Abans de respondre aixògalvanització question, let's have a basic understanding the composition of the steel. First of all, the predominant element is iron, and it's soft and ductile. By adding around 0.5 percent carbon to iron, it can dramatically increase tensile strength of the iron. In addition to carbon, manganese also increases hot working properties and increases hardenability and toughness of steel. In general, alloy steels have different material properties which lead to different mechanical performance. Oxygen reacts with and consumes the carbon, to solve this problem, silicon is added to iron as a deoxidizing agent. With excessive quantity of silicon, it can create rough galvanization coating.
El fòsfor està permès en un nivell màxim de fins al {{0}},04 per cent, si és superior al 0,04 per cent, pot augmentar la tendència de l'acer a trencar-se durant la soldadura i crear recobriments galvanitzats rugosos. Tant el fòsfor com el silici poden actuar de manera sinèrgica per provocar un creixement ràpid de les capes d'aliatge de zinc-ferro en recobriments galvanitzats perquè actuen com a catalitzadors durant el procés de galvanització. La foto següent és un exemple típic de superfície de galvanització rugosa causada per un contingut excessiu de silici.
Per aconseguir una superfície de galvanització sòlida, segons ASTM A385 o ISO 1461, aquests elements importants s'han de mantenir al nivell que s'indica a continuació:
√ Ccontingut en arbon inferior al 0,25 per cent
√ Manganès inferior a l'1,3 per cent,
√ Silicon menys de {{0}}.04 per cent o entre 0,15 per cent i 0,24 per cent
√ Phòsfor inferior al 0,04 per cent .
To visualize the importance of these elements, you can see the chart below how they affect hot dip galvanization (HDG) quality.
Composició elemental recomanada per a la galvanització per immersió en calent segons ASTM A385 3.2 | ||
| Element | Percentatge recomanat per a HDG | Notes |
| Si | < 0.04="" %="" or="" 0.15%="" -=""> | Els acers Sandelin i els acers alts en Si poden produir recobriments gruixuts, mats i/o rugosos |
| P | <> | P >0,04% produeix recobriments gruixuts i rugosos susceptibles de delaminació |
| Si equivalent | < 0.04="" %="" or="" 0.15%="" -=""> | Si Equivalent 0.04% - 0.15% or >0.22 per cent pot produir recobriments gruixuts, mats i/o rugosos |
| C | <> | Check ultimate tensile strength for steels >1 per cent C |
| Mn | <> | Un alt Mn pot produir recobriments de color marronós i trencadissos |
Therefore, it is very import to evaluate the chemistry for an unfamiliar steel grade, the elemental composition of the steel in weight percentage should be estimated from the mill test reports for the heat. Where mill reports (or accurate mill reports) cannot be obtained, it is possible to locate a range for each element in the steel by looking up the chemical requirements tables within the steel grade specification or by acquiring ranges from the steel supplier. The elements listed in ASTM A385 should be compared according to the recommendations in order to achieve a coating which is of typical appearance and thickness. In particular, steels with silicon and phosphorus levels which are beyond the recommended levels are considered reactive steels. The silicon equivalent is used to evaluate the combined effect silicon and phosphorus have on the metallurgical reaction during hot-dip galvanizing. After the silicon equivalent has been calculated, look up the value on the Sandelin Curve to predict the reactivity of the steel.
Nota: Equivalent de silici=contingut Si més 2,5 (contingut P)
A continuació es mostra l'impacte sobre HDG per Silicon Equivalent
Equivalent de silici<0.04% or="" 0.15%="" -="" 0.22%="" -="" coating="" is="" of="" typical="" thickness="" and="">
L'equivalent de silici 0,04% - 0,15% (acer Sandelin) - varien en aspecte i desenvolupen recobriments més gruixuts. Si s'utilitza un bany de zinc aliat de níquel, l'efecte Sandelin es mitiga, donant lloc a una major probabilitat de recobriments brillants de gruix de recobriment típic.
Silicon equivalent >0,22 per cent - recobriments grisos mats i rugosos d'un gruix superior als requisits mínims. Vegeu la corba de Sandelin per determinar el nivell de reactivitat i el gruix de recobriment previst.
La qualitat i l'acabat superficial dels acers al carboni galvanitzats en calent estan directament relacionats amb la química dels components que s'estan processant. Fonamentalment, dos elements, el Silici (Si) i el Fòsfor (P) dins de l'acer al carboni, influeixen en l'acabat superficial en termes d'uniformitat, color (gris mat brillant o mat) i suavitat relativa.
Carbon steels fall into two broad categories,
√ Aluminum killed steels, < 0.03%="" si,="" 0.01="" to="" 0.03%,="" phosphorous="" (p)="0.015%" maximum="" (50="" to="" 65µm,="" more="" flexible="" and="" "short="" term="" shiny="">
√ Silicon killed steels, >{{0}}.03 per cent de Si, 0,15 a 0,25 per cent i fòsfor (P)<0.02% maximum="" (normally="">120µm with a dull grey surface finish, but brittle when over 200µm)
The term "killed steels" relates to degassing and de-oxidizing of carbon steels during their manufacturing process. In the steel making process aluminum or silicon is used to remove oxygen and contaminants from the steel. This is referred to as "aluminum killed" or "silicon killed" steel. The effect is that aluminum killed steel will have low amounts of silicon and more aluminum. With silicon killed steel the reverse applies. Aluminum killed (Si range of 0.008 In the steel making process aluminum or silicon is used to remove oxygen and contaminants from the steel. This is referred to as "aluminum killed" or "silicon killed" steel. The effect is that aluminum killed steel will have low amounts of silicon and more aluminum. With silicon killed steel the reverse applies. Aluminum killed (Si range of 0.008% to 0.03%) and is less reactive when immersed in molten zinc. Silicon killed steel, Si >0,03 per cent és més reactiu i, per tant, s'hauria de controlar mitjançant l'especificació d'acer amb un rang de Si especificat. L'alumini en acer té poc efecte sobre la reactivitat amb el zinc fos. El fòsfor en la química de l'acer té un efecte addicional sobre la seva reactivitat dins del zinc fos. Per controlar els gruixos del recobriment i l'acabat superficial, s'ha d'especificar el fòsfor<0.02% and="" if="" possible=""><0.01%. phosphorous="" content="" above="" 0.02%="" will="" cause="" coatings="" to="" be="" excessively="" thick="" (=""> 200µm), brittle and subject to chipping and / or flaking due to mechanical damage. A steel chemistry analysis certificate should be obtained from steel suppliers, particularly when large quantities of material are to be hot dip galvanized. % to 0.03%) and is less reactive when immersed in molten zinc. Silicon killed steel, Si >0,03 per cent és més reactiu i, per tant, s'hauria de controlar mitjançant l'especificació d'acer amb un rang de Si especificat. L'alumini en acer té poc efecte sobre la reactivitat amb el zinc fos. Però, el fòsfor en la química de l'acer té un efecte addicional sobre la seva reactivitat dins del zinc fos. Per controlar els gruixos del recobriment i l'acabat superficial, s'ha d'especificar el fòsfor<0.02% and="" if="" possible=""><0.01%. phosphorous="" content="" above="" 0.02%="" will="" cause="" coatings="" to="" be="" excessively="" thick="" (=""> 200µm), brittle and subject to chipping and / or flaking due to mechanical damage. A steel chemistry analysis certificate should be obtained from steel suppliers, particularly when large quantities of material are to be hot dip galvanized.
Research studies have shown that cold and hot rolled steel can behave somewhat differently in hot dip galvanizing, which is discussed below. When the finish requirements on the galvanized surface are "normal", an aluminum killed steel with silicon plus phosphorus content < 0.03="" %="" is="" adequate="" and="" gives="" acceptable="" surface="" finish="" on="" both="" cold="" and="" hot="" rolled="" steel.="" if="" the="" appearance="" of="" the="" galvanized="" surface="" is="" of="" high="" importance,="" for="" example="" in="" architectural="" applications,="" the="" following="" expression="" shall="" be="" used="" for="" cold="" rolled="" steel:="" si="">< 0.03="" and="" si="" +="" 2.5="" x="" p="">< 0.04="" weight="">
Per a l'acer laminat en calent, el contingut de silici és encara més crític, però el contingut de fòsfor és de menys importància, i es podria utilitzar la següent expressió: Si< 0.02="" and="" si="" +="" 2.5="" x="" p="">< 0.09="" weight="">
The influence of silicon and phosphorus content when galvanizing cold or hot rolled steel is expressed below.
La influència del silici i el fòsfor en la galvanització de l'acer laminat en fred | ||
| Silicon plus Phosphrous | Acer laminat en fred | Acer laminat en calent |
| Si més P<> | Acceptable surface finish in most cases. Shiny coating. If the appearance of the galvanized surface is very important, the following expression should be used | Acceptable surface finish in most cases. Shiny coating. If the appearance of the galvanized surface is very important, the following expression should be used |
| Si< 0,03="" and="" si+2,5p=""><> | Si< 0,02="" and="" si+2,5p=""><> | |
| 0,03 <> | No és adequat | No és adequat |
| {{0}},15 Less than or equal to Si Less than or equal to 0,21 | Thicker coatings than in standard. | Recobriments més gruixuts que els estàndards. |
| L'oxidació interna pot canviar la reactivitat. | ||
| {{0}},22 Less than or equal to Si Less than or equal to 0,28 | Significant thicker coatings than in standard. | Significant thicker coatings than in standard. |
| Aspecte gris. | Aspecte gris. | |
| {{0}},29 Less than or equal to Si Less than or equal to 0,35 | Thick coatings that may be brittle. Grey appearance. | Thick coatings that may be brittle. Grey appearance. |
What if silicon content is not in line with the above suggested range, and what could happen to the galvanization? The answer lies in the Sandelin Curve The conclusion is rather clear, and slope is very steep when the silicon is around {{0}}.05 percent , and this level of silicon content usually lead to very thick zinc coating thickness with dull grey appearance. Based upon Sandelin Curve, the recommended limits for silicon is either below 0.04 percent or between 0.15 percent and 0.24 percent .
In reality, many galvanizers don't really care that much about chemical composition because they have limited knowledge about Sandelin Curve, and therefore, they primarily use dipping time to control the galvanization thickness. Aquesta pràctica incorrecta normalment condueix a moltesdefectes de galvanització, such as rough surface, thin galvanization coating which is not in line with ISO 1461 requirement.
El contingut de silici adequat pot tenir un gruix de recobriment molt sòlid i de gruix, i pot proporcionar una bona protecció contra la corrosió i, per tant, en alta mar,agricultura, aqüicultura, telecomunicació, mineria i túnel projects require very accurate control on silicon content.
De fet, un certificat de fàbrica d'acer ajudarà els galvanitzadors a filtrar el material no qualificatgalvanització en calent, and it will save costs for both galvanizers and the steel fabricators.
