KR20110027075A - Steel sheet having excellent spot weldabity, strength and elongation for automobile and method for manufacturing the same - Google Patents

Abstract

PURPOSE: A steel sheet with excellent spot-welding performance, strength, and elongation for a vehicle and a manufacturing method thereof are provided to enhance the stability of a vehicle body and to reduce weight using the steel sheet as a reinforcing member and a shock absorbing member. CONSTITUTION: A steel sheet with excellent spot-welding performance, strength, and elongation for a vehicle contains C(Carbon) of 0.06~0.22wt.%, Si(Silicon) of 0.8~2.0wt.%, Mn(Manganese) of 1~3wt.%, P(Phosphorus) of 0.03wt.% or less, S(Sulfur) of 0.008wt.%, N(Nitrogen) of 0.013~0.03wt.%, Al(Aluminum) of 0.05wt.% or less, Ti(Titanium) of 0.005~0.02wt.%, B(Boron) of 20ppm or less, Sb(Stibium) of 0.01~0.03wt.%, Fe, and inevitable impurities. The steel sheet satisfies tensile strength(MPa) * elongation ratio(%) >= 20000 (MPa*%) and a ductility ratio >=0.4.

Description

Automotive sheet having excellent spot weldability, strength and elongation and manufacturing method {STEEL SHEET HAVING EXCELLENT SPOT WELDABITY, STRENGTH AND ELONGATION FOR AUTOMOBILE AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a steel sheet for automobiles and a method of manufacturing the same, which can be used for automobile reinforcement and shock absorbers, and more particularly, to control the component system and manufacturing conditions of the steel sheet to increase the strength and elongation, and also improve the spot weldability. It relates to a steel sheet for automobiles and a manufacturing method thereof.

In recent years, due to the complexity and integration of automobile molded products, automotive steel sheets are required to have a higher level of formability. In particular, the bumper reinforcement or shock absorber in the door is a part that is closely related to the passenger's safety when the vehicle collides with a tensile strength of 780 MPa and an elongation of 24% or more. It is anticipated that the parts having will be required.

In addition, as the environmental pollution problem caused by automobile exhaust gas has emerged, technology development for improving fuel economy has been made, and research for achieving automobile weight reduction using ultra high strength steel has been increasing. In order to manufacture a high strength steel sheet, the content of C has been used to increase the content of the steel sheet, but due to the increase in the content of C, the spot weldability is deteriorated, causing problems such as fracture of the weld.

In addition, studies are being conducted to improve strength and elongation at the same time. For this purpose, steel sheets containing a large amount of retained austenite have been proposed. The steel sheet is not only excellent in uniform ductility due to the increase in ductility as the residual austenite is transformed into martensite by processing, and also in the local deformation as the residual austenite is transformed into martensite when subjected to local compression pressure such as drawing. The resistance to this rapidly increases. For this reason, there is a characteristic that drawing processing is possible even if the (222) aggregate structure is not developed, such as an ultra low carbon cold rolled steel sheet. Therefore, if the steel sheet containing a large amount of residual austenite having excellent ductility can be applied to the workpiece for drawing, its field of application will be considerably broadened.

A steel sheet containing a large amount of the retained austenite described above is usually formed by adding Si and Mn to carbon steel to form austenite during annealing, and to maintain a constant bainite temperature during the cooling process to simultaneously increase strength and ductility. It is prepared by the method. The residual austenite prepared by this method is transformed into martensite during plastic deformation, thereby increasing ductility by relieving stress concentration due to plastic organic transformation with increasing strength, which is transformed plastic steel (TRIP). It is called, and is used as high strength steel which has high strength and ductility.

Japanese Unexamined Patent Publication No. 193-070886 discloses C: 0.05 to 0.3%, Si: 2.0% or less, Mn: 0.5 to 0.4%, P: 0.1% or less, S: 0.1% or less, Ni: 5.0% or less, Al: 0.1 or less. ~ 2.0%, N: 0.01% or less, satisfying the component system of Si (%) + Al (%) ≥ 0.5, Mn (%) + 1/3 Ni (%) ≥ 1.0, and containing 5% or more of retained austenite And the slab of the composition is hot rolled to 300-720 ° C., cold rolled at a reduction ratio of 30-80%, and then heated in a temperature range of Ac1 transformation point or more to Ac3 transformation point in the continuous annealing process, and during cooling. It is proposed to maintain the temperature in the temperature range of 350 ~ 550 ℃ 30 seconds or more, or slow cooling at a cooling rate of 400 ℃ / min or less. However, the present invention has a problem that can not significantly improve the spot welding characteristics.

In addition, Japanese Patent Laid-Open No. 2008-214752 discloses C: 0.05 to 0.30%, Si: 0.80 to 2.50%, Mn: 0.8 to 3.00%, P: 0.003 to 0.100%, S: 0.010% or less, and Al: 0.010 to 0.50. % And N: 0.007% or less, TS ≥ 590 MPa, TS x EL ≥ 20000 MPa *%, and a method for producing a hot-dip galvanized steel sheet having an explosion-produced current in weldability is 6.25 kA or more.

In Japanese Unexamined Patent Application Publication No. 2001-152287, C: 0.05-0.14%, Si: 0.50-1.50%, Mn: 0.80-2.00%, Mo: 0.01-0.25%, respectively, and the steel structure is ferrite, bainite. And a three-phase structure composed of residual austenite, each phase having an area ratio, ferrite: 50 to 90%, residual austenite: 3 to 9%, and bainite: the production of high strength cold rolled steel sheet excellent in spot weldability. The method is presented, and the strength of the steel of the invention is 780 MPa or less, the strength is low.

In Japanese Laid-Open Patent Publication No. 2004-269920, mass%, C: 0.13 to 0.15%, Si: 1-2%, Mn: 1.7 to 2.2%, P: 0.02% or less, S: 0.01% or less, sol. Al: 0.01 ~ 0.5% and N: 0.005% or less, high ductility high strength cold rolled steel sheet with residual austenitic phase of 5% or more and tensile strength of 80kg / mm ² or more and 1 of Ti, Nb, Zr, V At least 750 to 870 ° C. after cold rolling and cold rolling, which may contain at least one of 0.001 to 0.04%, or Cr: 0.01 to 1.0%, Mo: 0.01 to 0.5%, and B: 0.0001 to 0.0020%. After heating and holding for 10 sec or more, cooling is performed at a rate of 20 ° C./sec or less to 700 to 600 ° C., and cooling to 10 ° C./sec or more to 500 to 350 ° C. and then maintained for 60 sec or more, and then cooled again. . However, the range of the C component of the present invention is very narrow to 0.13 ~ 0.15 is not useful.

The above inventions mainly focus on lowering the lower limit of the C content and adding other alloying elements in order to simultaneously increase spot weldability and ductility, but there is an uneconomical problem due to the addition of alloys. Elongation could not be increased together. Therefore, research on this is required.

The present invention provides a steel sheet for automobiles and a method of manufacturing the same, which improves strength and elongation by stabilizing residual austenite in the microstructure of the steel sheet by controlling the component system and manufacturing conditions of the steel sheet, and improving the spot weldability with a low C content. I would like to.

The present invention, in one embodiment, in weight%, C: 0.06 to 0.22%, Si: 0.8 to 2.0%, Mn: 1 to 3%, P: 0.03% or less, S: 0.008% or less, N: 0.013 to 0.03 %, Al: 0.05% or less, Ti: 0.005 ~ 0.02%, B: 20ppm or less, Sb: 0.01 ~ 0.03%, balance Fe and other unavoidable impurities, tensile strength (MPa) * elongation (%) ≥20000 ( The present invention relates to a steel sheet for automobiles having excellent spot weldability, strength and elongation that satisfy MPa *%) and ductility ratio ≥ 0.4.

The steel sheet preferably satisfies tensile strength (MPa) ≥ 780 MPa.

The microstructure of the steel sheet is a percentage of ferrite, preferably 35% or more, martensite 5% or less, bainite 20 to 50% and residual austenite 3 to 10%.

The grain size of the ferrite is preferably 10㎛ or less.

The steel sheet may be a cold rolled steel sheet or a galvanized steel sheet.

As another embodiment of the present invention, in weight%, C: 0.06 to 0.22%, Si: 0.8 to 2.0%, Mn: 1-3%, P: 0.03% or less, S: 0.008% or less, N: 0.013 to 0.03 %, Al: 0.05% or less, Ti: 0.005% to 0.02%, B: 20ppm or less, Sb: 0.01% to 0.03%, steel slab containing residual Fe and other unavoidable impurities, heated to 1100 to 1250 ° C and 860 to 920 ° C. Hot-rolling at the step of producing a hot-rolled steel sheet; Cooling and winding the hot rolled steel sheet in the range of bainite transformation start temperature (Bs) to martensite transformation start temperature (Ms) + 20 ° C. at a cooling rate of 10 to 30 ° C./sec immediately after hot rolling; Cold rolling to a cold rolling rate of 30 to 70% to produce a cold rolled steel sheet; Continuously annealing the cold rolled steel sheet for more than 50 seconds in the range of 720 to 850 ° C. after the cold rolling; And it relates to a method for manufacturing a steel sheet for automobiles including a final cooling step of maintaining 60 seconds or more after cooling in the 300 ~ 450 ℃ range at a cooling rate of 10 ~ 200 ℃ / sec.

As a hot rolled steel sheet satisfying the above component system, the microstructure of the hot rolled steel sheet is preferably a ferrite and bainite two-phase structure.

It may include a step of pickling after the winding step.

After the final cooling step, it may include the step of plating the cold rolled steel sheet.

The present invention can provide a steel sheet having a strength * elongation of more than 20000 and a ductility ratio of 0.4 or more, through which it can be used as a reinforcing material for automobiles and shock absorbers, and because the general drawing process is possible, When used in place of some of the parts used, the stability and weight reduction of the car body can be expected.

The present invention improves the spot weldability by lowering the C content, and the addition of N to prevent the decrease in strength and elongation at the same time to bring stability and fractional increase of residual austenite, thereby increasing the strength and elongation, Sb By preventing decarburization during the annealing of the surface, the residual austenite fraction on the surface can be properly maintained, thereby producing a steel sheet excellent in spot weldability, strength and ductility.

Hereinafter, the component system and composition range of the steel sheet of the present invention will be described.

C (carbon): 0.06 to 0.22 wt%

C is the most important component in high strength TRIP steels and is closely related to strength and ductility, and affects the residual austenite fraction and stabilization. The higher the C content, the higher the fraction of retained austenite and the better the stability.

When the content of C is less than 0.06% by weight, little residual austenite is formed, and thus strength and ductility cannot be improved at the same time. Therefore, the content of C is preferably contained at least 0.06% by weight, more preferably at least 0.08% by weight in order to provide a higher strength TRIP steel. However, if the C content exceeds 0.22% by weight, it is easy to manufacture ultra-high strength TRIP steel with a tensile strength of 980 MPa or more. There is a disadvantage that is significantly reduced. Therefore, the content of C is preferably limited to 0.06 to 0.22% by weight.

Si (silicon): 0.8-2.0 wt%

Si plays a role in securing the amount of solid carbon which is essential for inhibiting carbide formation and inducing transformation organic plasticity (TRIP). In addition, Si facilitates floating separation of inclusions during steelmaking and increases the fluidity of the weld metal during welding. In low carbon TRIP steel, when the Si content is 0.8 wt% or more, carbide formation can be greatly suppressed by Si and the stability of residual austenite can be improved. However, when the content of Si exceeds 2.0% by weight, the hot rolled scale is greatly induced, the wettability is greatly deteriorated, the plating property is degraded, and the weldability is also degraded. Therefore, the content of Si is preferably limited to 0.8 to 2.0% by weight.

Mn (manganese): 1-3 wt%

In the present invention, Mn may serve to increase the hardenability and expand the temperature range in which austenite is formed. In addition, Mn facilitates the formation of low-temperature transformation phases such as acicular ferrite and bainite by increasing hardenability, increases strength, and stabilizes austenite, and is very effective for easily retaining austenite formed during annealing. to be.

In the steel of the present invention having a low content of C, Mn is preferably contained by 1% by weight or more in order to secure the strength of the steel and secure the TRIP characteristics. However, when the Mn content is more than 3% by weight, the weldability is greatly reduced, the martensite phase is sharply increased, and cracks are likely to occur at edges during cold rolling. In addition, the composition of slag changes during steelmaking, increasing the refractory erosion and forming manganese oxide in the grain boundary near the surface layer of the steel in the heating step before hot rolling, causing surface defects after hot rolling. In the hot rolling, a segregation zone is formed in the center of the sheet, and hydrogen formation is caused by inclusions. Therefore, the content of Mn is preferably limited to 1 to 3% by weight.

P (phosphorus): 0.03% or less

P is an element that is inevitably contained during manufacture, and it is preferable to control it as low as possible, and in principle, it is possible to limit the content of P to 0%, but it is inevitably added in the manufacturing process. It is important to manage the upper limit, and if the content exceeds 0.03% by weight, it combines with Mn to form a non-metallic inclusion, thereby greatly reducing the toughness and strength of the steel. Therefore, the content of P is preferably limited to 0.03% by weight or less.

S: 0.008 wt% or less

S is an element that is inevitably contained in the production, it is preferable to suppress the content as much as possible. In theory, it is possible to limit the content of S to 0%, but inevitably be added in the manufacturing process. It is important to manage the upper limit, and if the content exceeds 0.008% by weight, it forms a non-metallic inclusion by combining with Mn and the like, and thus, the problem of greatly reducing the toughness and strength of the steel is limited. It is desirable to.

Al (aluminum): 0.05 wt% or less (excluding 0)

In the present invention, the content of Al is preferably limited to the minimum. In general, the addition of Al stabilizes residual austenite and is effective in improving delayed fracture characteristics by hydrogen embrittlement. However, the oxidation characteristics may be formed in the steel and plating properties may be degraded. In addition, since the slab may cause high temperature embrittlement, in the present invention, it is preferable to limit the content of Al to 0.05% by weight or less.

N (nitrogen): 0.013-0.03 weight%

In general, N is dissolved in the steel and then precipitates to increase the strength of the steel, which is superior to C. The dissolved N component plays a similar role to C when the C content is low to ensure the stability of residual austenite. In order to obtain the effect, a content of 0.013% by weight or more is required. However, when the content of N exceeds 0.03% by weight, there is a problem in that defects such as bubbles are generated on the surface by N which is not dissolved. Therefore, the content of N is preferably limited to 0.013 to 0.03% by weight.

Ti (titanium): 0.005 to 0.02 wt%

If the content of Ti exceeds 0.02% by weight, the effect of addition is no longer expected and the formation of carbides increases and the content of solid solution carbon decreases. Therefore, the content of Ti is preferably limited to 0.005 to 0.02% by weight.

B (boron): 20 ppm or less (excluding 0)

B can improve hardenability even if small amount is added to steel. However, when the content of B exceeds 20 ppm, the recrystallization temperature is raised to deteriorate the weldability. Therefore, the content of B is preferably limited to 20 ppm or less.

Sb (antimony): 0.01-0.03% by weight

When the Sb content is 0.01% by weight or more, the surface properties are improved and surface decarburization is prevented. However, when the content of Sb is more than 0.03% by weight, the surface properties are rather deteriorated due to thickening. Therefore, the content of Sb is preferably limited to 0.01 to 0.03% by weight.

The steel sheet satisfying the above component type and composition range preferably satisfies tensile strength (MPa) * elongation (%) ≧ 20000 (MPa *%) and ductility ratio ≧ 0.4. In addition, it is desirable to satisfy the tensile strength (MPa) ≥ 780 MPa. It is preferred that the present invention have physical properties in the range shown in FIG.

Hereinafter, the manufacturing method of this invention is demonstrated.

In the present invention, the steel slab that satisfies the above component system and composition range is heated to a temperature range of 1100 ~ 1250 ℃, hot finish rolling in the range of 860 ~ 920 ℃ to produce a hot rolled steel sheet. Immediately after hot rolling, the hot rolled steel sheet is wound after cooling the bainite transformation start temperature (Bs) to martensite transformation start temperature (Ms) + 20 ° C at a cooling rate in the range of 10 to 30 ° C / sec. After that, it can be cooled to room temperature.

Controlling the cooling rate and the end temperature after hot rolling is to control the microstructure of the hot rolled steel sheet as a composite of ferrite and bainite to secure strength and elongation through uniform distribution of residual austenite after cold rolling and annealing. .

In the manufacture of conventional TRIP steel and high-strength steel, the microstructure of the hot rolled steel sheet is not specified. Generally, hot rolling and annealing heat treatment are performed on hot-rolled structures of ferrite and pearlite structures. However, the present inventors have confirmed that there are various advantages when introducing some of the microstructure of the hot rolled steel sheet to the bainite through various experiments.

In particular, when pearlite is present in the hot-rolled steel sheet, coarse carbides grow in the pearlite during annealing, thereby delaying austenite transformation during annealing, thereby making it difficult to form sufficient retained austenite. However, when bainite is present in the steel, fine carbides are easily dissolved during annealing heating, so that there is no delay of austenite transformation during annealing and it is uniformly distributed.

In addition, if martensite is present in the steel, there is a problem that cracks easily occur at edges during cold rolling. Therefore, it is preferable to limit the microstructure of the hot rolled steel sheet to ferrite and bainite, and to provide cooling conditions for securing the microstructure.

Cooling should be started immediately after hot rolling to prevent the occurrence of pearlite during continuous cooling. In addition, when the cooling rate is less than 10 ℃ / sec Perlite may occur during continuous cooling, if it exceeds 30 ℃ / sec ferrite transformation hardly occurs, it is easy to crack the edge portion during cold rolling of the steel.

The coiling temperature is determined in the range of bainite transformation start temperature (Bs) to martensite transformation start temperature (Ms) + 20 ° C. or less so that martensite does not occur and bainite is sufficiently generated. The bainite transformation start temperature (Bs) and martensite transformation start temperature (Ms) may be experimentally obtained through a phase transformation test, and then the winding temperature may be set.

The hot rolled steel sheet may be cooled to room temperature after winding up. You can add pickling. The hot rolled steel sheet may be cold rolled in a range of 30 to 70% of a cold rolling rate, thereby manufacturing a cold rolled steel sheet.

When the cold reduction rate is less than 30%, the thickness reduction effect due to cold rolling is small, and when it exceeds 70%, the cold load rate is 30 because the rolling load increases and the possibility of cracking at the edge part is high. It is preferable to limit to ˜70%.

It is preferable to anneal the cold rolled steel sheet in the temperature range of 720 ~ 850 ℃ fast C and Mn diffusion rate is necessary, because the fast reaction of the distribution of C and Mn, at this temperature. Austenite is easily formed during annealing. When the annealing temperature is less than 720 ℃, it is difficult to secure the amount of carbon required to stabilize the austenite for strength and ductility increase, and the austenite transformation fraction is too small to secure sufficient TRIP characteristics. On the other hand, when the temperature exceeds 850 ° C., diffusion of Si is promoted to prevent the precipitation of carbides, thereby making it difficult to secure the stability of austenite. Therefore, the annealing temperature is preferably limited to 720 ~ 850 ℃.

The annealing time is the time necessary to obtain the equilibrium at the annealing temperature, and if it is maintained for more than 50 seconds, the austenite can reach the equilibrium sufficiently in the temperature range. However, the time that can be applied to continuous annealing is preferably 50 seconds to 300 seconds.

After the annealing step is cooled to 300 ~ 450 ℃ at a cooling rate of 10 ~ 200 ℃ / sec and maintained for 60 seconds or more. After the final cooling to produce a cold rolled steel sheet. In addition, the cold-rolled steel sheet produced by the above method can be galvanized in accordance with a conventional method to produce a galvanized steel sheet.

In the present invention, the microstructure of the hot rolled steel sheet is composed of ferrite and bainite. Irrespective of each phase ratio, it does not matter if it is a structure containing ferrite and bainite. In the annealing heat treatment step after cold rolling the hot rolled steel sheet, the fine carbide of bainite is easily re-used so that there is no delay in the austenite transformation process that occurs later, and uniform transformation is possible because the austenite transforms around the evenly dispersed fine carbide first. And, the average diameter of the transformed austenite can be made fine to 1㎛ or less.

Therefore, in the final microstructure after completion of annealing, the distribution of residual austenite and formation of other microstructures are very uniform, and the average ferrite grain size is 10 µm or less. The microstructure of the final steel sheet may include, as a percentage, ferrite 35% or more, martensite 5% or less, bainite 20-50% and residual austenite 3-10%.

Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example)

The steel slab satisfying the component system and composition range shown in Table 1 was heated to 1100 ~ 1250 ℃ and hot-rolled rolling at 860 ~ 920 ℃ to prepare a hot rolled steel sheet having a thickness of about 2.7mm. The hot rolled steel sheet was cooled in a range of bainite transformation start temperature (Bs) to martensite transformation start temperature (Ms) + 20 ° C at a cooling rate of 10 to 30 ° C / sec, and then wound and air cooled to room temperature.

Here, the winding temperature range was determined from the Ms and Bs temperatures tested for each emphasis range, the winding temperature, hot finish rolling temperature, cooling rate and the microstructure of the hot rolled steel sheet are shown in Table 2 below.

The microstructure of the hot rolled steel sheet was observed with an optical microscope, and the fraction of each microstructure was analyzed by Image Analysis System after 2% Nital etching.

The hot rolled steel sheet was wound up to room temperature after cooling, pickled with hydrochloric acid, and cold rolled at a cold reduction rate of 55%. The thickness of the cold rolled steel sheet was about 1.2 mm.

The cold rolled steel sheet was annealed at 800 ° C. for 90 seconds, and immediately cooled to 350 ° C. at a cooling rate of 20 ° C./sec immediately after annealing, maintained for 300 seconds, and finally cooled to room temperature at a cooling rate of 3 ° C./sec.

Tensile Strength (TS), Elongation (T-El), Cross Tension Strength (CTS), and Tensile Shear Strength (TSS) for cold rolled steel sheets manufactured under the above conditions ) Was measured and shown in Table 3 below.

In addition, the annealed cold rolled steel sheet was observed using a scanning electron microscope (FE-SEM JEOL JSM-7001F), and image analysis of the results and residual austenite phase using EBSD (Electron Back Scattered Diffractometer, Hikari System, TSL ver 5.2) This is the result of the analysis. F is the ferrite phase, B is the bainite phase, M is martensite phase and RA is the residual austenite phase fraction.

In addition, the cross tensile strength (CTS) and shear tensile strength (TSS) tests of the spot welding materials shown in Table 3 were carried out by welding the cold rolled steel sheet with a welding tester of 60 Hz, single phase AC, and maximum pressure of 60 kN. The electrode material was RWMA Class. II (Cu-Cr) was used and the shape was Dome radius type and electrode pressure was 3.5kN. During welding, the average diameter of the nugget was 4.5 ~ 5.0mm, and the diameter of the nugget was the average of long and short axis.

A cruciform tensile test piece was prepared by spot welding the center part by overlapping two plate-shaped specimens 150 mm long and 45 mm wide in a cross shape. In addition, the shear tensile test piece was prepared by spot welding two plate-shaped specimens of length 100mm and width 30mm in a straight line so that the length of the ends overlap 30mm.

Steel grade C Mn Si P S Al Ti Cr Mo B Sb N Ceq
One 0.22 1.9 1.28 0.012 0.003 0.45 0.017 0.04 - 0.001 0.02 0.004 0.59
2 0.24 1.8 1.11 0.009 0.003 0.52 0.015 0.04 - 0.001 0.019 0.004 0.5863
3 0.23 1.8 1.47 0.009 0.003 0.04 0.015 0.03 - 0.001 0.022 0.004 0.5913
4 0.25 1.6 0.52 0.008 0.003 1.01 0.014 0.02 - 0.001 0.02 0.005 0.5383
5 0.3 1.6 1.49 0.012 0.003 0.03 0.01 0.02 - 0.001 0.021 0.004 0.6288
6 0.17 2.7 1.23 0.01 0.003 0.38 0.015 0.02 0.02 0.001 0.022 0.004 0.6713
7 0.15 2.88 1.02 0.009 0.003 0.52 0.016 0.1 0.02 0.001 0.017 0.005 0.6725
8 0.1 2.9 1.52 0.009 0.003 0.03 0.014 0.03 0.02 0.001 0.02 0.004 0.6467
9 0.12 2.8 0.56 0.012 0.003 0.99 0.02 0.04 0.02 0.001 0.025 0.005 0.59
10 0.08 3 1.54 0.010 0.003 0.03 0.014 0.02 0.02 0.001 0.015 0.004 0.6442
11 0.08 2.5 1.5 0.014 0.003 0.021 0.015 0.03 - 0.001 0.022 0.015 0.5092
12 0.06 2.7 1.6 0.015 0.003 0.02 0.019 0.03 - 0.001 0.022 0.013 0.51
13 0.1 2.5 1.3 0.012 0.003 0.018 0.02 0.04 - 0.001 0.022 0.014 0.5708
14 0.12 2.2 1.4 0.01 0.003 0.019 0.019 0.04 - 0.001 0.029 0.017 0.545
15 0.14 2.3 1.3 0.012 0.003 0.015 0.02 0.04 - 0.001 0.025 0.018 0.5942

(The unit of the composition range for each component is weight percent.)

Steel grades 1 to 10 are steel grades outside the composition range of the present invention, and steel grades 11 to 15 are steel grades having the composition range of the present invention.

Steel grade Hot Finish Rolling Temperature (℃) Average cooling speed
(℃ / sec) Coiling temperature
(℃) Hot Rolled Steel Microstructure Remarks One 900 20 600 90% F + 10% P Comparative Example 1 2 890 20 600 80% F + 10% P Comparative Example 2 3 900 22 600 85% F + 15% P Comparative Example 3 4 890 15 560 70% F + 20% P + 10% B Comparative Example 4 5 910 20 500 60% B + 40% F Comparative Example 5 6 890 25 600 80% F + 20% P Comparative Example 6 7 900 20 500 50% B + 20% F + 30% P Comparative Example 7 8 900 18 560 60% F + 30% P + 10% B Comparative Example 8 9 900 20 500 50% B + 45% F Comparative Example 9 10 900 25 600 80% F + 20% P Comparative Example 10 11 910 22 550 50% B + 50% F Inventive Example 1 11 910 50 700 70% F + 30% P Comparative Example 11 12 900 25 560 45% B + 55% F Inventive Example 2 12 900 45 700 80% F + 20% P Comparative Example 12 13 890 25 550 55% B + 45% F Inventive Example 3 13 890 60 700 70% F + 30% P Comparative Example 13 14 890 25 540 60% B + 40% F Honorable 4 14 890 50 690 70% F + 30% P Comparative Example 14 15 900 25 560 35% B + 55% F + 10% P Inventory 5 15 900 55 700 60% F + 40% P Comparative Example 15

(F means ferrite, P means pearlite, and B means bainite.)

Comparative Examples 11 to 15 satisfy the ingredient system and the composition range of the present invention, but the cooling rate does not satisfy the range of the present invention. can confirm.

division TS (MPa) T-El (%) TS * T-El (MPa *%) TSS (KN / spot) CTS (KN / spot) CTS / TSS Comparative Example 1 830 22.3 18509 13.9 4.9 0.353 Comparative Example 2 930 18.0 16740 14.2 4.6 0.324 Comparative Example 3 970 19.4 18818 14.8 4.6 0.311 Comparative Example 4 981 19.2 18835 14.7 4.8 0.327 Comparative Example 5 1080 28.0 30240 15.2 4.4 0.289 Comparative Example 6 1040 15.2 15808 14.7 4.8 0.327 Comparative Example 7 1058 19.0 20102 15.8 5.1 0.323 Comparative Example 8 1023 17.6 18005 16.2 5.3 0.327 Comparative Example 9 1038 15.3 15881 16.0 5.2 0.325 Comparative Example 10 1018 18.2 18528 15.4 5.2 0.338 Inventive Example 1 798 25.1 20030 13.2 7.9 0.598 Comparative Example 11 751 23.6 17724 13.4 7.8 0.582 Inventive Example 2 793 25.8 20459 13.0 7.8 0.600 Comparative Example 12 742 24.1 17882 12.9 7.6 0.589 Inventive Example 3 987 20.3 20036 14.4 6.8 0.472 Comparative Example 13 942 18.1 17050 14.2 6.9 0.486 Honorable 4 974 21.2 20649 15.2 7.3 0.480 Comparative Example 14 920 19.9 18308 15.1 6.9 0.457 Inventory 5 1102 18.8 20718 16.3 7.1 0.436 Comparative Example 15 982 16.4 16105 15.1 6.8 0.450

It can be seen that the shear tensile strength (TSS) tends to increase in proportion to the tensile strength of the steel, and the cross tensile strength (CTS) tends to increase in inverse proportion to the carbon content. Therefore, the CTS / TSS value (ductility ratio), which is the ratio of these two values, can be used as a representative value representing the spot welding characteristics of the steel.

Figure 1 shows the range of spot welding properties and strength * elongation of the invention example, as can be seen through this, it can be seen that the CTS / TSS value of the invention example is higher than the comparative example. And, in the case of the invention example it can be seen that due to the low C content, the ductility ratio has all 0.4 or more.

From the measurement results of the tensile strength and the elongation values shown in Table 3, it can be seen that the tensile strength and the elongation of the inventive example showed superior results compared to the strength and the elongation of the comparative example. In addition, it can be seen that the invention examples all exhibit excellent spot welding characteristics.

1 is a graph showing strength * elongation and ductility ratio, and is a graph showing a range of strength * elongation and ductility ratio intended in the present invention.

Claims (10)

By weight%, C: 0.06-0.22%, Si: 0.8-2.0%, Mn: 1-3%, P: 0.03% or less, S: 0.008% or less, N: 0.013-0.03%, Al: 0.05% or less, Ti: 0.005 ~ 0.02%, B: 20ppm or less, Sb: 0.01 ~ 0.03%, balance Fe and other unavoidable impurities, tensile strength (MPa) * elongation (%) ≥20000 (MPa *%) and ductility ratio≥ Automotive steel sheet with excellent spot weldability, strength and elongation satisfying 0.4. The steel sheet for automobiles of claim 1, wherein the steel sheet satisfies tensile strength (MPa) ≥ 780 MPa. According to claim 1, wherein the microstructure of the steel sheet as a phase fraction, ferrite 35% or more, martensite 5% or less, bainite 20 to 50% and residual austenite 3 to 10% Automotive steel plate with high strength and elongation. 4. The steel sheet for automobiles having excellent spot weldability, strength and elongation of claim 3, wherein the grain size of the ferrite is 10 µm or less. The steel sheet for automobiles according to any one of claims 1 to 4, wherein the steel sheet is a cold rolled steel sheet or a galvanized steel sheet. The steel sheet for automobiles having excellent spot weldability, strength and elongation according to claim 1, wherein the hot rolled steel sheet satisfies the component system, and the microstructure of the hot rolled steel sheet is a ferrite and bainite two-phase structure. By weight%, C: 0.06-0.22%, Si: 0.8-2.0%, Mn: 1-3%, P: 0.03% or less, S: 0.008% or less, N: 0.013-0.03%, Al: 0.05% or less, Steel slab containing Ti: 0.005 ~ 0.02%, B: 20ppm or less, Sb: 0.01 ~ 0.03%, balance Fe and other unavoidable impurities is heated to 1100 ~ 1250 ℃ and hot-rolled at 860 ~ 920 ℃ to hot rolled steel sheet. Manufacturing step; Cooling and winding the hot rolled steel sheet in the range of bainite transformation start temperature (Bs) to martensite transformation start temperature (Ms) + 20 ° C. at a cooling rate of 10 to 30 ° C./sec immediately after hot rolling; Cold rolling the hot rolled steel sheet in the range of 30 to 70% of a cold rolling rate; Continuous annealing the cold rolled steel sheet 50 seconds or more after cooling in the 720 ~ 850 ℃ range after the cold rolling; And After annealing after cooling in the range 300 ~ 450 ℃ at a cooling rate of 10 ~ 200 ℃ / sec after the final cooling step to maintain 60 seconds or more excellent weldability, strength and elongation manufacturing method of the automotive steel sheet. The method of claim 7, wherein the steel sheet for automobiles having excellent spot weldability, strength, and elongation, which includes pickling a wound steel sheet after the winding step. The method of claim 7, wherein after the final cooling step, the cold rolled steel sheet comprises plating the cold rolled steel sheet. The method of claim 7, wherein the microstructure of the hot rolled steel sheet manufactured after hot rolling is a ferrite and bainite two-phase structure.

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