JPH04202643A - Stainless steel having high strength and high toughness and its production - Google Patents

Abstract

PURPOSE:To easily provide high strength and high toughness by means of age hardening by specifying solution treatment temp., before final cold rolling, for a steel in which chemical components and the composition of nonmetallic inclusions remaining as impurities are specified, respectively. CONSTITUTION:A stainless steel has a composition consisting of, by weight, 0.050-0.150% C, 1.5-3.0% Si, 0.1-2.0% Mn, 4.0-8.0% Ni, 13.0-17.0% Cr, 1.0-3.0% Cu, 0.050-0.150% N, 0.0005-0.0050% S, 0.0005-0.0025% solAl, 0.0020-0.0130% O, and the balance Fe with inevitable impurities. The composition of nonmetallic inclusions remaining as impurities is regulated so that it lies within the region enclosed with a quadrilateral connecting the points 1-4 in the Al2O3-MnO-SiO2 ternary diagram. Solution treatment, before final cold rolling, for this steel is exerted at 900-1050 deg.C, by which austenite crystalline grain size number is regulated to 8-11.5. Strain induced martensite is formed by 60-95% by means of cold rolling. Then, age hardening is done by means of heat treatment at 400-600 deg.C for 0.5-5min.

Description

Detailed Description of the Invention "Objective of the Invention" (Industrial Application Field) The present invention is applicable to stainless steel springs and ultra-thin internal materials which have high strength as well as corrosion resistance, and which require excellent ductility and toughness such as stretchability. Peripheral cutter (Inner dia+neter 5a1
The present invention relates to a high-strength, high-toughness stainless steel used for the substrate of 1 blade (ID blade) and a method for manufacturing the same.

(Prior Art) Conventionally, semi-austenitic stainless steel, precipitation hardening stainless steel, and the like have been used as stainless steel spring materials used as ID blade substrates.

Semi-austenitic stainless steel exhibits an austenite phase in the solution treatment state, and is 5US301, SU
This is typified by S304. This type of steel can obtain high strength by producing strain-induced martensite through cold working. However, in order to obtain such high strength, it is necessary to increase the amount of cold working, and there are manufacturing problems such as increasing the capacity of the rolling mill. Further, when the cold rolling rate is increased, problems such as increased anisotropy and decreased toughness also occur. Japanese Patent Application Laid-open No. 62-60824 and Japanese Patent Publication No. 2-448 have been disclosed as this kind of rope.
Although 91 has gained considerable strength through work hardening, its tensile strength as cold-rolled reaches more than 180 kg f/m'', which poses a problem in rolling workability.It is also very important as an ID blade substrate. It is thought that the ductility and toughness are deteriorated because the ductility and toughness have not been evaluated and the morphology of inclusions has not been controlled.

Semi-austenitic precipitation-hardening stainless steel is produced by cold working after solution treatment to form a two-phase structure of an austenite phase and a martensitic phase, and then further precipitation hardening by aging heat treatment, and is typified by SO3631.

5US631 is Ni3Aj by adding AI! Because it is age hardened, alumina-based nonmetallic inclusions and AI! Inclusions such as N are likely to form. When used in an extremely thin and stretched state, cracks may occur starting from these inclusions, and this type of material cannot be used as an ID blade. Also, JP-A-61-29536 and JP-A-63-3176 have been disclosed as this kind of rope.
No. 28 was age-hardened by combined addition of Si and Cu, and H
A certain degree of high strength was obtained with v=580. but,
The ductility and toughness of overhangs, etc., are not at a satisfactory level, and attempts are being made to remove surface defects by polishing before final pressure adjustment, but this only complicates the manufacturing process and improves the toughness only slightly.

(Problem B to be Solved by the Invention) As described above, conventional high-strength stainless steels have had problems such as poor workability in manufacturing or significantly poor ductility and toughness. In other words, semi-austenite type JP-A-62-60
824 and Japanese Patent Publication No. 2-44891 obtain their strength through work hardening by cold rolling, so their strength as cold rolled is high and their rolling workability is poor.

In addition, semi-austenitic precipitation hardening type JP-A-61-2
9536 and JP-A-63-317628 have achieved considerable strength, but since the wood ductility and toughness have not been improved by controlling the shape of inclusions, surface defects may be the starting point when used as an ID blade. Problems such as cracking occurred.

(Means for Solving the Problems) The present invention was made to solve the above-mentioned problems, and is relatively easy to manufacture. The Company provides steel and its manufacturing method.

That is, in members such as ID blade substrates that are extremely thin and are used under tension stress, it is necessary to eliminate surface defects such as slivers and grain boundaries that are preferentially corroded during pickling. As a result of repeated research by the present inventors, we have found that nonmetallic inclusions that inevitably enter the Al zOt-Mn
Point 1 of O-5iO2 ternary system phase diagram (A A' g0
32Q%i%, MnO14wt%, Sing 66ht
%), point 2 (A/z(h 8.5 wt%, MnO
41wt%, Sing 50.5wt$) Point 3 (8l
zox 9wt%, MnO 44wt%, Si
ng 47wtX), point 4 (Al zch
By making the composition into a rectangular inner square region connecting 33wt%, MnO21ht%, and 5jCh40ht, the ductility and toughness of the high-strength material were dramatically increased, leading to the invention of high-strength and high-toughness stainless steel. The reason for the improvement in ductility and toughness is not only the reduction of surface defects such as slivers, but also
This is thought to be due to a reduction in stress concentration due to spherical inclusions inside the steel sheet.

In addition, in semi-austenitic precipitation-hardening stainless steel with combined addition of St and Cu, if the austenite grain size number is adjusted to 8 or more and 11.5 or less before final cold rolling, it will have extremely excellent age hardenability and will not only prevent overhang etc. We found that it has excellent ductility and toughness. Although the reason for this is not necessarily clear, it is thought that the amount of work hardening of austenite increases and martensite is uniformly and finely dispersed. In other words, austenite becomes stable due to grain refinement, martensitic transformation is suppressed in the initial stage of rolling, and as austenite is work hardened, lattice defects increase. Due to the increase in lattice defects in austenite and the fineness of austenite grains, martensite is uniformly and finely dispersed, and precipitates caused by subsequent aging are also uniformly and finely dispersed, which is thought to provide excellent age hardenability. It will be done.

Since the steel of the present invention has excellent age hardenability, it is possible to keep the hardness low before aging heat treatment, and the rolling workability is improved, as described below.

(11C: 0.050-0.150wt%, Si: 1.
5 to 3.0 wt%, Mn: 0.1 to 2.0 wt%,
Ni: 4.0-8.0ivt%, Cr: 13.0-17
．． 0wt%, Cu: 1.0-3.0wt%, N: 0.0
50-0.150wt%, S: 0.0005-0.00
50ivtz, sol, A1: 0.0005-0.
0025wt%, 0:0.0020~0.0130wt
%, the balance consists of Fe and unavoidable impurities,
The composition of the nonmetallic inclusions remaining as the inevitable impurities is surrounded by a quadrilateral connecting points 1, 2, 3 and 4 in the ternary phase diagram of the AlzO=-MnO-5iO2 system in Figure 1. High strength, high toughness stainless steel characterized by a composition in the region.

The manufacturing method thereof is as follows.

(2) Solution treatment of stainless steel having the composition described in item (1) above before final cold rolling to 900 to 1
The austenite grain size number is adjusted to 8 or more and 11.5 or less, and then 60 to 95% of deformation-induced martensite is generated by cold rolling, and then 0.5 to 0.5 at 400 to 600 °C. A method for producing high-strength, high-toughness stainless steel, characterized by age hardening by heat treatment for ~5 minutes.

(3) Solution treatment of stainless steel having the composition described in item (1) above before final cold rolling to 900 to 1
The austenite grain size number is adjusted to 8 to 11.5 at a temperature of 050°C, followed by cold rolling to generate 80 to 95% deformation-induced martensite, and then rolled at 400 to 600°C to 0.5 to 11.5. A method for producing high-strength, high-toughness stainless steel, which is characterized by age hardening by heat treatment for 5 minutes.

(Function) First, the reason for limiting the composition range of the steel of the present invention as described above will be explained as follows.

C is an austenite-forming element, and is required in an amount of 0.050 ivt% or more in order to suppress δ-ferrite and strengthen deformation-induced martensite. However, 0.
If it exceeds 150 wt%, Cr carbide will precipitate and corrosion resistance and toughness will deteriorate, so the content is set to 0.150 wt% or less.

1. Si is used for solid solution strengthening of austenite and deformation-induced martensite. : Mt% or more is required, but if added in excess, 6-ferrite will precipitate and hot workability will deteriorate, so the upper limit is set at 3.0 wt%.

Mn is an austenite-forming element, and is required in an amount of 0.1 wt% or more in order to form a single austenite phase through solution treatment. However, 2. If it exceeds 0.1w, the formation of deformation-induced martensite will be extremely suppressed.
The content was set to be t% or more and 2.0wt% or less.

Ni is an austenite-forming element, and is required in an amount of 4.0 wt% or more in order to suppress the formation of δ-ferrite and form a single austenite phase in the solution treatment state.

However, if it exceeds 8.0 wt%, austenite becomes stabilized and martensite cannot be obtained by processing at room temperature, so 4.
The content was set to be 0 wt% or more and 8.0 tst% or less.

Cr acts on corrosion resistance and is required in an amount of 13.0 wt% or more in order to characterize it as stainless steel. Also, 17.0
If it exceeds wt%, δ-ferrite is generated, so 13
．． 0-17. ht%.

Cu is required to be present in an amount of 1.0 wt% or more in order to cause age hardening, but since excessive addition deteriorates hot workability and causes cracking, the upper limit is set to 3.0 wt%.

N is 0.050-t to strengthen martensite
% or more, but adding too much can cause blowholes during casting, so it was set at 0.150 wt% or more.

S forms MnS and causes a decrease in ductility, so 0.0
It is necessary to keep it below 0.050 wt%. However, 0.00
Since it is practically difficult to set the content to less than 0.05 wt%, it was set to 0.0005% to 0.0050 wt%.

sol and Al determine the number and composition of nonmetallic inclusions, and if it exceeds 0.0025wt%, the oxygen content in the steel becomes 0.002(ht%t%), and the number of inclusions decreases, but the inclusions The composition is AI! 203 series, which causes surface defects and cracks during use. 0.0005wt
% less sol, Aj! Then, the amount of oxygen in molten steel is 0.
0130111t%, the number of inclusions increases, and the inclusion composition is high melting point MnO-3iO□ binary system, Cr,
O, and the hot stretchability decreases. Therefore, as shown in Figure 1, A with a low melting point and hot stretchability
sol, A1 is 0.0005 wt% or more, 0.00 to form lzOz-FInO-SiO2-based inclusions.
25wt% or less, and 0 is 0.0020 to 0.0130
-t%.

The remainder of the steel used in the present invention 6 other than the above-mentioned component elements is basically Fe, but Ca and REM are added for the purpose of controlling the form of sulfides and improving hot workability. It is possible to contain unavoidable impurities of the target B.

Steel with the above chemical composition exhibits austenite Mim in the solution treatment state. In order to make the strain-induced martensite during cold working fine, it is necessary to make the austenite grains fine by solution treatment before final rolling, so the solution treatment temperature is set to 1050'C or less. However, 900℃
If the temperature is lower than this, carbides and the like will precipitate and the toughness will decrease, so the solution treatment temperature was set at 900°C to 1050°C, and the austenite grain size was set at 8 or more and 1165 or less.

By performing cold working (rolling) of 30% or more after solution treatment, 60% or more of deformation-induced martensite is generated in a uniform and fine form. In addition, since the surface defects of the steel plate were eliminated by controlling the form of inclusions, even if the amount of martensite was increased, as shown in the example below, the decrease in toughness was slight, and in order to achieve high strength, the toughness decreased by more than 80%. It is extremely effective to set the amount of martensite to .

A high-strength stainless steel with excellent toughness can be obtained by performing heat treatment at 400° C. to 600° C. for a short time of 0.5 minutes or more and 5 minutes or less.

(Example) The present inventors melted 11 types of steel having chemical components as shown in Table 1 below. That is, steels A to F are steels that satisfy the provisions of the present invention, and steels 1 to 5 are comparative steels.

After hot rolling the above-mentioned steel in Table 1, pickling, cold rolling, and solution treatment are repeated, and the solution treatment before the final cold rolling is carried out once.
The austenite crystal grain size was measured at 000°C for 1 minute. After that, it was cold rolled at each cold rolling rate shown in Table 2, and the plate thickness was 0.
．． A 1 m steel plate was used.

Then, an aging treatment was performed at 425° C. for 1 minute. The Vickers hardness was measured to determine the strength of the obtained steel plate, and a small punch test was conducted to determine the toughness and ductility, and the amount of work required to reach breakage was measured.

In addition, the amount of martensite was measured by XvA diffraction. The results are shown in Table 2.

As is clear from Table 2 above, the inventive steel A-F has a ΔHv (age hardening amount - hardness after aging - hardness as rolled) of 60 or more, and Hv after aging is 550 or more. High strength can be obtained. Moreover, the hardness of the steel of the present invention as cold rolled is 500 Hv or less, which is advantageous for rolling extremely thin and wide materials. In addition, the punch test workload is 20kgf.
-m or higher toughness.

On the other hand, in comparison fjll~3, the age hardening amount ΔHν is 50 or less, and in order to make the hardness Hv after aging 550 or more, the hardness Hv as cold rolled must be 500 or more, which causes problems in the rolling operation. . In addition, in Comparative Steels 4 and 5, the age hardening amount is ΔHv of 60 or more, which is comparable to that of the steel of the present invention, but since the amounts of sol, Al, and 0 are outside the range specified by the present invention, many spherical inclusions are formed. However, the punch test work load was less than 15 kgf-m, and the toughness was poor.

FIG. 2 shows the number of spherical inclusions after hot rolling of the present invention @A-F and comparative steels 4 and 5 with respect to the sol and Al amounts. In addition, here, the ratio d/l of the width d in the direction orthogonal to the rolling direction and the length l along the rolling direction is
0.3 or more are considered spherical inclusions. In the range of sol and Aji amounts of 0.0005 wt% or more and 0.0025 wt% or less as specified in the present invention, spherical inclusions are
/cIA or less, and most of the inclusions are A 1 t
Al z03-MnO containing about 5 to 20% o3
The -5iO□-based inclusions were elongated in the rolling direction.

On the other hand, Comparative Steels 4 and 5, which had sol and AI contents outside the specified ranges of the present invention, had more than 100 pieces/- of spherical inclusions, and had a high Alzo's content.

Figure 3 shows 2 for the invention steels A-F and comparative steels 4 and 5.
The amount of punch test work shown in the table is shown in relation to the amounts of sol and Al. In addition, all hardnesses are Hv=560
That's about it. As shown in FIG. 2, β20. Comparative steels 4 and 5, which have a content outside the range specified by the present invention, have deteriorated toughness because they contain many spherical inclusions, and the punch test work is 15 kgf·n or less.

Figure 4 shows that the present invention 111A and comparative steel 4 were subjected to solution treatment at 1000°C for 1 minute, cold rolled at various rolling rates to a plate thickness of 0.1 mm, and then treated at 425°C for 1 minute. This figure shows the relationship between the amount of martensite, hardness, and punch test work when aging heat treatment is performed. The hardness of both steel types increases with the increase in the amount of martensite, and the inventive steel A and comparative steel 4 show comparable values.

On the other hand, the punch test work decreases as the martensite content increases for both steel types, but in the invention steel A, the punch test work hardly changes when the martensite content exceeds 70%. is 9
Even if it reaches 5%, a punch test workload of about 20 kgf-m can be obtained. On the other hand, in Comparative Steel 4, which contains many spherical inclusions, when the amount of martensite exceeds 70%, the punch test work decreases rapidly, and when the amount of martensite exceeds 90%.
If it exceeds , the punch test workload becomes almost 0.

Figure 5 shows the austenite grain size of invention steel A changed by solution treatment before final cold rolling, then 40% cold rolling to obtain plate thicknesses of o and iw, and then aging at 425°C for 1 minute. The punch test work amount when heat treatment is performed is shown in relation to the austenite grain size number. When the grain size is 11.5 or more, carbides and the like precipitate, resulting in a decrease in toughness. Furthermore, when the austenite grains become so coarse as to have a particle size number of less than 8, the deformation-induced martensite caused by subsequent cold rolling does not become fine, and the toughness decreases. On the other hand, in the range of the present invention where the austenite grain size number is 8 or more and 11.5 or less, high toughness is achieved with a punch test work of 20 kgf-m or more.

"Effects of the Invention" As explained above, according to the present invention, cold rolling can be performed with good workability, and high strength and high toughness stainless steel can be obtained by aging heat treatment, which can be used for stainless steel springs and poles. This invention can provide a steel plate suitable for use as a substrate for a cutter with a thin inner circumferential blade, and is industrially highly effective.

[Brief explanation of the drawing]

Figure 1 shows 74120. -MnO-SiO2 system ternary phase diagram, the liquidus temperature of inclusions and point 1 (A
12Q. 20-t%, MnO 14wt%, 5i(h 66wt%
), point 2 (A' zO+ 8.5wt%, MnO41
wt%, SiO2 50.5wt%), point 3 (Aj!
zo39wt%, MnO44ivt%, Sin, 47
wtχ), point 4 (A 1 z(h 33wt%, Mn
Figure 2 shows the number of spherical inclusions, sol, and l amount after hot rolling of Example steel according to the present invention and Comparative steels 4 and 5. A chart showing the relationship, FIG. 3, shows the punch test workload, sol, Aj of the example steel according to the present invention and comparative steels 4 and 5.
A diagram showing the relationship between the amount of i, FIG. 4 is Example Steel A according to the present invention.
Comparative steel 4 martensite content and aging treatment (425℃×
Figure 5 is a graph showing the relationship between hardness after 1 minute) and punch test workload, and Figure 5 is a graph showing the relationship between hardness after 1 minute) and punch test workload, and Fig. 5 is a graph showing the relationship between hardness after 1 minute) and punch test workload. , which is a chart showing the punch test workload of the specimens further subjected to aging treatment at 425° C. for 1 minute versus austenite grain size number. Figure 1 Figure 2 sot At lp, pffLl Figure 3 sol AI Tpp31 Figure 5 Austenite grain size number

Claims (3)

[Claims] (1), C: 0.050-0.150wt%, Si: 1
．． 5 to 3.0 wt%, Mn: 0.1 to 2.0 wt%, N
i: 4.0 to 8.0 wt%, Cr: 13.0 to 17.0
wt%, Cu: 1.0 to 3.0 wt%, N: 0.050
~0.150wt%, S:0.0005~0.0050wt%, sol. Al: 0.0005-0.0025wt%, O
:0.0020 to 0.0130wt%, the balance consists of Fe and unavoidable impurities, and the composition of the nonmetallic inclusions remaining as the unavoidable impurities is the Al_2O_3-MnO-SiO_2 system ternary phase diagram shown in Figure 1. A high-strength, high-toughness stainless steel characterized by having a composition of a region surrounded by a quadrilateral connecting points 1, 2, 3, and 4. (2) Solution treatment of stainless steel having the composition according to claim 1 before final cold rolling to 900 to 105
The austenite grain size number is adjusted to 8 or more and 11.5 or less at a temperature of 0°C, followed by cold rolling to generate 60-95% deformation-induced martensite, and then 0.5% at 400-600°C. A method for producing high-strength, high-toughness stainless steel, characterized by age hardening by heat treatment for ~5 minutes. (3) Solution treatment of stainless steel having the composition according to claim 1 before final cold rolling to 900 to 105
The austenite grain size number is adjusted to 8 or more and 11.5 or less, followed by cold rolling to generate 80 to 95% deformation-induced martensite, and then 4
A method for producing high-strength, high-toughness stainless steel, which comprises aging hardening by heat treatment at 00 to 600°C for 0.5 to 5 minutes.