ÜRKMEZ SAC DEMİR
The Siemens-Martin method has been used for many years in the production of steels used in ships. Oxidation and smelting as may be required are carried out in the vessel-shaped chamber of the Siemens-Martin furnace. Very high temperatures can be reached with the regenerative combustion system. In this system, the preheated air and flammable gas are blown through separate channels by using the heat of the flue gases, creating a flame on the furnace charge. The oxidation time is considerably longer than other methods. The most important event is the conversion of carbon into CO gas. Thanks to the rising gas bubbles, that is, the boiling event, the bath is thoroughly mixed. The oxidation reaction becomes homogeneous in the entire melt.
Scrap is used with solid or liquid pig iron as raw material. The carbon in pig iron reduces the iron oxides (rust, canvas) contained in the scrap. Since the heat is given from the outside, the amount of scrap in the mixture can be kept high. The quality of the steel also directly depends on the amount of scrap melted. Therefore, rolling mill residues or other high-quality scraps can be utilized in high-quality steel manufacturing.
The reaction ability of the slag, which is heated from the top and hot, is very high. Therefore, very low phosphorus (0.02%), sulfur (0.03%) and nitrogen ratios can be obtained in basic quarries. Alloying elements in scrap can be easily dissolved in liquid metal. The long oxidation time allows the composition to be provided very precisely. There is no freezing of the melt due to external heating. Alloying elements can also be added after oxidation.
In this method, which is newer than the Siemens-Martin method, oxygen is used to increase the reaction rate.
Oxidation is carried out in convectors with a basic lining. Therefore, it is possible to produce steel from pig iron, which is rich in phosphorus. The required air is blown onto the liquid peak from the bottom of the converter, which contains several channels. In this method, not only silicon and manganese, but also carbon is burned before phosphorus, so the desired amount of carbon can be achieved by adding ferro manganese or spiegel after oxidation.
The amounts of phosphorus and nitrogen in Thomas steel are quite high. The production of “Thomas” steel has been largely discontinued in Germany, as phosphorus and nitrogen cause embrittlement. Extreme care must be taken when modifying or repairing systems using Thomas steel. In Thomas steel, in connection with the mechanical properties, the weldability is also quite bad.
Phosphorus burns before carbon, so the amount of phosphorus is much reduced. Up to 25% scrap can be added to the bathroom. Another economic advantage is the freedom in the choice of the peak type. By oxygen blowing method, high quality, low-solid, weldable steels are produced economically. This method replaces the Siemens-Martin method today.
The energy required in this method is provided by arc or induction. For the production of high alloy steels;
The oxygen activity in the stove should be kept as low as possible. Otherwise, the alloying element will also burn. High furnace temperatures must be achievable. Thus, alloying elements enter the bath more easily.
These conditions can only be achieved by the method of electric melting. There is no problem of increased solids due to fuel. The required oxidation process is done with iron ore or oxygen. In high-strength and high-alloy steels, the amount of dissolved gas should be very small, especially in terms of sufficient toughness. For this purpose, hydrogen and nitrogen are much more difficult to remove than oxygen.
In the melting electrode arc method, the steel electrode, which is continuously moved towards the metal bath, is melted under vacuum with the electric arc it creates. Reaction products in gaseous or liquid form leave steel more easily in this environment. Sulfur and phosphorus, on the other hand, are chemically removed. The steels obtained by this method have very good resistance to dynamic stresses and notch striking toughness.
Most of the steel used in shipbuilding is offered to the market as rolled. However, normalization heat treatment is required when materials with high notch strength or high-strength steels are desired for critical areas of the ship.
The thinning of the internal structure is achieved by the contribution of aluminum or other structure thinning elements in proportions of steel measurements.
Completely killed, fine-grained normalized carbon steel can be obtained in accordance with a controlled chemical combination, and steels with high notch impact resistance can be obtained if desired.
Harder and higher strength steels can be obtained by quenching and tempering processes, but these processes are used for low-alloy or unalloyed steels.
Sheets are obtained by three types of processes, these are edge-cut plates, flat rolling plates, or continuous strip rolling plates.
Edge cut sheets have the feature of rolling in both directions. The process of rolling in both directions is called cross rolling. Finishing processes include cooling, trimming, paving, stamping, cutting and checking. Steels made in this type generally have very good notch strength in the longitudinal and transverse directions.
Flat rolled sheets have low elongation in the transverse direction because they are not rolled in both directions. In such steel producing factories, there are two mills, one in the horizontal and the other in the vertical direction. The vertical rolling roller achieves the transverse dimension of the plate without side cutting.
The continuous strip rolling process is mostly used in the production of thin and wide sheets. Occasionally, billets undergo rolling in both directions to a certain degree before undergoing continuous strip rolling. In this case, the qualities of the plates in both directions are between those of steels made by edge-cutting and continuous strip-rolling operations.
The selection of high-strength steels in terms of the desired mechanical properties and weldability should be done with great care. The use of high-strength steel sheets on ships can be divided into two groups:
- High-strength carbon steels with a yield limit of 345 MPA = 50,000 psi = 35.2 kg/mm²
- Rapidly cooled and tempered steels by low-alloy irrigation with yield limit up to 690 MPA =100,000 psi =70.4 kg/mm²
High-strength steels are generally used when:
- Reducing the weight of the steel hull of the ship
- Avoid the use of too thick sheets in areas where high stresses occur
Low alloy nickel steels are used where there are high stresses because they have superior notch strength at low temperatures, especially in places where temperatures below –57 degrees Celsius are required.
The notch hardness or crash resistance of the material is defined as the energy it absorbs in plastic deformation under load. This property of the material increases its resistance to sudden cracking during metallurgical or mechanical cracks or the formation of notches. The material is defined as brittle if little or no energy has been ingested as plastic before cracking and the rupture is of the slit type. Since this property of steel is highly dependent on temperature, the conversion temperature has been a criterion for notch hardness.
In reality, the transformation occurs in a narrow temperature zone. A notch impact test is applied to the part to measure the notch hardness or strength.
From a metallurgical point of view, thicker plates for a steel of the same group or cast are more susceptible to the notching effect than thin plates. This is due to changes in finishing temperatures during rolling in more steel mills. Increasing the plate thickness from 12.5 mm to 38 mm increases the impact conversion temperature by 10 to 20 degrees in some cases.
In certain places, this thickness effect can be eliminated by using steel with greater notch hardness for thicker plates. Despite everything, it is necessary to be very careful in the use of thick plates, regardless of their location.
When the plates are rolled, the internal structure stretches in the direction of rolling. As a result, the notch striking values of the longitudinal test pieces are found to be higher than those of the transverse test pieces. Therefore, when winding the coating sheets of the ships, care should be taken to ensure that the plates are longitudinal in the bow and stern directions. Thus, the stresses that will come in the direction of the longitudinal rolling are acted upon.
Throughout the thickness, that is, in the direction perpendicular to the plate surface, the notch hardness of the boards is less. In addition, the tensile strength and flexibility of the board decreases in the direction of thickness. For this reason, designs that do not require high breaking stresses to be carried in the direction of the thickness of the plates should be used.
It causes different damages to the material in cold forming. First of all, as a result of fluttering or overshaping, defects such as cracks occur on the surface of the material. Secondly, a decrease in notch strength and hardness is observed in extremely cold-formed parts. In some steels, an adverse effect may occur as a result of the continuity of stresses, which is accelerated by moderate heating.
In cold forming, the decrease in notch hardness occurs when the steel is stretched by more than approximately 3%. Rolling of cardinal cast sheets or siyer-stringer cast sheets does not cause any problems. However, rolling thick sheets into small radii affects the notch hardness.
Although strain aging of hull steel is generally a rare event, the necessary precautions should be taken as “Bessemer” steel should not be used for any structural element that needs cold forming, steel loses more of its flexibility in aging as a result of strain. Smoothing the edges cut in the scissors, especially on thick plates, eliminates the possibility of any cracking starting from them.
There is little evidence that fatigue was an important element in the structure of the ship. It is known that the fatigue properties of high-strength steels are not more than those of normal-strength steels. For this reason, there is no significant increase in fatigue strength due to the increase in yield strength. Some designs that use high-strength steels are sometimes not very advanced when their yield points are compared. Therefore, when high-strength steels are used, the design of some critical details should be given due importance.
The American Bureau of Shipping (ABS) and the American Society for Testing and Materials (ASTM) have issued similar rules for marine steels. In our country, these standards are determined by Türk Loydu.
Note:
- Sheets thicker than 51 mm must be made according to specially approved specifications.
- There may be situations where special materials need to be used.
- Vessel half-length bottom sheet up to 51 mm thickness is acceptable.
- If a double base has been applied in accordance with the rules, a maximum of 19 mm is acceptable for bilge acre sheet.
The yield limit is between 345 and 485 and notch requests have also been determined. This specification also covers the type of normalized steel similar to ASTM A537. Fast-cooled HY80 and HY100 steels through low-alloy irrigation are given in MIL-S–16216. ABS and the U.S. Coastal Quard organization have issued rules for the transportation of liquefied gases such as liquefied petroleum gas (LPG) and liquefied natural gas (LNG) at low temperatures. In general, there are four types of service temperature ranges for steel sheets to be used in primary load tanks.
- For service temperatures above 0 ºC, normal strength sheets of type B and D of ABS can be used as rolled.
- For service temperatures between 0 ºC and -18 ºC, ABS type E and CS steel sheets can be used.
- Steel to be used for service temperatures between -18 ºC and -57 ºC must comply with the requirements of the 5.5 ºC notch impact test at degrees below operating temperature.
- For temperatures between -57 ºC and -196 ºC, it is desirable to use austenitic stainless steels, nickel alloy steels or aluminum alloy steels.
Depending on the operating temperatures, the nickel content in nickel alloy steels can vary between 2.25-9.0%. A203 types with a nickel percentage of ASTM of 2.25 can be used down to –62 ºC. Types A353 and A553 with a nickel percentage of 9.0 can be used down to -196 ºC. The notch properties of the base material, the weld metal and the heat transfer zone must comply with the specified requirements.
At service temperatures lower than -18 ºC, impact notch tests are required for the weld metal, melting line, heat transfer zone in each welding method. Since there are lower service temperatures in the second-degree tanks built separately from the hull structure in the areas near these tanks and in the first-degree tanks in the hull structure, it is desirable to use the type of steel with superior notch hardness compared to normal boat steel.
The designer must select materials that will withstand cracking under normal service conditions. For example, where there are different discontinuities and where there are high stresses, steels with greater notch hardness should be used. Likewise, the notch hardness of the materials that will work in places exposed to lower temperatures than normal should be quite high.
Although the selection of materials is largely limited by the rules of classification societies (Türk Loydu classification rules are valid in our country), it should be considered that there are many different types of steel to be used for structural work. Some types make the result even more pronounced by using methods such as controlled temperature rolling, structure thinning or normalization.
Increasingly, high-strength steels are used in the outer hull, decks, bow and stern masts and cargo rigging masts of merchant ships. Different types of these steels are available in a wide range of areas as they are rolled, normalized, or rapidly cooled and tempered by irrigation.
In addition to the tensile strength properties, the notch hardness, fatigue and weldability properties must be predetermined for a specific application in the selection of steel. Of course, the latest approval must be obtained from the classification societies. Sometimes, these organizations may require a chemical analysis of the type of steel being used, tests to demonstrate the suitability of the design and material (including the weld metal), or both.
In some cases, the selection of materials depends on the availability of special welding methods and qualified welders. The use of high-strength steels or normal-strength steels is highly dependent on the availability of vehicles, materials and workmanship. In order to make repairs that may arise in case of any damage, it is desirable to have pictures on the boat showing where high-strength steels are used, which are quickly cooled and tempered by irrigation. Along with these pictures, the methods used in the construction of the material and the recommended repair methods should also be given.
The following methods are applied to adjust the strength, toughness properties and grain sizes of steel parts used for shipbuilding.
Increasingly, high-strength steels are used in the outer hull, decks, bow and stern masts and cargo rigging masts of merchant ships. Different types of these steels are offered in a wide range of areas as they are rolled, normalized or rapidly cooled and tempered by irrigation.
In addition to the tensile strength properties, the notch hardness, fatigue and weldability properties must be predetermined for a specific application in the selection of steel. Of course, the latest approval must be obtained from the classification societies. Sometimes, these organizations may require a chemical analysis of the type of steel being used, tests to demonstrate the suitability of the design and material (including the weld metal), or both.
In some cases, the selection of materials depends on the availability of special welding methods and qualified welders. The use of high-strength steels or normal-strength steels is highly dependent on the availability of vehicles, materials and workmanship. In order to make repairs that may arise in case of any damage, it is desirable to have pictures on the boat showing where high-strength steels are used, which are quickly cooled and tempered by irrigation. Along with these pictures, the methods used in the construction of the material and the recommended repair methods should also be given.