JPH0132301B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a structural steel that exhibits high working strength, good weldability up to a given carbon content (0.3% by weight), and resists atmospheric corrosion, which steel is capable of resisting large cyclic stresses and It is particularly intended for the construction of structures and frames exposed to adverse weather conditions, above-ground or underwater structures, vehicles, machinery and machine elements, substructures and superstructures for railways, etc. The current economic situation, which in particular requires a general reduction in energy and material consumption, means that the properties of conventional steels are no longer satisfactory in all industrial sectors, especially in the exploration, production and transport of hydrocarbons. This has made it necessary to meet technical and economic requirements that have not been possible and thus, in a sense, have been a brake on development in these fields. developing useful construction and manufacturing methods and standard technologies, applying new techniques and technical solutions and, to a certain extent, exploiting underground resources hitherto untapped for technical and economic reasons; This is only conceivable unless new steels are available that exhibit favorable composite properties and sufficient fatigue strength for industrial changes and must be produced at sufficiently low cost to be widely used. be. Therefore, new steels can be developed which offer greater reliability and withstand current stresses even though the cross-sectional area of the structure and therefore the weight of the structure itself are significantly smaller in order to meet the requirements for energy and material saving. This conversion to new steels also eliminates certain costs associated with manufacturing products made from traditional steels. It was essential to develop a new steel that would not exceed Structural steels are already known which exhibit good mechanical properties and good weldability under suitable conditions. In the field of weldable steels, for example, the following steels can be listed: US T1, RQC-100,
HY and NAXTRA, or Japanese HT, HW,
KLN and RIVER-ACE. The chemical composition of these steels is characterized by the following components: : That is,
By weight, 0.10-0.23% C, 0.50-1.50% Mn,
0.60-1.50% Cr and 1.0-9.5% Ni, and some steels also contain 0.50-1.00% Mo, 0.08-0.15%
of V, 0.003-0.04% B and 0.5-0.7% Cu. The characteristics of these steels are that their apparent elastic limit (load for 0.2% permanent elongation) is 51.0 to 71.4 kg/
mm ² and that their plasticity makes them suitable for industrial deformation. The fatigue strength limit is 20.4 to 40.8 Kg/mm 2 for stress R = ^-1 and 25.5 to 51.0 Kg/mm ² for stress R = 0 in the case of fracture that occurs after applying a stress of 10 ⁵ ( (for non-welded specimens). Some of these steels exhibit some degree of atmospheric corrosion resistance. However, the disadvantage of these steels is that good properties can only be imparted to them by quenching and tempering treatments carried out in special equipment. Therefore, the mechanical properties are obtained as a result of the quenching and tempering process, which not only limits the number of product shapes that can be made with the mechanical properties, but also because the quenching reduces the uniformity. This makes the mechanical properties very unstable, and the production costs are high due to the limited throughput capacity of the quenching and tempering equipment and the complexity and high cost of this equipment. This results in costs that are several times higher than it would take to produce regular steel. Materials in the condition of being quenched and tempered pose additional difficulties in industrial deformation, especially in hot cutting, forming welded joints and hot bending. Therefore, despite its excellent mechanical properties, the applications of steels subjected to quenching and tempering are limited by the number of desired cross-sectional shapes, lack of uniformity of mechanical properties, and difficulties with respect to deformation. It is very limited due to its size and high price. In the field of non-weldable steels, known steels with excellent mechanical properties include:
For example, the following steels may be listed:
En and AISI-V developed in the US, Soviet Union
GhNW, British ReX, Melt-A and HST, or West German CSV4 and MOG. The chemical composition of these steels is characterized by the following components:
i.e., by weight, 0.2-0.6% C, 0.2-1.6% Si,
0.3-1.6% Mn, 0.3-5.0% Mo and 0.1-1.0%
V, but some steels also contain 1.5 to 3.0% W and
It also contains 0.1-0.3% Ti. These steels are characterized by their apparent elastic limit (load for 0.2% permanent elongation) of 132.6 to 163.2 Kg/mm ² and ultimate tensile strength of 173.4 to 204.0 Kg/mm when subjected to quenching and tempering treatment. ² , the corresponding elongation is 7 to 10% and the impact strength in a notched or isot test piece is 0.71 to 2.0 Kg·m/cm ² . Their fatigue strength limit is the stress ^R =
0 to 40.8 to 81.6 Kg/ ^mm2 . The disadvantage of these steels is that their above-mentioned properties appear only after quenching and tempering treatment,
This treatment greatly limits their use due to the difficulty of deformation (casting scale, forging scale, bending, machinability) and also makes these steels rather brittle as well as susceptible to notch effects. Moreover, their manufacturing costs, due to their high alloying element content, make them practically unsuitable for use on a large industrial scale. Therefore, currently known structural steels have fairly good mechanical properties in both weldable and non-weldable areas, since they have been subjected to a heat treatment, i.e. quenching followed by tempering, with the addition of alloying elements such as those mentioned above. Show characteristics. However, this method of increasing strength limits the number of product geometries that can be manufactured, and hardened structural elements are difficult to cut with regular machine tools, and furthermore, the hardened structural elements are difficult to cut with regular machine tools, In the case of structural elements subjected to high quenching temperatures, high quenching temperatures cause decarburization, bending and possibly cracking. Special equipment is required to produce these steels, which further increases costs and makes them impractical for use on a large industrial scale. The combination of the disadvantages mentioned above leads to the use value of these steels being considerably reduced, despite their clearly favorable mechanical properties. The object of the present invention is to provide a structural steel that resists wear and atmospheric corrosion and exhibits good weldability up to a predetermined carbon content (0.3% by weight), the fatigue strength limit and apparent elastic limit of which are lower than those of conventional steels. Structures, frames, above-ground or underwater structures that are larger than those of steel and are exposed to greater cyclic stresses and adverse weather conditions;
As a base material for making vehicles, machines and machine elements,
The objective is to develop structural steels that can be used without hardening thanks to various strengthening mechanisms. The present invention aims to achieve the above-mentioned object by providing a structural steel which exhibits high fatigue strength and good weldability up to a carbon content of 0.3% by weight and resists atmospheric corrosion, with a carbon content of up to 1.6% by weight. , 0.3-3.0% Mn,
Maximum 1.8% Si, 0.6-4.0% each Cu and Ni, maximum 3.0% Mo, 0.02-0.4% each Nb and V, maximum 0.006% B, maximum 0.4% Zr, 0.02- 0.2%
Al, 0.005~0.2% N, 0.0001~0.005% Ca,
This is achieved by providing a structural steel with a maximum of 0.25% Pb, a maximum of 0.1% S, and the balance Fe and unavoidable impurities. The reason why the component composition (weight %) in the present invention is limited as described above is as follows. In general, strength increases with increasing C content, but further increases in strength beyond 1.6% result in lower toughness, so the upper limit is limited to 1.6% to obtain the desired strength. On the other hand, weldability decreases with increasing C content, so if it is desired that the inventive steel be a weldable steel rather than having high strength, good welding can be achieved up to a C content of 0.3%. Show your gender. Mn increases strength without impairing weldability, and S is added to improve machinability.
It has the effect of preventing red heat embrittlement caused by S by being finely and uniformly dispersed in the form of MnS. At least 0.3% Mn is required to obtain these effects. However, if the content exceeds 3.0%, machinability deteriorates. Si is an effective element for deoxidizing, and is effective for increasing strength by solid solution in the matrix. However, if the content exceeds 1.8%, the toughness decreases. Cu is effective for increasing atmospheric corrosion resistance and obtaining high strength. At least 0.6% Cu is required to obtain these effects. but,
If the content exceeds 4.0%, hot brittleness will occur and surface cracks will occur during hot rolling, so the upper limit is limited to 4.0%. Ni prevents hot embrittlement that may occur due to the presence of Cu, and therefore substantially
It is added in an equivalent amount. Ni also strengthens the matrix and improves toughness at low temperatures. To obtain these effects, Ni must be at least 0.6
% contained. However, even if it is contained in an amount of 4.0% or more, no further effect can be obtained and the cost only increases. Mo is an effective element for increasing strength and impact strength, but no further effects can be obtained even if it is contained in an amount of 3.0% or more. Both Nb and V are carbide-forming elements and have the effect of refining crystal grains. For this purpose, each element should be contained in an amount of at least 0.02%, but if excessively contained, the toughness will deteriorate, so the upper limit is limited to 0.4% each. B is effective in increasing strength without impairing toughness and weldability. However, if it is contained in excess
The upper limit is limited to 0.006% because Fe ₂ B is generated and causes red-hot brittleness. Zr is effective in deoxidizing and desulfurizing. Furthermore, Zr
improves corrosion resistance and refines grains to increase strength, elongation, and reduction of area. However, even if it is contained in an amount of 0.4% or more, no further effect can be obtained and the cost will only increase. Al is very effective as a deoxidizing agent, and N
It is effective for grain refinement because it combines with AlN to form finely precipitated AlN. To that end, at least
0.02% is necessary, but if it is contained more than 0.2%, it will make the steel brittle. N strengthens and stabilizes austenite to increase its strength at room temperature, and for this purpose, at least 0.005% is required. However, if it is contained in an amount of 0.2% or more, the toughness, especially at low temperatures, deteriorates, which is not preferable. Ca acts as a deoxidizing agent and has the effect of controlling the shape of nonmetallic inclusions and improving ductility. To obtain these effects, at least
0.0001% is required. However, even if it is contained in an amount of 0.005% or more, no further effect can be obtained, and workability is rather deteriorated. Pb does not form a solid solution in steel, and when dispersed finely and uniformly, it is very effective in improving machinability. In order to obtain this effect, the steel of the present invention contains Pb in addition to S. However, if the content exceeds 0.25%, the toughness deteriorates. The steel of the present invention contains S in order to improve machinability to some extent. However, if the content exceeds 0.1%, the toughness deteriorates. Some of the alloying elements form multicomponent intermetallic compounds when they are present in the proportions according to the invention;
A part of the compound already forms nuclei of a critical size that are effective for crystallization during the casting stage, and another part forms a solid solution in an interstitial manner, thus creating a prestress in the iron crystal lattice. Increase the number of lattice defects. Other alloying elements give rise to the precipitation of intermetallic compounds with high shear strength, which increase and at the same time stabilize the internal stress of the iron crystal lattice. As the number of critical size nuclei increases, the crystallization capacity of the melt appearing during casting becomes much greater, thus shortening the solidification time and reducing the coarsening of the primary crystals, thus increasing the grain size. Enrichment of undesirable impurities at the grain boundaries is reduced because the interfacial area is significantly increased. The advantageous properties and proportions of the components in the alloy system according to the invention create the following thermodynamic, thermokinetic and crystal nucleation conditions during solid solution, solidification, recrystallization and hot deformation: That is, the condition is such that the arrangement of the components in interstitial solid solution, the amount of these components, and thus the number and degree of prestressed crystal grains are clearly increased. Metallurgically induced dislocations that favor and dominate the formation of intermetallic precipitates and their dispersion due to an increase in the number of crystal lattices exhibiting prestress due to interstitial solid solutions and the extent of such stress. The number of is increased significantly, which greatly enhances the effectiveness of the precipitate's ability to immobilize dislocations when the load-induced dislocation front attempts to move. The components according to the invention and their preferred proportions thus automatically ensure the good metallurgical properties of the steel and the positive effect of the various reinforcing fibers and their The combined action increases the practical mechanical strength and fatigue strength limits of the steel. The chemical composition of the steel of the present invention also includes Cu, which is enriched on the surface of the steel without combining with iron. Therefore, over a long period of time, a dense and indestructible protective layer is formed on the surface of the steel by the action of the atmosphere, and this protective layer eliminates the possibility of corrosion due to dirt and paints. By improving the adhesion of steel to steel, it protects the steel from the corrosive effects of the surrounding environment and fluids. The steel according to the invention can be successfully welded up to a given carbon content (0.3% by weight) by applying appropriate heat, in which case the properties of the heat-affected zone are identical to those of the base metal. Since the steel of the present invention does not require a reducing atmosphere, it can be produced using ordinary equipment.
Further, the processing can be carried out by rolling or forging to give the steel any desired dimensions and shape using hot forming methods, and thus mass production is possible without using any special equipment. The steel of the invention exhibits excellent mechanical properties without quenching, while at the same time being amenable to common deformation and assembly techniques. In the field of non-weldable steels, the deformability or machinability can be adjusted by heat treatment, and the hardness after cutting can be adjusted by low temperature heat treatment. The price of the steel according to the invention as a base material is thus not subject to the costs of the quenching and tempering treatment carried out in special liquids and the cost of the equipment required for this treatment. Moreover, the manufacturing costs of products made from the steel of the invention do not exceed the costs of conventional products. For this reason, the technical effects brought about by the steel according to the invention due to its high fatigue strength and elastic limits (i.e. reduced energy consumption and weight, etc.)
The economic benefits derived from this are virtually unaffected by the costs associated with manufacturing and using new matrix materials. The invention will be better understood from the following detailed description of the examples given as non-limiting illustrations of the manufacture of steels and their mechanical properties. EXAMPLE 1 As an example, three charges of the steel of the invention in the field of weldable steels were prepared. These charges were made in a 60t electric arc furnace and then smelted in a ladle metallurgical apparatus. Casting is carried out in a continuous casting machine equipped with four molds with a cross-sectional shape of 240 x 240 mm.The barets thus made are rolled under normal conditions to produce a 20 mm diameter steel bar, which is then cooled in a cooling device. did. The test results of these charges according to the present invention are listed below. 1.1 Chemical composition of charge (wt%)

[Table] In these examples, the meanings of the following abbreviations are as follows. R _p Elastic limit (Load for 0.2% permanent elongation) R _n Breaking load A ₅ Elongation Z Reduction of area KCU Impact value (Value of Charpy impact test using U-notched specimen: +20℃, -40℃ is test temperature) 1.2 Mechanical properties

[Table] Note 1: Rolled state without heat treatment Note 2: Air cooling after holding at 500℃ for 90 minutes Note 3: Air cooling after holding at 1250℃ for 45 minutes
1.3 Weldability Tests were carried out on specimens made of Charge 2 and welded in an inert atmosphere from 12 mm thick plates. The plates received no heat treatment before or after welding. Plate thickness = V = 12 mm Welding type = butt welding (60° bevel angle) Welding heat input = 3000 joules/cm Number of welds = 3 + 1 Inert atmosphere = CO ₂ Welding rod = the material itself with a diameter of 1.6 mm 1.31 Tensile test R _p = 80.0Kg/mm ² R _n = 92.1Kg/mm ² A ₅ = 16% Z = 52% Fracture occurred outside the weld. 1.32 Plasticity of heat affected zone

【table】

【table】 1.4 Corrosion resistance against air (Measured in the air at an industrial site)

[Table] 1.5 Fatigue strength Fatigue or durability tests are performed using the Schenk-Erlinger test, which operates on the resonance principle.
The test was carried out on a type fatigue testing machine. In this case, both the static load component and the oscillating load (±Fa) were applied by springs supported by a common load head. The static load was generated and adjusted by a threaded rod, and the oscillating spring was excited by an electric motor. The vibrations generated by the rotation of the eccentric wheel actuate the pulsator at the resonance point, and this pulsator has a range of 0 to 20000Kg.
(20mega−Pond) static load and ±10000Kg
(10mP) cyclic load was generated. Diameter 20mm made by rolling from charge 2
of steel bars were fatigue tested. The results of the control tensile test conducted on the rolled test piece without heat treatment are the results of the fifth test.
Shown in the table.

[Table] For comparison, tests using the same method and equivalent test pieces were conducted on 42CD4 steel. The chemical composition of the steel used as a basis for this comparison is shown in Table 6, and the mechanical properties are shown in Table 5.

【table】 1.51 Degree of load for fatigue tests

[Table] 1.52 Stress applied to the test piece corresponding to the above load level or class

【table】 1.53 Fatigue test results

[Table] 1.54 Interpretation of Fatigue Test Results Comparing the test results obtained by applying the same stress to 42DC4 steel and the steel made from Charge 2 according to the present invention, the probability of failure of 50% was determined based on the comparison. It can be seen that in the case of steel (42CD4 steel) a stress of approximately 60,000 corresponds to the probability value, whereas in the case of the steel according to the invention a stress of approximately 700,000 corresponds. As is clear from the comparison of the results obtained using the same test method, the life of the steel according to the present invention is approximately 10 times longer than that of the conventional steel used as a standard for comparison, even under the same stress. By comparing the fatigue strength or durable load corresponding to a probability of failure of 50%, that is, by comparing two straight lines (so-called S-N curves) indicating the fatigue strength of the two materials at that probability value, the It can be seen that the load that can be supported is approximately twice that of 42CD4 steel.

【table】

Table: Example 2 As an example, two charges of the steel according to the invention in the field of non-weldable steels were prepared. These charges were made in a 65t electric arc furnace and then smelted in a ladle metallurgy apparatus. Casting was carried out in a continuous casting apparatus equipped with a mold having a cross-sectional shape of 240 x 240 mm. A steel bar with a diameter of 20 mm was produced from the thus produced billet by rolling under normal conditions, and this was air-cooled in a cooling device. The test results are shown below. 2.1 Chemical composition of charge (wt%)

【table】 2.2 Mechanical properties

[Table] 2.3 Fatigue Strength The fatigue test tests the characteristics of the steel of the present invention when a vibration load that changes over time is applied. The test method used was the Locati method, which tests the strength against combined bending and torsion loads, as well as the usual fatigue test in which torsional fatigue specimens are subjected to cyclic torsional loads. The results were processed by a computer and the vibration intensity was finally calculated. In the Charge 4 fatigue test, specimens made from rolled and stress-relieved steel bars with a diameter of 40 mm were used. Table 13 shows the results of a static mechanical strength test of the steel bar rolled from this charge 4.

[Table] 2.31 Fatigue test with cyclic torsional loading This test was to obtain a Wo¨hler diagram for a combined loading of bending and symmetrical vibration. 2.32 Loading degree or class for fatigue tests with cyclic torsional loading

【table】 2.33 Factors in fatigue testing with cyclic torsional loading

【table】 2.34 Test results

[Table] 2.35 Data regarding the distribution of fatigue test results using a cyclic torsional load after they are processed by a computer

[Table] 2.36 Fatigue test with torsional loading This test was to obtain a Wo¨hler diagram for a combined torsional and symmetrical vibration load. 2.37 Degree or class of load for fatigue tests due to torsional loading

[Table] 2.38 Factors for fatigue tests using torsional loads Table 19 Initial torque M = 2.00 Kg・m Initial stress T = 29.7 Kg/mm ² Value of degree or class of stress or load Ta =
1.50Kg/mm ² Number of stress N ₁ = 10 ⁵ Number of cycles △N = 10 ⁵ 2.39 Results of fatigue test using torsional load

[Table] 2.4 Dynamic strength values against vibration obtained based on fatigue tests using cyclic torsional loading

[Table] 2.5 Vibration strength values obtained based on fatigue tests using torsional loads

[Table] 2.6 Interpretation of Results Vibration strength obtained for a 40 mm diameter steel bar of the invention steel that was not subjected to quenching and subsequent tempering treatment, that is, Rvh = 38.0 to 45.0 Kg/mm ² and Tv = 25.9 to The value of 26.0 Kg/mm ² corresponds to the vibration strength value of conventional spring steels that have been hardened or hardened and subsequently tempered. It should be noted that in the fatigue test with cyclic torsional loading, with an appropriate preload generated by a stress of approximately 44.9 Kg/ ^mm2 ,
The strength value against vibration has been significantly improved by several million orders of magnitude. Therefore, when the steel of the invention is used in structures such as frames, its strength against vibrations is significantly increased, and this is the essence of the very useful properties of the steel of the invention.

Claims (1)

[Claims] 1 Structural steel that exhibits high fatigue strength, good weldability up to a carbon content of 0.3% by weight, and resists atmospheric corrosion, with the maximum
1.6% C, 0.3-3.0% Mn, maximum 1.8% Si,
0.6~4.0% Cu and Ni each, maximum 3.0% Mo,
0.02~0.4% Nb and V each, maximum 0.006% B,
Maximum 0.4% Zr, 0.02~0.2% Al, 0.005~0.2%
N, 0.0001~0.005% Ca, maximum 0.25% Pb,
Structural steel consisting of a maximum of 0.1% S and the balance Fe and unavoidable impurities.