JPH02163343A - High-manganese dual-phase steel for structural use - Google Patents

Abstract

PURPOSE:To provide a structural steel having high mechanical strength and superior weldability while obviating the necessity of the addition of many strengthening alloying elements by specifying respective contents of C and Mn as principal components. CONSTITUTION:A high-Mn dual-phase steel has a basic composition consisting of, by weight, 0.02-0.5% C, 2.0-5.0% Mn, and the balance Fe with inevitable impurities. If necessary, 0.2-1.5% Si or further 0.02-0.5% of one or more elements among Mo, Nb, Ta, W, Cr, Ti, V, and B are incorporated to the above composition. By this composition, the dual-phase steel having a high strength superior to that of low-C or low-alloy steel and excellent in ductility can be obtained by means of air cooling alone from a relatively low annealing temp. of about 700-800 deg.C. Further, mechanical properties in a weld zone practically equal to those in an unweld zone can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a dual-phase steel having excellent mechanical properties and good weldability as a structural steel.

[Conventional technology]

Rolled steel materials used for general or welded structures such as ship hulls, bridges, pressure vessels, and automobiles have excellent mechanical strength, toughness, and good weldability. Economic efficiency is required, such as being possible.

In order to improve the strength of carbon steel, it is easiest to increase the carbon content, but as the carbon content increases, ductility such as elongation and reduction of area decreases, and weldability deteriorates. For this reason, in particular, considering weldability, one or more alloying elements such as silicon, manganese, nickel chromium, and copper are added in small amounts to a low-carbon base material to utilize solid solution strengthening or crystal refinement. Low-carbon, low-alloy steel that has been strengthened has come to be widely used as low-alloy high-strength steel (H3LA steel).

However, low-alloy high-strength steel used in the as-rolled, unheated state retains its ferrite-pearlite structure and is strengthened by adding silicon, manganese, and other alloying elements, resulting in a tensile strength of 60 kgf 7 mm. For high-strength steel with higher strength,
By further increasing the amount of alloying elements such as nickel and chromium and performing quenching and tempering, the tensile strength due to the tempered martensitic phase can be increased to 100 kgf/ffl.
It is intended to be strengthened to about 11".

On the other hand, a steel with a two-phase structure in which martensitic phase is dispersed in a ferrite matrix through special heat treatment using the components of such high-strength copper rings has been developed, which maintains strength while maintaining ductility and has good weldability. Therefore, it is attracting attention for its use in reducing the weight of structural parts. For example, 0.05% C-0 exhibiting a tensile strength of 600 MPa and an elongation of 20%
,02%Si-0,32%Mn-remaining Fe is annealed in a two-phase region of ferrite and austenite at approximately 790°C and then rapidly cooled to change the austenite to martensite. Compatible steel has a tensile strength of 65
It has an elongation of about 30% at 0 MPa, and has good weldability due to its low carbon content.

[Problem to be solved by the invention]

However, in order to further improve the high strength that is the first requirement of the high-strength steel, there is a tendency to increase the types and amounts of expensive alloying elements in the tempered high-strength steel. However, it is difficult to achieve this goal without sacrificing ductility, and welding cracks are likely to occur during welding. Even at already traditional intensity levels,
Hardening and cold cracking are likely to occur due to rapid heating and cooling of the weld heat-affected zone, and in order to prevent such embrittlement of the weld zone, preheating is required and there are also heat input restrictions. Furthermore, even in the above-mentioned low-alloy duplex steel, the amount of carbon added is kept to a low value, and the types and amounts of expensive alloying elements such as nickel, chromium, and vanadium are increased to ensure strength. In addition to requiring expensive heat treatment including quenching treatment, the product is expensive compared to the performance obtained.For example, it is used for lightweighting automobiles, which was originally intended when duplex steel was developed, i.e., thin sheets for car bodies. There were problems with economic efficiency, such as not being suitable for use as a. Additionally, if the amount of alloying elements mentioned above increases, the carbon equivalent, which is a guideline for weld hardening, will exceed the standard amount by about 0.4%, causing the weld heat-affected zone to harden and become brittle during welding. for,
There was a problem in that it was difficult to obtain a higher strength alloy steel that was practical.

Therefore, the present invention does not require the addition of large amounts of expensive reinforcing alloy elements, is easy to temper heat treatment, can obtain even higher mechanical strength without significantly impairing ductility, and has good weldability. The objective is to provide structural steel that is low in production cost and highly practical.

[Means to solve the problem]

To achieve the above object, the present invention provides, as weight %,
Carbon 0.02-0.5% and manganese 2.0-5.0%
Structural cylindrical manganese duplex stainless steel containing, preferably further containing 0.2 to 1.5% silicon, and more preferably further containing a member of the group consisting of molybdenum, niobium, tantalum, tungsten, chromium, titanium, vanadium and boron. We propose a structural cylindrical manganese duplex steel containing 0.02 to 0.5% of one or more of these elements.

The present invention will be explained in detail below.

The high manganese duplex steel of the present invention is produced by charging the raw material into a normal steelmaking furnace such as a converter, electric furnace, or high-frequency induction furnace, melting it, adjusting the composition, and casting. It is obtained by rolling to obtain a predetermined shape and then subjecting it to heat treatment. The heat treatment is carried out by annealing by holding in a ferrite-austenite two-phase region at 700 to 800'C for about 10 minutes, followed by gradual or rapid cooling such as air cooling, water quenching, oil quenching, etc.

In the case of the high manganese duplex steel of the present invention, the transformation temperatures A1 and A3 are significantly lowered due to the high manganese content, and the isothermal transformation curve and line are shifted to the long time side.
Duplex steel can be obtained not only by rapid cooling such as water quenching from a relatively low annealing temperature range of °C, but also by slow cooling such as air cooling. Its two-phase structure consists of a relatively soft ferrite matrix with a hard martensitic phase in the form of fine needles.
It is in a dispersed state in the form of particles or islands. The high manganese duplex steel of the present invention has high strength and ductility based on its structure, and can easily obtain tensile strength of 1,000 MPa or more and elongation at break of about 20% or more in the hardened state, and can be tempered. By doing this, the tensile strength was increased to 700MPa.
It is possible to have an elongation at break of 40% or more while maintaining a certain level.

The high manganese duplex steel of the present invention also has the characteristics of a typical duplex steel, in which the stress strain curve obtained by tensile measurements is continuous and does not exhibit abrupt yielding phenomena, and the tensile strength The ratio of yield strength to yield stress ratio is also 0.4 to 0.
7 and has good cold workability. moreover,
Strain aging, or baking, also has hardening properties, in that after a slight strain is applied, the load is removed and the yield strength increases significantly upon short heating at a relatively low temperature.

Although the high manganese duplex steel of the present invention has a high manganese content, it has excellent weldability, which is a major feature and an excellent advantage. That is, conventionally, for welding, the carbon equivalent of steel needs to be 0.4 or less, but for example, 0.1%C-3%Mn-0.5, which is also the basic dual phase steel of the present invention.
%Si-Fe alloy has good weldability even though its carbon equivalent is 0.6. This is T.I.G.
It has also been confirmed by welding and electron beam welding. In either case, the welded part is allowed to cool in argon gas or vacuum, but the structure of the welded part obtained at this time always shows a structure unique to duplex steels, in which the martensitic phase is finely dispersed in the ferrite matrix. There is.

If the cooling rate is high, the amount of martensitic phase increases, but when cooling is allowed to occur, its volume fraction is always less than 50% of the total. The mechanical strength of the weld is therefore increased, but it still retains considerable ductility. Therefore, in tensile measurements, a test piece that has been successfully welded will break outside the weld and exhibit approximately the same tensile properties as a non-welded test piece. Additionally, if the weld is further tempered by heating it to around 700°C for 10 minutes and then air-cooled, the amount of martensitic phase in the weld will be reduced, making the entire test piece a highly ductile steel. .

Next, the reason for limiting the content of each element in the high manganese duplex steel of the present invention will be explained.

Carbon is a useful element that improves strength, but 0.5%
In the above case, the mechanical properties become too brittle for use as a structural material, so the upper limit was set at 0.5%. Further, the lower limit was set at 0.02%, at which the reinforcing effect of carbon was observed.

In the present invention, manganese, along with carbon, is the most important basic element that improves the hardenability and strengthens the obtained duplex steel. However, if it is too large, the retained austenite phase will increase and the steel will become a different type of steel. Since it is no longer a dual-phase steel in the sense of 2 and also has a tendency to become brittle, the upper limit was set at 5% in consideration of economic efficiency. In addition, although there are slight variations due to other additive elements, the two-phase structure intended by the present invention can be obtained by air cooling from the two-phase region, and Considering that the upper limit is approximately 1.7%, the lower limit was specifically set to 2%.

Silicon has a reinforcing effect, but if it exceeds 1.5%, the strength will actually decrease, and if it is 0.2%, it will not show any significant reinforcing effect, so the preferred range is 0.2 to 1.5%. It was adopted.

High-melting point metals such as molybdenum, tantalum, and niobium are also effective reinforcing agents, but they are expensive elements, and it becomes difficult to obtain ductility if the amount is increased, so the upper limit is 0.5 mm, and 0.02% is considered to have a reinforcing effect. was set as the lower limit.

〔Example〕

Using various combinations of Armco iron, electrolytic manganese, 4% carbon-iron alloy, and ferroalloy as raw materials, various alloy steels are melted in a high-frequency induction furnace, and cast into a water-cooled mold under an argon gas atmosphere to produce a 2 kg ingot. And so. The components of the melted alloy steel are shown in Table 1. The ingot is 2 at 1,100″C.
A homogenization treatment was carried out under a flow of argon gas for 0 h, followed by hot rolling at 1,100°C to form 20 mm thick blocks of 5
．． The material was reduced to 5M, further reduced to 5 pupils by cold rolling, and then reduced to a tetravalent flat plate by hot rolling at 900°C, which was then air cooled to room temperature. This flat plate was mechanically cut to a thickness of 3InI11, an effective length of 32mm, and a width of 6.25mm.
A large number of flat plate tensile test pieces were prepared and subjected to appropriate tests.

[Table 1] Regarding the steel test piece of Alloy Steel No. 1, Ac, transformation temperature and Ac=transformation temperature were measured in advance with an expansion needle, and they were 680°C and 830°C, respectively. Alloy steel number 1
of steel specimens were divided into two groups, each with 70
After heat annealing at temperatures of 0 °C, 750 °C, and 800 °C for 10 minutes, the test pieces of the first group were allowed to cool in air to room temperature, and the test pieces of the second group were annealed. Quenched in room temperature water.

The metal structure after heat treatment was confirmed by scanning electron microscopy to be a two-phase fiber with martensite dispersed in a ferrite matrix. Further, the test piece was subjected to tensile measurement using an Instron testing machine, and the tensile measurement value was obtained from a stress stress curve.

The relationship between annealing temperature and tensile properties, that is, strength and elongation, is shown in Figure 1. In the figure, the solid line is for air cooling, the dotted line is for water cooling, σ8 is tensile strength, and σ is 0.2%.
Indicates permanent elongation yield strength, A8 is elongation at break and Au is uniform
Shows elongation.

It has been found that by adding 2% or more manganese to carbon steel, that is, a larger amount than ordinary structural manganese steel, the transformation temperatures A and A are significantly lowered, and the isothermal transformation curve is shifted to the longer time side. Therefore, in addition to rapid cooling such as water quenching from a relatively low temperature of 700 to 800"C,
Even by slow cooling such as cooling in air, the martensite phase is finely dispersed in the ferrite matrix.
A duplex steel with high strength and ductility was easily obtained.

As shown in the figure, for alloy steel number 1 steel, 70'C
, 750°C and 800″C, the tensile strength is approximately 550 MPa and 700 MPa, respectively.
At MPa and 650MPa, the elongation at break is 32, respectively.
%.

25% and 23% were obtained. This steel was heated to 700°C17
When quenched in room temperature water at 50°C and 800″C, the tensile strength is approximately 800 MPa and 90 MPa, respectively.
Although the elongation at break was high at 0 MPa and 1,100 MPa, the elongation at break decreased to 19%, 15%, and 10%, respectively. That is, in the duplex steel of the present invention, mechanical properties such as strength, ductility, and workability can be varied over a fairly wide range by adjusting the components and heat treatment.

Wikikyo I Test specimens of alloy steel numbers 2 to 5 with different carbon contents were
As in the case of test 1 above, ? After heat annealing at temperatures of 00"C, 750°C and 800°C for 10 minutes, the specimens were air cooled to room temperature. After confirming that the test pieces were duplex steel, tensile measurements were performed. The resulting tensile strength The relationship between the properties, that is, strength, elongation, and carbon content in the steel, is determined by the alloy steel number I.
Figures 2 (a), (b), and (c) show the results classified by annealing temperature, including the results for steel. In the figure, σ8 indicates tensile strength, σ indicates 0.2% permanent elongation yield strength, AI indicates elongation at break, and Au indicates uniform elongation.

As described above, it can be seen that the tensile properties of the dual phase steel of the present invention vary significantly depending on the carbon content.

That is, when the carbon content was 0.2% or less, the ductility was considerably high and the elongation at break could be maintained at 20% or more.

As the carbon content increases, strength increases and ductility decreases. However, if the carbon content is still below 0.5%, 700 to 7
Air cooling from 50°C increases tensile strength to 1,200~
The ductility was set to 1,400 MPa and the elongation at break was approximately 5%.

As in the case of Test 1, test specimens of alloy steel Nos. 6 to 9 with different silicon contents were heated at 700'C, 750
°C and 800''C for 10 minutes, and then air cooled to room temperature.After confirming that the obtained test piece was a duplex steel, tensile measurements were performed.The obtained tensile properties, i.e. The relationship between strength, elongation, and silicon content in steel is shown in Figures 3 (a), (b), and (C), classified by annealing temperature, including the results for the steel of alloy steel number 1. In Figure 1, the solid line is for air cooling, the dotted line is for water cooling, σ□ is tensile strength, σ is 0.2% permanent elongation yield strength, AI is elongation at break, and Au is uniform -Represents elongation.

As described above, by adding 0.2% or more of silicon, the elongation at break decreases slightly, but the tensile strength increases. That is, for steel with a silicon content of 0.5 to 1.2%, the angle of 700°
C and air cooling from 750°C, the elongation at break was 20, respectively.
% and tensile strength is 800MP while keeping it at 15%.
a and 1,000 MPa.

In addition, by quenching in room temperature water at 800"C, the tensile strength was increased to 1,000 while maintaining the elongation at break at approximately 10%.
M P a was shown.

Wipe-out soil Molybdenum, niobium, tantalum, vanadium.

700"C for specimens of alloy steel No. 10-16 with addition of titanium, chromium and tungsten.

After heat annealing at 750°C and 800°C for 10 minutes, test pieces of alloy steel No. 10 and steels 13 to 16 were air cooled and water quenched.
and 12 steel test pieces were air cooled. According to microscopic examination, the obtained test pieces all showed two-phase fibers in which martensite was further refined and distributed. The relationship between the tensile properties, that is, strength, elongation, and annealing temperature obtained for these test pieces is shown in Figures 4 (a), (b), and (c).
), (ct), (e), (r), (g). In the figure, the solid line shows the case of air cooling and the dotted line shows the case of water cooling, σ is tensile strength, σ is 0.2% permanent elongation yield strength, A, I are elongation at break, and AlJ is average elongation. represents.

That is, about 0.1% C, about 3% Mn, about 0.5% Si, and about 0.3% chromium in steel with the remaining Fe as the basic components.
%, for steel with approximately 0.1% of high melting point metal added, 7
The tensile strength is approximately 8 by air cooling from the annealing temperature of 00°C.
00-1,000 MPa and elongation at break of about 20% were obtained. When the annealing temperature was increased to 750°C, the elongation at break decreased only slightly, and the tensile strength further increased. The water-quenched samples showed a slight decrease in elongation at break depending on the annealing temperature, but the tensile strength increased and reached 1200 MPa when water-quenched from 800°C.

Thus, the duplex steels of the present invention include molybdenum, niobium, tantalum, vanadium, titanium, and chromium.

By adding small amounts of high melting point metals such as tungsten and boron, the martensite phase dispersed in the ferrite matrix is commonly made even finer, further increasing the mechanical strength.

Alloy steel number 19 with addition of molybdenum or vanadium
and 13 steel specimens at 800°C.
After annealing for 0 minutes and quenching in room temperature water, tensile measurements were performed on some of them, and on the others, 700'C for 1 hour for alloy steel number 1g, and 1 hour at 700'C for alloy steel number 13. was tempered at 700°C for 30 minutes, air cooled to room temperature, and subjected to tensile measurement. FIG. 5 shows the relationship between the tempering time and tensile properties, that is, strength and elongation, for alloy steel No. 19. In the figure, σ8
is tensile strength, σ is 0.2% permanent elongation yield strength, A,
l stands for elongation at break and A stands for average elongation.

As described above, the ductility of the dual-phase steel of the present invention can be greatly improved by tempering treatment. In other words, a duplex steel that showed a tensile strength of 1,200 MPa and a breaking elongation of 10% when quenched from 800°C had a tensile strength of about 700 MPa after tempering, but a breaking elongation of about 40%.
showed a large increase.

Although not shown, alloy steel No. 13 showed 1,150 MPa and elongation at break of about 13% in the hardened state, but after tempering, the elongation at break increased to about 30% against the tensile strength of about 750 MPa. did.

A test piece of alloy steel No. 14 containing titanium was cut and subjected to electron beam welding in vacuum using three passes. After cooling, a tensile measurement was performed and the test piece broke outside the weld.

Tensile property values are 0.2% permanent elongation yield strength 580MP
a, tensile strength 917 MPa, average elongation 10.8
% and elongation at break was 18.3%. The micro-Vickers hardness in the base metal region of this welded test piece was 290 and the martensite volume fraction was 30%, whereas the micro-Vickers hardness in the welded region was 340 and the martensite volume fraction was 42%.

In addition, the tensile property value of the original test piece that was not welded was
0.2% permanent elongation yield strength 507 MPa, tensile strength 890 MPa, average elongation 11.3% and elongation at break 19
．． It was 9%.

As described above, the welded area was slightly hardened due to the increase in the martensite phase, but the martensite volume ratio remained at 42%, and the mechanical properties of the welded specimen were almost the same as those of the unwelded specimen. It was shown that the weldability was excellent.

〔Effect of the invention〕

As is clear from the above description, according to the present invention, the following effects can be obtained.

With only air cooling from a relatively low annealing temperature of 700 to 800°C, it has higher strength and ductility than conventional low carbon low alloy steel.
A practically advantageous duplex steel can be obtained.

Good weldability is achieved despite the high manganese content, and the mechanical properties of the welded area are comparable to those of the non-welded area.

A simple composition based on carbon-manganese-iron is sufficient, and mechanical strength can be increased by adding a small amount of silicon without impairing ductility.Additionally, small amounts of high melting point metals such as molybdenum, niobium, etc. can be added to further improve mechanical strength. It is possible to improve the target strength.

The basic additive components carbon and manganese, as well as silicon, are inexpensive elements, and only a small amount of expensive high-melting point metals such as molybdenum can be added, and since refining is easy, manufacturing costs are low.

In particular, since the raw materials are cheap and duplex steel with high strength and ductility and excellent weldability can be obtained by air cooling from annealing temperature, it can be mass-produced and has a wide range of applications including automobile body applications. This yields highly economical high-strength steel suitable for

Even higher strength can be obtained by rapid cooling from a relatively low annealing temperature as described above, and high ductility can be obtained by simple tempering.

As a duplex steel, it has a characteristically low yield stress ratio, making it advantageous for processing.

The yield strength can be increased by utilizing the strain aging properties of the dual-phase steel.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between annealing temperature and tensile properties in the high manganese duplex steel of the present invention, and FIG. 2(a). (b) and (C) are diagrams showing the relationship between carbon content and tensile properties in the high manganese duplex steel of the present invention shown at different annealing temperatures, and Figures 3 (a), (b), and (C) are Figures 4(a) and 4(a) are diagrams showing the relationship between silicon content and tensile properties in the high manganese duplex steel of the present invention shown at different annealing temperatures.
b). (C), (d), (e), (Members are diagrams showing the relationship between the annealing temperature and tensile properties of the high manganese duplex steel of the present invention, and Figure 5 is a diagram showing the relationship between the annealing temperature and tensile properties of the high manganese duplex steel of the present invention. It is a diagram showing the relationship between tempering time and tensile properties in compatible steels, and in common to each diagram, the solid line indicates the case of air cooling, the dotted line indicates the case of water cooling, σ7 is the tensile strength, and σ5 is 0.2%. Perpetual elongation yield strength, A3 is the elongation at break, and A is the line showing the average elongation.

Claims (3)

[Claims] (1) A high manganese duplex stainless steel for structural use, characterized in that it contains 0.02 to 0.5% carbon and 2.0 to 5.0% manganese, with the remainder consisting of iron and unavoidable impurities. (2) Contains 0.02 to 0.5% carbon, 2.0 to 5.0% manganese, and 0.2 to 1.5% silicon as weight%,
High manganese duplex steel for structural use, characterized in that the remainder consists of iron and unavoidable impurities. (3) In addition to carbon 0.02 to 0.5%, manganese 2.0 to 5.0%, and silicon 0.2 to 1.5%, molybdenum, niobium, tantalum, tungsten, chromium, Containing 0.02 to 0.5% of one or more elements from the group consisting of titanium, vanadium and boron,
High manganese duplex steel for structural use, characterized in that the remainder consists of iron and unavoidable impurities.