CABINETS FOR HOUSEHOLD APPLIANCES HAVING HIGH ENDURANCE AND REDUCED THICKNESS

Description

Field of the Invention

[0001] The present invention relates in general to the cabinets for household appliances produced from steel, and more particularly from the high strength low alloy steels (HSLA) with a cold rolling process that imparts high endurance with reduced weight and thickness to the resulting cabinet. Thereby, the invention in this sense makes it possible provide a new application area for the steels of HSLA class, and this can considerably reduce the costs in the sector of the household appliances.

Background of the Invention

[0002] There are various types of steels having a variety of chemical compositions and mechanical properties. Some of them, especially the conventional stainless steels, such as those having the standards of EN10130 (Cold Rolled Low Carbon Steel Flat Products for Cold Forming-Technical Delivery Conditions) and EN10209 (Cold Rolled Low Carbon Steel Flat Products for Vitreous Enameling Technical Delivery Conditions) have been traditionally used in manufacturing of the different components of the household appliances. The aforesaid commercial grade steels, however mostly fail to provide the sufficient endurance within a low thickness range below 0.4 mm. Therefore, there are the continuous attempts in household appliance industry for lightening overall weight of the cabinets while keeping the mechanical endurance and manufacturing properties in an acceptable level.

[0003] In household appliances such as refrigerators, dish washers and washing machines, it is particularly important for the planar sheet materials used in manufacturing of cabinets to obtain the desired radius of bending angles, reduced counter effect after compressing polyurethane, high durability in freefall tests and resistance to twisting in steel sheets. These sheets after the production and reformation steps need to be allowing a bending angle of at least 90° and should not be exhibiting the effect of spring back. On the other hand, the isolation material such as polyurethane which is filled in the lateral spaces of the cabinets creates a coercive field forcing the metal sheet outwardly to bend with a wavy shape. Yield strength and tensile strength are the additional requirements that need to be met by the steel used in manufacturing of the household appliances. Therefore, there is a continuous need in the household appliance industry to provide new materials with improved properties and lower cost.

[0004] There are different kinds of steels having various qualities with improved properties used in different sectors. Steels of TRIP grade (Transformation Induced Plasticity) and DP grade (Dual Phase), for instance provide excellent strength and ductility with improved formability. Steels of MART grade on the other hand exhibit ultra-high tensile strength being however of low ductility requiring hot treatments. These steels find application areas particularly in automotive and are not suitable for the household appliances either because of the mechanical properties or their costs comparatively higher than the other traditional steels. Therefore, it is particularly important to provide low cost steel sheets for use in household appliances while having the desired mechanical properties in lower thicknesses such as 0.2 to 0.4 mm.

[0005] A method of producing a stainless steel sheet is disclosed in GB 1190638-A wherein stainless steel sheet is produced by pickling, cold rolling and then annealing. Said document is actually related to a process for producing a billet, slab or plate of a mild steel for lining of refrigerator bodies.

[0006] CN 2144280-Y discloses a sealing device for attracting between stainless steel refrigerator door and body. The thickness of the stainless stripe is between 0.4 mm and 3.0 mm.

[0007] CN 103074546-A discloses a cold rolled strip steel for a condenser tube of a refrigerator and a manufacturing method for said cold rolled steel strip. The chemical components of said cold rolled strip steel comprises by weight 0.001 to 0.003 percent of C, no more than 0.03 percent of Si, 0.10 to 0.20 percent Mn, no more than 0.015 percent of P, no more than 0.008 percent of S, 0.015 to 0.060 percent of Al, 0.0004 to 0.001 percent of B and 0.020 to 0.040 percent of Ti, with the balance being Fe and inevitable impurity elements.

[0008] The present invention provides a novel and cost effective method for producing cabinets of the household appliances in order to meet the expectations in this industry. In particular, there are provided cabinets for the household appliances with reduced thickness and improved mechanical properties which has not been achieved so far with cold rolled steels of HSLA series.

Detailed Description of the Invention

[0009] The present invention is related to the cabinets for household appliances and a method for producing the same by using steels of specific types belonging to HSLA class. This class of steels are well defined in the state of the art as well as in the internationally recognized standards (EN 10149-2) with a large variety of members in different grades. They have been finding application areas particularly in metal profiles, safety barriers, machinery and automotive. Nonetheless, they have not been considered in household appliances because of their mechanical features such as formability, weight and endurance insufficiently observed in lower thickness ranges such as 0.2 to 0.4 mm. Therefore, it would be a considerable development to adapt steels of HSLA group to the cabinets of household appliances in this cost sensitive sector, because the steels of HSLA series have reasonable prices as compared to other high quality steels.

[0010] In the context of the present invention, the term "cabinet" refers to the external and internal casing or housing as well as the door panels of the household appliances which are made of planar sheets of steel. The term "household appliance" on the other hand refers to the electrical home appliances including refrigerators, dish washers, washing machines, drying machines, as well as ovens especially of the built-in type.

[0011] The inventors have investigated a variety of steels including HSLAs in different qualities for use in the cabinets for household appliances, and unexpectedly noted that only a few of the steels have successfully met the requirements. Refrigerators have comparatively bigger size than the other household appliances and therefore it is easier to observe the mechanical deformation both in the freefall tests and polyurethane pumping tests. The specific HSLA steel types found to be advantageous and satisfactory in these tests were the steels of S315MC, S355MC, S420MC, S460MC, S500MC, S550MC and S600MC series which are actually hot rolled steels as commercially available in the market.

[0012] HSLA steel grades mentioned above having the properties of high strength and low alloy are considered to provide high lightening ratio because the said steels ensure high extension values without losing their strength. In the scope of the present invention, it is aimed to manufacture cabinets for household appliances featuring H260LA, H300LA, H340LA, H380LA, H420LA, H460LA, H500LA grades from the specific HSLA steels mentioned above, in order to obtain a sheet thickness of 0.2 mm to 0.4 mm, and more particularly around 0.4 mm without having quality problems. It is therefore, one of the aspects of the present invention to provide a new process for conversion of HSLA steels mentioned in the above context to the steels having one of the grades of H260LA, H300LA, H340LA, H380LA, H420LA, H460LA, H500LA with a thickness of 0.2 mm to 0.4 mm, and more particularly around 0.4 mm.

[0013] High-strength low-alloy steel (HSLA) is a type of alloy steel that provides better mechanical properties and greater resistance to corrosion than carbon steel. HSLA steels differ from other steels in that they are not made to meet a specific chemical composition but rather to have specific mechanical properties. They have a carbon content between 0.05-0.25% to retain formability and weldability whereas the specific HSLAs in the present invention have a max. of 0.12% C content. Other alloying elements include up to 2.0% manganese and small quantities of copper, nickel, niobium, nitrogen, vanadium, chromium, molybdenum, titanium, calcium, rare earth elements, or zirconium. HSLA steel cross-sections and structures are usually 20 to 30% lighter than a carbon steel with the same strength.

[0014] The hot rolled HSLA steel sheets S315MC, S355MC, S420MC, S460MC, S500MC, S550MC, S600MC comprise maximum 0.12 weight percent of C; 0.5 weight percent of Si; 1.9 weight percent or less of Mn; 0.25 weight percent of P; 0.02 weight percent or less of S; 0.2 weight percent of V; 0.09 weight percent of Nb; 0.15 weight percent of Ti; and minimum 0.015 weight percent of Al. S600MC also comprises maximum 0.5 weight percent of Mo, and 0.005 weight percent of B. The cold rolled H260LA, H300LA, H340LA, H380LA, H420LA, H460LA, H500LA steels comprise maximum 0.38-0.45 weight percent of C; 0.38-0.45 weight percent of Si; 0.5-0.8 weight percent of Mn; 0.03 weight percent or less of P; 0.03 weight percent or less of S; 0.03 weight percent or less of Ni; 0.15-0.25 weight percent of Mo; and 0.9-1.2 weight percent of Cr.

[0015] Table I shows an exemplary breakdown of the desired mechanical properties of the hot rolled HSLA steel sheets used in the context of the present invention.

Table I. Mechanical properties of the HSLA steel sheets

Quality	Yield Strength (Mpa) (min)	Tensile Strength (Mpa)	% Elongation (min)
S315MC	315	390-510	20
S355MC	355	430-550	19
S420MC	420	480-620	16
S460MC	460	520-670	14
S500MC	500	600-760	12
S550MC	550	650-820	12
S600MC	600	700-880	11

[0016] The hot rolled HSLA steels used in the context of the present invention preferably possess the following parameters in their chemical structure.

Table II. Chemical requirements of the HSLA steel sheets

Quality	C max %	Mn max %	Si max %	P max %	S max %	Al total min %	Nb max %	V max %	Ti max %	Mo max %	B max %
S315MC	0,12	1,3	0,5	0,250	0,02	0,015	0,09	0,2	0,15	-	-
S355MC	0,12	1,5	0,5	0,250	0,02	0,015	0,09	0,2	0,15	-	-
S420MC	0,12	1,6	0,5	0,250	0,015	0,015	0,09	0,2	0,15	-	-
S460MC	0,12	1,6	0,5	0,250	0,015	0,015	0,09	0,2	0,15	-	-
S500MC	0,12	1,7	0,5	0,250	0,015	0,015	0,09	0,2	0,15	-	-
S550MC	0,12	1,8	0,5	0,250	0,015	0,015	0,09	0,2	0,15	-	-
S600MC	0,12	1,9	0,5	0,250	0,015	0,015	0,09	0,2	0,15	0,5	0,005

[0017] The HSLA steels of S315MC, S355MC, S420MC, S460MC, S500MC, S550MC and S600MC series need to be treated with a process according to the instant invention in order to impart the desired mechanical strength as well as the surface and bending properties to the steel sheets, whereby the sheets are eventually treated with a reshaping step to produce a cabinet for the household appliance.

[0018] Accordingly, the method for producing a cabinet for a household appliance comprises the steps of:

• providing a steel sheet of HSLA class having one of the grades selected from the group of S315MC, S355MC, S420MC, S460MC, S500MC, S550MC and S600MC,
• cold rolling of the steel sheet to have a final thickness of 0.2 mm to 0.4 mm, whereby the thickness reduction in the first pass mill is not more than 0.5 mm,
• annealing the steel sheet and treating the same with skin pass milling,
• reshaping of the steel sheet, and bending the same to form a panel, and
• using said panel to form a cabinet for a household appliance.

[0019] The cold rolling step, especially the treatment in the first pass mill in the process outlined above is noted to be critical because a thickness reduction of more than 0.5 mm not only affect the tensile strength but also increases the bending radius which is a critical parameter in the reshaping step to form the cabinets, especially in a lower thickness range such as 0.2 mm to 0.4 mm as mentioned hereinbelow. Without wishing to be bound by a theory, the inventors think that an extreme reduction of thickness more than 0.5 mm with a high force of rolling causes irreversibly phase transformations from crystallinity to amorphous form within the internal structure of the material. Therefore, it is highly preferred to apply a max. rolling load of 1500 tons in the very first pass milling.

[0020] In preferred embodiments, the method according to the present invention further comprises the step of pickling or acid wash before the cold rolling to remove any dirtiness or rust from surface of the steel.

[0021] It is also aimed generally to convert the specific HSLA sheets to the steels having one of the grades of H260LA, H300LA, H340LA, H380LA, H420LA, H460LA and H500LA in the aforementioned process, because of their advantageous properties for household appliances. On the other hand, the cold rolling can advantageously involve reversible cold milling (RCM) as conventionally known in the art.

[0022] It is also preferred during cold rolling, to use low diameter rolls, and preferably said diameter is particularly determined in between 378-395 mm. Additionally, roughness (R_a) values of the working rolls may be varied between 0,40 and 0,60 µm. Reduction ratios between passes maybe varied between 9% and 25% and the preferred number of passes is at least 7. Higher reduction ratios are used for the first and single passes, while lower passing ratios are used for the last and double passes.

[0023] The preferred annealing conditions are set as 14-15 hours in hydrogen atmosphere at about 670- 680 °C. Curved rolls can be used during annealing and the coils can be wet-tempered. In addition, a solution can be used in order to decrease the friction between sheet surfaces and roll surfaces.

Claims

1. A method for producing a cabinet for a household appliance comprising the steps of:

providing a steel sheet of HSLA class having one of the grades selected from the group of S315MC, S355MC, S420MC, S460MC, S500MC, S550MC and S600MC, cold rolling of the steel sheet to have a final thickness of 0.2 mm to 0.4 mm, whereby the thickness reduction in the first pass mill is not more than 0.5 mm, annealing the steel sheet and treating of the same with skin pass milling, reshaping of the steel sheet, and bending the same to form a panel, and using said panel to produce a cabinet for a household appliance.

2. A method according to claim 1 wherein the said steel of HSLA class is converted to a steel sheet of a grade selected from the group of H260LA, H300LA, H340LA, H380LA, H420LA, H460LA, and H500LA.

3. A method according to claim 1 wherein the method further comprises the step of pickling or acid treatment before the cold rolling step.

4. A method according to claim 1 wherein the cold rolling is carried out with reversible cold milling.

5. A method according to claim 1 wherein the steel of HSLA series comprises maximum 0.12 weight percent of C; 0.5 weight percent of Si; 1.9 weight percent or less of Mn; 0.025 weight percent of P; 0.02 weight percent or less of S; 0.2 weight percent of V; 0.09 weight percent of Nb; 0.15 weight percent of Ti; and minimum 0.015 weight percent of Al.

6. A method according to claim 5 wherein said steel comprises maximum 0.5 weight percent of Mo, and 0.005 weight percent of B.

7. A method according to claim 1 wherein the final thickness of the steel sheet obtained in the cold rolling step is about 0.4 mm.

8. A method according to claim 1 wherein the max. rolling load in the first pass milling is 1500 tons.

9. A method according to claim 1 wherein the annealing is carried out under hydrogen atmosphere at a temperature ranging from 670 to 680°C for 14 to 15 hours.

10. A method according to claim 1 wherein the rolls used in the cold rolling step have a surface roughness (R_a) ranging from 0.40 to 0.60 µm.

11. A method according to claim 1 wherein the household appliance is a refrigerator, dish washer or washing machine.