JP2017504724A - Hot-rolled steel sheet excellent in weldability and burring property and method for producing the same - Google Patents

Abstract

One aspect of the present invention provides a hot-rolled high burring steel that is excellent in electric resistance weldability and easy in welding work, and a method for manufacturing the same. [Selection] Figure 1

Description

  The present invention relates to a hot-rolled steel sheet excellent in weldability and burring properties and a method for producing the same.

Recently, in the automotive industry, in order to achieve both weight reduction for reducing CO ₂ emissions and strengthening of the vehicle body to improve collision stability, application of high strength steel plate to the vehicle body Is spreading. A high-strength steel sheet is an excellent material for the cost of realizing such conflicting demands, and its application amount is expected to increase gradually in order to comply with regulations that will become stricter in the future. In particular, as the interest in crash stability increases, its role is becoming increasingly important.

  In general, the higher the strength of a material, the worse the moldability. There are no exceptions in steel materials, and attempts have been made to achieve both high strength and high ductility. In addition to ductility, the properties required for materials used in automobile parts include burring workability. However, since burring workability tends to decrease with increasing strength, improvement of burring workability is also a problem regarding application of high-strength steel sheets to automobile parts. On the other hand, automotive parts are assembled by welding such as spots, arcs, plasmas, lasers, etc., with members processed by press molding or the like. Recently, there are also cases where the steel plates are joined by welding and then press-formed. The strength of the weld during molding or when assembled and used as a part is very important in terms of molding limits and stability. Therefore, in the application of high-strength steel sheets to automobile parts and the like, the strength of the welded portion as well as its burring workability becomes an important examination subject.

In Patent Document 1, after the hot rolling, the ferrite-bainite structure is formed by using three-stage cooling that is cooled to a temperature of around 700 ° C., then air-cooled for a certain time, and then cooled and wound again. A technology to improve stretch flangeability is proposed. Further, in Patent Document 2, the ferrite-bainite structure is controlled. At this time, the ferrite ratio is set to 80% or more, and the ratio of the short diameter (ds) to the long diameter (dl) of the crystal grains is 0. A technique for producing a hot-rolled steel sheet having a strength of 69 kg / cm ² or more and excellent stretch ratio and stretch flangeability is proposed so that the number of crystal grains 1 is 80% or more.

  However, the technique as described above produces ferrite-bainite dual-phase composite steel by mainly utilizing alloy components such as Si, Mn, Al, and Mo. However, such an alloy component has a problem that resistance heating is increased by increasing the electrical resistance during electrical resistance welding, or cold welding occurs when working with a lower input current value. Moreover, Si, Mn, Al, etc. have the problem of forming an oxide at the time of welding and reducing the soundness of a welded part.

Japanese Patent Publication No. Hei 6-293910 Korean Patent Publication No. 2003-0055339

  One aspect of the present invention is to provide a hot-rolled steel sheet having excellent electrical resistance weldability, which facilitates welding work and excellent burring properties, and a method for manufacturing the hot-rolled steel sheet.

  One aspect of the present invention is, by weight, C: 0.03-0.1%, Si: 0.01-1.2%, Mn: 1.2-1.9%, Al: 0.01- 0.08%, Cr: 0.005-0.8%, Mo: 0.01-0.12%, P: 0.01-0.05%, S: 0.001-0.005%, N : 0.001 to 0.01%, balance Fe and other inevitable impurities are included, each of the above components satisfies the following formula 1, and the product of tensile strength and stretch flangeability is 48000 or more, and is excellent in weldability and burring properties Provide hot rolled steel sheet.

10.646 + 0.2 [C] +0.25 [Si] +0.3 [Mn] −0.1 [Cr] +0.55 [Al] +0.2 [Mo] −4.23 [Ti] −2.5 [Nb] -2.9 [V] ≦ 11.1 [Formula (1)]
However, in the said Numerical formula 1, C, Si, Mn, Cr, Al, Mo, Ti, Nb, and V show the content (weight%) of an applicable element, respectively.

  The method for producing a hot-rolled steel sheet excellent in weldability and burring properties, which is another aspect of the present invention, is C: 0.03 to 0.1%, Si: 0.01 to 1.2% by weight%. , Mn: 1.2 to 1.9%, Al: 0.01 to 0.08%, Cr: 0.005 to 0.8%, Mo: 0.01 to 0.12%, P: 0.01 A steel slab containing -0.05%, S: 0.001-0.005%, N: 0.001-0.01%, the balance Fe and other inevitable impurities, each of the above components satisfying the following formula 1. A step of re-heating the steel slab at 1200 to 1300 ° C., a step of hot rolling the re-heated steel slab at a finish rolling temperature of 850 to 1000 ° C. to obtain a steel plate, and the hot A step of primarily cooling the rolled steel sheet to a temperature of 500 to 750 ° C. at a cooling rate of 10 to 100 ° C./second; The including chilled steel sheet comprising the steps of cooling 4-10 seconds, the steps of the secondary cooling at a cooling rate of the air-cooled steel sheet to a temperature of 300 to 500 ° C. 10 to 100 ° C. / sec, the.

10.646 + 0.2 [C] +0.25 [Si] +0.3 [Mn] −0.1 [Cr] +0.55 [Al] +0.2 [Mo] −4.23 [Ti] −2.5 [Nb] -2.9 [V] ≦ 11.1 [Formula (1)]
However, in the said Numerical formula 1, C, Si, Mn, Cr, Al, Mo, Ti, Nb, and V show the content (weight%) of an applicable element, respectively.

  Furthermore, the means for solving the problems described above do not enumerate all the features of the present invention. The various features of the present invention and the advantages and effects thereof can be understood in more detail with reference to the following specific embodiments.

  According to the present invention, by optimizing the steel composition and the hot-rolled structure, the product of tensile strength and stretch flangeability is 48000 or more, and the soundness of the welded part during welding is improved, so that the weldability and burring are improved. A hot-rolled steel sheet having excellent properties can be secured.

It is a graph which shows the value when it substitutes for the product of the tensile strength (TS) and stretch flangeability (HER) of all the steels of an Example, and Numerical formula 1.

  The present inventors have studied to develop a hot-rolled steel sheet that can overcome the problems that the above-described techniques could not solve, and as a result, by controlling the steel composition components, microstructure, and process conditions. The present inventors have confirmed that a hot-rolled steel sheet having excellent weldability and burring properties can be produced, and have reached the present invention.

  Hereinafter, the hot-rolled steel sheet excellent in weldability and burring properties, which is one aspect of the present invention, will be described in detail.

  One aspect of the present invention is, by weight, C: 0.03-0.1%, Si: 0.01-1.2%, Mn: 1.2-1.9%, Al: 0.01- 0.08%, Cr: 0.005-0.8%, Mo: 0.01-0.12%, P: 0.01-0.05%, S: 0.001-0.005%, N : 0.001 to 0.01%, balance Fe and other inevitable impurities are included, each of the above components satisfies the following formula 1, and the product of tensile strength and stretch flangeability is 48000 or more, and is excellent in weldability and burring properties Provide hot rolled steel sheet.

10.646 + 0.2 [C] +0.25 [Si] +0.3 [Mn] −0.1 [Cr] +0.55 [Al] +0.2 [Mo] −4.23 [Ti] −2.5 [Nb] -2.9 [V] ≦ 11.1 [Formula (1)]
However, in the said Numerical formula 1, C, Si, Mn, Cr, Al, Mo, Ti, Nb, and V show the content (weight%) of an applicable element, respectively.

Carbon C: 0.03 to 0.1% by weight
C is the most economical and effective element for strengthening steel. As the amount of carbon added increases, the martensite fraction of the ferrite-martensite composite structure steel increases and the tensile strength increases. When the carbon content is less than 0.03% by weight, it is not easy to form a martensite phase during cooling after hot rolling. On the other hand, when the carbon content exceeds 0.1% by weight, the strength is excessively increased, and there is a problem that weldability, formability and toughness are deteriorated. Accordingly, the C content is preferably 0.03 to 0.1% by weight.

Silicon (Si): 0.01 to 1.2% by weight
Si has the effect of deoxidizing molten steel, strengthening solid solution, and promoting ferrite transformation during cooling after hot rolling as a ferrite stabilizing element, so it constitutes the base of ferritic martensitic composite structure steel It is an effective element for increasing the ferrite fraction. When the silicon content is less than 0.01% by weight, it is difficult to produce the base structure into a ferrite structure because the effect of stabilizing the ferrite is small. On the other hand, when the silicon content exceeds 1.2% by weight, not only the red scale is formed on the steel plate surface during hot rolling and the surface quality of the steel plate becomes very poor, but also ductility and weldability. There is also a problem of lowering. Therefore, the silicon content is preferably 0.01 to 1.2% by weight.

Manganese (Mn): 1.2 to 1.9% by weight
Mn is an element effective for solid solution strengthening of steel, similar to Si, and has the effect of facilitating the formation of a bainite phase during cooling after hot rolling by increasing the hardenability of the steel. In this invention, in order to show such an effect, it is preferable to contain 1.2 weight% or more. However, if the manganese content exceeds 1.9% by weight, it is difficult to ensure a proper fraction of ferrite as a base structure by greatly delaying ferrite transformation. In addition, there is a problem in that a segregation part is greatly developed at the center of the thickness during casting of the slab in the continuous casting process, thereby impairing the weldability of the final product. Accordingly, the Mn content is preferably 1.2 to 1.9% by weight.

Aluminum (Sol. Al): 0.01 to 0.08% by weight
Al is a component added mainly for deoxidation, and has an effect of helping to form a ferrite phase in steel during cooling after hot rolling as a ferrite stabilizing element. When the aluminum content is less than 0.01% by weight, the effect intended by the present invention cannot be sufficiently ensured. On the other hand, if the aluminum content exceeds 0.08% by weight, defects such as corner cracks are likely to occur in the slab during continuous casting, and surface defects are generated after hot rolling to deteriorate the surface quality. There is a problem. Therefore, the aluminum content is preferably 0.01 to 0.08% by weight.

Chromium (Cr): 0.005 to 0.8% by weight
Cr plays a role of strengthening the solid solution of the steel and delaying the transformation of the ferrite phase during cooling to help the formation of bainite. In order to secure the effect intended by the present invention, it is preferably contained in an amount of 0.005% by weight or more. On the other hand, if the chromium content exceeds 0.8% by weight, the ferrite transformation is greatly delayed, and the stretch ratio decreases due to an increase in the martensite fraction more than necessary. Therefore, the Cr content is preferably 0.005 to 0.8% by weight.

Molybdenum (Mo): 0.01 to 0.12% by weight
Mo has an effect of increasing the hardenability of steel and facilitating the formation of a bainite structure. In order to show such an effect of the present invention, it is preferably contained in an amount of 0.01% by weight or more. On the other hand, if the molybdenum content exceeds 0.12% by weight, the hardenability increases too much, which deteriorates the weldability, which is economically disadvantageous. Therefore, the Mo content is preferably 0.01 to 0.12% by weight.

Phosphorus (P): 0.01 to 0.05% by weight
P, like Si, has an effect of promoting solid solution strengthening and ferrite transformation. When the phosphorus content is less than 0.01% by weight, it is not sufficient to obtain the strength to be secured by the present invention. On the other hand, if the phosphorus content exceeds 0.05% by weight, the ductility may decrease due to band organization by microsegregation. Therefore, it is preferable that the P is contained in an amount of 0.01 to 0.05% by weight.

Nitrogen (N): 0.001 to 0.01% by weight
N is a typical solid solution strengthening element together with C, and forms coarse precipitates together with Ti, Al and the like. In general, the solid solution strengthening effect of N is superior to that of carbon, but there is a problem that as the amount of N in the steel increases, the toughness greatly decreases. When the nitrogen content is less than 0.001% by weight, it takes a lot of time during the steelmaking operation and the productivity is lowered. On the other hand, if the nitrogen content exceeds 0.01% by weight, the risk of brittleness greatly increases. Therefore, the nitrogen is preferably contained in an amount of 0.001 to 0.01% by weight.

  The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, impurities that are not intended from the raw material or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities can be understood by any engineer in the normal manufacturing process, the entire contents thereof are not specifically mentioned herein.

  However, sulfur is an impurity that is generally mentioned. This will be briefly described as follows.

Sulfur (S): 0.001 to 0.005%
The sulfur is inevitably contained as an impurity and forms a non-metallic inclusion by combining with Mn and the like, and as a result, the stretch flangeability of the steel is greatly reduced. . Theoretically, the sulfur content is advantageously limited to 0% by weight, but is inevitably contained in the production process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the sulfur content is preferably limited to 0.01% by weight.

  Furthermore, when the steel material of the present invention additionally adds one or more elements selected from the group consisting of niobium (Nb), titanium (Ti), and vanadium (V) described later, the effects of the present invention are achieved. Further improvement can be achieved. More preferably, it contains 0.001 to 0.15% by weight of one or more elements selected from the above group.

  Ti is present as TiN in steel and has an effect of suppressing the growth of crystal grains in the heating process for hot rolling. In addition, Ti remaining after reacting with nitrogen is dissolved in the steel and bonded to carbon, so that TiC precipitates are formed, which is a useful component for improving the strength of the steel.

  Nb and V are effective for forming fine grains by forming carbides in the steel, and can form fine precipitates to improve the strength and toughness of the steel. In order to stabilize solid solution elements such as C and N that increase the electrical specific resistance, there is also an effect that the development of local sparks during electrical resistance welding is alleviated and softening of the weld is suppressed.

  Furthermore, in order to improve weldability, it is preferable that the C, Si, Mn, Cr, Al, Mo, Ti, Nb, and V satisfy the following formula 1. If 11.1 set as the boundary in Formula 1 is exceeded, the electrical resistivity at high temperatures may be high and weldability may be significantly reduced. The lower limit is not particularly limited, but the lower limit can be limited to 10.5 in consideration of stretch flangeability and stretch ratio. When added below, there is a problem that the strength or the stretch ratio is rapidly inferior.

10.646 + 0.2 [C] +0.25 [Si] +0.3 [Mn] −0.1 [Cr] +0.55 [Al] +0.2 [Mo] −4.23 [Ti] −2.5 [Nb] -2.9 [V] ≦ 11.1 [Formula (1)]
However, in the said Numerical formula 1, C, Si, Mn, Cr, Al, Mo, Ti, Nb, and V show the content (weight%) of an applicable element, respectively.

  The hot rolled high burring steel preferably has a product of tensile strength and stretch flangeability of 48000 or more.

  Moreover, it is preferable that the said hot-rolled steel plate contains the fine structure containing a bainite phase in a ferrite structure. By including such a fine structure, there is an effect of preventing the formation of coarse carbides and preventing the breakage at the interface during plastic deformation. Furthermore, it is more preferable that the bainite phase fraction is contained in the ferrite structure in an amount of 5 to 20%. When the fraction of the bainite phase contained is less than 5%, the strength that the present invention intends to secure cannot be ensured. On the other hand, if it exceeds 20%, the stretch ratio becomes inferior.

  Hereinafter, a method for producing a hot-rolled steel sheet excellent in weldability and burring properties, which is another aspect of the present invention, will be described in detail.

  The method for producing a hot-rolled steel sheet excellent in weldability and burring properties, which is another aspect of the present invention, is C: 0.03 to 0.1%, Si: 0.01 to 1.2% by weight%. , Mn: 1.2 to 1.9%, Al: 0.01 to 0.08%, Cr: 0.005 to 0.8%, Mo: 0.01 to 0.12%, P: 0.01 A steel slab containing -0.05%, S: 0.001-0.005%, N: 0.001-0.01%, the balance Fe and other inevitable impurities, each of the above components satisfying the following formula 1. A step of re-heating the steel slab at 1200 to 1300 ° C., a step of hot rolling the re-heated steel slab at a finish rolling temperature of 850 to 1000 ° C. to obtain a steel plate, and the hot A step of primarily cooling the rolled steel sheet to a temperature of 500 to 750 ° C. at a cooling rate of 10 to 100 ° C./second; The including chilled steel sheet comprising the steps of cooling 4-10 seconds, the steps of the secondary cooling at a cooling rate of the air-cooled steel sheet to a temperature of 300 to 500 ° C. 10 to 100 ° C. / sec, the.

10.646 + 0.2 [C] +0.25 [Si] +0.3 [Mn] −0.1 [Cr] +0.55 [Al] +0.2 [Mo] −4.23 [Ti] −2.5 [Nb] -2.9 [V] ≦ 11.1 [Formula (1)]
However, in the said Numerical formula 1, C, Si, Mn, Cr, Al, Mo, Ti, Nb, and V show the content (weight%) of an applicable element, respectively.

(Reheating stage)
It is preferable to reheat the slab satisfying the above component system at 1200 to 1300 ° C. When the reheating temperature is lower than 1200 ° C., the precipitates are not sufficiently re-dissolved, and thus precipitates such as NbC and TiC may decrease in the step after hot rolling. On the other hand, when the temperature exceeds 1300 ° C., the strength may decrease due to abnormal grain growth of austenite crystal grains. Therefore, the reheating temperature of the slab is preferably limited to 1200 to 1300 ° C.

(Hot rolling stage)
Hot rolling can be performed on the slab reheated as described above. At this time, the finish rolling is preferably performed at 850 to 1000 ° C. When the hot finish rolling temperature is less than 850 ° C., the rolling load may be greatly increased. On the other hand, when the hot finish rolling temperature exceeds 1000 ° C., the structure of the steel sheet becomes coarse, the steel material becomes brittle, the scale becomes thick, and the surface quality such as high-temperature rollability scale defects may deteriorate. Therefore, the hot finish rolling is preferably limited to 850 to 1000 ° C.

(Primary cooling stage)
It is preferable to primarily cool the steel sheet that has been hot-rolled as described above. Moreover, it is preferable to perform primary cooling until it reaches 500-750 degreeC from the said finish hot rolling temperature of the said hot-rolled steel plate. When the temperature at which the primary cooling is completed is less than 500 ° C., the microstructure in the steel has mostly bainite, so that the microstructure to be secured by the present invention cannot be secured sufficiently. On the other hand, when it exceeds 750 ° C., coarse ferrite and pearlite structures are formed, and the strength of the steel may be reduced. Moreover, it is preferable to perform primary cooling at a cooling rate of 10 to 100 ° C./second. When the temperature is less than 10 ° C./second, the ferrite crystal grains are coarsened, and the precipitation is also coarsened, so that there is a problem in securing the strength to be obtained by the present invention. Moreover, when it exceeds 100 degreeC / second, the shape of a hot-rolled sheet may become inferior.

(Air cooling stage)
It is preferable to air-cool the steel plate cooled as described above. The cooled steel sheet is preferably air-cooled for 4 to 10 seconds. When air cooling is performed in less than 4 seconds, there is a problem that the ductility is greatly lowered because a ferrite structure cannot be sufficiently formed. On the other hand, when air cooling is performed after exceeding 10 seconds, the ferrite fraction increases and the martensite fraction decreases, so that the strength and the stretch ratio to be secured by the present invention cannot be sufficiently secured. .

(Secondary cooling stage)
It is preferable to secondarily cool the steel sheet that has been air-cooled as described above. Moreover, it is preferable to perform secondary cooling at a cooling rate of 10 to 100 ° C./second until reaching 300 to 500 ° C. from the temperature of the air-cooled steel sheet. When the temperature at which the secondary cooling is finished is less than 300 ° C., a martensite phase is formed, and stretch flangeability may be inferior. On the other hand, when it exceeds 500 ° C., coarse carbides are formed, and the stretch flangeability may be inferior. Moreover, it is preferable to perform primary cooling at a cooling rate of 10 to 100 ° C./second. When it is less than 10 ° C./second, there is a problem in securing the stretch flangeability that the present invention intends to secure by forming coarse carbides. Moreover, when it exceeds 100 degreeC / second, there exists a problem that formation of a hot-rolled board will become defect.

  After the cooling step, the method may further include winding the cooled steel sheet to facilitate storage and movement.

  After naturally cooling the steel plate produced by the method as described above, the pickled steel plate can be produced by further including a step of pickling, removing the scale of the surface layer portion, and applying oil.

  Moreover, after reheating the said pickled steel plate at 450-480 degreeC, the hot dip galvanized steel plate can be manufactured by further including the step which lets a hot dip galvanizing bath pass.

  Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for illustrating the present invention in more detail and are not intended to limit the scope of rights of the present invention. This is because the scope of rights of the present invention is determined by matters described in the claims and matters reasonably inferred therefrom.

  A steel slab satisfying the component system shown in Table 1 below was heated at 1250 ° C., and hot finish rolling was performed at the temperature shown in Table 2 below. Thereafter, primary cooling is performed at a cooling rate of 70 ° C./second to a temperature of 680 ° C., air cooling is performed for 6 seconds, and then secondary cooling is performed at a cooling rate of 70 ° C./second to a temperature of 450 ° C. Winding was performed at the temperature shown in FIG.

  Evaluate the yield strength (YS), tensile strength (TS), fracture elongation (T-El), stretch flangeability (Hole Expanding Ratio, HER) of the final hot-rolled steel sheet obtained by completing the winding process. did. It is shown in Table 2 below.

  The bainite fraction in the ferrite structure was analyzed by an image analyzer after observing the final hot-rolled steel sheet at 500 magnifications using an optical microscope. Table 2 below shows the phase fraction.

  Furthermore, the weldability of the final hot-rolled steel sheet is also shown in Table 2 below. Regarding the weldability, after welding was performed under the conditions shown in Table 3, the strength of the welded portion of the steel sheet was measured by a uniaxial tensile test method. At this time, when the welded portion was broken, it was evaluated that the weldability was poor.

  As shown in Table 2 above, Invention Examples 1 to 6 are heats having high product values of tensile strength and stretch flangeability and excellent weldability by satisfying the component ranges and manufacturing conditions proposed by the present invention. It can be confirmed that a rolled steel sheet is secured.

  On the other hand, since Comparative Examples 1, 4, 5, 6, and 7 did not satisfy Formula 1, it can be confirmed that the weldability is inferior.

  In Comparative Examples 2, 3 and 8, the product of the tensile strength and the stretch flangeability satisfies the value of the range proposed by the present invention, but since Formula 1 is not satisfied, it can be confirmed that the weldability is inferior.

Claims (7)

By weight, C: 0.03-0.1%, Si: 0.01-1.2%, Mn: 1.2-1.9%, Al: 0.01-0.08%, Cr: 0.005-0.8%, Mo: 0.01-0.12%, P: 0.01-0.05%, S: 0.001-0.005%, N: 0.001-0. A hot-rolled steel sheet excellent in weldability and burring properties, containing 01%, the remainder Fe and other inevitable impurities, each of the above components satisfying the following formula 1 and the product of tensile strength and stretch flangeability is 48000 or more.
10.646 + 0.2 [C] +0.25 [Si] +0.3 [Mn] −0.1 [Cr] +0.55 [Al] +0.2 [Mo] −4.23 [Ti] −2.5 [Nb] -2.9 [V] ≦ 11.1 [Formula (1)]
(However, in Formula 1, C, Si, Mn, Cr, Al, Mo, Ti, Nb, and V are the contents (% by weight) of the corresponding elements, respectively)   2. The weldability and burring property according to claim 1, further comprising 0.001 to 0.15 wt% of one or more selected from Nb, Ti, and V in total including the weight of Nb, Ti, and V. 3. Excellent hot-rolled steel sheet.   The hot-rolled steel sheet excellent in weldability and burring properties according to claim 1, wherein the ferrite structure has a microstructure containing a bainite phase fraction of 5 to 20%. By weight, C: 0.03-0.1%, Si: 0.01-1.2%, Mn: 1.2-1.9%, Al: 0.01-0.08%, Cr: 0.005-0.8%, Mo: 0.01-0.12%, P: 0.01-0.05%, S: 0.001-0.005%, N: 0.001-0. Providing a steel slab containing 01%, the balance Fe and other inevitable impurities, each of the above components satisfying the following formula 1.
Reheating the steel slab at 1200-1300 ° C .;
Hot rolling the reheated steel slab at a finish rolling temperature of 850 to 1000 ° C. to obtain a steel plate;
Primary cooling the hot-rolled steel sheet to a temperature of 500 to 750 ° C. at a cooling rate of 10 to 100 ° C./second;
Air-cooling the cooled steel sheet for 4 to 10 seconds;
Secondary cooling the air-cooled steel plate to a temperature of 300 to 500 ° C. at a cooling rate of 10 to 100 ° C./second, and a method for producing a hot-rolled steel plate having excellent weldability and burring properties.
10.646 + 0.2 [C] +0.25 [Si] +0.3 [Mn] −0.1 [Cr] +0.55 [Al] +0.2 [Mo] −4.23 [Ti] −2.5 [Nb] -2.9 [V] ≦ 11.1 [Formula (1)]
(However, in Formula 1, C, Si, Mn, Cr, Al, Mo, Ti, Nb, and V are the contents (% by weight) of the corresponding elements, respectively)   The welding according to claim 4, wherein the slab further includes at least one selected from Nb, Ti, and V in a range of 0.001 to 0.15 wt% in total of Nb, Ti, and V. For producing hot-rolled steel sheet having excellent properties and burring properties.   The method for producing a hot-rolled steel sheet having excellent weldability and burring properties according to claim 4, further comprising a step of pickling after naturally cooling the steel sheet.   The method for producing a hot-rolled steel sheet having excellent weldability and burring properties according to claim 6, further comprising a step of reheating at 450 to 480 ° C. and performing hot dip galvanization after the pickling step.

Images