KR20190078233A - Laser hardening low carbon steel sheet and manufacturing the same - Google Patents

Abstract

The laser-curable low-carbon steel sheet according to an embodiment of the present invention may include 0.02 to 0.05% of C, 1.0% or less of Si (excluding 0%), 3.0% or less of Mn (excluding 0% Of the steel sheet, and the steel sheet comprises a heat deforming portion formed on a part of the steel sheet, the heat deforming portion including at least one of martensite (Al), 5.0%, P: not more than 0.02% (excluding 0%), S: not more than 0.01% And 50% by volume or more of bainite.

Description

TECHNICAL FIELD [0001] The present invention relates to a laser-hardened low-carbon steel sheet,

And more particularly, to a laser-curable low carbon steel sheet and a manufacturing method thereof. More specifically, the present invention relates to a laser-hardened low-carbon steel sheet capable of easily forming a local hard phase through laser heat treatment and having corrosion resistance in an atmospheric environment, and a method of manufacturing the same.

Recently, cold rolled steel sheets used in household appliances such as TVs, refrigerators, and washing machines are required to be manufactured to reduce production costs, weight, and thickness. However, even if it is made thinner, it is inevitably required to have a high strength in order to have the same rigidity. Even if the strength is increased, internal heat deformation, deformation due to residual stress, and buckle problem of the steel plate become an issue.

As an example of a solution to this problem, an attempt has been made to secure a rigidity by attaching a bracket to the inside of the steel panel. However, in this case, not only the additional cost is incurred but also the production cost and productivity are lowered, There is a drawback that it becomes complicated.

On the other hand, as an alternative, a method of responding to lowering rigidity by raising the strength of the entire steel sheet by using a high-strength DP (Dual Phase) steel mainly used in automotive steel sheets as a panel of household appliances is also emerging. There is a disadvantage in that the weldability is poor due to an excessive amount of the alloy element and the residual stress generated by the uneven cooling of the plate during the process of forming the low temperature structure by quenching causes surface defects such as waves and buckles And the like.

And more particularly, to a laser-curable low carbon steel sheet and a manufacturing method thereof. More specifically, the present invention relates to a laser-hardened low-carbon steel sheet capable of easily forming a local hard phase through laser heat treatment and having corrosion resistance in an atmospheric environment, and a method of manufacturing the same.

The laser-curable low-carbon steel sheet according to an embodiment of the present invention may include 0.02 to 0.05% of C, 1.0% or less of Si (excluding 0%), 3.0% or less of Mn (excluding 0% Of the steel sheet, and the steel sheet comprises a heat deforming portion formed on a part of the steel sheet, the heat deforming portion including at least one of martensite (Al), 5.0%, P: not more than 0.02% (excluding 0%), S: not more than 0.01% And 50% by volume or more of bainite.

The laser-curable low carbon steel sheet according to an embodiment of the present invention may further include at least one of 0.001 to 0.03% by weight of Mo and 0.001 to 0.003% by weight of B.

The laser-curable low carbon steel sheet according to an embodiment of the present invention may further include at least one of Cu: 0.1 to 0.5 wt%, Ni: 0.1 to 0.5 wt%, and Co: 0.01 to 0.1 wt%.

The hot deformed portion may contain 30 to 99% by volume of martensite and 1 to 40% by volume of bainite.

A plurality of heat deforming portions may be formed, and a distance between the heat deforming portions may be 6 to 12 mm.

The area of each of the thermal deformations may be less than or equal to 50 mm ² .

The thickness of the steel sheet may be more than 1.0 mm and not more than 3.0 mm.

A method of manufacturing a laser-curable low-carbon steel sheet according to an embodiment of the present invention includes: 0.02 to 0.05% of C, 1.0% or less of Si (excluding 0%), 3.0% (Excluding 0%), S: not more than 0.01% (excluding 0%), and the remainder Fe and unavoidable impurities are contained in an amount of 1.0 to 5.0%, Mo: 0.001 to 0.03%, B: 0.001 to 0.003% Preparing a steel sheet; Irradiating a part of the steel sheet with a laser beam; .

The method may further include the step of imparting tension to the steel sheet before the step of irradiating the part of the steel sheet with the laser beam.

In the step of irradiating a part of the steel sheet with the laser beam, the surface irradiated with the laser beam can be heated to a temperature of (Ac3 + 150) DEG C or higher.

In the step of irradiating a part of the steel sheet with the laser beam, the output of the laser beam may be 5 to 10 kW.

The method may further include the step of cooling the steel sheet after the step of irradiating a part of the steel sheet with the laser beam.

The low carbon steel sheet according to the present invention is excellent in both ductility and rigidity and can be preferably used as a panel of a home appliance such as a TV.

According to an embodiment of the present invention, it is possible to form a hard phase by laser treatment even for a low-carbon steel sheet having a large thickness.

Further, according to the embodiment of the present invention, it is possible to provide a low carbon steel sheet having excellent ductility and rigidity through laser treatment and having corrosion resistance in an atmospheric environment.

1 is a schematic view schematically showing a surface (ND surface) of a steel sheet according to an embodiment of the present invention.
2 is a schematic view schematically showing a section (TD surface) of a steel sheet according to an embodiment of the present invention.
3 is an optical image of a cross section (TD surface) of a heat deforming portion among the steel sheets of Inventive Example 1. FIG.

The terms first, second and third, etc. are used to describe various portions, components, regions, layers and / or sections, but are not limited thereto. These terms are only used to distinguish any moiety, element, region, layer or section from another moiety, moiety, region, layer or section. Thus, a first portion, component, region, layer or section described below may be referred to as a second portion, component, region, layer or section without departing from the scope of the present invention.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. The singular forms as used herein include plural forms as long as the phrases do not expressly express the opposite meaning thereto. Means that a particular feature, region, integer, step, operation, element and / or component is specified and that the presence or absence of other features, regions, integers, steps, operations, elements, and / It does not exclude addition.

When referring to a portion as being "on" or "on" another portion, it may be directly on or over another portion, or may involve another portion therebetween. In contrast, when referring to a part being "directly above" another part, no other part is interposed therebetween.

Unless otherwise defined, all terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Commonly used predefined terms are further interpreted as having a meaning consistent with the relevant technical literature and the present disclosure, and are not to be construed as ideal or very formal meanings unless defined otherwise.

Unless otherwise stated,% means% by weight, and 1 ppm is 0.0001% by weight.

In an embodiment of the present invention, the term further includes an additional element, which means that an additional amount of the additional element is substituted for the remaining iron (Fe).

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily carry out the present invention. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

1 is a schematic view of a curing type low carbon steel sheet 10 according to an embodiment of the present invention. As shown in FIG. 1, the curable low carbon steel sheet 10 according to an embodiment of the present invention includes a heat deforming portion 20 formed on a part of a steel sheet.

First, an alloy component of the low carbon steel sheet 10 according to an embodiment of the present invention will be described. This alloy component is equally contained in the low carbon steel sheet 10 and the heat deforming portion 20.

The laser-curable low-carbon steel sheet according to an embodiment of the present invention may include 0.02 to 0.05% of C, 1.0% or less of Si (excluding 0%), 3.0% or less of Mn (excluding 0% 5.0%, P: 0.02% or less (excluding 0%), S: 0.01% or less (excluding 0%), and the balance Fe and unavoidable impurities.

Hereinafter, the content of each element will be described in detail.

C: 0.02 to 0.05 wt%

Carbon (C) in steel plays a role in securing strength by being employed in steel. When the content of carbon is too small, the ability to cure by carbon is insufficient and no carbide is formed. Even if rapid heating and rapid cooling are applied, low temperature structure such as martensite and bainite is not formed, It can be difficult. On the other hand, when the carbon content is too large, it does not meet the object of the present invention to improve the strength by using the low carbon steel sheet. More specifically, carbon may be included in an amount of 0.02 to 0.045% by weight.

Si: 1.0 wt% or less

Silicon (Si) helps to improve the strength of steel by solid solution strengthening, but when added in large amounts, the surface quality is lowered due to an increase in scale defects, so that the content can be limited to 1.0 wt% or less. More specifically 0.1% by weight or less.

Mn: 3.0 wt% or less

Manganese (Mn) improves steel strength and hardenability by solid solution strengthening. However, if the content is excessive, segregation such as center segregation or micro segregation becomes serious and adversely affects quality, so that the content can be limited to 3.0 wt% or less. And more specifically 2.0% by weight or less.

Cr: 1.0% to 5.0%

Cr (Cr) is injected into the steel to improve the strength of the steel after heat treatment, improve hardenability, and improve atmospheric corrosion resistance. When the content of Cr is too small, the above-mentioned effect can not be sufficiently obtained. Also, if the Cr content is excessive, there is a risk of sheet breakage by causing defects in the production process. Therefore, Cr may be included in the above-mentioned range. More specifically, Cr may be contained in an amount of 1.5 to 4.0% by weight.

P: not more than 0.02% by weight (excluding 0% by weight)

Phosphorus (P) is an element which contributes to strength improvement by solid solution strengthening. However, when the content is excessive, it deteriorates the impact characteristics of steel and significantly reduces sulfuric acid corrosion resistance, so that the content thereof is limited to 0.02 wt% desirable. More specifically, P may be contained in an amount of 0.01% by weight or less.

S: not more than 0.01% by weight (including 0% by weight)

Sulfur (S) is an impurity inevitably contained in the steel, and is an element that causes hot brittleness. Therefore, it is preferable to control the content as low as possible, and it is preferable to limit the content to 0.01 wt% or less Do. More specifically, S may be contained in an amount of 0.005% by weight or less.

The laser-curable low carbon steel sheet according to an embodiment of the present invention may further include at least one of 0.001 to 0.03% by weight of Mo and 0.001 to 0.003% by weight of B.

Mo: 0.001 to 0.03 wt%

Molybdenum (Mo) is an element that helps improve the hardenability of steel. In particular, when elements such as Mn and Cr, which can increase the hardenability, are lacking, Mo can work together with B to help improve hardenability. Therefore, by adding Mo in an amount of 0.001% by weight or more, the above effects can be obtained. However, if the content of Mo is too large, the effect of improving the hardenability may be deteriorated. Therefore, when Mo is further added, it can be added in the above-mentioned range. More specifically, 0.01 to 0.25% by weight of Mo may be further added.

B: 0.001 to 0.003 wt%

Boron (B) is an element that helps to improve hardenability by segregation in the grain boundary with Mo. Therefore, by adding B in an amount of 0.001% by weight or more, the above effects can be obtained. If B is added excessively, B may cause grain boundary embrittlement. Therefore, when B is further added, it can be added in the above-mentioned range. More specifically, 0.0015 to 0.0025% by weight of B may be further added.

Means that one or more of Mo: 0.001 to 0.03% by weight and B: 0.001 to 0.003% by weight means to contain Mo alone, B alone or Mo and B at the same time.

The laser-curable low carbon steel sheet according to an embodiment of the present invention may further include at least one of Cu: 0.1 to 0.5 wt%, Ni: 0.1 to 0.5 wt%, and Co: 0.01 to 0.1 wt%.

Cu: 0.1 to 0.5 wt%

Copper (Cu) is an element added to ensure corrosion resistance. The above effect can be obtained by adding Cu in an amount of 0.1 wt% or more. If Cu is added in excess, the brittleness and spreadability may be deteriorated. Therefore, when Cu is further added, it can be added in the range described above. More specifically, it may contain 0.15 to 0.4 wt% of Cu.

Ni: 0.1 to 0.5 wt%

Nickel (Ni) is an element that prevents surface cracking. By adding at least 0.1% by weight of Ni, the above effects can be obtained. When Ni is added in an excessive amount, there is a fear of formation of Cr-based precipitates and the like. Therefore, when Ni is further added, it can be added in the above-mentioned range. More specifically, it may contain 0.1 to 0.3% by weight of Ni.

Co: 0.01 to 0.1 wt%

Cobalt (Co) is an element added to improve corrosion resistance. By adding at least 0.01% by weight of Co, the above effect can be obtained. Even if Co is added in excess, there is a limit to the improvement of the effect. Therefore, when Co is further added, it can be added in the above-mentioned range. And more specifically 0.03 to 0.07% by weight of Co.

The rest of the composition is Fe. On the other hand, addition of an effective component other than the above-mentioned composition is not excluded.

Returning to the description of FIG. 1 again, the thermally deformable portion 20 contains 50 vol% or more of martensite and bainite as its sum. As described above, since the low-temperature structure such as martensite and bainite exists in the island shape, the ductility, rigidity and corrosion resistance of the steel sheet 10 can be improved at the same time. When the sum of martensite and bainite is too small, it is not possible to sufficiently improve the ductility, rigidity and corrosion resistance of the steel sheet 10. Specifically, the thermally deformable portion 20 may contain 70 vol% or more of martensite and bainite as a sum. More specifically, the thermally deformable portion 20 may contain 80 vol% or more of martensite and bainite as a sum.

More specifically, the thermally deformable portion 20 may contain 30 to 99% by volume of martensite and 1 to 40% by volume of bainite. When martensite and bainite are included in the above-mentioned range, ductility, rigidity and corrosion resistance can be further improved. The thermally deformable portion 20 includes ferrite, pearlite, and austenite as the remainder.

The steel sheet other than the heat deformable portion 20 is less than martensite and bainite and is distinguished from the heat deformable portion 20 in that it contains a large amount of ferrite. Specifically, the steel sheet other than the heat deformable portion may contain 50 vol% or more of ferrite. More specifically, the steel sheet other than the heat deformable portion may contain not less than 75 vol% of ferrite. More specifically, the steel sheet other than the heat deformable portion may contain not less than 95% by volume of ferrite.

As shown in FIG. 1, a plurality of the heat deformable portions 20 are formed, and the interval D between the heat deformable portions 20 may be 6 to 12 mm. 1, the shape of the heat deforming portion is shown as a circular shape, but the present invention is not limited thereto, and an elliptical shape, a polygonal shape, or the like can be freely deformed. The distance D between the heat deformable portions 20 means a straight gap between the heat deformable portions 20 adjacent to each other. When the interval D between the heat deformable portions 20 is too narrow, the heat emission is not sufficient and the self-lighting effect may not be properly displayed. When the distance D between the heat deformable parts 20 is too large, the heat deformable parts 20 are formed too little, and the desired ductility, rigidity and corrosion resistance can not be sufficiently improved. More specifically, the interval D between the heat deformable portions 20 may be 7 to 10 mm.

The area of each of the thermally deformable portions 20 may be 50 mm ² or less. In this case, the area means the area on the rolled surface (ND surface). If the area of each of the heat deforming portions 20 is too large, the heat releasing is not sufficient at the center of the heat deforming portion 20, so that the self-smoothing effect may not be properly displayed. On the other hand, as the area of the thermally deformable portion 20 is smaller, it is advantageous for ensuring the self-cheating effect, and the lower limit of the area of the thermally deformable portion 20 is not particularly limited. More specifically 10 to 45 mm < ² >.

As shown in FIG. 2, the thermally deformable portion 20 may be formed to penetrate the thickness of the steel plate 10.

The thickness of the steel sheet may be more than 1.0 mm and not more than 3.0 mm.

In one embodiment of the present invention, the phase transformation can be efficiently performed by appropriately controlling the alloy content in the steel sheet. Therefore, even if the thickness of the steel sheet 10 is thick, the heat deformable portion 20 is appropriately formed, and the ductility, rigidity and corrosion resistance are greatly improved.

On the other hand, if the alloy content in the steel sheet can not be properly controlled, the phase transformation does not take place efficiently, and the thermal deformation part is not properly formed. As a result, the ductility, rigidity and corrosion resistance improvement effect can not be properly obtained.

The laser-curable low-carbon steel sheet according to an embodiment of the present invention is excellent in strength and elongation, and has corrosion resistance. Specifically, the yield strength of the steel sheet may be 300 MPa or more, the tensile strength may be 450 MPa or more, and the elongation may be 30% or more. Corrosion resistance can be less than 0.15 mm and maximum corrosion rate less than 2.0 mm / year after immersing for 500 hours in JASO M 611-92 Method A method.

A method of manufacturing a laser-curable low-carbon steel sheet according to an embodiment of the present invention includes: 0.02 to 0.05% of C, 1.0% or less of Si (excluding 0%), 3.0% (Excluding 0%), S: not more than 0.01% (excluding 0%), and the remainder Fe and unavoidable impurities are contained in an amount of 1.0 to 5.0%, Mo: 0.001 to 0.03%, B: 0.001 to 0.003% Preparing a steel sheet; Irradiating a part of the steel sheet with a laser beam; .

Hereinafter, each step will be described in detail.

Since the alloy components of the steel sheet are the same as those described above, the overlapping description is omitted. In one embodiment of the present invention, a steel having a temperature of Ac 3 + 150 ° C not exceeding 1050 ° C may be used as a raw material of the invention.

The steel sheet before laser beam irradiation may contain 50 vol% or more of ferrite. More specifically, the steel sheet may contain at least 75 vol% of ferrite. More specifically, the steel sheet may contain 95 vol% or more of ferrite. The residual structure outside the ferrite structure is not particularly limited, and may include, for example, a pearlite structure.

Next, a laser beam is irradiated.

The method may further include imparting a tension before the step of irradiating the laser beam.

Thermal deformation may occur in the course of manufacturing a laser low carbon steel sheet because a local temperature deviation occurs when the laser beam is irradiated. At this time, when the step of applying the tensile force in the rolling direction (RD direction) of the low carbon steel sheet before the laser beam irradiation is performed, there is an advantage that the occurrence of thermal deformation can be effectively suppressed.

The surface irradiated with the laser beam during the laser beam irradiation can be heated to a temperature of (Ac3 + 150) DEG C or higher. This is to completely transform the microstructure of the low carbon steel sheet irradiated with the laser beam into austenite. For reference, Ac3 can be calculated from Equation 1 below.

[Formula 1]

(Wt%) + 100 x (wt% Ti) + 68 x (wt% Si) + 50 x (wt% (wt% Al) - 11 x (wt% Cr) + 67 x (wt% Mo) - 33 x (wt% Ni)

(Here, the parentheses denote the weight% of the corresponding element, respectively, and do not include the corresponding element.)

The output of the laser beam may be between 5 and 10 kW. If the output of the laser beam is too low, the steel sheet may not be properly heated. If the output of the laser beam is too high, the surface of the steel sheet may be damaged.

When irradiating the laser beam, it is possible to irradiate a plurality of laser beams. In this case, it is preferable to control the interval between the plurality of laser beams to 6 to 12 mm. If the distance between the plurality of laser beams is excessively narrow, it is difficult to release heat from the region irradiated with the laser beam to the surrounding region, so that it is difficult to secure a critical cooling rate necessary for forming martensite and bainite, It may be difficult to enjoy the chirping effect. On the other hand, if the interval between the plurality of laser beams is too wide, the heat deformable portion 20 is formed too little, and the desired ductility, rigidity and corrosion resistance can not be sufficiently improved. More specifically, the interval between the plurality of laser beams may be 7 to 10 mm.

The irradiation area of the laser beam may be 50 mm ² or less. When the area is too large, it is difficult to release the heat to the peripheral portion in the central portion of the region irradiated with the laser beam, so that it is difficult to secure the critical cooling rate necessary for forming the martensite and bainite, and thereby the self- It can be difficult. On the other hand, as the irradiation area of the laser beam is smaller, it is advantageous for securing self-etching effect, and the lower limit of the irradiation area of the laser beam is not particularly limited. More specifically 10 to 45 mm < ² >.

Thereafter, the method may further include cooling the steel sheet. When the heated steel sheet is cooled by irradiation of the laser beam, the heated steel sheet is cooled to a temperature above the critical cooling rate which is the minimum cooling rate at which martensite or bainite is formed, and the structure of the steel sheet is transformed from austenite to martensite and bainite . At this time, the sum of the transformed martensite and bainite is 50 vol% or more.

In order to form a sufficient hard phase (martensite, bainite) at the cooling rate during laser heat treatment, it is necessary to secure sufficient hardenability of the steel species. For this purpose, it is necessary to satisfy the alloying elements proposed in one embodiment of the present invention.

The step of cooling is not particularly limited, and the effect of the present invention can be fully enjoyed by simply leaving the locally heated steel sheet in the air. However, in order to ensure sufficient rigidity of the low-carbon steel sheet, when it is assumed that a laser beam is continuously irradiated a plurality of times over the entire steel sheet, one laser beam is irradiated and then the next laser beam is irradiated The cooling time is preferably 1 second or more. This is to maximize the self-etching effect by keeping the temperature of the entire steel sheet below the martensitic transformation temperature.

The laser irradiation step and the cooling step may be repeated two or more times. When laser patterning is repeatedly performed, there is an advantage that a steel sheet excellent in both ductility and rigidity can be obtained in the continuous process.

Hereinafter, preferred embodiments and comparative examples of the present invention will be described. However, the following examples are only a preferred embodiment of the present invention, and the present invention is not limited to the following examples.

Experimental Example  1: Experiment according to alloy composition

First, a low carbon steel sheet (thickness: 2.0 mm) having the composition shown in Table 1 and having a ferrite single phase structure was prepared.

(weight%) C Mn Cr Mo B Cu Ni Co Inventory 1 0.04 One 3 0.02 0.002 0.3 0.15 0.05 Inventory 2 0.04 0.2 3 0.02 0.002 0.3 0.15 0.05 Inventory 3 0.04 One 2 0.02 0.002 0.3 0.15 0.05 Honorable 4 0.04 One 1.5 0.02 0.002 0.3 0.15 0.05 Inventory 5 0.02 One 2 0 0 0.3 0.15 0.05 Comparative Example 1 0.04 One 0.01 0.02 0.002 0.3 0.15 0.05 Comparative Example 2 0.04 0.2 0.01 0 0 0.3 0.15 0.05 Comparative Example 3 0.01 One 3 0.02 0.002 0.3 0.15 0.05 Comparative Example 4 0.002 One 3 0.02 0.002 0.3 0.15 0.05

The low-carbon steel sheet was placed between a pay-off reel and a skin pass roll, and a laser patterning heat treatment was performed in a state in which the tension was applied in the longitudinal direction. More specifically, a plurality of laser beams were simultaneously irradiated in the width direction of the steel sheet, and a plurality of laser beams were irradiated simultaneously in the width direction of the steel sheet at a position spaced apart by 8 mm in the longitudinal direction of the steel sheet after 2 seconds. At this time, the interval between the plurality of laser beams was 8 mm, and the irradiation area of the laser beam was made constant at 30 mm ² .

On the other hand, the surface temperature of the low carbon steel sheet irradiated with the laser beam was controlled to be 1050 占 폚. The irradiation of the laser beam was carried out in a continuous wave mode with a high power diode laser of 6 kW for 0.11 second (possible temperature increase rate is 1,000 to 10,000 K / s).

After measuring the mechanical properties of the obtained laser-curable steel sheet, the results are shown in Table 2 below.

The mechanical properties obtained from the tensile test in Table 2 are the results obtained when the entire specimen is subjected to the tensile test after the patterning treatment and the fraction of the microstructure is determined by measuring the percentage of the volume passing through the entire thickness at the position irradiated with laser This is a result.

Table 3 shows the results of corrosion resistance evaluation. To evaluate the corrosion resistance, a solution of JASO M 611-92 Method A was utilized. The composition of ions in the solution was controlled by the content of SO ₄ ^{2 -} , SO ₃ ^2- , NO ₃ ^- , Cl ^- , and CH ₃ COO ^- in 1 L of distilled water to the contents of 600 ppm, 600 ppm, 20 ppm, 100 ppm, Ammonia water was used to adjust the pH to 4 by adjusting the content of NH ₄ ⁺ in the solution. Table 3 shows the maximum corrosion depth and corrosion loss of the test specimens for a long period of immersion (500 hr) in the above-mentioned solution.

Martensite fraction
(volume%) Bainite fraction
(volume%) Ferrite fraction
(volume%) Perlite fraction
(volume%) YS (MPa) TS (MPa) El (%) Inventory 1 98.5 1.3 0.01 0 491 664 31 Inventory 2 92 6.3 1.53 0 473 644 33 Inventory 3 93.5 5.36 1.1 0 461 631 32 Honorable 4 53.5 17.08 29.36 0 393 553 37 Inventory 5 32.01 33.25 34.73 0 335 485 39 Comparative Example 1 0 0 98.24 0.03 241 384 43 Comparative Example 2 0 0 98.97 1.03 215 347 44 Comparative Example 3 0 0 99.01 0 209 339 40 Comparative Example 4 0 0 100 0 190 300 45

Maximum corrosion depth (mm) Maximum corrosion rate (mm / year) Inventory 1 0.098 1.716 Inventory 2 0.056 0.981 Inventory 3 0.102 1.787 Honorable 4 0.112 1.962 Inventory 5 0.108 1.892 Comparative Example 1 0.253 4.43 Comparative Example 2 0.308 5.39 Comparative Example 3 0.097 1.699 Comparative Example 4 0.085 1.489

As can be seen from Tables 1 and 2, martensite and bainite were suitably formed in the heat-deforming portions of Inventive Examples 1 to 4 satisfying the alloy composition, and the tensile strength was higher than that of 500 MPa. % Elongation can be obtained. Example 5 shows that the tensile strength is somewhat less than 500 MPa, but the elongation is superior to the others.

However, in the comparative example, martensite and bainite, which are hard phases, were not observed in the portion subjected to the laser heat treatment, and it was confirmed that the mechanical properties were deteriorated. This is because the Cr alloy, which is an element capable of securing the hardenability, is insufficient, and sufficient curing is not ensured enough to form a hard phase even if transformation has not occurred during the time of laser heat treatment or transformation occurs. Even when Cr is contained in a proper amount, sufficient curing ability can not be secured when the content of C is 0.1 wt% or less.

As can be seen from Table 3, Examples 1 to 5 exhibited excellent atmospheric corrosion properties compared to normal carbon steels having a maximum corrosion rate not exceeding 2.0 mm / year. On the contrary, in the case of Comparative Examples 1 and 2 in which the content of Cr was small, the corrosion rate exceeded 4.0.

Experimental Example  2: Experiment according to steel sheet thickness

The material and microstructure fraction of each layer were measured using the component system of Comparative Example 1, which had the lowest hardness among Examples 5 and Comparative Examples, and the results are shown in Table 4.

Steel grade Steel plate thickness (mm) Martensite fraction (volume%) Bainite fraction (vol%) Ferrite fraction (vol%) Perlite fraction (vol%) YS TS (MPa) (Mpa) Inventory 5 2 32.01 33.25 34.73 0 335 485 1.5 35.2 33.25 31.53 0 340 502 One 36.25 31.8 31.34 0 345 505 0.5 36.36 30.25 32.53 0 348 508 Comparative Example 1 3 0 0 99.1 0 248 388 2 0 0 98.9 0 252 399 1.5 0 13.25 85.6 0 282 412 One 5.2 15.3 78.95 0 290 420 0.5 10.01 22.5 67.35 0 323 460

As shown in Table 4, in Inventive Example 5, regardless of the steel sheet thickness, martensite and bainite are produced in a large amount, and it can be confirmed that the strength is excellent.

On the other hand, in Comparative Example 1, when the thickness of the steel sheet is not less than 1.5 mm, no martensite is produced at all and the strength is very poor.

When the thickness is 1 mm or less, it can be confirmed that the fraction of martensite and bainite is 50% by volume or less and the strength is poor.

In Comparative Example 1, too little Cr was contained, and when the thickness was too thick, sufficient transformation occurred during the time of laser heat treatment, or even if transformation occurred, sufficient hardening ability to form a hard phase could not be secured, Martensite and bainite were only partially formed in the vicinity of the surface even if the thickness was thin.

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 present invention as defined by the following claims. As will be understood by those skilled in the art. It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive.

Claims (12)

(Excluding 0%), Mn: 3.0% or less (excluding 0%), Cr: 1.0 to 5.0%, P: 0.02% or less (excluding 0% , S: not more than 0.01% (excluding 0%) and the balance Fe and unavoidable impurities,
And a heat deforming portion formed on a part of the steel sheet,
Wherein the heat deforming portion comprises at least 50 vol% of martensite and bainite as the sum. The method according to claim 1,
0.001 to 0.03% by weight of Mo, and 0.001 to 0.003% by weight of B, based on the total weight of the laser-curable low-carbon steel sheet. The method according to claim 1,
0.1 to 0.5% by weight of Cu, 0.1 to 0.5% by weight of Ni and 0.01 to 0.1% by weight of Co, based on the total weight of the laser-curable low-carbon steel sheet. The method according to claim 1,
Wherein the thermally deformable portion comprises 30 to 99% by volume of the martensite and 1 to 40% by volume of the bainite. The method according to claim 1,
A laser-curable low-carbon steel sheet having a plurality of the heat-deformable portions and an interval between the heat-deformable portions of 6 to 12 mm. 6. The method of claim 5,
And the area of each of the heat deforming portions is 50 mm ² or less. The method according to claim 1,
The thickness of the steel sheet is more than 1.0 mm and not more than 3.0 mm. (Excluding 0%), Mn: 3.0% or less (excluding 0%), Cr: 1.0 to 5.0%, P: 0.02% or less (excluding 0% , S: not more than 0.01% (excluding 0%), and the balance Fe and unavoidable impurities; And
And irradiating a part of the steel sheet with a laser beam. 9. The method of claim 8,
Further comprising the step of applying a tensile force to the steel sheet before the step of irradiating a part of the steel sheet with a laser beam. 9. The method of claim 8,
Wherein the surface of the steel sheet on which the laser beam is irradiated is heated to a temperature of (Ac3 + 150) DEG C or higher in the step of irradiating a part of the steel sheet with a laser beam. 9. The method of claim 8,
The method of manufacturing a laser-curable low-carbon steel sheet according to claim 1, wherein the step of irradiating a part of the steel sheet with a laser beam is such that the output of the laser beam is 5 to 10 kW. 9. The method of claim 8,
Further comprising the step of cooling the steel plate after the step of irradiating a part of the steel plate with a laser beam.

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