KR20130056053A

KR20130056053A - Thin steel sheet having ultra high strength and manufacturing method of the same

Info

Publication number: KR20130056053A
Application number: KR1020110121785A
Authority: KR
Inventors: 조원태; 서석종; 김성우; 주세돈
Original assignee: 주식회사 포스코
Priority date: 2011-11-21
Filing date: 2011-11-21
Publication date: 2013-05-29

Abstract

The manufacturing method of the ultra-high strength steel sheet, which is one side of the present invention, is% by weight, C: 0.1-0.3%, Si: 0.1-1.5%, Mn: 2.0-3.0%, P: 0.001-0.10%, S: 0.010% Al: 0.01 to 0.1%, Cr: 0.3 to 1.0%, B: 0.0010 to 0.0030%, Ti: 0.01 to 0.1%, N: 0.001 to 0.01%, Nb: 0.02 to 0.05%, balance Fe and unavoidable impurities Winding up a hot rolled steel sheet including; And cold rolling the wound steel sheet, wherein the coiling temperature (Coiling Temperature, CT) in the winding step is represented by Equation 1: 141-90.2C + 6.27 Si-14.6Mn-5.47Cr-0.0826CT <45 2: 1030C + 35.9Si + 246Mn + 189Cr + 0.487CT-489 <830 is characterized in that it is determined to satisfy.

Description

Ultra High Strength Steel Sheet and Manufacturing Method {THIN STEEL SHEET HAVING ULTRA HIGH STRENGTH AND MANUFACTURING METHOD OF THE SAME}

The present invention relates to an ultra-high strength steel sheet and a method of manufacturing the same that can be used for steel sheets for automobiles.

Recently, steel sheets for automobiles are increasing the adoption of ultra-high strength steels in order to regulate fuel efficiency and secure crash safety for passengers for global environmental conservation. In order to manufacture such high-strength steel, it is not easy to secure sufficient strength and ductility only with steel using general solid solution strengthening or steel using precipitation strengthening.

In order to solve this problem, the proposed technique relates to metamorphic reinforcing steel using metamorphic tissue. Such transformation steel is also called abnormal phase steel (Dual Phase Steel, hereinafter referred to as DP steel), Complex Phase Steel (hereinafter also referred to as CP steel), and Transformation Induced Plasticity Steel (hereinafter referred to as TRIP steel). ).

DP steel is a kind of steel in which hard martensite is dispersed finely homogeneously in soft ferrite to ensure high strength and ductility. Examining the conventionally proposed technique for the manufacturing method of such DP steel, Patent Documents 1 and 2 propose a method for producing DP steel with a tensile strength of 980 MPa or more by controlling the component system and the manufacturing conditions, Patent Document 3 is the minor hardening characteristics The excellent tensile strength of 980MPa or more cold rolled steel sheet production method is presented, and Patent Document 4 provides a method of manufacturing a cold rolled steel sheet of 1470MPa or more tensile strength through heat treatment after quenching.,

In addition, CP steel includes two or three phases of ferrite, martensite and bainite, and is a steel grade including precipitation hardening elements such as Ti and Nb for strength improvement. TRIP steel is a steel grade that secures strength and ductility by causing martensitic transformation when micro homogeneously dispersed residual austenite is processed at room temperature. Examining the conventionally proposed technique for the production method of such steel, Patent Documents 5 and 6 propose a method for producing cold rolled steel sheet and plated steel sheet of TRIP steel having a tensile strength of 1180 MPa or more containing residual austenite.

However, the existing techniques are mostly manufacturing methods for DP steel and TRIP steel, in particular, the main characteristics to be focused on high tensile strength and elongation or formability. However, in the case of ultra high strength steel having a tensile strength of 1 GPa or more, the component system is designed so that the hardenability is large to secure the strength, and thus it has a very high strength even after hot rolling, which is an intermediate stage of actual product production. As a result, the load applied to the rolling mill during cold rolling is severe, and the thickness and width of the ultra-high strength cold rolled steel sheet that can be produced are limited, thereby lowering the stability of operation and ultimately leading to a decrease in productivity.

Japanese Patent Publication No. 2003-413431 Japanese Patent Publication No. 2005-286118 Japanese Patent Publication No. 2003-056900 Japanese Patent Publication No. 2008-261188 Japanese Patent Application Laid-Open No. 2008-068058 Japanese Patent Publication No. 2008-235092

The present invention is to provide a method for producing an ultra-high strength thin steel sheet having a tensile strength of 1 GPa or more and improved productivity.

The manufacturing method of the ultra-high strength steel sheet, which is one side of the present invention, is% by weight, C: 0.1-0.3%, Si: 0.1-1.5%, Mn: 2.0-3.0%, P: 0.001-0.10%, S: 0.010% Al: 0.01 to 0.1%, Cr: 0.3 to 1.0%, B: 0.0010 to 0.0030%, Ti: 0.01 to 0.1%, N: 0.001 to 0.01%, Nb: 0.02 to 0.05%, balance Fe and unavoidable impurities Winding up a hot rolled steel sheet including; And cold rolling the wound steel sheet, wherein the coiling temperature (CT) in the winding step may satisfy the following Equations 1 and 2 below.

Relationship 1: 141-90.2C + 6.27Si-14.6Mn-5.47Cr-0.0826CT <45

Relationship 2: 1030C + 35.9Si + 246Mn + 189Cr + 0.487CT-489 <830

Ultra high strength steel sheet is another aspect of the present invention by weight%, C: 0.1 ~ 0.3%, Si: 0.1 ~ 1.5%, Mn: 2.0 ~ 3.0%, P: 0.001 ~ 0.10%, S: 0.010% or less, Al: 0.01-0.1%, Cr: 0.3-1.0%, B: 0.0010-0.0030%, Ti: 0.01-0.1%, N: 0.001-0.01%, Nb: 0.02-0.05%, balance Fe and unavoidable impurities The microstructure may include bainite 40-70% and residual ferrite and martensite.

According to an aspect of the present invention, by controlling the component system, and by setting the component system and the winding temperature to satisfy the relation 1 and 2 relating to the ferrite phase fraction and the yield strength of the steel after hot rolling, by reducing the load during cold rolling It is possible to provide an ultra high strength steel sheet having excellent productivity and a tensile strength of 1 GPa or more.

The inventors of the present invention have repeatedly studied to derive a manufacturing method of ultra-high strength thin steel sheet which can improve productivity while securing ultra high strength, and accordingly, control the component system and manufacturing conditions appropriately, in particular, control the winding temperature to fine structure By controlling the ferrite fraction in the middle to lower the yield strength, it was confirmed that it is possible to increase the productivity by reducing the load during cold rolling and led to the present invention.

Hereinafter, the manufacturing method of the ultra-high strength steel sheet which is one side of the present invention will be described in detail.

Carbon (C): 0.1 to 0.3 wt%

C is an important element added to secure strength in metamorphic steel. When the content of C is less than 0.1% by weight, the above-mentioned effect can not be ensured and it is difficult to ensure a tensile strength of 1 GPa or more as intended by the present invention. On the other hand, when the content of C is more than 0.3% by weight, ductility, bending workability, and weldability of the steel sheet are poor, which is difficult to apply to automotive steel sheets. Therefore, the content of C is preferably controlled to 0.1 to 0.3 wt%.

Silicon (Si): 0.1 to 1.5 wt%

Si is an element capable of improving the strength and elongation of a steel material. When the content of Si is less than 0.1% by weight, the above-mentioned effect can not be ensured. On the other hand, when the content exceeds 1.5% by weight, not only surface scale defects are caused in relation to the surface quality, but also oxides which cause unplated plated steel sheets are formed on the surface, and surface defects such as unplated and plated- &Lt; / RTI > Therefore, the Si content is preferably controlled to 0.1 to 1.5 wt%.

Manganese (Mn): 2.0 to 3.0 wt%

Mn is an element that can play a large role in enhancing solubility in the presence of steel. When the content of Mn is less than 2.0% by weight, it is difficult to secure the strength intended for the present invention. On the other hand, when the content of Mn is more than 3.0% by weight, there is a high possibility that problems such as weldability and cold rolling load increase are caused, and also surface defects of the coated steel sheet can be caused by formation of coarse annealing agglomerates . Therefore, the content of Mn is preferably controlled to 2.0 to 3.0 wt%.

Phosphorus (P): 0.001 to 0.10 wt%

P is an element that can play a role in reinforcing the steel sheet. If the content of P is less than 0.001% by weight, the effect to be described can not be obtained and the production cost may be a problem. On the other hand. If the content exceeds 0.10% by weight, press formability may deteriorate and brittleness of steel may occur. Therefore, the content of P is preferably controlled to 0.001 to 0.10% by weight.

Sulfur (S): 0.010 wt% or less

S is an impurity contained inevitably and is an element that inhibits the ductility and weldability of the steel sheet. In theory, the content of S is advantageously limited to 0%, but inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, the upper limit of the S content in the present invention is preferably controlled to 0.010% by weight.

Aluminum (Al): 0.01 to 0.1 wt%

Al is an element effective in improving martensite hardenability by binding to oxygen in steel to deoxidize and distributing C in ferrite to austenite like Si. When the content of Al is less than 0.01% by weight, it is difficult to secure the above-described effects. On the other hand, when the content of Al exceeds 0.1 wt%, the slab surface quality is lowered and the manufacturing cost is increased. Therefore, the content of Al is preferably controlled to 0.01 to 0.1 wt%.

Cr (Cr): 0.3 to 1.0 wt%

Cr is an element added to improve the hardenability of a steel and ensure strength. In particular, the present invention serves as an element for inducing bainite formation through ferrite transformation delay. When the content of Cr is less than 0.3% by weight, the above-mentioned effect can not be exhibited. On the other hand, when the content exceeds 1.0% by weight, the effect is saturated and the cold rolling load is increased, and the manufacturing cost is greatly increased. Therefore, the content of Cr is preferably controlled to 0.3 to 1.0% by weight.

Boron (B): 0.0010 to 0.0030 wt%

B is a component that delays the transformation of austenite into pearlite during cooling during annealing, and is an element that inhibits ferrite formation and promotes the formation of bainite. When the content of B is less than 0.0010 wt%, the above-mentioned effect can not be exhibited. On the other hand, if the content is more than 0.0030 wt%, not only the effect is saturated due to grain boundary segregation of B, but also formation of excessive surface contaminants can lead to plating defects. Therefore, the content of B is preferably controlled to 0.0010 to 0.0030% by weight.

Titanium (Ti): 0.01 to 0.1 wt%

Ti is an element added for increasing the strength of the steel sheet and for scavenging N present in the steel. When the content of Ti is less than 0.01% by weight, it is difficult to secure such effect. On the other hand, if the content of Ti exceeds 0.1 wt%, process defects such as clogging of the nozzle during the continuous casting process may be caused. Therefore, the content of Ti is preferably controlled to 0.01 to 0.1 wt%.

Nitrogen (N): 0.001-0.01 wt%

N is a solid solution strengthening element capable of increasing the strength of the steel sheet, and is generally an element mixed from the atmosphere. Its content should be controlled by the steelmaking process degassing process. When the content of N is less than 0.001% by weight, excessive degassing treatment is required, causing an increase in manufacturing cost. When the content of N exceeds 0.01% by weight, high temperature ductility is reduced due to excessive formation of precipitates such as AlN and TiN. Therefore, the content of N is preferably controlled to 0.001 to 0.01 wt%.

Niobium (Nb): 0.02 to 0.05 wt%

Nb is an element added for increasing the strength of the steel sheet and for refining the crystal grains. When the content of Nb is less than 0.02% by weight, it is difficult to secure the above-mentioned effect. On the other hand, when the content of Nb is more than 0.05% by weight, the production cost is increased and the bending workability and ductility can be lowered due to excessive precipitates. Therefore, the content of Nb is preferably controlled to 0.02 to 0.05 wt%.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

Additionally, the Ti and N preferably satisfy 3.4 = Ti / N. Ti in the steel is an element added for increasing the strength of the steel sheet and scavenging of N present in the steel, and the content thereof preferably satisfies Ti / N ≧ 3.4. When the Ti / N ratio is less than 3.4, the Ti addition amount is insufficient compared to the amount of dissolved N, and the strength increase effect due to the addition of B is decreased due to the formation of BN due to the residual N, and the strength may be lowered. In addition, the upper limit of the Ti / N ratio is not particularly limited, but the upper limit may be controlled to 10 in consideration of the possibility of denitrification treatment cost and clogging of nozzles in the performance process.

The steel sheet, which is one side of the present invention, includes the component system and may include one or two or more of Mo, V, and W.

Molybdenum (Mo), vanadium (V) and tungsten (W): 0.01 to 0.2% by weight, respectively

When Mo is less than 0.01% by weight, it is difficult to obtain an effect of increasing the strength and grain refinement. When the Mo content exceeds 0.2% by weight, the production cost is excessively increased . Therefore, the contents of Mo, V and W are preferably controlled to 0.01 to 0.2 wt%, respectively.

In addition, the Nb, Mo, V and W is preferably satisfied the relationship of 0.02 = Nb + 0.2 (Mo + V + W), when less than 0.02 it is difficult to expect the effect of grain refinement and precipitation strengthening. The upper limit is not particularly limited, but the upper limit can be controlled to 0.05 in consideration of the manufacturing cost in terms of the effect.

A hot rolled steel sheet satisfying the above-described component system is produced, and the hot rolled steel sheet is wound up. At this time, the coiling temperature (Coiling Temperature, CT) is preferably determined to satisfy the following relations 1 and 2.

Relationship 1: 141-90.2C + 6.27Si-14.6Mn-5.47Cr-0.0826CT <45

Relationship 2: 1030C + 35.9Si + 246Mn + 189Cr + 0.487CT-489 <830

The relational expression 1 is an expression related to the ferrite phase fraction in the microstructure of the hot rolled steel sheet, and is a relational expression derived through experiments while changing C, Si, Mn, Cr, and CT temperatures. When the value of the relational expression 1 is 45 or more, the fraction of the ferrite as the soft phase is high, and the hot rolled strength may be lowered. However, the width direction yield strength deviation between the width direction center part controlled by the target winding temperature and the edge part which is generally controlled to be lower than the target winding temperature to increase the fraction of the secondary phase (150-200 ° C lower than the target winding temperature). Since it becomes deeper, cold rolling property can be reduced.

The relational expression 2 is an expression related to the yield strength of the hot rolled steel sheet, and is a relational expression derived through experiments while changing C, Si, Mn, Cr, and CT temperatures. When the value of the relation 2 is greater than 830, the unit roll force of the cold rolling mill exceeds about 1.9 ton / mm in normal operating conditions, so the width of the rollable steel cannot exceed 1100 mm. There is a problem that is difficult to meet the needs of general customers.

Cold rolled steel sheet wound in the above temperature range. After cold rolling, annealing and cooling may be performed to prepare an ultra high strength cold rolled steel sheet. Here, when the annealing temperature is less than 770 ° C, the ferrite structure fraction exceeds 40%, making it difficult to secure the strength of the steel sheet, and the bending workability is lowered. On the other hand, if the annealing temperature exceeds 850 DEG C, the bending workability of the steel sheet is improved, but the amount of surface enrichment such as Si, Mn, B generated at the high temperature annealing is greatly increased, there is a problem. Therefore, the annealing step is preferably performed at 770 to 850 ° C.

Further, it is preferable that the annealing step is carried out under an atmospheric condition composed of a hydrogen concentration of 5 to 50% and the balance of nitrogen. When the hydrogen concentration is less than 5%, surface hydrates such as Si, Mn, and B in the steel having large oxygen affinity are easily generated, which may cause dent and plating defects. On the other hand, when the hydrogen concentration exceeds 50%, the increase in the above-mentioned effect is insignificant compared with the manufacturing cost. Nitrogen used as a remainder material prevents formation of a surface agglomeration of a steel sheet and is low in manufacturing cost and can be suitably used as an atmospheric gas.

It is preferable to perform cooling after the annealing. The cooling may be two stage cooling. As for primary cooling, it is desirable to perform cooling at a speed | rate of 100-600 degreeC / min. When the cooling rate is less than 100 ° C / min, the strength intended for the present invention can not be secured due to the formation of ferrite and pearlite. On the other hand, when the cooling rate exceeds 600 캜 / min, the elongation rate is lowered due to the formation of an excessive hard phase, and problems such as shape defects may occur. However, it is preferable that the cooling stop temperature is controlled in the range of the martensitic transformation starting temperature (Ms) to the bainite transformation starting temperature (Bs). Through the cooling, an appropriate range of microstructure fraction can be secured. In the method for manufacturing a cold rolled steel sheet of the present invention, after the process, the cooled steel sheet may be secondly cooled at a rate of 10 ° C./min or less. If the cooling rate exceeds 10 ° C / min it is not possible to secure more than 40% bainite, there is a problem that the bending workability is reduced.

In addition, as another embodiment of the present invention, the cold rolled steel sheet may be annealed, cooled, and subjected to hot dip galvanizing to prepare an ultra high strength hot dip galvanized steel sheet.

Here, the same process is performed until the above-mentioned annealing and primary cooling. The primary cooled steel sheet is preferably immersed in a zinc plating bath to be galvanized. At this time, the temperature of the zinc plating bath is preferably 480 to 520 ° C. If the temperature of the plating bath is less than 480 DEG C, the formation of the alloying inhibition layer may be insufficient and the plating may be peeled off. If the temperature exceeds 520 DEG C, a problem arises that the generation of dross increases.

Hereinafter, another aspect of the ultra-high strength steel sheet is described in detail.

The thin steel sheet satisfying the above component system can secure a tensile strength of 1 GPa or more. The microstructure of the steel sheet preferably contains 40 to 70% by area of bainite and residual ferrite and martensite. When the content of bainite is less than 40% by area, the bending workability decreases. When the bainite content exceeds 70% by area, it is difficult to realize a high tensile strength of 1 GPa or more. The fraction of the ferrite and martensite structure is not particularly limited. However, in order to secure excellent strength, ductility and bending workability, it is preferable that the ferrite structure and the martensite structure have a range of 10 to 40% by area and 15 to 30% Do. Here, the steel sheet may be a cold rolled steel sheet or a hot dip galvanized steel sheet.

In addition, the steel sheet may be wound at a winding temperature (CT) that satisfies the following relations 1 and 2.

Relationship 1: 141-90.2C + 6.27Si-14.6Mn-5.47Cr-0.0826CT <45

Relationship 2: 1030C + 35.9Si + 246Mn + 189Cr + 0.487CT-489 <830

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

(Example)

To prepare a slab satisfying the component system shown in Table 1, after hot rolling, it is wound at the winding temperature (CT) shown in Table 1, followed by cold rolling. After annealing at 780 ~ 850 ℃ cooled to a temperature higher than the martensite starting temperature.

The values of relations 1 and 2 are calculated and shown in Table 2 below. After winding, the ferrite phase fraction, tensile strength, yield strength and elongation of the microstructure of the steel sheet were measured and shown in Table 2 together. In addition, if the deviation of the yield strength of the steel sheet edge portion and the center portion is 80MPa or more, the material deviation is severe enough to lower the cold rolling property. Therefore, each deviation is measured to be good if the 80MPa or more and less than 80MPa. It is shown together in Table 2 below. Additionally, the hot rolled steel sheet was cold rolled, and the tensile strength, yield strength and elongation of the cold rolled steel sheet were measured and shown in Table 2 together.

(However, the unit of each element is weight%. In addition, CT is a coiling temperature and a unit is ° C.)

(Relationship 1: 141-90.2C + 6.27Si-14.6Mn-5.47Cr-0.0826CT, Relationship 2: 1030C + 35.9Si + 246Mn + 189Cr + 0.487CT-489)

As shown in Table 2, Inventive Examples 1 to 10 satisfy the component systems controlled by the present invention, relations 1 and 2, and satisfy the relations 1 and 2 related to the ferrite phase fraction and yield strength of the hot rolled material. After the annealing, the final cold rolled steel sheet was able to confirm that the tensile strength of 1GPa or more, which is the intended purpose of the present invention.

On the other hand, Comparative Examples 1 to 3 is a comparative example outside the range of components limited in the present invention, the ferrite fraction of the hot rolled steel sheet exceeds the relationship 1, and after annealing, the steel material of the present invention is not able to secure more than 1Gpa target. It was. Comparative Examples 4 to 6 satisfy the range of components limited in the present invention, but did not satisfy the relational formula (2). Finally, Comparative Example 6 is out of the ferrite phase percentage range of the relational formula 1, and the material deviation in the width direction was severe.

Claims

By weight%, C: 0.1-0.3%, Si: 0.1-1.5%, Mn: 2.0-3.0%, P: 0.001-0.10%, S: 0.010% or less, Al: 0.01-0.1%, Cr: 0.3-1.0 Winding a hot rolled steel sheet containing%, B: 0.0010-0.0030%, Ti: 0.01-0.1%, N: 0.001-0.01%, Nb: 0.02-0.05%, balance Fe and unavoidable impurities; And cold rolling the wound steel sheet, wherein the winding temperature (Coiling Temperature, CT) is determined to satisfy the following relations 1 and 2 in the winding step.
Relationship 1: 141-90.2C + 6.27Si-14.6Mn-5.47Cr-0.0826CT <45
Relationship 2: 1030C + 35.9Si + 246Mn + 189Cr + 0.487CT-489 <830

The method of claim 1, wherein the Ti and N satisfy 3.4 ≦ Ti / N.

The method according to claim 1, wherein the steel sheet further comprises one or two or more of Mo: 0.01 ~ 0.2%, V: 0.01 ~ 0.2% and W: 0.01 ~ 0.2%, the Nb, Mo, V and W is A method for producing ultra-high strength thin steel sheet, characterized by satisfying 0.02 ≦ Nb + 0.2 (Mo + V + W).

The method of claim 1, further comprising: annealing the cold rolled steel sheet after the cold rolling;
The annealing temperature of the annealing step is 770 ~ 850 ℃, the atmosphere gas in the annealing furnace is hydrogen concentration of 5 to 50%, the balance is nitrogen,
A primary cooling step of cooling the annealed steel sheet at a cooling rate of 100 to 600 ° C./min to stop cooling at a martensite transformation start temperature (Ms) to bainite transformation start temperature (Bs); And
And a second cooling step of cooling the cooled steel sheet at a cooling rate of 10 ° C./min or less.

The method of claim 1, further comprising: annealing the cold rolled steel sheet after the cold rolling;
The annealing temperature of the annealing step is 770 ~ 850 ℃, the atmosphere gas in the annealing furnace is hydrogen concentration of 5 to 50%, the balance is nitrogen,
Cooling the annealed steel sheet at a cooling rate of 100 to 600 ° C./min to stop cooling at a martensite transformation start temperature (Ms) to bainite transformation start temperature (Bs); And
The method of manufacturing an ultra-high strength steel sheet further comprising the step of introducing a hot dip galvanized steel sheet into the plating bath at 480 ~ 520 ℃.

By weight%, C: 0.1-0.3%, Si: 0.1-1.5%, Mn: 2.0-3.0%, P: 0.001-0.10%, S: 0.010% or less, Al: 0.01-0.1%, Cr: 0.3-1.0 %, B: 0.0010-0.0030%, Ti: 0.01 ~ 0.1%, N: 0.001 ~ 0.01%, Nb: 0.02 ~ 0.05%, balance Fe and inevitable impurities, microstructure includes bainite 40-70% and balance Ultra-high strength steel sheet, characterized in that it contains ferrite and martensite.

The ultra-high strength steel sheet according to claim 6, wherein Ti and N satisfy 3.4 ≦ Ti / N.

The steel sheet according to claim 6, wherein the steel sheet contains at least one of Mo: 0.01 to 0.2%, V: 0.01 to 0.2%, and W: 0.01 to 0.2%, and Nb, Mo, V, and W are 0.02 ≦ Ultra high strength steel sheet characterized by satisfying Nb + 0.2 (Mo + V + W).

The ultra-high strength steel sheet according to claim 6, wherein the steel sheet is wound at a winding temperature CT satisfying the following relations 1 and 2.
Relationship 1: 141-90.2C + 6.27Si-14.6Mn-5.47Cr-0.0826CT <45
Relationship 2: 1030C + 35.9Si + 246Mn + 189Cr + 0.487CT-489 <830

The ultra-high strength steel sheet according to claim 6, wherein the steel sheet is a cold rolled steel sheet or a hot dip galvanized steel sheet.