Rare-earth microalloyed steel and control method

Abstract

Provided in the present application are a rare-earth microalloyed steel and a control process. The steel has a special microstructure, and the microstructure comprises a rare earth-rich nanocluster having a diameter of 1-50 nm. The nanocluster has the same crystal structure type as a matrix. The rare earth-rich nanocluster inhibits the segregation of the elements S, P and As on a grain boundary, and obviously improves the fatigue life of the steel. In addition, a rare-earth solid solution also directly affects a phase change dynamics process so that the diffusion-type phase change starting temperature in the steel changes at least to 2° C., and even changes to 40-60° C. in some kinds of steel, thereby greatly improving the mechanical properties thereof, and providing a foundation for the development of more kinds of high-performance steel.

Claims

1-20. (canceled)

21. A rare-earth microalloyed steel, wherein the steel has a microstructure comprising rare earth-rich nanoclusters with diameters of 1-50 nm, the rare earth-rich nanoclusters having the same crystal structure type as the matrix are nano-scale particle groups formed by the aggregation of several to hundreds of rare earth atoms, and the diameters of the rare earth-rich nanoclusters are directly proportional to the residual amount T.sub.RE of rare earth elements in the steel, but inversely proportional to the total oxygen content in the steel; essentials for controlling the preparation of the rare-earth microalloyed steel comprise: controlling a total oxygen content T.sub.[O]m of molten steel to be within 50 ppm, T.sub.[S]≤50 ppm; controlling a total oxygen content of the rare earth metal added in the molten steel is less than 60 ppm; controlling the temperature of the molten steel to exceed its liquidus line T.sub.m+(20-100°) C. when adding the rare earth metal; and controlling the RH or VD deep vacuum cycle time after the addition of the high-purity rare earth metal to be more than 10 min and the Ar gas soft blowing time to be more than 15 min.

22. The rare-earth microalloyed steel according to claim 21, wherein the vacancies in the Fe matrix form rare earth-vacancy pairs with a number of rare earth atoms, so that a number of rare earth atoms around the vacancies are regularly arranged, thereby forming a microstructure of rare earth-rich nanoclusters, and the presence of a single Fe vacancy helps stabilize local rare earth-rich nanoclusters consisting of up to 14 rare earth atoms.

23. The rare-earth microalloyed steel according to claim 21, wherein the rare earth-rich nanoclusters have diameters of 2-50 nm.

24. The rare-earth microalloyed steel according to claim 21, wherein the residual amount T.sub.RE of rare earth elements in the microalloyed steel is 30-1000 ppm.

25. The rare-earth microalloyed steel according to claim 21, wherein the residual amount T.sub.RE of rare earth elements in the microalloyed steel is 30-600 ppm.

26. The rare-earth microalloyed steel according to claim 21, wherein the residual amount T.sub.RE of rare earth elements in the microalloyed steel is 50-500 ppm.

27. The rare-earth microalloyed steel according to claim 21, wherein the change in the initial temperature of the diffusion-type phase transition of the rare-earth microalloyed steel satisfies the following table: TABLE-US-00009 Change in initial temperature of diffusion-type phase Types transition/° C. Plain carbon steel At least 2° C. Low alloy steel with an alloy At least 5° C. content of not more than 10 wt % Medium-high alloy steel with an At least 10° C. alloy content of more than 10 wt %

28. The rare-earth microalloyed steel according to claim 27, wherein the change in the initial temperature of the diffusion-type phase transition of the rare-earth microalloyed steel satisfies the following table: TABLE-US-00010 Change in initial temperature of diffusion-type phase Types transition/° C. Plain carbon steel 10-50° C. Low alloy steel with an alloy 20-60° C. content of not more than 10 wt % Medium-high alloy steel with an 25-60° C. alloy content of more than 10 wt %

29. The rare-earth microalloyed steel according to claim 27, wherein the initial temperature of ferrite phase transition in rare-earth microalloyed plain carbon steel decreases by 20-50° C.; and the initial temperature of bainite transformation in rare-earth microalloyed low alloy steel decreases by 30-60° C.

30. The rare-earth microalloyed steel according to claim 27, wherein the number and diameters of the rare earth-rich nanoclusters in the rare-earth microalloyed steel are directly proportional to the change of the initial temperature of the phase transition.

31. A process for controlling the rare-earth microalloyed steel according to claim 21, comprising the steps of: (1) controlling a total oxygen content T.sub.[O]m of molten steel to be within 50 ppm, and T.sub.[S]≤50 ppm; (2) adding a rare earth metal with a total oxygen content of less than 60 ppm into the molten steel, wherein the addition amount of the rare earth metal satisfies W.sub.RE>α×T.sub.[O]+T.sub.[S], and the value of α is 6-30; T.sub.[O]m is the total oxygen content in the steel, and T.sub.[S] is the total sulphur content in the steel; controlling the temperature of the molten steel to exceed its liquidus line T.sub.m+(20-100°) C. when adding the rare earth metal; controlling the RH or VD deep vacuum cycle time after the addition of the high-purity rare earth metal to be more than 10 min and the Ar gas soft blowing time to be more than 15 min; and (3) protecting the molten steel containing the rare earth metal from air to control the residual amount T.sub.RE of the rare earth metal in the liquid steel to be 30-1000 ppm.

32. The process for controlling the rare-earth microalloyed steel according to claim 31, wherein the total oxygen content T.sub.[O]m in step (1) to be within 25 ppm.

33. The process for controlling the rare-earth microalloyed steel according to claim 31, wherein the value of α is 8-20 in step (2); the rare earth metal is added in one time or step by step in two or more times, wherein the time interval between the two steps of rare earth addition is not less than 1 minute and not more than 10 minutes.

34. The rare-earth microalloyed steel according to claim 22, wherein the rare earth-rich nanoclusters have diameters of 2-50 nm.

35. The rare-earth microalloyed steel according to claim 22, wherein the residual amount T.sub.RE of rare earth elements in the microalloyed steel is 30-1000 ppm.

36. The rare-earth microalloyed steel according to claim 22, wherein the residual amount T.sub.RE of rare earth elements in the microalloyed steel is 30-600 ppm.

37. The rare-earth microalloyed steel according to claim 22, wherein the residual amount T.sub.RE of rare earth elements in the microalloyed steel is 50-500 ppm.

38. The rare-earth microalloyed steel according to claim 22, wherein the change in the initial temperature of the diffusion-type phase transition of the rare-earth microalloyed steel satisfies the following table: TABLE-US-00011 Change in initial temperature of diffusion-type phase Types transition/° C. Plain carbon steel At least 2° C. Low alloy steel with an alloy At least 5° C. content of not more than 10 wt % Medium-high alloy steel with an At least 10° C. alloy content of more than 10 wt %

39. The process for controlling the rare-earth microalloyed steel according to claim 32, wherein the value of a is 8-20 in step (2); the rare earth metal is added in one time or step by step in two or more times, wherein the time interval between the two steps of rare earth addition is not less than 1 minute and not more than 10 minutes.

40. The process of claim 31, wherein the vacancies in the Fe matrix form rare earth-vacancy pairs with the number of rare earth atoms, so that the number of rare earth atoms around the vacancies are regularly arranged, thereby forming the microstructure of rare earth-rich nanoclusters, and the presence of a single Fe vacancy helps stabilize local rare earth-rich nanoclusters consisting of up to 14 rare earth atoms.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1(a) is a high resolution HAADF-STEM image of a phase in the RE microalloyed steel of Embodiment 1 in the present application;

[0021] FIG. 1(b) is a diffraction pattern of an area A in FIG. 1(a);

[0022] FIG. 1(c) is a diffraction pattern of an area B in FIG. 1(a);

[0023] FIG. 2 shows effects of solid solution rare earth elements on the initial temperature (Fs) of ferrite phase transition in the RE microalloyed steel of Embodiment 1 at a cooling rate of 2.5° C./s;

[0024] FIG. 3 shows a high resolution HAADF-STEM image of a phase in the RE microalloyed steel of Embodiment 2 herein;

[0025] FIG. 4 shows effects of solid solution rare earth elements on the initial temperature of the phase transition of granular bainite in the RE microalloyed steel of Embodiment 2 at a cooling rate of 2.5° C./s.

DETAILED DESCRIPTION OF THE INVENTION

[0026] The present application is described in further details below with reference to specific embodiments, but the scope of protection of the present application is not limited thereto.

Embodiment 1

[0027] A rare earth microalloying method for plain carbon steel has a production process route being vacuum induction melting (VIM).fwdarw.ingot casting.fwdarw.forging.fwdarw.rolling, including the following steps:

(1) Raw materials such as pure iron, Mn—Fe and Si—Fe are preferably selected, with the purity of the raw materials controlled, and the raw materials are smelted in a vacuum induction melting (VIM) furnace; the selection of raw materials ensures that the total oxygen content of the metal mother liquor after melting down is less than 25 ppm; the VIM process is performed by using 30% power*0.1-0.5 h, 50% power*0.2-0.5 h and 80% power, respectively; after the metal smelting in the crucible, the temperature is measured by a thermocouple; when the temperature is more than 1560° C., a high-purity rare earth mischmetal (mainly La—Ce alloy) is added into a vacuum chamber, wherein T[O]r in the rare earth alloy is less than 60 ppm, and the particle size of the rare earth mischmetal is 1-10 mm; when a rare earth mischmetal is added, the molten steel has a total oxygen content of T[O].sub.m≤25 ppm and a total sulfur content of T[S]≤50 ppm, and is further cast into a steel ingot; wherein, the rare earth mischmetal has an addition amount of W.sub.RE>α×T[O].sub.m+T[S].
(2) The above-mentioned steel ingot is forged into a rectangular bar with a cross section of 50 mm*80 mm, and then the bar is heated to 1170-1210° C. and rolled into a plate with a thickness of 3-8 mm.
(3) Their composition (shown in Table 2), structure and properties are sampled and tested.

TABLE-US-00002 TABLE 2 Composition of steels of Comparative Example 1 and Embodiment 1 Steel C Si Mn T[S] P Als T[O] H N T[La] T[Ce] Comparative 0.12- 0.1- 1.2- ≤0.005 ≤0.005 0.0015- ≤25 ppm ≤1.0 ppm 10-30 ppm — — Example 1 0.25 0.4 1.9 0.0085 Embodiment 1 0.12- 0.1- 1.2- ≤0.005 ≤0.005 0.0015- ≤25 ppm ≤1.0 ppm 10-30 ppm 0.012 0.024 0.25 0.4 1.9 0.0085 Note: in Table 1, all the components except O, H and N being in ppm by weight are in % by weight, and the balances are Fe and inevitable impurity elements. In Comparative Embodiment 1, no rare earth elements are added.

[0028] High-brightness rare earth-rich nanoclusters with radii of 2-4 nm are also observed experimentally by the characterization of high-resolution High Angle Annular Dark Field (HAADF) of a spherical aberration-corrected electron transmission microscope, as shown by a closed circle A in FIG. 1(e). As shown in FIG. 1(f), these nanoclusters are isostructural with bcc Fe (FIG. 1(g)), but there is significant lattice distortion to the Fe matrix. FIG. 2(a) shows that, at a cooling rate of 2.5° C./s, the initial temperature (Fs) of the ferrite phase transition of RE microalloyed steel decreases from 755° C. to 707° C. when the RE content is 360 ppm (i. e. the total amount of La and Ce rare earth elements) in the RE microalloyed steel, that is to say, the initial temperature decreases by 48° C., which greatly improves the hardening ability of the steel, thus affecting its mechanical properties. Analytically, it is concluded that the addition of RE elements not only results in higher diffusion energy barrier, but also affects the migration energy barrier of carbon atoms at the most adjacent gap position; and it also has a great effect on the migration energy barrier of carbon atoms at the second/third adjacent gap positions, thus significantly slowing down the diffusion of carbon atoms. At a cooling rate of 2.5° C./s, when the RE content is 360 ppm, the decrease in Fs is close to 48° C. [FIG. 2(a)], mainly because there is not enough time for carbon atoms to diffuse by the phase transition process at such a fast cooling rate; and the effect of RE elements on carbon diffusion is very significant, and such a low solubility of RE elements can effectively lead to a significant change in the temperature of Fs, eventually leading to important changes in the microstructure and mechanical properties.

Embodiment 2

[0029] A rare earth microalloying method for a low alloy steel has a production process route being LF smelting.fwdarw.VD refining.fwdarw.continuous casting, including the following steps.

(1) Al deoxidation at LF station+diffusion deoxidation. The slag alkalinity is controlled to be more than 4.5, and the white slag is kept for more than 30 min, so as to carry out deep deoxidation and desulphurization, making the total sulphur content not more than 15 ppm and the total oxygen content not more than 25 ppm, further achieving more solid solution after adding the rare earth elements.
(2) After LF refining and before VD treatment, a rare earth mischmetal is added into the ladle via the slag layer (T[O]r<60 ppm in the rare earth mischmetal); the addition amounts of the rare earth mischmetal in Embodiments 2A and 2B are 300 ppm and 680 ppm respectively; and the temperature of the molten steel before adding the rare earth mischmetal is controlled at 1550° C. or above.
(3) After rare earth addition, the VD deep vacuum time is not less than 15 min; and the soft blowing time after breaking VD vacuum is not less than 15 min.
(4) In the continuous casting process, the whole nitrogen increasing amount of the large ladle-tundish-crystallizer is controlled to be no more than 5 ppm so as to prevent rare earth burning caused by secondary oxidation;
(5) The continuous casting samples (shown in Table 3) are analyzed by its composition (shown in Table 3), structure and performances.

TABLE-US-00003 TABLE 3 Composition of steels of Comparative Example 2 and Embodiment 2 Steel C Si Mn Cr Mo V P T[S] T[RE] T[O] Comparative 0.10- 0.03- 0.45- 1.8- 0.6- 0.2- ≤0.008 ≤0.0015 — ≤25 Example 2 0.18 0.15 0.65 2.6 1.2 0.3 Embodiment 2A 0.10- 0.03- 0.45- 1.8- 0.6- 0.2- ≤0.008 ≤0.0015 0.020 ≤25 0.18 0.15 0.65 2.6 1.2 0.3 Embodiment 2B 0.10- 0.03- 0.45- 1.8- 0.6- 0.2- ≤0.008 ≤0.0015 0.048 ≤25 0.18 0.15 0.65 2.6 1.2 0.3 Note: all the components except O being in ppm by weight are in % by weight, and the balances are Fe and inevitable impurity elements. In Comparative Embodiment 2, no rare earth elements are added.

[0030] High-brightness rare earth-rich nanoclusters with sizes of 4-8 nm are also observed experimentally in the sample of Embodiment 2A (rare earth elements of 200 ppm) by the characterization of high-resolution High Angle Annular Dark Field (HAADF) of a spherical aberration-corrected electron transmission microscope, as shown in FIG. 3. The high-resolution images show that these nanoclusters are isostructural with the bcc matrix but have obvious lattice distortion to the Fe matrix.

[0031] FIG. 4 shows that, at a cooling rate of 2.5° C./s, with residual RE contents of 200 ppm and 480 ppm, the initial temperature of the phase transition of granular bainite in the RE microalloyed steel decreases from 573° C. to 536° C. and 543° C.; and the reduction of the initial temperature closes to 37° C. and 30° C., respectively, which will greatly improves the hardening ability of the steel, thus affecting its mechanical properties. The reason is that the addition of RE elements not only results in a higher diffusion energy barrier, but more importantly affects the migration energy barrier of carbon atoms at the most adjacent gap position; and it also has great effects on the migration energy barrier of carbon atoms at the second/third adjacent gap positions, thus significantly slowing down the diffusion of carbon atoms.

Embodiment 3

[0032] A rare earth microalloying method for a low-alloy steel has a production process route being LF smelting.fwdarw.RH refining.fwdarw.ingot casting.fwdarw.forging, including the following steps:

(1) The alloy composition is adjusted at the LF station. The slag alkalinity is controlled to be more than 5, and the white slag is kept for more than 40 min, so as to carry out deep deoxidation and desulphurization, making the oxygen and sulfur contents both less than 20 ppm.
(2) After LF refining, when the vacuum degree of RH treatment reaches 200 Pa or less, a rare earth mischmetal (T[O]r<60 ppm in the rare earth mischmetal) is directly added into the molten steel by the RH overhead storage bin; the addition amounts of the rare earth mischmetal in Embodiments 3A and 3B are 500 ppm and 1500 ppm respectively, wherein the rare earth mischmetal in Embodiment 3B is added in two times, with 1000 ppm added for the first time, and 500 ppm addition after 3 minutes, and the temperature of the molten steel is controlled to be above 1530° C. before adding the rare earth mischmetal; and after rare earth addion, the RH deep vacuum time is not less than 12 min, and the soft blowing time after breaking vacuum is not less than 15 min.
(3) The molten steel is poured into an ingot mold, cooled and solidified into an ingot.
(4) The ingot is forged to prepare a metal bar with diameters of 100-350 mm, and its composition (shown in Table 4), structure and properties are tested.

TABLE-US-00004 TABLE 4 Composition of steels of Comparative Example 3 and Embodiment 3 Steel C Si Mn Cr Mo V P T[S] T[RE] T[O] Comparative 0.25- 0.95- 0.3- 4.5- 1.2- 0.8- ≤0.02 ≤0.005 — ≤12 Example 3 0.60 1.1 0.45 5.5 1.6 1.1 Embodiment 3A 0.25- 0.95- 0.3- 4.5- 1.2- 0.8- ≤0.02 ≤0.005 0.042 ≤12 0.60 1.1 0.45 5.5 1.6 1.1 Embodiment 3B 0.25- 0.95- 0.3- 4.5- 1.2- 0.8- ≤0.02 ≤0.005 0.102 ≤50 0.60 1.1 0.45 5.5 1.6 1.1 Note: all the components except O being in ppm by weight are in % by weight in Table 4, and the balances are Fe and inevitable impurity elements. In Comparative Embodiment 3, no rare earth elements are added.

[0033] High brightness rare earth-rich nanoclusters ranging in sizes from 2 to 25 nm and from 25 to 50 nm are observed experimentally in samples of Embodiment 3A (residual amount of rare earth elements is 420 ppm) and Embodiment 3B (residual amount of rare earth elements is 1020 ppm), respectively, by the characterization of high-resolution High Angle Annular Dark Field (HAADF) of a spherical aberration-corrected electron transmission microscope. The high-resolution images show that these nanoclusters are isostructural with the bcc matrix but have obvious lattice distortion to the Fe matrix.

[0034] By performing phase transition point tests on the samples of Embodiments 3 and 3B described above, it is found that the diffusion-type phase transition temperature changes by 15° C. and 40° C., respectively.

Embodiment 4

[0035] A rare earth microalloying method for high-end bearing steel has a production process route being LF smelting.fwdarw.RH refining.fwdarw.continuous casting.fwdarw.rolling, including the following steps.

(1) The slag system is reasonably adjusted, and the slag alkalinity is more than 6; during the LF refining, it ensures white slag time more than 15 min, stable slag alkalinity not less than 5, the total oxygen content T[O] not more than 15 ppm and the total sulfur content T[S] less than 0.003% by using Al pre-deoxidation.
(2) In the RH refining, the components are not adjusted as much as possible, and all the component adjustments shall be completed at LF station; after RH vacuum treatment for 10 min, a high-purity rare earth mischmetal (T[O]r<60 ppm in the rare earth mischmetal) is added into the overhead storage bin, and the addition amount of the high-purity rare earth mischmetal satisfies WRE>α×T[O]+T[S], wherein a is a correction coefficient and the value is 6-30, preferably 8-20; T[O] is the total oxygen content in the steel, and T[S] is the total sulfur content in the steel; after the addition of the high-purity rare earth mischmetal, the RH deep vacuum cycle time is guaranteed to be more than 10 min, and the soft blowing time of Ar gas is guaranteed to be more than 20 min; the formed rare-earth oxysulfides/rare-earth sulfides is partially floated to reduce the number of inclusions; the superheat is controlled between 25° C. and 40° C., and the superheat control is increased by 5° C. to 10° C. compared with the conventional superheat control so as to prevent nozzle clogging; and the Al content at the end point of RH refining is controlled between 0.015% and 0.030%.
(3) High-purity rare earth mischmetal is added by selecting the subsequent furnace of the whole pouring, and the rare earth in Embodiments 4A, 4B, and 4C are added in amounts of 100 ppm, 500 ppm, and 1200 ppm, respectively, in which the rare earth of Embodiment 4C being added in two times, with 700 ppm added in the first time, and 500 ppm in the second time, at an interval of 4 minutes.
(4) The gas tightness between the big ladle-tundish-crystallizer and the thickness of the liquid surface covering agent of the tundish are strengthened in continuous casting; the argon purging of the tundish liquid surface is strengthened to avoid air suction in the continuous casting process; the amount of nitrogen increase is controlled within 5 ppm in the whole continuous casting process, inhibiting the formation of TiN inclusions and ensuring the purity of the steel; the content of MgO in the working layer of the tundish is controlled to be more than 85%; the SiO.sub.2 content of a ladle shroud, a tundish stopper and a submerged nozzle is less than 5%, so as to ensure the compactness and corrosion-resistance of the tundish and the anti-scouring and erosion resistance of the three-major-items; and continuous casting is performed at a constant casting speed, then rolled into a rectangular billet with a diameter of 320*480 mm.
(5) The rectangular continuous casting billet is heated to 1150-1250° C., passed through a continues rolling mill and rolled into bars with diameters of 90-210 mm; and it is sampled for composition testing (shown in Table 5).

TABLE-US-00005 TABLE 5 Composition of steels of Comparative Example 4 and Embodiment 4 Steel C Si Mn Cr P T[S] T[RE] T[O] Comparative 0.9-1.1 0.15-0.35 0.25-0.45 1.4-1.65 ≤0.01 ≤0.005 — ≤40 Example 4 Embodiment 4A 0.9-1.1 0.15-0.35 0.25-0.45 1.4-1.65 ≤0.01 ≤0.005 0.007 ≤40 Embodiment 4B 0.9-1.1 0.15-0.35 0.25-0.45 1.4-1.65 ≤0.01 ≤0.005 0.035 ≤40 Embodiment 4C 0.9-1.1 0.15-0.35 0.25-0.45 1.4-1.65 ≤0.01 ≤0.005 0.098 ≤40 Note: all the components except O being in ppm by weight are in % by weight in Table 5, and the balances are Fe and inevitable impurity elements. In Comparative Embodiment 4, no rare earth elements are added.

[0036] For analytical test of the rolled materials with the four components above, the size of the rare earth-rich nanoclusters and the change of the diffusion-type phase transition temperature are shown in Table 6. It can be seen that, with the increase of the residual rare earth elements T[RE] in the steel, the size of the rare earth-rich nanoclusters increases, the influence on the diffusion-type phase transition points increases, and the temperature of the phase transition correspondingly increases.

TABLE-US-00006 TABLE 6 Analytical Test Results Rare earth-rich Change in diffusion nanocluster diameter type phase transition Steel (nm) point (° C.) T[RE] Comparative — — — Example 4 Embodiment 4A 1-5 2 0.007 Embodiment 4B  5-20 25 0.035 Embodiment 4C 20-50 60 0.098

Embodiment 5

[0037] A rare earth microalloying method for high-quality stainless steel has a production process route being LF smelting.fwdarw.VD refining.fwdarw.ingot casting.fwdarw.forging, including the following steps.

(1) The alloy composition is adjusted at the LF station. The slag alkalinity is controlled to be more than 3, and the white slag is kept for more than 35 min, so as to carry out deep deoxidation and desulphurization, making the total oxygen content not more than 25 ppm and the total sulfur content not more than 30 ppm.
(2) After LF refining, a rare earth mischmetal (T[O]r<60 ppm in the rare earth mischmetal) is rapidly added into the ladle via the slag surface before VD treatment; the addition amounts of the rare earth in Embodiments 5A and 5B are 400 ppm and 750 ppm, respectively; and after adding the rare earth mischmetal, the deep vacuum time of VD is 15 min, and the soft blowing time after breaking VD vacuum is 25 min.
(3) The molten steel is respectively poured into ingot molds of 5-30 t in weight, cooled and solidified into ingots.
(4) The ingot is subjected to forging processing to prepare a rectangular billet having a cross-sectional size of 280×450 mm, and its composition (shown in Table 7) and properties (shown in Table 8) are tested.

TABLE-US-00007 TABLE 7 Composition of steels of Comparative Example 5 and Embodiment 5 Steel C Si Mn Cr P T[S] T[RE] T[O] Comparative 0.25-0.4 0.3-0.6 0.4-0.65 11-15 ≤0.02 ≤0.003 — ≤30 Example 5 Embodiment 5A 0.25-0.4 0.3-0.6 0.4-0.65 11-15 ≤0.02 ≤0.003 0.032 ≤30 Embodiment 5B 0.25-0.4 0.3-0.6 0.4-0.65 11-15 ≤0.02 ≤0.003 0.067 ≤25 Note: all the components except O being in ppm by weight are in % by weight in Table 7, and the balances are Fe and inevitable impurity elements. In Comparative Embodiment 5, no rare earth elements are added.

[0038] For analytical test of the rolled materials with the three components above, the size of the rare earth-rich nanoclusters and the change of the diffusion-type phase transition temperature are shown in Table 8. It can be seen that the size of the rare earth-rich nanoclusters tends to increase with the increase of the residual amount T[RE] of rare earth elements in the steel; and the influence on the diffusion-type phase transition points increases, and the temperature of the phase transition increases accordingly. The size of the rare earth-rich nanoclusters is directly proportional to the residual amount T[RE] of rare earth elements in the steel. However, the size of the rare earth-rich nanoclusters tends to decrease with the increase of the total oxygen content in the steel, the relationship between them is in inverse ratio.

TABLE-US-00008 TABLE 8 Analytical Test Results Rare Change in earth-rich diffusion nanocluster type phase diameter transition Steel (nm) point (° C.) T[RE] T[O] Comparative — — — ≤30 ppm Example 5 Embodiment 5A  4-15 12 0.032 ≤30 ppm Embodiment 5B 15-42 23 0.067 ≤25 ppm

[0039] The above embodiments are merely preferred embodiments of the present application and are not to be construed as limiting the scope of the present application. It should be noted that those skilled in the art can make various changes, substitutions and alterations herein which fall in the scope of protection of this application without departing from the spirit and scope of the invention.