KR100951297B1 - A wire rod free from heat treament having high toughness for cold forging and method for manufacturing the same - Google Patents

Abstract

In the present invention, there is provided a cold toughened high toughness heat treatment omission wire rod used for fastening mechanical structures or bolts for automobile parts and the like and a method of manufacturing the same.

High toughness heat treatment omitted wire rod for cold rolling of the present invention is C: 0.05 ~ 0.20%, Si: 0.01 ~ 0.1%, Mn: 1.0 ~ 2.0%, Nb: 0.01 ~ 0.05%, Mo: 0.01 ~ 0.1%, N: 0.003-0.03%, P: 0.035% or less (does not contain 0%), S: 0.040% or less (does not contain 0%), B: 0.001-0.004%, and Ti: 0.01-0.1 1 or 2 or more selected from the group consisting of%, consisting of the remaining Fe and other impurities, the bainitic ferrite (area fraction 85-90%) and the area fraction is 10-15% It is characterized by having a M / A (martensite and residual austenite mixture) phase structure.

Cold Rolling Mill, Cold Finishing, Bainistic, Ferrite, Toughness

Description

High toughness heat treatment omission wire rod for cold press and manufacturing method {A WIRE ROD FREE FROM HEAT TREAMENT HAVING HIGH TOUGHNESS FOR COLD FORGING AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a cold toughened high toughness heat treatment omission wire rod used for fastening mechanical structures or bolts for automobile parts, and a method for manufacturing the same. Specifically, the present invention relates to a microstructure obtained by imparting an optimum alloy component, an effective rolling and cooling rate. Through appropriate control, the present invention relates to a wire rod which can be cold pressed without any heat treatment until the manufacture of the parts, and a method of manufacturing the same.

In recent years, the trend of technology development of cold-rolled wire rods has been focused on developing high strength and high toughness wires, as well as omitting wire rods that omit heat treatment and processing processes. In line with the trend of cost reduction and energy reduction, the development of low-cost wire rods containing extremely high amounts of expensive elements is emerging.

In the case of the conventional cold-rolled heat-treated wire rod, the wire is subjected to spheroidizing heat treatment in the wire state, followed by sizing purpose, followed by spheroidizing heat treatment, bolt forming, quenching, and soaking process, and thus final tempered martensite. Has a microstructure of. Therefore, the strength of the bolt is determined by the composition, quenching, and heat treatment process, and most commonly, about two heat treatments are applied until the final product stage. However, as a result of cost increase due to heat treatment, there is a need for development of heat-treated wire rods.

Therefore, after manufacture, wire rods have been developed in which all heat treatments have been completely omitted, and this type of wire rod is often referred to as an unstructured wire rod. Products having these characteristics can be manufactured directly by sizing purposes after wire fabrication, and only through bolt forming and final processing. The biggest feature of these heat-treated wires is to reduce the energy consumed in the heat-treatment process and lower the cost of the product. Since heat-treatment is not given, quench cracks, warpage and other defects due to heat treatment do not occur. There are advantages. However, the strength is not secured by heat treatment, but the strength is obtained through cold working such as drawing, so that ductility reduction and toughness reduction have been most problematic.

On the premise of such mechanical characteristics, the technology to reduce the content gap by reducing the content of expensive added elements and to optimize the rolling conditions, and to improve the strength ratio while securing high strength level by cold drawing without heat treatment. This was developed. In addition, by establishing a technique for removing stress in a series of cold drawing processes, it is possible to cold draw from a coil material rather than a bar material, thereby eliminating stress removal annealing and improving yield. For example, in the automobile steering rack parts that have been required to withstand strength, such as fatigue strength, conventionally, heat treatment type products by quenching and tempering have been adopted. Partial progress was made.

However, in the case of these methods, it is possible to secure a high strength to a large extent through high strength, but there is a problem in that the application field is limited because problems of ductility reduction and impact characteristics decrease due to drawing and cold forging.

That is, the technologies for wire rods that can omit the conventional heat treatment process are mainly focused on increasing the strength, and the effect on the impact toughness is still insufficient.

Therefore, even if the heat treatment step is omitted, it is necessary to develop a wire rod that can secure high strength as well as high toughness, and through the present invention, it is possible to manufacture a heat-treated omitted wire rod having excellent high toughness.

An object of the present invention is to provide a wire rod used for fastening mechanical structures or bolts for automobile parts, and the like.

In order to achieve the above object, the high toughness heat treatment omission wire rod for cold rolling of the present invention is C: 0.05 to 0.20%, Si: 0.01 to 0.1%, Mn: 1.0 to 2.0%, Nb: 0.01 to 0.05%, and Mo : 0.01 to 0.1%, N: 0.003 to 0.03%, P: 0.035% or less (not including 0%), S: 0.040% or less (not including 0%), B: 0.001 to 0.004% And Ti: one or two or more selected from the group consisting of 0.01 to 0.1%, and are composed of the remaining Fe and other impurities, and the area fraction is 85 to 90% bainitic ferrite and area fraction It has a 10% to 15% M / A (martensite and residual austenite mixture) phase structure.

Furthermore, the method for producing a high toughness heat treatment omission wire rod for cold press of the present invention is C: 0.05 to 0.20%, Si: 0.01 to 0.1%, Mn: 1.0 to 2.0%, Nb: 0.01 to 0.05%, Mo: 0.01 to 0.1%, N: 0.003 to 0.03%, P: 0.035% or less (not including 0%), S: 0.040% or less (not including 0%), B: 0.001 to 0.004% and Ti: 1 or 2 or more selected from the group consisting of 0.01 to 0.1%, and the billet composed of the remaining Fe and other impurities 30 minutes or more at A _e3 + 150 ℃ ~ A _e3 + 250 ℃ Heating to 30 minutes or less;

Cooling the heated billet to 5 to 15 ° _C./s and rolling at A _e3 + 50 ° C. to A _e3 + 150 ° C .;

After the rolling, the rapid heating and cooling once in a 350 ± 20 ℃ ~ 550 ± 20 ℃ section of the cooling is made to the step of cooling to 150 ℃ ± 20 ℃ section at an average of 5 ~ 30 ℃ / s.

The present invention is a cold-rolled wire rod that can be applied to fastening mechanical structures or bolts for automobile parts, etc., and has excellent high toughness even if the heat treatment is omitted by forming bainitic ferrite and M / A (martensite and residual austenite mixed) structure. There is a useful effect in manufacturing the wire rod to ensure the.

Hereinafter, the composition range of the steel component of the present invention will be described.

C: 0.05-0.20% (less than weight%)

The reason for limiting the content of C to 0.05 to 0.20% is that when the content exceeds 0.20%, the tendency of ferrite and pearlite microstructure formation becomes stronger under the influence of C. If the content is less than 0.05%, the content of the wire after hot rolling This is because the tensile strength is not sufficiently secured.

Si: 0.01 ~ 0.1%

The reason for limiting the content of silicon to 0.01 ~ 0.1% is as follows. If the Si content is more than 0.1%, the work hardening phenomenon during the cold drawing and pressing process occurs rapidly, which is a problem in the workability, and less than 0.01% can not secure sufficient strength of the bolt.

Mn: 1.0-2.0%

Mn is an element that forms a solid solution to form a solid solution to strengthen the solid solution to obtain a required strength without a decrease in ductility, the content is limited to 1.0 to 2.0%. When Mn is added in excess of 2.0%, Mn segregation is more harmful to product characteristics than solid solution strengthening effect. Macro segregation and micro segregation are more likely to occur depending on the segregation mechanism when steel solidifies. Due to the relatively low diffusion coefficient, the segregation zone is promoted and the hardenability improvement is the main cause of the core martensite. In addition, when the Mn is added less than 1.0%, the effect of segregation zone due to Mn segregation is hardly expected, but it is difficult to expect the effect of guaranteeing strength and improving toughness by solid solution strengthening.

Nb: 0.01 ~ 0.05%

Nb limits its content to 0.01 to 0.05%. Since it is an expensive element, it is not added in excess of 0.05% for the purpose of increasing the optimum hardenability. If the content is less than 0.01%, the minimum limit is reduced because the effect of delaying the formation of ferrite and pearlite by synergistic interaction with B is reduced. It is preferable to set it as 0.01%.

Mo: 0.01 ~ 0.1%

It also shows the effect of delaying the production of pearlite, and serves to double the Nb addition effect. Although it is an expensive element, the effect on quenchability is great, so it is necessary to add 0.01% or more, and when it exceeds 0.1%, the effect on quenchability and mutual synergy with Nb are insufficient, so the amount is limited as above. .

N: 0.003-0.03%

The content of N is limited to 0.003 to 0.03%. If the content is less than 0.003%, not only is it ineffective for producing Ti nitride, but the effect of adding Ti disappears, and if it exceeds 0.03%, Ti nitride is formed and the remaining N forms a compound with B to reduce the effect of adding B. Because.

P: 0.035% or less (does not contain 0%), S: 0.040% or less (does not contain 0%)

The contents of P and S are limited to 0.035% or less and 0.040% or less, respectively. Since P is the main cause of deterioration of toughness due to segregation at grain boundary, the upper limit thereof is limited to 0.035%, and S is grain boundary segregation with low melting point element to reduce toughness and form an emulsion, thereby delaying fracture resistance and stress relaxation. It is preferable to limit the upper limit to 0.040% because it adversely affects the characteristics.

B: 0.001-0.004%

B is a grain boundary strengthening element for improving the hardenability and delayed fracture resistance in the present invention to limit the content of B to 0.001 ~ 0.004%. If the content is less than 0.001%, B atoms have insufficient effect of improving grain strength or hardenability due to grain boundary segregation during heat treatment.If the content is higher than 0.004%, the effect is saturated and the nitride of B is precipitated at grain boundary, which lowers grain strength. Because.

Ti: 0.01 ~ 0.1%

The content of Ti is limited to 0.01 to 0.1%. If the content is less than 0.01%, the effect of improving the corrosion resistance is insufficient and it is difficult to form Ti nitride, which prevents the formation of B nitride to improve the hardenability of B. If the content exceeds 0.1%, the effect is saturated and coarse Ti-based nitride This is because it is harmful to fatigue characteristics by forming.

Hereinafter, the structure of the wire rod of the present invention will be described.

The wire rod of the present invention has a structure of bainitic ferrite having an area fraction of 85 to 90% and M / A (mixed martensite and residual austenite) having an area fraction of 10 to 15%. The tissue is a needle-like or spherical microstructure as shown in Figure 1, it can be obtained a high tensile strength and ductility in the area fraction. That is, when the area fraction of bainitic ferrite exceeds 90%, the tensile strength is lower than 800 MPa, and when less than 85%, impact toughness is lower than 150J.

In addition, in the case of bainitic ferrite tissue, the average grain size is 200 μm or less. This is because when the average grain size of bainitic ferrite exceeds 200 µm, the impact toughness becomes poor due to a decrease in tensile strength and a decrease in ductility.

Hereinafter, the manufacturing method of the wire rod of this invention is demonstrated.

(1) heating at A _e3 + 150 ° C. to A _e3 + 250 ° C. for at least 30 minutes and at most 1 hour 30 minutes.

The heating at this temperature is maintained in the austenite single phase, and is a range in which the austenite grains are not coarsened, and the temperature at which the segregation, carbides, and inclusions remaining can be effectively dissolved. When A _e3 + 250 ° C., the austenite grains become very coarse, and the tendency of coarsening of the final microstructure formed after cooling is not strong, and thus high strength and high toughness wires cannot be obtained. Ae ₃ If it is less than + 150 ° C, the effect of heating cannot be obtained. Therefore, heating at A _e3 + 150 ° C to A _e3 + 250 ° C is preferable. If the heating time is less than 30 minutes, there is a problem that the overall temperature cannot be made uniform, and when heated for more than 1 hour 30 minutes not only increases the possibility of coarsening of austenite grains, but also significantly reduces productivity.

(2) cooling the billet to 5-15 ° _C./s and rolling at A _e3 + 50 ° C.-A _e3 + 150 ° C .;

The cooling rate is limited to the purpose of minimizing the transformation of the microstructure in the cooling step before hot rolling. If the cooling rate before hot rolling is 5 ° C / s or less, productivity is reduced, and additional equipment is required to maintain slow cooling. In addition, as in the case where the heating time is maintained for a long time, there is a fear that the strength and toughness of the wire rod after the hot rolling is completed. On the other hand, if the cooling rate exceeds 15 ° C / s, the driving force of the transformation of the billet before rolling increases, which increases the possibility of the appearance of new microstructures during rolling, and the serious problem of resetting the rolling temperature to a lower temperature. Will result.

In addition, in the rolling temperature range of A _e3 + 50 ° C to A _e3 + 150 ° C, the appearance of microstructures due to deformation during rolling is suppressed, and only sizing rolling is possible without recrystallization. At temperatures below A _e3 + 50 ° C., it is not possible to obtain the microstructure of the present invention close to the dynamic recrystallization temperature, and there is a great possibility of securing a general soft ferrite. At temperatures exceeding A _e3 + 150 ° C., a problem arises in which heating is required after cooling.

(3) rapid heating and cooling once in a section of 350 ± 20 ° C. to 550 ± 20 ° C. to cool to 150 ° C. ± 20 ° C. at an average of 5 to 30 ° C./s.

The cooling rate is intended to effectively generate the microstructure of the present invention after rolling in accordance with the above-mentioned hot rolling conditions. Rapid cooling and cooling once in the above temperature range during cooling, while raising the temperature to 550 ± 20 ℃ by using a heating device near 350 ± 20 ℃ during cooling, the development of M / A (martensite and residual austenite mixture) phase And the fraction can be made 10% or less. That is, this section is closely related to the formation of the M / A phase, and it is possible to obtain the M / A phase which enables the improvement of tensile strength and toughness without changing the bainitic ferrite already generated during cooling. Specifically, some residual austenite which cannot be transformed into bainitic ferrite during transformation is formed into a mixed phase such as M / A phase.

In addition, the average speed cooled from 150 degreeC ± 20 degreeC immediately after hot rolling shall be 5-30 degreeC / s. This means that when the average cooling rate is less than 5 ° C / s, the fraction of bainitic ferrite is too reduced, the mechanical properties are not good, the bainitic ferrite of the present invention is not secured, soft ferrite is produced. On the contrary, if it exceeds 30 ° C./s, the fraction of bainitic ferrite may increase too much, resulting in poor mechanical properties or a large amount of hard martensite, which may lead to a serious decrease in ductility. In addition, the end temperature of the cooling is preferably finished in the range of 150 ° C ± 20 ° C. It is almost impossible to control the cooling rate in the range below this section, and because the abnormal microstructure may occur at the temperature exceeding this section, the stability of the generated microstructure is not secured.

The tensile strength of the wire rod obtained through the above component range and manufacturing method is preferably 800MPa or more and impact toughness of 150J or more.

Hereinafter, the present invention will be described in detail with reference to the following examples, which are only preferred embodiments of the present invention, and the scope of the present invention is not limited by the description of these embodiments.

Example 1

Hereinafter, the manufacturing method of this invention is demonstrated to the invention material and comparative materials 1, 2, and 3 of this invention, and the example is demonstrated.

Table 1 below shows the hot rolled manufacturing method of the inventive material and the comparative materials 1, 2, and 3, in the case of the inventive material, the conditions of the present invention are applied, and the comparative materials 1, 2, and 3 represent the conventional manufacturing conditions. It is applied as is. However, due to the difference in composition, the A _e3 temperature is different for each condition.

Steel grade A _e3 temperature ( ^o C) Billet heating condition Billet cooling rate Billet rolling condition Cooling condition after rolling Invention 875 1112 ^o C, 1h 20min 8 ~ 11 ^o C / s 964 ^o C, final diameter 16 mm Cool down to 27 ^o C / s on average up to 165 ^o C Comparative material 1 827 1090 ^o C, 1h 30min 10 ~ 19 ^o C / s 945 ^o C, final diameter 16 mm Cool down to 17 ~ 27 ^o C / s to room temperature Comparative material 2 786 1107 ^o C, 1h 26min 8 ~ 10 ^o C / s 998 ^o C, final diameter 16 mm Cool down to room temperature 5 ~ 17 ^o C / s Comparative material 3 775 1119 ^o C, 1h 45min 11 ~ 17 ^o C / s 982 ^o C, final diameter 16 mm Cool down to 12 ~ 19 ^o C / s to room temperature

Table 2 below shows the main components of the invention and comparative materials 1, 2 and 3 (not the composition range, but analyzed by wet analysis). Comparative material 1 and comparative material 2 are general wires of similar strength level, which are manufactured by hot drawing, cold drawing and cold working, and then quenched and treated. Comparative material 3 is a bar for wire rods. It is used without. Inventive material and comparative material 3 require the composition to control the strength of the material by using work hardening without undergoing heat treatment, and comparative material 1 and comparative material 2 undergo quenching and sour heat treatment, so A composition that can be obtained is necessary.

Table 3 below shows the microstructure, tensile strength and elongation, and impact toughness of each process step after hot rolling of the inventive and comparative materials. However, the diameter of the object was made the same at 16 mm.

Steel grade Chemical composition (% by weight) C Si Mn P S Cr Ni V Nb Mo N Ti B Invention 0.11 0.047 1.52 0.013 0.013 - - 0.028 0.050 0.0046 0.018 0.0019 Comparative Material 1 0.20 0.20 1.20 0.013 0.015 - - - - 0.0050 0.018 0.0018 Comparative Material 2 0.41 0.21 0.74 0.013 0.012 0.152 0.013 - - - 0.0042 0.018 - Comparative Material 3 0.47 0.27 1.31 0.010 0.046 0.10 0.15 0.10 - - 0.0018 0.023

Invention Comparative Material 1 Comparative Material 2 Comparative Material 3 Microstructure after Hot Rolling Bainistic Ferrite (87%) + M / A Phase (13%) Ferrite (65%) Pearlite (35%) Ferrite (59%) Pearlite (41%) Ferrite (55%) Pearlite (45%) Tensile Strength After Hot Rolling (MPa) 595 635 645 660 Elongation after Hot Rolling (%) 62 42 35 22 U-notch impact toughness after hot rolling (J) 320 115 120 95 Whether spheroidization heat treatment before cold drawing skip 680 ^o C, 20hr 680 ^o C, 20hr skip Tensile Strength after Cold Drawing 10% (MPa) 672 706 710 765 Tensile strength after cold drawing 20% (MPa) 798 789 798 852 Tensile strength after cold drawing 30% (MPa) 906 816 836 912 U-notch impact toughness after cold drawing (J) 258 89 91 72 Whether to heat treatment after bolt processing skip 880 ^o C, 2hr quench and 475 ^o C, 2hr 880 ^o C, 2hr quench and 475 ^o C, 2hr skip Final bolt product microstructure Bainistic Ferrite (87%) + M / A Phase (13%) Temper Martensite Temper Martensite Ferrite (55%) Pearlite (45%) Final Bolt Product Tensile Strength (MPa) 910 890 905 920 Final Bolt Product Elongation (%) 38 16 12 8 End bolt product U-notch impact toughness (J) 226 110 102 63

As can be seen from the results of Table 2, it can be seen that the mechanical properties after the final component is manufactured are significantly superior to the other comparative materials.

Example 2

Table 4 below shows the remaining conditions of the manufacturing method of Example 1 using the various billets having the composition of the present invention, the difference in the fraction of the bainitic ferrite and the M / A phase of the wire rod when only the average cooling rate is different Size is shown. As can be seen from the results of Table 4, the area fraction and average size of M / A phases are in proportion to each other, and the smaller the area fraction, the smaller the size.

Table 5 below shows the mechanical properties according to the area fraction of the bainitic ferrite and M / A phases of the respective wire rods obtained in Table 4.

Average cooling rate Bainitic ferrite fraction (%) Fraction (%) and average size (μm) of M / A phase 1 ℃ / s 73 27, 30 3 ℃ / s 80 20, 20 5 ℃ / s 85 15, 12 24 ℃ / s 87 13, 12 30 ℃ / s 90 10, 10 42 ℃ / s 96 4, 9 53 ℃ / s 99 1, 5

Tensile Strength After Hot Rolling (MPa) Elongation after Hot Rolling (%) U-notch impact toughness after hot rolling (J) Tensile Strength (MPa) of Final Product Elongation (%) of final bolt product End bolt product U-notch impact toughness (J) Bainistic Ferrite (73%) + M / A Phase (27%) 988 22 125 950 25 132 Bainistic Ferrite (80%) + M / A Phase (20%) 698 35 136 923 16 98 Bainistic Ferrite (85%) + M / A Phase (15%) 668 48 286 935 21 184 Bainistic Ferrite (87%) + M / A Phase (13%) 595 62 320 910 38 226 Bainistic Ferrite (90%) + M / A Phase (10%) 512 67 409 821 45 324 Bainistic Ferrite (96%) + M / A Phase (4%) 509 23 187 756 32 298 Bainistic Ferrite (99%) + M / A Phase (1%) 523 18 169 732 29 169

As can be seen from the results of Table 5, when the area fraction of the microstructure is 85 to 90% of bainitic ferrite, the strength is more than 800 MPa, and the impact toughness is also high. However, when the area fraction of bainitic ferrite exceeds 90%, the tensile strength is low, and when the bainitic ferrite is less than 85%, the strength is high, but the impact toughness is low.

Example 3

In Table 6 below, the remaining conditions in the manufacturing method of Example 1 using the various billets having the composition of the present invention are the same, the size of the bainitic ferrite and the presence or absence of the M / A phase of the wire rod obtained only by hot rolling temperature and The main mechanical properties are shown.

Hot rolling temperature Size of bainitic ferrite after hot rolling Presence of M / A Tensile Strength After Hot Rolling (MPa) Elongation after Hot Rolling (%) U ~ notch impact toughness after hot rolling (J) A _e3 ~ 25 ℃ No bainitic ferrite X 450 43 298 A _e3 No bainitic ferrite X 465 45 265 A _e3 + 25 ℃ Fine ferrite O 475 50 275 A _e3 + 50 ℃ ~ A _e3 + 150 ℃ About 200㎛ O 595 62 320 A _e3 + 175 ℃ About 500㎛ X 502 52 198 A _e3 + 200 ℃ About 1000㎛ X 519 49 186 A _e3 + 225 ° C No bainitic ferrite X 511 35 178

As can be seen from the results of Table 6, various microstructures are formed according to hot rolling conditions, and the microstructure and mechanical properties of the present invention can be obtained only in the temperature range limited by the present invention. When the hot rolling temperature is higher than A _e3 + 150 ° C., austenitic coarsening occurs and the impact toughness is remarkably reduced. M / A phase is not obtained under conditions below A _e3 + 50 ° C., and bainitic ferrite structures are also obtained. It can be seen that it is not supported.

Example 4

In Table 7 below, the remaining conditions of the manufacturing method of Example 1 using the various billets having the composition of the present invention are the same, and the size of the bainitic ferrite and the presence / absence of M / A phases of the wire rod obtained by different rapid heating temperatures are shown. Indicated.

Rapid heating temperature range Size of bainitic ferrite after hot rolling Presence of M / A Remarks 150 ± 20 ℃ ~ 250 ± 20 ℃ About 200㎛ X Not created on M / A 250 ± 20 ℃ ~ 350 ± 20 ℃ About 200㎛ X Not created on M / A 350 ± 20 ℃ ~ 450 ± 20 ℃ About 200㎛ O Proper M / A Phase Fraction 450 ± 20 ℃ ~ 550 ± 20 ℃ About 200㎛ O Proper M / A Phase Fraction 550 ± 20 ℃ ~ 650 ± 20 ℃ it does not exist O M / A exists but mechanical properties are poor due to high fraction

As can be seen from the results of Table 7, the grain size of the bainitic ferrite tissue becomes about 200 µm in the temperature range of 350 ± 20 ℃ to 550 ± 20 ℃ of the present invention, it can be seen that the M / A phase is formed. have.

1 shows the microstructure of the present invention.

Figure 2 shows the pattern of billet heating, hot rolling and cooling to obtain the microstructure of the present invention.

Claims (4)

By weight%, C: 0.05-0.20%, Si: 0.01-0.1%, Mn: 1.0-2.0%, Nb: 0.01-0.05%, Mo: 0.01-0.1%, N: 0.003-0.03%, P: 0.035% or less (Not including 0%), S: 0.040% or less (not including 0%), B: 0.001% to 0.004% and Ti: 0.01% to 0.1% selected from the group consisting of Bainitic ferrite with 85 to 90% area fraction and M / A with 10-15% area fraction (mixed martensite and residual austenite) A high toughness heat treatment omission wire rod for cold rolling having a phase structure. 2. The high toughness heat treatment-free wire for cold rolling as claimed in claim 1, wherein the bainitic ferrite has an average grain size of 200 mu m or less. The high toughness heat treatment-free wire for cold rolling as claimed in claim 1, wherein the tensile strength of the wire is 800 MPa or more and the impact toughness is 150 J or more. delete

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