Steel for cutting tools

Abstract

To provide a steel for edged tools that has increased hardness and enhanced edge sharpness, while providing excellent corrosion resistance, toughness, and wear resistance.SOLUTION: A steel for edged tools contains, in mass%, C: 1.30-1.70%, Cr: 14.00-18.00%, Mo: 2.00-2.80%, at least one of V, W, Nb: a total of 2.10-3.60%, with the balance being Fe and inevitable impurities.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a martensitic cutlery steel. [Background technology]

[0002] As is well known, sharp cutting ability and durability are required for bladed tools such as knives, kitchen knives, hairdressing scissors, and medical scalpels. Conventionally, martensitic stainless steels such as SUS420J2 and SUS440C have been used as materials for these blades, but this blade steel must have various properties, such as hardness to form a sharp cutting edge and suppress deformation of the cutting edge, appropriate toughness to suppress chipping of the sharpened cutting edge, wear resistance to maintain the sharp cutting ability, and corrosion resistance to suppress rusting of the cutting edge.

[0003] Generally, the higher the C (carbon) content of stainless steel, the higher the hardness after quenching and tempering tends to be, but if the C content is increased to further increase the hardness, Cr (chromium), which forms a passive film and improves corrosion resistance, bonds with C to form Cr carbides, resulting in a side effect of reduced corrosion resistance. Therefore, in the past, other carbide-forming elements such as Mo (molybdenum), V (vanadium), and W (tungsten) were added and the amounts added were adjusted to achieve both hardness and corrosion resistance, while balancing other properties such as toughness and wear resistance (for example, Patent Documents 1 and 2 below). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2002-212679 A [Patent Document 2] Japanese Patent Application Publication No. 10-1703 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in recent years, especially in the case of high-end cutlery, there has been a demand for even better sharpness, and cutlery with sharper cutting edge angles (for example, a small cutting edge angle of about 16 degrees) has been manufactured. In order to form and maintain such a sharp cutting edge, the steel needs to have not only higher hardness and toughness, but also sufficient corrosion resistance and wear resistance, but conventional cutlery steels have the problem of not being able to fully meet such demands.

[0006] For example, as shown in Figure 1, when sharpening a blade 1 made using conventional blade steel, if the cutting edge angle is approximately 24 degrees (Figure 1(a)), it is possible to properly form the cutting edge 11. However, if the cutting edge angle is approximately 16 degrees (Figure 1(b)), when the cutting edge 11 is sharpened against a whetstone, the cutting edge 11 plastically deforms toward the opposite side of the whetstone, making it difficult to properly form a sharp cutting edge 11.

[0007] In order to form and maintain such a sharp cutting edge, mechanical properties are required that ensure that the cutting edge will not chip or bend even when sharpened. To achieve this, greater base hardness and fine dispersion of metal carbides that contribute to wear resistance but do not impede sharpening of the blade are required. However, in conventional blade steels, the balance between strengthening the base material and strengthening the dispersion of metal carbides was not taken into consideration, and the addition of excessive amounts of carbon and metal elements sacrificed base hardness and corrosion resistance.

[0008] The present invention was made in consideration of the above-mentioned problems with conventional blade steels, and has an object to provide a blade steel that has higher hardness and excellent sharpening properties, while also having excellent corrosion resistance, toughness, and wear resistance. [Means for solving the problem]

[0009] Cr is an important element that dissolves in the steel base material to improve corrosion resistance. At the same time, it also forms complex carbides (M3C, M7C3, M 23Cr is also a carbide-forming element that contributes to improving hardness and wear resistance by forming complex carbides (C6, etc.), but the ratio of the solid solution concentration of Cr in complex carbides to the solid solution concentration in the steel matrix (distribution coefficient) tends to be larger than other carbide-forming elements such as Mo, V, and W. Conventionally, the formation of coarse complex carbides was suppressed by reducing the amount of C added, and the solid solution concentration of Cr in the matrix was ensured to maintain corrosion resistance. However, when the amount of C added is small, the matrix is ​​not solid solution strengthened by quenching heat treatment, and high hardness cannot be obtained.

[0010] Therefore, the inventors focused on the carbide formation tendency of added elements such as Cr, Mo, V, W, and Nb (niobium), and came up with the idea of ​​suppressing the formation of coarse complex carbides by adjusting the added amounts of V, W, and Nb, which have a relatively high carbide formation tendency.Furthermore, they discovered that in order to improve the cutting performance of steel for blades, it is necessary to improve not only the hardness of the steel as a whole, which contains various carbides, but also the hardness of the steel base material itself in particular.As a result of extensive research, the inventors have completed the present invention.

[0011] That is, the steel for cutlery according to the present invention contains, by mass%, C: 1.30 to 1.70%, Si:0.19~0.34%, Mn:0.10~0.31%, It is characterized by a composition consisting of Cr: 14.00-18.00%, Mo: 2.00-2.80%, one or more of V, W and Nb: 2.10-3.60% in total, and the remainder being Fe and unavoidable impurities.

[0012] Further, the cutlery steel according to the present invention is characterized in that it satisfies the relationship: -1.2%≦Mo%-(V+W+Nb)%≦-0.2%.

[0013] Moreover, the steel for cutlery according to the present invention is characterized in that the V content is 1.50 to 2.10%, the W content is 0.50 to 1.40%, and the Nb content is 0.20 to 0.60%. Effect of the Invention

[0014] The blade steel of the present invention contains specified amounts of V, W, and Nb, which have a relatively high tendency to form carbides. This effectively suppresses the formation of complex carbides that reduce corrosion resistance and are harmful when sharpening a blade, while at the same time ensuring the hardness of the steel base itself, resulting in a blade with higher hardness and excellent sharpening properties, while also exhibiting excellent corrosion resistance, toughness, and wear resistance. [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 is an enlarged schematic cross-sectional view of the cutting edge of a blade made using conventional blade steel, with (a) a cutting edge angle of 24 degrees and (b) a cutting edge angle of 16 degrees. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Hereinafter, an embodiment of the cutlery steel according to the present invention will be described. First, the reasons for limiting the components constituting the cutlery steel according to the present invention will be described.

[0017] C (carbon): 1.30~1.70% Carbon (C) is an element that dissolves in the steel base material to increase hardness, and also combines with Cr, Mo, V, W, and Nb to form high-hardness carbides to further increase hardness. If C falls below 1.30%, it is not possible to ensure the hardness of the steel base material required to sharpen a sharper cutting edge (e.g., a small blade angle of about 16 degrees) and suppress deformation of the cutting edge. On the other hand, if C exceeds 1.70%, the amount of carbides becomes excessively large, which reduces toughness and reduces corrosion resistance due to an insufficient amount of Cr, Mo, etc. dissolved in the base material.

[0018] Cr (chromium): 14.00-18.00% Cr (chromium) is an element that dissolves in the steel base material to form a passive film that improves corrosion resistance, and also forms carbides to improve hardness and wear resistance. If the Cr content is less than 14.00%, the corrosion resistance required for knives and other bladed tools cannot be ensured, and if it exceeds 18.00%, the amount of carbides becomes excessively large, reducing toughness.

[0019] Mo (molybdenum): 2.00-2.80% Mo (molybdenum) is an element that forms a stable passive film together with Cr to improve corrosion resistance, and also forms harder carbides to improve hardness and wear resistance. If Mo is less than 2.00%, the necessary corrosion resistance, hardness, and wear resistance cannot be ensured, and secondary hardening by tempering cannot be obtained sufficiently. On the other hand, if it exceeds 2.80%, the amount of carbides becomes excessively large, and toughness decreases.

[0020] V (vanadium): 1.50-2.10% Vanadium (V) is an element that forms hard carbides to improve hardness and wear resistance, and refines crystal grains to improve toughness. If V is less than 1.50%, the necessary hardness and wear resistance cannot be ensured, and if it exceeds 2.10%, the amount of carbides becomes excessively large, reducing toughness and worsening workability during plastic processing.

[0021] W (tungsten): 0.50 to 1.40% W (tungsten) is an element that forms hard carbides to improve hardness and wear resistance, and refines crystal grains to improve toughness. If W is less than 0.50%, the necessary hardness and wear resistance cannot be ensured, and if it exceeds 1.40%, the amount of carbides becomes excessively large, reducing toughness and worsening workability during plastic processing.

[0022] Nb (niobium): 0.20 to 0.60% Nb (niobium) is an element that forms hard carbides to improve hardness and wear resistance, and refines crystal grains to improve toughness. If the Nb content is less than 0.20%, the necessary hardness and wear resistance cannot be secured, and if it exceeds 0.60%, an excessive amount of Nb carbide is generated, reducing the amount of C required for matrix solid solution strengthening, and reducing the matrix solid solution strengthening ability.

[0023] One or more of V, W, and Nb: 2.10 to 3.60% in total V, W, and Nb are elements with a higher tendency to form carbides than Cr, and the formation of carbides by these elements can suppress the formation of Cr carbides, preventing a shortage of Cr in the base material and ensuring the necessary corrosion resistance. In particular, Nb has a higher tendency to form carbides than other elements, so it can more effectively suppress the formation of Cr carbides and improve corrosion resistance. If the total of one or more of V, W, and Nb is below 2.10%, the formation of Cr carbides cannot be effectively suppressed, and corrosion resistance decreases. On the other hand, if it exceeds 3.60%, the amount of carbides becomes excessively large, the amount of dissolved C during quenching decreases, the hardness of the steel base material decreases, and the sharpness of the cutting edge decreases.

[0024] -1.2%≦Mo%-(V+W+Nb)%≦-0.2% Mo, like V, W, and Nb, is an element that forms hard carbides to improve hardness and wear resistance, and also forms a stable passive film together with Cr to improve corrosion resistance. If the difference between the amount of Mo added and the total amount of V, W, and Nb added is within the range of -1.2% to -0.2%, better corrosion resistance and cutting performance are exhibited.

[0025] The cutlery steel according to the present invention does not intentionally contain Co (cobalt), which dissolves in the steel matrix and promotes matrix strengthening, except as an unavoidable impurity. This is because the cutlery steel according to the present invention can ensure the necessary matrix strengthening ability by suppressing the formation of Cr carbides through the addition of specified amounts of V, W, and Nb, as described above. Therefore, the cutlery steel according to the present invention does not substantially contain Co, which is designated as a conflict mineral, and thus realizes a sustainable composition design.

[0026] Furthermore, the cutlery steel according to the present invention can be manufactured by a powder metallurgy method in addition to the conventional melting method. The cutlery steel manufactured by the powder metallurgy method can have fine crystal grains and a large amount of carbides uniformly and finely distributed, so that it can achieve both high levels of hardness and toughness. EXAMPLES

[0027] Examples 1 to 8 and Comparative Examples 1 to 10 having the compositions shown in Table 1 were produced, and tests were carried out on each steel type regarding hardness, bending strength (toughness), wear resistance, cutting performance, and corrosion resistance. The results are shown in Table 2.

[0028] For each steel type, test pieces were taken from the steels processed to a thickness of 2 mm and quenched and tempered. The quenching and tempering temperatures were set to the temperature at which the maximum hardness of each steel type was obtained. Comparative Examples 9 and 10 (SUS440C) were manufactured by the melting method, while the others were manufactured by the powder metallurgy method.

[0029] [Table 1]

[0030] [Table 2]

[0031] "hardness" The hardness was measured on the polished surface of the test piece using a Vickers hardness tester (load 500 grams).

[0032] "Bending strength" A test piece adjusted to 2 mm x 50 mm was held at one end by a jig, and the strength at the time of break was measured.

[0033] "Wear resistance" Three test pieces (total area 1050 mm2) adjusted to 5 mm x 70 mm were polished for 15 minutes with a polishing machine at a rotation speed of 200 rpm and a load of approximately 200 N, using emery paper (#80) as the mating material, and the total weight loss (amount of wear) of the test pieces was then measured. The less the amount of wear, the better the wear resistance.

[0034] "Cutting performance" Two types of blades (cutting edge angle 16±1 degree and cutting edge angle 24±1 degree) made under the same conditions were fixed to a sharpness testing machine, and a number of sheets of 7.5 mm wide paper equivalent to newspaper were stacked and a load of approximately 750 grams was applied while the blade was moved back and forth over a distance of 20 mm, and the number of sheets of paper cut was measured.

[0035] "Corrosion resistance" A test piece adjusted to 18 mm x 18 mm was embedded in epoxy resin, and the end surface was polished to a roughness of 1000. A salt spray test was performed under the condition of continuous spraying of 5% NaCl aqueous solution at 35°C and 75 kPa for 24 hours. After the test, the depth of each pitting corrosion that occurred in an area of ​​14 mm x 14 mm excluding the ridge of the polished surface of the test piece was measured. The smaller the maximum pitting corrosion depth of each test piece, the less grinding amount can be reduced when sharpening the blade, and the easier the sharpening work can be performed. In addition, the conventionally used pitting corrosion index (PRE value = Cr% + 3.3Mo% + 16N%) is shown in Table 2 for each steel type of Examples 1 to 8 and Comparative Examples 1 to 10. This PRE value is an index that predicts corrosion resistance from the components of the steel material, but the test results of the present Examples and Comparative Examples have overturned the prediction based on this PRE value.

[0036] As shown in Tables 1 and 2, in Examples 1 to 8 according to the present invention, the total amount of V, W, and Nb, which are elements with a higher tendency to form carbides than Cr, is in the range of 2.10 to 3.60%, so that the formation of complex carbides can be effectively suppressed, and the excellent corrosion resistance required for blades is exhibited, while the hardness of the steel base material itself can be secured, and even sharper blades can be properly sharpened, resulting in excellent cutting performance. In addition, since various carbides are not excessively formed, appropriate toughness and wear resistance are exhibited.

[0037] In particular, in Examples 2 to 5 and 8, the difference between the amount of Mo added and the total amount of V, W, and Nb added is in the range of -1.2% to -0.2%, and therefore the steel exhibits higher hardness and a better balance of corrosion resistance, toughness, and wear resistance required of blade steel.

[0038] In contrast, in Comparative Example 1, the total amount of V, W, and Nb added is within the range of the present invention, but the amount of Cr added is below the range of the present invention. Therefore, although the material exhibits high hardness, the maximum pitting depth and the total pitting depth are larger than those of Examples 1 to 8, and the corrosion resistance is inferior.

[0039] In addition, in Comparative Example 2, the total amount of V, W, and Nb added is within the range of the present invention, but the amount of C added is below the range of the present invention, so that although Comparative Example 2 shows high corrosion resistance, it is inferior in hardness and cutting performance to Examples 1 to 8. In particular, in Comparative Example 2, although the Vickers hardness of the entire steel containing various carbides is similar to that of Comparative Example 10 (SUS440C), the hardness of the steel base itself is insufficient due to a decrease in the amount of dissolved C in the steel base, so although it is possible to form a cutting edge with a cutter angle of 24±1 degrees, it is difficult to properly form a sharper cutting edge with a cutter angle of 16±1 degrees, and the cutting performance when the cutter angle is 16±1 degrees is significantly inferior, with the number of cut sheets being 0.

[0040] Also, in Comparative Example 6, the total amount of V, W, and Nb added is within the range of the present invention, but the amount of Mo added is below the range of the present invention, so that the hardness and cutting performance are inferior to Examples 1-8.

[0041] In Comparative Examples 3 and 4, although the amounts of C and Cr added are significantly higher than the range of the present invention, the total amount of V, W, and Nb added is lower than the range of the present invention. As a result, the Cr carbide suppression effect is not sufficient, and the corrosion resistance is significantly inferior to Examples 1 to 8, resulting in general corrosion.

[0042] In addition, in Comparative Examples 7, 9, and 10, the total amount of V, W, and Nb added is below the range of the present invention, so the Cr carbide suppression effect is not sufficient, and the corrosion resistance is inferior to Examples 1 to 8. In particular, although Comparative Example 7 exhibits a high pitting corrosion index (PRE value) of 26.2, the maximum pitting corrosion depth and the total pitting corrosion depth are larger than those of Examples 1 to 8, and the corrosion resistance as a blade is inferior.

[0043] Comparative Example 5, in which the total amount of V, W, and Nb added exceeds the range of the present invention, exhibits high corrosion resistance, but has poorer hardness than Examples 1 to 8. In Comparative Example 5, the amount of carbides is excessively large, so that although the Vickers hardness of the entire steel containing various carbides is similar to that of Comparative Example 10 (SUS440C), the hardness of the steel base itself is insufficient due to a decrease in the amount of dissolved C in the steel base, and therefore the cutting performance when the cutting edge angle is 16±1 degree is inferior to that when the cutting edge angle is 24±1 degree.

[0044] In addition, in Comparative Example 8, the total amount of V, W, and Nb added exceeds the range of the present invention, and therefore the hardness is inferior to Examples 1 to 8, even though the amounts of C and Cr added exceed the range of the present invention.

[0045] As is clear from these test results, according to the present invention, by taking advantage of the relatively high carbide formation tendency of V, W, and Nb, it is possible to ensure the necessary corrosion resistance by suppressing the formation of complex carbides that reduce corrosion resistance and are harmful when sharpening a blade, while at the same time ensuring the hardness of the steel base itself, thereby providing a knife steel that has higher hardness and excellent blade sharpening properties, as well as excellent corrosion resistance, toughness, and wear resistance. [Explanation of symbols]

[0046] 1. Knives 11 Cutting Edge

Claims

1. A steel for cutlery characterized by having a composition, in mass%, of 1.30-1.70% C, 0.19-0.34% Si, 0.10-0.31% Mn, 14.00-18.00% Cr, 2.00-2.80% Mo, 2.10-3.60% in total of one or more of V, W and Nb, with the remainder being Fe and unavoidable impurities.

2. 2. The steel for cutlery according to claim 1, characterized in that the relationship is -1.2%≦Mo%-(V+W+Nb)%≦-0.2%.

3. 3. The steel for cutlery according to claim 1 or 2, characterized in that V: 1.50 to 2.10%, W: 0.50 to 1.40%, and Nb: 0.20 to 0.60%.

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