HOT-ROLLED, LOW-TEMPERATURE-RESISTANT, H-SHAPED STEEL WITH GRADE OF YIELD STRENGTH OF 420 MPA AND PREPARATION METHOD THEREFOR

Abstract

Provided are a hot-rolled, low-temperature-resistant, H-shaped steel with a grade of 420 MPa and a preparation method therefor. The H-shaped steel comprises the following chemical components in percentages by weight: C: 0.08-0.10%, Si0.2%, Mn: 1.25-1.45%, V: 0.03-0.045%, Ti: 0.015-0.025%, Cr: 0.15-0.30%, Als: 0.02-0.04%, N: 0.007-0.01%, P0.008%, S0.005%, O0.004%, and the balance being Fe and inevitable impurities. In the preparation method, the characteristic of the flange of the small-specification H-shaped steel being thin in rectangular blank rolling is combined, the design of a low content of C suitable for normalizing rolling being in cooperation with a V micro-alloyed component is used, an appropriate amount of Cr is added to control the cooling rate, and the situation whereby the low-temperature impact toughness of steel deteriorates due to the occurrence of abnormal structures such as widmanstatten is avoided, thus a stably controlled, high-strength and high-toughness, hot-rolled, H-shaped steel with a grade of 420 MPa or more is obtained on a hot-rolled H-shaped steel rolling mill.

Claims

1. A low-temperature-resistant, H-shaped steel with a grade of yield strength of 420 MPa, wherein the H-shaped steel comprises the following chemical components in percentages by weight: C: 0.08-0.10%, Si0.2%, Mn: 1.25-1.45%, V: 0.03-0.045%, Ti: 0.015-0.025%, Cr: 0.15-0.30%, Als: 0.02-0.04%, N: 0.007-0.01%, P0.008%, S0.005%, O0.004%, and the balance being Fe and inevitable impurities.

2. The low-temperature-resistant, H-shaped steel with a grade of yield strength of 420 MPa according to claim 1, wherein the H-shaped steel comprises the following chemical components in percentages by weight: P+S0.01%.

3. A preparation method for a low-temperature-resistant, H-shaped steel with a grade of yield strength of 420 MPa, comprising the following steps: (1) pretreatment and repeated dephosphorization of molten iron: in a tapping process of a blast furnace, molten iron in the blast furnace flows into the first molten iron ladle, with continuous injection of the molten iron in the blast furnace, a dephosphorization agent is blown into the molten iron ladle to perform dephosphorization, a level in the molten iron ladle keeps rising, when a level of the molten iron exceeds an upper edge of a tapping hole by 20-30 cm, the tapping hole is opened, the molten iron flows into the second molten iron ladle through the tapping hole, blowing and dephosphorization operations are performed again in the second molten iron ladle, a tapping hole is also opened in a side of the second molten iron ladle at a position 20-30 cm below a steel slag interface of the molten iron ladle, molten iron dephosphorized at an upper portion of the second molten iron ladle flows downward into the third molten iron ladle through the tapping hole, this process is repeated, molten iron flows downward into an N.sup.th molten iron ladle through a tapping hole, an Nth dephosphorization operation is performed on the molten iron, and a phosphorus content in the molten iron is reduced below 0.02%, wherein 4N6; and every time dephosphorization is performed for 30 to 40 min, one slag exchange operation is performed on the first molten iron ladle to an (N1).sup.th molten iron ladle; (2) converter smelting: a final phosphorus content is controlled below 0.007%; and (3) steel ladle argon blowing, RH/LF refining, rectangular continuous casting blank casting, slow cooling or hot delivery and charging in a continuous casting blank slow cooling pit, section steel line semi-continuous rolling, and dense slow cooling in a cooling bed are performed, wherein in a rolling process, a soaking temperature of a heating furnace is 1210-1250 C., and a casting blank in-furnace time is 140-180 min; and a finish initial rolling temperature is 1000-1050 C., and a finish final rolling temperature is 890-930 C.

4. The preparation method according to claim 1, wherein the first molten iron ladle to the (N1).sup.th molten iron ladle in step (1) have nominal capacities of 30-50 t molten iron and depths of 1.5-2 m, a tapping hole is opened in a sidewall of a molten iron ladle at a position 50-60 cm away from a top of the molten iron ladle, the tapping hole extends outward by 20-30 cm, a sliding plate is used to control opening and closing of the tapping hole, and a capacity of the N.sup.th molten iron ladle matches an engineering tonnage of a converter; and compression ratios in the last two passes of finish rolling in step (4) are 6-12%.

5. The preparation method according to claim 1, wherein blowing points of the dephosphorization agent are an injection point of the molten iron in the blast furnace into the first molten iron ladle and an injection point of molten iron in an upper level of molten iron ladle into a lower level of molten iron ladle.

6. The preparation method according to claim 1, wherein the dephosphorization agent is a mixture of sludge pellets and lime, a mass ratio of the sludge pellets to the lime is 1:1, the sludge pellets comprise the following components in percentages by mass: CaO: 2-8%, Fe.sub.2O.sub.3: 85-92%, and SiO.sub.2: 1-5%, the lime comprises the following components in percentages by mass: CaO: 85-90%, Fe.sub.2O.sub.3: 0-3%, SiO.sub.2: 0-10%, and MgO: 0-10%, and an addition amount of the dephosphorization agent to the molten iron ladle is 2-3 kg/ton iron molten iron ladle.

7. The preparation method according to claim 1, wherein a carrier gas during the blowing of the dephosphorization agent is oxygen, a blowing pressure is 0.2-0.3 Mpa, and a blowing flow rate is 2-4 m.sup.3/min.

8. The preparation method according to claim 1, wherein in the molten iron ladles, a position of a next molten iron ladle is lower than that of a previous molten iron ladle by 60-70 cm.

9. The preparation method according to claim 1, wherein the slag exchange operation is: furnace slag in the first molten iron ladle is poured out, furnace slag at a top of the second molten iron ladle is poured into the first molten iron ladle, and furnace slag at a top of the third molten iron ladle is poured into the second molten iron ladle, until furnace slag at a top of the (N1).sup.th molten iron ladle is poured into an (N2).sup.th molten iron ladle.

10. The preparation method according to claim 9, wherein a slag tap is opened in the top of each of the first molten iron ladle to the (N1).sup.th molten iron ladle, the slag tap extends outward by 20-30 mm, and a position difference between the slag tap and the tapping hole in a ladle wall of the molten iron ladle is one fourth of a circumference; and a hydraulic elevation apparatus and a rotation apparatus are configured in a ladle car of each of the first molten iron ladle to the (N1).sup.th molten iron ladle, the molten iron ladle is elevated by 1-2 m, and the molten iron ladle is rotatable around a central axis by 90 degrees.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0065] FIG. 1 is an image of a metallurgical structure (200) of a high-toughness, low-temperature-resistant, H-shaped steel with a grade of yield strength of 420 MPa prepared in Example 2 of the present invention;

[0066] FIG. 2 is a schematic diagram of multi-level dephosphorization according to the present application;

[0067] FIG. 3 is a schematic structural diagram of a small-scale molten iron ladle; and

[0068] FIG. 4 is a schematic structural side view of a small-scale molten iron ladle.

REFERENCE NUMERALS

[0069] 1. blast furnace, 2. first molten iron ladle, 3. second molten iron ladle, 4. third molten iron ladle, 5. fourth molten iron ladle, 6. fifth molten iron ladle, 7. powder injection desulfuration apparatus, 8. tapping hole, and 9. slag tap.

DETAILED DESCRIPTION

[0070] The present invention is described below by using specific embodiments. It needs to be pointed out that the embodiments are only used to further describe the present invention, but are not used to limit the scope of protection of the present invention. Nonessential changes and adjustments made by others according to the present invention still belong to the scope of protection of the present invention.

[0071] Continuous casting blanks in the following embodiments are all prepared according to the following process procedure: According to set ranges of chemical components (Table 1), molten iron in a blast furnace are used as raw materials. Through dephosphorization, KR dephosphorization, converter smelting, and refining in a tapping process of the blast furnace, contents of C, Si, Mn, S, P, and the like are adjusted and micro-alloyed. Continuous casting and direct heating or soaking of casting blanks are performed after the contents of the components reach target values. Preparation steps in Examples 14 are as follows:

[0072] The steel undergoes dephosphorization and KR molten iron pretreatment and desulfuration.fwdarw.converter smelting.fwdarw.steel ladle argon blowing.fwdarw.refining.fwdarw.continuous casting.fwdarw.haped steel line rolling.fwdarw.online cooling.fwdarw.slow cooling in a cooling bed in the tapping process of the blast furnace. Section steel line rolling includes two phases of rolling: rough rolling and finish rolling. Priority is given to temperature control in a hot-rolling working procedure, an outer side of a flange is detected for a final rolling temperature, and a rolled rolling steel is naturally cooled in a cooling bed. Chemical components and specific processes in Examples 1-4 are shown in Table 1 below.

TABLE-US-00001 TABLE 1 Chemical components (wt %, the balance being iron) Item C Si Mn P S Cr V Ti Al N Example 1 0.08 0.15 1.30 0.007 0.002 0.28 0.03 0.015 0.025 0.008 Example 2 0.09 0.20 1.45 0.006 0.003 0.30 0.03 0.025 0.026 0.009 Example 3 0.08 0.20 1.28 0.006 0.004 0.25 0.04 0.018 0.03 0.008 Example 4 0.10 0.15 1.39 0.005 0.003 0.30 0.035 0.019 0.033 0.01

[0073] A specific dephosphorization process in the tapping process of the blast furnace is as follows: [0074] Step (1): In the tapping process of the blast furnace, the molten iron in the blast furnace flows into the first molten iron ladle, the first molten iron ladle has a nominal capacity of 30 t molten iron and a depth of 1.5 m, a tapping hole is opened in a sidewall of a molten iron ladle at a position 50 cm away from a top of the molten iron ladle, the tapping hole extends outward by 20 cm, and a sliding plate is used to control opening and closing of the tapping hole. With the continuous injection of the molten iron in the blast furnace, when a volume of the molten iron in the first molten iron ladle exceeds 50%, the dephosphorization agent is blown into the molten iron ladle to perform dephosphorization. In this case, the tapping hole is in a closed state. A level in the molten iron ladle keeps rising. When a level of the molten iron exceeds an upper edge of the tapping hole by 20 cm, the tapping hole is opened, and the molten iron flows into the second molten iron ladle through the tapping hole. A flow rate at which the molten iron flows from the first molten iron ladle into the second molten iron ladle is the same as a flow rate at which the molten iron in the blast furnace flows into the first molten iron ladle. The level of the molten iron is a stable state of neither rising nor dropping. A dephosphorization reaction occurs continuously at a steel slag interface of the first molten iron ladle. Phosphorus in molten iron near the steel slag interface is continuously removed. first molten iron ladle the upper portion Molten iron that is sufficiently dephosphorized flows into the second molten iron ladle through the tapping hole, as shown in FIGS. 2-4.

[0075] Blowing points of the dephosphorization agent include an injection point of the molten iron in the blast furnace into the first molten iron ladle and an injection point of molten iron in an upper level of molten iron ladle into a lower level of molten iron ladle. Molten iron at this position is intensely stirred, and dephosphorization has a good dynamic condition.

[0076] The dephosphorization agent is a mixture of sludge pellets and lime, an addition amount of the dephosphorization agent to the first molten iron ladle is 2 kg/ton iron molten iron ladle, a mass ratio of the sludge pellets to the lime is 1:1, the sludge pellets include the following components in percentages by mass: CaO: 5%, Fe.sub.2O.sub.3: 88%, and SiO.sub.2: 3%, and the balance being impurities. The lime includes the following components in percentages by mass: CaO: 90%, Fe.sub.2O.sub.3: 1%, SiO.sub.2: 3%, and MgO: 5%, and the balance being impurities.

[0077] A carrier gas during the blowing of the dephosphorization agent is oxygen, a blowing pressure is 0.2 Mpa, and a blowing flow rate is 2 m.sup.3/min.

[0078] A position of the second molten iron ladle is lower than that of the first molten iron ladle by 60 cm, and the molten iron flows from the first molten iron ladle into the second molten iron ladle through the tapping hole under the action of gravity. [0079] Step (2): Molten iron ladle size parameters and dephosphorization process parameters of the second molten iron ladle, the third molten iron ladle, and the fourth molten iron ladle are identical with those of the first molten iron ladle, and blowing and dephosphorization operations are performed in all the second, third, and fourth molten iron ladles.

[0080] Molten iron that is sufficiently dephosphorized near a steel slag interface of the second molten iron ladle flows into the third molten iron ladle through a tapping hole under the action of gravity, and molten iron that is sufficiently dephosphorized near a steel slag interface of the third molten iron ladle flows into the fourth molten iron ladle through a tapping hole under the action of gravity. Molten iron that is sufficiently dephosphorized near a steel slag interface of the fourth molten iron ladle flows into the fifth molten iron ladle through a tapping hole under the action of gravity.

[0081] Positions of the third molten iron ladle, the fourth molten iron ladle, and the fifth molten iron ladle are respectively lower than those of the second molten iron ladle, the third molten iron ladle, and the fourth molten iron ladle by 60 cm.

[0082] Components and temperatures of molten iron are detected. Dephosphorization rates and temperatures of molten iron in four small-scale molten iron ladles are shown in the following table:

TABLE-US-00002 TABLE 2 Dephosphorization rates and temperatures of molten iron in molten iron ladles Molten First Second Third Fourth Fifth iron in molten molten molten molten molten the blast iron iron iron iron iron furnace ladle ladle ladle ladle ladle Phosphorus 0.163 0.116 0.069 0.035 0.018 0.017 content (% in molten iron Molten iron 1520 1483 1471 1454 1437 1426 temperature ( C.)

[0083] As can be seen from the foregoing table, after dephosphorization in the four small-scale molten iron ladles, the phosphorus content in the molten iron is reduced from 0.163% to 0.017%, a dephosphorization rate of the molten iron is 89.6%, and the dephosphorization rate of the molten iron is high. The temperature of the molten iron is reduced from 1520 C. to 1426 C., a temperature drop of the molten iron is 94 C., and the temperature drop of the molten iron is small. The temperature of the molten iron after dephosphorization is much higher than a temperature requirement for entering a converter (higher than 1250 C.).

[0084] The fifth molten iron ladle is a large-volume molten iron ladle. A capacity of the fifth molten iron ladle matches an engineering tonnage of a converter. The molten iron in the fifth molten iron ladle is transported into a KR working position for desulfuration. After the desulfuration is completed, the molten iron is transported into the converter to perform normal smelting. A final phosphorus content of converter smelting is controlled below 0.007%.

[0085] Every time normal dephosphorization is performed for 30 to 40 min, one slag exchange operation is performed on the first to fourth molten iron ladles. A slag exchange process of the molten iron ladles is: furnace slag in the first molten iron ladle is poured out, furnace slag at a top of the second molten iron ladle is poured into the first molten iron ladle, furnace slag at a top of the third molten iron ladle is poured into the second molten iron ladle, and furnace slag at a top of the fourth molten iron ladle is poured into the third molten iron ladle. Because a multi-level dephosphorization process is used, a phosphorus content in the lower level of molten iron ladle is lower than that in the upper level of molten iron ladle. Under a process condition of sufficient dephosphorization, a distribution ratio of phosphorus in slag iron basically remains unchanged. That is, the phosphorus content in slag in a lower level of steel ladle is also lower than that in an upper level of steel ladle. Component detection is performed on furnace slag in the molten iron ladles. Components of furnace slag in the four small-scale molten iron ladles are shown in the following table.

TABLE-US-00003 TABLE 3 Components of furnace slag in molten iron ladles Furnace First Second Third Fourth slag in molten molten molten molten the blast iron iron iron iron furnace ladle ladle ladle ladle CaO (%) 41 37 43 45 45 SiO.sub.2 (%) 35 31 30 29 31 Al.sub.2O.sub.3 (%) 14 11 7 5 5 MgO (%) 6 5 5 6 5 S (%) 0.9 0.1 0.2 0.1 0.1 P.sub.2O.sub.5 (%) 0.13 0.91 0.64 0.47 0.30 T. Fe (total 0.3 8 11 13 14 iron) (%)

[0086] As can be seen from the foregoing table, a phosphorus content in furnace slag in a lower level of steel ladle is lower than a phosphorus content in furnace slag in an upper level of steel ladle. The furnace slag in the lower level of steel ladle has higher basicity and a stronger oxidizing ability (T.Fe may be converted into FeO). Therefore, after furnace slag at a top of the lower level of molten iron ladle is poured into the upper level of molten iron ladle, the furnace slag can still produce a good dephosphorization effect.

[0087] A slag tap is opened in the top of each of the first to fourth molten iron ladles, the slag tap extends outward by 20 mm, furnace slag in a molten iron ladle can flow out through the slag tap, and a position difference between the slag tap and the tapping hole in a ladle wall of the molten iron ladle is one fourth of a circumference.

[0088] A hydraulic elevation apparatus and a rotation apparatus are configured in a ladle car of each of the first molten iron ladle to the fourth molten iron ladle, the molten iron ladle may be elevated by 1.5 m, and the molten iron ladle is rotatable around a central axis by 90 degrees.

[0089] During the slag exchange operation, the tapping hole is first closed, and then the rotation apparatuses on the ladle cars of the molten iron ladles are used to make the upper level of molten iron ladle and the lower level of molten iron ladle rotate around the central axis by 90 degrees, to avoid contacting the tapping hole during elevation of the steel ladles. Then the hydraulic elevation apparatuses are used to lower the upper level of molten iron ladle and elevate the lower level of molten iron ladle. When the lower level of molten iron ladle is higher than the upper level of molten iron ladle and furnace slag in the lower level of molten iron ladle can flow into the upper level of molten iron ladle through the slag tap, the slag tap is opened, so that the furnace slag flows from the lower level of molten iron ladle into the upper level of molten iron ladle. After the furnace slag has flowed completely, the slag tap is closed.

[0090] Hot-rolling process conditions in Examples 14 are shown in Table 2. The used standard is BS EN ISO 377-1997 Location and preparation of samples and test pieces for mechanical testing. For experimental methods of a yield strength, a tensile strength, and an elongation, refer to the standard ISO6892-1-2009 Metallic materialsTensile testingMethod of test at room temperature. For an impact energy experiment method, refer to the standard ISO 148-1 Metallic materialsCharpy pendulum impact test. Results are shown in Table 3.

TABLE-US-00004 TABLE 4 Hot-rolling process in examples Heating Holding Final rolling Flange temperature time temperature thickness t Item ( C.) (min) ( C.) mm Example 1 1230 140 930 15 Example 2 1220 145 910 12.7 Example 3 1210 170 900 11 Example 4 1230 160 880 10

TABLE-US-00005 TABLE 5 Mechanical performance experiment results in examples Yield Tensile 50 C. strength strength Elongation Akv Item (MPa) (MPa) (%) (J) Example 1 440 560 25 180 Example 2 445 600 20 190 Example 3 450 580 22 205 Example 4 455 590 26 220

[0091] As can be seen from the table, yield strengths in Examples 1-4 in the present invention are kept at a 440 MPa grade. The steels have good extensibility, and have high impact energy at 50 C. The steels can meet use conditions of preparing ocean engineering members in extremely low-temperature environment, and are suitable for manufacturing support structure members such as ocean oil platforms and ocean transport ships that have high low-temperature toughness requirements.

[0092] As can be seen from FIG. 1, the structure in the present application is a granular bainite+ferrite structure. When a P content is reduced, for an H-shaped steel with a grade of strength of 420 MPa, while the strength is met, and the toughness, especially low-temperature toughness, is improved. Conventional technical knowledge in the field may be used for content that is not described in detail in the present invention.

[0093] Finally, it should be noted that the foregoing embodiments are merely intended for describing the technical solutions of the present invention rather than limiting the present invention. Although the present invention is described in detail with reference to the embodiments, persons of ordinary skill in the art should understand that they may still make modifications or equivalent replacements to the technical features of the present invention without departing from the spirit and scope of the technical solutions of the technical solutions of the present invention. These modifications or equivalent replacements shall all fall within the scope of the claims the present invention.