KR20160078851A - Forming part having excellent corrosion resistance and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a hot formed part having excellent corrosion resistance and a manufacturing method thereof, allowing an oxidized layer, which is uniform and compact, to be distributed on the hot formed part, thereby securing superior corrosion resistance. According to an embodiment of the present invention, the hot press formed part with superior corrosion resistance comprises: 0.1-0.4 wt% of carbon (C); 0.6-3.0 wt% of silicon (Si); 0.1-3 wt% of manganese (Mn); 0.03 wt% of less (excluding 0 wt%) of phosphorus (P); 0.02 wt% or less (excluding 0 wt%) of sulfur; 0.001-1.0 wt% of aluminum; 3-10 wt% of chromium (Cr); and the remaining consisting of iron (Fe) and inevitable impurities, wherein a Si and Al oxidized layer is formed on the part, and a Cr and Mn oxidized layer is formed on the Si and Al oxidized layer.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-formed member having excellent corrosion resistance,

The present invention relates to a hot-formed member excellent in corrosion resistance and a method of manufacturing the same.

The hot forming technology is known as a very superior technology for securing ultra high strength for light weight of vehicles and improvement of crashworthiness which are required in automobile companies. That is, the hot forming technique can solve the problem of moldability and shape dynamics in the cold forming according to the ultrahigh strength of the steel material.

In addition, patents and techniques for securing the corrosion resistance of automobile members have been developed, and Patent Document 1 discloses a technique relating to a method of manufacturing an aluminum-plated steel material for hot forming.

The above technique is characterized in that a hot-formed member can be manufactured by ensuring heat resistance after hot forming by aluminum plating. However, since the aluminum plating layer has no sacrificial corrosion resistance, the above-mentioned technique has a problem that the desired corrosion resistance can not be secured when the base steel is exposed to the outside by a weld, a blank edge, scratch or the like.

In addition, in the case of a cold-rolled steel sheet for hot-forming which can not be plated, welding can be performed only when the oxide layer formed during hot forming is removed through shot blast process, thereby causing additional cost problems and preventing corrosion resistance at all.

Therefore, there is a demand for development of a hot-formed member having excellent corrosion resistance and a manufacturing method thereof.

Patent Document 1: United States Patent No. 6296805

The present invention provides a hot-formed member excellent in corrosion resistance that can be suitably used for an automobile member or the like by appropriately controlling the composition of the alloy and distributing the oxide layer uniformly and densely during the heat treatment for hot forming in accordance with the above requirements, The purpose is to provide.

One aspect of the present invention is a steel sheet comprising, by weight%, 0.1 to 0.4% of C, 0.6 to 3.0% of Si, 0.1 to 3% of Mn, 0.03% or less of P (not including 0%) and 0.02% or less of S (Including 0% is not included), 0.001 to 1.0% of Al, 3 to 10% of Cr, and the balance of Fe and other unavoidable impurities, wherein Si and Al oxide layers are formed on the member, And a Cr and Mn oxide layer formed on the Cr and Mn oxide layers.

In another aspect of the present invention, there is provided a ferritic stainless steel comprising 0.1 to 0.4% of C, 0.6 to 3.0% of Si, 0.1 to 3% of Mn, 0.03% or less of P (0% : A steel slab containing 0.02% or less (not including 0%), 0.001 to 1.0% of Al, 3 to 10% of Cr, the balance Fe and other unavoidable impurities at 1000 to 1300 캜; Hot rolling the steel slab to obtain a hot-rolled steel sheet; Winding the hot-rolled steel sheet; Hot rolling the cold rolled steel sheet to obtain a cold rolled steel sheet; Heating the annealed cold rolled steel sheet at a rate of 1 to 100 ° C / sec to austenite single phase inverse temperature of 750 to 1000 ° C and holding it for 1 to 1000 seconds to perform heat treatment; And a step of hot-forming and cooling the heat-treated cold-rolled steel sheet, and a method of manufacturing a hot-formed member excellent in corrosion resistance.

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The various features of the present invention and the advantages and effects thereof can be understood in more detail with reference to the following specific embodiments.

The present invention relates to a hot-formed member excellent in corrosion resistance and a method of manufacturing the same, and has an effect of ensuring excellent corrosion resistance by distributing a uniform and dense oxide layer on a hot-formed member in a thickness of 10 to 500 nm. Furthermore, in the case of a member subjected to heat treatment using a cold-rolled steel sheet, shot blasting can be omitted, which simplifies the process and reduces the cost.

1 is a photograph of a surface oxide structure of Inventive Example 1 taken by using a transmission electron microscope (TEM).
Fig. 2 is a photograph of surfaces of Inventive Example 1 and Comparative Example 3; Fig.

The inventors of the present invention have succeeded in producing a hot-formed member having excellent corrosion resistance by appropriately controlling the alloy composition and uniformly and densely distributing the oxide layer during the heat treatment for hot forming. In the case of the heat treated member using the cold- The present invention has been completed.

Hereinafter, a hot-formed member having excellent corrosion resistance according to one aspect of the present invention will be described in detail.

The hot-formed member having excellent corrosion resistance according to one aspect of the present invention is characterized by containing 0.1 to 0.4% of C, 0.6 to 3.0% of Si, 0.1 to 3% of Mn, 0.03% or less of P (0% , Si: not more than 0.02% (not including 0%), Al: 0.001 to 1.0%, Cr: 3 to 10%, balance Fe and other unavoidable impurities, And Cr and Mn oxide layers are formed on the Si and Al oxide layers.

Hereinafter, the reason for controlling the alloy composition of the member will be described. Hereinafter, the content of each element is expressed in weight%.

C: 0.1 to 0.4 wt%

C is an element essential for securing the strength of martensite. When the content is less than 0.1%, it is difficult to obtain a predetermined strength. When the content is more than 0.4%, impact toughness of the slab is reduced, The weldability of the member tends to deteriorate. Therefore, the C content is preferably 0.1 to 0.4%.

Si: 0.6 to 3.0 wt%

The Si is one of the elements that play an important role in the present invention. In order to allow Si oxide to be generated on the surface layer of the heat treatment member, the Si content is preferably 0.6% or more (more preferably 0.8% And even more preferably 1.0% or more). On the other hand, when the Si content exceeds 3.0%, excessive Si oxide is formed on the surface of the steel, which causes a problem of deteriorating the pickling property in cold rolling. Therefore, the content of Si is preferably 3.0% or less (more preferably 2.5% or less, still more preferably 2.0% or less)

Mn: 0.1 to 3.0 wt%

The Mn is added not only to suppress undesired ferrite or bainite transformation in the hot-formed member but also to allow Mn oxide to be produced in the surface layer of the heat treatment member. If the Mn content is less than 0.1%, it is difficult to secure sufficient effects as described above. Therefore, the Mn content is preferably 0.1% or more (more preferably 0.3% or more, still more preferably 0.5% or more). However, when the Mn content exceeds 3.0%, not only the characteristics are saturated but also the steel plate production cost may increase, so that it is preferable to add Mn of 3.0% or less (more preferably 2.5% or less, 2% or less).

P: not more than 0.03% by weight (not including 0%)

It is preferable that P is an element which interferes with the characteristics of the heat treatment member as a grain boundary segregation element and is added as little as possible. Therefore, it is important to manage the upper limit, and when the P content exceeds 0.03%, there arises a problem that the bending property, the impact property, the weldability, and the like of the member are heated. Therefore, the upper limit is preferably limited to 0.03%.

S: 0.02% by weight or less (0% is not included)

The S exists as an impurity in the steel and is present as a sulfide such as MnS, and is preferably added as little as possible as an element inhibiting the bendability and weldability of the member. Therefore, it is important to manage the upper limit. When the S content exceeds 0.02%, there arises a problem in heat resistance of the member such as bending property and weldability. Therefore, the upper limit is preferably limited to 0.02%.

Al: 0.001 to 1.0 wt%

Al is an element capable of forming an oxide on the surface layer of the heat treatment member similarly to Si. In order to achieve this, Al is preferably added in an amount of 0.001% or more (more preferably 0.01% or more). However, if the content exceeds 1.0%, the above effect is saturated and the austenite single-phase reverse temperature is elevated to make the surface characteristics of the heat treatment member dull and dull, so it is preferable that the content is 1.0% or less Preferably not more than 0.5%, more preferably not more than 0.1%).

Cr: 3 to 10 wt%

The Cr is added not only to improve the hardenability of the hot-formed member but also to allow the Cr oxide to be formed on the surface layer of the heat treated member. When the Cr content is less than 3%, it is difficult to form a sufficient oxide layer. Therefore, it is preferable to add Cr oxide in an amount of 3% or more (more preferably 3.5% or more) so that Cr oxide can be uniformly and densely distributed on the surface of the heat- , And even more preferably 4% or more). On the other hand, when the Cr content exceeds 10.0%, not only the characteristics are saturated but also the steel plate production cost increases. Therefore, the Cr content is preferably 10.0% or less (more preferably 9% or less, even more preferably 8% Below).

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.

It is possible to uniformly and densely distribute the oxide layer during the heat treatment for hot forming of the hot-formed steel sheet satisfying the alloy composition described above, thereby making it possible to manufacture a hot-formed member excellent in corrosion resistance.

Hereinafter, the oxide layer of the hot-formed member according to the present invention will be described in detail.

The hot-formed member having excellent corrosion resistance according to the present invention satisfies the above-mentioned component system, and Si and Al oxide layers are formed on the member, and Cr and Mn oxide layers are formed on the Si and Al oxide layers.

For example, as shown in Fig. 1, Si and Al oxide layers are formed on the member, and Cr and Mn oxide layers are formed on the Si and Al oxide layers.

At this time, it is preferable that the combined thickness of the Cr and Mn oxide layers and the Si and Al oxide layers is 10 to 500 nm or less and is densely and uniformly distributed. By securing such an oxide layer, unlike a conventionally produced hot-formed member, scaling is hardly formed, and scale adhesion is also excellent. This effect can be confirmed through the photograph of FIG.

When the combined thickness of the Cr and Mn oxide layers and the Si and Al oxide layers is less than 10 nm, sufficient corrosion resistance can not be secured. Therefore, the lower limit is preferably 10 nm (more preferably 30 nm, still more preferably 50 nm ).

On the other hand, when the combined thickness of the Cr and Mn oxide layers and the Si and Al oxide layers exceeds 500 nm, the effect of improving the corrosion resistance is not only saturated but also an excessive amount of ferroalloys is required. Therefore, the upper limit is preferably 500 nm More preferably 400 nm, even more preferably 300 nm).

Therefore, when a hot-formed member is manufactured from a cold-rolled steel sheet, a shot blasting process is not required, and the oxide layer formed on the surface layer of the member greatly contributes to improvement in corrosion resistance.

At this time, the thickness of the Si and Al oxide layers may be 2 to 30 nm. If the thickness is less than 2 nm, it is difficult to secure sufficient corrosion resistance. If the thickness is more than 30 nm, excessive Si and / or Al must be added, thereby deteriorating the weldability.

The thickness of the Cr and Mn oxide layers may be 8 to 470 nm. When the thickness is 8 nm or less, there is a problem in ensuring corrosion resistance. When the thickness is more than 470 nm, Cr and / or Mn must be excessively injected.

Hereinafter, a method for manufacturing a hot-formed member having excellent corrosion resistance, which is another aspect of the present invention, will be described in detail.

A method of manufacturing a hot-formed member having excellent corrosion resistance, which is another aspect of the present invention, is characterized by comprising 0.1 to 0.4% of C, 0.6 to 3.0% of Si, 0.1 to 3% of Mn, 0.03% or less of P Steel slab containing S: not more than 0.02% (0% is not included), Al: 0.001 to 1.0%, Cr: 3 to 10%, balance Fe and other unavoidable impurities to 1000 to 1300 Lt; 0 >C; Hot rolling the steel slab to obtain a hot-rolled steel sheet; Winding the hot-rolled steel sheet; Hot rolling the cold rolled steel sheet to obtain a cold rolled steel sheet; Heating the annealed cold rolled steel sheet at a rate of 1 to 100 ° C / sec to austenite single phase inverse temperature of 750 to 1000 ° C and holding it for 1 to 1000 seconds to perform heat treatment; And hot-forming and cooling the heat-treated cold-rolled steel sheet.

Hot-rolled  step

The steel slab satisfying the above-mentioned composition is heated at 1000 to 1300 캜 and hot-rolled to obtain a hot-rolled steel sheet.

The hot rolling finishing temperature is not limited in the present invention as long as it does not hinder the cold rolling property and the like, but it is preferably carried out at a temperature of Ar3 transformation point to 1000 deg. When the finishing temperature is higher than 1000 ° C, an excessive scale is generated on the surface, which may lead to surface defects due to poor pickling properties when the cold rolled steel sheet is produced.

Winding  step

The coiling temperature is not limited to a large extent in the present invention as long as it does not hinder cold rolling property, but it is preferable to wind at a temperature of 500 to 750 캜.

If the coiling temperature is less than 500 캜, there is a problem that the cold rolling property is remarkably deteriorated due to an excessive material deviation in the width direction. If the coiling temperature is more than 750 캜, dent can be induced in the annealing step However, there is a difficulty in securing the surface hardening ability by excessive ferrite formation in the surface layer. Therefore, the coiling temperature is preferably 500 to 750 占 폚.

Step of Obtaining Cold Rolled Steel Sheet

The wound hot rolled steel sheet is pickled, cold rolled and annealed to obtain a cold rolled steel sheet. The production method of cold rolling is not particularly limited, and a cold rolled steel sheet can be produced by a general manufacturing method.

Heat-treated  step

The steel sheet for hot forming according to the present invention is heated to a temperature of from 750 to 1000 占 폚 at a rate of 1 to 100 占 폚 / sec and held for 1 to 1000 seconds.

Although the rate of temperature increase is not limited to a great extent, it is difficult to ensure sufficient productivity of a hot-formed member at a rate of less than 1 ° C / second. When the rate exceeds 100 ° C / second, an excessive facility investment costs are required. .

When the austenite single-phase reverse temperature is lower than 750 ° C, it is difficult to secure sufficient martensite in the base steel sheet in the case of a cold rolled steel sheet, and it is difficult to sufficiently secure predetermined oxides in the surface layer. desirable. However, when the temperature exceeds 1000 ° C, the effect is saturated and the equipment maintenance cost is excessively increased. Therefore, the upper limit is preferably 1000 ° C.

Cooling step

The heat-treated cold-rolled steel sheet is hot-formed and cooled at a cooling rate of 1 to 1000 ° C / sec.

Although the cooling rate is not limited to a large extent, the productivity may be lowered at a temperature lower than 1 ° C. When the heating rate is higher than 1000 ° C / sec, desirable.

The hot-formed member having the surface texture as shown in FIG. 1 can be manufactured through the above-described heat treatment and cooling steps.

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)

The steel slab formed as shown in Table 1 below was vacuum-melted and heated at a reheating temperature of 1200 占 폚 for 2 hours and hot rolling was performed. At this time, the hot rolling finish temperature was 900 占 폚 and the coiling temperature was 600 占 폚. . The hot-rolled steel sheet was pickled and cold-rolled to a final thickness of 1.5 mm at a cold rolling reduction of 50%, followed by annealing to produce a cold-rolled steel sheet.

The cold-rolled steel sheet was heat-treated for 5 minutes in a heating furnace heated to 900 ° C, transferred to a mold having a HAT shape, and hot formed and quenched to prepare a hot-formed member.

The tensile strength, Cr and Mn oxide thickness, Si and Al oxide thickness, Shot Blast process requirement, and corrosion depth of the member were measured and are shown in Table 2 below.

Mechanical properties were measured using the tensile test specimen of JIS Z 2201 5 from the above member. Transmission Electron Microscope (TEM) specimens were prepared by using FIB (Focused Ion Beam) Thickness and component distribution were analyzed. In addition, the corrosion depth was measured 57 times by CCT (salt cyclone corrosion test) using salt spray, and the corrosion depth was measured at 3 mm intervals at 2 mm intervals.

division Chemical composition (% by weight) C Si Mn P S Al Cr Inventory 1 0.20 1.5 0.9 0.012 0.006 0.05 7 Inventory 2 0.30 0.8 1.5 0.010 0.004 0.15 4.5 Comparative Example 1 0.22 0.1 1.2 0.016 0.005 0.02 0.1

division Characteristics of hot-formed member TS (MPa) (Cr, Mn) Oxide thickness (nm) (Si, Al) oxide thickness (nm) Shot Blast Corrosion depth (mm) Inventory 1 1582 137 15 X 0.05 Inventory 2 1915 95 7 X 0.07 Comparative Example 1 1536 <1 <1 ○ 0.6

In the case of Inventive Examples 1 and 2 according to the present invention, a dense and uniform oxide layer can be observed. Therefore, it is not necessary to carry out the Shot Blast process, and sufficient corrosion resistance can be ensured from the oxide layer with a corrosion depth of 0.05 mm and 0.77 mm.

On the other hand, in Comparative Example 1, a sufficient oxide layer was not formed, a shot blast process was required, and the corrosion depth was reduced to 0.6 mm.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

Claims (8)

The steel sheet according to any one of claims 1 to 3, wherein 0.1 to 0.4% of C, 0.6 to 3.0% of Si, 0.1 to 3% of Mn, 0.03% or less of P (not including 0%) and 0.02% , Al: 0.001 to 1.0%, Cr: 3 to 10%, balance Fe and other unavoidable impurities,
Si and Al oxide layers are formed on the member,
Wherein the Cr and Mn oxide layers are formed on the Si and Al oxide layers, and the corrosion resistance is excellent.
The method according to claim 1,
Wherein the total thickness of the Cr and Mn oxide layers and the Si and Al oxide layers is 10 to 500 nm or less.
The method according to claim 1,
Wherein the thickness of the Si and Al oxide layers is 2 to 30 nm and the corrosion resistance is excellent.
The method according to claim 1,
Wherein the Cr and Mn oxide layers have a thickness of 8 to 470 nm and excellent corrosion resistance.
The steel sheet according to any one of claims 1 to 3, wherein 0.1 to 0.4% of C, 0.6 to 3.0% of Si, 0.1 to 3% of Mn, 0.03% or less of P (not including 0%) and 0.02% Heating a steel slab containing from 0.001 to 1.0% of Al, from 3 to 10% of Cr, the balance Fe and other unavoidable impurities at 1000 to 1300 캜;
Hot rolling the steel slab to obtain a hot-rolled steel sheet;
Winding the hot-rolled steel sheet;
Hot rolling the cold rolled steel sheet to obtain a cold rolled steel sheet;
Heating the annealed cold rolled steel sheet at a rate of 1 to 100 ° C / sec to austenite single phase inverse temperature of 750 to 1000 ° C and holding it for 1 to 1000 seconds to perform heat treatment; And
And hot-molding the heat-treated cold-rolled steel sheet and cooling the heat-treated cold-rolled steel sheet.
6. The method of claim 5,
Wherein the hot rolling is a hot-rolling step of finishing hot rolling at an Ar3 transformation point to 1000 占 폚.
6. The method of claim 5,
Wherein the winding is performed in a temperature range of 500 to 750 占 폚.
6. The method of claim 5,
Wherein the cooling is performed at a cooling rate of 1 to 1000 占 폚 / sec.

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