CN105734412B - Hot-rolled steel sheet for hot press forming with small material variation and excellent formability and corrosion resistance, molded article using the same, and manufacturing method thereof - Google Patents

Abstract

The invention discloses a hot-rolled steel sheet for hot-press forming with small material deviation and excellent formability and corrosion resistance, a hot-press formed product using the hot-rolled steel sheet, and a manufacturing method of the hot-press formed product and the hot-press formed product. One aspect of the present invention provides a hot-rolled steel sheet for hot press forming, including, in 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, 0.001 to 0.05% of Al, 0.006 to 0.02% of N, 0.0001 to 0.005% of B, less than 0.1% of Cr, and 0.05% or less of Mo, and including 0.060 to 1% in total of one or more selected from Cu and Ni, and including 0.025 to 0.09% in total of one or more selected from Ti and Nb, and including the balance of Fe and unavoidable impurities.

Description

Hot-rolled steel sheet for hot press forming with small material variation and excellent formability and corrosion resistance Molded article using same and manufacturing method thereof

technical field

The invention relates to a material with small deviation and good formability A hot-rolled steel sheet for hot-press forming excellent in corrosion resistance, a hot-press-formed product using the same, and a method for producing the same.

Background technique

In recent years, in order to improve the fuel consumption efficiency and fuel efficiency of automobiles, the strengthening of automobile structural parts is being accelerated, and the multifunctionalization of said automobile structural parts is also being accelerated in order to meet the various characteristics required by each part. .

Especially for automobile steering parts, not only excellent strength but also excellent fatigue resistance and corrosion resistance are required, and hot-rolled steel sheets are generally used as steel sheets for automobile steering parts. In addition, such a steering member is made into a tubular shape, and various forming processes and heat treatments are performed to make the steering member. However, it has been reported that, in most cases, process cracks occur during the forming and drawing of the tube. This is known to be caused by a number of reasons, but has been reported to be primarily related to the cleanliness of the steel and the mechanical physical properties of the steel.

In contrast, Patent Documents 1 to 3 disclose high-strength hot-press molded products excellent in properties such as formability, weldability, and fatigue properties. However, the technique relates to a thermoformed product obtained by thermoforming a cold-rolled or hot-rolled steel sheet obtained through an existing rolling process, wherein There is no disclosure of any treatment process for reducing material variation in the width or length direction of the steel sheet before hot press forming, and therefore, it is considered that problems such as poor processing frequently occur when steering components are manufactured by the above-mentioned technique.

[Prior Art Literature]

【Patent Literature】

(Patent Document 1) Korean Laid-Open Patent Publication No. 10-2011-0053474

(Patent Document 2) Korean Granted Patent Publication No. 10-1291010

(Patent Document 3) Japanese Laid-Open Patent Publication No. 2005-097725

Contents of the invention

The technical problem to be solved in the present invention

An object of the present invention is to provide a hot-rolled steel sheet for hot-press forming with less material variation and excellent formability and corrosion resistance, a hot-press-formed product using the same, and a method for producing the same.

Technical means to solve technical problems

In order to achieve the above object, one aspect of the present invention provides a hot-rolled steel sheet for hot-press forming, the hot-rolled steel sheet for hot-press forming contains 0.2-0.3% of C, 1.2-1.8% of Mn , 0.01-0.5% of Si, 0.015% or less of P, 0.004% or less of S, 0.001-0.05% of Al, 0.006-0.02% of N, 0.0001-0.005% of B, less than 0.1% of Cr and less than 0.05% Mo, and contains a total of 0.060 to 1% of one or more selected from Cu and Ni, and contains a total of 0.025 to 0.09% of one or more selected from Ti and Nb, and contains the balance of Fe and Unavoidable impurities. In addition, the content of N, B, Ti and Nb satisfies the following relational formula 1.

[relational expression 1]

0.3≤[(mol%N)/{(mol%B)+(mol%Ti)+(mol%Nb)}]≤1.6

In the relational formula 1, the parentheses represent the values obtained by dividing the weight % of the corresponding element by the atomic weight of the corresponding element, respectively.

In addition, another aspect of the present invention provides a method for manufacturing a hot-rolled steel sheet for hot-press forming, the method for manufacturing a hot-rolled steel sheet for hot-press forming includes the following steps: performing continuous casting with molten steel at a speed of 4 to 7 mpm , so as to obtain a thin slab, the molten steel contains 0.2-0.3% of C, 1.2-1.8% of Mn, 0.01-0.5% of Si, 0.015% or less of P, 0.004% or less of S, 0.001- 0.05% of Al, 0.006 to 0.02% of N, 0.0001 to 0.005% of B, less than 0.1% of Cr, and less than 0.05% of Mo, and containing a total of 0.060 to 1% of one or more selected from Cu and Ni , and contains a total of 0.025 to 0.09% of one or more selected from Ti and Nb, and contains the balance of Fe and unavoidable impurities, in addition, the content of N, B, Ti and Nb satisfies the following relationship Formula 1: Rough rolling and finishing rolling of the thin slab at a constant speed within the range of 200-600mpm , when performing the above finish rolling, hot rolling is carried out at 800-950°C to obtain a hot-rolled steel sheet; the above-mentioned hot-rolled steel sheet is water-cooled at a speed of 0.5°C/second or more until it reaches 600-730°C; The above-mentioned cooled hot-rolled steel sheet is coiled after being air-cooled.

[relational expression 1]

0.3≤[(mol%N)/{(mol%B)+(mol%Ti)+(mol%Nb)}]≤1.6

In the relational formula 1, the parentheses represent the values obtained by dividing the weight % of the corresponding element by the atomic weight of the corresponding element, respectively.

In addition, another aspect of the present invention provides a thermoformed product, the thermoformed product comprises 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, 0.001 to 0.05% of Al, 0.006 to 0.02% of N, 0.0001 to 0.005% of B, less than 0.1% of Cr and less than 0.05% of Mo, and containing a total of 0.060 to 1% of one or more selected from Cu and Ni, and containing a total of 0.025 to 0.09% of one or more selected from Ti and Nb, and containing the balance of Fe and unavoidable impurities. In addition, the N , B, Ti and Nb contents satisfy the following relational formula 1. In addition, the microstructure is measured by area content In total, it contains 90-98% martensite and 2-10% retained austenite.

[relational expression 1]

0.3≤[(mol%N)/{(mol%B)+(mol%Ti)+(mol%Nb)}]≤1.6

In the relational formula 1, the parentheses represent the values obtained by dividing the weight % of the corresponding element by the atomic weight of the corresponding element, respectively.

In addition, another aspect of the present invention provides a method for manufacturing a hot-press formed product, the method for manufacturing a hot-press formed product includes the following steps: performing continuous casting with molten steel at a speed of 4 to 7 mpm to obtain a thin slab, The molten steel contains 0.2-0.3% of C, 1.2-1.8% of Mn, 0.01-0.5% of Si, 0.015% or less of P, 0.004% or less of S, 0.001-0.05% of Al, 0.006% ~0.02% of N, 0.0001 to 0.005% of B, less than 0.1% of Cr and 0.05% or less of Mo, and contain a total of 0.060 to 1% of one or more selected from Cu and Ni, and contain a total of 0.025 ~0.09% of one or more selected from Ti and Nb, and contains the balance of Fe and unavoidable impurities. In addition, the content of N, B, Ti and Nb satisfies the following relational formula 1; Rough rolling and finish rolling are carried out on the thin slab at a constant speed within the range of 600 mpm, and hot rolling is carried out at 800-950° C. to obtain a hot-rolled steel plate during the above-mentioned finish rolling; Cooling with water at a speed above °C/s until it reaches 600-730 °C; air cooling the cooled hot-rolled steel plate, and then coiling; heating the above-mentioned coiled hot-rolled steel plate to 750-1000 °C The heating temperature; the above-mentioned heated hot-rolled steel sheet is maintained at the above-mentioned heating temperature for 1 to 10 minutes;

[relational expression 1]

0.3≤[(mol%N)/{(mol%B)+(mol%Ti)+(mol%Nb)}]≤1.6

In the relational formula 1, the parentheses represent the values obtained by dividing the weight % of the corresponding element by the atomic weight of the corresponding element, respectively.

In addition, the technical features of the present invention are not fully listed in the means for solving the above technical problems. Referring to the following specific implementation manners, the various features of the present invention and the advantages and effects brought by the features can be understood in more detail.

The effect of the invention

The hot-rolled steel sheet for hot press forming according to the present invention has the advantage that the variation in quality is very small.

The thermoformed article according to the present invention has the advantage of being very excellent in formability and corrosion resistance.

Description of drawings

Fig. 1 is a schematic diagram for explaining a mini-mill process used in the present invention.

FIG. 2 shows the results of measuring the material variation in the width direction of the hot-rolled steel sheet according to Invention Example 5 of the present invention.

Fig. 3 is a photograph of the microstructure of the molded article of Invention Example 1 of the present invention observed with an optical microscope.

detailed description

Next, a hot-rolled steel sheet for hot press forming, which is an aspect of the present invention with little material variation and excellent formability and corrosion resistance, will be described in detail.

Carbon (C): 0.2 to 0.3% by weight

Carbon is an element used to form carbides in steel or solid-solve in ferrite to increase the strength of hot-rolled steel sheets. In order to obtain such an effect in the present invention, the carbon is preferably contained in an amount of 0.2% by weight or more, more preferably in an amount of 0.24% by weight or more. However, when the carbon content is too much, a very hard (hard) structure will be formed on the surface of the cast sheet or strip during the continuous casting and rolling process, resulting in surface cracks, or the yield strength of the hot-rolled steel plate may be affected. And the tensile strength is too high, so that when the tube is formed by using multi roll (multi roll) at room temperature, it will cause poor processing. Therefore, the carbon content is preferably 0.3% by weight or less, more preferably 0.26% by weight or less.

Manganese (Mn): 1.2 to 1.8% by weight

Manganese plays the role of suppressing the formation of ferrite and facilitating the formation of low-temperature transformation phase by improving the stability of austenite, thereby increasing the strength of steel. In the present invention, in order to obtain these effects, manganese is preferably contained in an amount of 1.2% by weight or more, more preferably in an amount of 1.4% by weight or more. However, when the manganese content is too high, segregation bands will be formed inside and/or outside the continuous casting slab and hot-rolled steel plate, and cracks will be caused during the continuous casting operation and/or hot rolling operation. And spread, so that the material deviation of the hot-rolled steel plate. Furthermore, as described above, when the material variation occurs in the hot-rolled steel sheet, when the hot-press-formed product is formed into a pipe after hot-press forming, processing defects occur frequently. Therefore, it is preferable to contain the manganese in an amount of 1.8% by weight or less, more preferably in an amount of 1.6% by weight or less.

Silicon (Si): 0.01 to 0.5% by weight

Silicon is an element added to improve the strength and ductility of the hot-rolled steel sheet. In the present invention, in order to obtain such an effect, the silicon is preferably contained in an amount of 0.01% by weight or more, more preferably in an amount of 0.05% by weight or more. However, when the silicon content is too high, a large amount of oxide is formed on the surface, thereby causing surface defects such as dents. 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, an unavoidable impurity in steel, segregates to grain boundaries and/or phase boundaries and becomes an element that causes brittleness. Therefore, it is preferable to keep its content as low as possible. Theoretically, it is advantageous to control the phosphorus content to 0%, but phosphorus is inevitably contained in the manufacturing process. Therefore, it is important to control its upper limit value. In the present invention, the upper limit of the phosphorus content is controlled to be 0.015% by weight.

Sulfur (S): 0.004% by weight or less

Sulfur is an inevitable impurity in steel. This element combines with Mn in steel to form non-metallic inclusions MnS, and segregates during continuous casting and solidification to become the main cause of high-temperature cracks. Therefore, preferably, its content is controlled as low as possible. Theoretically, it is advantageous to limit the sulfur content to 0%, but sulfur will inevitably be contained in the manufacturing process. Therefore, it is important to control its upper limit value. In the present invention, the upper limit of the sulfur content is controlled to be 0.004% by weight.

Aluminum (Al): 0.001 to 0.05% by weight

Aluminum is an element that improves the ductility of steel by inhibiting the formation of carbides. In the present invention, in order to obtain such an effect, it is preferable to contain aluminum in an amount of 0.001% by weight or more. However, when the aluminum content is too high, the aluminum reacts with nitrogen in the steel to form AlN, which causes corner cracks in the manufacture of thin slabs, degrading the quality of slabs or steel sheets. Therefore, the aluminum content is preferably 0.05% by weight or less.

Nitrogen (N): 0.006 to 0.02% by weight

Nitrogen is generally classified as an unavoidable impurity in steel, however, nitrogen in the present invention is an element intentionally added for the formation of nitrides and stabilization of austenite in steel. In addition, the nitrogen can increase the hardness of the martensite formed by rapid cooling after hot press forming, thereby improving the tensile strength and fatigue resistance of the hot press formed product. In order to obtain these effects in the present invention, the nitrogen is preferably contained in an amount of 0.006% by weight or more, more preferably in an amount of 0.008% by weight or more. However, when the content is too large, the content of the precipitation elements required for precipitation strengthening in the steel will be reduced, thereby reducing the strength and fatigue resistance of the hot-rolled steel sheet. Therefore, the nitrogen is preferably contained in an amount of 0.02% by weight or less, more preferably in an amount of 0.012% by weight or less.

Boron (B): 0.0001 to 0.005% by weight

Boron is used to increase the hardenability of steel. In addition, when hot-rolled steel sheets are manufactured through a short-flow steelmaking process, since the content of solid-solution nitrogen in the steel is high, the precipitation strengthening effect of the precipitate-forming elements will be reduced, resulting in deterioration of the strength and workability of the steel. reduce. However, the boron forms nitrides by reacting with solid-solution nitrogen in the steel, so that the reduction in strength and workability of the hot-rolled steel sheet can be minimized. In order to obtain these effects in the present invention, the boron is preferably contained in an amount of 0.0001% by weight or more, more preferably in an amount of 0.0002% by weight or more. However, when the boron content is too large, the recrystallization temperature of austenite is increased, thereby causing deterioration of weldability. Therefore, the boron is preferably contained in an amount of 0.005% by weight or less, and more preferably in an amount of 0.003% by weight or less.

Chromium (Cr): less than 0.1% (including 0%)

Chromium contributes to the improvement of the strength of the hot-rolled steel sheet, but it is not intentionally added in the present invention. In addition, when the chromium content is too high, the formability of the hot-rolled steel sheet is improved, resulting in deterioration of formability. Therefore, it is preferable to control its content in a low range as much as possible. In addition, in the present invention, the content of chromium is controlled in a range of less than 0.1%.

Mo: 0.05% or less (including 0%)

Molybdenum also contributes to the improvement of the strength of the hot-rolled steel sheet, but like chromium, the molybdenum is not intentionally added in the present invention. In addition, when the molybdenum content is too high, the formability of the hot-rolled steel sheet is improved, resulting in deterioration of formability. Therefore, it is preferable to control its content as low as possible. In the present invention, the molybdenum content is controlled below 0.05%.

One or more types selected from copper (Cu) and nickel (Ni): 0.060 to 1% by weight in total

Copper and nickel are elements that contribute to improving the corrosion resistance of hot-rolled steel sheets and molded products. In order to obtain these effects in the present invention, at least one element of copper (Cu) and nickel (Ni) is contained, and the sum of their contents is preferably 0.060% by weight or more. However, when the content is too high, it will be concentrated on the surface in a liquid state during the slab manufacturing process, causing slab defects, and will remain scale on the surface of the hot-rolled steel sheet, thereby reducing 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.

One or more types selected from titanium (Ti) and niobium (Nb): 0.025 to 0.09% by weight in total

Titanium and niobium are elements that form precipitates (TiC, TiCN, TiNbCN, NbC, etc.) and are used to increase the strength of steel. In order to obtain these effects in the present invention, at least one element of titanium (Ti) and niobium (Nb) is contained, and the sum of these elements is preferably 0.025% by weight or more, more preferably 0.04% by weight or more. However, if the content is too high, not only will the production cost of the hot-rolled steel sheet increase, but also coarse crystal precipitates will be formed in the steel during continuous casting, thereby reducing the effect of precipitation strengthening. Furthermore, the excessive increase in the tensile strength of the hot-rolled steel sheet causes deterioration in formability. Therefore, the sum of these contents is preferably 0.09% by weight or less, more preferably 0.06% by weight or less.

Other than the above composition, the rest is Fe. However, in the usual manufacturing process, undesired impurities will inevitably be mixed in from raw materials or the surrounding environment, so the mixing of these impurities cannot be ruled out. These impurities are well known to those skilled in the art, therefore not all content about impurities is mentioned in this description.

When designing an alloy of a steel material having the above composition range, it is preferable that the contents of N, B, Ti, and Nb satisfy the following relational expression 1. Relational expression 1 is obtained by parameterizing the precipitation strengthening effect due to the formation of precipitates. When the value is too low, it is difficult to ensure the fatigue resistance of the final molded product due to the low nitrogen content in the steel. Therefore, it is preferable to control this value to 0.3 or more. However, when this value is too high, it becomes difficult to ensure the desired yield strength in the final molded product due to insufficient content of precipitate-forming elements. Therefore, it is preferable to control this value to 1.6 or less, more preferably to 1.4 or less.

[relational expression 1]

0.3≤[(mol%N)/{(mol%B)+(mol%Ti)+(mol%Nb)}]≤1.6

In the relational formula 1, the parentheses represent the values obtained by dividing the weight % of the corresponding element by the atomic weight of the corresponding element, respectively.

According to a specific embodiment of the present invention, when designing an alloy of a steel material having the above-mentioned composition range, it is preferable to satisfy a carbon equivalent (Ceq) of 0.4-0.7, more preferably 0.5-0.6. If the carbon equivalent is less than 0.4, it will be difficult to ensure the expected strength of the hot-rolled steel sheet. If it exceeds 0.7, the hardness difference between the welded heat-affected zone and the welded heat-affected zone will be too large due to the high strength of the welded zone, which may cause Weld cracks. In addition, the carbon equivalent is defined by the following formula 1, and when the alloy elements constituting the following formula 1 do not exist, it is regarded as 0 and calculated.

[Formula 1]

Ceq=(wt%C)+(wt%Mn)/6+{(wt%Cr)+(wt%Mo)+(wt%V)}/5+{(wt%Ni)+(wt%Cu) }/15

In the above formula 1, the parentheses represent the weight % of the corresponding elements, respectively.

The microstructure of the cold-rolled steel sheet of the present invention may contain 40 to 70% of ferrite and 30 to 60% of pearlite, more preferably 50 to 60% of ferrite in terms of area content. body and 40-50% pearlite. By securing such a fine structure, it is possible to secure a tensile strength of 500 to 600 MPa or more, a yield ratio of 0.5 to 0.6, and an elongation of 20 to 30% or more.

Hereinafter, a method for manufacturing a hot-rolled steel sheet for hot press forming with less variation in material and excellent formability and corrosion resistance according to another aspect of the present invention will be described in detail.

First, the short-flow steelmaking process using the endless continuous rolling method through the thin slab continuous casting and rolling production process will be described in detail.

Fig. 1 is a schematic diagram for explaining a short-flow steelmaking process used in the present invention. As shown in Fig. 1, the short-flow steelmaking process used in the present invention is composed of continuous casting, rough rolling, finish rolling, cooling and coiling steps, after that, cold rolling and continuous annealing steps are implemented by conventional equipment, thereby producing Get cold rolled steel. In this case, it is characterized in that the operating conditions of each step in the short-flow steelmaking process are controlled, and the driving speed (mass flow rate) of rough rolling-finish rolling-coiling is controlled to the same level to perform uniform rolling, thereby The hot-rolled steel sheet is obtained by a batchwise hot rolling method using a coil box, or by a continuous method without using a coil box.

Next, the short-flow steelmaking process of Fig. 1 will be described in more detail. In the continuous casting machine 10, a thin slab a having a thickness of 30 to 150 mm is obtained. It has a considerably thinner thickness than a slab having a thickness of 200 mm or more produced by a conventional continuous rolling caster, and such a slab is called a thin slab. Since the thin slab is directly transferred to the rough rolling mill 20 for rough rolling through a continuous process, the heat source of the slab itself can be directly used, thereby saving energy. The migration process of microstructure and precipitate formation that may be generated in the process is different from the existing rolling (mill), so that the mechanical and physical properties of the finally manufactured steel plate are different. In addition, when the thickness of the thin slab exceeds 150 mm, the difference from conventional rolling becomes small, and if it is less than 30 mm, the temperature of the slab drops sharply, making it difficult to form a uniform structure. In order to solve these problems, it is possible to provide additional heating equipment, but this becomes a factor that increases the production cost, so it is preferable to avoid this as much as possible.

In addition, the thin slab is rolled to the expected final thickness in the roughing rolling unit 20 and the finishing rolling unit 50, and after being cooled by the output roller table (ROT) 60, it is cooled in the winder 70 at a certain temperature. Coiled to be made into hot-rolled steel sheets. As mentioned above, the present invention is characterized in that the driving speeds of the rough rolling mill 20-finish rolling mill 50-coiler 60 are controlled at the same level to perform constant-speed rolling. In the case of a difference between the continuous casting speed and the rolling speed, in order to make up for the difference, a coil uncoiling box 40 can be set in front of the finishing rolling unit 50, and the strip (barplate) b can be subjected to the first step through the induction heater 30. One winding.

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

First, after preparing molten steel satisfying the aforementioned alloy composition, continuous casting is performed in the continuous casting machine 10 at a speed of 4 to 7 meters per minute (meter per minute) to obtain a thin slab. The reason for controlling the casting speed to 4 m/min or more is that since the casting and rolling processes are carried out successively, in order to ensure the target rolling temperature, a casting speed higher than a certain level is required. However, when the casting speed is too fast, the operation success rate may be reduced due to the instability of the liquid steel level. Therefore, it is preferable to control the casting speed below 7 m/min.

Afterwards, the thin slab obtained through the above-mentioned continuous casting is subjected to rough rolling by a rough rolling block 20 composed of 2 to 4 rolling stands, and then in the finish rolling mill 60, the bar obtained through the above-mentioned rough rolling is subjected to rough rolling. The sheet b was subjected to finish rolling to obtain a hot-rolled steel sheet.

At this time, it is preferable to control by uniform rolling so as to have the same mass flow (mass flow) from the continuous casting to the coiling process, and it is preferable to control the rolling speed in the range of 200 to 600 m/min within, more preferably controlled within the range of 300 to 500 m/min. The reason for such control is that when the rolling speed is too slow, it will be difficult to ensure the temperature of the hot-rolled steel plate. Operational accidents, and it is difficult to control the hot rolling temperature to the target temperature.

In this case, the finish rolling temperature during the finish rolling is preferably 800 to 950°C. When the finish rolling temperature is less than 800°C, the load in the rolling equipment will increase significantly. On the other hand, when the finish rolling temperature exceeds 950°C, the stability of the endless continuous rolling operation will decrease and the rolling Generation of scale defects.

According to a specific embodiment of the present invention, the surface temperature of the thin slab at the entrance of the rough rolling mill (ie, the starting temperature of the thin slab during rough rolling) may be 1000-1200°C, more preferably 1000-1100°C. When the surface temperature of the thin slab is lower than 1000°C, the rough rolling load will increase, and cracks may be generated at the edge of the strip during the rough rolling, which in this case may cause the edge of the hot-rolled steel plate Departmental defects. In addition, when the surface temperature of the thin slab exceeds 1200° C., the surface quality may be deteriorated due to hot rolling scale, or the shape of the slab may be deformed due to unsolidified cast slab.

In addition, according to a specific embodiment of the present invention, the cumulative reduction during rough rolling may be 60-90%, more preferably 70-80%. In rough rolling, the higher the cumulative rolling reduction, the more advantageous it is to manufacture the steel sheet with excellent surface quality targeted by the present invention. In addition, during rough rolling, the higher the cumulative reduction ratio, the more it helps to distribute the continuous casting microstructure and alloy components formed inside the continuously cast slab (thin slab) evenly. In order to ensure this effect, it is preferable to control the cumulative reduction ratio to 60% or more. However, if the cumulative reduction ratio is too high, the rolling deformation resistance will increase, which will cause a problem of difficulty in handling. Therefore, it is preferable to control the cumulative reduction ratio to 90% or less.

Thereafter, water cooling is performed at a rate of 0.5° C./second or more in the output roller table (ROT) 60 to 600 to 730° C., and air cooling is performed before winding in the winder 70 .

The water cooling is performed to delay the transformation of austenite into ferrite. Therefore, when the water cooling is performed, the water cooling termination temperature is preferably 600-730°C, more preferably 650-700°C. If the water cooling termination temperature is less than 600°C, irregularly shaped ferrite may be formed, and the transformation of ferrite in the water cooling zone may be terminated, which may cause the shape of the coil to deteriorate. On the other hand, if it is higher than 730°C, it may cause deterioration of the surface quality of the hot-rolled steel sheet.

In addition, in the water cooling, the water cooling rate is preferably 0.5°C/sec or higher, more preferably 10°C/sec or higher. If the water cooling rate is less than 0.5°C/sec, thick hot-rolled scale will be formed again, and the composition of the scale may change, making it difficult to remove the scale. In addition, the faster the water cooling rate is, the more favorable it is to delay the transformation of ferrite, so the upper limit is not particularly limited.

The present invention may further include a step of pickling the hot-rolled steel sheet after the above-mentioned coiling, by which the scale formed on the surface of the hot-rolled steel sheet can be removed. All methods generally used in this technical field can be utilized for the said pickling process.

Hereinafter, the heat-press molded article of another aspect of the present invention will be described in detail.

Another aspect of the present invention is a thermoformed article having the aforementioned component system, characterized in that the microstructure of the thermoformed article contains 90 to 98 area % of martensite and 2 to 10 area % of residual austenitic.

According to a specific embodiment of the present invention, the maximum size of the martensite packet of the molded product may be 5-35 μm, more preferably 5-25 μm. Here, the martensite bundle refers to a group of laths and block martensites having the same crystal orientation.

An advantage of the thermoformed article according to the invention is that it has very good strength. According to a specific embodiment of the present invention, the yield strength of the thermoformed product may be above 1000 MPa, and the tensile strength may be above 1470 MPa.

Hereinafter, a method for manufacturing a thermoformed article according to another aspect of the present invention will be described in detail.

First, a hot-rolled steel sheet is prepared by the aforementioned method. Thereafter, for hot press forming, the hot-rolled steel sheet is heated to the temperature region of the austenite single-phase region.

In this case, the heating temperature (heating end temperature) is preferably 750 to 1000°C, more preferably 850 to 950°C. If the heating temperature is lower than 750°C, there will be residual ferrite due to insufficient transformation of austenite, or the temperature before hot forming will be lowered by the mold to form ferrite, which will lead to heat loss. The strength of the molded article after press molding is lowered or the bending workability is lowered. On the other hand, if it exceeds 1000°C, not only will the productivity decrease, but also oxides will be excessively formed on the surface layer, or a decarburized layer will be formed on the surface of the steel sheet, thereby deteriorating the surface quality of the molded product after hot press forming, or It may cause a decrease in surface hardness and a decrease in corrosion resistance.

According to a specific embodiment of the present invention, the heating rate during the heating may be 1-100°C/sec, more preferably 3-20°C/sec. If the heating rate is less than 1° C./sec, productivity will be lowered. On the other hand, if it exceeds 100° C./sec, it will be difficult to secure a uniform temperature distribution over the entire thickness of the hot-rolled steel sheet.

Thereafter, the above-mentioned heated hot-rolled steel sheet is maintained at the above-mentioned heating temperature (heating termination temperature) for 1 to 10 minutes, more preferably for 5 to 6 minutes. This step is a step to ensure the austenite structure with uniform size and area content. If the maintenance time is less than 1 minute, an uneven austenite structure may be formed in the thickness center area of the hot-rolled steel plate. On the other hand , If it exceeds 10 minutes, it will lead to a decrease in the productivity of the molded product, and an excessively deep decarburization layer will be formed on the surface of the steel sheet, so that it may be difficult to secure the martensitic structure after rapid cooling.

Thereafter, the hot-rolled steel sheet maintained at the above-mentioned heating temperature is press-formed using a die, and rapidly cooled. At this time, molding and rapid cooling using a mold can be performed as long as a conventional hot press molding method is used, and therefore, it is not particularly limited in the present invention.

According to a specific embodiment of the present invention, the rapid cooling rate during the rapid cooling may be 10° C./s, more preferably 30° C./s. If the rapid cooling rate is less than 10°C/sec, a uniform martensitic structure cannot be secured over the entire thickness of the hot-rolled steel sheet, and it becomes difficult to secure the quality of the final molded product. In addition, the faster the rapid cooling rate is, the more favorable it is to ensure a uniform martensitic structure, so the upper limit is not particularly limited.

According to a specific embodiment of the present invention, it may further include the step of performing tempering heat treatment at a temperature range of 100-350° C. after the rapid cooling. When performing the tempering treatment as described above, part of the martensite structure is transformed into tempered martensite, and in this case, toughness can be imparted to the final molded product.

Next, the present invention will be described in more detail by way of examples. However, the description of these examples is only for illustrating the implementation of the present invention, and therefore, the present invention is not limited to these examples. The scope of rights of the present invention is determined by the content described in the claims and the content reasonably derived therefrom.

(Example)

After preparing molten steel with the composition of the following Tables 1 and 2, continuous casting was carried out according to the conditions recorded in Table 3 to manufacture a thin slab with a thickness of 90 mm, and the above-mentioned thin slab was subjected to rough rolling, finish rolling, Cooling and coiling treatment to produce a hot-rolled steel plate with a thickness of 4.5-5.0 mm. At this time, the reduction ratio during rough rolling is set to 80%, the rolling speed during rolling is set to 400 m/min, the water cooling rate is set to 20°C/s, and the water cooling termination temperature is set to 650°C.

Thereafter, the microstructure of the thus-produced hot-rolled steel sheet was analyzed, and the material was measured. The results are shown in Table 4 below. At this time, the measurement of the material of the steel plate was performed as follows. That is, the JIS No. 5 test piece was selected and measured in a direction perpendicular to the rolling direction at 1/4 of the width direction. In Table 4 below, YS, TS, T.El, and YR represent yield strength, tensile strength, elongation, and yield ratio, respectively.

Thereafter, the formability and corrosion resistance of the obtained hot-rolled steel sheets were evaluated, and the results are shown in Table 4 below. The formability was analyzed by the naked eye and a solid microscope by performing a U-shaped bending test, and the case where no crack occurred was evaluated as "○", and the case where cracks occurred was evaluated as "×". Corrosion resistance was evaluated in the following manner. After the X-shaped scratches are drawn on the surface of the steel plate, the width of the scratches in the salt spray test for 480 hours in a spray atmosphere of 5% NaCl is measured, and the average value of the width of the scratches is 3mm The following cases were evaluated as "◯", and the case where the average value of the width of scratches was larger than 3 mm was evaluated as "×".

Table 1

Table 2

table 3

Table 4

Referring to Table 4, it can be confirmed that Inventive Examples 1 to 7 satisfying all the conditions disclosed in the present invention exhibit very excellent moldability and corrosion resistance of molded articles. In addition, in Comparative Examples 1 and 2, it was confirmed that although the contents of Ti and Nb did not satisfy the range disclosed in the present invention, the formability and corrosion resistance of the hot-rolled steel sheet were excellent. On the other hand, for Comparative Examples 3 to 5, the content of Cr and/or Mo was too much, thereby exhibiting relatively poor formability and corrosion resistance.

In addition, FIG. 2 shows the results of measuring the material variation in the width direction of the hot-rolled steel sheet according to Invention Example 5 of the present invention. 2 is the result of measuring the yield strength, tensile strength and elongation of each cut test piece after cutting the hot-rolled steel plate of Invention Example 5 into 50 test pieces along the width direction. Referring to FIG. 2 , it can be confirmed with the naked eye that the hot-rolled steel sheet of the present invention has very little material variation in the width direction perpendicular to the rolling direction. From this, it can be seen that the hot-rolled steel sheet of the present invention has almost no difference in material due to the difference in position, and therefore has the effect of improving the manufacturing error rate of the formed pipe.

Thereafter, each of the obtained hot-rolled steel sheets was heated to a heating temperature of 900° C. at a rate of 10° C./sec and maintained for 5 minutes, and then press-formed and rapidly cooled to manufacture molded products. Thereafter, the microstructure of the molded article thus obtained was analyzed, and the material was measured. The results are shown in Table 5 below. In this case, the material measurement method is as described above.

table 5

Referring to Table 5, it can be confirmed that the molded articles of Invention Examples 1 to 7 satisfying all the conditions disclosed in the present invention are very excellent in strength. In addition, in Comparative Examples 3 to 5, it was confirmed that although the content of Cr and/or Mo did not satisfy the range disclosed in the present invention, the strength of the molded product was excellent. On the other hand, although the contents of Ti and Nb in Comparative Examples 1 and 2 did not satisfy the range disclosed in the present invention, they showed relatively poor strength of molded products.

In addition, FIG. 3 is a photograph of the fine structure of the molded article of Invention Example 1 of the present invention observed with an optical microscope. More specifically, in order to measure the size of martensite bundles, after using a scanning electron microscope with an electron backscatter diffraction (Electron Backscatter Diffraction, EBSD) analysis function to measure the grain orientation difference angle (misorientation angle) of the crystal grains, a blue coarse A line marks a structure having a grain boundary with a grain orientation difference angle of 15° or more on a photograph observed with an optical microscope. Referring to FIG. 3 , it can be confirmed with the naked eye that the maximum size of the martensite bundles of the molded article of the present invention is 35 μm or less.

Claims (12)

1. A hot-rolled steel sheet for hot-press forming, characterized in that, the hot-rolled steel sheet for hot-press forming comprises 0.2-0.3% of C, 1.2-1.8% of Mn, 0.01-0.5% of Si, 0.015% or less of P, 0.004% or less of S, 0.001 to 0.05% of Al, 0.006 to 0.02% of N, 0.0001 to 0.005% of B, less than 0.1% of Cr and less than 0.05% of Mo, and the total It is 0.060 to 1% of one or more selected from Cu and Ni, and contains a total of 0.025 to 0.09% of one or more selected from Ti and Nb, and contains the balance of Fe and unavoidable impurities, The microstructure of the hot-rolled steel sheet for hot-press forming includes 40-70% of ferrite and 30-60% of pearlite in terms of area content, In addition, the content of N, B, Ti and Nb satisfies the following relational formula 1, [relational expression 1] 0.3≤[(mol%N)/{(mol%B)+(mol%Ti)+(mol%Nb)}]≤1.6 In the relational formula 1, the parentheses represent the values obtained by dividing the weight % of the corresponding element by the atomic weight of the corresponding element, respectively. 2 . The hot-rolled steel sheet for hot-press forming according to claim 1 , wherein the carbon equivalent of the hot-rolled steel sheet is 0.4 to 0.7. 3. A method for manufacturing a hot-rolled steel sheet for hot-press forming, characterized in that, the method for manufacturing a hot-rolled steel sheet for hot-press forming comprises the following steps: continuous casting with molten steel at a speed of 4 to 7 m/min, Thereby obtaining a thin slab having a thickness of 30 to 150 mm, the molten steel contains 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% The following S, 0.001 to 0.05% of Al, 0.006 to 0.02% of N, 0.0001 to 0.005% of B, less than 0.1% of Cr and 0.05% or less of Mo, and a total of 0.060 to 1% of selected from Cu and One or more types of Ni, and contain a total of 0.025 to 0.09% of one or more selected from Ti and Nb, and contain the balance of Fe and unavoidable impurities. In addition, the N, B, Ti, and Nb The content satisfies the following relational formula 1; Carrying out rough rolling and finishing rolling on the thin slab at a constant speed in the range of 200-600 m/min, and performing hot rolling at 800-950°C during the finishing rolling, so as to obtain a hot-rolled steel plate; Water cooling the hot-rolled steel plate above at a rate of 0.5°C/s or more until it reaches 600-730°C; Coiling the cooled hot-rolled steel plate after air cooling; [relational expression 1] 0.3≤[(mol%N)/{(mol%B)+(mol%Ti)+(mol%Nb)}]≤1.6 In the relational formula 1, the parentheses represent the values obtained by dividing the weight % of the corresponding element by the atomic weight of the corresponding element, respectively. 4 . The method for producing a hot-rolled steel sheet for hot press forming according to claim 3 , wherein when the rough rolling is performed, the rolling start temperature of the thin slab is 1000 to 1200° C. 4 . 5 . The method for producing a hot-rolled steel sheet for hot press forming according to claim 3 , wherein the reduction ratio is 60 to 90% during the rough rolling. 6 . 6 . The method for manufacturing a hot-rolled steel sheet for hot-press forming according to claim 3 , further comprising the step of pickling the hot-rolled steel sheet after the coiling. 7. A hot-pressed formed product, characterized in that it is a hot-pressed formed product obtained by hot-pressing the hot-rolled steel sheet according to claim 1 or 2, and the microstructure includes 90 to 98 % martensite and 2-10% retained austenite. 8 . The thermoformed product according to claim 7 , wherein the maximum size of the martensite bundles of the formed product is 5 to 35 μm. 9. The thermoformed article according to claim 7, characterized in that, the yield strength of the molded article is above 1000 MPa, and the tensile strength is above 1470 MPa. 10. A method for manufacturing a hot-pressed product, characterized in that the method for manufacturing a hot-pressed product comprises the following steps: continuous casting with molten steel at a speed of 4 to 7 m/min to obtain a 30 to 150 mm A thin slab of thickness, the molten steel contains 0.2-0.3% of C, 1.2-1.8% of Mn, 0.01-0.5% of Si, 0.015% or less of P, 0.004% or less of S, 0.001- 0.05% of Al, 0.006 to 0.02% of N, 0.0001 to 0.005% of B, less than 0.1% of Cr, and less than 0.05% of Mo, and containing a total of 0.060 to 1% of one or more selected from Cu and Ni , and contains a total of 0.025 to 0.09% of one or more selected from Ti and Nb, and contains the balance of Fe and unavoidable impurities, in addition, the content of N, B, Ti and Nb satisfies the following relationship Formula 1; Carrying out rough rolling and finishing rolling on the thin slab at a constant speed in the range of 200-600 m/min, and performing hot rolling at 800-950°C during the finishing rolling, so as to obtain a hot-rolled steel plate; Water cooling the hot-rolled steel plate above at a rate of 0.5°C/s or more until it reaches 600-730°C; After the above-mentioned cooled hot-rolled steel plate is air-cooled, it is coiled; heating the coiled hot-rolled steel plate to a heating temperature of 750-1000°C; Maintaining the above-mentioned heated hot-rolled steel sheet at the above-mentioned heating temperature for 1 to 10 minutes; and performing rapid cooling while the hot-rolled steel sheet maintained at the above-mentioned heating temperature is press-formed; [relational expression 1] 0.3≤[(mol%N)/{(mol%B)+(mol%Ti)+(mol%Nb)}]≤1.6 In the relational formula 1, the parentheses represent the values obtained by dividing the weight % of the corresponding element by the atomic weight of the corresponding element, respectively. 11. The method for manufacturing a hot-press-formed article according to claim 10, wherein when heating the coiled hot-rolled steel sheet, the heating rate is 1 to 100°C/sec. 12. The method for manufacturing a hot-press-formed product according to claim 10, wherein the cooling rate when the hot-rolled steel sheet maintained at the heating temperature is press-formed and cooled is 10° C./second or more .