The iron-carbon alloy phase diagram is a diagram describing the relationship between different compositions, temperatures, and structures of iron-carbon alloys under extremely slow heating or extremely slow cooling conditions. Its premise is:
①The composition of iron and carbon;
② is the process of organization formation in an extremely slow transition process;
③Fe3C is actually a metastable phase, which can be decomposed into iron solid solution and graphite under certain conditions, so the iron-carbon phase diagram actually contains a dual phase diagram of the iron-graphite phase diagram;
④ Iron-carbon alloys with more than 6.69% are not included in the research scope because of their brittleness and no use value.
In the heat treatment process, the actual use of ABN in the upper left corner of the phase diagram is of little significance, so the organization can be ignored. However, the GPQ phase zone has practical meaning and need not be ignored.
There are mainly 6 characteristic lines in the phase diagram, which are as follows.
ACD line: the liquidus line of iron-carbon alloy;
AECF line: solid phase line of iron-carbon alloy;
GS line: the transition line where austenite precipitates to ferrite during cooling;
ES line: the solubility line of carbon in austenite;
ECF horizontal line: eutectic transition line, L4.3→A0.77+Fe3C, transition temperature 1148℃;
PSK horizontal line: eutectoid transformation line, A0.77→Fe0.0218+Fe3C, transformation temperature 727℃.
【1】
What is the main purpose of hot-dip galvanized sheet?
Answer: Hot-dip galvanized sheets are mainly used in construction, home appliances, automobiles, machinery, electronics, light industry and other industries
【2】
What types of galvanizing methods are currently available in the world?
Answer: There are three types of methods: electro-galvanizing, hot-dip galvanizing and galvanizing.
【3】
Which two types of hot-dip galvanizing can be divided into different annealing methods?
Answer: It can be divided into two types: in-line annealing and out-of-line annealing, which are also called shielding gas method and flux method respectively.
[4] What are the commonly used steel grades for
hot-dip galvanized sheets?
Answer: Product categories: General commodity rolls (CQ), structural galvanized sheets (HSLA), deep drawing hot-dip galvanized sheets (DDQ), bake hardening hot-dip galvanized sheets (BH), dual-phase steel (DP), TRIP steel (transformation induced plasticity steel), etc.
[5] What are the types of galvanizing annealing furnace?
Answer: There are three types: vertical annealing furnace, horizontal annealing furnace and vertical and horizontal annealing furnace.
[6] How many cooling methods are usually used for cooling towers?
Answer: There are two types: air-cooled and water-cooled.
[7] What are the main defects of hot-dip galvanizing?
Answer: Mainly include: shedding, scratches, passivation spots, zinc particles, thick edges, air knife streaks, air knife scratches, exposed steel, inclusions, mechanical damage, poor performance of the steel base, wave edges, buckling, size Inconsistent, embossing, zinc layer thickness inconsistency, roll printing, etc.
[8] Known: The production specification is not 0.75×1050mm, and the coil weight is 5 tons. What is the length of the coil? (The specific gravity of galvanized sheet is 7.6g/cm3)
Solution: L=G/(h×b×p)=(5×1000)/(0.75×1.050×7.6)=834.42m
Answer: The strip length of the coil is 834.42m.
[9] What are the main reasons for the zinc layer to fall off?
Answer: The main reasons for the zinc layer to fall off are: surface oxidation, silicon compounds, too dirty cold rolled emulsion, too high NOF oxidizing atmosphere and protective gas dew point, unreasonable air-fuel ratio, low hydrogen flow, oxygen infiltration in the furnace, The temperature of the strip steel entering the pot is low, the furnace pressure in the RWP section is low and the furnace door suction, the furnace temperature in the NOF section is low, the grease does not evaporate, the aluminum content of the zinc pot is low, the unit speed is too fast, the reduction is insufficient, and the zinc liquid The medium residence time is too short and the coating is too thick. [10] What are the causes of white rust and dark spots?
Answer: Dark spots are formed by further oxidation of white rust. The main causes of white rust are:
(1) Poor passivation, insufficient or uneven passivation film thickness;
(2) No oil on the surface or residual moisture on the surface of the strip;
(3) The surface of the strip steel contains moisture during coiling;
(4) The passivation is not completely dried;
(5) Moisture or rain during transportation or storage;
(6) The finished product has been stored for too long;
(7) The galvanized sheet is in contact with or stored together with other corrosive media such as acid and alkali.
[11] How long can the finished product be stored in the warehouse? why?
Answer: It is allowed to store for three months to avoid oxidation due to long storage.
[12] What is the length tolerance of the galvanized steel sheet?
Answer: The length tolerance is not allowed to have negative values, and the maximum is not allowed to exceed +6mm.
[13] What is a big spangle? What is a small spangle?
Answer: Large spangles are normal spangles. Large spangles can be produced according to the normal production process. The crystal nucleus diameter is not less than 0.2mm; the crystal nucleus diameter is less than 0.2mm called small spangles. This kind of spangles is generally unusable Identify with the naked eye.
[14] How to protect the hot-dip galvanized layer?
Answer: Because zinc can form a film with good corrosion resistance on the surface in a corrosive environment. It not only protects the zinc layer itself, but also protects the steel base.
[15] What is the principle of passivation of hot-dip galvanized sheet?
Answer: Chromate passivation treatment on galvanized sheet can form a passivation film, the chemical reaction formula is as follows: Zn+H2GrO4-ZnGrO2=H2
The trivalent chromium in the solution passivation group is difficult to dissolve in water, and its chemical properties are inactive, which acts as a skeleton. The hexavalent chromium is easily soluble in water and can play a re-passivation effect when the passivation film is scratched. It has the healing effect of passive film. Therefore, within a certain limit, the passivation film can prevent steam or humid air from directly corroding the galvanized sheet, and the Dun zinc layer plays a protective role.
[16] What are the methods for testing the corrosion resistance of hot-dip galvanized sheets?
Answer: There are three methods to test the corrosion resistance of hot-dip galvanized sheets:
(1) Salt spray test; (2) Humidity test; (3) Corrosion test.
【17】Why do hot-dip galvanized products need anti-rust treatment?
Answer: When the hot-dip galvanized sheet is in humid air, especially when the air contains acidic substances such as SiO2, CO2, NO2, and NO, the surface of the galvanized layer will quickly form loose white rust. The main components of white rust are ZnO and Zn(OH)2. This white rust not only affects the appearance, but also brings great difficulties to future use.
[18] What are the causes of white rust?
Answer: The causes of white rust are:
(1) Hot-dip galvanizing is put together with other corrosive media such as acid, alkali and salt;
(2) The passivation film or the anti-film is destroyed;
(3) The passivation or oiling effect is not good;
(4) The storage warehouse is not well ventilated and humid;
(5) The galvanized sheet is sprayed with water during transportation;
(6) Transport at low temperature and store at high temperature to form condensate.
[19] How to calculate the thickness of the coating?
Answer: The coating thickness formula is: Dz = Gz (Sz*dz) where: Dz―――coating thickness mm, Gz――coating weight per square meter, g,
Sz―――coating area mm2, dz―――zinc specific gravity
The above formula can be simplified to: Dz=0.000139Gz
[20] What are the causes of zinc particles?
Answer: (1) The problem of the original board is mainly the cleanliness of the original board and the iron powder content of the original board;
(2) The temperature of the zinc liquid is too high, causing the bottom dross to float;
(3) Too high Al content affects the solubility of iron.
【twenty one】
What is the meaning of hot-dip galvanized sheet finishing?
Answer: With the continuous expansion of the use of hot-dip galvanized sheets, in the modern continuous hot-dip galvanizing unit for strip steel, the following goals can be achieved by smoothing the strip:
(1) Improve the straightness and flatness of the sheet, and at the same time, the zinc particles on the surface can be flattened to make the surface of the strip smooth, which is particularly advantageous for the subsequent deep drawing and other occasions with higher precision.
(2) The smoothing roller used is pre-shot peened, so the smoothed plated surface has a certain roughness. It can improve the adhesion of the coating and can store a certain amount of grease, which is beneficial to the lubrication of the die during deep drawing processing;
(3) For the plated plate that will be painted later, although the surface is controlled to be small spangles, the small spangles can still be exposed through the paint layer. Therefore, for some high-demand galvanized sheets, the surface of the small spangles needs to be smoothed. In this way, the galvanized sheet can obtain a more uniform silver-white appearance.
(4) Through smoothing, the lower yield point can be lowered, and the yielding platform disappears or is less obvious, which can prevent slip lines from appearing during subsequent drawing or deep drawing processing, and improve the possibility of deep drawing. The editor of China Steel Pipe Information Port learned
