JPH03146642A - High strength ferritic stainless steel - Google Patents

Abstract

PURPOSE:To improve strength by using a conventional steel as a base, increasing or adding solid solution-strengthening-type elements, such as C and Si, by proper amounts, and controlling respective contents of the above elements so that relations among them satisfy inequality in the steel used after work hardening. CONSTITUTION:This ferritic stainless steel has a composition which consists of, by weight, 0.01-0.20% C, <=2.0% Si, <=3.0% Mn, 0.15-2.0% Cu, 15-25% Cr, 0.05-0.20% N, and the balance Fe and in which respective contents of Mn, Cu, C, N, and Si satisfy the relations in an inequality. Moreover, in addition to the above elements, 1.0-3.0% Ni and/or 0.5-2.0% Mo can further be added. When this steel is used, e.g. for spokes of a bicycle wheel, tensile strength after drawing at 90% draft reaches >=130kgf/mm<2> and the occurrence of deterioration in ductility can be practically prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to ferritic stainless steel that is used after being strengthened through processing, such as in the spokes of bicycle wheels.

[Theft] bλ reform - As bicycles become more sophisticated, lightweight bodies are required, but spokes, which are the main parts that make up wheels,
There is a need to reduce weight by increasing its strength. Spokes support the bicycle body on the wheel rim by their tensile stress.

Therefore, first of all, spoke materials are required to withstand repeated loads over a long period of time, and high strength against fatigue is the most required characteristic of spoke materials.

Fatigue strength is usually proportional to tensile strength, so high tensile strength is specifically required. In addition, ductility is also required because it is dangerous if the material suddenly breaks when an impact is applied. Furthermore, since bicycles are of course exposed to wind and rain, corrosion resistance is also required.

Due to the requirement for corrosion resistance, stainless steel can be considered, but martensitic stainless steel such as 5O8403
Not only does it have poor corrosion resistance, but thin materials such as spokes tend to bend significantly during quenching and tempering.
It's not appropriate. Austenitic stainless steels such as 5O8304 have satisfactory properties in terms of strength and corrosion resistance, but are difficult to employ for general purposes due to cost.

Therefore, conventionally, 5O543 has been used as a spoke material that satisfies the above basic characteristics and is reasonable from a cost perspective.
The material used was drawn from 0 grade ferritic stainless steel wire.

And the spoke material is 90% for 5tlS430 wire.
The final product diameter is 2.
The one finished to the size of mm is used. However, with ordinary 808430 steel, a tensile strength of only about 90 kgf/mm'' can be obtained at a working degree of about 90%, which does not quite satisfy the demands of the recent advanced market for lightweight cars. It is also possible to increase the strength by adding a small amount of solid solution strengthening elements to 5US430, but even in this case, the tensile strength only increases by about 120 kgf/mm2, which is lower than the required level for spoke strength of 130 kgf in recent years.
/mm2.

It is an object of the present invention to solve these problems and provide a high-strength ferritic stainless steel that is particularly strengthened through processing.

In order to achieve the above object, the high strength ferritic stainless steel according to the present invention has a weight ratio of C0.01 to C0.01.
20%, Si2.0% or less, Mn3.0% or less, Cu
0.15-2.0%, Cr15-25%, N 0.0
5~0°20%, the balance consists of Fe and impurity elements, and each content of Mn, Cu, C, N and SL is 4X (Mn + Cu) + 55X (C + N) + 1
It is characterized by satisfying the relational expression 8XS i≧20.

In order to achieve the same purpose and to improve corrosion resistance, in addition to the above components, the second invention steel also contains 1.0 to 3.0% Ni and 0.5 to 2% by weight of Ni. .0
% of MO.

In addition to high strength, in order to make the steel even more ductile, the third
In the invention steel, the grain size is adjusted to be fine with grain size number 9 or more in the state after rolling. Moreover, in order to improve corrosion resistance and ductility at the same time, in the fourth invention steel, in addition to the above basic components, 1.
It contained 0 to 3.0% Ni and 0.5 to 2.0% Mo, and was adjusted to have a fine grain size of 9 or more in terms of grain size number after rolling.

Production-1- The basic idea of the present invention is based on conventional ferritic stainless steel, and C, SL, Mn, and Cu.

Strength is improved by increasing or newly adding a solid solution strengthening element called N, and ductility is also ensured by making crystal grains finer by adding N. Here, if the amount of these elements is simply increased or added, although the strength is improved, the material becomes unusable in other properties such as ductility, workability, and corrosion resistance.

Therefore, the present inventors conducted various research with the aim of creating a ferritic stainless steel that maintains these properties at a level that can be used in practical use, and that also has higher strength than conventional steels, on the premise of cold working. As a result, by limiting the component range of each element individually as described above, and at the same time specifying the relational expression that takes into account the contribution of each element to strengthening as described above, all contradictory characteristics can be satisfied. I found out that this happens. The reasons for limiting each component will be described below.

Since C is an element that contributes most to improving strength, the lower limit was set at 0.01% to ensure a predetermined strength. However, if the content exceeds 0.20%, the cold workability and corrosion resistance will be significantly deteriorated, so the upper limit was set in this way.

Si is added as a deoxidizing element during steel manufacturing, but it is also an element that increases the strength of ferrite by forming a solid solution therein. In the present invention, it was decided to contain 2.0% or less of SL in order to actively utilize its strength-improving effect. However, excessive addition leads to a decrease in ductility and toughness, so taking into account that cold working will be performed,
The upper limit was set at 2.0% to ensure workability and to prevent a decrease in ductility after working. In addition, in order to further facilitate workability and ensure ductility, it is desirable that the content be 1.0% or less as long as the above relational expression is satisfied.

In addition to being used as a deoxidizing agent during steel manufacturing, Mn is also an element that contributes to solid solution and strengthening, so we decided to include it far beyond the upper limit of the conventional 308430 steel composition range to take advantage of its strengthening effect. . However, since Mn is an austenite-forming element and also reduces corrosion resistance and hot workability, it is undesirable to contain Mn in an amount of 3% or more. Note that in order to stabilize this reinforcing effect, it is desirable to limit the content to a range of 0.5 to 2.0%.

Cu is also a solid solution strengthening element, and in the present invention, in order to actively utilize its strengthening effect, it is essential to add 0.15% or more. However, Cu is an austenite-forming element,
Excessive content causes a decrease in ductility especially when hot, making processing such as rolling difficult, and also causes a decrease in ductility when cold, so the content was set to 2.0% or less.

Cr is a basic element that imparts corrosion resistance to stainless steel, and in order to obtain this effect, it must be contained in an amount of 15% or more. However, even if the content is 25% or more, the effect of improving corrosion resistance is hardly improved, so the upper limit was set in consideration of cost. A more limited range is preferably about 16 to 23%.

Like N-tc, it exhibits a remarkable strengthening effect by penetrating between the lattices of ferrite, and also has the effect of improving ductility by forming carbonitrides and contributing to grain refinement. In the present invention, in order to actively utilize its strengthening effect and crystal grain refining effect, 0.05%
I decided to add the above.

However, excessive addition causes difficulties during steel manufacturing (occurrence of blowholes during solidification) and during rolling (deterioration of hot workability), so in the present invention, the upper limit is set at 0.20%.

Ni is also an element that increases the strength of ferrite by solid solution, but it also has a great effect of improving corrosion resistance.

From the viewpoint of actively utilizing both of these characteristics, the content was decided to be 1.0% or more. However, since Ni is an austenite-forming element and is an expensive element, adding a large amount of Ni will not comply with the above-mentioned purpose of excluding austenitic stainless steel. Therefore, the upper limit was set at 3.0%.

Like Ni, MO utilizes its solid solution strengthening effect and corrosion resistance improving effect. When adding 0.5% or less,
These effects cannot be fully expected.

However, when considering the application target of the steel of the present invention, 2°0%
The above additions do not make a large contribution to improving corrosion resistance, but are disadvantageous in terms of cost.

The reasons for limiting the individual component ranges of each element in the first and second invention steels are as described above, but in the present invention, the contents of M n , Cu , C, N and Si are further reduced to 4X ( Mn+Cu)+55X(C+N)+18XS i
It is specified that the relational expression ≧2° is satisfied. The coefficient of content of each element in this formula is determined by considering the contribution to ferrite strengthening. For example, C
The reason why the coefficient 55 for C and N is set larger than the coefficient 4 or 18 for other elements is to take into consideration that even a small amount of C and N greatly contributes to improving the strength of ferrite.

Therefore, in the steel of the present invention, the desired properties cannot be obtained even if the amount of each element is added close to the lower limit of the specified component range, and the sum after multiplying these by a specified coefficient is the specified value. (20) or more.

In the third and fourth invention steels, the grain size was adjusted to be fine with a grain size number of 9 or more, and this was specified in order to provide sufficient ductility as well as strength. Note that this is a regulation for the state after rolling and before cold working. Under the same rolling conditions, grain refinement can be easily achieved by adding N as described above;
), the youngest third and fourth invention steels can be manufactured more reliably. Note that if strain remains due to low-temperature rolling, an annealing step may be performed, but in this case, annealing is performed at a low temperature to prevent crystal grain growth.

Below, the characteristics of specific examples of the steel of the present invention will be explained in comparison with comparative steel and conventional steel. Table 1 shows the chemical components of the invention steel, comparative steel, and conventional steel. Regarding the invention steel, the calculation results using the above formula are also shown. In Table 1, A, B and C
are the first and third invention steels, and D, E, and F are the second and fourth invention steels.

"Conventional Steel" Toshita Kogyo Hi Jha 5US430a Tearari,
G and H, which were used as "comparative steels", were made by simply adding Cu and Ni to 5US430.

These test steels were hot rolled to a diameter of 6.5 mm.

The rolling temperature at this time was approximately 950°C.
, B, C, D, E, and F, in addition to the normal rolling, those were also prepared by lowering the rolling end temperature (approximately 880°C) and further low-temperature annealing (approximately 800°C). Ferrite grain size after rolling of these test steels (measured by JI
SG 0552) are shown in Table 2. While the grain size numbers of the other test steels did not exceed 9, the grain size numbers of the third and fourth invention steels subjected to low-temperature rolling were 9 or more.

Thereafter, the φ6.5 mm rolled material was drawn to a diameter of φ1.97 mm (processing rate 91.9%) to obtain a spoke material.

For each sample steel, the drawing process was performed (
A tensile test was conducted with a diameter of 1.97 mm. Table 2 shows the tensile strength and reduction of area values at that time. All of the steels of the present invention have a tensile strength of 130 kgf/mm2 or more, which is sufficient as a lightweight bicycle spoke material. In contrast, the "comparative steel" weighs 120 kg.
Although it exceeds f/mm2, it does not reach the required strength. ``Conventional steel J 5US430 has even lower strength.Also, when looking at the reduction of area, which is a measure of ductility, the steel of the present invention has a slightly smaller value compared to conventional steel 5US430, but compared to the comparative steel ``comparative steel'', which has high strength. '', which shows almost the same value, and it is judged that the value is sufficiently durable for practical use.

In order to evaluate the corrosion resistance, a salt spray test in accordance with JIS Z2371 was conducted on the spoke materials manufactured from each sample steel after being drawn. After 48 hours, the surface condition was visually inspected: ◎No second-shot rusting, ○=slight rusting (rusting rate 3% or less), △=slightly rusting (rusting rate 3% or more),
It was evaluated in three stages. The results are shown in Table 2. All of the steels of the present invention have a low rusting rate, and when exposed to wind and rain,
It was confirmed that it has sufficient corrosion resistance as a car spoke material.

In particular, it has been revealed that the second and fourth invention steels containing Cu or Ni exhibit almost no rusting and are further excellent in corrosion resistance.

(Left below) Table 4X (Mn+Cu)+55X(C+N)+18×Si
2nd table ◎ = No rusting, O = Rusting rate 3% or less, Δ = Rusting rate 3%
As explained above and more than one hour with J, in the present invention, C15L, Mn, C
U, Cr, and N are selected, and the content range of each of these elements is specified, with the main focus on improving strength, and on the condition that properties such as ductility and corrosion resistance after processing are not impaired. Mn, Cu, C, N, Si
The content of is specified so that the value of the formula that takes into account the contribution of each element to ferrite strengthening is a predetermined value or more. Therefore, the strength after 90% drawing satisfies the required value of 130 kgf/mm2 or more as expected, and the ductility hardly decreases. Note that, as in the third and fourth invention steels, the ductility is further improved by adjusting the grain size after rolling to be equal to or greater than a predetermined value. Furthermore, regarding corrosion resistance, which is the most important characteristic of stainless steel, even the first invention steel can achieve performance equivalent to that of conventional ferritic stainless steel.

The second invention steel to which Ni and Mo are added has even better corrosion resistance when used for a long period of time.

Claims (4)

[Claims] (1) Weight ratio: C0.01-0.20%, Si2
．． 0% or less, Mn 3.0% or less, Cu 0.15-2.0
%, Cr15-25%, N0.05-0.20%, the balance consists of Fe and impurity elements, and Mn, C
Each content of u, C, N and Si is 4×(Mn+Cu)+55×(C+N)+18×Si≧
High-strength ferritic stainless steel that satisfies the relational expression 20. (2) The high strength according to claim 1, further containing one or two of 1.0 to 3.0% Ni and 0.5 to 2.0% Mo in addition to the above elements. Ferritic stainless steel. (3) The high-strength ferritic stainless steel according to claim 1, which is adjusted to have fine grains with a grain size number of 9 or more in the state after rolling. (4) The high-strength ferritic stainless steel according to claim 2, wherein the high-strength ferritic stainless steel is adjusted to have fine grains with a grain size number of 9 or more in the state after rolling.