JPH07316740A - High strength stainless steel with composite phase structure and its production - Google Patents

Abstract

PURPOSE:To impart wear resistance to a stainless steel with a mixed composite phase structure combining ductility and strength by economical means. CONSTITUTION:A high strength stainless steel with a composite phase structure having a compsn., among various components contained in the steel, contg., as essential componental contents,<=0.20% C, 10.0 to 20.0% Cr, <=4.0% Ni and <=0.15% N and having a mixed composite phase structure of 20 to 95vol.% martensitic phase and the balance substantial ferritic phase, and in which the surface layer part of a depth of at least 1mum from the outermost surface layer is coated with the layer of a martensitic single phase is prepd. Thus, the stainless steel excellent in workability and wear resistance can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength dual-phase stainless steel having a martensite single-phase structure in the surface layer and a mixed structure of ferrite and martensite in the interior. In the material form of steel strips or steel plates used for punching and press forming as high strength members such as parts and springs, the surface layer has martensite and the inside has a mixed structure of ferrite and martensite. The present invention relates to a high-strength stainless steel having a surface hardness that exhibits sufficient wear resistance while having good formability.

[0002]

2. Description of the Related Art As a high strength stainless steel, martensitic stainless steel has been well known.
Further, austenitic stainless steels such as SUS301 and SUS304, which are cold-rolled and are strengthened by work hardening, are also widely used.

Furthermore, the inventors of the present invention have disclosed JP-A-63-7338, JP-A-63-169330-5, and JP-A-1-172524-5.
In Japanese Patent Publication, a slab of steel whose composition is adjusted to exhibit a ferrite + austenite structure at high temperature is basically formed into a steel strip through hot rolling and cold rolling, and a ferrite + austenite two-phase structure is used as a finish heat treatment. Present A
c Duct and high-strength dual-phase chrome stainless steel with a mixed microstructure of ferrite + martensite, which is obtained by performing continuous heat treatment of heating and holding at an appropriate temperature range of ₁ point or more and cooling at an appropriate cooling rate ( Obi) is proposed.

[0004]

Martensitic stainless steel can be imparted with high strength by quenching treatment or quenching / tempering treatment. After being subjected to these heat treatments, the martensitic stainless steel basically has a martensitic structure as its name suggests, and it has extremely high strength (hardness) and excellent wear resistance.
On the other hand, the growth is very low.

Therefore, since martensitic stainless steel has extremely poor workability after quenching (tempering) treatment, it is annealed from the material manufacturer, that is, a state of soft ferrite structure with low strength and hardness. It is often shipped as a (steel strip or steel plate) and processed by a processing maker after it is formed into a product shape, and then quenched and tempered.

When performing such heat treatment after processing,
When an oxide film is formed on the surface, it is often not preferable for stainless steel where surface beauty is important. As a countermeasure, a process such as heat treatment in a vacuum or an inert gas atmosphere, or pickling or polishing after the heat treatment is required. In addition, if heat treatment is performed, some degree of deformation or dimensional change cannot be avoided, and post-treatment such as shape correction may be necessary in terms of product dimensional accuracy.

For this reason, the use of martensitic stainless steel causes an increase in the burden on the processing maker side, and there is a problem that the cost increase of the final product cannot be avoided.

On the other hand, since austenitic stainless steel is generally soft in the state of solution heat treatment, in order to increase strength, temper rolling (cold rolling) is further performed after solution heat treatment to increase strength by work hardening. However, it is difficult to obtain a material with a large thickness and high strength by this cold rolling method. That is, in order to secure the cold rolling ratio required to obtain a desired high strength in a thick plate, it is necessary to increase the plate thickness before cold rolling in accordance with the product plate thickness. There is a limit to the initial thickness that can be passed through. Further, austenitic stainless steel has a substantial problem that it is expensive because it contains a large amount of expensive Ni.

The dual phase chromium stainless steel proposed by the present inventors in the above publications has a good strength-ductility balance, has small in-plane anisotropy of strength and ductility, and has low yield strength and low yield ratio. It has sufficient characteristics as a high-strength molding material. However, since this dual-phase stainless steel contains a soft ferrite phase as part of its metallographic structure, the surface of the martensitic stainless steel is hard chrome plated regardless of the quenching / tempering material or the base metal. It cannot be denied that the surface wear resistance is inferior to that of materials.
However, martensitic stainless steel has the above-mentioned problems, and when the hard chrome plating material peels off the plating, the effect is naturally lost, and the hard chrome plating itself causes a cost increase.

An object of the present invention is to solve various problems of these conventional high strength stainless steels, and the base material itself has high strength and has workability such as punchability, press formability and bendability. The aim is to obtain economical stainless steel that is excellent and at the same time has good wear resistance.

[0011]

According to the present invention, among various components contained in steel, C: 0.20% or less, Cr:
10.0 to 20.0%, Ni: 4.0% or less, N: 0.15
% Or less as an essential component amount and 20 to 95 vol.%
Is a stainless steel having a multi-phase structure in which the martensite phase and the rest are substantially mixed with the ferrite phase, and only the surface layer portion having a depth of at least 1 μm from the outermost surface is covered with the martensite single phase layer. To provide high strength duplex stainless steel.

This high-strength dual-phase stainless steel has a γmax value of 20 in which the contents of various components in the steel are represented by the following equation.
The temperature is adjusted to 95 or less, and the steel (particularly steel strip) whose composition is adjusted in this way is heated to an Ac ₁ point or higher in an atmosphere containing nitrogen and having a dew point adjusted to 0 ° C or less. By heating and then performing a heat treatment (particularly continuous heat treatment of the steel strip) at a cooling rate sufficient to transform the austenite phase to the martensite phase from this heating temperature, only the surface layer part is the martensite single phase and the inside is It can have a multiphase structure.

Γ max = 420 (% C) +470 (% N) +
23 (% Ni) +7 (% Mn) +9 (% Cu) -11.5
(% Cr) -11.5 (% Si) +189.

[0014]

In the high ductility and high strength stainless steel strip of the mixed multiphase structure of the martensite phase and the ferrite phase which has been previously proposed, the present inventors have kept the mixed multiphase structure inside and at least 1 μm from the outermost surface. If the martensite single phase is formed only in the surface layer at the depth of, the steel strip will have sufficient formability during punching and press forming, and the wear resistance It was found that the scratch resistance) was significantly improved.

Therefore, the high-strength stainless steel of the present invention is excellent in wear resistance and scratch resistance in addition to good ductility and workability. Therefore, the steel strip or steel plate material is used for potential parts, precision machine parts, etc. When applied to manufacturing, it can exhibit properties that conventional materials do not have.

In order to produce a high-ductility / high-strength stainless steel strip or steel sheet having excellent wear resistance (scratch resistance) according to the present invention, the above-mentioned γmax is adjusted to a range of 20 to 95. In contrast, the final finishing heat treatment (Ac ₁
The heat treatment may be performed under an atmosphere containing nitrogen and having a dew point adjusted to 0 ° C. or lower. This atmosphere control does not affect the internal multiphase structure or the steel strip or It was found that the martensite single phase can be formed only in the very surface layer of the steel sheet.

The stainless steel of the present invention contains C: 0.20% or less and Cr: among the various general components contained in the steel.
10.0 to 20.0%, Ni: 4.0% or less, N: 0.15
% Or less must be contained as an essential component amount. The reason is as follows. In this specification, the content (rate) of the components in the steel is expressed in mass% unless otherwise specified.

Since C is a strong austenite forming element and has a large martensite strengthening ability, the amount of martensite after heat treatment for heating to a temperature of A _C1 or higher can be changed. , The steel of the present invention effectively controls the strength and strengthens the strength. However, if the amount of C is too large, the amount of martensite after heat treatment will be too large, and in some cases it will be 100% martensite, and the hardness of the martensite phase will also be very high, so high strength can be obtained but ductility will decrease. To do. Further, when the amount of C is large, a large amount of carbide is generated after the heat treatment, which causes deterioration of corrosion resistance and toughness. For this reason, the C content is preferably 0.20% or less.

In order to maintain the corrosion resistance as stainless steel, Cr should be contained in a minimum amount of at least 10.0%, but if the Cr content is too high, the toughness decreases and the Cr content increases. Along with this, C, Ni, Mn, which are necessary to obtain a high strength by producing a martensite phase,
Since the amount of austenite forming elements such as Cu and N increases, it is preferable to set the upper limit to 20.0%.

When Ni is contained, as described above, the adverse effect caused by the inclusion of a large amount of C can be avoided, and excessive Cr can be suppressed, while it is effective to secure the amount of martensite after heat treatment as an austenite forming element replacing C. Act on. Ni is also an element that lowers the A _C1 point, and effectively acts to reduce the heat treatment temperature. However, since an increase in Ni causes an increase in cost, it is preferable to set 4.0% as the upper limit.
Note that Mn and Cu also have the same effect as Ni,
Part of Ni can be replaced with Mn and Cu.

N, like C, is an austenite-forming element, but it is difficult to add a large amount at the time of steelmaking because of the solubility, and addition of a large amount causes an increase in surface defects, so 0.15%. The following is good.

Γ max according to the above equation is an index corresponding to the maximum austenite amount (vol.%) At high temperature. The stainless steel of the present invention has a two-phase structure of ferrite and austenite at a temperature of Ac ₁ point or higher, and austenite transforms to martensite in the process of cooling at a cooling rate above a certain level. It also affects the quantity.

In the composition of components where γ max is less than 20, A _C1
Even if heated above the point, the amount of martensite formed after cooling is small and the structure mainly consists of the ferrite phase, so high strength cannot be obtained. When γmax is less than 20, it takes a long time to form a martensite single-phase layer on the surface by heat treatment in a controlled atmosphere described later, which impairs the manufacturability of the steel of the present invention. On the other hand, if γmax is too high, the amount of martensite after heat treatment becomes close to 100%, and the characteristic of dual-phase stainless steel that achieves both high strength and workability is lost, so the upper limit of γmax is set to 95.

The steel of the present invention has the above-mentioned essential components C and C.
If r, Ni, N content requirements are satisfied, and if γmax is in the range of 20 to 95, corrosion resistance, heat resistance,
The inclusion of various elements added to improve various properties such as hot workability and the inclusion of impurity elements mixed in during manufacture are allowed. For example, Si of 2.0% or less, Mn of 4.0% or less,
P less than 0.04%, S less than 0.010%, 0.03%
B below, Cu below 4.0%, A1 below 1.0%
Mo less than 3.0%, REM less than 0.20% (rare earth elements such as La and Ce), Y less than 0.20%, 0.2%.
The inclusion of Ca of 10% or less and Mg of 0.10% or less is allowed.

By adjusting the above component systems and performing heat treatment for heating and cooling at an Ac of ₁ point or more, a strength having a multiphase structure in which a martensite phase of 20 to 95 vol.% And the balance substantially a ferrite phase are mixed. Stainless steel with excellent ductility can be obtained. By using the martensite single-phase layer as the surface layer of this mixed multi-phase steel, wear resistance and flaw resistance can be significantly improved without impairing the ductility and workability.

In order to obtain the effects of imparting wear resistance and scratch resistance, it is necessary that the depth of at least 1 μm or more from the outermost surface is a martensite single phase. Of course, the inside must remain the mixed multiphase structure. This is because if the whole becomes a martensitic single phase, it becomes a mere martensitic stainless steel, and the characteristic of the present invention that it is excellent in ductility and various workability is impaired. The ratio of the thickness of the martensite single-phase layer in the surface layer to the thickness of the mixed multi-phase microstructure layer inside may be appropriately selected depending on the plate thickness, the ratio of ferrite in the mixed multi-phase microstructure, and the application of the material steel sheet. However, in any case, it is preferable that the thickness of the martensite single phase layer does not exceed 1/3 of the whole.

The fact that the interior has a mixed multi-phase structure of ferrite and martensite means that the base material itself has high strength and is excellent in workability such as punchability, press formability, bendability and ductility, and ductility. Is essential for Even if the surface has a hard martensite single-phase layer, if the interior is a soft ferrite phase, the strength of the steel itself is insufficient and it cannot be applied as a strength component. The ratio of ferrite phase to martensite phase is
It is also related, but it is preferable that the martensite phase is approximately 20 to 95%.

Next, the heat treatment conditions for obtaining the microstructure of the steel of the present invention and their effects will be described. It is one of the great features of the present invention to perform the heat treatment of heating to a temperature of A _C1 point or higher in a controlled atmosphere. When high temperature heat treatment is performed in an atmosphere containing nitrogen, nitrogen is absorbed from the surface of the steel. This nitrogen increases γmax in the surface layer and contributes to making the surface layer a single phase of martensite. In order to utilize this effect, it is a necessary condition that the atmosphere contains nitrogen, but even if nitrogen is included, for example, in normal air, a large amount of oxygen is present and the surface is oxidized first, Not enough nitrogen is absorbed.
Therefore, it is necessary to keep the dew point at 0 ° C. or lower to suppress surface oxidation during high temperature heating. Specific examples of the atmosphere composition include 100% nitrogen and a nitrogen-hydrogen mixed atmosphere containing 10% or more nitrogen.

The A _C1 point is generally defined as the temperature at which an austenite phase begins to form when the steel is heated from a low temperature, and the ferrite single phase region on the low temperature side and the (ferrite + austenite) 2 on the higher temperature side. It is the boundary temperature of the phase region.
In the present invention, it is an absolute condition to heat to the _{AC 1} point or higher in order to obtain a mixed structure of ferrite and martensite after heat treatment. The A _C1 point of the steel of the present invention varies depending on the component balance, but is generally in the range of 600 to 900 ° C.
Since the above-mentioned absorption of nitrogen is accelerated at higher temperatures, the higher the heat treatment temperature is in that sense, the upper limit temperature is about 1200 ° C from the viewpoint of energy saving. The structure of the steel before applying the heat treatment of heating to A _C1 point or higher in this controlled atmosphere is not particularly limited, and may be a ferrite single-phase structure (some may include carbides) or ferrite and martensite. It may be a mixed organization with the site.

The present invention will be further described below with reference to examples.

[0031]

EXAMPLE Steels having the chemical composition shown in Table 1 were vacuum melted to produce a slab having a thickness of 165 mm, a width of 200 mm and a weight of 400 kg. This was heated to 1200 ° C and hot-rolled at 920 ° C to a plate thickness of 3.6 mm. After that, hot-rolled sheet annealing was performed by heating to 810 ° C for 6 hours, after descaling, it was cold-rolled to a sheet thickness of 0.7 mm.

Each of the obtained cold-rolled steel strips, except for steel No. 7, was heated to 1000 ° C. for 1 minute in a mixed atmosphere of 25% nitrogen and 75% hydrogen with a dew point of −40 ° C., and then cooled. Continuous annealing was performed.

Steel No. 7 is a martensitic stainless steel, SUS420J2, which was batch treated to 1000 ° C.
After heating by quenching for minutes and quenching, heating at 400 ° C for 30 minutes and tempering by air cooling were performed.

Steel Nos. 1, 2 and 4 were also subjected to other heat treatment conditions. That is, Steel Nos. 1 and 4 are 750 ° C in a mixed atmosphere of 25% nitrogen + 75% hydrogen with a dew point of -40 ° C.
After soaking for 1 minute (at a temperature not reaching the Ac ₁ point), continuous annealing was performed for cooling. Steel No. 2 was continuously annealed by heating it to 1000 ° C for 1 minute in the air and then cooling it. In addition, in all of those heated in the air, the oxide film on the surface was removed by pickling.

Specimens were taken from each of the obtained heat-treated steel strips, the microstructure of the cross section was examined, and the tensile properties, the cross-sectional hardness of the central portion of the plate thickness, and the scratch resistance were evaluated. The results are shown in Table 2 together with the heat treatment conditions. In addition, as for the scratch resistance, a chromium steel ball (JIS B1501, HV: 820) with a diameter of 6.3 mm can be loaded under a load of 100.
The sample was pressed against the sample surface at g and moved back and forth with a stroke of 25 mm, and the number of reciprocations at which scratches were generated on the sample surface was evaluated.

[0036]

[Table 1]

[0037]

[Table 2]

As can be seen from Table 2, all of Examples A to F of the present invention having the composition requirements and the structural requirements specified in the present invention exhibit high strength and good ductility, and have a scratch resistance. It is also excellent in sex. FIG. 1 shows, as a representative example, a photograph of the cross-sectional structure of the surface of the steel sheet of Inventive Example B. The white part is the ferrite phase, the black part is the martensite phase, the surface part is the single phase of the martensite phase, and the inside has a structure in which the ferrite and martensite phases are mixed. Recognize.

On the other hand, in Comparative Examples a and c, the compositional requirements are within the range specified in the present invention, but the heat treatment temperature is 750 ° C., which is below the A _C1 point of each steel. , The internal structure becomes a structure of ferrite + carbide, and therefore the strength is low. Also, at this heat treatment temperature, the amount of nitrogen absorbed was small, so that no martensite single-phase layer was formed on the surface layer, and the scratch resistance was poor.

In Comparative Example b, the heat treatment for heating to A _C1 point or more is performed in the atmosphere, so that the inside of the plate has a mixed multi-phase structure and is excellent in strength and ductility, but the martensite is formed in the surface layer portion. The single-phase layer is not formed and the scratch resistance is not sufficient.

Comparative Example d is martensitic stainless steel SUS420J2, which has high strength and excellent scratch resistance, but has extremely low elongation and poor workability.

The steel of Comparative Example e is intended for steel No. 8 having a γmax smaller than the range specified in the present invention, but martensite is not formed under the heat treatment conditions and the martensite of the surface layer portion is not formed. The site single phase layer is not formed either. As a result, it is inferior in strength and scratch resistance.

FIG. 2 is a photograph showing the surface of the sample (Comparative Example b) in which a flaw was generated by the flaw test, but a very clear scratch was observed.

[0044]

As described above, according to the present invention,
High-strength dual-phase stainless steel with good ductility, workability, wear resistance, and flaw resistance is provided in the form of steel plate or steel strip material. Applications of multi-phase stainless steel can be expanded in the field.

[Brief description of drawings]

FIG. 1 is a metallurgical micrograph showing a metal structure of a cross section of a steel sheet of Inventive Example B in Table 2.

FIG. 2 is a photograph showing the metallographic structure of the steel sheet surface when the steel sheet of Comparative Example in Table 2 was subjected to a flaw resistance test.

Claims (5)

[Claims] 1. Among various components contained in steel, C:
0.20% or less, Cr: 10.0 to 20.0%, Ni: 4.
A stainless steel containing 0% or less and N: 0.15% or less as an essential component and having a multi-phase structure in which 20 to 95 vol.% Of a martensite phase and the balance substantially a ferrite phase are mixed. , A high-strength dual-phase stainless steel in which the surface layer portion having a depth of at least 1 μm from the outermost surface is covered with a martensite single-phase layer. 2. The high-strength multiphase microstructure according to claim 1, wherein the contents of various components contained in the steel are adjusted so that the value of γmax represented by the following formula is 20 or more and 95 or less. Stainless steel, γmax = 420 (% C) +470 (% N) +23 (% N
i) +7 (% Mn) +9 (% Cu) -11.5 (% Cr)
-11.5 (% Si) +189. 3. The stainless steel according to claim 1, wherein the form of steel is a steel strip or a steel plate. 4. Among various components contained in steel, C:
0.20% or less, Cr: 10.0 to 20.0%, Ni: 4.
Nitrogen is added to stainless steel containing 0% or less and N: 0.15% or less as essential components, and the component amounts are adjusted so that the value of γmax represented by the following formula is 20 or more and 95 or less. A heat treatment of heating to a temperature of Ac ₁ point or higher in an atmosphere containing dew point of 0 ° C or lower and cooling from this heating temperature at a cooling rate sufficient to transform the austenite phase to the martensite phase. A method for producing a high-strength dual-phase stainless steel having a martensite single phase in the surface layer and a mixed microstructure of ferrite and martensite in the inside, γmax = 420 (% C) +470 (% N) +23 (% N
i) +7 (% Mn) +9 (% Cu) -11.5 (% Cr)
-11.5 (% Si) +189. 5. The process of heating to a temperature of Ac ₁ point or higher in an atmosphere in which the form of steel is a steel strip, which contains nitrogen and whose dew point is adjusted to 0 ° C. or lower, is carried out in a continuous annealing furnace for the steel strip. The manufacturing method according to claim 4.