JP2826819B2 - Method for producing high-strength stainless steel with excellent workability and no welding softening - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a high-strength stainless steel material having excellent workability and resistance to welding softening. <Prior art and its problems> Existing high-strength stainless steels are roughly classified into (1) martensitic stainless steel, (2) work-hardening austenitic stainless steel, and (3) precipitation-hardening stainless steel. Martensitic stainless steel is mainly composed of Fe-Cr-C, and is substantially an austenitic single phase in a quenching temperature (900 to 1100 ° C, but varies depending on the contents of Cr and C). It is a so-called "hardening" steel whose transformation start point (Ms point) is above room temperature. This type of steel is hard and poor in workability in a quenched state or a quenched and tempered state. For this reason, processing such as bending, cutting, and cutting is usually performed in an annealed state, and after a desired shape is given, heat treatment such as quenching and tempering is performed to impart high strength. However, large members have drawbacks in that heat treatment is difficult, welding cracks easily occur during welding, and tempering heat treatment must be performed on the welded portion after welding. When considering the use of martensitic stainless steel as a structural member, steel which exhibits a lath martensite phase in a quenched state with a composition in which C is kept low can be considered as a means for compensating for the above-mentioned disadvantages. For example,
The steel shown in 35447 is an example of this. One example of steel within the scope of the claims of this publication is “Nissin Steelmaking Technique” (Showa 50
No. 33) published in December 2012, and its composition is C: 0.0
32%, Si: 0.75%, Mn: 0.14%, Ni: 4.01%, Cr: 12.4%, Ti: 0.
31%, this material was about 108kgf / mm ² tensile strength, have an elongation of about 6%, yet it has been shown that welding softening is small. Low weld softening and high tensile strength are preferred for welded structural materials, but because of low elongation,
For example, cracks are likely to occur even in mild processing, which is unsatisfactory as a structural processing material. Work hardening type austenitic stainless steel is SUS30
A stainless steel having a metastable austenite phase such as 1, 201, 304, or 202, which is strengthened by cold working. The mechanical properties by this strengthening method are specified in JIS G 4307. For example, for 1 / 2H of SUS301, proof stress 77kgf / mm ²
As described above, the tensile strength is specified as 105 kgf / mm ² or more and the elongation is 10% or more, and both the tensile strength and the elongation show large values. However, this type of material has heat input such as welding,
The disadvantage is that the weld softens. In some cases, a Cr deficiency layer is formed in the weld heat affected zone due to precipitation of Cr carbide, which may cause intergranular stress corrosion cracking. Precipitation hardening stainless steels are classified into martensitic, ferritic, austenitic, etc., depending on the structure of the matrix.
Contains one or more of i, Nb, Cu, Mo, V, Ta, etc.
By aging the supersaturated solid solution, an intermetallic compound is precipitated and strengthened. These steels difference matrix, such as by the content of the element contributing to the age hardening, mechanical properties after aging treatment is different, 140~190kgf / mm ² tensile strength, an elongation of 2-5% . When considering the use of these steels as structural members, processing or welding is generally performed before aging treatment, but it is difficult to perform aging treatment for large structures. As described above, none of the steels conventionally known as high-strength stainless steels have all of strength, workability, and welding softening resistance. <Means for Solving the Problems> Research has been already conducted to obtain a new high-strength stainless steel material free from such disadvantages, and a patent application has been filed by the applicant of the present invention as Japanese Patent Application No. 61-192107. . This method is to bring steel of specific composition to a temperature of 550-675 ° C.
The heat treatment was performed for 30 hours, but after conducting various experimental studies, it was possible to reduce the heat treatment time without deteriorating the characteristics required for the product by shifting the heat treatment temperature to the higher temperature side. Thus, the present invention was completed. <Constitution of the Invention> In the present invention, C: 0.10% or less Si: more than 1.0 to 4.5% Mn: 5.0% or less P: 0.060% or less S: 0.030% or less Cr: 10.0 to 17.0% Ni: 3.0 to 10.0% N: 0.10 % Or less, with the balance being Fe and unavoidable impurities, and having a Ni eq value defined by Ni eq = Ni + Mn + 0.5Cr + 0.3Si + 20 (C + N) within the range of 13.0 to 17.5. Material, or any of their annealed materials
High-strength stainless steel material with excellent workability and no weld softening, consisting of a dual phase structure of martensite phase and fine austenite phase, which is subjected to a short-time heat treatment within a temperature range of 650 to 750 ° C for 10 minutes or less. And a method for producing the same. The present invention also provides, in addition to the above composition, one or more of Cu, Mo, W, Co in total of 4% or less, and / or Ti, Nb, V, Zr, Al, B in total of 1% or less. And a method for manufacturing a similar steel material using steel containing at least one of Ta and Ta as a raw material. In that case Ni
The definition of eq is modified according to the components. Cu, Mo, W, Co 1
If more than one kind is included, Nieq = Ni + Mn + 0.5Cr + 0.3Si + 20 (C + N) + Cu + Mo + W + 0.2Co, and if one or more of Ti, Nb, V, Zr, Al, B, and Ta is contained, Nieq = Ni + Mn + 0.5Cr + 0.3Si, one or more of Cu, Mo, W, Co, and Ti, Nb, V, Zr, Al,
When one or more of B and Ta is contained, Nieq = Ni + Mn + 0.5Cr + 0.3Si + Cu + Mo + W + 0.2Co. Steel of the present invention steel, the composition is within the above range,
And, by adjusting the composition so that Ni eq defined as above becomes the above numerical value, in the state of hot rolling, in the state of cold rolling, substantially in any of the annealed state It exhibits a structure composed of a martensite phase. In the method of the present invention, the material as hot rolled, the material as cold rolled, and the material annealed after cold rolling can be subjected to a short-time heat treatment at 650 ° C. to 750 ° C. for 10 minutes or less to reduce the austenite reverse transformation. And that it can be stabilized. At present, the mechanism and reason for such a transformation are not well understood, but it has been confirmed that this change occurs with reproducibility. No attempt has been made to modify martensitic stainless steel by such treatment. Steel of the invention has a 100 kgf / mm ² about intensity levels,
Shows about 20% elongation and no weld softening. The reasons for limiting the composition of the base steel of the method of the present invention are as follows. C: C is an austenite-forming element, is effective in forming an austenite phase at a high temperature, and is effective in strengthening a reverse transformed austenite phase and a martensite phase after heat treatment. However, since the corrosion resistance of the welded part is deteriorated, the limit is 0.10%. N: N is an austenite forming element like C, is effective in forming an austenite phase at high temperature, increases the strength of the reverse transformed austenite phase after heat treatment, and is effective in strengthening, but when too much, the elongation rate is reduced. 0.1% is made the upper limit because it lowers. Si: Si is an effective element for strengthening the reverse transformed austenite phase after heat treatment, and is also effective for widening the allowable range of temperature during heat treatment. For this purpose, it is better to contain more than 1.0%. However, an excessive amount promotes solidification cracking at the time of solidification or welding, so the upper limit is set to 4.5%. Mn: Mn is an austenite-forming element and is an element necessary for adjusting the Ms point. However, if it is too much, it will be harmful in steel making, so the upper limit is 5%. Cr: Cr is a basic component that imparts corrosion resistance. If it is less than 10%, it has no effect, while if it exceeds 17%, a large amount of austenite forming element is required to form an austenitic single phase at high temperature, and as a result, Since the desired tissue cannot be obtained when brought to room temperature, the upper limit is 17%. Ni: Ni is an austenite-forming element, and is an element necessary for making the austenite single phase at a high temperature and adjusting the Ms point. The required Ni content varies depending on the content of other elements. At least about 3% is required for austenite single phase formation and Ms point adjustment at high temperature. However, even if the amount of other components is reduced, if Ni exceeds 10%, a desired structure can be obtained. Disappears. P: P is an unavoidable impurity that is mixed in from the raw material and the auxiliary raw material at the time of smelting, but if contained too much, it makes the steel brittle, so the upper limit is 0.06%. S: S is also an unavoidable impurity that is mixed in from the raw material and the auxiliary raw material at the time of smelting, but if contained too much, it makes the steel brittle, so the upper limit is 0.03%. Cu: Cu is an element originally effective for improving corrosion resistance, but is effective for lowering the Ms point in the present invention. If it exceeds about 4%, the hot workability is significantly impaired, so the upper limit is 4%. Mo: Mo also improves corrosion resistance, increases the strength of reverse transformed austenite, and is effective in lowering the Ms point,
Since it is an expensive material and too much increases the price of steel, it is limited to 4%. W: W is an element effective for improving corrosion resistance and strength, and is effective for lowering the Ms point. However, if it is too large, it increases the price of the material, so it is limited to 4%. Co: Co has a large austenitizing effect at high temperatures, and M
Lower s point (Must be large for austenitizing effect
Do not excessively lower the s point). Although it is a very effective element for adjusting the composition of a system having a large Cr content, if it is too large, it increases the price of steel, so it is limited to 4%. The above four elements are effective in adjusting the martensite forming ability in relation to other components while improving corrosion resistance in common. It is equivalent in this sense. Ti: Ti is a carbide-forming element and is an element effective in suppressing the formation of a Cr deficiency layer due to precipitation of Cr carbide during welding and in suppressing the growth of crystal grains in a reverse transformed austenite layer. Or 1% due to scum formation during welding. Nb: Nb is an element effective in suppressing the formation of a Cr deficiency layer due to precipitation of Cr carbide during welding and suppressing the growth of crystal grains in a reverse transformed austenite layer.However, if it is too much, solidification cracking during casting or welding will occur. Not only is it accelerated, but it also impairs the ductility of the material, so the upper limit is 1%. V: V is effective in suppressing the Cr deficiency layer due to precipitation of Cr carbide during welding and in suppressing the growth of crystal grains in the reverse transformed austenite layer. However, if too large, the ductility of the material is impaired, so the upper limit is 1%. Zr: Zr is an element that is effective in suppressing the formation of a Cr-depleted layer due to precipitation of Cr carbide during welding and suppressing the growth of crystal grains in the reverse transformed austenite phase. Of non-metallic inclusions and impairs the ductility and surface properties of steel, so the upper limit is 1%. Al: Al fixes N in the steel and has a remarkable effect of suppressing the crystal grain growth of the reverse transformed austenite phase. However, if it is too much, the flow of molten metal at the time of welding becomes poor, and the welding operation becomes difficult.
% As the upper limit. B: B is effective for suppressing the crystal grain growth of the reverse transformed austenite phase and improving the hot workability. However, if it is too large, it impairs the ductility of the steel, so the upper limit is 1%. Ta: Ta is an element effective in suppressing the formation of a Cr-depleted layer due to precipitation of Cr carbide during welding and suppressing the growth of crystal grains in the reverse transformed austenite phase.However, if too large, solidification cracking during casting or welding can be prevented. Not only is it accelerated, but it also impairs the ductility of the material, so the upper limit is 1%. The above seven elements are carbide and nitride forming elements,
In each case, the growth of the inverse transformed austenite crystal grains is suppressed, and the effect is remarkable. It is an equivalent in that sense. The reasons for limiting the nickel equivalent value (Ni eq) are as follows. In the material steel of the method of the present invention, the martensitic transformation end temperature must be near room temperature (150 to -10 ° C). The material steel of the method of the present invention is an austenitic single phase or an austenitic phase containing a small amount (approximately 10%) of a δ ferrite phase in a high-temperature region where it is exposed during hot rolling, annealing or welding. Therefore, when brought to room temperature, it must have a substantially martensitic structure. "Substantially" means a small amount (approximately 25%)
Means that austenite and a small amount (approximately 10%) of ferrite may be present. It is not necessary to consider the amount of such retained austenite and δ-ferrite too strictly. In the material steel of the present invention, various elements are alloyed, but the present inventors, as long as the composition complies with the limitation of nickel equivalent (Ni eq) defined in the above-described composition table,
It has been discovered that at room temperature it has a substantially martensitic structure and can achieve the objects of the invention described at the outset. That is, even in the composition range described above, the steel having a nickel equivalent value defined below 13 is too high in Ms point, so that a desired high elongation cannot be achieved even by performing the heat treatment specified in the present invention. Further, when steel having a value greater than 17.5 is subjected to a heat history such as welding, a welded portion is softened, and a desired high-strength member cannot be obtained. Although it is not necessary to explain the Ni eq equation further, the Ni eq is converted to the amount of Ni by determining the coefficient based on the austenite forming ability of Ni in consideration of the contribution of each component element to the transformation. Since the seven elements below Ti are neutral with respect to the above properties and cancel the austenite-forming ability of C and N,
In compositions containing them, these elements and C and N are not taken into account. The reasons for limiting the heat treatment conditions in the method of the present invention are as follows. Steel is a martensite (lath martensite) structure in annealed condition has a 100 kgf / mm ² approximately tensile strength,
The elongation is at most about 6%, and it is hard to say that it has satisfactory workability. A part of martensite is reverse-transformed into an austenite phase by maintaining the temperature range of 650 ° C. to 750 ° C. for 10 minutes or less. This inversely transformed austenite is somewhat more or less structurally stable, and the entire amount does not necessarily return to martensite due to subsequent cooling, and may remain as austenite. In any case, this heat treatment provides great ductility without significantly lowering the strength (proof stress). 65
If the temperature is lower than 0 ° C, the effect of providing this ductility is small. The time is appropriately selected according to the size of the material to be heat-treated, but it has been found that within 10 minutes is sufficient in consideration of the productivity and the characteristics required for the product. When the material reaches a predetermined temperature, a slight decrease in ductility is observed even if the soaking time is not taken, but it has been confirmed that a material having sufficient mechanical properties intended in the present invention can be obtained. . In the heat treatment method of the present invention, the atmosphere is selected according to the required surface characteristics. That is, when a bright surface is required, an atmosphere such as hydrogen, hydrogen and nitrogen, and hydrogen and an inert gas is selected. On the other hand, when the scale is removed by pickling after the heat treatment of the present invention, the combustion atmosphere of a combustible oil such as heavy oil, kerosene, or light oil, or a combustible gas such as propane, and the atmosphere, nitrogen, or an inert gas. It is confirmed that when the heat treatment of the present invention is performed in these atmospheres, the mechanical properties after the heat treatment are not affected at all. As the heat treatment equipment, a batch furnace such as a bell-type annealing furnace, and a continuous heat treatment equipment such as a continuous bright annealing line and a continuous annealing pickling line can be used. Hereinafter, the present invention will be specifically illustrated by examples with reference to the drawings. The sample steel was melted in a 30 kg vacuum high-frequency furnace by the usual method,
110mm square, upper surface 120mm square, 290mm height ingot, 1250 ℃
After forging with a 35mm thick and 155mm wide plate,
0mm x 150mm plate, heat treated at 1250 ° C for 3 hours, thickness
The sample was hot-rolled to 6 mm, and a part thereof was subjected to a test as a hot-rolled material (a). The other part was annealed at 1030 ° C for 10 minutes, pickled, and cold rolled, partly made into a 1 mm thick plate, and used as a cold rolled material (b) under 83% reduction, and the other part was 2 mm thick And then cold-rolled with an intermediate annealed pickling to obtain a 1mm thick plate as a cold-rolled material (c) under 50% pressure reduction. The rest was further annealed at 1030 ° C for 1.5 minutes. And then pickled and subjected to a test as an annealed material (d). The operation of sample preparation is illustrated in FIG. The compositions of the inventive sample and the comparative sample are shown in Table 1. Samples Nos. 3 to 33 are the material steels of the method of the present invention. No.A〜
F is a material steel of a comparative example, and its composition is within a specified composition range, but the nickel equivalent value is less than 13 in AD, and exceeds 17.5 in EF. The test of the mechanical properties was carried out using No. 5 and No. 13B specimens specified in JIS Z 2201. The amount of martensite was measured by a sample vibration magnetometer. Table 2 summarizes the mechanical properties and the amount of martensite of the samples heat-treated in the air atmosphere. In Table 2, what is referred to as the conventional method is that which is not subjected to the heat treatment according to the present invention. According to Table 2, a sufficiently high intensity of lath martensite 73~118kgf / mm ² steel with yield strength with tissue, 98~127kgf / mm ² in tensile strength in annealed condition not subjected to heat treatment according to the invention Although it has a level, the elongation is at most 7%, which is significantly lower than those of the samples E and F which are 20% cold-rolled steel sheets. Even in the sample subjected to the heat treatment of the present invention, the elongation of the comparative material steel is slightly improved, but is at most 8%. Some of the samples of the method of the present invention have a slight decrease in proof stress, but generally show a marked increase in elongation while maintaining the proof stress. Table 3 shows the mechanical properties and the amount of martensite when the 83% cold-rolled material (b) was heat-treated under different conditions. Comparative examples in Table 3 are those in which the heat treatment temperature exceeds the upper limit of the method of the present invention. According to Table 3, it can be seen that the upper limit temperature of the heat treatment has a limit around 750 ° C. Table 4 shows the mechanical properties when the heat treatment of the present invention was performed at 675 ° C. for 2 minutes in various atmospheres. As can be seen from Table 4, the heat treatment of the present invention is effective even when the atmosphere is changed. The welding softening test was performed by TIG welding on a 1 mm thick plate with a current of 60
A, a bead was placed at a speed of 400 mm / min. The result is the second
Shown in the figure. The sample of the present invention (13, 17) is used as a 50% cold rolled material (c).
The samples were heat-treated at 650 ° C. for 10 minutes, and the comparative samples (E, F) are 20% cold-rolled materials. As can be seen from the figure, the sample of the present invention has no apparent softening of the weld. <Effects of the Invention> As described in the above examples, according to the method of the present invention, a material which did not exist conventionally, that is, having a strength level of about 100 kgf / mm ² and an elongation of about 20%, It is possible to manufacture stainless steel without a decrease in weld strength. 1) Steel belt material that is straightened by elongation deformation after welding, 2) Bending and tensile straightening, and welding of parts after these processes Vehicle underframe material that requires processing and straightening to correct deformation due to thermal strain, 3) container material that requires high strength and welding softening resistance, 4) blade material, 5)
In addition, the effects of the present invention are extremely large in fields such as structural members requiring high strength, workability, and resistance to welding softening.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a system diagram showing a method for preparing a sample according to the present invention. FIG. 2 is a graph showing the softening of the welds of the sample of the present invention and the comparative sample.

──────────────────────────────────────────────────の Continuing from the front page (72) Inventor Takashi Igawa 4976 Tomita, Onan, Shinnanyo-shi Nisshin Steel Corporation Shunan Laboratory (56) References JP-A-57-16154 (JP, A) JP 42 -16870 (JP, B1) JP 38-1960 (JP, B1) U.S. Pat. No. 3,783,367 (US, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C21D 6/00 102

Claims (1)

(57) [Claims] C: 0.10% or less Si: 1.0 to 4.5% Mn: 5.0% or less P: 0.060% or less S: 0.030% or less Cr: 10.0 to 17.0% Ni: 3.0 to 10.0% N: 0.10% or less, with the balance Fe Of hot rolled steel, cold rolled steel, or annealed steel, which consist of unavoidable impurities and whose Ni eq defined by Ni eq = Ni + Mn + 0.5Cr + 0.3Si + 20 (C + N) is in the range of 13.0 to 17.5 Either
High-strength stainless steel material with excellent workability and no weld softening, consisting of a dual phase structure of martensite phase and fine austenite phase, which is subjected to a short-time heat treatment within a temperature range of 650 to 750 ° C for 10 minutes or less. Manufacturing method. 2. C: 0.10% or less Si: 1.0 to 4.5% Mn: 5.0% or less P: 0.060% or less S: 0.030% or less Cr: 10.0 to 17.0% Ni: 3.0 to 10.0% N: 0.10% or less Cu, Mo, W, One or more types of Co contain 4.0% or less in total, and the balance is composed of Fe and unavoidable impurities, and the value of Ni eq defined by Ni eq = Ni + Mn + 0.5Cr + 0.3Si + 20 (C + N) + Cu + Mo + W + 0.2Co is 13.0 to 17.5. Hot or cold rolled steel, or any of their annealed materials
A high-strength stainless steel material with excellent workability and no weld softening, consisting of a dual phase structure of martensite phase and fine austenite phase, which is subjected to a short-time heat treatment within 10 minutes within a temperature range of 650 to 750 ° C. Manufacturing method. 3. C: 0.10% or less Si: 1.0 to 4.5% Mn: 5.0% or less P: 0.060% or less S: 0.030% or less Cr: 10.0 to 17.0% Ni: 3.0 to 10.0% N: 0.10% or less Ti, Nb, V, One or more of Zr, Al, B, and Ta containing 1.0% or less in total, the balance being Fe and unavoidable impurities, and the value of Ni eq defined by Ni eq = Ni + Mn + 0.5Cr + 0.3Si is in the range of 13.0 to 17.5 In either hot rolled, cold rolled, or annealed steel
High-strength stainless steel material with excellent workability and no weld softening, consisting of a dual phase structure of martensite phase and fine austenite phase, which is subjected to a short-time heat treatment within a temperature range of 650 to 750 ° C for 10 minutes or less. Manufacturing method. 4. C: 0.10% or less Si: 1.0 to 4.5% Mn: 5.0% or less P: 0.060% or less S: 0.030% or less Cr: 10.0 to 17.0% Ni: 3.0 to 10.0% N: 0.10% or less Cu, Mo, W, 4.0% or less in total of one or more types of Co Contains 1.0% or less in total of one or more types of Ti, Nb, V, Zr, Al, B, and Ta, consisting of the balance Fe and unavoidable impurities, Ni eq = Ni + Mn + 0.5Cr + 0 .3Si + Cu + Mo + W + 0.2Co Ni eq value is in the range of 13.0 to 17.5, either hot rolled or cold rolled steel or annealed steel
High-strength stainless steel material with excellent workability and no weld softening, consisting of a dual phase structure of martensite phase and fine austenite phase, which is subjected to a short-time heat treatment within a temperature range of 650 to 750 ° C for 10 minutes or less. Manufacturing method.