GB2246579A - High toughness non-refined steels and method for manufacturing them - Google Patents

Abstract

A high toughness non-refined steel consisting of 0.30% to 0.55% C, 0.15% to 0.45% Si, 0.60% to 1.55% Mn, no more than 0.050% S, 0% to 0.30% Cr, 0.01% to 0.05% Al, 0.05% to 0.15% V, Nb or mixtures thereof, 0.0% to 0.03% Ti, 0.0% to 0.003 B, 0.2923a to 0,02% N (a is equivalent to the amount of said Ti) and the balance Fe and impurities contained inevitably in manufacturing the steel. The steel may contain additionally at least one element selected from the group consisting of calcium, tellurium, cerium and other rare earth metals, misch metal, and mixtures thereof, in amounts of 0.0001 weight % to 0.04 weight %. These elements function to control shapes of inclusions. The steels exhibit a tensile strength of 75 Kgf/mm<2> and a toughness of 7Kgf.m/cm<2>. <IMAGE>

Description

1 - - 1 h --- HIGH TOUGHNESS NON-REFINED STEELS AND METHOD FOR

MANUFACTURING THEM

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to non-refined steels having at least the same mechanical properties as those of refined steels and a method for manufacturing them. In particular, the present invention relates to high toughness non-refined steels having improved toughness and tensile strength and a method for manufacturing them.

Description of the Prior Art

Generally, non-refined steels means steels which have the mechanical properties similar to those of refined steels, without being subjected to a refining treatment. This refining treatment means the heat treatment for improving mechanical properties of steels by quenching and tempering works, in manufacturing steels.

exhibit toughness very lower than that of refined steels that their applications are limited to the cases of These non-refined steels so no requiring high toughness and only requiring high strength in manufacturing mechanical elements.

Taking into consideration that the necessity of energy 1 i 1 i saving is bbing stressed again all over the world, due to Gulf War, it is strongly desired to extend the application of nonrefined steels to machine structural steels, in order to save energy consumed in refining treatment of steels. To this end, it is necessary to improve toughness which is poor in cases of nonrefined steels.

Conventionally, there have been proposed uses of a steel composition containing carbon in an amount of about 0.45 weight % and a steel composition containing carbon in an amount of about 0.03 weight % to about 0.25 weight % and chromium in an amount of about 1.5 weight % to about 2.0 weight %, in order to obtain tensile strength of 75 Kgf/mm2 in non-refined steels. When non-refined low carbon steels are used for accomplishing the above purposes, it is difficult to practice a high frequency hardening process for improving wear resistance. In order to obtain high strength, a separate cooling equipment is also necessary.

There has been also proposed a method for improving toughness of steel structure by the addition of Mn in an amount of no more than 1.55 weight %. However, this increased amount of Mn causes the reduction of machinability of the produced steel. In order to improve the machinability, the steel composition is added with large amounts of elements such as S, Pb or Bi. The addition of such elements may result in the reduction of toughness of the produced steel. These 1 2 i 1 1 i 1 1 1 1 i 1 i i i i i 1 1 i i 1 elements are subjected to a plastic deformation in hot working of the produced steel and remained in the alloy structure as inclusions of an A- type which has a linear shape.

According to their shapes, inclusions are classified into an A-type having a linear shape which is darkly shown in FIG. 5, a B-type having a polygonal shape, and a C-type having a spherical shape which is darkly shown in FIG. 4. The A-type inclusions have a directional property and thus effect on the steel material to highly reduce its mechanical properties such as toughness and fatigue strength. For example, the steel having the structure shown in FIG. 5 exhibits a low impact value of only about 3.9 K9f.m/cm2 at UE20, so that it is very brittle, over that of having the spherical C-type inclusion shown in FIG. 4. Accordingly, amounts and shapes of the inclusions should be carefully controlled.

In order to solve the above-mentioned problems, a nonrefined steel of bainite structure has been proposed. In manufacture of this steel, a quenching work is required. If the structure includes over 50 weight % of bainite, the impact value of the steel is reduced. Therefore, the application is limited. For solving this problem, additions of calcium or rare earth metals have been proposed. However, this proposal could not show definite composition of added elements, as well as any substantial improvements of the mechanical properties.

3 1 SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a high toughness non-refined steel which has improvements of toughness and tensile strength, enabling the above-mentioned disadvantages encountered in the prior arts.

Another object of the present invention is to provide a method for manufacturing a high toughness nonrefined steel which has improvements of toughness and tensile strength.

In one aspect, the present invention provides a high toughness nonrefined steel consisting of 0.30 % to 0.45 % C, 0.15 % to 0.35 % Si, 1.0 % to 1.55 % Mn, no more than 0.050 % S, no more than 0.30 % Cr, 0.01 % to 0.05 % AI, 0.05 % to 0. 15 % V, Nb or mixtures thereof, 0.0 % to 0.03 % Ti, 0.0005 %'to 0.003 % B, 0.2923Ti to 0.02 % N and the balance Fe and impurities contained inevitably in manufacturing the steel, all percentages being based on the weight of the steel, and having tensile strength of 75 Kgf/mm2 and toughness of 7 K9f. m/cm2.

In another aspect, the present invention provides a high toughness nonrefined steel consisting of 0.30 % to 0.55 % C, 0.15 % to 0.45 % Si, 0.60 % to 1.55 % Mn, no more than 0.050 % 8, 0 to 0.30 % Cr, 0.01 % to 0.05 % AI, 0.05 % to 0.15 % V, Nb or mixtures thereof, 0.0 % to 0.03 % Ti, no more than 0.02 % N and the balance Fe and impurities contained 1 4 i i i i i i 1 i J 1 i j i 1 i 1 i 1 1 i 1 i i i 1 1 inevitably in manufacturing the steel, all percentages being based on the weight of the steel, and having tensile strength of 75 Kgf2 and toughness of 7 Kgf.m/CM2.

In another aspect, the present invention also provides a method for manufacturing a high toughness non-refined steel comprising the steps of: preparing a composition consisting of 0.30% to 0.55% C, 0.15% to 0.45% Si, 0.60% to 1.55% Mn, no more than 0.050% S, 0% to 0.30% Cr, 0.01% to 0.05% Al, 0.05% to 0.15% V, Nb or mixtures thereof, 0.0% to 0.03% Ti, 0.0005% to 0.003% B, 0.2923a to 0.02% N (a is equivalent to the amount of said Ti) and the balance Fe, all percentages being based on the weight of the steel; casting the composition into ingots or bloom by treating it in a conventional furnace and under conventional melting condition; rolling the ingots or bloom to have a certain thickness under the condition that it is heated to the temperatures of above Ac3 and no more than 1,300'C, depending on the composition and the shape of the final product; and cooling controllably the rolled material from the temperatures of 80CC to 9500C to the temperatures of 50CC to 55CC, at cooling rates of 1CC/min to 1SO'C/min.

If the compositions according to the present invention are not controlled under the above severe thermal condition, the obtained non-refined steels exhibit hardly high toughness, high tensile strength and high frequency hardening capability.

14_ BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical microscopic photograph (X 400) of a high toughness non-refined steel in accordance with an embodiment of the present invention; FIG. 2 is a SEM (Scanning Electron Microscope) photograph (X 550) of the high toughness non-refined steel in accordance with the embodiment of the present invention; FIG. 3 is an optical microscopic photograph (X 400) of a high toughness non-refined steel in accordance with another embodiment of the present invention; FIG. 4 is an optical microscopic photograph (X 400) showing shapes of inclusions presented in a high toughness non-refined steel in accordance with the present invention; and FIG. 5 is an optical microscopic photograph (X 400) showing shapes of inclusions presented in a high toughness non-refined steel in accordance with the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Non-refined steels manufactured in accordance with the present invention basically have the increased hardness by virtue of the precipitation hardening of V and/or Nb which is appeared after the transformation at Ac3 and the increased 1 1 i 1 1 1 toughness by virtue of the grain refinement which is accomplished by carbon nitride precipitation which restrains the growth of austenite crystal grains in heating.

Ingredients of the steels according to the present invention are numerically limited as follows.

Carbon is an essential element for obtaining strength and hardness required in the steels. In the case when the composition of steel contains boron, carbon should be contained in an amount of at least 0.30 weight % (in some cases, at least 0.35 weight %) in the composition, so as to provide tensile strength of 75 K9f/mJ Under the condition that the composition contains boron, carbon in excess of 0.45 weight % reduces abruptly boron effect and thus adversely effects on the improvements of strength and toughness. This is because the boron effect gradually decreases as the content of carbon increases, although boron contributes to forming ferrite in the steel structure to assist the improvement of toughness and boron carbide contributes to improving strength by virtue of precipitation hardening thereof. If the composition contains no boron and carbon in excess of 0.55 weight %, toughness and weldability may be poor, so that the product can not be used for structural steels.

Silicon functions as a deoxidizer in steel manufacturing process and provides ferrite hardening effect. However, silicon in excess of 0.45 weight % (in some cases, 0.35 weight 7 1 J 1 %) promotes the transformation of proeutectic ferrite which effects adversely on toughness. Accordingly, the content of silicon should be limited to a maximum of 0.45 weight %.

Manganese is a cheap alloying element contributing to improving strength and providing toughness and also an essential element adapted as desulphurizer in the steel manufacturing process. In particular, manganese is an essential element, in that the present invention is intended to improve toughness by using MnS. Since the non-refined steel produced in accordance with the present invention requires the improvement of strength due to a small carbon content, manganese of at least 0.60 weight % (in some cases, at least 1.0 weight %) should be contained. Also, the content of manganese should be limited to a maximum of 1.55 weight %. In exceeding 1.55 weight %, cuttability and weldability become poor.

Sulfur is inevitably contained in the composition during the steel manufacturing process and forms a sulfide having a low plastic deformation temperature. In conventional steels, therefore, sulfur is normally contained in an amount of no more than 0.025 weight %. In the present invention, however, sulfur of a maximum of 0.050 weight % is used, because it provides the cuttability and the effect of forming ferrite in granular pearlite and thus functions to prevent decrease of toughness which is a typical disadvantage of non-refined 1 8 i i i i i 1 1 i i 1 i i 1 1 i 1 steel s. In exceeding 0.050 weight %, sulfur rather effects adversely an toughness and strength.

Chromium is presented as a solid solution in ferrite to strengthen it. In cases of the composition containing a small carbon, the addition of chromium in a small amount provides a advantageous result. The content of chromium should be limited to a maximum of 0.3 weight %. In exceeding 0.3 weight toughness rather becomes poor.

Aluminum is used in the steel manufacturing process because it has strong deoxidization effect. Also, aluminum remained in the steel contributes to improving toughness and refining crystal grains by virtue of the effects of dispersion type compounds and nitrides. When the composition contains aluminum in an amount of less than 0.01 weight %, insufficient deoxidization is obtained. In exceeding 0.075 weight % (in - in some cases, 0.05 -,,eiqht %), aluminum is contained in S'02 a small amount so that it is subjected to a plastic deformation, thereby causing cuttability and cleanliness to be poor. Accordingly, the content of aluminum should be limited to a minimum of '0.01 weight % and a maximum of 0.075 weight %.

Vanadium contributes to improving strength and toughness by forming carbides and nitrides. Even in a small amount, vanadium provides effectively a required strength.

Niobium contributes greatly to refining crystal grains. This is because niobium restrains recrystallization of 9 i austeni te i n hot worki ng and f orms a f i ne preci pi tati on by i ts transformation, so as to improve strength. Niobium improves strength and toughness, in cooperation with vanadium. In practice, niobium and vanadium may be used a l one, respectively, or in combination. The total content of these elements should be limited to a maximum of 0.15 weight %, because excessive addition thereof may result in decreasing weldability. In some cases, undesirable results such as poor strength and toughness may occur when the content is less than 0.05 weight %.

Boron functions to pr.ovide the effects of promoting ferrite formation and improving hardenability of non-refined steels. Accordingly, suitable addition of boron will result in obtaining advantageous effects. At least 0.0005 weight % of boron should be added to the composition. In exceeding 0,003 weight %, boron may be effective no longer and rather functions to reduce toughness.

Titanium has strong bonding strength with nitrogen and thus forms nitride. In particular, titanium contributes to refining granular austenite, thereby enabling toughness to be significantly improved. In exceeding 0. 03 weight %, titanium may not provide the abovementioned effects. In some cases, strength may be decreased when the content of titanium is less than 0.01 weight %.

If desired, metal elements such as calcium, tellurium, '10 1 i 1 1 k ceri um and other rare earth metal s, mi sch metal, and mi xtures thereof may be added to the composition. These elements are adapted to control shapes of inclusions, in particular the shape of MnS, so as to improve material anisotropy and impact toughness after hot working. Such effect can be obtained when the elements are added in an amount of at least 0. 0001 weight %. In exceeding 0.04 weight %, the effect is increased no longer. Accordingly, the content is limited to a minimum of 0.0001 weight % and a maximum of 0.04 weight %.

Together with vanadium, carbon forms carbide such as V,C3 or VBC7 or nitride such as VN. Carbon is also presented in the form of NbC or Nb(CN) by reacting with niobium, and in the form of TiC or Ti(CN) by reacting with titanium. A trace of carbon together with boron may form Fe3(C,B).

Nitrogen forms TiN or Ti(CN), together with titanium and also bonds with aluminum to form AlN. Together with vanadium, nitrogen forms VN or V(CN). A trace of BN is also formed. Depending on the addition order of above alloying elements, however, major nitrogen may form BN. Accordingly, boron is normally added in the composition finall.y, in order to restrain the formation of BN.

These carbides and nitrides have their high formation temperatures and thus greatly effect on rising of recrystallization temperature and grain refinement. Fine carbide also functions effectively to strengthen ferrite 11 1 1 i 1 1 1 i matrix.

Among them, TiN has the highest formation temperature to be precipitated at temperatures of 1 450 C to 1, 100 C, so that it functions to provide nuclear forming positions of austenite crystals.

At lower temperature, Ti(CN) is precipitated. NbV and VN are precipitated at temperatures of 950 C to 800 C and at temperatures of 900 C to 750 C, respectively. At slightly lower temperature, VC is precipitated.

Although Aluminum also forms AIN, together with nitrogen, the amount of formed AIN is little and insufficient to provide a desired effect. Once formed, however, A1N functions effectively, in that it has a similar formation temperature to that of TiN.

On the other hand, manganese reduces the activities of carbon and nitrogen. In order to increase the activities of carbon and nitrogen and thus ensure the effects of carbide and nitride, it is required to add elements such as vanadium and niobium which elements function to increase the activities of carbon and nitrogen. In this connection, vanadium is preferred, in that it is of interstitial fine grains which can be easily diffused and dispersed, as compared with niobium.

In addition of using the above-mentioned carbide and nitride in providing nuclear forming positions of austenite crystals, the present invention uses MnS in promoting the A2 i 1 i 1 1 1 1 i formation of intergranular ferrite which is effective for providing toughness. As more or less excessive sulfur is added to the composition, in accordance with the present invention, over the conventional steels, MnS can function to provide nuclear forming positions of intergranular ferrite. In order to minimize the decrease of toughness caused by MnS, shapes of inclusions are controlled by providing calcium, tellurium, cerium or other rare earth metals, or misch metal in accordance with the present invention.

According to the present invention, the content of the inclusion shape control element is limited to a minimum of 0.0001 weight % and a maximum of 0.04 weight %. Now, more detailed description will be made with respect to the inclusions.

Inclusions may be formed in the steel manufacturing process using a converter, an electric furnace, or a vacuum melting furnace. Factors forming inclusions are classified into internal factors and external factors. Inclusions formed by internal factors include deoxidized products, such as Si02 and MnO, which can be formed by virtue of the deoxidization, sulfides such as MnS, nitrides, and combinations thereof. Inclusions formed by external factors include silicic acid refractory which is produced by the reaction between molten steel and the refractory constituting furnace walls. According to shapes, inclusions are also classified into an A13 f 4 type having a linear shape, a B-type having a polygonal shape, and a C- type having a spherical shape.

In deoxidized killed steels, particularly Si-Mn base killed steels, oxide base inclusions such as S'02, MnO and Mnsilicate are formed. In case of Al base killed steels, A1203 base inclusions are mainly formed. These inclusions have spherical or hexagonal shapes at the beginning of solidification. However, oxides of aluminum, titanium and chromium are distributed under the aggregated condition, because of having high melting points. On the other hand, oxides and sulfides having lower melting points are subjected to plastic deformations in hot workings such as hot rolling or forging, and thereby elongated into linear shapes. These linear inclusions reduce toughness and cuttability of steels, because of having physical properties different from that of matrix structure. Accordingly, the present invention uses aluminum for the deoxidization. A1203 which is a deoxidized product formed in the deoxidization process has relatively small grain sizes and high hardness, so that no plastic deformation thereof occur even in hot working. As a result, the inclusion is hardly deformed into linear shape.

On the other hand, a major part of inclusions include sulfides which are mainly MnS. In the presence of elements such as aluminum, titanium or chromium, a trace of sulfides such as A12S, C3S, CrS and TiS may be formed.

1 14 1 i 1 i i 1 i i J i 1 1 i i i i 1 f A description will now be made with respect to a method for manufacturing non-refined steels by using the abovementioned compositions under the condition of controlling shapes of formed inclusions in accordance with the preser)t invention.

First, the alloy steel composition as mentioned above is melted in a converter, an electric furnace or a vacuum melting furnace which are maintained at a certain temperature. Even though there is no inclusion of impurities such as S and P, using the electric furnace causes the increase of the amounts of 02 and N2 of air flowing into the furnace and contacting with the melt. Accordingly, it is necessary to use a suitable deoxidizer such as aluminum or to remove gases by utilizing a vacuum melting method.

When products resulted from the deoxidization using aluminum are oxide base inclusions such as A1203 and SiO,, there is no problem, in that they have small grain sizes and high hardness, and thus relatively weak effects on the mechanical properties of steels, in the presence of small amounts. However, sulfide base inclusions should be controlled in their shapes, since they have large grain sizes and irregular shapes including a linear shape. Therefore, the present invention is mainly intended to control the shapes of sulfide base-inclusions.

To this end, the inclusion shape control elements as, 15 mentioned above are added to the composition in amounts of 0.0001 weight % to 0.04 weight %, in accordance with the present invention. These elements function to deoxidize the steel to be manufactured and control the shapes of sulfides. In this connection, if the deoxidization of the steel carried out by using elements such as silicon, manganese and aluminum, and then calcium, tellurium, rare earth metals such as cerium, or misch metal are added to the composition of steel, the effect of controlling the shapes of inclusions is improved.

That is, the inclusion shape control elements form sulfides which surround oxides bonded with high melting point oxides such as A12031 the product resulted from the deoxidization, so as to form totally spherical inclusions. These spherical inclusions are hardly subjected to a plastic deformation in hot working process, and thus remained as the C-type inclusions having a generally spherical shape which is darkly shown in FIG. 4. The steel including the spherical inclusions exhibits the impact value of about 8.6 Kgf.M/CM2 at UE20, and thus the improved toughness.

In addition of providing the improved toughness resulted from the inclusion shape controlling of the inclusion shape control elements, the present invention improves the mechanical properties of steel by alloying elements. That is, as the alloy steel manufactured as above is subjected to a hot working process, AIN or TiN are precipitated at the 1 16 t i 1 i i j i i 1 1 1 i i temperature above 1,300 C. These precipitations function as nuclears for austenite grains. Upon rolling or forging the steel at the temperature below 1 000 'C, compounds such as NbC, VN and VCN are precipitated in turns. In this case, NbC and VN functions to restrain coarseness of austenite grains and thereby improve grain refinement thereof, and lowering of recrystalization temperature.

On the other hand, austenite grains grown in the above process have the polygonal shape similar to the round shape. Upon cooling the steel below the temperature corresponding to Ar3 transformation point, VCN is intergranularly precipitated. Thus, intergranular precipitation hardening effect is obtained. Ferrite which has been precipitated due to the transformation of steel in the previous cooling process is subjected to a work hardening, thereby significantly improving the strength. Thereafter, as the steel is subjected to a cooling below the temperature corresponding to Ar, transformation point, ferrite constituting pearlite is hardened, by virtue of its work hardening phenomenon caused by a forging work, thereby improving fatigue strength of steel.

As apparent from the above description, the present invention comprises adding calcium, tellurium, cerium or other rare earth metals, or misch metal, alone or in combination, as inclusion shape control element or elements, in certain amounts, so that sulfides such as MnS surround A1203 which is 17 i z the product resulted from the deoxidization effect of aluminum, thereby causing the inclusions to have spherical shapes. Thus, toughness of result alloy steels is not reduced. By virtue of the addition of alloying elements such as Nb, V a6d N, granular refinement is promoted. These alloying elements also function to provide precipitation hardening effect, thereby avoiding lowering of recrystalization temperature in hot working process. Thus, work hardening effect of ferrite is obtained, thereby significantly improving the strength of result steel. Consequently, it is possible to obtain superior non-refined steels having the mechanical properties equivalent to those of refined steels, without requiring any refining treatment. the manufacturing process of non-refined steels can be also efficiently carried out.

Thus, the present invention overcomes the disadvantages encountered in providing low carbon and high alloy steels.

The present invention will be understood more readily with reference to the following examples; however these examples are intended to illustrate the invention and are not to be construed to limit the scope of the present invention.

EXAMPLES

Various compositions as presented in TABLE 1 were prepared and cast into square ingots of 100 mm X 100 mm by 1 18 1 i i i i i i 1 1 using a laboratory melting furnace. After controllably forged at temperatures of 1,300 C to 920 C, all ingots were rolled to have the diameter of 50 mm. The rolled steel specimens were cooled from about 920 C to about 520 C at a cooling rate of about 60 C/min. Then, the samples were subjected to conventional tensile and impact toughness tests. The results were presented in TABLE 2.

1 19 TABLE 1 (weight %) Kind c si Mn S Al v Nb N T i B A B Cr Other 0.32 0.26 0.89 0.032 0.027 0.06 0.003 0.0082 0.011 0.0018 0.18 0.34 0.22 1.28 0.046 0.026 0.08 0.003 0.0104 0.015 0.0029 - 0.002 M.M 0 0 c 0.50 0.24 0.87 0.046 0.026 0.09 - 0.0091 0.017 - 0.17 - 1) D 0.45 0.25 0.92 0.022 0.039 0.10 - 0.0089 0.012 - 0.12 0.002 0 Te E 0.37 0.21 1.35 0.025 0.031 0.31 - 0.0105 - - 0.20 - - 2) C) F 0.33 0.25 0.92 0.025 0.020 0.06 0.0113 0.009 0.0020 G 0.44 0.29 1.50 0.017 0.032 0.12 0.001 0.0094 - - 0.16 - 2) H 0.46 0.23 1.05 0.021 0.041 0.09 - 0.0076 - - 0.10 0.001 2) Ca 1 0.37 0.21 1.35 0.025 0.031 0.13 - 0.015 - - 0.20 0 Other: Inclusion shape control element MI.)M: Misch metal 2): Present invention: Prior art

1, ik TABLE 2 (Tensile Test Specimens, diameter of reduced section 14 mm and gage length 50 mm, Impact Test Specimens, 2 mm U notched charpy specimen) Kind Tensile Yield Elongation Reduction Impact Strength Strength of Area Strength K9f /mm2(MPa) K9f.m/cm2 (20 C) A 78.2(767) 53.4(524) 23 50 10.1 B 82.3(807) 51.6(506) 18 42 12.3 C 94.4(926) 62.4(612) 19 49 8.1 D 88.6(860) 54.1(530) 17 45 9.5 E 83.0(814) 58.1(570) 23 53 7.9 F 75.4(739) 48.5(476) 24 48 5.5 G 96.8(930) 64.3(631) 22 49 6.7 H 92.8(910) 56.6(555) 15 33 5.5 1 83.0(814) 58.1(570) 23 53 7.9 : Subjected to hot working, maintained at 900 C and for 2 hours, and then air cooled.

As apparent from TABLES 1 and 2, non-refined steels of the present invention exhibit high toughness of the same level as those of refined steels. It is possible to save costs and labors, by virtue of eliminating the refinement. Thus, nonrefined steels of the present invention is superior, in terms of the manufacture cost and the application, over conventional refined steels and non-refined steels.

Referring to the annexed drawings, it could be found that the steel structures of the present invention were dense sufficient to provide desired toughness.

Although the preferred embodiments of the invention have been disclosed for illustrative purpose, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

1 22 i i 1 i 1 i i 1 i 1 1 1 1 1 i i i 1 X.

Claims (3)

WHAT IS CLAIMED IS:

1 A high toughness non-refined steel consisting of 0.30% to 0.45% C, 0.15% to 0.35% Si, 1.0% to 1.55% Mn, no more than 0.050% S, no more than 0.30% Cr, 0.01% to 0.05% Al, 0.05% to 0.15% V, Nb or mixtures thereof, 0.0% to 0.03% Ti, 0.0005% to 0.003% B, 0.2923a to 0.02% N (a is equivalent to the amount of said Ti) and the balance Fe and impurities contained inevitably in manufacturing the steel, all percentages being based on the weight of the steel, and having tensile strength of 75 Kgf/mm2 and toughness of 7 Kgf m/cm2.

2 A high toughness non-refined steel consisting of 0.30% to 0.55% C, 0.15% to 0.45% Si, 0.60% to 1.55% Mn, no more than 0.050% S, 0 to 0.30% Cr, 0.01% to 0.05% Al, 0.05% to 0.15% V, Nb or mixtures thereof, 0.0% to 0.03% Ti, no more than 0.02% N and the balance Fe and impurities contained inevitably in manufacturing the steel, all percentages being based on the weight of the steel, and having tensile strength of 75 Kgf/mM2 and toughness of 7 Kgf.m/cm2.

3 A high toughness non-refined steel in accordance with Claim 1, wherein said steel contains additionally at least one element selected from the group consisting of calcium, tellurium, cerium and other rare earth metals, misch metal, and mixtures 1 24 thereof, in amounts of 0.0001 weight % to 0.04 weight 4 A method for manufacturing a high toughness non-refined steel comprising the steps of: preparing a composition consisting of 0.30% to 0. 55% C, 0.15% to 0.45% Si, 0.60% to 1.55% Mn, no more than 0.050% S, 0% to 0.30% Cr, 0.01% to 0.05% Al, 0.05% to 0.15% V, Nb or mixtures thereof, 0. 0% to 0.03% Ti, 0.0005% to 0.003% B, 0.2923a to 0.02% N (a is equivalent to the amount of said Ti) and the balance Fe, all percentages being based on the weight of the steel; casting the composition into ingots or bloom by treating it in a conventional furnace and under conventional melting condition; rolling the ingots or bloom to have a certain size under the condition that it is heated to the temperatures of above Ac3 and no more than 1,3000C, depending on the composition and the shape of the final product; and cooling controllably the rolled material from the temperatures of 8000C to 9500C to the temperatures of 5000C to 5500C, at cooling rates of 10'C/min to 150'C/min.

A method in accordance with Claim 4, further comprising adding at least one element selected from the group consisting of calcium, tellurium, cerium and other rare earth metals, misch metal, and mixtures thereof, in amounts of j, 1 i j i 1 i i i i 1 I i 1 j 0.0001 weight % to 0.04 weight %, to the composition.

1 25 Published 1992 at The Patent Office, Concept House, Cardiff Road, Newport. Gwent NP9 I RH. Further copies may be obtained from Sales Branch. Unit 6. Nine Mile Point. Cwrnfelinfach. Cross Keys, Newport. NPI. 7RZ. Printed by Multiplex techniques ltd, St Mary Cray, Kent.