JPH04362153A - High hardness weather resistant steel excellent in cold workability - Google Patents

Abstract

PURPOSE:To obtain a steel good in cold workability, face roughness on a cutting face is equal to that of a steel for structural purpose, having surface hardness equal to that of a steel for structural purpose subjected to surface hardening and furthermore free from the generation of rust unless it is used under a corrosive condition in particular. CONSTITUTION:The above steel contains, as alloy components, 0.45 to 0.65% C, <=0.25% Si, <=0.25% Mn, <=0.6% Ni, 4.0 to 11.0% Cr, 0.2 to 1.0% Mo, <=0.03% Al and 0.001 to 0.020% Ca, in which impurities are regulated to <=0.03% P, <=0.005% S and <=0.005% O and the balance substantially Fe. If desired, one or two kinds of 0.05 to 0.5% V and 0.05 to 0.5% Nb and/or 0.01 to 0.05% REM (rare earth metal) may be added thereto.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention has good cold workability, low roughness of the cut surface, high hardness and easy hardening, and corrosion resistance to the extent that it will not rust even under mildly corrosive usage conditions. Regarding weather-resistant steel.

0002

2. Description of the Related Art With the diversification and spread of various precision machines, OA equipment, and electronic equipment, the demand for high-hardness precision parts with a certain degree of corrosion resistance has increased.

[0003] Such parts require a hardness of HRC 58 or higher and about 60, so conventionally they have been manufactured by subjecting structural steel to carburizing treatment or by using bearing steel. However, even parts made of relatively weather-resistant materials such as SUJ2 (bearing steel) will rust over a long period of time. As a countermeasure against this problem, plating with Ni or the like is used, but the quality of plating varies widely and there is a concern about peeling, so it cannot be said that the reliability is sufficiently high.

[0004] If high C stainless steel such as US440C is used instead of plating, the hardness and corrosion resistance can be satisfied, but the material is expensive and has poor cold workability, making it difficult to cold forge or cold profile drawing. It cannot be implemented industrially. Moreover, when precision cutting or grinding is performed, the machined surface is rough,
There is a problem with lack of accuracy. If machining accuracy is insufficient, equipment using the parts may not be able to demonstrate the required performance.

On the other hand, if it is sufficient that rust does not occur in a normal indoor environment, a corrosion resistance lower than that of ordinary stainless steel is sufficient.

[0006]

[Problems to be Solved by the Invention] In view of the current state of the art as described above, the object of the present invention is to provide a material for precision mechanical parts that harmonizes mechanical properties and corrosion resistance. It exhibits cold workability close to that of structural steel and surface roughness during precision cutting and grinding, and has superior corrosion resistance than structural steel, does not rust during normal indoor use, and has an HRC of 58 or higher. The object of the present invention is to provide a weathering steel that is easy to harden and has a hardness of .

[0007]

[Means for Solving the Problems] The high hardness weathering steel provided by the present invention, which has excellent cold workability and is suitable as a precision parts material, basically has C: 0.45 to 0.65%, Si:
0.25% or less, Mn: 0.25% or less, Ni: 0.6
% or less, Cr: 4.0-11.0%, Mo: 0.2-1
．． 0%, Al: 0.03% or less and Ca: 0.001
Contains ~0.020%, P: 0.03% or less, S: 0
．． 0.005% or less, O: 0.005% or less, and the balance is substantially Fe.

[0008] In addition to the above basic composition, this alloy can contain one or both of the following additional elements. 1) V: 0.05-0.5% and Nb: 0.05
~0.5% of one or two types, and 2) REM: 0.01~0.05% (REM is L
It may be any one kind of lanthanide rare earth such as a, Ce, etc., or it may be two or more kinds, for example, a misch metal mainly composed of Ce+La. )

[0009]

[Operation] The alloy design of the high hardness weathering steel with excellent cold workability according to the present invention is based on the following idea.
In other words, in order to obtain a certain level of corrosion resistance, a higher Cr content than in structural steel is used, and Mo is added, and C and Cr are added to suppress the formation of coarse carbides that impair corrosion resistance.
limited the amount of This limit on the amount of C and Cr is
This is to limit the increase in quenching temperature. On the other hand, in order to obtain the desired strength, it was made possible to ensure the amount of solid solution C during quenching. In order to improve cold workability and precision machinability, C and Cr were selected to suppress the formation of coarse carbides in the spheroidized structure, and the amounts of Si and Mn were kept within a range that did not cause hardening of the matrix.
For precision grindability, it is important to prevent the formation of undissolved coarse carbides.

The alloy composition determined in this way will be explained below from the viewpoint of the reasons for limiting each component.

[0011] C: 0.45 to 0.65% by quenching,
C makes the matrix a martensitic structure, and
It is solid-dissolved in it to strengthen it. In order to satisfy the condition of HRC≧58, 0.45% or more is required, and 0.45% or more is required.
When it exceeds 65%, undissolved coarse carbides are generated, impairing cold workability and precision machinability.

Si: 0.25% or less Mn: 0.25% or less Both are used as deoxidizing agents, but they strengthen the ferrite phase of the matrix in a spheroidized structure as a solid solution and lower cold workability. The upper limits of 0.25% for both were determined in consideration of such effects and manufacturing costs.

Ni: 0.6% or less Ni is often mixed in from raw material scrap during the production of special steel, and has the effect of slightly improving the ductility of the martensitic phase, but in large amounts it makes softening annealing difficult.

Cr: 4.0 to 11.0% Cr forms a solid solution in the martensite phase of the matrix and forms a protective film on the surface of the material, playing a central role in imparting corrosion resistance. Carbides are produced during tempering and annealing, softening the material. To obtain corrosion resistance that does not rust in a relatively mild environment, C
It is sufficient if r is added in an amount of 4% or more. Corrosion resistance increases as the Cr content increases, but in the C content range described above, coarse carbides are likely to form, which impairs cold workability, and undissolved carbides remain, making it difficult to determine hardness. Therefore, the upper limit was set at 11%.

[0015] Mo: 0.2 to 1.0% Mo is an element that plays a role in corrosion resistance together with Cr, and is said to have the function of stabilizing and repairing the protective film. This effect can be obtained by adding 0.2% or more, while adding more than 1% impairs cold workability and increases cost.

[0016] Al: 0.03% or less In order to achieve the O amount regulation described later without using excessive amounts of Si and Mn, Al must be used as a deoxidizing agent, but if the amount is too large, it becomes The above limitations exist because the formation of metal inclusions Al2O3 impairs cold workability and corrosion resistance.

Ca: 0.001-0.020% Addition of a small amount exceeding the above lower limit has deoxidizing and desulfurizing effects, improves hot and cold workability, and improves machinability. improve slightly. Even if it is added in excess of 0.02%, the effect is saturated.

[0018] P: 0.03% or less Since it impairs corrosion resistance and toughness and ductility, it should be reduced to within the above limits.

S: 0.005% or less impairs hot workability, corrosion resistance, and cold workability, so the above limit was set.

[0020] O: 0.005% or less forms oxides and causes the presence of nonmetallic inclusions.
Since nonmetallic inclusions reduce cold workability and impair fatigue strength and toughness and ductility, the O content must be kept within the above range.

The functions of optionally added alloying elements and the reasons for limiting the composition are as follows.

[0022] V: 0.05~0.5% Nb: 0.05~0.5% Both form stable fine carbides, refine crystal grains, and improve strength and toughness and ductility. It is advisable to add it when desired improvement is desired. Addition of less than 0.05% will have a low effect, and addition of more than 0.5% will not only impair cold workability but also reduce toughness and ductility.

REM: Has deoxidation and desulfurization of 0.01 to 0.05%, improves hot workability, and improves corrosion resistance, especially oxidation resistance. A content of less than 0.01% is not effective, while a content of more than 0.05% saturates the effect and at the same time reduces hot workability.

[0024]

[Example] Steel having the alloy composition shown in Table 1 was melted. This is made into a wire rod with a diameter of 20 mm by hot forging and rolling.
Spheroidizing annealing was performed. The treated materials were subjected to hardness measurements, cold workability evaluations through upsetting tests, and external machining tests using automatic lathes.

Furthermore, the spheroidized annealed material was subjected to quenching and low-temperature tempering, and its hardness, corrosion resistance, and oxidation loss were examined.
The results are shown in Table 2. In Table 2, the deformation resistance and cracking reduction ratio were evaluated by an upsetting test, and the deformation resistance was data at a reduction ratio of 63%. The machined surface roughness is 4 when machined with an automatic lathe.
This is the result of examining the 00th sample.
Corrosion resistance was evaluated by the presence or absence of pitting after a dry/wet repeated test of 4 cycles of wet (50° C., 98% RH, 4 hrs) and dry (70° C., 2 hrs). The oxidation loss was measured after being maintained in the atmosphere at 500°C for 50 hours.

[0026]

[Table 1]

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

[Table 2]

[0030]

[Effects of the Invention] The weathering steel of the present invention has good cold workability due to the selection of a unique alloy composition, the surface roughness of the cut surface is equivalent to that of structural steel, and the weathering steel has a structure that has been subjected to surface hardening treatment. This material meets the requirements of having a surface hardness comparable to that of commercial steel, and not rusting unless used under corrosive conditions.

Therefore, this alloy is suitable as a material for cold forging, and is also useful as a material for manufacturing precision parts such as shafts, rollers, and small screws constituting various precision machines.

Claims (4)

[Claims] [Claim 1] C: 0.45-0.65%, Si: 0
．． 25% or less, Mn: 0.25% or less, Ni: 0.6%
Below, Cr: 4.0-11.0%, Mo: 0.2-1.
0%, Al: 0.03% or less and Ca: 0.001~
Contains 0.020%, P: 0.03% or less, S: 0.
0.005% or less, O: 0.005% or less, and the balance is substantially Fe, and the high hardness weathering steel has excellent cold workability. [Claim 2] C: 0.45-0.65%, Si: 0
．． 25% or less, Mn: 0.25% or less, Ni: 0.6%
Below, Cr: 4.0-11.0%, Mo: 0.2-1.
0%, Al: 0.03% or less and Ca: 0.001~
In addition to 0.020%, it contains one or two of V: 0.05 to 0.5% and Nb: 0.05 to 0.5%,
P: 0.03% or less, S: 0.005% or less, O: 0.
A high hardness weathering steel having excellent cold workability and having an alloy composition of 0.005% or less and the balance substantially consisting of Fe. [Claim 3] C: 0.45-0.65%, Si: 0
．． 25% or less, Mn: 0.25% or less, Ni: 0.6%
Below, Cr: 4.0-11.0%, Mo: 0.2-1.
0%, Al: 0.03% or less and Ca: 0.001~
In addition to 0.020%, REM: 0.01-0.05%
P: 0.03% or less, S: 0.005% or less, O: 0.005% or less, and the balance is substantially Fe.
A high-hardness, weather-resistant steel with excellent cold workability that has an alloy composition of [Claim 4] C: 0.45-0.65%, Si: 0
．． 25% or less, Mn: 0.25% or less, Ni: 0.6%
Below, Cr: 4.0-11.0%, Mo: 0.2-1.
0%, Al: 0.03% or less and Ca: 0.001~
In addition to 0.020%, it contains one or two of V: 0.05-0.5% and Nb: 0.05-0.5%, and REM: 0.01-0.05%. , P: 0.03
% or less, S: 0.005% or less, O: 0.005% or less, and the balance is substantially Fe, and the high hardness weathering steel has excellent cold workability.