EP1127951A1

EP1127951A1 - Highly cleaned steel

Info

Publication number: EP1127951A1
Application number: EP00939092A
Authority: EP
Inventors: Wataru Nippon Steel Corp.kimitsu Works YAMADA; Seiki Nippon Steel Corp. Kimitsu Works NISHIDA; Satoshi Nippon Steel Corp. Kimitsu Work SUGIMARU; Shinjiro Nippon Steel Corp. Kimitsu Works UEYAMA; Hiroshi Nippon Steel Corp. Kimitsu Works YATABE
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1999-06-16
Filing date: 2000-06-16
Publication date: 2001-08-29
Also published as: CA2340688A1; BR0006880A; CN1313912A; KR20010086358A; WO2000077270A1

Abstract

The present invention provides a super-clean steel excellent in cold workability and fatigue properties even with a reduced use of Ca and Mg ferroalloys,
characterized in that, among the 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, the number of inclusions having the composition A1 specified below accounts for 20% or more, and the total number of inclusions having the composition A1 or B1 specified below accounts for 80% or more, and that, regarding the non-metallic inclusions satisfying the relation 1/d ≦ 5 and having the composition A1 specified below, d is 40µm or less.
Composition A1: containing over 60% of SiO₂.
Composition B1: containing one or both of 50% or less of CaO and 15% or less of MgO, in addition to 20 to 60% of SiO₂ and 10 to 80% of MnO.

Description

FIELD OF ART

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.

BACKGROUND ART

It is well known that hard non-metallic inclusions are harmful to steels for thin flat springs and tire cords, which steels undergo intensive cold working such as cold rolling and wire drawing, and to steels for valve springs requiring high fatigue strength, since fractures propagate with the hard non-metallic inclusions acting as starting points. It is possible, as a countermeasure against the above problem, to stretch inclusions through hot rolling and cold rolling or wire drawing by softening them and to reduce the size of the inclusions. Japanese Examined Patent Publication No. S54-7252 discloses, for instance, a method to compose inclusions mainly of spessartite and to satisfy the formula, Al₂O₃/SiO₂ + Al₂O₃ + MnO = 0.15 to 0.40. The inclusions shown therein, however, exist across a zone where corundum appears as a primary crystal. For this reason, it is difficult to prevent extremely hard and harmful corundum from forming in actual manufacturing processes and thus the proposed method is incapable of achieving satisfactory effects.
Further, 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 SiO₂ 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 SiO₂, 30% or less of Al₂O₃, 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.
For the purpose of softening inclusions in steel, 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.

DISCLOSURE OF THE INVENTION

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 Al₂O₃ in inclusions to the utmost. The gist of the first present invention is as follows:
(1) A super-clean steel excellent in cold workability and fatigue properties, characterized in that, among the 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, the number of inclusions having the composition A1 specified below accounts for 20% or more, and the total number of inclusions having the composition A1 or B1 specified below accounts for 80% or more, and that, regarding the non-metallic inclusions satisfying the relation 1/d ≦ 5 and having the composition A1 specified below, d is 40µm or less.
Composition A1: containing over 60% of SiO₂.
Composition B1: containing one or both of 50% or less of CaO and 15% or less of MgO, in addition to 20 to 60% of SiO₂ and 10 to 80% of MnO. Here, the composition of non-metallic inclusions is determined on the basis that the sum of the amounts of SiO₂, MnO, CaO, MgO and Al₂O₃ is 100. The same applies also to the present invention described below.
(2) A super-clean steel excellent in cold workability and fatigue properties according to item (1), characterized in that the number of non-metallic inclusions with their length (1) and width (d) satisfying the relation 1/d ≦ 5 and having the composition A1 specified above is 1 piece/mm² or less.
(3) A super-clean steel excellent in cold workability and fatigue properties, characterized in that 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 40% or less of CaO and 12% or less of MgO, in addition to 30% or more of SiO₂ and 8 to 65% of MnO, and that regarding the non-metallic inclusions satisfying the relation 1/d ≦ 5, d is 40µm or less. Here, the average composition of non-metallic inclusions is determined by averaging the numbers of the non-metallic inclusions whose compositions are analyzed within one visual field at an L section of a rolled steel material. The same applies also to the present invention described below.The second present invention is to carry out compound deoxidation for actively making inclusions contain CaO, MgO and Al₂O₃. The gist of the second present invention is as follows:
(4) A super-clean steel excellent in cold workability and fatigue properties, characterized in that, among the 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, the number of inclusions having the composition A2 specified below accounts for 20% or more, and the total number of inclusions having the composition A2 or B2 specified below accounts for 80% or more, and that regarding the non-metallic inclusions satisfying the relation 1/d ≦ 5 and having the composition A2 specified below, d is 40µm or less.
Composition A2: containing over 75% of SiO₂.
Composition B2: containing one or both of 50% or less of CaO and 15% or less of MgO, in addition to 35 to 75% of SiO₂ and 30% or less of Al₂O₃.
(5) A super-clean steel excellent in cold workability and fatigue properties according to item (4), characterized in that the number of non-metallic inclusions with their length (1) and width (d) satisfying the relation 1/d ≦ 5 and having the composition A2 specified above is 1 piece/mm² or less.
(6) A super-clean steel excellent in cold workability and fatigue properties, characterized in that 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 40% or less of CaO and 12% or less of MgO, in addition to 43% or more of SiO₂ and 24% or less of Al₂O₃, and that, regarding the non-metallic inclusions satisfying the relation 1/d ≦ 5, d is 40µm or less. In the chemical composition of a steel according to the present invention, it is necessary to contain 0.1% or more of Si and Mn for controlling the composition of inclusions, but there is no specific limitations regarding other elements. Thus, the present invention is applicable to various steels such as low carbon steels, high carbon steels and austenitic stainless steels whereto alloying elements are added in accordance with requirements. The chemical composition of a steel according to the present invention will more specifically be described hereunder.
(7) A super-clean steel excellent in cold workability and fatigue properties according to any one of items (1) to (6), characterized by containing, in terms of weight percent, 0.4 to 1.2% of C, 0.1 to 1.5% of Si and 0.1 to 1.5% of Mn.
(8) A super-clean steel excellent in cold workability and fatigue properties according to any one of items (1) to (6), characterized by containing, in terms of weight percent, 0.4 to 1.2% of C, 0.1 to 1.5% of Si and 0.1 to 1.5% of Mn, and one or more types of 0.05 to 1.0% 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.

BEST MODE FOR CARRYING OUT THE INVENTION

Since low-melting-point inclusions in a hot rolled steel material soften more than the steel material at a rolling temperature, the inclusions stretch longitudinally. It is therefore possible to judge the extent of softening of the inclusions by measuring 1/d, the ratio of their length (1) and width (d), at an L section of a rolled steel material. Inclusions having a large value of 1/d, specifically 1/d > 5, are highly stretchable and harmless since they are stretched during rolling. On the other hand, with regard to inclusions having a small value of 1/d, it is impossible to tell by the value of 1/d alone whether or not the inclusions are harmful, because some of them are broken down and dispersed into small and harmless fragments while others survive intact and remain harmful when subjected to cold rolling or wire drawing after hot rolling.
The aforesaid conventional technologies soften the inclusions satisfying the relation 1/d ≦ 5 by forming a complex composition therein. Here, the SiO₂ 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 SiO₂ inclusions are formed when the SiO₂ 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 SiO₂ content, they do not cause any harm during cold rolling or wire drawing after hot rolling if their size is small. Though SiO₂ inclusions are hard, they are softer than CaO, MgO or Al₂O₃ 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 ≦ 40µm. It is further preferable to control the size of high SiO₂ inclusions satisfying the relation 1/d ≦ 5 to the range of d ≦ 20µm.
In the present invention, 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, and the composition A (A1, A2) denotes the composition range of the inclusions having higher SiO₂ contents than the inclusions of the composition B.
In the both first and second present inventions, among the number of non-metallic inclusions satisfying the relation 1/d ≦ 5, 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 total number of the inclusions having the composition A or B is controlled so as to account for 80% or more is that the inclusions not conforming to the composition A or B are hard even though they are the inclusions of, for example, CaO, MgO or Al₂O₃ system and, when the number of these hard inclusions exceeds 20%, cold workability and fatigue properties of a steel material deteriorate.
Further, 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.
The reasons for specifying the range of the chemical compositions B are described below with regards to the first and second present inventions.
The reason why the 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 SiO₂ and 10 to 80% of MnO is as follows:
When SiO₂ 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 SiO₂ content over 60% coincides with the range of the chemical composition A1, which range of SiO₂ 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. What is important in the present invention is, however, that hard inclusions are prevented from forming by keeping 10 to 80% of MnO by properly controlling addition of the Ca and Mg ferroalloys, despite the fact that MnO tends to disappear when Ca and Mg ferroalloys are added. With CaO content exceeding 50%, hard CaO inclusions are formed and, with MgO exceeding 15%, hard MgO inclusions are formed, and thus the envisaged object is not achieved in either case. It is preferable that CaO content is 5% or more for securing the inclusion softening effect of the compound deoxidation. Likewise, it is preferable that MgO content is 3% or more for securing the inclusion softening effect of the compound deoxidation.
It is preferable to eliminate Al₂O₃ as much as possible for preventing hard inclusions from forming. The first present invention does not involve A1, but roughly 20% or less of Al₂O₃ 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 Al₂O₃ is present, and thus 20% or less of Al₂O₃ is permissible.
The reason why the 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 SiO₂ and 30% or less of Al₂O₃ is as follows:
Even when Si, Ca, Mg, Al and other deoxidizing elements prone to form hard inclusions are used, it is possible to render the inclusions very soft by making CaO, MgO or Al₂O₃ coexist with a certain content range of SiO₂. When the content of SiO₂ is below 35%, hard CaO, MgO or Al₂O₃ inclusions are formed, and any of them cannot be broken down into sufficiently small fragments by hot rolling or cold working. The range of SiO₂ content exceeding 75% is the range of the chemical composition A2, which range has conventionally been avoided as the one where hard inclusions are formed. In addition, hard CaO, MgO or Al₂O₃ inclusions and their composite inclusions are formed, respectively, with CaO exceeding 50%, MgO exceeding 15% or Al₂O₃ exceeding 30%. It is preferable that CaO content is 5% or more for securing the inclusion softening effect of the compound deoxidation. Likewise, it is preferable that 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 Al₂O₃ 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 Al₂O₃ is made comparatively high as in the present invention. In addition, 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 Al₂O₃. Since Al₂O₃ is actively added according to the second present invention, 5% or more of Al₂O₃ 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 40µm. Although 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 40µm, the inclusion softening effect is not hindered.
Large inclusions having d exceeding 40µm are mainly composed of primary deoxidation products formed in molten steel after deoxidation. In the case of the present invention where 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 > 40µm turn into stretched inclusions satisfying 1/d > 5. As described above, 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 40µm.
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/mm² or less in a field of view (5.5 mm x 11 mm) under microscopic observations, more preferably to 0.5 piece/mm² or less.
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. Here, 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.
In the first present invention, 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 SiO₂ and 8 to 65% of MnO, and d is 40µm 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 SiO₂ 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 SiO₂ 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. The reason why d of the non-metallic inclusions satisfying 1/d ≦ 5 has to be 40µm or less is as described before.
It is preferable that CaO content is 5% or more for securing the inclusion softening effect of the compound deoxidation. Likewise, it is preferable that 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 SiO₂ 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%.
In the second present invention, 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 SiO₂, 24% or less of Al₂O₃, 40% or less of CaO and 12% or less of MgO, and d is 40µm 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 SiO₂ content in the average chemical composition becomes 43% or more. Hard CaO, MgO or Al₂O₃ inclusions and their composite inclusions are formed, respectively, with CaO exceeding 40%, MgO exceeding 12% or Al₂O₃ 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 40µm or less is as described before.
It is preferable that CaO content is 5% or more for securing the inclusion softening effect of the compound deoxidation. Likewise, it is preferable that 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 SiO₂ content exceed 75%. In this case, the upper limits of Al₂O₃, CaO and MgO become 17%, 20% and 15%, respectively.
As described above, 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. When 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, on the other hand, 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.
Since Cu improves scale properties and corrosion fatigue properties of wire, its addition by 0.05%, where its 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 γ 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.
In addition, regarding high carbon steels, it is preferable to control the content each of P and S to 0.02% or less since either of them deteriorates not only the wire drawing property but also the ductility after wire drawing.
Another field of application where the present invention is effective is 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.

EXAMPLE

(Example 1)

Steels of the chemical compositions shown in Tables 1 and 2 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 one or both of Ca and Mg. Non-metallic inclusions at an L section of each of the steels were examined after hot-rolling into wires at a reduction of 80% or more. In this example, 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 (which average life is longer than standard die service life) are marked with ○ meaning good, and those showing die service lives below the average are marked with × meaning poor. As for 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. In No. 22, d of the non-metallic inclusions satisfying 1/d ≦ 5 and conforming to the chemical composition A1 exceeded 40µm and the wire breakage ratio was poor. In No. 23, 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. In No. 24, 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. In No. 25, Mn was high and, as a result, the ratio of the non-metallic inclusions conforming to the chemical composition A1 or B1 was too small and the die service life was poor. In No. 26, Si was low and, as a result, the ratio of the inclusions conforming to the chemical composition A1 was too small and the wire breakage ratio was poor. No. 27 had the effects of the present invention showing a good wire breakage ratio, but its Mn content was low and, consequently, the number of inclusions was outside the range defined in claim 2, showing a poor die service life. In No. 28, the number of inclusions was outside the range defined in claim 2, and the die service life was poor. In No. 29, d of the non-metallic inclusions satisfying 1/d ≦ 5 and conforming to the chemical composition A1 exceeded 40µm and the wire breakage ratio was poor.
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 γ 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.

	No.	Tensile test result	Fatigue test result
		Diameter (mm)	Tensile strength (MPa)	Reduction of area (%)	Fatigue limit stress / Tensile strength
Invented material	2	0.302	3425	39.8	0.291
Comparative material	23	0.301	3483	38.6	0.253

(Example 2)

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. In No. 52, d of the non-metallic inclusions satisfying 1/d ≦ 5 and conforming to the chemical composition A2 exceeded 40µm and the wire breakage ratio was poor. In No. 53, the ratio of the inclusions conforming to the chemical composition A2 or B2 was too small and the die service life was poor. In No. 54, 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. In No. 55, 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. In No. 56, 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. In No. 57, 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 40µm and, what is more, the number of inclusions was outside the range defined in claim 5, and the die service life was poor. In No. 58, the number of inclusions was outside the range defined in claim 5, and the die service life was poor. In No. 59, d of the non-metallic inclusions satisfying 1/d ≦ 5 and conforming to the chemical composition A2 exceeded 40µm and the wire breakage ratio was poor.

INDUSTRIAL APPLICABILITY

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.

Claims

A super-clean steel excellent in cold workability and fatigue properties, characterized in that; among the 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, the number of inclusions having the composition A1 specified below accounts for 20% or more, and the total number of inclusions having the composition A1 or B1 specified below accounts for 80% or more, and that; regarding the non-metallic inclusions satisfying the relation 1/d ≦ 5 and having the composition Al specified below, d is 40µm or less;
composition A1: containing over 60% of SiO₂,
composition B1: containing one or both of 50% or less of CaO and 15% or less of MgO, in addition to 20 to 60% of SiO₂ and 10 to 80% of MnO;
wherein the composition of non-metallic inclusions is determined on the basis that the sum of the amounts of SiO₂, MnO, CaO, MgO and Al₂O₃ is 100.
A super-clean steel excellent in cold workability and fatigue properties according to claim 1, characterized in that the number of non-metallic inclusions with their length (1) and width (d) satisfying the relation 1/d ≦ 5 and having the composition A1 specified above is 1 piece/mm² or less.
A super-clean steel excellent in cold workability and fatigue properties, characterized in that; 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 40% or less of CaO and 12% or less of MgO, in addition to 30% or more of SiO₂ and 8 to 65% of MnO, and that; regarding the non-metallic inclusions satisfying the relation 1/d ≦ 5, d is 40µm or less;
wherein the average composition of non-metallic inclusions is determined by averaging the numbers of the non-metallic inclusions whose compositions are analyzed within one visual field at an L section of a rolled steel material.
A super-clean steel excellent in cold workability and fatigue properties, characterized in that; among the 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, the number of inclusions having the composition A2 specified below accounts for 20% or more, and the total number of inclusions having the composition A2 or B2 specified below accounts for 80% or more, and that; regarding the non-metallic inclusions satisfying the relation 1/d ≦ 5 and having the composition A2 specified below, d is 40µm or less;
composition A2: containing over 75% of SiO₂,
composition B2: containing one or both of 50% or less of CaO and 15% or less of MgO, in addition to 35 to 75% of SiO₂ and 30% or less of Al₂O₃.
A super-clean steel excellent in cold workability and fatigue properties according to claim 4, characterized in that the number of non-metallic inclusions with their length (1) and width (d) satisfying the relation 1/d ≦ 5 and having the composition A2 specified above is 1 piece/mm² or less.
A super-clean steel excellent in cold workability and fatigue properties, characterized in that; 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 40% or less of CaO and 12% or less of MgO, in addition to 43% or more of SiO₂ and 24% or less of Al₂O₃, and that; regarding the non-metallic inclusions satisfying the relation 1/d ≦ 5, d is 40µm or less.
A super-clean steel excellent in cold workability and fatigue properties according to any one of claims 1 to 6, characterized by containing, in terms of weight percent, 0.4 to 1.2% of C, 0.1 to 1.5% of Si and 0.1 to 1.5% of Mn.
A super-clean steel excellent in cold workability and fatigue properties according to any one of claims 1 to 6, characterized by containing, in terms of weight percent, 0.4 to 1.2% of C, 0.1 to 1.5% of Si and 0.1 to 1.5% of Mn, and one or more types of 0.05 to 1.0% 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.