KR101677351B1 - Hot rolled steel sheet for hot press forming having low deviation of mechanical property and excellent formability and corrosion resistance, hot pressed part using the same and method for manufacturing thereof - Google Patents

Abstract

A hot-rolled steel sheet for hot press forming, which has a small material variation, excellent in toughness and corrosion resistance, a hot-press formed article using the same, and a method for producing the same. An aspect of the present invention relates to a steel sheet comprising, by weight, 0.2 to 0.3% of C, 1.2 to 1.8% of Mn, 0.01 to 0.5% of Si, 0.015% or less of P, 0.004% or less of S, 0.006 to 0.02% of N, 0.0001 to 0.005% of B, less than 0.1% of Cr and not more than 0.05% of Mo and 0.060 to 1% in total of at least one selected from the group consisting of Cu and Ni Ti, and Nb, the balance being Fe and inevitable impurities, wherein the contents of N, B, Ti and Nb satisfy the following relational expression 1: A hot-rolled steel sheet for molding is provided.
[Relation 1]
0.3? [(Mol% N) / {(mol% B) + (mol% Ti) + (mol% Nb)}]
(Wherein the parentheses represent the weight% of the element divided by the atomic weight of the element)

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet for hot-press forming, which is excellent in toughness and corrosion resistance, PRESSED PART USING THE SAME AND METHOD FOR MANUFACTURING THEREOF}

The present invention relates to a hot-rolled steel sheet for hot-press forming, which has a small material variation, excellent toughness and corrosion resistance, a hot-press molded article using the same, and a method of manufacturing the same.

In recent years, the strength of automobile structural members has been rapidly increasing to improve the fuel consumption efficiency and fuel efficiency of automobiles, and multifunctionalization is rapidly proceeding to meet the various characteristics required for each part.

In particular, automobile steering parts are required to have excellent strength, fatigue durability and corrosion resistance, and hot rolled steel sheets are generally applied to steel sheets for automobile steering parts. Meanwhile, such steering parts are manufactured in the form of a pipe, and they are manufactured as steering parts through various molding processes and heat treatments. In many cases, it is reported that a problem occurs in manufacturing cracks during piping and drawing. It is known to be caused by various causes, but it is reported to be mainly related to the cleanliness of the steel and the mechanical properties of the steel.

In this connection, Patent Documents 1 to 3 disclose a high-strength hot-press molded article excellent in moldability, weldability, fatigue characteristics, and the like. However, the above-mentioned techniques relate to a hot-press molded product produced by hot-forming a cold-rolled or hot-rolled steel sheet obtained by a conventional milling process, and any means for reducing the material deviation in the width direction or the longitudinal direction of the steel sheet before hot- Therefore, it is expected that, when the steering parts are manufactured by the above techniques, defective machining or the like frequently occurs.

Korean Patent Publication No. 10-2011-0053474 Korean Patent Registration No. 10-1291010 Japanese Laid-Open Patent Publication No. 2005-097725

An aspect of the present invention is to provide a hot-rolled steel sheet for hot-press forming, which has a small material variation, excellent toughness and corrosion resistance, a hot-press formed article using the same, and a method for producing the same.

In order to accomplish the above object, according to one aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising: 0.2 to 0.3% of C, 1.2 to 1.8% of Mn, 0.01 to 0.5% of Si, , At least one selected from the group consisting of 0.001 to 0.05% of Al, 0.006 to 0.02% of N, 0.0001 to 0.005% of B, 0.1% or less of Cr and 0.05% or less of Mo, Wherein the content of N, B, Ti and Nb is 0.060 to 1% in total, and contains 0.025 to 0.09% in total of at least one selected from the group consisting of Ti and Nb, the balance Fe and inevitable impurities, A hot-rolled steel sheet for hot press forming satisfying the following relational expression (1).

[Relation 1]

0.3? [(Mol% N) / {(mol% B) + (mol% Ti) + (mol% Nb)}]

(Wherein the parentheses represent the weight% of the element divided by the atomic weight of the element)

In another aspect of the present invention, there is provided a ferritic stainless steel comprising 0.2 to 0.3% of C, 1.2 to 1.8% of Mn, 0.01 to 0.5% of Si, 0.015% or less of P, 0.004% or less of S, At least one selected from the group consisting of Cu and Ni, in an amount of 0.060 to 1% in total, including 0.001 to 0.05% of N, 0.006 to 0.02% of B, 0.0001 to 0.005% of B, Wherein the content of N, B, Ti and Nb is in the range of 0.02 to 0.09% in total of at least one selected from the group consisting of Ti and Nb and the balance Fe and inevitable impurities, Continuously satisfying molten steel at a rate of 4 to 7 mpm to obtain a thin slab; Subjecting the thin slab to rough rolling and finish rolling at a constant speed within a range of 200 to 600 mpm and hot rolling at 800 to 950 占 폚 to obtain a hot rolled steel sheet at the finish rolling; Cooling the hot-rolled steel sheet to a temperature of 600 to 730 ° C at a rate of 0.5 ° C / sec or more; And a step of cooling the cooled hot-rolled steel sheet after air-cooling and winding the hot-rolled steel sheet.

[Relation 1]

0.3? [(Mol% N) / {(mol% B) + (mol% Ti) + (mol% Nb)}]

(Wherein the parentheses represent the weight% of the element divided by the atomic weight of the element)

In another aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising: 0.2 to 0.3% of C, 1.2 to 1.8% of Mn, 0.01 to 0.5% of Si, 0.001 to 0.05% of N, 0.006 to 0.02% of B, 0.0001 to 0.005% of B, less than 0.1% of Cr and not more than 0.05% of Mo, and at least one selected from the group consisting of Cu and Ni in a total amount of 0.060 to 1 %, And contains at least 0.025 to 0.09% of at least one selected from the group consisting of Ti and Nb, and the balance Fe and inevitable impurities, wherein the contents of N, B, Ti and Nb satisfy the following relational expression 1 , And the microstructure provides a hot press molded article comprising 90 to 98% of martensite and 2 to 10% of retained austenite in an area fraction.

[Relation 1]

0.3? [(Mol% N) / {(mol% B) + (mol% Ti) + (mol% Nb)}]

(Wherein the parentheses represent the weight% of the element divided by the atomic weight of the element)

In another aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising: 0.2 to 0.3% of C, 1.2 to 1.8% of Mn, 0.01 to 0.5% of Si, 0.001 to 0.05% of N, 0.006 to 0.02% of B, 0.0001 to 0.005% of B, less than 0.1% of Cr and not more than 0.05% of Mo, and at least one selected from the group consisting of Cu and Ni in a total amount of 0.060 to 1 %, And contains at least 0.025 to 0.09% of at least one selected from the group consisting of Ti and Nb, and the balance Fe and inevitable impurities, wherein the contents of N, B, Ti and Nb satisfy the following relational expression 1 Is continuously cast at a rate of 4 to 7 mpm to obtain a thin slab; Subjecting the thin slab to rough rolling and finish rolling at a constant speed within a range of 200 to 600 mpm and hot rolling at 800 to 950 占 폚 to obtain a hot rolled steel sheet at the finish rolling; Cooling the hot-rolled steel sheet to a temperature of 600 to 730 ° C at a rate of 0.5 ° C / sec or more; Cooling the cooled hot-rolled steel sheet after cooling it; Heating the wound hot rolled steel sheet to a heating temperature of 750 to 1000 占 폚; Maintaining the heated hot-rolled steel sheet at the heating temperature for 1 to 10 minutes; And pressing the hot rolled steel sheet held at the heating temperature and quenching the hot rolled steel sheet.

[Relation 1]

0.3? [(Mol% N) / {(mol% B) + (mol% Ti) + (mol% Nb)}]

(Wherein the parentheses represent the weight% of the element divided by the atomic weight of the element)

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS The various features and advantages and effects of the present invention will become more fully understood with reference to the following specific embodiments.

The hot-rolled steel sheet for hot-press forming according to the present invention is advantageous in that the material deviation is very small.

There is an advantage that the hot press molded article according to the present invention has very excellent toughness and corrosion resistance.

FIG. 1 is a schematic view for explaining a mini-mill process applied to the present invention.
2 is a graph showing a result of measurement of the material deviation in the width direction of the hot-rolled steel sheet according to Inventive Example 5 of the present invention.
3 is a photograph of the microstructure of a molded article according to Inventive Example 1 of the present invention observed by an optical microscope.

Hereinafter, the hot-rolled steel sheet for hot-press forming, which is one aspect of the present invention and has a small material deviation and is excellent in toughness and corrosion resistance, will be described in detail.

Carbon (C): 0.2 to 0.3 wt%

Carbon is an element that forms a carbide in steel or is dissolved in ferrite and contributes to the improvement of strength of hot-rolled steel sheet. In order to exhibit such an effect in the present invention, the content is preferably 0.2 wt% or more, more preferably 0.24 wt% or more. However, if the content is excessive, a hard texture is formed on the surface of the cast steel or bar plate during the performance and rolling, resulting in surface cracking, or the yield strength and tensile strength of the hot-rolled steel sheet are too high, Defects may occur during pipe making using rolls. Therefore, the carbon content is preferably 0.3 wt% or less, and more preferably 0.26 wt% or less.

Manganese (Mn): 1.2 to 1.8 wt%

Manganese inhibits ferrite formation and increases the austenite stability, facilitating the formation of low temperature transformation phases, thereby increasing the strength of the steel. In order to exhibit such an effect in the present invention, the content is preferably 1.2% by weight or more, more preferably 1.4% by weight or more. However, if the content is excessive, segregation bars may be formed inside and / or outside the performance slab and hot rolled steel sheet, and cracks may be generated and propagated during performance and / or hot rolling operations There is a problem of causing a material deviation of the hot-rolled steel sheet. Further, when the material deviation of the hot-rolled steel sheet occurs as described above, there arises a problem that a high frequency of machining defects is caused at the time of piping of the hot-pressed product after hot press forming. Accordingly, the content of manganese is preferably 1.8 wt% or less, more preferably 1.6 wt% or less.

Silicon (Si): 0.01 to 0.5 wt%

Silicon is an element added to improve the strength or ductility of hot-rolled steel sheets. In order to exhibit such an effect in the present invention, it is preferably contained in an amount of 0.01 wt% or more, more preferably 0.05 wt% or more. However, when the content is excessive, a large amount of surface oxide is formed to cause surface defects such as dent. Therefore, the silicon content is preferably 0.5% by weight or less, more preferably 0.2% by weight or less.

Phosphorus (P): 0.015% by weight or less

Phosphorus is an impurity that is inevitably contained in the steel and is an element which causes segregation in grain boundaries and / or intergranular grain boundaries and causes brittleness. Therefore, it is preferable to control the content to a minimum. Theoretically, it is preferable to limit the phosphorus content to 0%, but it is inevitably contained inevitably in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the phosphorus content is controlled to 0.015 wt%.

Sulfur (S): not more than 0.004% by weight

Since sulfur is an impurity inevitably contained in the steel, it forms an MnS nonmetallic inclusion by binding with Mn in the steel, segregates during performance solidification, and is a main cause of causing high-temperature cracking, so that the content thereof is preferably controlled to be as low as possible . Theoretically, it is advantageous to limit the content of sulfur to 0%, but it is inevitably contained inevitably in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the sulfur content is controlled to 0.004% by weight.

Aluminum (Al): 0.001 to 0.05 wt%

Aluminum is an element contributing to improvement of steel ductility by inhibiting carbide formation. In order to exhibit such an effect in the present invention, it is preferable that it is contained in an amount of 0.001% by weight or more. However, when the content is excessive, AlN is formed by reacting with nitrogen in the steel, thereby causing corner cracks in manufacturing thin slabs, thereby deteriorating the quality of slabs or steel sheets. Therefore, the aluminum content is preferably 0.05 wt% or less.

Nitrogen (N): 0.006 to 0.02 wt%

Nitrogen is generally classified as impurities inevitably contained in steel, but in the present invention, it is an element intentionally added for nitride formation and austenite stabilization in steel. Furthermore, the nitrogen serves to increase the hardness of martensite formed by rapid cooling after hot press forming, thereby improving the tensile strength and fatigue durability of the hot press molded article. In order to exhibit such an effect in the present invention, the content is preferably 0.006% by weight or more, more preferably 0.008% by weight or more. However, if the content is excessive, the content of the precipitation elements in the steel required for precipitation strengthening is reduced, resulting in a decrease in strength and fatigue durability of the hot-rolled steel sheet. Accordingly, the nitrogen content is preferably 0.02 wt% or less, and more preferably 0.02 wt% or less.

Boron (B): 0.0001 to 0.005% by weight

Boron contributes to improving the hardenability of the steel. On the other hand, when the hot-rolled steel sheet is produced by the min-milling process, the nitrogen content in the steel is high, and the precipitation strengthening effect due to the precipitate-forming element is reduced to lower the strength and workability of the steel. It is possible to minimize the deterioration of the strength and workability of the hot-rolled steel sheet. In order to exhibit such an effect in the present invention, the content is preferably 0.0001% by weight or more, more preferably 0.0002% by weight or more. However, when the content is excessive, the austenite recrystallization temperature is raised to deteriorate the weldability. Therefore, the content of boron is preferably 0.005% by weight or less, and more preferably 0.003% by weight or less.

Cr: less than 0.1% (including 0%)

Chromium helps improve the strength of the hot-rolled steel sheet, but is not intentionally added in the present invention. Moreover, when the chromium content is excessive, the material of the hot-rolled steel sheet becomes high, which causes a problem of deteriorating the toughness. Therefore, it is preferable to control the content as low as possible. In the present invention, the chromium content is controlled to be less than 0.1%.

Mo: 0.05% or less (including 0%)

Molybdenum also helps to improve the strength of the hot-rolled steel sheet, but like chromium, it is not intentionally added in the present invention. Further, when the content of molybdenum is excessive, there is a problem that the material of the hot-rolled steel sheet becomes high and the toughness deteriorates. Therefore, it is preferable to control the content as low as possible. In the present invention, the molybdenum content is controlled to be 0.05% or less.

At least one selected from the group consisting of copper (Cu) and nickel (Ni): 0.060 to 1.0 wt%

Copper and nickel are elements contributing to the improvement of corrosion resistance of hot-rolled steel sheets and molded products. In order to exhibit such effects in the present invention, at least one element selected from the group consisting of copper (Cu) and nickel (Ni) is preferably contained in an amount of 0.060 wt% or more. However, if the content is excessive, the slab may be concentrated into a liquid phase on the surface during the slab manufacturing process to cause a defective cast steel, and the scale may remain on the surface of the hot-rolled steel sheet to lower the pickling quality. Therefore, the sum of these contents is preferably 1.0% by weight or less, more preferably 0.8% by weight or less.

At least one selected from the group consisting of titanium (Ti) and niobium (Nb): 0.025 to 0.09 wt%

Titanium and niobium are elements forming precipitates (TiC, TiCN, TiNbCN, NbC, etc.) and contribute to the improvement of steel strength. In order to exhibit such an effect in the present invention, at least one element selected from the group consisting of titanium (Ti) and niobium (Nb) is included, and the sum of the contents is preferably 0.025% by weight or more, more preferably 0.04% . However, when the content is excessive, not only the production cost of hot-rolled steel sheet is increased, but also the precipitation hardening effect is lowered due to the precipitation of coarse precipitates in steel during performance, and the tensile strength of hot- There is a problem of deterioration. Therefore, the sum of these contents is preferably 0.09% by weight or less, more preferably 0.06% by weight or less.

The rest of the composition is 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 referred to in this specification, as they are known to one of ordinary skill in the art.

When designing an alloy of a steel material having the above-mentioned composition range, it is preferable that the content of N, B, Ti and Nb satisfy the following relational expression (1). The following relational formula 1 is obtained by factoring precipitation strengthening effect by precipitate formation. When the value is too low, it is difficult to secure fatigue durability of the final molded product due to the low nitrogen content in the steel. Therefore, it is preferable to control it to 0.3 or more. However, when the value is too high, the content of the precipitate-forming elements is insufficient and it is difficult to secure the yield strength required of the final molded product. Therefore, it is preferable to control to 1.6 or less, more preferably to 1.4 or less.

[Relation 1]

0.3? [(Mol% N) / {(mol% B) + (mol% Ti) + (mol% Nb)}]

(Wherein the parentheses represent the weight% of the element divided by the atomic weight of the element)

According to one embodiment of the present invention, when designing an alloy of a steel material having the above-described composition range, the carbon equivalent (Ceq) is preferably 0.4 to 0.7, more preferably 0.5 to 0.6 . If the carbon equivalent is less than 0.4, it may be difficult to secure the strength of the desired hot-rolled steel sheet. If the carbon equivalent is more than 0.7, the strength of the welded portion is too high, and the difference in hardness from the welded heat affected portion is too large. On the other hand, the carbon equivalent is defined by the following equation (1), and when the alloy element constituting the following equation (1) is absent, it is calculated as 0.

[Formula 1]

(Wt% Ni) + (wt% Cu) Ceq = (wt% C) + (wt% Mn) / 6 + } / 15

(Where the parentheses indicate the weight% of the corresponding element, respectively)

The microstructure of the hot-rolled steel sheet according to the present invention may contain 40 to 70% of ferrite and 30 to 60 of pearlite, more preferably 50 to 60% of ferrite, And 40 to 50 pearlite. By securing the microstructure as described above, a tensile strength of 500 to 600 MPa, a yield ratio of 0.5 to 0.6, and an elongation of 20 to 30% can be secured.

Hereinafter, a method of manufacturing a hot-rolled steel sheet for hot press forming which is less in material deviation and superior in toughness and corrosion resistance, which is another aspect of the present invention, will be described in detail.

First, a mini-milling process using continuous continuous rolling by thin-slab and rolling direct-rolling processes will be described in detail.

FIG. 1 is a schematic view for explaining a mini-mill process applied to the present invention. As shown in FIG. 1, the mini-mill process applied to the present invention comprises continuous casting, rough rolling, finishing rolling, cooling and winding steps, and then subjected to cold rolling and continuous annealing steps through ordinary equipment, . At this time, an intermittent hot rolling method using a coil box is carried out by controlling the operating conditions of each step in the mini-mill process so that the driving speed (mass flow rate) of the rough rolling-finishing rolling- Or a hot-rolled steel sheet is obtained in a continuous manner without using a coil box.

1, the thin slab a having a thickness of 30 to 150 mm is obtained in the continuous casting machine 10. This is considerably thinner than a slab having a thickness of 200 mm or more produced by a continuous casting machine of a conventional mill, and this slab is called a thin slab. Since the thin slab is transferred to the roughing mill 20 in a continuous process and then roughly rolled, the heat source of the slab itself can be used as it is and energy can be saved. As a result, fine slabs The transition process of the texture and precipitate is different from that of the conventional mill, and the mechanical properties of the final steel sheet are changed. On the other hand, when the thickness of the thin slab is more than 150 mm, the difference with respect to the existing mill is small. When the thickness of the thin slab is less than 30 mm, the temperature of the slab falls sharply and it is difficult to form a uniform structure. In order to solve this problem, it is possible to additionally provide a heating facility, but this is a factor for improving the production cost, so it is preferable to exclude it.

The thin slabs are further rolled to a desired final thickness at roughing and finishing mills 20 and 50, cooled through a runout table (ROT) 60 and then wound at a constant temperature in a winder 70 Hot-rolled steel sheets. As described above, the present invention is characterized in that the operation speed of the coarse rolling mill (20) - finishing mill (50) - coiler (60) is controlled to be the same, A coil box 40 is provided in front of the finish rolling mill 50 and a bar plate b having passed through the induction heater 30 is firstly wound to compensate for this difference It is possible.

Hereinafter, specific operating conditions of each step will be described in detail.

First, molten steel satisfying the alloy composition described above is prepared, and then continuously cast in a continuous casting machine 10 at a rate of 4 to 7 mpm (meter per minute) to obtain a thin slab. The reason why the casting speed is controlled to be 4 mpm or more is that a casting speed higher than a certain level is required to secure the target rolling temperature because the casting and rolling processes are connected. However, when the casting speed is excessively high, there is a possibility that the operation success rate due to instability of the molten steel bath surface is reduced. Therefore, it is preferable to control the casting speed to 7 mpm or less.

Thereafter, the thin slabs obtained by the continuous casting are rough-rolled by a roughing mill 20 composed of two to four rolling stands, and then the bar-shaped b obtained by the rough rolling is transferred to a finishing mill 60 Followed by finish rolling to obtain a hot-rolled steel sheet.

At this time, it is preferable to control the mass flow to be the same mass flow from the continuous casting to the winding process through the constant velocity rolling. It is preferable to control the rolling speed within the range of 200 to 600 mpm and within the range of 300 to 500 mpm It is more preferable to control it. This is because it is difficult to secure the temperature of the hot-rolled steel sheet when the rolling speed is excessively low, and it is difficult to control the hot-rolled steel sheet temperature to the target temperature when the rolling speed is excessively fast, to be.

At this time, the finish rolling temperature during the finish rolling is preferably 800 to 950 ° C. If the finishing rolling temperature is lower than 800 ° C, the load on the rolling mill increases greatly. On the other hand, when the finishing rolling temperature exceeds 950 ° C, the stability of continuous continuous rolling operation is lowered and the occurrence of hot rolling scale defects increases.

According to an embodiment of the present invention, the surface temperature of the thin slab at the side of the rough rolling mill may be 1000 to 1200 ° C, have. If the surface temperature of the thin slab is less than 1000 캜, the rough rolling load may increase and cracks may be generated in the bar plate edge portion in the rough rolling process. In this case, there is a fear that the edge portion of the hot rolled steel sheet may be defective. On the other hand, when the surface temperature of the thin slab is more than 1200 ° C, the surface quality may be deteriorated due to the generation of the hot-rolled scale or the slab shape may be deformed due to the non-solidification of the cast steel.

Also, according to an embodiment of the present invention, the cumulative rolling reduction during the rough rolling may be 60 to 90%, and more preferably 70 to 80%. The higher the cumulative reduction ratio in rough rolling, the more advantageous is the production of a steel sheet having the desired excellent surface quality in the present invention. In addition, the higher the cumulative reduction ratio in the rough rolling, the more uniform the distribution of the microstructure and the alloy composition formed in the cast slab (thin slab). In order to secure such effect, it is desirable to control the cumulative reduction ratio to 60% or more. However, if the cumulative reduction ratio is excessively high, the rolling deformation resistance may become large, which may cause difficulty in operation. Therefore, the cumulative reduction ratio is preferably controlled to 90% or less.

Thereafter, the hot-rolled steel sheet is cooled from 600 to 730 DEG C at a rate of 0.5 DEG C / sec or more in the run-out table (ROT) 60, air-cooled and then wound by a winder 70.

The water-cooling is performed to retard the transformation of austenite into ferrite. The water-cooling termination temperature during the water-cooling is preferably 600 to 730 ° C, more preferably 650 to 700 ° C. If the water-cooling end temperature is lower than 600 ° C, irregularly shaped ferrite may be formed. In this case, the ferrite transformation may end in the water-cooling section and the coil shape may deteriorate. On the other hand, There is a possibility that the surface quality of the steel sheet is deteriorated.

On the other hand, in the water-cooling, the water-cooling rate is preferably 0.5 ° C / sec or more, more preferably 10 ° C / sec or more. If the water-cooling rate is less than 0.5 DEG C / sec, the hot-rolled scale is again formed thick, and there is a possibility that the scale composition that makes descaling difficult is changed. On the other hand, the higher the water-cooling rate, the better the delay of ferrite transformation, so the upper limit is not particularly limited.

The step of picking up the hot-rolled steel sheet may further include the step of picking up the hot-rolled steel sheet, whereby the scale formed on the surface of the hot-rolled steel sheet can be removed. The pickling process may be carried out by any of the usual methods in the art.

Hereinafter, a hot press molded article which is another aspect of the present invention will be described in detail.

The hot press molded article according to another aspect of the present invention has the above-mentioned component system, and its microstructure contains martensite of 90 to 98 area% and retained austenite of 2 to 10 area% .

According to an embodiment of the present invention, the maximum size of the martensite packet of the molded article may be 5 to 35 탆, and more preferably 5 to 25 탆. Here, the martensite packet means a cluster of lath and block martensite having the same crystal orientation.

The hot-pressed product according to the present invention has an advantage that the strength is extremely excellent. According to an embodiment of the present invention, the hot-pressed product may have a yield strength of 1000 MPa or more and a tensile strength of 1470 MPa or more.

Hereinafter, a method of manufacturing a hot press molded article, which is another aspect of the present invention, will be described in detail.

First, a hot-rolled steel sheet is prepared by the above-described method. Then, for the hot press forming, the hot-rolled steel sheet is heated to the austenite single-phase reverse temperature region.

At this time, the heating temperature (heating end temperature) is preferably 750 to 1000 占 폚, and more preferably 850 to 950 占 폚. If the heating temperature is less than 750 占 폚, the austenite transformation is insufficient and the residual ferrite is present, or the ferrite formation due to the temperature lowering before the hot forming by the mold lowers the strength of the molded product after hot press forming, May be deteriorated. On the other hand, when the temperature is higher than 1000 ° C., not only the productivity is lowered but also the oxide is excessively formed on the surface layer or the decarburized layer is formed on the surface of the steel sheet to degrade the surface quality of the molded product after hot press forming, There is a possibility of causing deterioration.

According to an embodiment of the present invention, the heating rate may be 1 to 100 ° C / sec, and more preferably 3 to 20 ° C / sec. If the heating rate is less than 1 占 폚 / sec, the productivity is deteriorated. On the other hand, when the heating rate is more than 100 占 폚 / sec, it is difficult to secure a uniform temperature distribution over the entire thickness of the hot-rolled steel sheet.

Thereafter, the heated hot-rolled steel sheet is maintained at the heating temperature (heating termination temperature) for 1 to 10 minutes, more preferably for 5 to 6 minutes. This step is carried out in order to secure a uniform size and fraction of austenite structure. If the holding time is less than 1 minute, a heterogeneous austenite structure may be formed at the center of the thickness of the hot-rolled steel sheet, If it exceeds 10 minutes, it may be difficult to secure a martensite structure after quenching due to a decrease in the productivity of molded product production and a decarbonization layer having an excessive depth on the surface of the steel sheet.

Thereafter, the hot-rolled steel sheet held at the above-mentioned heating temperature is press-molded by a die and rapidly cooled. At this time, the molding and quenching by the mold is sufficient by a conventional hot press forming method, and therefore, the present invention is not particularly limited thereto.

According to an embodiment of the present invention, the quenching rate during quenching may be 10 ° C / sec or more, more preferably 30 ° C / sec or more. If the quenching rate is less than 10 ° C / sec, a uniform martensite structure can not be ensured over the entire thickness of the steel sheet, and it may be difficult to secure the quality of the final molded article. On the other hand, as the quenching rate is higher, it is advantageous to obtain a uniform martensite structure, so the upper limit is not particularly limited.

According to an embodiment of the present invention, the quenching may further include a tempering heat treatment at a temperature ranging from 100 to 350 ° C. In the above tempering treatment, some martensite structure is transformed into tempering martensite. In this case, toughness can be imparted to the final molded product.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the description of these embodiments is intended only to illustrate the practice of the present invention, but the present invention is not limited thereto. And the scope of the present invention is determined by the matters described in the claims and the matters reasonably deduced therefrom.

( Example )

Hot rolled slabs of 90 mm in thickness were prepared by continuously casting molten steel having the compositions shown in Tables 1 and 2 under the conditions described in Table 3, and the slabs were subjected to rough rolling, finish rolling, cooling, A hot-rolled steel sheet having a thickness of 5.0 mm was produced. At this time, the rolling reduction rate in rough rolling was 80%, the rolling speed in rolling was 400 mpm, the water cooling rate was 20 ° C / sec, and the water cooling end temperature was constant at 650 ° C.

Then, the microstructure of the steel sheet was analyzed for the thus-prepared hot-rolled steel sheet, and the material thereof was measured. The results are shown in Table 4 below. At this time, the measurement of the material of the steel sheet was conducted by taking JIS No. 5 specimen in a direction perpendicular to the rolling direction at a quarter point in the width direction. On the other hand, in Table 4, YS, TS, T. El and YR mean yield strength, tensile strength, elongation and yield ratio, respectively.

Then, the produced hot-rolled steel sheet was evaluated for toughness and corrosion resistance, and the results are shown in Table 4 below. The biaxiality was evaluated by visual and microscopic observation whether cracks were generated by U-shaped bending test, and it was evaluated as "○" when no crack occurred and "×" when crack occurred. The corrosion resistance was evaluated by measuring the width of the scratch when the surface of the steel sheet was subjected to X-ray scratch test and then sprayed in a spraying atmosphere of NaCl 5% for 480 hours. When the average value of the scratch width was 3 mm or less, Was evaluated as "x" when the average value of the widths of the test pieces exceeded 3 mm.

Steel grade Alloy composition (% by weight) C Mn Si P S Al N B A 0.230 1.570 0.053 0.0150 0.0037 0.012 0.0075 0.0022 B 0.233 1.545 0.191 0.0066 0.0012 0.011 0.0064 0.0020 C 0.253 1.602 0.200 0.0056 0.0014 0.010 0.0065 0.0023 D 0.251 1.606 0.061 0.0150 0.0032 0.016 0.0085 0.0014 E 0.230 1.500 0.051 0.0150 0.0040 0.003 0.0085 0.0011 F 0.216 1.240 0.043 0.0140 0.0030 0.004 0.0097 0.0020 G 0.270 1.179 0.185 0.0152 0.0010 0.021 0.0080 0.0023 H 0.255 1.242 0.206 0.0101 0.0015 0.023 0.0036 0.0020 I 0.237 1.233 0.188 0.0140 0.0022 0.027 0.0038 0.0020

Steel grade Alloy composition (% by weight) Ceq ① Cr Mo Cu Ni Ti Nb A 0.098 0.020 0.305 0.000 0.0390 0.0100 0.536 0.476 B 0.035 0.000 0.306 0.000 0.0390 0.0100 0.518 0.413 C 0.051 0.000 0.329 0.282 0.0532 0.0100 0.571 0.324 D 0.090 0.000 0.060 0.000 0.0480 0.0000 0.541 0.536 E 0.097 0.020 0.100 0.000 0.0130 0.0000 0.510 1.626 F 0.098 0.018 0.307 0.000 0.0100 0.0000 0.466 1.759 G 0.150 0.140 0.053 0.000 0.0440 0.0000 0.528 0.505 H 0.150 0.000 0.050 0.000 0.0380 0.0030 0.495 0.254 I 0.140 0.130 0.029 0.000 0.0410 0.0030 0.498 0.253 (Mol% N) / (mol% B) + (mol% Ti) + (mol% Nb)}]

Steel grade Casting speed (mpm) Night slab
Inlet temperature (℃) Finishing rolling temperature (캜) Coiling temperature (캜) Remarks A 5.0 1111 903 700 Inventory 1 5.0 1085 903 650 Inventory 2 5.0 1097 903 600 Inventory 3 5.0 1093 809 700 Honorable 4 B 4.9 1073 868 700 Inventory 5 C 4.9 1089 845 650 Inventory 6 D 4.9 1069 854 650 Honorable 7 E 4.9 1089 853 650 Comparative Example 1 F 4.9 1082 878 700 Comparative Example 2 G 4.9 1050 850 713 Comparative Example 3 H 4.9 1077 896 691 Comparative Example 4 I 4.9 1096 884 695 Comparative Example 5

Steel grade Microstructure (area%) YS
(MPa) TS
(MPa) T.El
(%) YR Coarseness Corrosion resistance Remarks A F66 + P34 333 537 27.5 0.62 ○ ○ Inventory 1 F61 + P39 349 543 27.0 0.64 ○ ○ Inventory 2 F46 + P54 410 598 20.7 0.69 ○ ○ Inventory 3 F58 + P42 352 534 26.3 0.66 ○ ○ Honorable 4 B F63 + P37 366 564 27.0 0.65 ○ ○ Inventory 5 C F62 + P38 407 596 26.0 0.68 ○ ○ Inventory 6 D F65 + P35 372 582 23.7 0.64 ○ ○ Honorable 7 E F62 + P38 361 532 30.0 0.68 ○ ○ Comparative Example 1 F F70 + P30 363 524 28.0 0.69 ○ ○ Comparative Example 2 G F66 + P34 460 645 21.3 0.71 × × Comparative Example 3 H F68 + P32 496 688 20.0 0.72 × × Comparative Example 4 I F67 + P33 498 687 20.0 0.72 × × Comparative Example 5 Among the above microstructures, F means ferrite and P means pearlite.

Referring to Table 4, Examples 1 to 7, which satisfy all of the conditions proposed by the present invention, show that the molded article has excellent toughness and corrosion resistance. On the other hand, in the case of Comparative Examples 1 and 2, it was confirmed that the content of Ti and Nb did not satisfy the range suggested by the present invention, but the hot-rolled steel sheet had excellent toughness and corrosion resistance. On the other hand, in the case of Comparative Examples 3 to 5, the content of Cr and / or Mo was excessive and the toughness and corrosion resistance were poor.

FIG. 2 is a graph showing the results of measurement of the material deviation in the width direction of the hot-rolled steel sheet according to Inventive Example 5 of the present invention. FIG. 2 shows the results of measuring the yield strength, tensile strength and elongation of each of the cut specimens after cutting the hot-rolled steel sheet according to Inventive Example 5 into a total of 50 specimens in the width direction. Referring to FIG. 2, the hot-rolled steel sheet according to the present invention can visually confirm that the material deviation in the width direction perpendicular to the rolling direction is very small. Therefore, according to the present invention, there is no difference in the material of the hot-rolled steel sheet according to the position, so that the manufacturing error rate of the tubular pipe can be increased.

Then, each of the hot-rolled steel sheets thus produced was heated to a heating temperature of 900 캜 at a rate of 10 캜 / sec, held for 5 minutes, and quenched at the same time as press molding to produce a molded article. Then, microstructures were analyzed and the materials were measured on the thus-formed molded article, and the results are shown in Table 5 below. At this time, the material measurement method is as described above.

Steel grade Microstructure (area%) Martensite
Packet maximum size (㎛) YS
(MPa) TS
(MPa) T.El
(%) YR Remarks A M95 + A5 18.7 1016 1498 8.4 0.68 Inventory 1 M96 + A4 17.5 1023 1496 6.2 0.68 Inventory 2 M95 + A5 19.8 1019 1478 7.1 0.69 Inventory 3 M95 + A5 18.7 1012 1466 7.3 0.69 Honorable 4 B M96 + A4 16.3 1051 1566 9.7 0.67 Inventory 5 C M96 + A4 15.2 1110 1625 9.6 0.68 Inventory 6 D M95 + A5 16.3 1111 1577 7.1 0.70 Honorable 7 E M95 + A5 21.0 729 1183 5.1 0.68 Comparative Example 1 F M95 + A5 21.6 664 1105 5.3 0.68 Comparative Example 2 G M95 + A5 14.8 1141 1666 6.6 0.68 Comparative Example 3 H M95 + A5 14.2 1172 1648 6.5 0.71 Comparative Example 4 I M95 + A5 14.4 1174 1675 6.2 0.70 Comparative Example 5 In the above microstructure, M means martensite and A means residual austenite.

Referring to Table 5, Examples 1 to 7, which satisfy all of the conditions proposed by the present invention, can confirm that the strength of the molded article is extremely excellent. On the other hand, in the case of Comparative Examples 3 to 5, it was confirmed that the content of Cr and / or Mo did not satisfy the range suggested by the present invention, but the strength of the molded product was excellent. On the other hand, in Comparative Examples 1 and 2, the content of Ti and Nb did not satisfy the range proposed by the present invention, but the strength of the molded article was poor.

On the other hand, Fig. 3 is a photograph of the microstructure of the molded article according to Inventive Example 1 of the present invention observed with an optical microscope. More specifically, in order to measure the size of the martensite packet, a photograph was observed with an optical microscope, and the grain orientation angle was measured using a scanning electron microscope equipped with an EBSD (Electron Backscatter Diffraction) analysis function And grain boundaries having a crystal grain orientation angle of 15 degrees or more are shown in blue thick lines. Referring to FIG. 3, the molded article according to the present invention can visually confirm that the maximum packet size of martensite is 35 μm or less.

Claims (14)

0.001 to 0.05% of Al, 0.001 to 0.05% of Al, 0.006 to 0.02% of N, 0.001 to 0.03% of Cr, 0.2 to 0.3% of C, 1.2 to 1.8% of Mn, 0.01 to 0.5% , B: 0.0001 to 0.005%, Cu: 0.060 to 1%, Cr: less than 0.1% and Mo: 0.05% or less, and 0.025 to 0.09% in total of at least one selected from the group consisting of Ti and Nb , The balance Fe and unavoidable impurities,
Wherein the content of N, B, Ti and Nb satisfies the following relational expression 1 and has a tensile strength of 500 to 600 MPa and a yield ratio of 0.69 or less and a difference between the maximum value and the minimum value of the tensile strength in the transverse direction is within 100 MPa Hot - rolled steel sheet for hot press forming.
[Relation 1]
0.3? [(Mol% N) / {(mol% B) + (mol% Ti) + (mol% Nb)}]
(Wherein the parentheses represent the weight% of the element divided by the atomic weight of the element)
The method according to claim 1,
Wherein the carbon equivalent of the hot-rolled steel sheet is 0.4 to 0.7.
The method according to claim 1,
Wherein the microstructure of the hot-rolled steel sheet comprises an area fraction of 40 to 70% of ferrite and 30 to 60 of pearlite.
0.001 to 0.05% of Al, 0.001 to 0.05% of Al, 0.006 to 0.02% of N, 0.001 to 0.03% of Cr, 0.2 to 0.3% of C, 1.2 to 1.8% of Mn, 0.01 to 0.5% , B: 0.0001 to 0.005%, Cu: 0.060 to 1%, Cr: less than 0.1% and Mo: 0.05% or less, and 0.025 to 0.09% in total of at least one selected from the group consisting of Ti and Nb , The balance Fe, and unavoidable impurities, wherein the content of N, B, Ti and Nb is continuously cast at a rate of 4 to 7 mpm in molten steel satisfying the following relational expression 1 to obtain a thin slab having a thickness of 30 to 150 mm;
The thin slab is subjected to rough rolling and finish rolling at a constant speed within a range of 200 to 600 mpm, hot rolling at a reduction ratio of 60 to 90% in the rough rolling, hot rolling at 800 to 950 캜 during the final rolling, ;
Cooling the hot-rolled steel sheet at a rate of 0.5 ° C / sec or higher to 600 to 730 ° C; And
And cooling and coiling the cooled hot rolled steel sheet,
Wherein the hot rolled steel sheet has a tensile strength of 500 to 600 MPa and a yield ratio of 0.69 or less and a difference between a maximum value and a minimum value of the tensile strength in the width direction is within 100 MPa.
[Relation 1]
0.3? [(Mol% N) / {(mol% B) + (mol% Ti) + (mol% Nb)}]
(Wherein the parentheses represent the weight% of the element divided by the atomic weight of the element)
delete 5. The method of claim 4,
Wherein the drawing temperature of the thin slab is 1000 to 1200 DEG C during the rough rolling.
delete 5. The method of claim 4,
Further comprising the step of pickling the hot-rolled steel sheet after the winding.
A hot-pressed product obtained by hot-pressing a hot-rolled steel sheet according to any one of claims 1 to 3,
A hot press molded product comprising 90 to 98% of martensite and 2 to 10% of retained austenite in an area fraction thereof.
10. The method of claim 9,
Wherein the maximum size of the martensite packet of the molded article is 5 to 35 占 퐉.
10. The method of claim 9,
The yield strength of the molded article is 1000 MPa or more, and the tensile strength is 1470 MPa or more.
0.001 to 0.05% of Al, 0.001 to 0.05% of Al, 0.006 to 0.02% of N, 0.001 to 0.03% of Cr, 0.2 to 0.3% of C, 1.2 to 1.8% of Mn, 0.01 to 0.5% , B: 0.0001 to 0.005%, Cu: 0.060 to 1%, Cr: less than 0.1% and Mo: 0.05% or less, and 0.025 to 0.09% in total of at least one selected from the group consisting of Ti and Nb , The balance Fe, and unavoidable impurities, wherein the content of N, B, Ti and Nb is continuously cast at a rate of 4 to 7 mpm in molten steel satisfying the following relational expression 1 to obtain a thin slab having a thickness of 30 to 150 mm;
The thin slab is subjected to rough rolling and finish rolling at a constant speed within a range of 200 to 600 mpm, hot rolling at a reduction ratio of 60 to 90% in the rough rolling, hot rolling at 800 to 950 캜 during the final rolling, ;
Cooling the hot-rolled steel sheet at a rate of 0.5 ° C / sec or higher to 600 to 730 ° C;
Cooling the cooled hot-rolled steel sheet after cooling it;
Heating the wound hot rolled steel sheet to a heating temperature of 750 to 1000 占 폚;
Maintaining the heated hot-rolled steel sheet at the heating temperature for 1 to 10 minutes; And
Pressing the hot rolled steel sheet held at the heating temperature and quenching it,
Wherein the hot rolled steel sheet has a tensile strength of 500 to 600 MPa and a yield ratio of 0.69 or less and a difference between a maximum value and a minimum value of the tensile strength in the width direction is within 100 MPa.
[Relation 1]
0.3? [(Mol% N) / {(mol% B) + (mol% Ti) + (mol% Nb)}]
(Wherein the parentheses represent the weight% of the element divided by the atomic weight of the element)
13. The method of claim 12,
Wherein the heated hot rolled steel sheet is heated at a heating rate of 1 to 100 占 폚 / sec.
13. The method of claim 12,
Wherein the hot-rolled steel sheet held at the heating temperature is press-molded and the cooling rate is 10 ° C / sec or more at the time of cooling.

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