UA77352C2 - Process for production diphasic structure steel and galvanizing diphasic steel belt - Google Patents

Abstract

Dual phase steel sheet is made using a time/temperature cycle including a soak at the temperature of about 727?C (1340?F)-775?C(1425?F) and a maturing at 454-493?C (850-920?F), where the steel has the composition in weight percent, carbon: 0,02-0,20; aluminum: 0,010-0,150; titanium: 0,01 max; silicon: 0,5 max; phosphorous: 0,060 max; sulfur: 0,030 max; manganese: 1,5-2,40; chromium: 0,03-1,50; molybdenum: 0,03-1,50; on condition that the amounts of manganese, chromium and molybdenum have the ratio: (Mn+6Cr+10Mo)= at least 3,5%. The sheet in the form of a strip treated in a continuous galvanizing or galvannealing line, and the product of predominantly ferrite and martensite content is obtained.

Description

Description of the invention

Two-phase galvanized steel strip is produced using a thermal profile associated with a 2-layer sequence that includes isothermal aging and aging. At the entrance to the coating bath, the strip has a temperature close to that of the molten metal.

The galvanizing procedure, according to which the steel strip is subjected to heat treatment and covered with metal, is well known and developed. Usually, a cold-rolled steel strip is heated to enter the intercritical mode (between As) and Aso) to form austenite, after which it is cooled in such a way that part 70 of the austenite turns into martensite with the formation of a microstructure of ferrite and martensite. Additives such as Mp, 5i, St and Mo are added to the steel to promote the formation of martensite. This is followed by various procedures, one of which is described in U.S. Patent 6,312,536. According to this patent, cold-rolled steel strip is used as a base for hot-dip galvanizing, the steel strip having a certain composition which is said to be conducive to forming under the conditions of the microstructure process, consisting mainly of ferrite and martensite. This patent describes a galvanized two-phase product.

According to O5 6 312 536), two-phase galvanized steel strip is made by keeping cold-rolled steel strip at a temperature of 7802 (14362E) or higher, usually for 10-40 seconds. and then cooling at a rate of at least 59 per second, usually 20-402C/s, followed by introduction into the electroplating bath at a temperature of 4602 (8602P). According to this patent, steel should have the following composition (by weight):

Carbon 0.02-0.20

Titanium 0.01 (max.)

Phosphorus 0.060 (max.)

Manganese 1.5-2.40 ch 29 Molybdenum 0.03-1.50 Ge)

Aluminum 0.010-0.150

Silicon 0.04 (max.) dom o oyulyu schu, provided that the amounts of manganese, chromium and molybdenum are in the ratios:

ZMp6Stg-Mo: 8,190o (max.) o

MPi-6Sg-10Mo: at least 3.595. h-

According to patent 05 6 312 536) it is noted that the initial heat treatment (holding) is carried out at a temperature

From at least 78092 (14362) (see col. 5, lines 64-64; col. b, lines 2-4: "To obtain the desired microstructure and stable forming, it is necessary to heat the steel strip to 7802С or higher, that is, above the Asi point on approximately 502C. The duration of heating should exceed 10 seconds to obtain the desired microstructure of ferrite and austenite". Further, in the description of the process, it is noted that the steel sheet is cooled to "0 temperature of the electrolyzer (usually 440-4702C (824-8782P)) with an average cooling rate higher than 19C/s in s and passed through an electrolyzer. After electrodeposition, cooling at a rate of at least 5 oC/s also gives the desired microstructure, preferably ferrite-martensitic. Alternatively, the sheet with a galvanic coating before "" cooling can be heated according to the alloying procedure (galvanic normalization) after metal coating, but before final cooling.

The authors 05 6 312 536) do not assume the possibility of obtaining a two-phase product without creating a high temperature at the aging stage or that a certain aging stage after low-temperature aging can ensure the formation of the desired microstructure.

It was found that, contrary to the statements (at 05 6 312 536), not only is it not mandatory to maintain

Ge) temperature of the initial heat treatment at the level of 7802 (14362E) or higher, but the desired two-phase microstructure can be obtained by maintaining the temperature during the initial heat treatment (holding) in the range from i-th Asin4a5eg (82C), but from at least 13402 (72722) to Ason1352E ( 5722), but not higher than 14252E (77592). There is no need to maintain a temperature of 7802C or higher, provided that the rest of the procedure is followed. In the following, we will call the initial processing "weathering". The process according to the invention depends not only on a lower holding temperature; holding at a temperature from Аа 4144452Е (822) to 14252Р, usually at 1340 (72720С)-14202Е (77322) must be combined with subsequent essentially isothermal heat treatment, which is called

GF! aging stage, at a temperature in the range of 850-920 (454-4932C). During this operation, the sheet is kept at a temperature of 850-9202E (454-4932С), that is, at 885-352E, for 20-100 seconds. before cooling to room temperature. The rate of this cooling should be at least 59C/s. It should be noted that (at 05 6 312 536) the description of the process does not mention the aging stage of any duration at any temperature. 60 We have found that if the steel specified (in 05 6 312 536) is tempered, according to this patent, at higher temperatures, for example 14749 (79822), the resulting steel will not have the desired predominantly ferrite-martensitic microstructure , and will contain a significant amount of bainite and/or pearlite.

Therefore, the lower temperature limit for the aging operation is As 41459, but at least 13409 in5 (7272С), since for almost all steels the composition of As is at least 12952E (70290).

The composition of the steel sheet must be identical to the one given (at 05 6 312 5361: -Д-

Carbon 0.02-0.20

Titanium 0.01 (max.)

Phosphorus 0.060 (max.)

Manganese 1.5-2.40

Molybdenum 0.03-1.50

Aluminum 0.010-0.150

Silicon 0.04 (max.) 70 Sulfur 0.030 (max.)

Chromium 0.030-1.50, provided that the amounts of manganese, chromium and molybdenum are in the ratio:

Mpypi6Sg-10Mo: at least 3.595.

According to the invention, the silicon content should be up to 0.595 and the carbon content should be 0.03-0.1295, although the carbon content can be the same as 05 6 312 5361.

Such a composition with modifications will be called Composition A.

The object of the invention is a method of producing a two-phase steel sheet, which includes keeping the steel sheet at a temperature ranging from As-1452E, but at least 13402E (72722) to As--1352E, but at most 14259 (77522), for 20-90 seconds. cooling the sheet at a rate not lower than 12С/s to a temperature of 454-49392С and holding the sheet at a temperature within the range of 850-9202Е (454-493223) for 20-100 seconds. The holding operation may precede the hot dip or may begin with the hot dip since the galvanizing tank temperature is also in the range of 850-920 (454-4932C). Immediately after the holding operation, regardless of whether the sheet is galvanized or not, the sheet can be cooled to room temperature at a rate of su 2p5 of at least 59C/sec. In another option, after applying the coating, the sheet can be subjected to the usual galvanic normalization, i.e. heating the sheet for 5-20 seconds. to a temperature typically no higher than about 9602 (51622) followed by cooling at a rate of at least 5 esC/s.

The galvanic normalization cycle according to the invention and thermogalvanization cycles for comparison are given in

Fig. b with

The hot dip operation is carried out more or less traditionally, that is, the steel is brought into contact with the molten galvanizing metal for 5 seconds; although in some cases this time can be shortened, a significant increase in this time is also acceptable, but does not improve the results. The steel tape usually has a CO thickness of about 0.7mm to about 2.5mm, and the coating thickness is usually about 1Ωm. M

After the aging and coating operation, the coated steel can either be cooled to room temperature as described here or subjected to galvanic normalization (see above). Following the procedure described above, a product with a microstructure containing mainly ferrite and martensite is obtained.

In commercial applications, hot dip galvanizing is usually performed as a substantially continuous process using rolls of steel strip typically 1000-6000 feet long (305-1830m). The method according to the invention provides more convenient control of the process not only because the holding operation is carried out at a lower temperature, but also because the temperature of the strip is easier to maintain - the same at the entrance to the hot dip tank and at the exit from it, without worrying about significant heat exchange h between the steel strip and the zinc tank, which can heat up the molten zinc and limit productivity.

When applied to a line of continuous steel strip galvanization, which includes a strip feed unit and a hot-vanishing bath, the invention includes feeding a roll of cold-rolled strip of Composition A into the heated zone of the galvanizing line, continuously passing the strip through this zone to heat it to a temperature in the range of -1 AsinabBov (82), but at least 13409 (72722) to Ason1359E (5722), but at most 14259 (7752С), passing the tape Through the holding zone with a temperature from As /1452E (822), but at least 13409 and (7272С) to Aso -1359E (5722), but at most 14259 (77522) for 20-9Osec., passing the strip through the "sl 20 cooling zone at a rate of 12C/s, stopping the cooling of the strip when its temperature drops to a temperature in the range of 885-352E, but also -302E from the temperature of the galvanizing bath (preferably -202E, best 4-102E from the temperature of the bath), keeping the tape at a temperature within "302E from the temperature of the galvanizing bath (preferably 4202E, best 4-1 02E from the temperature of the bath) for 20-100 seconds, passing the tape through the electroplating bath, as an option, galvanic normalization of the coated tape and cooling the tape to the outside temperature. The temperature of the electroplating bath is approximately 8702 (850-920) (F) and may be located at the beginning of the holding zone or near the end of the holding zone, or anywhere in the holding zone or immediately after it. The time spent in the bath is usually 3-bsec., but it can vary, in particular, with an increase, perhaps up to 10 sec. As noted above, after immersing the steel in the bo/Chinkov bath and removing It from there, the strip can be heated in the usual way before cooling to room temperature to form a galvanically normalized coating, if desired.

In the drawings:

Fig. 1 - a general thermal cycle according to the invention,

Fig. 2 - tensile strength limit as a function of holding temperature and holding time for cycle Fig. 1, c5 Fig. 3 - yield ratio as a function of holding temperature,

Fig. 4 - the influence of the holding temperature on the flow ratio in the conditions described in Example 2,

Fig. 5 - another graph of the ratio of fluidity in the conditions described in Example 3,

Fig. 6 is a paradigm of the thermal cycle according to the invention.

Example 1

The steel samples were treated with different "holding" temperatures according to the general thermal cycle

Fig. 1 - one set of samples corresponds to a curve with a 35-second exposure, the second set of samples - a curve with a 70-second exposure. The samples were samples of cold-rolled steel of Composition A, described above, in particular, in it: carbon - 0.6795, My - 1.8195, St - 0.1895 and Mo - 0.1995 (by mass). The rest of the ingredients were typical for low-carbon mild steel. The holding temperatures were changed in steps of 20 9 (722) in the range of 70. 1330-15102E (720 - 822202). After cooling, the mechanical properties and microstructure of the modified samples were determined. Fig. 2 contains a graph of the dependence of the tensile strength limit (MMP) of the obtained product on the temperature of exposure and duration of exposure. In this case, the goal was a MMP of 600 MPa, which was achieved within the holding temperatures of 1350-1450 (732-7882C) and at both holding times.

The purpose of Example 1 was to obtain a predominantly ferrite-martensitic microstructure. The yield ratio, 75 i.e. the ratio of yield strength (MT) to MMR, indicates the presence or absence of a two-phase ferrite-martensitic microstructure. According to Example 1, a ferrite-martensitic microstructure is indicated by a flow ratio of 0.5 or less. If the flow ratio exceeds about 0.5, this indicates the presence of a significant volume fraction of such undesirable components as bainite, pearlite and/or Re 3 in the microstructure.

Fig. 3 contains a graph of the dependence of the flow ratio of the samples on the holding temperature at 35-second and 70-second holding times. It should be noted that a very low flow ratio for both curves of approximately 0.45 is achieved at temperatures of 1350-1430 F (732-7772C), which corresponds to optimal biphasicity within these limits of holding temperatures. Metallographic analysis of metal samples aged at temperatures of 1350-14309 (7332-7772) confirmed the ferrite-martensitic microstructure. Quantitative metallography using the point counting method showed martensite contents of 14.595 and 13.595 at exposures with durations of 70 and ZbBsec., respectively, at 8802 (30422) for steel that was exposed to 1390 RE (75520). o (The images were obtained by the Leper etching method, in which ferrite appears light gray, martensite appears white, and components such as bainite and pearlite appear black. At holding temperatures below about 13502 (7322), as expected, iron carbide (HezS) remains in the microstructure due to insufficient dissolution of carbide, and this limits the formation of martensite during cooling.

However, the appearance of bainite in the microstructure at holding temperatures above approximately 14309 (77722) is unexpected. For example, metallographic analysis shows a bainite content of 8.595 in steel aged at a temperature of 15109 (82222) with a duration of exposure of 7Osec. These data are very different from the data (05 6 312 o 536), according to which the ferrite-martensitic structure is formed at a holding temperature above 1436 FE - (77822). We found that a significant amount of bainite in the microstructure is formed when the heat treatment process is carried out at the normalization holding temperature within the recommended (5 b 312 536) limits and at a holding temperature close to 8802 (30422). For the steel used in this Example, the temperature limits of galvanic normalization necessary for the formation of a ferrite-martensitic microstructure were approximately 1350-1430 (732-7772C). Table 1 contains data that illustrate the relationship between « 70 thermal process, flow rate and microstructure components for this example at different - s temperature regimes of holding.

I" 4 - c. at 8 p.m

ГЯ6) Example 2

Another cold-rolled steel of Composition A was subjected to the thermal cycles described in Example 1 (Fig. 1). The composition of this steel was within the limits of Composition A and contained, in particular: C - 0.1295, Mn - 1.9695, St - 0.2495 and Mo - 0.1895. The rest of the bv ingredients were typical for low-carbon mild steel A1. Fig. 4 illustrates the effect of holding temperature on the yield ratio for this steel at a holding time of 7 seconds. at 8802 (30422). Curve

Ф, the dependence is similar to the curves of Fig. 3, and the crystallographic analysis showed the same metallogenic phenomena as in the previous example. As in the previous example, the predominant ferrite-martensitic structure was formed at temperatures of about 1350-14252E (732-7752C) and at a holding temperature of about 8802C (3042). 60 Example C

As in the previous examples, the third cold-rolled steel of Composition A was subjected to the thermal cycles shown in Fig.1. This steel contained C - 0.07695, My -1.8995, St - 01095, Mo - 0.09495 and 5i - 0.34 (by mass). The rest of the ingredients were typical for low carbon steel. After galvanic normalization, as in other examples, the resulting microstructures were determined. Fig. 5 contains the fluidity ratio of this material as a function of the holding temperature at a holding time of 7 seconds. The dependence curve is similar to the curve of the previous examples, with precise normalization limits (MN) that provide a two-phase ferrite-martensitic structure. However, the curves appear shifted to the right by about 30 2C, relative to the curves of the previous examples. The reason for this is that the temperature Ac) for this steel is higher than this temperature in the previous examples. Table C contains the holding temperatures required to form ferrite-martensite (FM) for

EACH of the steels, and their respective temperatures As; according to Andrews. The desired normalization limits appear as a function of the temperature As. It is natural that, according to these data, the limits of the holding temperature necessary to obtain biphasicity depend on the specific composition of the steel and are from As 44452E, but from at least 13402 (7272C) to Ason1352P, but not higher than 14252E (77522), with holding temperature in the region of 8809 (3042), i.e. (885-352). and

Example 4

Table C contains the mechanical properties of two more steels with lower carbon content than the previous steels. They were processed according to Fig. 1 at holding temperatures of 1365 (7392C), 1400 (7602) and 14759 (7982C) with a holding time of 7 seconds. at 8802E (3042C). The table also contains the necessary temperature limits for obtaining two-phase steel, calculated by Ac 3 according to Example 3. it should be noted that at holding temperatures of 1365 (739)-14002E (7602С), which lie within the desired limits of the holding temperature for both steels, were noted low characteristics of the flow ratio of ferrite-martensitic microstructures. For steels aged at a temperature of 14752E (7982C), which lies beyond these limits, the yield ratio is significantly higher due to the presence of bainite in the microstructure. Go)

C, uyu Mp, 96 | Stg, 96 Mo, 96) Asi, "ER | As11445-As14 Temperature | Yield strength MMTt | The ratio of c » u and in zv h

Example 5

The previous examples were based on laboratory studies, but rolling tests were also carried out, which confirmed the heat treatment schemes described above for the manufacture of a two-phase steel product 7-3) with both hot-dip galvanizing and galvanic normalization. Table 4 contains the results of such tests for galvanized steel. The steels given in the table have almost the same composition and, therefore, ;" the same temperatures Asi. From these temperatures, the temperature limits of exposure to the formation of biphasicity were calculated, namely, 1350-14402E (7332-7822). The temperature and duration of aging corresponded well to different steels, and the normalizing (aging) temperature is the main factor, which is different for different materials. Table -i also contains mechanical properties according to fluidity ratios. Steels 1-4 were aged according to the invention and had an expected yield ratio below 0.5. Metallographic analysis of steels 1-4 showed the presence of a ferrite-martensitic microstructure with a martensite content of 1595. Steel 5 was processed in a mode outside the desired

Ge) beyond the holding limits and obtained a relatively high flow ratio of approximately 0.61. Metallographic 5or analysis showed this material to contain bainite 1196. The same results were obtained for both hot-dip galvanizing and normalizing treatments.

Co., Ltd. (732-7822C) | (1732-7822 | (732-7822С | (732-7822С | (733-7832С dv

MM bob 610 614 618 538

Claims (20)

The formula of the invention 1. The method of manufacturing a sheet of steel in the initial two-phase microstructural stage containing, wt. 90: carbon - 0.02-0.20, aluminum - 0.010-0.150, titanium - maximum 0.01, silicon - maximum 0.5, phosphorus - maximum 0.060, sulfur - maximum 0.030, manganese - 1.5-2, 40, chromium - 0.030-1.50, molybdenum - 0.03-1.50, 70 provided that the amount of manganese, chromium and molybdenum is in the ratio: Мп-б6Сгт-10Мо - at least 3.590, which includes the aging of the specified steel sheet at a temperature in the range from A si1182C (452E), but at least 7272 (13402P), to As11572C (1352E), but not higher than 7752C (14252), for 20-90 s, cooling /5 sheet at a rate not lower than 19C/s to temperature 454-4932С (850-9202РЕ) and exposure of the sheet at a temperature within 4454-4932 (850-9202Р) for 20-100 seconds. 2. The method according to claim 1, characterized in that said steel sheet is a steel strip, and the method is implemented continuously on a strip at least 305 m (1000 feet) long. C. The method according to claim 1, which differs in that it includes coating said steel sheet in a 2o tank of molten galvanizing metal at a temperature in the range of 454-4932C (850-9202P) before, during or immediately after said exposure. 4. The method according to claim 3, which differs in that the temperature of the specified steel sheet during the specified coating is maintained with a maximum deviation of 11922 (4202E) from the temperature of the molten metal to minimize heat transfer between the specified steel strip and the specified molten metal. Ha 25 5. The method according to claim 1, which differs in that after its implementation, the specified steel sheet is cooled to the external temperature at a rate of at least 59C/s, and the two-phase microstructure is manifested as i) a microstructure with a preference for ferrite and martensite. 6. The method according to claim 1, which is characterized by the fact that it includes galvanic normalization of the specified steel sheet and cooling of the sheet covered in this way at a rate of at least 5 2C/s, the two-phase microstructure of C 30 manifests itself as a microstructure with a preference for ferrite and martensite. yu 7. The method according to claim 1, which differs in that the carbon content in the specified steel is 0.03-0.12 wt. 905. 8. A method of essentially continuous galvanizing of steel tape in a galvanization line with a galvanizing (ze) bath, which includes feeding a roll of steel tape containing, wt. for: carbon - 0.02-0.20, aluminum - 35 0.010-0.150, titanium - a maximum of 0.01, silicon - a maximum of 0.5, phosphorus - a maximum of 0.060, sulfur - a maximum of 0.030, manganese - 1.5 -2.40, chromium - 0.030-1.50, molybdenum - 0.03-1.50, provided that the amount of manganese, chromium and molybdenum is in the ratio: Mp-bSt-10Mo- at least 3.595, in the heating zone of the specified galvanizing line, continuous passing of the specified tape through this zone to heat it up to a temperature of 7272 (13402) - 77592 (14252E), passing the specified tape through a holding zone with a temperature of 40 in the range from 7272 (13409) to 7732 (14202) for 20- 90 s, passing the belt through the t s cooling zone to cool the specified belt at a rate higher than 12C/s, stopping the cooling of the specified "» belt when the temperature of the specified belt decreases to a temperature that differs by " -172C (4302) from the temperature the indicated galvanizing bath, keeping the indicated tape at temp raturi 4544-4932 (850-920) and within 172C (302) of the temperature of the indicated galvanizing bath for 20-100 s, passing the indicated tape through the indicated galvanizing bath and cooling the indicated tape to the external temperature. -AND 9. The method according to claim 8, which differs in that the time the specified tape is in the specified electroplating bath is 3-6 seconds. Mom 10. The method according to claim 8, which differs in that the specified cooling in the specified cooling zone 1,250 is carried out at a speed of 2-222С/s (5-409Р/s). "with 11. The method according to claim 8, which differs in that the specified tape enters the specified electroplating bath at a temperature within 62C (102E) of the temperature of the specified electroplating bath. 12. The method according to claim 8, which is characterized by the fact that the specified tape is introduced into the specified electroplating bath immediately after the termination of the specified cooling. 13. The method according to claim 8, which differs in that the specified tape is introduced into the specified electroplating bath (F) approximately at the end of the specified period of 20-100 seconds. Ge 14. The method according to claim 8, characterized in that the galvanized steel strip produced in this way has a predominantly ferrite-martensitic microstructure with less than 595 other morphological components. in 15. The method according to claim 8, which differs in that the carbon content in the specified steel strip is 0.03-0.12 wt. 9. 16. The method according to claim 8, which differs in that the specified steel strip is subjected to galvanic normalization before cooling to the external temperature. 17. The method of manufacturing a galvanized steel strip, which has a predominant ferrite-martensitic microstructure, but the specified steel contains, wt. 95: carbon - 0.02-0.20, aluminum - 0.010-0.150, titanium - maximum 0.01, silicon - maximum 0.5, phosphorus - maximum 0.060, sulfur - maximum 0.030, manganese - 1.5-2, 40, chrome - 0.030-1.50, molybdenum - 0.03-1.50, which includes keeping the specified steel strip at a temperature ranging from Asi82C (45"E), but at least 7272 (13402P), to Asi17572C (1359E), but not higher 7752С (14252Р), for at least 20 s, cooling the tape at a rate not lower than 1 2С/s, passing the specified tape through the galvanizing tank with a residence time of 2-9 s to coat the specified tape at any time with simultaneous exposure of the tape at a temperature of between 474-1922; (885-352) for 20-100 s and cooling the coated tape to outside temperature. 18. The method according to claim 17, which is characterized by the fact that the specified tape is subjected to galvanic normalization before cooling to the external temperature. 19. The method according to claim 17, which is characterized by the fact that the specified tape during the specified residence time has a temperature within 1122 (202) of the temperature of the galvanizing tank. 20. The method according to claim 17, which differs in that the specified tape during the specified residence time has a temperature within 62C (102E) of the temperature of the galvanizing tank. s (8) s IS) so cha i - - with . and? -I -I (95) p 50 Ko) F) ime) 60 b5