb, Bi and Sn content levels may be limited to traces resulting from melting, and each must not exceed 0.02% so as not to make hot transformations too difficult.
Controlling the N content level with respect to a well-defined level is an essential aspect of the invention. Like C, it makes it possible, when it is in a solid solution, to increase the hardness of the martensite without having the drawback of forming precipitates during solidification. If one does not wish to have an overly high C content level so as to avoid forming too many precipitates, adding N makes it possible to compensate the loss of hardness. Nitrides form at temperatures lower than carbides, which makes them easier to put in solution during austenitizing. The presence of N in solid solution also improves the corrosion resistance.
However, an excessive N content level no longer allows complete dissolution thereof during solidification, and leads to the formation of N2 bubbles, which form blowholes (pores) during solidification of the steel, detrimental to the internal health of the metal.
For these various reasons, the N content level must be comprised between 0.10 wt % and 0.20 wt %, preferably between 0.15 and 0.20 wt %.
The N content level must also satisfy a formula linking it to the Cr and C content levels.
Indeed, the hardness of the martensite depends on its C and N content levels. The inventors have shown that the hardening effects of these two elements are similar, and therefore that the hardness of the martensite depends on its overall C+N content level. It has been established by the inventors that the hardness after quenching and annealing will be sufficient if the following formula is respected:
C+N≥0.25%, preferably C+N≥0.30%
In one still more preferred embodiment of the invention, an even higher hardness is obtained after quenching and annealing if the following formula is respected:
C+N≥0.45%.
Three elements affect the corrosion resistance. Cr and N are beneficial, whereas C has a negative effect, since it is generally not possible to dissolve all of the Cr carbides during austenitizing, for productivity and cost reasons that, in industrial practice, limit the treatment duration and temperature. The undissolved Cr carbides reduce the Cr content level of the austenitic matrix, and thus reduce the corrosion content level.
From the study of the corrosion resistance of martensitic steels with different weight contents of Cr, N and C, the inventors have found a formula associating these various elements that makes it possible to ensure very good corrosion resistance.
Cr+16N−5C≥16.0%
One preferred, but non-mandatory, condition is that:
17Cr+500C+500N≤570%
This condition makes it possible to ensure that the temperature Ms will not be too high, since compliance therewith would represent a decrease in Ms of about 60° C. relative to what would be allowed by simultaneous satisfaction of the selected upper C, N and Cr content level limits.
Steels according to the invention have been subject to austenitizing tests at different temperatures before quenching in water at 20° C. with a cooling speed greater than 100° C./s, followed by annealing at 200° C., in order to vary the proportion of dissolved carbides, and consequently the carbon content level in the austenite, then in the martensite after quenching. The martensite level, as well as the Vickers hardness, were measured in order to trace the evolution of the hardness as a function of the martensite level, and the results are shown in
The obtainment of a high martensite level able to reach 100% can be better ensured if, after quenching to 20° C. or less, a cryogenic treatment is done, i.e., quenching is done in a medium at a very low temperature from −220 to −50° C., typically in liquid nitrogen at −196° C. or in carbon dioxide snow at −80° C., before performing annealing at 100-300° C.
When the martensite content level does not reach 100%, the remaining microstructure is typically made up essentially of residual austenite. There may also be ferrite.
As non-limiting examples, the following results will show the advantageous characteristics imparted by the invention.
The compositions of the different tested steel samples appear in table 1, expressed in weight percentages. The underlined values are those which do not comply with the invention. We have also reported the values of C+N, Cr+16N−5C and 17Cr+500C+500N for each sample.
13.4
13.8
0.478
13.7
13.9
14.3
0.465
0.520
18.5
579.6
582.6
607.8
581.5
0.002
0.225
12.32
0.003
12.29
0.003
11.36
13.68
15.96
0.032
14.51
0.253
602.7
637.8
636.0
0.226
14.86
15.65
0.235
0.217
After casting, these steels were heated to a temperature above 1100° C., hot rolled to a thickness of 3 mm, annealed at a temperature of 800° C., then pickled and cold rolled to a thickness of 1.5 mm.
The steel sheets were next annealed at a temperature of 800° C.
The annealed steel sheets next underwent an austenitizing treatment of 15 minutes at 1050° C., followed by quenching in water to a temperature of 20° C.
After cutting the sheets into two parts, one of the parts was next submerged, for 10 minutes, in a thermostated bath at −80° C., so as to be able to evaluate the effects of a cryogenic treatment, added to a mere quenching in water.
Annealing for 1 h at 200° C. was next done on each sheet part.
Table 2 shows the result of tests and observations done on these steels. The underlined values correspond to performance levels deemed insufficient.
The internal health is evaluated on a raw solidification state after pouring, knowing that the subsequent transformation operations will not damage it.
The martensite level is measured after quenching in water at 20° C. and after a cryogenic treatment by quenching at −80° C., this quenching, or the second of these quenching operations, having been followed by annealing at 200° C. When the martensite level is greater than or equal to 75% after quenching in water at 20° C., the other results given in table 2 relate to the state quenched at 20° C. followed by annealing at 200° C. When the martensite level is lower than or equal at 75% after quenching in water at 20° C., the other results given in table 2 relate to the state after a cryogenic treatment (quenching to a very low temperature, for example done in carbon dioxide snow) at −80° C., followed by annealing at 200° C.
The corrosion resistance is evaluated by an electrochemical corrosion test by pitting in an environment made up of NaCl 0.02M, at 23° C. and at a pH of 6.6. The electrochemical test done on 24 samples makes it possible to determine the potential E0.1 for which the elementary pitting probability is equal to 0.1 cm−2. The corrosion resistance is considered unsatisfactory if the potential E0.1 is less than 350 mV, measured relative to the calomel electrode saturated with KCl (350 mV/ECS). It is considered satisfactory if the potential E0.1 is comprised between 350 mV/ECS and 450 mV/ECS. It is considered very satisfactory if the potential E0.1 is greater than 450 mV/ECS.
The Vickers hardness is measured in the thickness on a mirror polished cut, under a load of 1 kg with a diamond pyramidal tip with a square base, according to standard EN ISO 6507. The mean of the obtained hardnesses is calculated by performing 10 imprints. The hardness is considered insufficient if the mean hardness is less than 500 HV. It is considered satisfactory if the mean hardness is comprised between 500 HV and 550 HV. It is considered very satisfactory if the mean hardness is comprised between 551 and 600 HV. It is considered excellent if the mean hardness is greater than 600 HV.
The polishability is evaluated by performing flat polishing at mid-thickness of the sample, successively using SiC 180, 320, 500, 800 and 1200 papers with a force of 30N, then polishing on sheet imbibed with diamond paste with particle size 3 μm, then 1 μm under a force of 20N. The surface is next observed by optical microscopy with a magnification of ×100. The polishability is considered insufficient if the flaw density, traditionally called “comet-tail”, is greater than or equal to 100/cm2. The polishability is considered satisfactory if this density is comprised between 10/cm2 and 99/cm2. The polishability is considered very satisfactory if this density is comprised between 1 and 9/cm2. The polishability is considered excellent if this density is less than 1/cm2.
The internal health is evaluated by observing a cut of the raw solidification steel by optical metallography with magnification ×25. The internal health is not satisfactory and is indicated by value “0” in table 2 if globular cavities (blowholes) reflecting the formation of nitrogen bubbles upon solidification are observed. Otherwise, the internal health is considered satisfactory and indicated by value “1” in table 2.
The martensite level is determined by X-ray diffraction by measuring the intensity of the characteristic rays of the martensite compared to the intensity of the characteristic rays of the austenite, knowing that, in all of the examined samples, these are the only two phases present. In general, it would not be ruled out that other phases may be observed marginally in samples according to the invention. It is the martensite level first and foremost that should be considered in the context of the invention.
A martensite level greater than or equal to 75% after quenching at 20° C. and annealing at 200° C., or greater than or equal to 75% after annealing at 20° C., cryogenic treatment at −80° C. and annealing at 200° C., is satisfactory. If a martensite level of 75% or more cannot be obtained by one of these treatments, the sample is considered unsatisfactory.
240
488
240
190
124
330
490
0
400
109
0
215
479
445
390
489
0
The steels according to the invention I1 to I6, as well as steels I8 to I0, combine good corrosion resistance, hardness and polishability properties, and have a good internal health, as well as a martensite level greater than or equal to 75% after quenching at 20° C.
Steel I7 according to the invention combines good corrosion resistance, hardness and polishability properties, and has a good internal health, as well as a martensite level greater than or equal to 75%, but on the condition that a cryogenic treatment is done at −80° C. Indeed, after a mere quenching in water at 20° C., the martensite level is still not sufficient, which is related to the presence of Cr at a level higher than that of the other samples according to the invention.
At a comparable level of N, one can see that the hardness increases between samples I1, I2, where C is between 0.10 and 0.20%, on the one hand, and samples I3, where C is between 0.20 and 0.30%, and above all I8, I9, I10, where C is between 0.30 and 0.35%, on the other hand.
I14, where C is still high and N is at the same level as in the previous cases, has a lower hardness than them, since the martensite fraction after quenching begins to decrease due to the decrease in the temperature Mf related to a high value of the sum 17Cr+500C+500N (see table 1). Also at comparable levels of N and other essential elements, one can see that the increase in Cr makes it possible to improve the corrosion resistance (see samples 18 and 19). Conversely, the increase in the Cr content level tends to decrease the hardness; see samples I8, I10 and I11, the compositions of which only differ significantly regarding Cr. Going beyond 18% Cr could increase the corrosion resistance, but would lead to decreased C and N content levels to retain a satisfactory Ms, and a correct hardness would no longer be insured.
Reference steels R1 to R3 have Cr and N content levels, as well as C+N and/or Cr+16N−5C sums, that are unsatisfactory, which does not allow sufficient corrosion resistance.
Reference steels R4 and R5 have insufficient Cr content levels. Without compensation by an addition of N, steel R4 also has an insufficient Cr+16N−5C combination leading to an unsatisfactory corrosion resistance. For steel R5, the compensation for the lack of Cr by adding N reestablishes a satisfactory corrosion resistance, but no longer makes it possible to ensure good internal health, since the Cr content level is no longer sufficient to allow complete dissolution of N in the liquid metal.
Reference steel R6 has too high a C content level and an insufficient N content level. The excessively high C content level does not have a sufficient polishability due to excessive carbide formation.
Reference steel R7 has too high a N content level, which damages the internal health. The same is true for reference steel R14. Reference steel R8 has an excessive C content level, which leads to poor polishability and an overly low martensite level, even after cryogenic quenching at −80° C. Reference steel R9 contains too much Cr, which leads to an insufficient martensite level, even after cryogenic quenching at −80° C.
Reference steels R10 and R11 have excessively low C content levels as well as insufficient C+N sums, leading to overly low hardnesses. Reference steels R12 and R13 would have compositions according to the invention on the individual content levels of each element, but their Cr+16N −5C content level, which is below 16.0%, is insufficient to guarantee a corrosion resistance as high as that of steels that comply with the invention on all points, including those which only slightly exceed the value of 16.0% for this sum Cr+16N−5C.
The steels according to the invention are used for good reason to produce cutting tools, for example scalpels, scissors, knife blades or circular blades for food processors.
| Number | Date | Country | Kind |
|---|---|---|---|
| PCT/IB2015/053144 | Apr 2015 | IB | international |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/059684 | 4/29/2016 | WO | 00 |
