Steel having high toughness in the heat-affected zone during welding

 

(57) Abstract:

Steel having high toughness in the heat-affected zone may be applied to ships, buildings, pressure vessels, pipelines, etc., Steel is an alloy system Ti-Mg-O containing at least 40 particles/mm2oxides and composite oxides of Ti and Mg having a size of 0.001 to 5.0 μm. Steel contains components in the following ratio, wt.%: carbon is 0.01 to 0.15, silicon is not more than 0.6, manganese 0.5 to 2.5, the phosphorus - not more than 0,030, sulfur, no more than 0.005, titanium from 0.005 to 0.025, aluminum is not more than 0.02, magnesium 0,0001-0,0010, oxygen 0,001-0,004, nitrogen 0,001-0,006, iron rest. The technical result consists in the fact that can be obtained steel having high toughness, which can markedly increased safety designs that use this steel. 2 C.p. f-crystals, 4 PL.

The invention relates to a steel having a high low-temperature toughness in the heat-affected zone (HAZ) during welding, and can be applied for structural steel materials that can be used in arc welding, electron beam welding, laser welding, etc.

More specifically, this invention relative to the Tali Ti and Mg, regulating the amount of O (oxygen) and fine dispersion of oxides and composite oxides of these elements.

One of the most important properties required for steel materials used for structures such as ships, buildings, pressure vessels, pipelines and so on, is the toughness in the HAZ. In recent years, has enjoyed great success technology of heat treatment processes, adjustable rolling and method of thermomechanical processing, therefore, the improvement of shock low temperature toughness of the steel material has become more feasible. However, the fine structure of the steel material is completely lost, because the heat-affected zone during welding reheated to a high temperature, and its microcrystalline structure, as a result, it becomes extremely coarse, which leads to a significant deterioration of the toughness in the heat-affected zone. Therefore, as a means of improving the structure in the heat-affected zone were considered and applied in practice: (1) technology restrictions escalation of austenitic grains by means of TiN and (2) technology education intergranular ferrite in the as in Ti steels oxide type, and in Vol. 79(1993), No. 10 describes the effect on the transformation of intergranular ferrite in the Ti-containing oxide steel. However, the level of toughness in the heat-affected zone obtained with the help of these technologies, is not quite satisfactory. Therefore, from the point of view of welding is highly desirable steel material, which has higher strength and can be used at low temperature and at a significant heat input.

The invention provides a steel material having high toughness in the heat-affected zone (for example, steel plate, hot rolled coil strip, high quality profile steel, steel pipes and so on ).

The inventors have conducted intensive studies of the chemical components of steel materials and their microstructure to improve toughness in the heat-affected zone and created a new steel having high toughness in the heat-affected zone.

The invention consists in a steel which contains, wt%:

C - 0,01 - 0,15

Si is Not more than 0.6

Mn - 0,5 - 2,5

P - Not more than 0,030

S - Not more than 0.005

Ti - 0,005-0,025

Al is Not more than 0.02

Nb: 0.005 to 0.10 to

V - 0,01 - 0,10

Ni - 0,05 - 2,0

Cu - 0,05 - 1,2

Cr - 0,05 - 1,0

Mo - 0,05 - 0,8

Fe and inevitable impurities - Rest

and it contains at least 40 particles/mm2oxides and composite oxides of Ti and Mg having a grain size of 0.001 to 5.0 μm.

When the above-described molten steel is, as Mg alloying additives used Mg in the shell of the iron foil.

Next will be explained the essence of the present invention.

The term"% " used in the following description means "weight%".

A feature of the invention consists in that the low carbon steel at the same time add traces of Ti and Mg, and in steel in regulating the amount of oxygen (O) finely dispersed oxides and composite oxides containing Ti and Mg (optionally containing MnS, CuS, TiN and so on).

The term "oxides and composite oxides containing Ti and Mg (optionally containing MnS, CuS, TiN and so on)" means here, mainly compounds such as oxides of Ti, Mg oxides or composite oxides of Ti and Mg in the steel, oxides and composite oxides of other elements such as Mn, Si, Al, Zr, etc., and compounds such as sulfides and composite sulfides Mn, Cu, Ca, Mg, etc., These compounds may optionally contain nits shall provide: (1) the formation of fine-grained intergranular ferrite in austenitic grains, which become large, and/or (2) the coarsening of austenite grains, they make the structure in the heat-affected zone of fine-grained and substantially increase toughness in the HAZ. In addition, the increase in toughness in the heat-affected zone can be achieved through the amount of Mg in the steel and form of Mg alloying additives. In other words, it was found that when pure Mg metal (at least 99%) in the shell of the iron foil add to the above item (1) exerts its influence, when the amount of Mg is not more than 0,0020% and above item (2) exerts its influence, when the amount of Mg exceeds 0,0020%. In addition, important factors are the size of the particles and the density of the composite oxides of Ti and Mg.

However, there is the case when, in addition to a compound of the oxides of Ti and Mg, there is an oxide of Mg separately and the amount of Mg is significant, and, in addition, there is the case when, in addition to a compound of the oxides of Ti and Mg, there is a titanium oxide separately, and the amount of Mg is small. However, problems do not arise as long as the size of the individual particles of oxides of Ti and Mg composite oxides of Ti and Mg amount of 0.001 to 5.0 μm, because they cocodimethylamine. The size of the particles oxidative oxides dispersed more and more thin, than the Ti oxide formed during the addition of only Ti, and their impact on the above paragraphs (1) and (2) is also larger. To obtain such effects, it is first necessary to limit the number of Ti and Mg, respectively, to values of 0.005 to 0.25% and from 0.0001 to 0,0010%. These quantities are minimum quantities required for fine dispersion of large quantities of composite oxides. The upper limit of Ti should be 0,025% in order to prevent the deterioration of low-temperature toughness due to the formation in the zone of thermal influence TiC, although the number of Ti changes in accordance with the amounts of O and N. In the production of steel is very difficult dispersing a large number of oxides of Mg, and for this reason, the upper limit of Mg is set to 0,0010%.

When the particle size of the composite oxide of Ti and Mg is less than about 0.001 μm, the oxide is so small that the effect of limiting escalation austenitic grain or the effect of formation of intergranular ferrite cannot be obtained. When the particle size exceeds 5.0 µm, the oxide is so large that the effect of limiting escalation austenitic grain or the effect of formation of intergranular ferrite cannot be avannah oxides is so minimal, the effect of intergranular transformation cannot be obtained. Therefore, the required density, component of at least 40 particles/mm2. To get a more fine-grained oxides of Ti and Mg in large quantities, it is important limiting the number of O (oxygen). When the amount Of is too small, a large amount of the composite oxide cannot be obtained, and Vice versa, when it is too large, the cleanliness of the steel is degraded. Therefore, the amount of oxygen should be between 0.001 and 0.004%.

Next will be explained the reasons for limiting the content of the constituent elements.

The number C is in the range from 0.01 to 0.15%. Carbon is an extremely effective element for increasing the strength of the steel and to obtain the cleaning effect of the crystal grains, it must at least 0,01%. When the amount of C is too large, the low temperature toughness of the base metal and heat-affected zone is extremely deteriorated. Therefore, the upper limit is set equal to 0.15%.

Silicon is an element, which is added for deoxidation and for increased strength. However, when its amount is too large the tion of 0.6%. The deoxidation of steel may be sufficient due to the Ti or Al, and therefore it is not always necessary to add Si.

Manganese is an essential element to guarantee the balance of strength and low-temperature impact toughness, and its lower limit is 0.5%. However, when the amount of Mn is too large, the hardenability of the steel is increased, therefore, deteriorates not only the toughness in the heat-affected zone, but also accelerated segregation in continuous casting (slab) and low-temperature toughness of the base metal is also deteriorating. Therefore, the upper limit is set equal to 2.5%.

The addition of Ti leads to the formation of fine-grained TiN, limits the coarsening of austenite grains during reheating of the slab and the heat-affected zone, makes the fine-grained microstructure and improves the low temperature toughness of the base metal and HAZ. When the amount of Al is small, Ti forms oxides, operates in HAZ as centres of education intergranular ferrite and makes fine-grained structure in the HAZ. To obtain this effect by adding Ti must be added at least 0.005% of Ti. However, if the amount of Ti is clickomania equal to 0.025%.

Aluminum is an element, which is usually contained in steel as rostislaleva element. However, when the amount of Al exceeds 0,02%, complex oxides of magnesium cannot be easily obtained. Therefore, the upper limit it is set to 0,020%. The deoxidation can be sufficiently achieved by Ti or Si, so you don't always need to add Al.

Magnesium is a highly rescission element and forms a fine-grained oxides, complex oxides containing traces of Ti and so on) when it reacts with oxygen. The oxides of Mg, thin-dispersed in the steel, are more stable even at high temperatures than TiN, limit the escalation of gamma grains throughout the zone of thermal influence or form a fine-grained intergranular ferrite inside the integrated austenitic grains and increase toughness in the HAZ. To obtain such effects is necessary at least 0.0001% per Mg. However, from the point of view of steel production in the steel is extremely difficult to add a large number of Mg. Therefore, its upper limit is set to 0,0010%.

To obtain a sufficiently fine-grained oxides while adding Ti and Mg is effective in the of aswat TiN, limits the coarsening of austenite grains during reheating of the slab and in the heat-affected zone during welding and improves the low temperature toughness of the base metal and HAZ. The minimum amount required for this purpose, is 0.001%. However, when the number N is too large, due to the formation of solid solution N, may be scratching the surface of the slab and the deterioration of the toughness in the heat affected zone. Therefore, the upper limit should be set equal to 0,006%.

In the present invention the number of P and S, which are contaminating impurities, limit accordingly to the level of not more than 0,030% and not more than 0.005%. The main reason is to further increase the low-temperature toughness of the base metal and HAZ. Reducing the number of P leads to a decrease segregation slab, prevents the destruction of grain boundary and improves the low temperature toughness. The decrease in the number of S leads to a decrease MnS, which is extended by means of an adjustable rolling, and increases toughness.

Next will be explained the purpose of the addition of Nb, V, Ni, Cu, Cr and Mo.

The main purpose of adding these items the bone, viscosity in the HAZ, and so on, and the increase in the volume of steel produced without deteriorating the excellent properties of the steel of the present invention. Therefore, their quantity must be naturally limited.

When the joint presence with Mo niobium limits the recrystallization of austenite during controlled rolling, making the crystal grains fine, improves the dispersion hardening and hardenability and makes the steel viscous and durable. You need at least 0.005% of Nb. However, when the number of added Nb is too large, it has an adverse effect on toughness in the HAZ. Therefore, its upper limit is set at 0.10%.

Vanadium has essentially the same impact that the Nb, but it seems that its effect is weaker than the influence of Nb. Must be added at least 0.01% of V, and from the viewpoint of toughness in HAZ upper limit set at 0.10%.

Nickel is added to increase strength and low temperature toughness. It was found that, compared with the addition of Mn, Cr and Mo, the addition of Ni forms fewer utverzhdenii reinforced structure, which is harmful to nicotine the trace amounts of Ni is also effective to improve toughness in the HAZ (especially effective amount of Ni for toughness in the HAZ is at least 0.3 percent). However, if the added amount is too large, deteriorating not only toughness in the HAZ, but also worsens the economic effect. Therefore, its upper limit is set equal to 2.0%. The addition of Ni is also effective to prevent cracking of the si during continuous casting and hot rolling. In this case, the Ni must be added in a quantity of at least 1/3 of the number of Cu.

Copper has essentially the same effect, which is Ni, and is effective to improve corrosion resistance and resistance to cracking caused by hydrogen. The addition of Cu in an amount of at least about 0.5% significantly increases the strength due to precipitation hardening. However, when it is added in an excessive amount, there is a reduction in toughness of the base metal and the toughness in the HAZ due to precipitation hardening, and due to precipitation hardening is the occurrence of cracking during hot rolling. Therefore, its upper limit is set equal to 1.2%.

Chromium increases the strength of the base metal and the welded part. However, when its amount is too large, udarn>Molybdenum severely limits the recrystallization of austenite during controlled rolling, when it is present together with Nb, and is also effective for improving the quality austenitic structure. However, excessive addition of Mo degrades toughness in the HAZ, and its upper limit is set equal to 0.80%.

The lower limit of each of the elements Ni, Cu, Cr and Mo is such minimum number, in which the impact on the material due to the addition of these elements becomes visible.

Next will be explained the size and number of particles of composite oxides of Ti and Mg.

When the particle size of the composite oxide of Ti and Mg is less than about 0.001 μm, the effect of formation of intergranular ferrite or the effect of limiting the escalation of austenitic grains cannot be obtained, while when it exceeds 5.0 µm, oxide particles become so large that the oxide does not provide the effect of formation of intergranular ferrite, and the effect of limiting the escalation of austenitic grains cannot be obtained.

When the density of particles of a composite oxide of Ti and Mg is less than 40 particles/mm2the number of dispersed oxide particles is small and the oxide particles are least 40 particles/mm2.

Incidentally, the density of the oxides of Ti and Mg or a compound oxide is determined by collecting samples from provisions amounting to 1/4 of the thickness of the irradiation beam diameter of 1 µm of the sample surface in the range of 0.5 mm X 0.5 mm with the use of MCA (Computer microanalyzer) and calculating the number of oxide particles per unit surface area.

Next will be explained Mg weld filler material. In the present invention as Mg filler material can be applied to metallic Mg (at least 99%), in the shell of the iron foil, and melt it in steel. If the metal Mg directly loaded into the molten steel, the reaction is so strong that the molten steel, in all probability, will be sprayed. Therefore, Mg metal wrapped in metal foil. The reason for using iron foil is that it prevents the impurities in the molten steel, and when the foil from the iron alloy has essentially the same composition that has a product, not a problem. In this case, as Mg filler material can be used Mg alloy, for example, Fe-Si-Mg or Ni-Mg alloy.

Through laboratory melting polycellulose foil. These ingots under different conditions were laminated in a plate, having a thickness of 13 to 30 mm, and studied their mechanical properties. Mechanical properties (yield strength: YS, ultimate tensile strength TS, the energy absorption of impact strength according to Charpy at - 40oC: vE-40and the transition temperature from ductile fracture to brittle by Sharpie: vTrs studied in the transverse direction. Toughness in the HAZ (impact strength Charpy at -20oC: vE-20) defined by replaying through HAZ equipment, reproducing the cycle of heating (maximum temperature: 1400oC, the cooling time from 800 to 500oC [t800-500]: 27). The size and number of particles of a composite oxide of Ti and Mg was studied by analysis of SMA using a beam diameter of 1 µm.

The oxide particles was determined by electron microscopy.

Examples are presented in table. 1. Steel sheets obtained in accordance with the present invention, had an impact strength Charpy in HAZ at -20oC, equal to at least 150 J (j.) and high toughness in the HAZ.

In contrast, since comparative steel contained inappropriate chemical components or had napthol is extremely low.

As the number O become N 15 small, the density of particles of a composite oxide of Ti and Mg is small, and impact strength Charpy in HAZ was low. Because the amount of Al in steel N 16 is too large, the density of particles of a composite oxide of Ti and Mg was barely noticeable and toughness in the HAZ was low. Since the amount of Ti in the steel N 17 was small, the density of the particles of complex oxide of Ti and Mg was little and impact strength Charpy in HAZ was low. Since the amount of Ti in the steel No. 18 was great, impact strength Charpy in HAZ was somewhat low. As the number Of steel No. 19 was great, the grain size of the particles of a composite oxide of Ti and Mg was large and impact strength Charpy in HAZ was low. As to steel N 20 Mg is not added, impact strength Charpy in HAZ was somewhat low (see tab. 2-4).

Using the invention it is possible to stably carry out the mass production of steel material, which has high toughness in the HAZ and can be applied to structures such as ships, buildings, pressure vessels, pipelines, etc., the result can be considerably improved the security of ships, buildings, pressure vessels and pipelines.

1. deposits, manganese, phosphorus, sulfur, titanium, aluminum, magnesium, oxygen, nitrogen and iron, characterized in that it contains components in the following ratio, wt.%:

Carbon - 0,01 - 0,15

Silicon is Not more than 0.6

Manganese - 0,5 - 2,5

Phosphorus - Not more than 0,030

Sulfur is Not more than 0.005

Titanium - 0,005 - 0,025

Aluminum is Not more than 0.02

Magnesium is 0.0001 - 0,0010

Oxygen - 0,001 - 0,004

Nitrogen - 0,001 - 0,006

Iron - Rest

it contains at least 40 particles/mm2oxides and (composite oxide of titanium and magnesium grain size of 0.001 to 5.0 μm.

2. Steel under item 1, characterized in that it further comprises at least one of the following components, wt%:

Niobium - 0,005 - 0,10

Vanadium - 0,01-0,10

Nickel - 0,05 - 2,0

Copper - 0,05 - 1,2

Chrome - 0,05 - 1,0

Molybdenum - 0,05 - 0,8

it contains at least 40 particles/mm2oxides and compound oxides of titanium and magnesium grain size of 0.001 to 5.0 μm.

3. Steel under item 1 or 2, characterized in that when it is received as magnesium filler material used magnesium metal in the shell of the iron foil.

 

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FIELD: ferrous metallurgy.

SUBSTANCE: invention provides round-profiled iron smelted from alloyed steel composed of, wt %: carbon 0.06-0.11, manganese 0.30-0.9, silicon 0.001-0.15, boron 0.0005-0.0050, vanadium 0.005-0.08, aluminum 0.02-0.06, titanium 0.01-0.04, sulfur 0.005-0.020, nitrogen 0.005-0.015, calcium 0.001-0.010, iron and unavoidable impurities - the balance. When following relationships are fulfilled: Ti/48+Al/27-N/14 ≥ 0.6 x 10-3; Mn+5.0C ≥ 0.80; Ca/S ≥ 0.065, rolled iron has following characteristics: maximum degree of pollution with nonmetal inclusions, in particular sulfides, oxides, silicates, and nitrides, does not exceed 3 points for each type of inclusions; longitudinally uniform spheroidized structure composed of at least 60% grainy perlite; effective grain size 5-10 points; diameter 10-16 mm; carbon-free layer not exceeding 1.0% of diameter; cold setting value at least 1/3 height; throughout hardenability in circles up to 16 mm in diameter; point of maximum load not higher than 500 MPa; relative elongation at least 22%; and relative contraction at least 70%.

EFFECT: ensured optimal conditions for cold die forging of high-strength geometrically complex fastening members and simultaneously improved steel hardenability characteristics.

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