JPS63210234A - Manufacture of high-strength stainless steel stock excellent in workability and free from softening by welding - Google Patents

Abstract

PURPOSE:To manufacture a high-strength stainless steel stock excellent in workability and free from softening in a weld zone, by subjecting a hot-rolled, cold-rolled, or annealed stock of a steel in which composition and Ni equivalent value are specified to heat treatment under specific conditions. CONSTITUTION:A steel which has a composition consisting of <=0.10% C, <=4.5% Si, <=5.0% Mn, <=0.060% P, <=0.030% S, 10.0-17.0% Cr, 3.0-10.0% Ni, <=0.10% N, and the balance Fe with inevitable impurities and in which the value of Nieq defined by an equation is regulated to 13.0-17.5 is cast. Any of the hot- rolled, cold-rolled, and annealed stocks of the steel with the above composition is subjected to heat treatment at 575-750 deg.C for <60min. In this way, the stainless steel stock composed of a martensitic single phase or a composite structure of martensitic single phase and fine austenitic phase and excellent in workability and resistance to softening by welding can be obtained. In necessary, proper amounts of one or more elements among Cu, Mo, W, and Co and one or more elements among Ti, Nb, V, Zr, Al, B, and Ta are incorporated into the above steel composition.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to a high-strength stainless fIJm material that has excellent workability and weld softening resistance.

<Prior art and its problems> Existing high-strength stainless steels are broadly classified into (1) martensitic stainless steels, (2) work-hardening austenitic stainless steels, and (3) precipitation-hardening stainless steels.

Martensitic stainless steel is mainly Fe-Cr-c
F9.2. In the quenching temperature range (900 to 1100°C, but varies depending on the content of Cr and C), it is essentially austenitic single phase, but at the martensitic transformation starting point (M s point) ) is above room temperature, making it a so-called "hardenable" steel.

This type of steel is hard and has poor workability in the hardened state or in the hardened and tempered state. Therefore, it is usually subjected to processing such as bending, cutting, cutting, etc. in the annealed state, and after the desired shape A is obtained, heat treatment such as quenching and tempering is performed to impart high strength.

However, large members are difficult to heat-treat, and have disadvantages such as weld cracking during welding, and the welded part must be tempered after welding.

When considering the use of martensitic stainless steel as a structural member, as a means to compensate for the above drawbacks,
A steel that has a composition with a low C content and exhibits a lath martensite phase in the quenched state is considered. For example, Tokuko Sho 51-3
Steel shown in No. 5447! ±This is an example. An example of steel within the scope of the claims of this publication is "Nissin Steel Technical Report" (
It was introduced in December 1975 issue No. 33), and its composition is C: 0.032"A, Si: 0.7
5X, Mn: 0.14X, Ni+4.01%, Cr:
12.4%, Ti:o, 31% Te, ko17) The material has been shown to have a tensile strength of approximately 108 kgf/mm2, an elongation of approximately 6%, and low welding softening. . Small weld softening and high tensile strength are desirable as welded structural materials, but the low elongation rate causes cracks, for example, even during light processing.1. Easy structural addition]-
It is unsatisfactory as a material.

Work-hardening austenitic stainless steel is 5US3
It is a stainless steel having a metastable austenite phase such as 01.201, 304, 202, etc., and is strengthened by cold working. The mechanical properties obtained by this strengthening method are J
Specified in IS G 4307. For example, SUS
301 1/2H has a cuff resistance of 7 kgf/mm2 or more and a tensile strength of 105 kgf/mm2. It is specified that the elongation is 10% or more, and both tensile strength and elongation show large values. However, this type of material has the disadvantage that the welded portion 11 softens when heat is input from welding or the like.

Further, in some cases, a Cr-depleted layer is formed in the weld heat-affected zone due to the precipitation of Cr carbides, and intergranular stress corrosion cracking may occur.

Precipitation hardening stainless steels are classified into martensitic, ferritic, austenitic, etc. depending on the matrix structure, but all contain Al, which contributes to age hardening.
A supersaturated solid solution containing one or more of Ti, Nb, Cu, Mo, V, Ta, etc. is aged and strengthened by precipitation of intermetallic compounds. The mechanical properties of these rusts after aging differ depending on the matrix, the content of elements that contribute to age hardening, etc., but it is 140 to 190 kgf/+nm 2
It has a tensile strength of , and an elongation of 2 to 5z.

When considering the use of these steels as structural members, it is common to process or weld them before aging treatment, but aging treatment is difficult for large structures.

As described above, none of the steels conventionally known as high-strength stainless steels have all of strength, workability, and weld softening resistance.

Research to obtain a new high-strength stainless steel material free of such defects has already been carried out, and one method has been patented as Japanese Patent Application No. 1987-192107 by the applicant of the present application. . This method uses steel with a specific composition of 550 to 675
℃ for 1 to 30 hours, but as a result of various experiments and studies, it was found that by shifting the heat treatment temperature to a higher temperature, it was possible to achieve long-lasting results without sacrificing the properties required for the product. The present invention was completed based on the finding that it is possible to shorten the heat treatment time.

<Structure of the Invention> The present invention includes C: 0.10% or less Si2 4.5% or less Mn: 5.0% or less P: 0.060% or less S: 0.030% or less Cr: 10.0 to 17 .0% Ni: 3.0-10.0% N: Contains 0.10% or less, the balance consists of Fe and inevitable impurities, NIBg-=Ni+Mn+0.5Cr+0.3Si
Ni defined as +20(fL+N), the value of y is 13.
Marten is produced by heat-treating hot-rolled, cold-rolled, or annealed steel in the temperature range of 575 to 750°C for less than 60 minutes. Provided is a method for manufacturing a high-strength stainless steel material that has a single-phase site structure or a multi-phase structure of a martensitic phase and a fine austenite phase, has excellent workability, and is free from welding softening.

In addition to the above composition, the present invention also provides a total of 4% or less (
1) One or more of Cu, Ma, W, Ca (1),
and/or 1% or less Ti, Nb, V in total,
Z r, A I.

A method for manufacturing a similar steel material made of steel containing one or more of B and Ta is provided. In that case, the definition of N1e4 is modified depending on the component. Cu, Mo, W.

If one or more types of CO are included, Nieg = Ni+Mn+0.5Cr+0.3Si
+20(C+N)+Cu+Mo+W 10, 2C:.

So, Ti, Nb, V, Zr, Al, B, T
When containing one or more types of a, N1 (H = Ni + Mn + 0.5Cr + 0.3Si, Cu, Mo, W, Co (1) one or more types, and Ti, Nb, V, Zr, AI, B, T
When containing 1 or more types of a0, Ni, 4 = Ni + Mn + 0.5Cr + 0.3S
i+Cu+Mo+W+0.2Co.

The steel of the present invention has a composition within the above-mentioned range, and by adjusting the composition so that N1cl defined as above becomes the above-mentioned value, the steel can be produced in the as-hot-rolled state. Both in the as-cold-rolled state and in the annealed state, it exhibits a structure consisting essentially of a martensitic phase.

The method of the present invention can be applied to materials as hot-rolled, materials as cold-rolled, and materials annealed after cold-rolling at 575°C to 750°C.
This is based on the novel finding that reverse austenite transformation can be caused and stabilized by heat treatment for less than a minute. At present, the mechanism and reason for such metamorphosis are not well understood, but it has been confirmed that this change occurs with reproducibility. No attempt has ever been made to modify and sharpen martensitic stainless steel by such a treatment.

The steel material of the present invention has a strength level of about 100 kgf/mm2, exhibits an elongation of about 20%, and has no 'lal soft weld.

The reason for limiting the composition of the raw material steel in the method of the present invention is as follows.

C: C is an austenite-forming element and is effective in forming an austenite phase at high temperatures and is effective in strengthening the reversely transformed austenite phase and martensite phase after heat treatment, but if it is too large, it will reduce the elongation rate and , since it deteriorates the corrosion resistance of the welded part, the limit is set at 0.10%.

Like C, NUN is an austenite-forming element, and is effective in forming an austenite phase at high temperatures, increasing the strength of the reverse-transformed austenite phase after heat treatment, and is effective in strengthening, or if too much, it reduces the elongation rate. Therefore, the upper limit is set at 0.1%.

Si: Si is an effective element because it strengthens the reversely transformed austenite phase after heat treatment and widens the temperature tolerance range during heat treatment, but if it is too large, it may cause solidification cracking during solidification or welding. Therefore, the upper limit is set at 4.5%.

Mn: Mn is an austenite-forming element, and Ms
It is an element necessary for adjusting the point, but if too much is used, it will be harmful during steel manufacturing, so the upper limit is set at 5%.

Cr: Cr is a basic component that imparts corrosion resistance,
If it is less than 10%, there is no effect, while if it exceeds 17%,
A large amount of austenite-forming elements is required to form a single austenite phase at high temperatures, and as a result, the desired structure cannot be obtained when brought to room temperature, so the upper limit is set at 17%.

Ni: Ni is an austenite-forming element and is an element necessary for forming austenite into a single phase at high temperatures and adjusting the Ms point. The necessary Ni content varies depending on the content of other elements. At least about 3% Ni is required to make the austenite single phase and adjust the Ms point at high temperatures, but even if the amounts of other components are reduced, if the Ni content exceeds about 10%, the desired structure cannot be obtained. It disappears.

P: P is an unavoidable impurity that is mixed in from raw materials and auxiliary raw materials during melting, but if it is included in a large amount, it will make the steel brittle, so the upper limit is set at 0.06%.

S: S is also an unavoidable impurity that gets mixed in from the EU raw material during melting, but if it is included in a large amount, it will make the steel brittle, so the upper limit is set at 0.03%.

Cu: Cu is originally an effective element for improving corrosion resistance, but in the present invention it is effective for lowering the Ms point. If it exceeds about 4%, hot workability will be significantly impaired, so the upper limit is set at 4%.

Mo: Mo is also effective in improving corrosion resistance, increasing the strength of reverse-transformed austenite, and lowering the Ms point, but it is an expensive material and too much will increase the price of the steel material, so it is limited to 4%. Ru.

W: W is an element effective in improving corrosion resistance and strength, and is effective in lowering the Ms point, but too much W increases the price of the material, so it is limited to 4%.

Co: Co has a large austenitizing effect in a high temperature range and lowers the Ms point (despite its large austenitizing effect, it does not excessively lower the Ms point). Although it is a very effective element for adjusting the composition of systems with a large Cr content, if it is present too much, the price of steel will increase, so it is limited to 4%.

The following four elements are commonly effective in improving corrosion resistance while adjusting the ability to form martensite in relation to other components. Equivalent in this sense.

Ti: Ti is a carbide forming element, and Cr during welding
This element is effective in suppressing the formation of a Cr-depleted layer due to the precipitation of carbides and the growth of grains in the reversely transformed austenite phase, but too much of it can cause surface defects and scum formation during welding. The upper limit is %.

Nb: Nb is Cr due to precipitation of Cr carbide during welding.
It is an element that is effective in suppressing the formation of a depleted layer and the growth of grains in the reversely transformed austenite phase, but if it is present in too much, it not only promotes solidification cracking during casting and welding, but also impairs the ductility of the material. The upper limit is 1%.

v: V is effective in suppressing the formation of a Cr-depleted layer due to Cr carbide precipitation during welding and the growth of grains in the reversely transformed austenite phase, but too much V impairs the ductility of the material, so the upper limit is set at 1%.

Zr: Zr is Cr due to precipitation of Cr and f oxides during welding.
This element is effective in suppressing the formation of r-depleted layers and the growth of grains in the reversely transformed austenite phase, but if it is present in too much, it may form oxide-based nonmetallic inclusions during casting or welding, which may reduce the ductility of the steel. The upper limit is set at 1% as it may damage the surface properties.

Al: Al has a remarkable effect of fixing N in the steel and suppressing the grain growth of the reversely transformed austenite phase, but if it is too large, leakage flow during welding will be poor and welding work will be difficult, so it should be kept at 1%. Upper limit.

B: B is effective in suppressing grain growth of the reversely transformed austenite phase and improving hot workability, but too much B impairs the ductility of the steel, so the upper limit is set at 1z.

Ta: Ta is Cr due to precipitation of Cr carbide during welding.
It is an element that is effective in suppressing the occurrence of deficient phases and grain growth of the reversely transformed austenite phase, but if it is present in too much, it not only promotes solidification cracking during casting and welding, but also impairs the ductility of the material. The upper limit is 1%.

The following seven elements are carbide- and nitride-forming elements, and all of them suppress the growth of crystal grains of reversely transformed austenite, and have a remarkable effect. In that sense, they are equivalent.

The reason for limiting the nickel equivalent value (Nle1) is as follows. In the steel material used in the method of the present invention, the martensitic transformation completion temperature must be close to room temperature (150 to -10°C). The material steel used in the method of the present invention has a single austenite phase or an austenite phase containing a small amount (approximately 10%) of a δ-ferrite phase in high-temperature regions such as those exposed during hot rolling, annealing, or welding. From the condition
When brought to room temperature, it should have a substantially martensitic structure. “Substantially” means a small amount (l
[25%) of austenite and a small amount (approximately 10%)
This means that ferrite may be present. It is not necessary to consider the amount of retained austenite and δ-ferrite too strictly.

Although various elements are alloyed in the steel material of the present invention, the inventors believe that as long as the composition complies with the composition table shown above and the defined nickel equivalent (Nicg-) limit, It has been discovered that the present invention can achieve the object of the present invention described at the beginning.

That is, even if the steel is within the above-mentioned composition range, the Ms point is too high for steels with a defined nickel equivalent value of less than 13, and the desired high elongation cannot be achieved even if the steel is subjected to the heat treatment specified in the present invention. In addition, if steel with a value greater than 17.5 is subjected to thermal history such as welding, the welded part will soften and the desired high-strength member cannot be obtained.There is no need to explain the formula for Nle# now. , a coefficient is determined based on the austenite forming ability of N1 in consideration of the degree of contribution of each component element to the transformation, and the coefficient is converted into the amount of Ni.

The seven elements below Ti are neutral with respect to the above properties and cancel the austenite forming ability of C and H, so
In compositions containing these elements, these elements and C and N are not taken into consideration.

The reasons for limiting the heat treatment conditions in the method of the present invention are as follows.

Steel that has a martensite (lath martensite) structure in an annealed state has a tensile strength of about 100 kgf/mm2, but its elongation rate is about 6% at most, and it cannot be said that it has satisfactory workability. 60 in the temperature range of 575-750℃
Hold for less than a minute to reverse transform some of the martensite to the austenite phase. This reverse-transformed austenite is structurally more or less stable, and the entire amount does not necessarily return to martensite upon subsequent cooling, and may remain as austenite. In any case, this heat treatment provides large ductility without significantly reducing strength (proof stress). If the temperature is lower than 575°C, the effect of providing this ductility is small, and if the temperature is higher than 750°C, the yield strength decreases and the ductility also decreases.

Although the time is appropriately selected according to the size of the material to be treated, etc., it has been found that less than 60 minutes is sufficient if productivity and characteristics required for the product are taken into consideration. It is recognized that once the material reaches a predetermined temperature, even if no soaking time is taken, the material has sufficient mechanical properties as intended in the present invention, although a slight decrease in ductility is observed. ing.

Furthermore, in the heat treatment method of the present invention, the atmosphere is selected depending on the required surface characteristics. That is, when a bright surface is required, an atmosphere of hydrogen, hydrogen and nitrogen, hydrogen and an inert gas, etc. is selected.

On the other hand, if scale removal is performed by pickling after the heat treatment of the present invention, the combustion atmosphere of flammable oils such as heavy oil, kerosene, and light oil and combustible gases such as propane, and the atmosphere,
Although an atmosphere such as nitrogen or an inert gas is selected, it has been confirmed that when the heat treatment of the present invention is carried out in these atmospheres, there is no effect on the mechanical properties after the heat treatment.

Regarding the heat treatment equipment, it is recommended to use a batch furnace such as a bell-type annealing furnace, and continuous heat treatment equipment such as a continuous bright annealing line and a continuous annealing pickling line.

Hereinafter, the present invention will be specifically illustrated by examples with reference to the drawings.

The sample steel was melted using a conventional method in a 30 kg high-vacuum furnace, and had a bottom surface of 110 mm square, a top surface of 120 mm square, and a height of 280 m.
IIl ingot, forged at 1250°C to a thickness of 35),
After making a board with a width of 155mm, cut it to 30mm x 15
Use a 0mm plate.

The material was soaked at 1250° C. for 3 hours, hot rolled to a thickness of 6 mm, and a portion thereof was subjected to a test as a hot rolled material (a). The other parts were annealed at 1030°C for 10 minutes, pickled, and then cold rolled. Some parts were made into plates with a thickness of 1+nm and were subjected to tests as cold-rolled materials (b) with a reduction of 83%, and the others with a thickness of 2H. A plate with a thickness of 11 mm is then roughly cold rolled with intermediate annealing and pickling in between.
The plate was subjected to a test as a cold-rolled material (C) with a reduction of 50%,
The remaining material was further annealed at 1030° C. for 1.5 minutes, pickled, and used as an annealed material (d) for testing. The procedure for preparing the sample is illustrated in FIG.

The compositions of the inventive sample and the comparative sample are shown in Table 1. Samples Nos. 1 to 33 are steel materials used in the method of the present invention.

No., A-F are steel materials of comparative examples, and their compositions are within the specified composition range, but the nickel equivalent value or A-D is 1.
It is less than 3, and exceeds 17.5 in E-F.

Mechanical property tests were conducted using No. 5 and No. 13B specimens specified in 1ISZ2201.

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

The mechanical properties and martensite content of the specimens heat-treated in the air atmosphere are summarized in Table 2. In Table 2, the conventional method refers to the method in which the heat treatment according to the present invention is not performed.

According to Table 2, the steel having a substantially lath martensite structure in the annealed state without the heat treatment according to the present invention has a yield strength of 73 to 118 kgf/+n+n2. 9 in tensile strength
Sample E has a high strength level of 8 to 127 kgf/m+n2, but the elongation is at most 7% and is a 20% cold rolled steel plate.
, significantly lower than that of F. Even with the heat-treated samples of the present invention, the elongation of the comparative steel material was slightly improved, but
It is 8% at most. Although some of the samples obtained by the method of the present invention have a slight decrease in yield strength, they generally show a significant increase in elongation while maintaining yield strength.

Table 3 shows the mechanical properties and the amount of martensite when the 83% cold rolled material (b) was heat treated under different conditions. The 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 heat treatment is critical 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 under various atmospheres. As seen in Table 4, it can be seen that 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 plate with a thickness of 1+++m.
The beads were placed at a current of 13 OA and a speed of 400 m+n/min. The results are shown in Figure 2. The figure is B1! The hardness distribution from the center is shown. Inventive samples (13, 17) are 50% cold-rolled material (c) at 650'C! The comparative samples (E, F) are 20% cold-rolled materials. As can be seen in the figure, there is clearly no softening of the weld zone in the sample of the present invention.

<Effects of the Invention> As explained in the above examples, according to the method of the present invention, a material that has not existed in the past, that is, 100 kgf
It is now possible to manufacture stainless steel with a strength level of about /mm2 and an elongation of about 20%, and without a decrease in strength at the welded part.1) Steel belt material that is straightened by elongation deformation after welding, ) Vehicle underframe materials that require bending, tensile straightening, welding of parts after these processes, and straightening to correct deformation due to thermal distortion, 3
) Container materials that require high strength and welding softening resistance, 4)
The present invention has brought extremely great effects to the fields of cutlery materials, 5) and other structural members that require high strength, workability, and weld softening resistance.

Special opening aG3-210234 (11)

[Brief explanation of the drawing]

FIG. 1 is a system diagram showing the method for preparing a sample in the present invention. FIG. 2 is a graph showing the softening of the welds of the inventive sample and the comparative sample.

Claims (1)

[Claims] 1. C: 0.10% or less Si: 4.5% or less 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: Contains 0.10% or less, the balance consists of Fe and unavoidable impurities, Ni_e_q=Ni+Mn+0.5Cr+0.3Si+
If the value of Ni_e_q defined by 20(C+N) is 13.
Marten is produced by heat-treating hot-rolled, cold-rolled, or annealed steel in the temperature range of 575 to 750°C for less than 60 minutes. A method for manufacturing high-strength stainless steel materials with excellent workability and no welding softening, consisting of a single phase structure or a multi-phase structure of a martensitic phase and a fine austenite phase. 2, C: 0.10% or less Si: 4.5% or less Mn: 5.0% or less P: 0.060% or less S: 0.030% or less Cr: 10.0-17.0% Ni: 3 .0 to 10.0% N: 0.10% or less Contains at least 4.0% in total of one or more of Cu, Mo, W, and Co, with the balance consisting of Fe and unavoidable impurities, Ni_e_q=Ni+Mn+0.5Cr+0. 3Si+
20(C+N)+Cu+Mo+W+0.2Co A hot-rolled, cold-rolled, or annealed steel with a Ni_e_q value defined by 13.0-17.5 is heated in a temperature range of 575-750°C. A high-strength stainless steel material with excellent workability and no welding softening, consisting of a single martensite phase or a multi-phase structure of a martensite phase and a fine austenite phase, is heat treated for less than 60 minutes. Production method. 3. C: 0.10% or less Si: 4.5% or less Mn: 5.0% or less P: 0.060% or less S: 0.030% or less Cr: 10.0-17.0% Ni: 3 .0 to 10.0% N: 0.10% or less Contains at least 1.0% in total of one or more of Ti, Nb, V, Zr, Al, B, Ta, and the balance consists of Fe and inevitable impurities, The value of Ni_e_q defined by Ni_e_q=Ni+Mn+0.5Cr+0.3Si is within the range of 13.0 to 17.5.
Weld softening with excellent workability, consisting of a single martensite phase or a multi-phase structure of a martensite phase and a fine austenite phase, which is performed by heat treatment within a temperature range of 75 to 750°C for less than 60 minutes. A method for manufacturing high-strength stainless steel materials without 4, C: 0.10% or less Si: 4.5% or less Mn: 5.0% or less P: 0.060% or less S: 0.030% or less Cr: 10.0-17.0% Ni: 3 .0 to 10.0% N: 0.10% or less Cu, Mo, W, Co total 4.0% or less Ti, Nb, V, Zr, Al, B, Ta one or more total Ni_e_q=Ni+Mn+0.5Cr+0.3Si+
Ni_e_q defined by Cu+Mo+W+0.2Co
A hot-rolled steel material whose value is within the range of 13.0 to 17.5,
A single martensite phase or a multi-phase structure of a martensite phase and a fine austenite phase, which is obtained by heat-treating either a cold-rolled material or an annealed material within a temperature range of 575 to 750°C for less than 60 minutes. A method for producing high-strength stainless steel material with excellent workability and no welding softening.