ULTRA-CLEAN RARE EARTH STEEL AND OCCLUDED FOREIGN SUBSTANCE MODIFICATION CONTROL METHOD

Abstract

A control process of inclusions in ultra-clean rare earth steel, wherein the content of rare earth elements REM in the ultra-clean rare earth steel, the total oxygen content T[O]m, the total sulfur content T[S]m in the steel, and the total oxygen content T[O]r in a rare earth metal or alloy added to the steel are controlled to satisfy the following formula: −500<REM−(m*T[O]m+n*T[O]r+k*T[S]m)<−30, where REM is the content of rare earth elements in the steel, in ppm; T[O]m is the total oxygen content in the steel, in ppm; T[O]r is the total oxygen content in a rare earth metal or alloy added to the steel, in ppm; T[S]m is the total sulfur content in the steel, in ppm; m is a first correction coefficient, with a value of 2-4.5;n is a second correction coefficient; and k is a third correction coefficient.

Claims

1. A control process of inclusions in ultra-clean rare earth steel, wherein the content of rare earth elements REM in the ultra-clean rare earth steel, the total oxygen content T[O]m, the total sulfur content T[S]m in the steel, and the total oxygen content T[O]r in a rare earth metal or alloy added to the steel are controlled to satisfy the following formula:
−500<REM−(m*T[O]m+n*T[O]r+k*T[S]m)<−30, where REM is the content of rare earth elements in the steel, in ppm; T[O]m is a total oxygen content in the steel, in ppm; T[O]r is a total oxygen content in a rare earth metal or alloy added to the steel, in ppm; T[S]m is a total sulfur content in the steel, in ppm; m is the first correction coefficient, with a value of 2-4.5; n is the second correction coefficient, with a value of 0.5-2.5; k is the third correction coefficient, with a value of 0.5-2.5; and the process comprises: a) guaranteeing white slag time to be 20 min or more, stabling slag alkalinity to be greater than 5, a total sulfur content T[S]m to be 90 ppm or less, and a total oxygen content T[O]m to be 25 ppm or less during LF refining; b) adding a high-purity rare earth metal or alloy before the LF refining departure or after at least 3 min of RH vacuum treatment, wherein a total oxygen content T[O]r in the high-purity rare earth metal or alloy is 50-200 ppm; c) after adding the high-purity rare earth metal or alloy, making RH or VD deep vacuum circulation time satisfy the following formula: T=(0.1-2.0)C.sub.RE+T.sub.0, where C.sub.RE is a content (in ppm) of rare earth elements in the steel, and T.sub.0 is a correction constant, with a value of 3-10 min; and making Ar gas soft blowing time satisfy the following formula: t=(0.05-3.0)C.sub.RE+t.sub.0, where C.sub.RE is a content (in ppm) of rare earth elements in the steel, and to is a correction constant, with a value of 5-10 min; and d) strengthening gas tightness between a ladle, a tundish and a crystallizer and thickness of a covering agent on a liquid surface of the tundish in the continuous casting, strengthening argon purging on the liquid surface of the tundish and controlling a N addition in the whole continuous casting to be within 8 ppm, wherein compared with a steel having the same composition but without rare earth elements, a superheat of casting is increased by 5-15° C.

2. The process according to claim 1, wherein the obtained steel contains 10-200 ppm of rare earth elements, wherein 50% or more of total number of inclusions in the steel are RE-oxygen-sulfides (RE.sub.2O.sub.2S) with a mean equivalent diameter D.sub.mean of 1-5 μm in a spherical shape or a spheroidal shape or a granular shape, and in dispersed distribution.

3. The process according to claim 2, wherein the content of RE-oxygen-sulfides (RE.sub.2O.sub.2S) accounts for 80% or more of total number of inclusions in the steel.

4. The process according to claim 2, wherein at least 90% of Al.sub.2O.sub.3 inclusions in the steel are modified into RE-oxygen-sulfides (RE.sub.2O.sub.2S).

5. The process according to claim 2, wherein at least 95% of Al.sub.2O.sub.3 inclusions in the steel are modified into RE-oxygen-sulfides (RE.sub.2O.sub.2S).

6. The process according to claim 1, wherein the steel contains 10-100 ppm of rare earth elements.

7. The process according to claim 1, wherein the steel contains 10-50 ppm of rare earth elements.

8. The process according to claim 1, wherein the mean equivalent diameter D.sub.mean of RE-oxygen-sulfide (RE.sub.2O.sub.2S) in the steel is 1-2 μm.

9. The process according to claim 1, wherein the steel is high-level bearing steel, gear steel, mold steel, stainless steel, steel for nuclear power, steel for automobile or ultra-high-strength steel.

10. The process according to claim 1, wherein the total number of inclusions in the steel comprise 50% or more of rare earth-oxygen-sulfides (RE.sub.2O.sub.2S), 50% or less of rare earth-sulfides, and 0-10% of Al.sub.2O.sub.3 inclusions.

11. The process according to claim 10, wherein the total number of inclusions in the steel comprise 85% or more of rare earth-oxygen-sulfides (RE.sub.2O.sub.2S), 10% or less of rare earth-sulfides, and 5% or less of Al.sub.2O.sub.3 inclusions.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0069] FIG. 1: effect of rare earth addition on tension-compression fatigue and rolling contact fatigue properties of bearing steel GCr15, wherein [0070] (a) shows the tension-compression fatigue life at a maximum stress load of ±800 MPa (2 Khz); and [0071] (b) shows the rolling contact fatigue life of the bearing steel at a load Fa of 8.82 KN and a rotating speed of 2000 r/min;

[0072] FIG. 2: effects of the rare earth on the modification of the inclusions, including mechanical properties of the inclusions, morphology and distribution of the inclusions, and morphology and distribution of the inclusions after fatigue failure, wherein [0073] (a) shows a comparison of statistics of the number of inclusions in the conventional GCr15 and RE-GCr15 steel ingots of the present application; [0074] (b) shows a comparison of nano-indentation test of rare earth-oxygen-sulfide and Al.sub.2O.sub.3; [0075] (c) shows SEM topography of the inclusions in the conventional GCr15 clean steel (without REM); [0076] (d) shows SEM topography of rare earth-oxygen-sulfide in the RE-GCr15 clean steel of the present application; [0077] (e) shows topography and diffraction pattern of the rare earth-oxygen-sulfide in the RE-GCr15 clean steel of the present application under TEM; and [0078] (f)-(g) show the inclusions and dislocation blocks around them after fatigue failure of the RE-GCr15 clean steel of the present application.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0079] The present application is further described below in combination with specific embodiments, but the scope of protection of the present application is not limited thereto.

Example 1

[0080] The present example provides a method of modifying inclusions in RE-GCr15 bearing steel, wherein a process flow is electric arc furnace smelting.fwdarw.LF refining.fwdarw.RH refining.fwdarw.continuous casting.fwdarw.heating.fwdarw.rolling, and includes the following steps: [0081] 1) electric arc furnace smelting; [0082] 2) LF refining: reasonably adjusting a refining slag system, stabilizing slag alkalinity to be greater than 5, ensuring white slag time to be 20 min or more, and controlling molten steel to have T[O]m of 10 ppm or less and content T[S]m to be not higher than 0.005%; [0083] 3) RH refining: adding a high-purity rare earth metal to an overhead bin after at least 5 min of RH vacuum treatment, with an addition amount of the high-purity rare earth satisfying the following formula:

W.sub.RE≥α×T[O]m+T[S]m,

where α is a correction coefficient, with a value of 6-30, preferably 8-20, T[O]m is a total oxygen content (ppm) in the steel, and T[S]m is a total sulfur content (ppm) in the steel; [0084] controlling T[O]r of the high-purity rare earth metal to be 60-200 ppm, ensuring RH deep vacuum circulation time to be 10 min or more after adding the high-purity rare earth metal, ensuring Ar gas soft blowing time to be 10 min or more, so that formed rare earth-oxygen-sulfide floats up, thereby reducing the number of inclusions, and controlling Al element content at the end of RH refining to be 0.015-0.030%, and rare earth element content in the molten steel to be 15-30 ppm; [0085] 4) strengthening gas tightness between a ladle, a tundish and a crystallizer and thickness of a covering agent on a liquid surface of the tundish in continuous casting, strengthening argon purging on the liquid surface of the tundish, to avoid gas suction during the continuous casting, controlling an N addition in the whole continuous casting to be within 8 ppm, to inhibit formation of TiN inclusions, and ensuring the purity of steel; controlling a superheat of casting to be 25-40° C., wherein the superheat controlled is increased by 5-20° C. than that of the conventional superheat control, with the purpose of preventing flocculation; controlling an MgO content of an working lining of the tundish to be more than 85%, and an SiO.sub.2 contents in a ladle long nozzle, a tundish stopper rod and a submerged nozzle to be less than 5%, to ensure compactness and corrosion resistance of the tundish and also erosion resistance and corrosion resistance of the three main components; and performing constant casting speed in the continuous casting; and [0086] 5) a conventional rolling process.

[0087] A plurality of samples were extracted from rolled products obtained in the present example, and the inclusions in the modified GCr15 steel were analyzed. Results show that: compared with the high-purity GCr15 steel without addition of rare earth, the modification of inclusions by adding the high-purity rare earth enables the RE-GCr15 steel to generate unprecedented excellent fatigue property, as shown in FIG. 1a, the addition of rare earth elements changes the law of the fatigue life, in the cyclic load tension/compression experiments of maximum stress of ±800 MPa and 20 kHz, the tension-compression fatigue life of the RE-GCr15 steel is improved to 4.1*10.sup.8 times and is more than 40 times that of the high-purity GCr15 steel (the tension-compression fatigue life reported on the literature is about 10 * 10.sup.6 times), and the addition of rare earth reduces the number of inclusions by 50% or more [FIG. 2(a)] and reduces the inclusions of 5 μm or more by at least 35%. In addition, as another important index of the bearing steel, the rolling contact fatigue life of the RE-GCr15 steel in FIG. 1b is also greatly improved. Under the conditions of an axial load Fa of 8.82 KN and a rotating speed of 2000 r/min, the rolling contact fatigue life of the RE-GCr15 steel is 3.08×10.sup.7, which is 910 ten thousand times higher than that of the high-purity GCr15 steel.

[0088] Conventional hard brittle Al.sub.2O.sub.3 oxides and strip-shaped MnS inclusions (>100 μm) are quite common in the high-purity GCr15 steel [FIG. 2(c)], whereas for rare earth-modified GCr15 steel, these conventional inclusions disappear dramatically, and are replaced by small-sized, spherical, dispersed Re-oxygen-sulfide and RE-sulfide with high typicality and regularity [FIG. 2(d)]. Further TEM observation shows that most of these rare earth-oxygen-sulfide inclusions are RE.sub.2O.sub.2S with a gentle boundary with the Fe matrix [FIG. 2(e)].

[0089] The RE.sub.2O.sub.2S inclusions have much lower elasticity, Young's modulus, shear modulus and hardness than the conventional Al.sub.2O.sub.3 inclusions, and these results are also confirmed by the current nano-indentation experiment measurements [FIG. 2(b)]. As the RE.sub.2O.sub.2S inclusions have better compatibility with the Fe matrix than the conventional hard Al.sub.2O.sub.3 inclusions, the non-uniform degree of internal micro-stress and strain concentration will be far lower than that of the conventional steel. As shown by the results of EDS and/or selected area diffraction patterns shown in f of FIG. 2, the composite inclusions consist of RE-O—S inclusions (≥85%) and/or O—Al—S-RE inclusions, rare earth-sulfide (≥10%), and a very small amount (≤5%) of Al.sub.2O.sub.3 inclusions (FIG. 2(f)), and after the tension-compression loading circulation, many dislocations appear inside the rare earth-oxygen-sulfide inclusions (FIG. 2(g), but laths in the matrix in the vicinity of the rare earth-oxygen-sulfide and the rare earth-sulfide are still intact, and boundaries between the laths are still clear; in contrast, Al.sub.2O.sub.3 particles almost have no dislocation inside, the laths crack, and the boundaries between them disappear. This comparison indicates that the rare earth-oxygen-sulfide has lower hardness than the Al.sub.2O.sub.3 inclusions and better plastic deformability, resulting in lower micro-stress/strain concentration at the boundaries, further reducing the cracking probability caused by strain concentration.

Example 2

[0090] The present example provides a method of modifying Al.sub.2O.sub.3 inclusions in IF steel, wherein a process flow is: molten iron reladling station—molten iron pretreatment—converter smelting—RH refining—continuous casting—hot rolling—acid pickling—cold rolling—annealing, and includes the following steps: [0091] 1) converter smelting: [0092] in the converter procedure, modifying ladle top slag, meanwhile without pre-deoxygenation and alloying of manganese in the converter procedure and an RH decarburization process, strictly controlling an oxygen content of molten steel to be 25 ppm or less in tundish, so as to improve the cleanness of IF steel; and strictly controlling a tapping temperature, a ladle temperature and a slag amount; [0093] 2) RH refining: [0094] modifying the ladle top slag in an RH refining process, and controlling an S content of the molten steel to be 0.003% or less when entering the RH refining; controlling the oxygen content before entering the RH refining and also before adding the high pure rare earth but after deoxygenation and alloying, wherein the total oxygen content T[O]m was not more than 20 ppm, and T[S]m was not more than 30 ppm in the molten steel before adding the high-purity rare earth; after vacuum decarburization, deoxygenation and alloying, and after at least 2 min of RH deep vacuum, adding the high-purity rare earth with a total oxygen content of 60-100 ppm to an overhead bin, after adding the high-purity rare earth, making RH deep vacuum bottom argon blowing time not less than 10 min, and negative pressure soft blowing time not less than 15 min after the vacuum being broken; [0095] 3) technical requirements in the continuous casting step: [0096] modifying the tundish top slag, and ensuring the tightness between the ladle, the tundish and the crystallizer, to avoid gas suction in the continuous casting process, and controlling the N addition in the whole continuous casting to be less than 8 ppm, wherein the superheat of casting was controlled to be increased by 5-15° C. compared with the conventional superheat, preventing the risk of flocculation; and controlling a constant casting speed in the continuous casting; and [0097] 4) conventional rolling and heat treatment processes.

[0098] A plurality of samples were extracted from annealed products obtained in the present example, and the modified IF steel was analyzed in detail in terms of components, gas content, morphology and size distribution of inclusions, and so on:

TABLE-US-00001 TABLE 1 Components and Contents of IF Steel Serial No. C Si Mn P T[S]m Al Ti T[O]m REM Comparative 0.001- 0.005- 0.01- ≤0.01 ≤0.003 0.01- 0.01- ≤0.0015 — Example 1 0.004 0.05 0.25 0.05 0.09 Example 2-1 0.001- 0.005- 0.01- ≤0.01 ≤0.003 0.01- 0.01- ≤0.0015 15- 0.004 0.05 0.25 0.05 0.09 20 Note: except that RE is in ppm, all of other elements are in wt %, and the balance is Fe and inevitable impurity elements; the components and the preparation and control process of Comparative Example 1 are the same as those of Example 2-1, but without REM.

TABLE-US-00002 TABLE 2 Typical Size and Number Distribution of Inclusions in IF Steel 1-2 μm 2-5 μm 5-10 μm Number number number number Total of and and and Serial No. number fields proportion proportion proportion Comparative 150 20 130 19 1 Example 1 (86.67%) (12.67%) (0.66%) Example 2-1 225 22 213 11 1 (94.67%) (4.89%) (0.44%)

TABLE-US-00003 TABLE 3 Typical Size and Number Distribution of Inclusions in IF Steel (continued Table 2) Dmax/ Dmin/ Dmean/ Dmax/ Area Serial No. μm μm μm Dmin proportion Comparative 1.890 1.027 1.464 1.841 0.146 Example 1 Example 2-1 1.817 1.044 1.431 1.741 0.139

[0099] In the present example, an appropriate amount of high-purity rare earth metal is added to the IF steel, then on the one hand, the number of fine inclusions of 1-2 μm level in the steel can be significantly increased by 8% (namely, from 86.67% to 94.67%), the number and proportion of fine inclusions of 5-10 μm level can be obviously decreased, the maximum diameter (1.464 μm.fwdarw.1.431 μm) of the inclusions can be slightly decreased, and compared with the IF steel without addition of rare earth, the number of inclusions (area proportion 0.146.fwdarw.0.139) is obviously decreased; on the other hand, adding an appropriate amount of RE to the IF steel can achieve the purpose of obviously modifying the inclusions, and in conjunction with SEM+EDS analysis, it is found that RE can modify large-size rod-like/clustered Al.sub.2O.sub.3 inclusions into O—Al—S-RE/RE-O—S compounds in a spheroidal shape, with finer size and in dispersed distribution; meanwhile, TiN and MnS inclusions lose the Al.sub.2O.sub.3 nucleation matrix, thus it is difficult for the nucleation to grow large, thereby reducing the cleavage effect and anisotropy of such inclusions on the matrix.

[0100] The distribution of the inclusions in the steel of Example 2-1 are characterized in that, in 22 fields, the total number of inclusions is less than 250, wherein the proportion of the inclusions with an equivalent diameter of 1-2 μm is 94.5% or greater, the proportion of the inclusions with an equivalent diameter of 2-5 μm is less than 5%, and the proportion of the inclusions with an equivalent diameter of 5-10 μm is less than 0.5%.

[0101] In conjunction with the testing results of tension test of JIS-5 sheet standard samples, it is confirmed that compared with the conventional IF steel, the RE-IF steel has the r value significantly increased by at least 25% (1.820.fwdarw.2.267), and meanwhile obviously improved the elongation and the product of strength and ductility without substantially changing the strength thereof.

TABLE-US-00004 TABLE 4 Typical Property Indexes of IF Steel Product of strength and Serial No. Rp.sub.0.2/Mpa Rm/Mpa Rp.sub.0.2/Rm A50/% r.sub.90/15 n.sub.10-20 ductility/MPa .Math. % Comparative 115.63 312.08 0.37 44.72 1.820 0.261 13957 Example 1 Example 2-1 99.28 294.67 0.34 50.36 2.267 0.269 14839

[0102] Example 3

[0103] The present example provides a method of modifying inclusions in ultra-high-strength F grade marine steel, wherein a process flow is: molten iron pretreatment—converter smelting LF refining—RH refining—continuous casting—rolling—quenching and tempering, and a control process is as follows: [0104] 1) smelting and rare earth addition parts: before adding the rare earth, ensuring white slag time in the LF refining to be 20 min or more, the molten steel to have a total oxygen content T[O]m of not higher than 10 ppm, and a content T[S]m not higher than 0.003%; adding the high-purity rare earth metal before the LF departure or after 3 min of RH net circulation, wherein the rare earth was added in a form of being cladded by a steel pipe of the same material or wrapped by an aluminum foil to the molten steel, for the purpose of avoiding oxidation or contacting with the steel slag during the addition of rare earth metal, the total oxygen content in the rare earth metal was 80-100 ppm, wherein the addition amount of rare earth in Example 3-2 was 2 times that in Example 3-1, and the rare earth of Example 3-2 could be added in two times; [0105] 2) after adding the rare earth, ensuring the RH deep vacuum time to be 15 min or more, after conventional Ca treatment at an RH negative pressure, ensuring the Ar gas soft blowing time to be 15 min or more; [0106] 3) in the continuous casting process, ensuring gas tightness between a ladle, a tundish and a crystallize, to avoid gas suction in the continuous casting process, and controlling the N addition in the whole continuous casting to be less than 5 ppm; and controlling the superheat of casting and the constant casting speed in the continuous casting, wherein the superheat was controlled to be increased by 5-15° C. compared with the conventional superheat; and [0107] 4) conventional rolling and tempering processes.

[0108] Through the above process control, a plurality of samples were extracted from tempered products obtained in the present example, and the modified ultra-high-strength steel was analyzed in detail in terms of component, gas content, morphology and size distribution of inclusions and so on:

TABLE-US-00005 TABLE 5 Components and Contents of Ultra-high-strength Steel Serial No. C Si Mn P T[S]m Nb Ni Al B T[O]m REM Comparative 0.05- 0.30- 1.2- ≤0.01 ≤0.003 0.10- 0.3- 0.01- 0.001- ≤0.0010 — Example 2 0.12 0.60 1.8 0.20 0.8 0.04 0.005 Example 3-1 0.05- 0.30- 1.2- ≤0.01 ≤0.003 0.10- 0.3- 0.01- 0.001- ≤0.0010 15- 0.12 0.60 1.8 0.20 0.8 0.04 0.005 20 Example 3-2 0.05- 0.30- 1.2- ≤0.01 ≤0.003 0.10- 0.3- 0.01- 0.001- ≤0.0010 30- 0.12 0.60 1.8 0.20 0.8 0.04 0.005 40

[0109] Note: except that RE is in ppm, all of other elements are in wt %, and the balance is Fe and inevitable impurity elements; the components and the preparation and control process of Comparative Example 2 are the same as those of Example 3-1 and Example 3-2, but without REM.

TABLE-US-00006 TABLE 6 Typical Size Distribution of Inclusions in Ultra-high-strength Steel 1-2 μm 2-5 μm 5-10 μm >10 μm Total Number number and number and number and number and Serial No. number of fields proportion proportion proportion proportion Comparative 529 20 34 358 122 15 Example 2 (6.43%) (67.67%) (23.06%) (2.84%) Example 3-1 400 20 47 248 86 19 (11.75%) (62.00%) (21.5%) (4.75%) Example 3-2 456 20 48 286 102 20 (10.53%) (62.72%) (22.37%) (4.39%)

TABLE-US-00007 TABLE 7 Typical Size Distribution of Inclusions in Ultra- high-strength Steel (continued Table 6) Dmax/ Dmin/ Dmean/ Dmax/ Area Serial No. μm μm μm Dmin proportion/‰ Comparative 34.40 1.78 4.37 19.33 0.45 Example 2 Example 3-1 31.46 1.78 4.02 17.67 0.36 Example 3-2 19.53 1.78 4.21 10.97 0.37

[0110] Study results indicate that as the addition amount of RE increases, the maximum diameter Dmax of the inclusions gradually decreases (34.fwdarw.31.fwdarw.19), and that the number of inclusions with a diameter of less than 2 μm increases by at least 4%, the total number of inclusions decreases by a mean of 18% (0.45% c->0.37% o); after the addition of RE, the mean equivalent diameter D.sub.mean of the inclusions is reduced by 8% (4.37-4.02), the maximum/minimum inclusion diameter is obviously reduced, and the area proportion of the inclusions is also reduced to different degrees.

[0111] Typical distribution of the inclusions in the steel of Example 3-1 and Example 3-2 is as follows: in 20 fields, the total number of inclusions is less than 500, wherein the proportion of the inclusions with an equivalent diameter of 1-2 μm is greater than 10.5%, the proportion of the inclusions with an equivalent diameter of 2-5 μm is 60-80%, the proportion of the inclusions with an equivalent diameter of 5-10 μm is less than 22.5%, and the proportion of the inclusions with an equivalent diameter of less than 10 μm is less than 5%.

[0112] In conjunction with the SEM+EDS analysis, there were large-sized Al.sub.2O.sub.3 cluster inclusions in the field of samples without adding RE, in which the large-size inclusions were comminuted, accompanied by strip-shaped MnS inclusions, while the inclusions in the samples of Example 3-1 and Example 3-2 with REM were mostly spherical or granular RE-O—S compounds, with a smaller size and in dispersed distribution.

TABLE-US-00008 TABLE 8 Typical Microstructure Comparison of Ultra-high-strength Steel Structure at Structure at position of position of ¼ plate ½ plate Category Surface structure thickness thickness Comparative a small amount of presence of P + F Example 2 bainite and ferrite microsegregation biphase Example 3-1 bainite slight P + F microsegregation biphase Example 3-2 increased content of Slight P + F bainite-like structure, microsegregation biphase and better structure uniformity from surface to center

TABLE-US-00009 TABLE 9 Typical Low Temperature Transverse and Longitudinal Impact Properties of Ultra-high-strength Steel Transverse impact energy/J Longitudinal impact energy/J Category 0° C. −20° C. −40° C. 0° C. −20° C. −40° C. Comparative 35 17 11 32 15 9 Example 2 Example 3-1 65 30 18 93 52 18 Example 3-2 80 45 30 102 74 45

[0113] Note: All the samples in Table 9 were taken at positions of ½ plate thickness.

[0114] The above analysis results indicate that, in the range of 0° C. to −40° C., compared with the F grade ultra-high-strength marine steel without addition of RE, the modification effect of the addition of an appropriate amount of high-purity rare earth metal on the inclusions can allow the low-temperature transverse and longitudinal impact energies of the F grade ultra-high-strength marine steel to be fully improved: at 0° C., the transverse impact energy is increased by at least 30 J, and the transverse impact energy is increased by at least 60 J; at −20° C., the transverse impact energy is increased by at least 13 J, and the longitudinal impact energy is increased by at least 35 J; at −40° C., the transverse impact energy is increased by at least 5 J, and the longitudinal impact energy is increased by at least 9 J; in particular, the improvement effect at the positions of ½ plate thickness is especially remarkable.

[0115] The examples above are merely preferred embodiments of the present application, but should not be construed as limitation on the scope of protection of the present application. It should be indicated that a person ordinarily skilled in the art still could make several modifications, substitutions and improvements without departing from the concept of the present application, all of which fall within the scope of protection of the present application.