CN101745610B - Method for casting high-strength aluminum alloy round ingot by continuous oil gas lubrication - Google Patents

Method for casting high-strength aluminum alloy round ingot by continuous oil gas lubrication Download PDF

Info

Publication number
CN101745610B
CN101745610B CN2008102441732A CN200810244173A CN101745610B CN 101745610 B CN101745610 B CN 101745610B CN 2008102441732 A CN2008102441732 A CN 2008102441732A CN 200810244173 A CN200810244173 A CN 200810244173A CN 101745610 B CN101745610 B CN 101745610B
Authority
CN
China
Prior art keywords
casting
ingot
melt
crystallizer
oil
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Active
Application number
CN2008102441732A
Other languages
Chinese (zh)
Other versions
CN101745610A (en
Inventor
郭世杰
王家淳
吴文祥
刘金炎
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
China Nonferrous Metals Processing Technology Co Ltd
Original Assignee
Suzhou Nonferrous Metal Research Institute Co Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Suzhou Nonferrous Metal Research Institute Co Ltd filed Critical Suzhou Nonferrous Metal Research Institute Co Ltd
Priority to CN2008102441732A priority Critical patent/CN101745610B/en
Publication of CN101745610A publication Critical patent/CN101745610A/en
Application granted granted Critical
Publication of CN101745610B publication Critical patent/CN101745610B/en
Active legal-status Critical Current
Anticipated expiration legal-status Critical

Links

Images

Landscapes

  • Continuous Casting (AREA)

Abstract

The invention relates to a method for casting a high-strength aluminum alloy round ingot by continuous oil gas lubrication, which comprises the following steps: firstly, melting an industrial pure aluminum ingot in an induction furnace; after the industrial pure aluminum ingot is molten, controlling the melt temperature at 700-730 DEG C, and adding an industrial pure zinc ingot and an industrial pure magnesium ingot; stirring the melt, heating up to 710-750 DEG C, and adding Al-50Cu, Al-10Zr, Al-10Mn and Al-5Cr intermediate alloy ingots; stirring and transferring the melt to a heat-insulation holding furnace for refining and removing slag; before casting, adding Al-5Ti-B poles into the furnace for thinning the melt; in the semi-continuous casting process, adopting a continuous oil gas lubrication crystallizer; in the casting process, controlling the crystallizer compressed air pressure at 0.1-0.4MPa and the lubrication oil pressure at 0.2-0.8MPa, mixing the compressed air and the lubrication oil, and forming an oil-gas cushion film on the inner wall of the crystallizer; in the initial stage of casting, controlling the casting speed at 10-30mm/min and the cooling water pressure at 0.08-0.1MPa; and after casting for a certain length, gradually increasing the casting speed and the cooling water pressure, and keeping the casting speed and the cooling water pressure constant. The method not only can be used for successfully casting a high-strength aluminum alloy cast ingot, but also can obviously improve surface quality of the cast ingot.

Description

Method for casting high-strength aluminum alloy round ingot by continuous oil gas lubrication
Technical field
The present invention relates to a kind of method for casting high-strength aluminum alloy round ingot by continuous oil gas lubrication, belong to the aluminum alloy semi-continuous casting technical field.
Background technology
High strength alumin ium alloy is that Al-Zn-Mg-Cu-Zr is an alloy, and combination property is more excellent, has higher structural strength, fracture toughness and stress corrosion resistant, is a kind of lightweight structural material that extensively adopt in current Aeronautics and Astronautics field.Yet the alloying element content of high strength alumin ium alloy is often higher, and for representative alloy 7050 wherein, the content of elements such as its Zn, Mg, Cu is especially up to 12%.In process of setting, the low melting point eutectic phase number at crystal boundary place increases, thus the ability drop of ingot casting opposing crackle.It is reported that hot tearing and the cold cracking phenomenon in casting later stage of high strength alumin ium alloy ingot casting in casting process is very serious, casting success ratio is lower, and the tearing tendency of ingot casting increases along with the increase of ingot casting specification.
In order to improve the lumber recovery of high strength alumin ium alloy ingot casting, often adopt the mode of fine aluminium shop fixtures, promptly at first cast one section commercial-purity aluminium ingot casting, and then the mode of carrying out normal alloy liquid casting prepares the high-strength alloy ingot casting at the casting initial stage.Adopt this technology to reduce the intensity of cooling of ingot casting bottom, reduced the bottom and solidified inner-stress value, can obtain the purpose of ingot casting bottom cracking to a certain extent.But, adopt fine aluminium shop fixtures technology to bring certain difficulty to casting operation, if the casting technique misoperation, the melt flow will be spread bottom aluminium liquid easily and splash and sneak into melt, the cutting output of increase ingot casting head.
In addition, high strength alumin ium alloy ingot casting often surface quality is relatively poor, and the phenomenon on the gross segregation of element and ingot casting surface " segregation knurl " is very serious, must be before the follow-up distortion of ingot casting with the surface segregation layer mill off, and the financial cost of ingot casting preparation is higher.
Summary of the invention
The present invention is directed to the problem that the high strength alumin ium alloy ingot casting is easy to crack and surface quality is relatively poor; A kind of method for casting high-strength aluminum alloy round ingot by continuous oil gas lubrication is provided; Adopting said method not only can successful High Strength Casting aluminium alloy cast ingot, and can significantly improve ingot surface quality.
The object of the invention is realized through following technical scheme:
Method for casting high-strength aluminum alloy round ingot by continuous oil gas lubrication specifically may further comprise the steps---
1) alloying and melting: the technical pure aluminium ingot is placed on melting in the induction furnace; Be controlled at melt temperature in 700 ℃~730 ℃ scopes after waiting to melt and add industrial-purity zinc, technical pure magnesium ingot; Stir melt and its temperature is increased to 710 ℃~750 ℃; In melt, add Al-50Cu, Al-10Zr, Al-10Mn, Al-5Cr intermediate alloy ingot respectively, after the stirring melt is gone to the resistance heated insulation and leave standstill stove, heat up and leave standstill;
2) melt treatment: melt temperature is remained on 720~750 ℃, adopt no sodium refining agent and slagging agent that melt is carried out refining and slag hitting and handle, left standstill 10~20 minutes after pulling down the oxidizing slag of bath surface; In stove is left standstill in insulation, add Al-5Ti-B line pole pair melt and carry out thinning processing, and left standstill 20~50 minutes, casting is also prepared in the melt temperature insulation in 690 ℃~730 ℃ scopes;
3) semi-continuous casting: the oil-air lubrication crystallizer is adopted in semi-continuous casting; The oil-air lubrication crystallizer comprises insulation hot tap, crystallizer body, cooling water cavity, air chamber, lubricant reservoir; Set inside cooling water cavity at the crystallizer body; Have air chamber and lubricant reservoir in the upper end of crystallizer body, the insulation hot tap is installed in the top of crystallizer body; At first open air chamber and lubricant reservoir before the casting beginning; And guarantee that air pressure range is 0.1~0.4MPa; The oil pressure scope is 0.2~0.8MPa; Compressed air in the air chamber and the lubricating oil in the lubricant reservoir mix and in crystallizer, encircle formation skim oil-air-cushion film, and liquid metal does not directly contact with the crystallizer inwall with ingot casting when making casting; Metal bath slowly flows into crystallizer during the casting beginning, and the dummy ingot base begins to descend with the speed of 10~30mm/min when crystallizer inner metal liquid position highly reaches 40~60mm, and the cooling water pressure in the cooling water cavity remains on 0.08~0.1MPa; Casting speed was increased to 40~80mm/min gradually after ingot casting length reached 100~200mm, and casting gets into the stabilization sub stage; When stable casting, keep the constant of oil pressure and air pressure and casting technological parameter.
Further, above-mentioned method for casting high-strength aluminum alloy round ingot by continuous oil gas lubrication, the interior lubricating oil of the compressed air in the step 3) air chamber and lubricant reservoir was with 1: 2~1: 4 mixed.
Further, above-mentioned method for casting high-strength aluminum alloy round ingot by continuous oil gas lubrication, the step 3) casting gets into the stabilization sub stage, and for φ 300mm ingot casting, the scope of casting speed is 40~100mm/min, and the scope of cooling water pressure normal value is 0.1~0.3MPa.
Substantive distinguishing features and obvious improvement that technical scheme of the present invention is outstanding are mainly reflected in:
The present invention applies continuous oil gas lubrication to the high strength alumin ium alloy ingot casting in casting process, in the surface quality that guarantees to have significantly improved when ingot casting is shaped smoothly ingot casting.Adopt this method not only can successful High Strength Casting aluminium alloy cast ingot, and the ingot casting flawless, and surface quality is more excellent.
Description of drawings
Below in conjunction with accompanying drawing technical scheme of the present invention is described further:
Fig. 1: the oil-air lubrication structure of mould sketch map that the present invention adopts.
The implication of each Reference numeral sees the following form among the figure:
Reference numeral Implication Reference numeral Implication Reference numeral Implication
1 The insulation hot tap 2 The crystallizer body 3 Cooling water cavity
4 Air chamber 5 Lubricant reservoir 6 Oil-air-cushion film
The specific embodiment
The high-strength aluminum alloy round ingot by continuous oil gas lubrication foundry technology process is: at first alloying and melting: the technical pure aluminium ingot is placed on melting in the induction furnace; Be controlled at melt temperature in 700~730 ℃ of scopes after waiting to melt and add industrial-purity zinc, technical pure magnesium ingot; Stir melt and its temperature is increased to 710~750 ℃; In melt, add Al-50Cu, Al-10Zr, Al-10Mn, Al-5Cr intermediate alloy ingot respectively; After the stirring melt is gone to the resistance heated insulation and leave standstill stove, heat up and leave standstill; Melt treatment again: melt temperature is remained on 720~750 ℃, adopt no sodium refining agent and slagging agent that melt is carried out refining and slag hitting is handled, left standstill 10~20 minutes after pulling down the oxidizing slag of bath surface; In stove is left standstill in insulation, add Al-5Ti-B line pole pair melt and carry out thinning processing, and left standstill 20~50 minutes, casting is also prepared in the melt temperature insulation in 690 ℃~730 ℃ scopes; Last semi-continuous casting: the oil-air lubrication crystallizer is adopted in semi-continuous casting; As shown in Figure 1; The oil-air lubrication crystallizer comprises insulation hot tap 1, crystallizer body 2, cooling water cavity 3, air chamber 4, lubricant reservoir 5; At the set inside cooling water cavity 3 of crystallizer body 2, have air chamber 4 and lubricant reservoir 5 in the upper end of crystallizer body 2, insulation hot tap 1 is installed in the top of crystallizer body 2; At first open air chamber 4 and lubricant reservoir 5 before the casting beginning, and guarantee that air pressure range is 0.1~0.4MPa, the oil pressure scope is 0.2~0.8MPa, and the interior lubricating oil of the compressed air in the air chamber 4 and lubricant reservoir 5 was with 1: 2~1: 4 mixed; And in crystallizer, encircling formation skim oil-air-cushion film 6, liquid metal does not directly contact with the crystallizer inwall with ingot casting when making casting; Metal bath slowly flows into crystallizer during the casting beginning, and the dummy ingot base begins to descend with the speed of 10~30mm/min when crystallizer inner metal liquid position highly reaches 40~60mm, and the cooling water pressure in the cooling water cavity remains on 0.08~0.1MPa; Casting speed and cooling water pressure all raise gradually behind the casting 100mm, and casting gets into the stabilization sub stage; For φ 300mm ingot casting, the scope of casting speed is 40~100mm/min, and the scope of cooling water pressure normal value is 0.1~0.3MPa; When stable casting, keep the constant of oil pressure and air pressure and casting technological parameter.
Said method is suitable for the high-strength aluminum alloy round ingot semi-continuous casting, casts instance with φ 350mm 7050 high strength alumin ium alloys, embodiment of the present invention is done further describing:
1. alloying and melting: the technical pure aluminium ingot is put into the induction furnace melting; Waiting to melt afterwards, melt temperature is controlled in 700~730 ℃ of scopes and adds industrial-purity zinc, technical pure magnesium ingot; In order to improve the recovery rate of magnesium metal; Magnesium ingot adds and fashionablely is pressed into aluminum melt with bell jar, after waiting to melt bell jar is taken out.Melt is also stirred in heating, treats to add when melt temperature is increased to 710~750 ℃ once more Al-50Cu, Al-10Zr, Al-10Mn, Al-5Cr intermediate alloy ingot, leaves standstill behind the stirring melt.
2. melt treatment: insulation is left standstill and is adopted no sodium refining agent and slagging agent that alloy melt is carried out refining and slag hitting processing when melt temperature is increased to 720 ℃ in the stove, pulls down surperficial oxidizing slag, leaves standstill 10~15 minutes.
3. semi-continuous casting: began preceding 30 minutes in semi-continuous casting;, insulation adds Al-5Ti-B line bar in leaving standstill stove; Open oil sources and source of the gas simultaneously; In Φ 350mm continuous lubricating crystallizer, feed lubricating oil and compressed air continuously, wherein air pressure is controlled at 0.2MPa, and lubrication pressure is controlled at 0.3MPa and remains constant.Begin casting when melt temperature is reduced to 690 ℃~700 ℃ in leaving standstill stove, 7050 alloy melts slowly flow into crystallizer and contact with oil-air-cushion film, cast that casting speed is controlled at 30mm/min when initial, and cooling water pressure is controlled at 0.1MPa.The long back of casting 200mm casting speed is increased to 50mm/min gradually, and cooling water pressure is increased to 0.17MPa gradually and keeps constant, accomplishes until casting process.The 7050 high strength alumin ium alloys circle ingot casting that adopts this mode to prepare, the ingot casting flawless, and surface quality is more excellent.
In sum, the present invention applies continuous oil gas lubrication to the high strength alumin ium alloy ingot casting in casting process, in the surface quality that guarantees to have significantly improved when ingot casting is shaped smoothly ingot casting.
Below only be concrete exemplary applications of the present invention, protection scope of the present invention is not constituted any limitation.All employing equivalents or equivalence are replaced and the technical scheme of formation, all drop within the rights protection scope of the present invention.

Claims (3)

1. method for casting high-strength aluminum alloy round ingot by continuous oil gas lubrication is characterized in that: may further comprise the steps---
1) alloying and melting: the technical pure aluminium ingot is placed on melting in the induction furnace; Be controlled at melt temperature in 700~730 ℃ of scopes after waiting to melt and add industrial-purity zinc, technical pure magnesium ingot; Stir melt and its temperature is increased to 710~750 ℃; In melt, add Al-50Cu, Al-10Zr, Al-10Mn, Al-5Cr intermediate alloy ingot respectively, after the stirring melt is gone to the resistance heated insulation and leave standstill stove, heat up and leave standstill;
2) melt treatment: melt temperature is remained on 720~750 ℃, adopt no sodium refining agent and slagging agent that melt is carried out refining and slag hitting and handle, left standstill 10~20 minutes after pulling down the oxidizing slag of bath surface; In stove is left standstill in insulation, add Al-5Ti-B line pole pair melt and carry out thinning processing, and left standstill 20~50 minutes, casting is also prepared in the melt temperature insulation in 690 ℃~730 ℃ scopes;
3) semi-continuous casting: the oil-air lubrication crystallizer is adopted in semi-continuous casting; The oil-air lubrication crystallizer comprises insulation hot tap, crystallizer body, cooling water cavity, air chamber, lubricant reservoir; Set inside cooling water cavity at the crystallizer body; Have air chamber and lubricant reservoir in the upper end of crystallizer body, the insulation hot tap is installed in the top of crystallizer body; At first open air chamber and lubricant reservoir before the casting beginning; Compressed air air pressure in the air chamber is at 0.1~0.4MPa; Lubricating oil oil pressure in the lubricant reservoir is at 0.2~0.8MPa, and the compressed air in the air chamber and lubricating oil in the lubricant reservoir mix also ring formation skim oil-air-cushion film in the crystallizer; Metal bath slowly flows into crystallizer during the casting beginning, and the dummy ingot base begins to descend with the speed of 10~30mm/min when crystallizer inner metal liquid position highly reaches 40~60mm, and the cooling water pressure in the cooling water cavity remains on 0.08~0.1MPa; Casting speed was increased to 40~80mm/min gradually after ingot casting length reached 100~200mm, and casting gets into the stabilization sub stage; When stable casting, keep oil pressure and air pressure and casting technological parameter constant.
2. method for casting high-strength aluminum alloy round ingot by continuous oil gas lubrication according to claim 1 is characterized in that: the interior lubricating oil of the compressed air in the step 3) air chamber and lubricant reservoir was with 1: 2~1: 4 mixed.
3. method for casting high-strength aluminum alloy round ingot by continuous oil gas lubrication according to claim 1 is characterized in that: the step 3) casting gets into the stabilization sub stage, and for φ 300mm ingot casting, casting speed is at 40~100mm/min, and cooling water pressure is at 0.1~0.3MPa.
CN2008102441732A 2008-12-18 2008-12-18 Method for casting high-strength aluminum alloy round ingot by continuous oil gas lubrication Active CN101745610B (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
CN2008102441732A CN101745610B (en) 2008-12-18 2008-12-18 Method for casting high-strength aluminum alloy round ingot by continuous oil gas lubrication

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
CN2008102441732A CN101745610B (en) 2008-12-18 2008-12-18 Method for casting high-strength aluminum alloy round ingot by continuous oil gas lubrication

Publications (2)

Publication Number Publication Date
CN101745610A CN101745610A (en) 2010-06-23
CN101745610B true CN101745610B (en) 2012-05-30

Family

ID=42473591

Family Applications (1)

Application Number Title Priority Date Filing Date
CN2008102441732A Active CN101745610B (en) 2008-12-18 2008-12-18 Method for casting high-strength aluminum alloy round ingot by continuous oil gas lubrication

Country Status (1)

Country Link
CN (1) CN101745610B (en)

Families Citing this family (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2014015539A1 (en) * 2012-07-26 2014-01-30 北京科技大学 Method of recycling of waste easy-open aluminum cans
CN103639386A (en) * 2013-12-20 2014-03-19 西南铝业(集团)有限责任公司 Aluminium alloy casting method used for bogie axle box body of railway vehicle
CN103990773B (en) * 2014-06-03 2016-05-11 攀钢集团攀枝花钢钒有限公司 Control the method for the dry oil lubrication of conticaster
CN104404415B (en) * 2014-12-12 2017-02-22 西南铝业(集团)有限责任公司 Preparation process of aviation aluminum alloy ingot and aluminum alloy ingot
CN108213370B (en) * 2018-01-02 2019-10-22 丛林集团有限公司 A kind of aluminium alloy round cast ingot oil-air lubrication film casting method
CN109894586A (en) * 2019-04-15 2019-06-18 中国科学院金属研究所 A kind of preparation method of low surface segregation layer thickness aluminium alloy continuous casting ingot
CN113426968B (en) * 2021-05-20 2022-12-13 山东创新金属科技有限公司 Aluminum alloy round cast ingot oil-gas lubricating film casting device and method
CN114012052B (en) * 2021-12-30 2022-05-03 东北大学 Horizontal continuous casting equipment for aluminum alloy cast ingot

Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP3121197B2 (en) * 1993-04-15 2000-12-25 三星電子株式会社 Fuzzy control encoding method and apparatus
CN1414127A (en) * 2002-10-11 2003-04-30 中国铝业股份有限公司 Production technology of large diameter aluminium alloy round cast ingot
CN1857828A (en) * 2006-05-19 2006-11-08 苏州有色金属加工研究院 Continuously lubricating crystallizer for semi-continuous casting of aluminium and aluminium alloy
CN101050499A (en) * 2007-05-16 2007-10-10 东北轻合金有限责任公司 7055 aluminum alloy in high intensity, and high toughness, and preparation method
CN101168187A (en) * 2007-11-26 2008-04-30 苏州有色金属研究院有限公司 Low-temperature intermediate frequency electromagnetic field adjuvant casting method for hard aluminum flat section ingot
CN101168810A (en) * 2007-11-16 2008-04-30 苏州有色金属研究院有限公司 High-strength high-modulus aluminum-base composite material and preparation method thereof

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP3121197B2 (en) * 1993-04-15 2000-12-25 三星電子株式会社 Fuzzy control encoding method and apparatus
CN1414127A (en) * 2002-10-11 2003-04-30 中国铝业股份有限公司 Production technology of large diameter aluminium alloy round cast ingot
CN1857828A (en) * 2006-05-19 2006-11-08 苏州有色金属加工研究院 Continuously lubricating crystallizer for semi-continuous casting of aluminium and aluminium alloy
CN101050499A (en) * 2007-05-16 2007-10-10 东北轻合金有限责任公司 7055 aluminum alloy in high intensity, and high toughness, and preparation method
CN101168810A (en) * 2007-11-16 2008-04-30 苏州有色金属研究院有限公司 High-strength high-modulus aluminum-base composite material and preparation method thereof
CN101168187A (en) * 2007-11-26 2008-04-30 苏州有色金属研究院有限公司 Low-temperature intermediate frequency electromagnetic field adjuvant casting method for hard aluminum flat section ingot

Also Published As

Publication number Publication date
CN101745610A (en) 2010-06-23

Similar Documents

Publication Publication Date Title
CN101745610B (en) Method for casting high-strength aluminum alloy round ingot by continuous oil gas lubrication
Maleki et al. Effects of squeeze casting parameters on density, macrostructure and hardness of LM13 alloy
CN106735003B (en) A kind of non-vacuum melting horizontal casting production technology of high-strength highly-conductive Cu-Cr-Zr alloy bar materials
CN103691909B (en) A kind of aluminium/magnesium solid-liquid composite casting forming method
CN103740957B (en) A kind of casting method of sacrificial aluminium alloy anode
CN105331909B (en) The still heat treatment method of semisolid Al-Si alloy rheo-diecasting part
CN114351017B (en) Casting method and application of high-toughness high-heat-conductivity aluminum alloy ingot
CN104550805A (en) Device and method for preparing magnesium alloy ingot blank for deformation
CN100575513C (en) A kind of heat resistance magnesium alloy and preparation method thereof
CN108405821A (en) The casting device and method of the big specification magnesium alloy slab ingot of flawless
CN105014020A (en) Device and method for preparing large-diameter semi-solid alloy billet
CN101173337B (en) Method for producing creep resistant magnesium alloy
CN117245064A (en) Process and equipment for controlling crystallization structure of integral casting roller
CN104264015A (en) Manufacturing method of high-strength antirust aluminum alloy slab ingot
CN100560251C (en) A kind of copper and method for continuously casting copper alloy
CN108188369A (en) A kind of semi-solid rheological molding method and device
CN104476128A (en) Method of manufacturing high-temperature alloy pipe blanks
CN1301166C (en) Preparation method of high speed steel blank and its equipment
CN108543921A (en) Strong shear is with electromagnetic field coordinate system for the device and method of big specification homogeneous ingot casting
CN101347833A (en) Sine waveform device for preparing semi-solid metal slurry
CN113322402A (en) High-strength corrosion-resistant wrought magnesium alloy
CN109022847B (en) Composite preparation method of high-performance rare earth magnesium alloy
CN101138782A (en) Hollow round ingot casting hot-top casting device and method for casting hollow tube thereof
CN105665669B (en) The preparation facilities and method of a kind of aluminium alloy compound pipe
CN102806329A (en) Continuous blank casting system capable of performing semi-solid processing on non-ferrous alloy

Legal Events

Date Code Title Description
C06 Publication
PB01 Publication
C10 Entry into substantive examination
SE01 Entry into force of request for substantive examination
C14 Grant of patent or utility model
GR01 Patent grant
TR01 Transfer of patent right
TR01 Transfer of patent right

Effective date of registration: 20171012

Address after: 471000 Henan Province, Luoyang city high tech Development Zone middle Ling Road

Patentee after: China Nonferrous Metals Processing Technology Co., Ltd.

Address before: Suzhou City, Jiangsu province 215021 Industrial Park No. 200 Shen Hu Road

Patentee before: Suzhou Non-ferrous Metal academy Co., Ltd.