Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon

Definitions

• the present invention relates to a super-clean steel excellent in cold workability and fatigue properties and, more specifically, to a super-clean steel having excellent performance when used for ultrahigh-tensile wire, ultrafine wire, high strength springs, and ultrathin flat springs.
• Japanese Examined Patent Publication No. H6-74484 discloses a steel wherein non-metallic inclusions with their length (1) and width (d) satisfying the relation 1/d ⁇ 5 at an L section of a rolled steel material comprise, as the average composition, one or both of 50% or less of CaO and 15% or less of MgO, in addition to 20 to 60% of SiO 2 and 10 to 80% of MnO. Additionally, Japanese Examined Patent Publication No.
• H6-74485 discloses a steel wherein non-metallic inclusions with their length (1) and width (d) satisfying the relation 1/d ⁇ 5 at an L section of a rolled steel material comprise, as the average composition, 35 to 75% of SiO 2 , 30% or less of Al 2 O 3 , 50% or less of CaO, and 25% or less of MgO. Although most inclusions in steel are well stretched by hot rolling, some of them remain not stretched sufficiently. According to the inventions disclosed in the above patent publications, it is possible to obtain a super-clean steel having excellent cold workability and fatigue properties through breaking down and dispersing even the non-metallic inclusions satisfying the relation 1/d ⁇ 5, which are not sufficiently stretched by hot rolling, in small fragments by cold rolling or wire drawing.
• the inventions disclosed in the Japanese Examined Patent Publication Nos. H6-74484 and H6-74485 form inclusions of complex composition through compound deoxidation by adding ferroalloys containing one or more of Ca, Mg and, if required, Al after adding Si, Mn and other necessary elements in molten steel. Since Ca and Mg ferroalloys added in molten steel are expensive, if the consumption of these costly ferroalloys can be reduced, then manufacturing costs preferably decrease.
• the object of the present invention is to provide a super-clean steel excellent in cold workability and fatigue properties even with the reduced consumption of Ca and Mg ferroalloys.
• the first present invention is to carry out compound deoxidation using Si, Mn and one or both of Ca and Mg, but not using Al in order to eliminate Al 2 O 3 in inclusions to the utmost.
• the gist of the first present invention is as follows:
• the aforesaid conventional technologies soften the inclusions satisfying the relation 1/d ⁇ 5 by forming a complex composition therein.
• the SiO 2 content in the inclusions is specified to be not more than 60% or not more than 75%. This is based on the understanding that hard SiO 2 inclusions are formed when the SiO 2 content exceeds the aforesaid concentration.
• the present inventors discovered through studies that, even though the inclusions satisfying the relation l/d ⁇ 5 have a high SiO 2 content, they do not cause any harm during cold rolling or wire drawing after hot rolling if their size is small. Though SiO 2 inclusions are hard, they are softer than CaO, MgO or Al 2 O 3 inclusions. Therefore, cold workability and fatigue properties of a steel material are kept sufficiently good if the size of the inclusions is controlled to the range of d ⁇ 40 ⁇ m. It is further preferable to control the size of high SiO 2 inclusions satisfying the relation 1/d ⁇ 5 to the range of d ⁇ 20 ⁇ m.
• the composition B (B1, B2) denotes the composition range of the inclusions which are sufficiently soft and are broken down and dispersed into small and harmless fragments by cold rolling or wire drawing
• the composition A (A1, A2) denotes the composition range of the inclusions having higher SiO 2 contents than the inclusions of the composition B.
• the number of inclusions having the composition A is controlled so as to account for 20% or more and the total number of inclusions having the composition A or B is controlled so as to account for 80% or more.
• the reason why the number of the inclusions having the composition A is controlled so as to account for 20% or more is that the number of the inclusions having the composition A increases with the decrease of the addition amount of Ca and Mg ferroalloys in molten steel, and, if the addition amount of Ca and Mg ferroalloys decreases to the extent that the number of the inclusions having the composition A accounts for 20% or more, the cost reduction effect which is an object of the present invention can be attained. A yet greater cost reduction effect can be obtained if the number of the inclusions having the composition A increases to the extent of accounting for 40% or more.
• chemical composition B1 is defined in the first present invention as comprising one or both of 50% or less of CaO and 15% or less of MgO in addition to 20 to 60% of SiO 2 and 10 to 80% of MnO is as follows:
• SiO 2 content is below 20%, hard CaO or MgO inclusions are formed, either of which inclusions cannot be sufficiently broken down by hot or cold rolling.
• the range of SiO 2 content over 60% coincides with the range of the chemical composition A1, which range of SiO 2 content has conventionally been avoided as the one where hard inclusions are formed.
• the chemical compositions of inclusions according to the present invention can be achieved by adding appropriate amounts of ferroalloys containing Ca and Mg after forming Mn-silicates through deoxidation with Si and Mn.
• the first present invention does not involve A1, but roughly 20% or less of Al 2 O 3 is inevitably formed even when deoxidation methods are properly controlled without using Al. Different from conventional technologies, however, when the chemical composition of inclusions conforms to the present invention, hard corundum or spinel is not formed even when the above level of Al 2 O 3 is present, and thus 20% or less of Al 2 O 3 is permissible.
• chemical composition B2 is defined in the second present invention as comprising one or both of 50% or less of CaO and 15% or less of MgO in addition to 35 to 75% of SiO 2 and 30% or less of Al 2 O 3 is as follows:
• CaO securing the inclusion softening effect of the compound deoxidation.
• MgO content is 3% or more for securing the inclusion softening effect of the compound deoxidation.
• a significant characteristic of the second present invention is that, even when CaO, MgO or Al 2 O 3 is actively added as described above, very stable manufacturing is viable without causing formation of corundum, spinel or other harmful hard inclusions as in the cases of conventional technologies.
• the reason why the content of MnO is not specifically defined is that MnO tends to disappear when Ca, Mg, Al or another strong deoxidizing element is added, and that the MnO content is usually 20% or less especially when the content of CaO, MgO or Al 2 O 3 is made comparatively high as in the present invention.
• MnO is effective for softening inclusions and thus its presence does not hinder the effects of the present invention, and this is another reason why MnO content is not specifically defined.
• No lower limit is specified regarding Al 2 O 3 . Since Al 2 O 3 is actively added according to the second present invention, 5% or more of Al 2 O 3 is usually included in the inclusions falling within the chemical composition B2.
• An important point in the present invention is to control the size of the inclusions satisfying the relation 1/d ⁇ 5 and falling within the chemical composition A1 or A2 so that d does not exceed 40 ⁇ m.
• the inclusions falling within the chemical composition A1 or A2 are somewhat harder than those falling within the chemical composition B1 or B2, when their size is so controlled that d does not exceed 40 ⁇ m, the inclusion softening effect is not hindered.
• Large inclusions having d exceeding 40 ⁇ m are mainly composed of primary deoxidation products formed in molten steel after deoxidation.
• Ca or Mg is used in the compound deoxidation so that the inclusions satisfying 1/d ⁇ 5 have a basic chemical composition conforming to the chemical composition B, said primary deoxidation products are softened in the end, and all the large inclusions satisfying d > 40 ⁇ m turn into stretched inclusions satisfying 1/d > 5.
• the present invention has successfully controlled the size of the inclusions satisfying 1/d ⁇ 5 and falling within the chemical composition A1 or A2 to the range where d does not exceed 40 ⁇ m.
• the present invention has succeeded in securing excellent cold workability and fatigue properties by controlling the size and chemical composition of inclusions as described above. According to the present invention it is further possible to improve the service life of wire drawing dies by reducing the number of the inclusions satisfying 1/d ⁇ 5 and conforming to the chemical composition A1 or A2 to 1 piece/mm 2 or less in a field of view (5.5 mm x 11 mm) under microscopic observations, more preferably to 0.5 piece/mm 2 or less.
• the present invention instead of specifying the ratio of the non-metallic inclusions falling within the chemical composition ranges A and the ratio of those falling within the chemical composition ranges A or B among those satisfying 1/d ⁇ 5 as described above, it is also possible to define the present invention by specifying average chemical composition of the non-metallic inclusions satisfying 1/d ⁇ 5, as described in items (3) and (6) above. More details are described hereafter.
• the average chemical composition of the non-metallic inclusions is obtained by averaging the numbers of non-metallic inclusions whose chemical composition is analyzed in one field of view at an L section of a rolled material.
• An appropriate size of a field of view is, for example, approximately 5.5 mm x 11 mm in the case of steel wire.
• the non-metallic inclusions whose length (1) and width (d) satisfy the relation 1/d ⁇ 5 comprise, as average chemical composition, one or both of 40% or less of CaO and 12% or less of MgO in addition to 30% or more of SiO 2 and 8 to 65% of MnO, and d is 40 ⁇ m or less regarding the non-metallic inclusions satisfying 1/d ⁇ 5.
• the cost reduction effect the present invention envisages is achieved when the addition of Ca or Mg ferroalloys is so decreased that SiO 2 content in the average chemical composition becomes 30% or more.
• MnO content is controlled to be 8% or more to prevent hard inclusions from forming.
• the upper limit of MnO is 65% in order to make the SiO 2 content 30% or more.
• Hard CaO inclusions form when Ca content exceeds 40% and hard MgO inclusions form when MgO content exceeds 12%, and the envisaged object cannot be achieved in either case.
• CaO content is 5% or more for securing the inclusion softening effect of the compound deoxidation.
• MgO content is 3% or more for securing the inclusion softening effect of the compound deoxidation.
• a further cost reduction can be obtained by making the SiO 2 content exceed 60%. In this case, both the upper limit of MnO and that of CaO are 32% and the same of MgO is 30%.
• the non-metallic inclusions whose length (1) and width (d) satisfy the relation 1/d ⁇ 5 comprise, as average chemical composition, 43% or more of SiO 2 , 24% or less of Al 2 O 3 , 40% or less of CaO and 12% or less of MgO, and d is 40 ⁇ m or less regarding the non-metallic inclusions satisfying 1/d ⁇ 5.
• the cost reduction effect the present invention envisages is achieved when the addition of Ca or Mg ferroalloys is so decreased that SiO 2 content in the average chemical composition becomes 43% or more.
• Hard CaO, MgO or Al 2 O 3 inclusions and their composite inclusions are formed, respectively, with CaO exceeding 40%, MgO exceeding 12% or Al 2 O 3 exceeding 24%, and the envisaged object cannot be achieved in any of these cases.
• the reason why d of the non-metallic inclusions satisfying 1/d ⁇ 5 has to be 40 ⁇ m or less is as described before.
• CaO content is 5% or more for securing the inclusion softening effect of the compound deoxidation.
• MgO content is 3% or more for securing the inclusion softening effect of the compound deoxidation.
• SiO 2 content exceed 75%. In this case, the upper limits of Al 2 O 3 , CaO and MgO become 17%, 20% and 15%, respectively.
• the present invention achieves excellent results in the applications where cold workability and fatigue properties as severe as conventional cases are required. Recently, however, larger diameter cords are used in some tire cord applications, wherein the required cold workability is a little more relaxed than before. With regards to the service life of drawing dies, also, improvements in lubrication and other factors have made it possible to continue drawing operations not affected by decrease in inclusion levels in steel materials.
• the super-clean steel according to the present invention has an excellent effect especially in these applications.
• the steel chemical composition is described hereafter. Since the present invention defines properties of inclusions, it is not necessary to specifically limit steel chemical composition. But, the fields of application of the present invention will be described hereunder.
• One example is steel wire and rods of carbon steel and low alloy carbon steel to be drawn for uses as wire, springs, etc. after hot rolling.
• the present invention is effective especially in extra fine soft wire and hard wire 0.3 mm or less in diameter for preventing disconnections during wire drawing and strand forming and in springs for enhancing fatigue strength.
• the steel materials used for these applications comprise, in weight, one or more of 0.05 to 0.5% of Cr, 0.05 to 1.0% of Ni, 0.05 to 1.0% of Cu, 0.001 to 0.01% of B, 0.001 to 0.2% of Ti, 0.001 to 0.2% of V, 0.001 to 0.2% of Nb, 0.05 to 1.0% of Mo and 0.1 to 2% of Co as required, in addition to 0.6 to 1.2% of C, 0.1 to 1.5% of Si and 0.1 to 1.5% of Mn.
• C is an economical and effective element to strengthen steel, and 0.4% or more of it is required to obtain the strength required for hard-steel wire.
• its content exceeds 1.2%, however, it decreases ductility of steel, resulting in embrittlement and difficulty in secondary working. For this reason, its content has to be 1.2% or less.
• Si and Mn are necessary for deoxidation and control of chemical composition of inclusions. Either of them is ineffective when added below 0.1%. Both the elements are also effective for strengthening steel, but steel becomes brittle when either of them exceeds 1.5%.
• Cr has to be controlled within a range from 0.05 to 1.0% because the least necessary amount for its effect to refine pearlite lamella and raise steel strength to show is 0.05% and thus a Cr addition of 0.05% or more is desirable. However, it deteriorates steel ductility when added beyond 1.0% and, for this reason, the upper limit of its addition is set at 1.0%.
• Ni strengthens steel through an effect similar to that of Cr, hence its addition by 0.05%, where the effect begins to show, or more, is desirable, but its content has to be 1.0% or less not to cause deterioration of ductility.
• B is an element to enhance hardenability of steel. According to the present invention, it is possible to raise steel strength by adding B, but its excessive addition deteriorates steel toughness through increased boron precipitation and, for this reason, its upper limit is set at 0.01%. Too small an addition of B does not bring about any effect, and its lower limit is set at 0.001%.
• Ti, Nb and V raise the strength of steel wire through precipitation hardening. None of them is effective when added below 0.001%, but they cause precipitation embrittlement when added beyond 0.2%. For this reason their respective contents have to be 0.2% or less. Addition of these elements is also effective for fining ⁇ grains during patenting.
• Mo is another element to enhance steel hardenability. According to the present invention, it is possible to raise steel strength by adding Mo, but its excessive addition raises steel hardness overly resulting in poor workability and, for this reason, the range of its content is specified as 0.05 to 1.0%. Co enhances steel ductility by suppressing the formation of proeutectoid cementite of supereutectoid steel.
• austenitic stainless steels which are used for extra thin flat springs with a thickness of 0.3 mm or less through cold rolling after hot rolling.
• the present invention is effective for enhancing fatigue strength of springs.
• the steel materials for this application comprise, typically, 0.15% or less of C, 0.1 to 1% of Si, 0.1 to 2% of Mn, 16 to 20% of Cr and 3.5 to 22% of Ni.
• Yet another field of application is low carbon steel sheets for deep drawing work, which are deep-drawn after being hot rolled, cold rolled into a thickness of 1.2 mm or less, annealed and skin pass rolled.
• the present invention is effective to prevent surface defects and enhance deep drawing property.
• the steel materials used for this application comprise, typically, 0.12% or less of C, 0.3% or less of Si, and 0.50% or less of Mn.
• the examination of the number and chemical composition of non-metallic inclusions at an L section was done in the following manner: a sample 0.5 m long was cut out from a coil of steel wire 5.5 mm in diameter; small specimens 11 mm long each were cut out from 10 places chosen at random along the length of each of the samples; and the entire surface of a longitudinal section of each of the small specimens including its longitudinal center line was inspected with an optical microscope.
• the wires were thereafter drawn for the purpose of evaluating die service life and wire breakage ratio during the drawing work.
• the evaluation results are shown also in Tables 1 and 2.
• the steel materials resulting in die service lives exceeding an average die service life of materials by conventional processes are marked with ⁇ meaning good, and those showing die service lives below the average are marked with ⁇ meaning poor.
• the wire breakage ratio the steel materials showing smaller wire breakage ratios than an average wire breakage ratio of materials by conventional processes (which average ratio is smaller than standard permissible breakage ratio) are marked with O meaning good, and those showing breakage ratios exceeding the average are marked with ⁇ meaning poor.
• Tables 3 and 4 show average chemical composition of the non-metallic inclusions satisfying 1/d ⁇ 5 at an L section of each of the steel wires shown in Tables 1 and 2.
• the left section of Tables 3 and 4 shows average chemical composition of all the non-metallic inclusions satisfying 1/d ⁇ 5, the center section the same of the non-metallic inclusions conforming to the chemical composition A1 among those satisfying 1/d ⁇ 5, and the right section the same of the non-metallic inclusions conforming to the chemical composition B1 among those satisfying 1/d ⁇ 5.
• Nos. 1 to 21 in Tables 1 and 3 are the steel materials according to the present invention. All of their parameters are within the ranges defined in accordance with the present invention, and they received good marks both in the wire breakage ratio and die service life.
• Nos. 22 to 29 in Tables 2 and 4 are comparative steel materials.
• d of the non-metallic inclusions satisfying 1/d ⁇ 5 and conforming to the chemical composition A1 exceeded 40 ⁇ m and the wire breakage ratio was poor.
• the ratio of the inclusions conforming to the chemical composition A1 and the ratio of those conforming to the chemical composition A1 or B1 were both too small and the die service life was poor.
• Si was high and, as a result, its ratio of the non-metallic inclusions conforming to the chemical composition A1 or B1 was too small and the die service life was poor.
• No. 22 to 29 in Tables 2 and 4 are comparative steel materials.
• d of the non-metallic inclusions satisfying 1/d ⁇ 5 and conforming to the chemical composition A1 exceeded 40 ⁇ m and the wire breakage ratio was poor.
• the ratio of the inclusions conforming to the chemical composition A1 and the ratio of those conforming to the chemical composition A1 or B1 were both too small and the die service life was poor.
• Si
• Material No. 2 according to the present invention shown in Table 1 and comparative material No. 23 shown in Table 2 were hot rolled into steel wires 5.5 mm in diameter, drawn into a diameter of 1.6 mm, heat-treated at 950°C to form ⁇ grains, and then immersed in a lead bath of 560°C for a final patenting, to make steel wires having pearlite structure.
• the wires thus obtained were then continuously drawn into a diameter of 0.3 mm, and fatigue properties of the product wires were compared through Hunter fatigue testing.
• Table 5 shows the tensile strength of the 0.3 mm diameter wires and results of their Hunter fatigue tests expressed in terms of fatigue limit stress. As seen in the table, there is no difference in the tensile strength between the material according to the present invention and the comparative material, but the material according to the present invention shows a higher fatigue limit stress than the comparative material at roughly the same strength.
• Steels of the chemical compositions shown in Tables 6 and 7 were produced by adding Si, Mn and other necessary component elements to molten steel at tapping from a 250-ton converter, and then adding ferroalloys containing Ca, Mg and A1.
• the steels thus obtained were hot-rolled into wires at a reduction of 80% or more.
• the same examination of inclusions at the L sections, wire drawing work and quality evaluations during wire drawing as in Example 1 were carried out. Difference from Example 1 is that A1 was actively added to the steels and that both Ca and Mg were added to all of the steels.
• Tables 8 and 9 show the average chemical composition of the non-metallic inclusions satisfying 1/d ⁇ 5 at an L section of each of the steel wires shown in Tables 6 and 7.
• the left section of Tables 8 and 9 shows the average chemical composition of all the non-metallic inclusions satisfying 1/d ⁇ 5, the center section the same of the non-metallic inclusions conforming to the chemical composition A2 among those satisfying 1/d ⁇ 5, and the right section the same of the non-metallic inclusions conforming to the chemical composition B2 among those satisfying 1/d ⁇ 5.
• Nos. 31 to 51 in Tables 6 and 8 are the steel materials according to the present invention. All of their parameters are within the ranges defined in accordance with the present invention, and they received good marks both in the wire breakage ratio and in the die service life.
• Nos. 52 to 59 in Tables 7 and 9 are comparative steel materials.
• d of the non-metallic inclusions satisfying 1/d ⁇ 5 and conforming to the chemical composition A2 exceeded 40 ⁇ m and the wire breakage ratio was poor.
• the ratio of the inclusions conforming to the chemical composition A2 or B2 was too small and the die service life was poor.
• Si was high and, as a result, the ratio of the non-metallic inclusions conforming to the chemical composition A2 and the ratio of those conforming to the chemical composition A2 or B2 were both too small and the die service life was poor.
• No. 52 d of the non-metallic inclusions satisfying 1/d ⁇ 5 and conforming to the chemical composition A2 exceeded 40 ⁇ m and the wire breakage ratio was poor.
• the ratio of the inclusions conforming to the chemical composition A2 or B2 was too small and the die service life was poor.
• Si was high and, as a result, the ratio of the non-metallic inclusions conforming to the chemical composition A2 and the
• Mn was high and, as a result, the ratio of the non-metallic inclusions conforming to the chemical composition A2 or B2 was too small and the die service life was poor.
• Si was low and, as a result, the ratio of the inclusions conforming to the chemical composition A2 was too small and the wire breakage ratio was poor.
• Mn was low and, as a result, d of the non-metallic inclusions satisfying 1/d ⁇ 5 and conforming to the chemical composition A2 exceeded 40 ⁇ m and, what is more, the number of inclusions was outside the range defined in claim 5, and the die service life was poor.
• the number of inclusions was outside the range defined in claim 5, and the die service life was poor.
• d of the non-metallic inclusions satisfying 1/d ⁇ 5 and conforming to the chemical composition A2 exceeded 40 ⁇ m and the wire breakage ratio was poor.
• the super-clean steel according to the present invention is excellent in cold workability and fatigue properties, has superior performance as a steel for extra thin plate springs, extra fine wire and high strength springs, and also has an excellent effect, due to reduced addition of expensive Ca and Mg ferroalloys, to allow manufacturing at low cost.

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• 2000
  • 2000-06-16 KR KR1020017002034A patent/KR20010086358A/ko not_active Application Discontinuation
  • 2000-06-16 CA CA002340688A patent/CA2340688A1/fr not_active Abandoned
  • 2000-06-16 CN CN00801137A patent/CN1313912A/zh active Pending
  • 2000-06-16 EP EP00939092A patent/EP1127951A1/fr not_active Withdrawn
  • 2000-06-16 WO PCT/JP2000/003975 patent/WO2000077270A1/fr not_active Application Discontinuation
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