JPS6347353A - Extra soft ferritic stainless steel - Google Patents

Abstract

PURPOSE:To obtain an extra soft ferritic stainless steel having <=140 hardness Hv and superior press workability by providing a compsn. contg. prescribed percentages of C, Si, Mn, P, S, etc., and one or more among Ti, Nb and V. CONSTITUTION:This extra soft ferritic stainless steel contains, by weight, <0.03% C, <=0.30% Si, <=1.5% Mn, <=0.04% P, <=0.15% S, <=1.0% Ni, <=0.50% Cu, <=0.60% Mo, 11.5-20% Cr, <=0.03% N and one or more among 0.005-0.20% each of Ti, Nb and V. The steel has the above-mentioned characteristics and gives a cold rolled material having superior surface properties. The cost of manufacture is low and the steel is superior to a ferritic stainless steel for Italian coins in every point, so it is used as an optimum coin material.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to ultra-soft ferritic stainless steel, particularly for use in cold pressing, such as coins, medals, game coins, keys, etc., which require more precise coining. This article relates to ultra-soft ferritic stainless steel, which is ideal as a material for various products that are needed.

[Conventional technology]

Magazine rChromium ReviewJ No. 1
According to the April 1983 issue, in 1979 there were 117
More than 55% of all newly minted coins in Kenya are made of stainless steel. This means that stainless steel is being looked at from the viewpoint of economy in coin production and durability in coin circulation.

Stainless steel coins have an attractive luster, excellent corrosion and abrasion resistance, and are economically advantageous compared to other materials such as copper alloys. The biggest problem in the application of coin manufacturing is that it is hard, so a large press capacity is required during coining (coining), and the life of the die during coining is short. This problem involves the various difficulties mentioned above.

The top 100 in Italy, which has been manufacturing coins made of stainless steel for the longest time, are listed in the third row from the bottom of Table 1.
Table 2 shows the compositional analysis value of Lila coin, and the coin has 7
Hardness H measured in recrystallized state after annealing at 50°C for 5 minutes
v was shown. Fig. 1 shows the surface shape of the coin showing the coining condition, and Fig. 2 shows the 75%Cu-25%N! This is the surface shape showing the coining status of Japan's 100 yen cupronickel coin.

As is clear from Table 2, the current Italian stainless steel for coins is hard with a hardness of approximately Hv163 in the annealed state. Coins made of steel have shallower and less clear patterns on their surfaces than Japanese 100 yen coins made of cupronickel.

Although the depth of the coin engraving can be improved by increasing the pressure of the coining press, this is not economically advantageous as it shortens the life of the expensive die. As described above, it should be understood that in order to fully utilize the various advantages of stainless steel as a material for coins, it is extremely important to first improve its hardness.

JP-A-55-89431 has been disclosed as a stainless steel for coins. The gist of Party B's invention is to reduce the amount of added elements other than Cr in ferritic stainless steel containing 12 to 18% Cr, and to improve ridging properties in the material processing process. During the hot rolling, the hot rolling finishing temperature was set at 800°C.
and the winding temperature is limited to 450°C or less.

However, reducing the content of other elements other than Cr has various problems. For example, reducing the CXN content increases costs, and suppressing Sl to a low level results in insufficient deoxidation, resulting in coin production. This can lead to deterioration of the surface quality, which is important for In addition, it is effective to reduce the hot-rolling finish by 1 degree and the coiling temperature to improve ridging properties.
15696, JP-A-52-66816, JP-A-58-5
6012, etc., but setting the coiling temperature of the hot-rolled material to a low temperature of 450°C or lower will result in extremely poor coin shape, which may be a fatal defect as a stainless steel for coins. be.

[Problem that the invention seeks to solve]

The purpose of the present invention is to solve the above-mentioned problems of the prior art of stainless steel processed by cold press forming, etc., such as stainless steel for coins, and to improve the hardness, which is the biggest problem, and to To provide soft ferritic stainless steel.

[Means for solving problems]

The gist of the present invention is as follows.

That is, in weight ratio: C: less than 0.03% Si: 0.30% or less Mn: 1.5% or less P: 0.04% or less S: 0.15% or less Ni: 1.0% or less Cu: Contains 0.50% or less Mo: 0.60% or less Cr: 1 1,5 to 2 0% N: 0.03% or less, furthermore contains Ti: 0.005 to 0.20%, Nb:
0.005-0.20%, V: 0.005-020%, the remainder consists of Fe and unavoidable impurities, and the hardness is 1 on the Vickers hardness scale.
40 or less and is an extremely soft ferritic stainless steel characterized by excellent coining workability.

In attempting to materialize ferritic stainless steel, the present inventors first prepared C, Si, Mn, P, and S.

The Hv hardness of 87 types of ferritic stainless steels containing various levels of the 10 elements Cr, Ni, Cu, Mo, and N and the grain size of all 20 to 30 μm are shown below. As a result of performing a correlation analysis with the equation (1) below, we were able to obtain the equation (1) below.

Hv=73.3-12.3 (XC)+22.7 (X
Si)+0.8 (XMn)+361 (XP)-55
,1(XS)+2.9(XCr)+2.6(X
Ni)+9.8 (XCu)+5.1 (XMo)+
370 (j+mL-...(1) From the above formula (1), 5O
In order to reduce the Hv hardness based on the concentration level of each element contained in ordinary ferritic stainless steel such as 3430 and the magnitude of the coefficient in equation (1) above, it is necessary to Si, P, Cu
It was found that five components, namely , Mo, and N, should be regulated.

By the way, C is generally an element that increases hardness, but in the above equation (1), the result is that it has a rather negative coefficient. The reason for this is that after the normal treatment of ferritic stainless steel, C is precipitated as Cr carbides such as Cr23C6, and therefore, the solid solution hardening effect inherent to C element does not occur, but rather the Cr that gives solid solution hardening effect does not occur. This is thought to be due to the fact that it provides a softening effect by substantially lowering the hardness.

Based on the above knowledge of the present invention, and by limiting the composition range as outlined above in consideration of economic efficiency during melting, cleanliness and surface properties of the material, extremely soft ferritic stainless steel can be produced industrially at low cost. The present invention has been completed by discovering that it is possible to do so.

The reason for limiting the composition of the ferritic stainless steel according to the present invention will be explained.

C: As mentioned above, it has become clear that C has a softening effect on hardness Z, contrary to what was generally thought in the past, through the formation of Cr carbides. Therefore, C does not need to be particularly regulated from the viewpoint of hardness at a normal addition level.

However, when applied to coins, it is required to have sufficient corrosion resistance during the circulation life of coins, which is about 15 to 20 years.

Sweat is thought to have the greatest effect on the corrosion resistance of coins. The present inventors found that Si: 0.15%, Mn: 0.
30%, P: 0.015%, S: 0.002%, N
i: 0.04%, Cu: 0.03%, Mo: 0.02%
, N: 0.015%, Cr: 12.5% and 17
5%, V: 0.15%, and the C content was varied from 0009 to 0065% for each Cr content level. The pitting potential was measured and the results are shown in FIG. For comparison, the results for the stainless steel currently in use for Italian coins are also shown with an O mark in the same figure.

The pitting potential was measured as follows.

That is, 7 g of NaCl, Ig of urea in 11 water,
S as a reference electrode with an artificial sweat solution containing 4 g of lactic acid.
Anodic polarization tests were performed using CE (saturated Amagi electrode). The measurement was performed by immersing the sample in the artificial sweat solution at 35°C, holding it at -500 mV using SCE for 10 minutes, holding it at the natural immersion potential for another 10 minutes, and then sweeping it for 1 minute until the total potential reached 1 V at 20 mV. did. On the anode polarization curve thus obtained, the potential at which rapid dissolution begins with the occurrence of pitting corrosion and the dissolution current density reaches 100 μA/c was defined as the pitting corrosion potential.

From Figure 3, it can be seen that the pitting corrosion potential in the artificial sweat solution is not affected by the Cr content, but is greatly influenced by the C content.
From the viewpoint of obtaining good corrosion resistance, the C content was limited to less than 0.03% in the present invention, which is less than 0003%, resulting in #4, which is superior to the current Italian coin stainless steel at a 175% Cr content level. It was found that it is edible.

Sl: Si is an essential component for deoxidizing during melting, but it has a strong effect of increasing hardness, so in the present invention, the content of Sl is kept to a minimum and its upper limit is limited to 0.30%.

Mn: As is clear from the above formula (1), Mn has a small effect of increasing hardness, and even with the addition of 1%, the hardness increases by 1 Hv.
It is weak and does not need to be particularly strictly regulated, but if it exceeds 15%, corrosion resistance will deteriorate, so the upper limit was limited to 15%.

P: As shown in formula (1) above, P has a very large hardness increase coefficient of 361, so it is preferable to reduce it as much as possible, but it is limited to 0.04% or less in consideration of the economic efficiency of deoxidation.

S: S has a negative coefficient in the above equation (1), and it is desirable to add it at a high concentration to reduce hardness, but if S is 0.15%
If the amount exceeds 0, the corrosion resistance will decrease, so set the upper limit to
．． It was limited to 15%.

Ni, Cu, Mo: Considering that the [v increase coefficients of Ni, Cu, and Mo in the above formula (1) are 2.6.9.8.5.1, the Hv hardness of these three elements is As a guideline, the total increase in is about 3 or less, Ni is 10% or less, Cu is 0.50%.
Hereinafter, Mo was limited to 0.60% or less.

Cr: Cr is the most important element for maintaining the corrosion resistance of ferritic stainless steel. If it is less than 11.5%, corrosion resistance cannot be maintained, and if it exceeds 20%, hot fF' Since the properties deteriorate, the content was limited to a range of 115% to 20%.

N: In the above formula (1), the Hv increase coefficient is the maximum of 370, and it is desirable to set it to a low value, but in the present invention, Ti5
Since N was fixed as a nitride by adding appropriate amounts of Nb, V, the upper limit was set to 0.03%, which does not increase costs, and was limited to 0.03% or less.

The steel of the present invention has the above C, Si, Mn, P, S, Ni, C
In addition to containing each component of U, Mo, Cr, and N in limited amounts, it also simultaneously contains one of the following limited amounts of Ti1NbSV, and the reasons for limiting these T11Nb and V are as follows. be.

Ti, Nb, V: The effects of Ti, Nb, V in the present invention are as described above (H
The purpose is to fix N, which has the greatest effect on v hardness, as TiN, NbN, and VN through its strong nitride-forming action, and to render its solid solution hardening action harmless.

In ferritic stainless steel, 0.20 to 0.60% of Ti, Nb, and V are sometimes added for the purpose of improving corrosion resistance. For ferritic stainless steel containing Ti1Nb1V in this range, the coefficients of these three elements for Hv hardness were determined using the same method used to determine equation (1) above, and the results were +112, +17.2, and +74, respectively.
It was calculated that it had a large hardening effect. However, when Ti, Nb, and V are added in an amount of 0.2% or less, the addition of Ti, Nb, and V in an amount less than about 3 times the amount of N in atomic percent substantially suppresses the solid solution hardening effect by converting solid solution nitrogen into nitrides. It has been found that solid solution Ti, Nb, and V, which exert a remarkable hardening effect, can be kept to a negligible level.

In addition, the addition of T1 causes problems such as clogging of the nozzle of the tundish and immersion tube during continuous casting after melting, and increases the frequency of ground defects. However, in the present invention, N is limited to 0.03% or less. Therefore, by adding Ti at 0.20% or less, such troubles can be prevented, so the upper limit was set at 0.20%.

In addition, the above-mentioned effects of Ti, Nb, and V addition can be expected to have a corresponding effect even with a small amount of addition, but in order to obtain a substantial effect, at least 0.0005% is required.
It was limited to a range of 5-020%.

Although it has been clarified that the relationship between the main element content and hardness of ferritic stainless steel can be expressed quantitatively by the above equation (1), the solid solution hardening of Ti, Nb, and V themselves can also be ignored in the present invention. It was possible to maintain the level. As a result, it has been found that the substantial relationship between hardness and components can be expressed by the following hardness index HvN, which ignores the N term in equation (1) and the hardening effects of Ti, Nb, and V.

Hardness index HvN=73.3-12.3 (XCr)+2
2.7 (XS i)+0.8 (XMnJ+361
(zP)-55,1(XS)+2.9(XCr)+
2.6 (Ni)+9. '8 (XCu)+
5.1 (xMo)・・・・・・・・・・・・(2) In the present invention, compared to existing conventional materials, coins, medals,
It is desirable to set the hardness index HvN of equation (2) to 140 or less as a value that can significantly reduce the coining pressure during coining processing of game coins, keys, etc.

A ferritic stainless steel that satisfies the above-mentioned limiting requirements of each component, the remainder consists of Fe and unavoidable impurities, and more preferably does not satisfy the above-mentioned hardness index: - The final cold-rolled and sintered sheet has an Hv hardness of 140. It was possible to ensure the following, and it was possible to obtain a ferritic stainless steel that was significantly softer than conventional ones.

〔Example〕

The seven original ferritic stainless steels of A-G according to the present invention as shown in Table 1 and SUS 430 as comparative steel
and a ferritic stainless steel that is the same as the ferritic stainless steel for Italian coins, and a ferritic stainless steel that is low carbon and low nitrogen, but contains Ti or Nb of 036% and 050%, respectively, which are excessively higher than the limited amounts according to the present invention. Comparative steel IH1■ was similarly melted in a high-frequency vacuum melting furnace, and each steel ingot weighed 30 kg.

All of these steel ingots were heated to 1250° C. under the same conditions and then hot-rolled to obtain hot-rolled sheets with a thickness of 3.

The hot rolling finishing temperature at that time was 830°C.

This hot-rolled sheet was annealed by a known method, cold-rolled, and finished annealed to obtain a cold-rolled and annealed pure steel sheet with a thickness of 1.2 m.

Generally, a material with a thickness of 12 to 27 mm is used as a coin material, but neither the present invention nor the comparative steel showed any deterioration in surface roughness due to ridging even after undergoing the above-mentioned processes.
No troubles occurred during subsequent trials of the test coin.

It has been found that there is no need to perform a process treatment under special conditions regarding turf ridging.

Table 2 shows the results of measuring the mechanical properties of Hv hardness, yield stress, tensile strength, and elongation for each of these test materials. As is clear from Table 2, the steel of the present invention has an Hv hardness of 119 to 135. Ali, comparison @5U
It can be seen that S430 (7) 157 is significantly softer than 163, the stainless steel for Italian coins.

The reason why the hardness of the comparative steels is significantly different from the hardness index according to formula 2) is thought to be that in these D steels, solid solution N, solid solution Ti, and Nb contribute to the hardness.

Next, among the present invention, Cr content 134% class and 18% class
Sample material A was used as a representative steel with different Cr levels in the 0% class.
, D, C, and F are selected from cupronickel, brass, aluminum, which have been widely used as materials for coins,
Comparative tests of corrosion resistance, wear resistance, and coining properties were conducted using nickel as a comparative material. Corrosion resistance was evaluated by the pitting potential in artificial sweat described above.

In addition, the wear resistance test was carried out using an Okoshi type wear resistance tester, and a load of 3
．． 2kg, wear distance 566.6m, wear rate 0.51
The specific wear amount was measured under the conditions of m/sea.

Furthermore, in order to measure the coining force during coining, 251II1 was used from the sample materials A, D, C, F according to the present invention and each comparative material.
After punching a 1φ blank and attaching ears using a pressure edge,
A coin was made using a die material of SKD 11 according to JIS G4404 with a coining depth of 250 μm, and the optimum coining force was measured from the occurrence of peripheral banners and the engraving of the pattern.

The cupronickel used as a comparative material was 75% Cu-25%.
The brass is a 70% Cu-30% Zn alloy. Comparative tests of corrosion resistance, abrasion resistance, and coining property using the optimum coining force using the steels of the present invention and comparative materials are shown in Table 3.

Table 3 As is clear from Table 3, the ferritic stainless steel specimens A, D, C, and F according to the present invention have the following superior properties compared to other comparative materials.

That is, (a) In terms of corrosion resistance, the steel of the present invention is far superior to other nonferrous coin materials such as cupronickel, brass, aluminum, and nickel; Almost equivalent to stainless steel for Italian coins.

(b) Regarding wear resistance, the steel of the present invention is superior to comparative non-ferrous coin materials and stainless steel for Italian coins, and is almost equivalent to 5US430.

(c) Regarding the optimum coining force, which is the most important characteristic for cold press coining of coin materials, etc., the steel of the present invention is superior to other stainless steels such as SUS 430, Italian coin stainless steel, etc., even without post-annealing. It has a significantly lower value than ferritic stainless steel, and when compared to other comparative non-ferrous coin materials, it is superior to brass and nickel, and close to the level of cupronickel and aluminum. This indicates that the material is ideal for press-forming applications. This is a result of the extremely soft nature of the steel of the present invention as shown in Tables 1 and 2.

〔Effect of the invention〕

The ferritic stainless steel according to the present invention has an appropriate composition, and in particular, for the purpose of the present invention to be extremely soft and highly corrosion resistant, Si, P, Cu, Mo, and N are reduced, and at the same time,
Furthermore, N1tTiN or Nb
Since the composition was fixed as N and VN and the solid solution hardening effect thereof was rendered harmless, the following effects could be achieved.

(a) Hardness is extremely soft, Hv120-14
0, so that the optimum coining force is very low.

(b) Corrosion resistance and abrasion resistance are also significantly superior to other non-ferrous coin materials.

(c) The surface quality of the cold-rolled material is excellent.

(d) Manufacturing costs are low.

(e) As a coin material, it is an optimal material superior to ferritic stainless steel for Italian coins in all respects.

[Brief explanation of the drawing]

Figure 1 is a measurement diagram of the front and back shapes of an Italian 100 lira coin made of ferritic stainless steel, and Figure 2 is a measurement diagram of the Japanese 100 lira coin made of 75% Cu-25% Ni cupronickel.
FIG. 3 is a diagram showing the relationship between pitting potential and carbon content in an artificial sweat solution.

Claims (1)

[Claims] (1) By weight C: less than 0.03% Si: 0.30% or less Mn: 1.5% or less P: 0.04% or less S: 0.15% or less Ni: 1.0% or less Cu : 0.50% or less Mo: 0.60% or less Cr: 11.5-20% N: 0.03% or less, furthermore contains Ti: 0.005-0.20%, Nb:
0.005-0.20%, V: 0.005-0.20%
An ultra-soft ferritic stainless steel characterized by containing one or more selected from the following, the remainder being Fe and unavoidable impurities, having a hardness of 140 or less on the Vickers hardness scale, and having excellent coining workability.