Process for preparing a high strength stainless steel having excellent workability and free form weld softening

Abstract

A stainless steel which exhibits substantially martensitic structure at room temperature was heated at a temperature of 550.degree. to 675.degree. for 1 to 30 hours. Then a reverse-transformed austenite phase appeared and a stainless steel having high strength and high elongation and being free from weld softening was obtained.

Description

FIELD OF THE INVENTION

This invention relates to a high strength stainless steel material having excellent workability and resistance to softening by welding.

BACKGROUND OF THE INVENTION

Conventional high strength stainless steels are roughly classified into (1) martensitic stainless steels, (2) work-hardenable austenitic stainless steels, and (3) precipitation-hardenable stainless steels.

Martensitic stainless steels mainly comprise Fe-Cr-C system and are substantially of single austenitic phase at the quenching temperature (which is 900.degree.-1100.degree. C., but varies depending on the content of Cr and C), but their martensite start point (Ms point) is higher than the room temperature range and they are so-called quench-hardenable steels.

These steels are hard and poor in workability in the quenched state or the quenched and tempered state. Therefore, in these steels, working such as bending, machining and cutting is carried out in the annealed state and high strength is provided by a heat-treatment such as quenching and tempering after the steel is shaped as desired. However, heat-treatment of large parts or members is difficult, and these steel materials are susceptible to weld cracking, and, therefore, tempering must be carried out after welding.

When martensitic stainless steels are to be used as structural members, the above-mentioned defects must be compensated for. To this end, a steel in which the C content is restricted lower so that a massive martensite phase appears in the quenched state has been considered. The steel of Japanese Patent Publication No. 51-35447 (1976) is an example of such a steel. A steel which falls within the claim of said patent publication is presented in No. 33 of "Nisshin Seiko Giho (Technical Reports of Nisshin Steel Co.)" (December 1975 issue). The composition thereof is: C: 0.032%, Si: 0.75%, Mn: 0.14%, Ni: 4.01%, Cr: 12.4%, and Ti: 0.31%. This material has a tensile strength of about 108 kgf/mm.sup.2 and an elongation of about 6%, and that is very low in weld softening. Although low weld softening and high tensile strength are desirable for a welded structural material, the steel is still unsatisfactory as a structural material to be worked since elongation is poor and cracking easily occurs even in light working.

Work-hardenable austentic stainless steels have the metastable austenitic phase as represented by AISI 301, 201, 304, 202, etc., and are hardened by cold working. Mechanical properties attained by this cold working are stipulated in JIS G 4307. For instance, in 1/2H of AISI 301, it is specified that yield strength is not less than 77 kgf/mm.sup.2, tensile strength is not less than 105 kgf/mm.sup.2 and elongation is not less than 10%. That is, both tensile strength and elongation are specified as being high. However, the materials of this class have a defect in that when they undergo heat input such as welding, the heated part or weld softens. Also in some cases, chromium carbide deposit in the part heated by welding, and chromium-poor layers are formed and thus intergranular stress corrosion cracking occurs.

Precipitation-hardenable stainless steels are classified into martensite type, ferrite type and austenite type in accordance with the structure of the matrix. But all of them contain at least one of Al, Ti, Nb, Cu, Mo, V, etc., which contribute to age-hardening, and the steels are hardened by precipitation of intermetallic compounds caused by aging from the super-saturated solid-solution state. These steels have a tensile strength of 140-190 kgf/mm.sup.2 and an elongation of 2-5%, depending upon the state of the matrix, contents of the elements which contribute to age-hardening, etc.

When these steels are used for structural members, generally working and welding are effected prior to age-hardening. However, it is difficult to age-harden larger structural members.

As has been described, the materials conventionally known as high strength stainless steels do not possess all of strength, workability and resistance to weld softening.

The object of the present invention is to provide a novel high strength steel material free from the above-described defects. The object is achieved by heating a steel material of a martensitic structure, which is in a specific composition range and that satisfies a specific composition relationship, to cause reverse austenitic transformation and stabilize the thus formed reverse-transformed austenite phase.

SUMMARY OF THE INVENTION

This invention provides process for preparing a high strength stainless steel material having excellent workability free from weld softening consisting of a single martensitic phase or a duplex phase structure of martensite and minute austenite, said process comprising heat-treating at a temperature of 550.degree. to 675.degree. C. for 1 to 30 hours a cold-rolled material of a steel essentially consisting of:

C: not more than 0.10% Si: 0.85-4.5% Mn: 0.20-5.0% P: not more than 0.060% S: not more than 0.030% Cr: 10.0-17.0% Ni: 3.0-8.0% N: not more than 0.10% and Fe and inevitable incidental impurities, wherein the Ni.sub.eq value defined as: Ni.sub.eq =Ni+Mn+0.5Cr+0.3Si+20(C+N) is in the range of 13.0-17.5.

This invention also provides processes for preparing similar steel materials using steels which contain in addition to the above-described components not more than 4% in total of at least one of Cu, Mo, W, and Co and/or not more than 1% in total of at least one of Ti, Nb, V, Zr, Al and B, wherein the definition of Ni.sub.eq is modified in accordance with the composition.

When at least one of Cu, Mo, W and Co is contained, the Ni.sub.eq value is defined as:

Ni.sub.eq =Ni+Mn+0.5Cr+0.3Si+20(C+N)+Cu+Mo+W+0.2Co

When at least one of Ti, Nb, V, Zr, Al and B is contained, the Ni.sub.eq value is defined as:

Ni.sub.eq =Ni+Mn+0.5Cr+0.3Si

When at least one of Cu, Mo, W and Co and at least one of Ti, Nb, V, Zr, Al and B are contained, the Ni.sub.eq value is defined as:

Ni.sub.eq =Ni+Mn+0.5Cr+0.3Si+Cu+Mo+W+0.2Co

The steel preferably contains 0.005-0.08% and more preferably 0.010-0.06% C; preferably 0.85-4.00% Si; preferably 0.30-4.50% and more preferably 0.40-4.0% Mn; preferably not more than 0.040% and more preferably not more than 0.035% P; preferably not more than 0.02% and more preferably not more than 0.015% S; preferably 11.0-16.0% and more preferably 12.0-15.0% Cr; preferably 3.5-7.5% and more preferably 4-7.5% Ni; preferably not more than 0.07% and more preferably not more than 0.05% N; preferably 0.5-3.5% and more preferably 1.0-3.0% of at least one of Cu, Mo, W and Co when contained; and preferably 0.1-0.8% and more preferably 0.15-0.8% of at least one of Ti, Nb, V, Zr, Al and B when contained.

The above-mentioned steel for the process of the present invention exhibits substantially martensitic structure in the cold-rolled state as a result of adjusting the composition so that the Ni.sub.eq value as defined above is in the above-defined range.

This invention is based on the inventors' finding that the above-mentioned steel, as cold-rolled, undergoes reverse austentic transformation and stabilized by heat-treating the steel at a temperature of 550.degree.-675.degree. C. for 1-30 hours. The mechanism involved and reason for it are not yet well understood, but it has been confirmed that this reverse austenitic transformation occurs with reproducibility. Modification of the properties of stainless steel of martensitic structure by such a treatment has never been attempted before.

The steel material of the present invention exhibits a strength level of about 100 kgf/mm.sup.2 and an elongation of about 20%, and does not suffer from weld softening.

The reason why the composition of the steel is defined as defined in the claim in the present invention is as follows:

C: C is an austenite former, and effective for formation of austenite phase at high temperatures, and is also effective for strengthening the reverse transformed austenite phase and martensite phase after the heat treatment. However, a larger amount of C impairs elongation, and deteriorates corrosion resistance of the weld. Therefore, it is limited to 0.10%.

N: Like C, N is an austenite former, effective for formation of the austenite phase at high temperatures, and also hardens the reverse transformed austenite phase, and is therefore, effective for strengthening the steel. However, a larger amount of N deteriorates elongation. Therefore, N is limited to 0.1%.

Si: Si is effective for strengthening the reverse transformed austenite after the heat treatment and is effective for broadening the allowable temperature range for heat treatment. For this purpose, at least 0.85% si is required. However, a larger amount of Si promotes solidification cracking when the steel is solidified or welded. Therefore, the upper limit of the Si content is defined as 4.5%.

Mn: Mn is an austenite former and necessary for adjustment of the Ms point. For this purpose, at least 0.2% Mn is required. But a larger amount of Mn causes troubles in the course of steelmaking and therefore its upper limit is defined as 5%.

Cr: Cr is a fundamental component for providing the steel with corrosion resistance. However, with less than 10%, no effect can be expected, while more than 17% of Cr requires a larger amount of austenite former elements in order to produce a single austenite phase at high temperatures. The upper limit of Cr is defined as 17% so that the desired structure is obtained when the steel is brought to room temperature.

Ni: Ni is an austenite former, and is necessary for obtaining a single austenite phase at high temperatures and adjustment of the Ms point. The Ni content depends on the contents of the other elements. At least about 3% of Ni is required for obtaining a single austenite phase at high temperatures and adjustment of the Ms point. Even if the contents of the other elements are reduced, more than 8% of Ni does not give the desired structure.

P: P is an inevitable impurity element incidental to principal and auxiliary raw materials. P makes steels brittle and therefore it is limited to 0.060% at the highest.

S: S is also an inevitable impurity element incidental to principal annd auxiliary raw materials in steelmaking. S also makes steels brittle and therefore it is limited to 0.030% at the highest.

Cu: Cu is inherently effective for improving corrosion resistance. In the present invention Cu is effective for lowering the Ms point. However, if it is contained in an amount in excess of about 4%, workability at high temperature is impaired. Therefore, its content is limited to 4%.

Mo: Mo improves corrosion resistance and is effective for strengthening the reverse transformed austenite and lowering the Ms point. However, Mo is an expensive element and its content is limited to 4% in consideration of the cost of the steel.

W: W is effective for improving corrosion resistance and strength of the steel, and is also effective for lowering the Ms point. However, the upper limit is defined as 4%, since it raises the cost of the steel if it is contained in a larger amount.

Co: Co has a high austenitizing effect at the high temperature range, and lowers the Ms point. (Although this element has high austenitizing effect, it does not lower the Ms point excessively.) Co is very effective for adjustment of composition in a high Cr content system. But the upper limit on the content thereof is defined as 4%, since it raises the cost of the steel if it is contained in a larger amount.

The last four elements mentioned above improve corrosion resistance and are effective for adjusting the martensite-forming ability of the steel in relation with the other components. They are equivalent in this sense.

Ti: Ti is a carbide-former and effective for preventing formation of Cr-poor layers caused by deposition of the carbide in welding and inhibition of grain growth of the reverse transformed austenite phase. However, if this is contained in a large amount, it may cause surface defects and may form a larger amount of scum in welding. Therefore, the Ti content is limited to 1%.

Nb: Nb is effective for preventing formation of Cr-poor layers caused by precipitation of Cr carbide in welding and inhibition of grain growth of the reverse transformed austenite phase. If it is contained in a larger amount, however, it promotes solidification cracking when cast or welded, and also impairs ductility of the steel material. Therefore its content is limited to 1%.

V: V is effective for preventing formation of Cr-poor layers and inhibition of grain growth of the reverse transformed austenite. If it is contained in a larger amount, however, it impairs ductility of the steel. Therefore, its content is limited to 1%.

Zr: Zr is effective for preventing formation of Cr-poor layers caused by deposition of carbide in welding and inhibition of grain growth of the reverse transformed austenite phase. If it is contained in a larger amount, however, oxide type non-metallic inclusions are formed in casting and welding, and the surface properties and ductility of the steel are impaired. Therefore, its content is limited to 1%.

Al: Al has a remarkable effect for fixing N in the molten steel and inhibiting grain growth of the reverse transformed austenite phase. If it is contained in a larger amount, it impairs flow of the molten metal in welding and thus makes the welding operation difficult. Therefore, the Al content is limited to 1%

B: B is effective for inhibition of grain growth of the reverse transformed austenite and improvement of hot workability of the steel. If it is contained in a larger amount, however, it impairs ductility of the steel. Therefore, its content is limited to 1%.

The last six elements mentioned above are carbide formers, and remarkably effective in inhibiting grain growth of the reverse transformed austenite. In this sense, these six elements are equivalent.

The reason for defining the nickel equivalent (Ni.sub.eq) as defined in the claims is as follows. In the steel used for the present invention, the temperature at which the martensite transformation is finished must be around room temperature (150.degree.-10.degree. C.). The steel used in the process of the present invention is of single austenite phase in the temperature range to which the steel is exposed during hot rolling, annealing or welding. But the steel must be substantially transformed into the martensite structure when the steel is brought down to room temperature from the above-mentioned condition. Here the term "substantially" means that a small amount (approximately 25%) of austenite may be retained. The amount of such remaining austenite need not be strictly considered.

In the steel used in the present invention, various elements are alloyed. We have found that insofar as the composition of the steel falls within the above-described composition range and that the nickel equivalent (Ni.sub.eq) thereof as defined above is in the above-described range, the steel is of substantially martensite structure at room temperature and the object of the invention as described in the beginning of this specification is achieved.

That is to say, even through the composition is within the above-defined range, if the nickel equivalent is less than 13, the Ms point is too high and the desired high elongation cannot be obtained even if the steel is heat-treated as defined above. If the nickel equivalent is greater than 17.5, the steel softens at the weld when it is welded, and thus the desired high strength members cannot be obtained. Needless to say, the formula for Ni.sub.eq was defined by considering the degree of contribution of each element to the austenite-martensite transformation and thus determining each coefficient as the equivalent of the Ni amount in comparison with the degree of the contribution of Ni. Ti and the five elements that follow are neutral with respect to the above-described property, and that cancel the austenite-forming ability of C and N. Therefore, in the steels which contain these elements, these elements and C and N are not taken into consideration.

The reason for defining the heat treatment conditions as defined in the present invention is as follows.

The steels which are of the martensite structure (massive martensite) in the annealed state have around 100 kgf/mm.sup.2 of tensile strength. But as their elongation is about 6% at the utmost, it cannot be said that they have satisfactory workability. When the steels are kept at a temperature in a range of 550.degree.-675.degree. C. for 1-30 hours so that part of martensite is reverse-transformed to austenite, the thus formed austenite is more or less stable as a structure, not all thereof returns to martensite in the cooling that follows, and may remain as austenite. At any rate, this heat-treatment confer high ductility to the steel without remarkably lowering strength (yield strength). At temperatures lower than 550.degree. C., the heat treatment does not effectively bring about this ductility, and at temperatures higher than 675.degree. C., yield strength as well as ductility are impaired.

The time of the heat treatment is suitably selected by taking the size of the material to be treated into consideration. A heat treatment over 30 hours is disadvantageous since it raises the cost of the steel.

The steel material of the present invention is suitable for manufacturing structural parts and members as well as steel belt. The steel material possesses high strength, high ductility and does not suffer weld softening.

Now the invention will be explained specifically by way of working examples with reference to the attached drawings.

BRIEF EXPLANATION OF THE ATTACHED DRAWINGS

FIG. 1 is a flow chart illustrating preparation of samples in the present invention, and FIG. 2 is a diagram showing the softening at the weld in samples of the present invention and comparative examples.

DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

Sample steel heats were prepared using a vacuum high frequency furnace of 30 kg capacity by the usual process, and cast into ingots 110.times.110 mm at the bottom plane, 120.times.120 mm at the top plane and 290 mm in height. The ingots were forged into plates 35 mm in thickness and 155 mm in width at 1250.degree. C., and the plates were machined into plates measuring 30 mm.times.150 mm. The plates were heated at 1250.degree. C. in a soaking pit and thereafter hot-rolled to 6 mm of thickness. A portion thereof was tested as hot-rolled samples (a), and the other portion was annealed at 1030.degree. C. for 10 minutes, pickled and cold-rolled into sheet of 1 mm thickness (83% reduction), a portion thereof was tested as cold-rolled sample (b). The remaining portion was cold-rolled to 2 mm thick sheets and further cold-rolled after intermediate annealing to 1 mm thick sheets (50% reduction) and a portion thereof was tested as 50% reduction cold-rolled sheet samples (c). The remaining portion was further annealed at 1030.degree. C. for 1.5 minutes and pickled. These were tested as annealed samples (d). Cold-rolled samples (b) and (c) were made according to the process of the invention. Procedures of preparing samples are illustrated in FIG. 1.

The compositions of the samples of this invention and the comparative samples are indicated in Table 1. Sample Nos. 4, 14-16, 24-28, and 30-32 are steels having a silicon content within the desired compositional range used in the process of this invention and Nos. A-F are steels of comparative examples. The compositions of these samples are similar to the defined composition range, but the nickel equivalent Ni.sub.eq of Samples A-D are less than 13 and those of Samples E-F is greater than 17.5.

Mechanical property tests were carried out using test pieces No. 5 and No. 13B stipulated in JIS Z 2201.

The amount of martensite was measured using a vibrating sample magnetometer.

Mechanical properties and the amount of martensite of the samples are summarized in Table 2. In Table 2, "Conventional Process" means that the heat treatment in accordance with this invention was not carried out.

According to Table 2, the steels which were not heat-treated in accordance with the present invention and exhibit a substantially massive martensite structure in the annealed state have high level strength such as yield strengths of 73-126 kgf/mm.sup.2 and tensile strengths of 94-135 kgf/mm.sup.2, but their elongation is at the utmost 7.0%. This is remarkably low in comparison with Sample E and F, which are 20% cold-rolled sheets. Even among the samples which underwent the heat treatment of the present invention, those of the comparative steels have only 8.5% elongation at the highest, though even this is some improvement. The samples of the present invention exhibit generally remarkable improvement in elongation while retaining yield strength, although some samples suffer slight decrease in yield strength.

The mechanical properties and the amounts of martensite when annealed samples (d) were heat-treated under various conditions are shown in Table 3. "Comparative Process" in Table 3 means examples in which samples were heat-treated at temperatures in excess of the heat treatment temperature range of the present invention. From Table 3, it is learned that there is a criticality for annealed samples (d) around the upper limit heat treatment temperature of 675.degree. C.

The welding test was carried out by laying a bead on 1 mm thick plates by TIG welding with 50 A electric current at a rate of 400 mm/min. The results are shown in FIG. 2. FIG. 2 shows hardness distribution profile from the center of beads. Sample 19 and 25, were heat-treated at 600.degree. C. for 20 hours. Comparative Sample E and F are 20% cold-rolled sheets. As seen in this figure, the sample 25 of the present invention obviously does not exhibit softening at the weld.

TABLE 1__________________________________________________________________________Chemical Composition and Ni.sub.eq of the Steels for the InventionProcess and Comparative Steels Composition (wt %)Sample Ti, Nb, Al,No. C Si Mn P S Cr Ni N Cu, Mo, Co, W Zr, B, V Ni.sub.eq__________________________________________________________________________Steels 1 0.060 0.25 1.58 0.027 0.009 12.96 3.52 0.012 13.1for the 2 0.010 0.27 1.14 0.031 0.010 13.04 4.00 0.076 13.5Invention 3 0.013 0.22 0.36 0.029 0.007 12.77 7.43 0.019 14.9Process 4 0.045 2.07 0.37 0.034 0.014 13.03 6.25 0.012 14.9 5 0.021 0.54 0.47 0.021 0.009 16.99 5.01 0.015 Co: 3.01 15.2 6 0.011 0.24 0.31 0.029 0.012 15.09 6.03 0.010 Cu: 2.12 16.5 7 0.007 0.28 0.27 0.019 0.007 12.91 7.47 0.011 W: 1.84 16.5 8 0.006 0.22 0.30 0.024 0.005 12.37 6.69 0.013 Mo: 2.60 16.2 9 0.019 0.41 0.33 0.027 0.004 13.82 7.12 0.014 Cu: 0.76, W: 1.08 17.0 10 0.013 0.26 3.80 0.020 0.006 12.87 3.03 0.019 Ti: 0.27 13.3 11 0.014 0.28 4.69 0.035 0.013 12.90 3.02 0.018 Ti: 0.16 14.2 12 0.030 0.25 2.87 0.022 0.009 12.99 4.98 0.015 Ti: 0.08 14.4 13 0.014 0.92 0.38 0.026 0.007 13.04 7.38 0.015 Ti: 0.15 14.6 14 0.011 2.02 0.37 0.030 0.012 13.04 7.31 0.014 Ti: 0.28 14.8 15 0.034 2.15 0.31 0.027 0.008 12.92 6.67 0.020 Nb: 0.41 14.1 16 0.026 0.85 0.30 0.031 0.010 15.62 6.94 0.013 Nb: 0.50 15.3 17 0.014 0.37 0.29 0.036 0.014 14.21 7.02 0.015 Ti: 0.67 14.5 18 0.020 0.65 0.39 0.029 0.006 14.08 6.60 0.027 Al: 0.77 14.2 19 0.015 0.32 0.46 0.025 0.004 13.87 7.00 0.016 B: 0.39 14.5 20 0.010 0.35 0.40 0.037 0.006 13.64 7.03 0.014 V: 0.47 14.4 21 0.011 0.30 0.35 0.026 0.011 13.72 6.89 0.010 Zr: 0.59 14.1 22 0.030 0.41 0.40 0.034 0.008 13.61 6.91 0.010 Ti: 0.50, Nb: 14.2 23 0.028 0.35 0.42 0.028 0.006 13.57 7.04 0.010 Ti: 0.32, Al: 14.4 24 0.056 1.90 0.39 0.021 0.005 13.06 6.25 0.013 Ti: 0.10, V: 13.8 25 0.040 1.44 0.29 0.018 0.006 14.61 7.36 0.010 Cu: 1.01 Ti: 0.49 16.4 26 0.038 1.52 0.21 0.027 0.011 13.87 7.02 0.008 Cu: 0.68, Mo: 1.02 Ti: 0.37 16.3 27 0.007 3.05 0.31 0.024 0.008 13.16 6.33 0.015 Mo: 1.17 Ti: 0.10, Nb: 15.3 28 0.007 2.04 0.30 0.032 0.012 12.18 5.40 0.011 Cu: 2.03 Nb: 0.45 14.4 29 0.013 0.37 0.26 0.028 0.008 13.09 7.00 0.010 W: 0.82 Ti: 0.58 14.7 30 0.010 2.56 0.24 0.028 0.005 14.56 6.37 0.009 Co: 2.31 Al: 0.70 15.1 31 0.040 1.39 0.27 0.037 0.009 12.82 6.12 0.014 Cu: 0.91, W: 1.97 Ti: 0.81 16.1 32 0.026 4.12 0.30 0.023 0.008 13.12 7.12 0.014 16.0Compara- A 0.035 0.21 0.16 0.021 0.004 11.79 4.42 0.009 Ti: 0.27 10.5tive B 0.046 0.31 0.21 0.018 0.006 11.52 5.01 0.013 Nb: 0.40 11.1Steels C 0.009 0.45 0.40 0.021 0.004 11.72 5.26 0.011 12.1 D 0.014 0.28 1.32 0.019 0.007 10.86 3.97 0.025 11.6 E 0.013 0.57 1.49 0.028 0.007 17.53 7.40 0.094 20.0 F 0.058 0.51 1.14 0.025 0.005 17.44 7.10 0.070 19.7__________________________________________________________________________ TABLE 2__________________________________________________________________________Mechanical Properties and Amount of Martensite of Comparative Products Conventional Process As Annealed and As Hot-Rolled Processes (d) and (a) As Annealed (d) 600.degree. C. .times. 10 hr. (As Annealed (d)) 600.degree. C. .times. 10 hr. (a)) .sigma..sub.0.2 .sigma..sub.B .sigma..sub.B .sigma..sub.0.2 .sigma..sub.B Sample4 (kg/ (kg/ El mar. .sigma..sub.0.2 (kg/ El mar. (kg/ (kg/ El mar. No. mm.sup.2) mm.sup.2) (%) Hv (%) (kg/mm.sup.2) mm.sup.2) (%) Hv (%) mm.sup.2) mm.sup.2) (%) Hv (%)__________________________________________________________________________Products 1 126 135 4.3 394 100 98 110 13.6 332 97 101 116 15.2 337 96of the 2 114 121 4.7 362 100 91 107 13.7 318 92 90 107 14.8 319 92Invention 3 77 100 5.9 328 100 71 84 17.4 271 87 73 87 16.8 280 88Process 4 88 129 7.0 331 100 74 93 18.7 294 85 76 90 17.9 301 86 5 76 99 5.7 304 97 72 94 16.2 300 86 72 90 16.5 297 85 6 75 96 5.2 297 93 69 89 18.4 288 79 70 91 18.6 291 76 7 81 97 5.2 300 94 74 97 17.7 305 72 74 95 17.1 298 71 8 80 94 6.1 307 98 77 89 18.2 291 80 75 91 18.3 293 79 9 82 96 5.8 297 90 79 91 17.9 298 73 78 92 18.1 296 75 10 77 96 4.0 303 100 77 83 16.1 273 95 75 83 16.4 275 94 11 75 98 5.8 293 95 75 80 16.7 303 80 74 81 17.1 272 81 12 73 94 6.2 306 94 70 91 18.4 275 73 70 93 18.76 289 75 13 74 106 6.4 322 99 71 91 18.9 272 81 73 94 18.2 287 79 14 75 103 6.4 315 98 71 90 19.7 289 84 73 89 19.0 286 82 15 91 118 5.7 336 94 76 92 18.2 301 79 78 93 18.5 293 79 16 83 105 6.2 319 92 75 94 17.6 304 72 75 96 17.9 300 72 17 86 108 4.2 322 100 79 98 17.4 312 93 78 98 18.1 315 91 18 89 109 5.1 327 100 82 97 18.2 317 85 83 99 18.7 304 85 19 92 116 4.7 334 96 80 102 16.8 321 82 81 99 16.8 319 82 20 91 114 4.2 329 100 78 99 17.3 306 91 78 95 17.6 304 92 21 98 111 4.0 317 100 85 109 18.6 330 94 86 101 19.2 326 93 22 79 98 5.6 299 100 74 89 19.3 276 96 75 88 19.0 281 95 23 84 103 6.1 309 100 72 91 18.5 284 95 70 85 18.2 276 93 24 96 129 6.2 336 91 74 93 18.1 306 74 74 91 18.7 283 78 25 93 115 3.9 327 91 85 124 16.1 323 70 85 119 16.2 311 73 26 95 119 4.2 333 93 82 119 17.9 320 71 81 108 17.4 306 70 27 95 121 5.0 327 97 84 116 16.8 319 79 83 112 17.0 311 78 28 98 126 4.7 337 100 87 112 17.5 325 96 86 115 18.0 323 91 29 83 110 5.1 316 100 78 95 18.1 307 94 80 95 17.7 311 93 30 94 120 5.7 329 98 83 107 17.3 312 85 82 105 17.9 320 83 31 87 114 6.2 324 94 80 97 18.4 305 81 76 93 19.0 298 74 32 89 132 3.9 395 91 105 114 16.3 369 90 98 113 16.7 358 92Compara- A 94 105 6.0 330 100 82 97 7.3 302 100 82 95 6.8 297 100tive B 96 103 5.7 326 100 85 95 7.4 304 100 83 96 7.0 300 100Products C 89 97 5.5 307 100 82 94 7.0 296 100 81 94 6.l7 294 100 D 95 105 5.9 320 100 83 95 7.2 297 100 85 98 5.9 307 100 E* 72 96 28.0 340 18 F* 70 109 26.3 342 21__________________________________________________________________________ Invention Process 600.degree. C. .times. 10 hr. ((b)) (83% 600.degree. C. .times. 10 hr. ((c)) (50% Cold) Sample .sigma..sub.0.2 .sigma..sub.B El mar. .sigma..sub.0.2 .sigma..sub.B El mar. No. (kg/mm.sup.2) (kg/mm.sup.2) (%) Hv (%) (kg/mm.sup.2) (kg/mm.sup.2) (%) Hv (%)__________________________________________________________________________ Steels 1 -- -- -- -- -- 107 112 14.6 330 95 for the 2 99 110 16.2 324 90 98 107 15.3 320 92 Invention 3 85 91 18.2 289 82 83 90 17.9 281 84 Process 4 89 102 19.3 317 81 87 98 18.9 314 84 5 84 97 17.8 301 73 83 95 16.8 293 79 6 83 95 19.1 289 70 81 92 18.9 289 72 7 88 96 18.2 295 67 85 93 17.9 291 70 8 89 95 20.4 307 73 86 91 19.1 302 71 9 91 95 21.7 302 70 90 93 20.8 300 72 10 81 89 20.3 294 91 81 85 19.6 281 93 11 80 84 23.1 279 76 80 87 22.1 281 79 12 76 82 20.6 275 68 76 81 19.8 286 67 13 79 84 21.2 290 70 77 82 21.0 285 71 14 91 92 20.7 291 75 86 93 20.5 289 78 15 91 93 20.2 288 76 91 95 20.8 300 82 16 80 84 19.4 279 65 80 86 19.0 284 69 17 82 85 19.3 281 83 81 87 19.1 289 84 18 88 91 21.3 294 80 89 94 21.5 299 80 19 89 94 23.4 297 67 86 93 22.7 300 69 20 86 91 20.1 292 84 82 90 19.8 288 84 21 88 93 21.8 298 85 86 92 21.2 295 85 22 83 87 20.9 281 87 81 86 20.0 291 86 23 85 90 21.4 289 81 83 89 20.8 295 80 24 80 83 20.2 275 68 81 85 20.5 287 67 25 89 93 19.3 291 64 86 90 19.1 287 65 26 86 91 20.0 287 63 85 91 20.8 293 66 27 85 94 19.4 295 68 84 92 19.0 290 70 28 89 93 21.4 306 74 89 96 20.9 310 73 29 85 88 19.1 297 73 84 95 19.0 305 70 30 86 93 19.7 301 68 85 91 19.3 295 69 31 80 84 20.5 279 60 83 90 20.0 276 64 32 114 117 16.8 384 85 105 110 16.5 369 88 Compara- A 85 99 8.3 298 100 84 103 8.0 315 100 tive B 89 94 8.1 305 100 89 97 8.4 302 100 Steels C 83 97 8.5 300 100 85 98 8.0 299 100 D 87 99 7.3 318 100 89 104 7.3 320 100 E* F*__________________________________________________________________________ *E, F: 20% coldrolled TABLE 3 Mechanical Properties and Amount of Martensite of the Comparative Process (Annealed Materials) As Annealed Process(d) 550.degree. C. .times. 30 hr. 575.degree. C. .times. hr. 600.degree. C. .times. 20 hr. Sample .sigma..sub.0.2 .sigma..sub.B El mar. .sigma..sub.0.2 .sigma..sub.B El mar. .sigma..sub .0.2 .sigma..sub.B El mar. No. (kg/mm.sup.2) (kg/mm.sup.2) (%) Hv (%) (kg/mm.sup.2) (kg/mm.sup.2) (%) Hv (%) (kg/mm.sup.2) (kg/mm.sup.2) (%) Hv (%) 3 74 85 15.7 286 98 77 86 15.3 290 96 71 84 17.3 276 85 4 80 87 16.1 291 98 95 106 15.4 311 97 75 89 18.4 291 82 6 75 83 15.4 290 94 82 91 14.9 300 90 68 90 18.5 293 75 9 82 97 16.4 313 97 89 101 15.5 322 90 75 87 18.0 284 72 12 75 84 15.8 287 97 75 83 14.9 283 97 70 90 18.9 285 73 13 80 85 16.8 291 98 83 88 15.3 298 96 72 93 18.7 281 75 14 89 95 16.1 322 98 100 105 15.1 340 96 73 88 19.3 283 80 18 91 99 16.0 320 96 104 110 15.0 337 93 82 95 19.1 300 84 25 84 111 16.3 334 96 89 110 15.6 330 90 86 117 17.4 328 71 28 85 110 15.7 326 98 91 106 14.8 325 95 82 108 18.5 321 89 31 87 98 16.9 300 98 82 101 15.4 316 97 78 96 19.4 303 72 32 97 119 13.3 356 100 98 117 15.3 358 95 100 113 17.3 353 92 As Annealed Process(d) Comparative Process 625.degree. C. .times. 1 hr. 675.degree. C. .times. 1 hr. 710.degree. C. .times. 1 hr. Sample .sigma..sub.0.2 .sigma..sub.B El mar. .sigma..sub.0.2 .sigma..sub.B El .sigma..sub.0.2 .sigma..sub.0.2 .sigma..sub.B El mar. No. (kg/mm.sup.2) (kg/mm.sup.2) (%) Hv (%) (kg/mm.sup.2) (kg/mm.sup.2) (%) Hv (%) (kg/mm.su p.2) (kg/mm.sup.2) (%) Hv (%) 3 67 93 16.2 304 80 64 91 14.9 298 83 52 88 11.0 281 78 4 67 97 16.7 319 76 65 93 16.3 305 82 54 98 10.7 287 79 6 65 87 17.1 285 70 63 89 16.9 292 73 50 85 11.6 261 66 9 74 88 16.9 290 69 71 89 16.4 289 70 59 87 10.9 272 64 12 68 85 18.0 288 68 64 91 17.5 290 70 50 81 12.1 250 62 13 72 90 18.9 283 71 73 92 16.3 281 71 56 87 11.3 269 70 14 74 85 19.6 279 73 71 93 16.9 283 69 60 85 12.5 278 71 18 79 94 19.8 306 80 68 94 16.4 295 75 61 91 10.8 290 76 25 83 114 18.1 319 67 79 109 15.9 309 72 61 110 11.7 286 65 28 82 105 18.5 318 88 81 103 15.7 308 71 58 99 10.8 276 83 31 74 90 20.5 294 66 75 93 17.0 289 69 57 85 12.6 271 62 32 101 116 20.2 363 91 75 115 14.0 321 78 68 117 9.2 310 70

Claims

1. A process for preparing a high strength stainless steel material having excellent workability free from weld softening consisting of a single martensitic phase or a duplex phase structure of martensite and minute austenite, said process comprising heat-treating at a temperature of 550.degree. to 675.degree. C. for 1 to 30 hours cold-rolled material of a steel wherein no annealing treatment is performed between a final cold rolling step and said heat-treatment step, said steel consisting essentially of:

C: not more than 0.10%

Si: 0.85-4.5%

Mn: 0.20-5.0%

P: not more than 0.060%

S: not more than 0.030%

Cr: 10.0-17.0%

Ni: 3.0-8.0%

N: not more than 0.10% and Fe and inevitable incidental impurities, wherein the Ni.sub.eq value is defined as:

Ni.sub.eq =Ni+Mn+0.5Cr+0.3Si+20(C+N) is in the range of 13.0-17.5.

2. The process for preparing a high strength steel material as set forth in claim 1, wherein the steel essentially consists of:

C: 0.005-0.08%

Si: 0.85-4.0%

Mn: 0.30-4.5%

P: not more than 0.04%

S: not more than 0.02%

Cr: 11.0-16.0%

Ni: 3.5-7.5%

N: not more than 0.07% and Fe and inevitable incidental impurities.

3. The process for preparing a high strength steel material as set forth in claim 2, wherein the steel essentially consists of:

C: 0.007-0.06%

Si: 0.85-4.0%

Mn: 0.40-4.0%

P: not more than 0.035%

S: not more than 0.015%

Cr: 12.0-15.0%

Ni: 4.0-7.5%

N: not more than 0.05% and Fe and inevitable incidental impurities.

4. Process for preparing a high strength stainless steel material having excellent workability and free from weld softening consisting of a single martensitic phase or a duplex phase structure of martensite and minute austenite, said process comprising heat-treating at a temperature of 550.degree. to 675.degree. C. for 1 to 30 hours a cold-rolled material of a steel wherein no annealing treatment is performed between a final cold rolling step annd said heat-treatment step, said steel consisting essentially of:

C: not more than 0.10%

Si: 0.85-4.5%

Mn: 0.2-5.0%

P: not more than 0.060%

S: not more than 0.030%

Cr: 10.0-17.0%

Ni: 3.0-8.0%

N: not more than 0.10% At least one of Cu, Mo, W and Co: not more than 4% in total and Fe and inevitable incidental impurities, wherein the Ni.sub.eq value defined as:

Ni.sub.eq =Ni+Mn+0.5Cr+0.3Si=20(C+N)+Cu+Mo+W+0.2Co is in the range of 13.0-17.5.

5. The process for preparing a high strength stainless steel material as set forth in claim 4, wherein the steel essentially consists of:

C: 0.005-0.08%

Si: 0.85-4.0%

Mn: 0.30-4.5%

P: not more than 0.04%

S: not more than 0.020%

Cr: 11.0-16.0%

Ni: 3.5-7.5%

N: not more than 0.07% At least one of Cu, Mo, W and Co: 0.5-3.5% in total and Fe and inevitable incidental impurities.

6. The process for preparing a high strength stainless steel material as set forth in claim 5, wherein the steel essentially consists of:

C: 0.007-0.06%

Si: 0.85-4.0%

Mn: 0.40-4.0%

P: not more than 0.035%

S: not more than 0.015%

Cr: 12.0-15.0%

Ni: 4.0-7.5%

N: not more than 0.05% At least one of Cu, Mo, W and Co: 1.0-3.0% in total and Fe and inevitable incidental impurities.

7. Process for preparing a high strength stainless steel material having excellent workability and free from weld softening consisting of a single martensitic phase or a duplex phase structure of martensite and minute austenite, said process comprising heat-treating at a temperature of 550.degree. to 675.degree. C. for 1 to 30 hours a cold-rolled material of a steel wherein no annealing treatment is performed between a final cold rolling step and said heat-treatment step, said steel consisting of:

C: not more than 0.10%

Si: 0.85-4.5%

Mn: 0.2-5.0%

P: not more than 0.060%

S: not more than 0.030%

Cr: 10.0-17.0%

Ni: 3.0-8.0%

N: not more than 0.10%

At least one of Ti, Nb, V, and Zr not more than 1% in total and Fe and inevitable incidental impurities, wherein the Ni.sub.eq value defined as:

Ni.sub.eq =Ni+Mn+0.5Cr+0.3Si is in the range of b 13.0-17.5.

8. The process for preparing a high strength stainless steel material as set forth in claim 7, wherein the steel essentially consists of:

C: 0.005-0.08%

Si: 0.85-4.0%

Mn: 0.30-4.5%

P: not more than 0.04%

S: not more than 0.02%

Cr: 11.0-16.0%

Ni: 3.5-7.5%

N: not more than 0.07% At least one of Ti, Nb, V, and Zr 0.1-0.8% in total and Fe and inevitable incidental impurities.

9. The process for preparing a high strength stainless steel material as set forth in claim 8, wherein the steel essentially consists of:

C: 0.007-0.06%

Si: 0.85-4.0%

Mn: 0.40-4.0%

P: not more than 0.035%

S: not more than 0.015%

Cr: 12.0-15.0%

Ni: 4.0-7.5%

N: not more than 0.5% At least one of Ti, Nb, V, and Zr, 0.15-0.8% in total and Fe and inevitable incidental impurities.

10. Process for preparing a high strength stainless steel material having excellent workability and free from weld softening consisting of a single martensitic phase or a duplex phase structure of martensite and minute austenite, said process comprising heat-treating at a temperature of 550.degree. to 675.degree. C. for 1 to 30 hours a cold-rolled material of a steel wherein no annealing treatment is performed between a final cold rolling step and said heat-treatment step, said steel consisting essentially of:

C: not more than 0.10%

Si: 0.85-4.5%

Mn: 0.20-5.0%

P: not more than 0.060%

S: not more than 0.030%

Cr: 10.0-17.0%

Ni: 3.0-8.0%

N: not more than 0.10%

At least one of Cu, Mo, W and Co: not more than 4% in total

At least one of Ti, Nb, V, and Zr not more than 1% in total and Fe and inevitable incidental impurities, wherein the Ni.sub.eq value defined as:

Ni.sub.eq +Ni+Mn+0.5Cr+0.3Si+Cu+Mo+W+0.2Co is in the range of 13.0-17.5.

11. The process for preparing a high strength stainless steel material as set forth in claim 10, wherein the steel essentially consists of:

C: 0.005-0.08%

Si: 0.85-4.0%

Mn: 0.30-4.5%

P: not more than 0.040%

S: not more than 0.020%

Cr: 11.0-16.0%

Ni: 3.5-7.5%

N: not more than 0.07%

At least one of Cu, Mo, W and Co: 0.5-3.5% in total

At least one of Ti, Nb, V, and Zr, 0.1-0.8% in total and Fe and inevitable incidental impurities.

12. The process for preparing a high strength stainless steel material as set forth in claim 11, wherein the steel essentially consists of:

C: 0.007-0.06%

Si: 0.85-4.0%

Mn: 0.40-4.0%

P: not more than 0.035%

S: not more than 0.015%

Cr: 12.0-15.0%

Ni: 4.0-7.5%

N: not more than 0.05%

At least one of Cu, Mo, W and Co: 1.0-3.0% in total

At least one of Ti, Nb, V, and 0.15-0.8% in total and Fe and inevitable incidental impurities.