KR20090072357A - High strength thin steel sheet excelling in weldability and process for producing the same - Google Patents

Abstract

A high strength thin steel sheet with excellent weldability and a manufacturing method thereof are provided to improve platability, weldability, bending workability and hole extension property and increase tensile strength above 800MPa to improve fuel efficiency and durability. A high strength thin steel sheet with excellent weldability includes C: 0.02-0.20 weight%, Si: 1.5 weight% or less, Mn: 1.5-3.0 weight%, P: 0.001-0.10 weight%, S: 0.010 weight% or less, Sol.Al: 0.01-0.40 weight%, N: 0.020 weight% or less, Cr: 0.3-1.5 weight%, B: 0.0010-0.0060 weight%, and Sb: 0.001-0.10 weight%. The high strength thin steel sheet includes one or more kinds selected among Ti: 0.003-0.08 weight%, Nb: 0.003-0.08 weight%, and Mo: 0.003-0.08 weight% and remnant Fe and other impurities. The Si,Mn,B,Sb,P,S satisfies 5<(Si/Mn+150B)/Sb<20 and C+Mn/20+Si/30+2P+4S<0.27.

Description

High Strength Thin Steel Sheet excelling In Weldability and Process For Producing The Same}

The present invention relates to a high strength steel sheet with a tensile strength of 800MPa or more and a method for manufacturing the same, which are mainly used for building materials, home appliances, and automobiles, and more particularly, excellent plating property, weldability, bending workability, and hole expandability with high tensile strength. It relates to a high-strength thin steel sheet having a manufacturing method.

Recently, steel sheets for automobiles are required to have higher strength steel sheets to improve fuel efficiency and durability, and in view of collision safety and passenger protection, high-strength steel sheets of 800 MPa or more are increasing as vehicle structural structures and reinforcement materials. However, since the high strength of the steel sheet causes a decrease in moldability and weldability, it is desirable to develop a material that compensates for this. To meet such demands, various composite steel sheets have been developed such as ferritic-martensitic dual phase steel and TRIP steel sheets using transformation-induced firing of residual austenite.

For example, Japanese Patent Application Laid-Open No. 6-145892 proposes a method for producing a steel sheet excellent in formability by controlling the chemical composition and the amount of retained austenite in the steel sheet. Japanese Patent No. 2660644 and Japanese Patent No. 2704350 propose a method for producing a high strength steel sheet having good press formability by controlling the chemical composition and the microstructure of the steel sheet. In addition, Japanese Patent No. 3317303 proposes a steel sheet having excellent workability, in particular, local drawing, containing 5% or more of retained austenite. However, most of these inventions have been developed to improve ductility, and sufficient consideration has not been given to bending workability, hole expandability, or weldability, which are important measures in actual part machining.

Among the required characteristics of the steel sheet, the most important characteristic of the body structure or reinforcement, in which a high strength steel sheet of 800 MPa or more is mainly used, is spot weldability. Structural or stiffeners serve to protect passengers by absorbing collision energy during a collision, and if the strength of the spot weld is insufficient, it breaks during collision and sufficient collision absorption energy cannot be obtained. Japanese Laid-Open Patent Publication No. 2003-193194 discloses a technique regarding high strength steel sheet considering weldability, but there is a problem in that it does not satisfy the weldability required by the market.

In addition, Japanese Laid-Open Patent Publication No. 2005-105367 proposes a technique that ensures weldability and ductility in steel of 780 Mpa or more. As described above, when manufacturing a high strength steel sheet of 800 MPa or more in a practical process, the cold rolling property is greatly reduced due to the high strength of the hot rolled sheet, which is an intermediate material, and the operability is greatly reduced because the quench heat treatment conditions must be applied during annealing heat treatment. There is a problem. In Japanese Unexamined Patent Application Publication No. 2005-105367, this has not been sufficiently examined.

An object of the present invention is to provide a steel sheet excellent in plating property, weldability, bending workability, and hole expandability in manufacturing a high strength steel sheet having a tensile strength of 800 MPa or more. Another object of the present invention is to provide a manufacturing method capable of securing the operability of such a steel sheet.

Steel sheet of the present invention for achieving the above object, by weight% C: 0.02 ~ 0.20%, Si: 1.5% or less, Mn: 1.5 ~ 3.0%, P: 0.001 ~ 0.10%, S: 0.010% or less, Sol. Al: 0.01-0.40%, N: 0.020% or less, Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, including Ti: 0.003-0.08%, Nb: 0.003- 0.08%, Mo: 0.003-0.08% of at least one selected from the group and the remaining Fe and other inevitable impurities,

Si, Mn, B, Sb, P, and S satisfy 5 <(Si / Mn + 150B) / Sb <20 and C + Mn / 20 + Si / 30 + 2P + 4S <0.27.

In addition, the method for producing the steel sheet, by weight% C: 0.02-0.20%, Si: 1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less, Sol.Al: 0.01 to 0.40%, N: 0.020% or less, Cr: 0.3 to 1.5%, B: 0.0010 to 0.0060%, Sb: 0.001 to 0.10%, including Ti: 0.003-0.08%, Nb: 0.003-0.08% , Mo: 0.003-0.08% of at least one selected from the group and the remaining Fe and other inevitable impurities,

Reheating the slab of steel where Si, Mn, B, Sb, P, S satisfy 5 <(Si / Mn + 150B) / Sb <20 and C + Mn / 20 + Si / 30 + 2P + 4S <0.27 After, rolling and winding so that the finish rolling exit temperature is between Ar3 transformation point ~ 950 ℃,

Cold rolling the wound hot rolled sheet at a reduction ratio of 40 to 80% after pickling,

The cold rolled sheet thus obtained is continuously annealed at a temperature range of 740 ° C. to 860 ° C., and is -5 LogCR + 25C-17Si + 40Cr + 13,000B> 30 in the range of 3 to 150 ° C / s cooling rate (CR). After cooling continuously to a temperature of 250 ~ 600 ℃ at a cooling rate that satisfies the condition of the cooling rate of 5 ℃ / min or more.

In the steel sheet manufactured according to the present invention, at least one selected from the group consisting of bainite and bainitic ferrite is 40% or more, and the rest is in the form of ferrite and martensite.

According to the present invention has a high strength of more than 800MPa tensile strength, there is a useful effect that can provide a steel sheet excellent in plating property, weldability, bending workability and hole expansion properties and a manufacturing method that can ensure the operation of such a steel sheet.

Hereinafter, the present invention will be described in detail.

The carbon (C) is preferably 0.02 to 0.20% by weight (hereinafter simply referred to as%).

Carbon in steel is an important element added to strengthen the metamorphic structure. However, when the amount exceeds 0.20%, the hole expandability and weldability are reduced, and when the amount is less than 0.02%, it is difficult to secure the strength.

Silicon (Si) is preferably 1.5% or less.

In steel, Si is an element that can be usefully used for improving strength, but it is usually 1.0% or less because it not only causes surface scale defects related to surface properties, but also degrades the surface properties of plated steel sheets and degrades chemical conversion. In many cases, due to the recent advances in plating technology, the steel content can be produced without major problems up to about 1.5%, so the content is limited to 1.5% or less.

As for manganese (Mn), 1.5 to 3.0% is preferable.

In the steel, Mn is an element having a very high solid solution effect and promotes the formation of a complex structure composed of ferrite and martensite. If the content is less than 1.5%, there is a difficulty in securing the target strength in the present invention, while if the content exceeds 3.0%, problems such as weldability and hot rolling property are likely to occur.

The phosphorus (P) is preferably 0.001 to 0.10%.

P in the steel is an element showing the effect of strengthening the steel sheet. If the content is less than 0.001%, the effect may not be secured, and it may cause a problem of manufacturing cost. On the other hand, excessive addition may deteriorate press formability and cause brittleness of steel.

Sulfur (S) is preferably 0.010% or less.

S in steel is an impurity element in steel and is an element that inhibits the ductility and weldability of the steel sheet. If the content exceeds 0.01%, there is a high possibility of inhibiting the ductility and weldability of the steel sheet.

As for soluble aluminum (Sol.Al), 0.01 to 0.4% is preferable.

Soluble Al in steel is an effective component for improving martensite hardenability by combining with oxygen in steel to deoxidize and distribute carbon in ferrite to austenite such as Si. If the content is less than 0.01%, the effect may not be secured, while if the content exceeds 0.4%, the effect may not only be saturated, but the manufacturing cost may increase.

Nitrogen (N) is preferably 0.020% or less.

N in the steel is an effective component to stabilize the austenite, and when it exceeds 0.020%, the austenite stability is greatly increased, which may prevent the formation of bainite, the microstructure to be formed in the steel of the present invention.

Chromium (Cr) is preferably 0.3 to 1.5%.

Cr in steel is a component added to improve the hardenability of steel and to secure high strength, and is an element that plays an important role as bainite formation promoting element in the present invention. When the content of Cr is less than 0.3%, it is difficult to secure the above effects, and when the content of Cr exceeds 1.50%, the effect is not only saturated but also economically disadvantageous.

As for boron (B), 0.0010 to 0.0060% is preferable.

Steel B is a component that delays the transformation of austenite into pearlite during cooling during annealing, and is added as an element that suppresses ferrite formation and promotes the formation of bainite. However, when the content of B is less than 0.0010%, it is difficult to obtain the above effects, and when the content of B is more than 0.0060%, excessive B may be concentrated on the surface, resulting in deterioration of plating adhesion.

Antimony (Sb) is preferably 0.001 to 0.1%.

Sb in steel is an essential component added in order to ensure the excellent plating property in this invention. Sb reduces surface defects by inhibiting surface concentration of oxides such as MnO, SiO ₂ , and Al ₂ O ₃ , and has an excellent effect of suppressing coarsening of surface concentrates due to temperature rise and hot rolling process changes. When the content of Sb is less than 0.001%, it is difficult to secure the above effects, and even if the amount of the addition thereof continues to increase, such an effect does not increase greatly, and may cause problems such as manufacturing cost and workability deterioration. The content is preferably limited to 0.005 to 0.1%.

In the present invention, one or two or more selected from Ti: 0.003 to 0.08%, Nb: 0.003 to 0.08%, and Mo: 0.003 to 0.08% can be added to the steel formed as described above to increase the strength and reduce the particle size. Can be.

When the lower limit of Ti Nb and Mo is less than 0.003%, it is difficult to secure the effect of increasing the strength and the particle size, and when the upper limit exceeds 0.08%, the softness is increased due to the increase in manufacturing cost and excessive precipitates. Can be greatly reduced.

The present invention is composed of Fe and other unavoidable impurities in addition to the above components.

According to the present invention, the alloy composition ratio of Si, Mn, B, Sb, P, and S in the alloy design of the steel sheet having the above component range preferably satisfies the following component relations (1) and (2).

[Relationship 1]

5 <(Si / Mn + 150B) / Sb <20

[Relationship 2]

C + Mn / 20 + Si / 30 + 2P + 4S <0.27

The relational expression 1 is obtained as an empirical figure of the component relationship which can ensure the surface quality. That is, Mn, Si, B, etc. in steel are elements having a feature of forming a concentrate on the surface during annealing, and the more the concentration of these elements, the lower the plating property. On the other hand, Sb is very advantageous in terms of surface quality because it plays a role in preventing grain boundary diffusion of the surface thickening elements. For example, when the value calculated by Equation 1 has a value between 5 and 20, it means that good surface quality can be secured.

On the other hand, relationship 2 is obtained as an empirical value of the component relationship that can ensure the weldability. That is, elements of C, Mn, Si, P, and S in steel serve to increase the carbon equivalent. As is well known, the higher the carbon equivalent, the lower the weldability. When the present invention steel is used by setting the condition that the welding failure does not occur during the spot welding, which is mainly the construction method is configured as shown in Equation 2. If the value calculated by Equation 2 exceeds 0.27, it means that the possibility of welding defects is increased.

In the steel sheet of the present invention, the structure is preferably at least 40% of at least one selected from bainite and bainitic ferrite, and the rest of which is in the form of ferrite and martensite. Ferrite and martensite are preferably 25% or less for ferrite and 35% or less for martensite.

Hereinafter, a method of manufacturing the steel formed as described above as a cold rolled steel sheet will be described in detail.

After reheating the slab of the composition in the alloy design method as described above is subjected to hot rolling. It is preferable to finish-roll in hot rolling so that exit temperature may be between Ar3 transformation point-950 degreeC. That is, if the hot finish rolling temperature is less than the Ar3 transformation point, the hot deformation resistance is likely to increase sharply, and manufacturing problems may occur. .

The hot rolled sheet produced in the above manner is cold rolled after pickling.

As for the reduction ratio in cold rolling, 40 to 80% is preferable. If the reduction ratio is less than 40%, there is a possibility that the recrystallization driving force is weakened to obtain a good recrystallized grain, and if the reduction ratio exceeds 80%, the rolling load increases rapidly.

Although the cold-rolled sheet obtained above is continuously annealed, the annealing temperature is preferably 740 ° C to 860 ° C. If the temperature is less than 740 ℃ during continuous annealing increases the risk of un-recrystallized grains, and if the temperature exceeds 860 ℃ may be due to the high temperature annealing operation with a large grain formation may be poor mailing.

After the continuous annealing, the cooling was continuously cooled to a temperature of 250 to 600 ° C. at a cooling rate exceeding 30 calculated by the following equation in the range of cooling rate (CR) of 3 to 150 ° C./s. Next, cool slowly at a cooling rate of 5 ° C / min or more. By continuous annealing under the above conditions, a high strength steel sheet having good plating property, weldability and hole expandability of tensile strength of 800 MPa or more can be easily manufactured.

[Relationship 3]

-5 LogCR + 25C-17Si + 40Cr + 13,000B> 30

Where CR is the cooling rate

If the cooling rate after the continuous annealing is lowered to less than 3 ° C / s, ferrite or pearlite is formed, it is difficult to secure the strength targeted in the present invention. In addition, if it is too high at 150 ° C / s or more, the hard phases such as martensite are excessively formed, which greatly degrades the bending workability and the hole expansion property, and is greatly concerned about the deterioration of the boardability due to poor shape during operation. It is preferable to cool in the range of a cooling rate (CR) of ˜150 ° C./s.

In addition, in order to achieve excellent bending workability and hole expandability, which is a characteristic of the present invention steel, the value calculated by the relational expression 3 must be applied at a cooling rate of more than 30. That is, when the value calculated by the relation 3 is less than 30, it is difficult to obtain more than 40% of the bainite or bainitic ferrite phase to be obtained as a desirable microstructure in the present invention steel. When the bainite-based structure becomes 40% or more, it is possible to manufacture a product having high strength of 800 MPa or more and excellent bending property and hole expansion property, which is a characteristic of the present invention steel.

On the other hand, it is desirable to make the cooling end temperature between 250 and 600 ° C. during cooling. If the cooling end temperature is less than 250 ° C., the risk of martensite is increased. This is because a large amount of soft phases such as lights are formed, and thus the material is difficult to achieve the target material.

The manufacturing method is equally applicable to not only cold rolled steel plate but also plated products such as GI and GA materials.

Hereinafter, the present invention will be described in more detail.

As shown in Table 1 below, the slab having the composition of the present invention was heated and extracted at a temperature of 1200 ° C., followed by hot rolling at a finish rolling temperature of 900 ° C., followed by rolling at a cold reduction rate of 55%. The annealing temperature and the cooling conditions in Table 2 were carried out to conduct a continuous annealing heat treatment (CR), and in the case of a plated product was subjected to hot dip galvanizing (GI) and alloying (GA) treatment to produce a product. Applied conditions and alloying time in continuous annealing are as follows.

-Annealing furnace atmosphere: N ₂ -10% H ₂ O (dew point -30 ℃)

-Annealing furnace heating rate: 3 ℃ / sec.

Annealing time: 90 sec.

Plating temperature: 460 ℃

Alloying time: 24 sec (for GA products)

As shown in Table 2, the plating properties (appearance, adhesion) and the material (tensile strength, hole expandability, bending workability) were measured and the results are shown together with the comparative material.

In Table 2, the appearance of the plating was set to o when unplated and other plating defects were not included, and the defect name was specified when plating defects occurred.

In Table 2, the evaluation of plating adhesion was performed by cutting the plated plate into 20 mm x 50 mm, and then performing a 60 ° bending test, and then applying the tape to the bent flat place to evaluate the width of the plated layer.

◎: No plating off or less than 1mm in width

(Circle): Falling plating width is within 1-3 mm

△: stripped plating width is within 3 ~ 5mm

X: The plating width which fell out is 5mm or more

In Table 2, the Hole Expansion Ratio (HER) is obtained by drilling a 10mm diameter hole in a 120 × 120mm specimen and extending the hole until a crack occurs using a punch with a 60 ° molding angle. This is obtained by calculating the expanded ratio of the hole. In addition, the bendability evaluation in Table 2 was evaluated as the smallest punch radius (mm) by bending the specimen with a 90-degree V-shaped punch.

River C Si Mn P S Al N Ti Nb Mo Cr B Sb Equation 1 calculated value Equation 2 calculated value One 0.06 0.1 2.5 0.01 0.004 0.035 0.005 0.02 0.05 0.03 0.9 0.0018 0.02 15.5 0.22 2 0.07 0.15 2.2 0.015 0.003 0.05 0.004 0.025 0.055 0.05 0.7 0.0023 0.03 13.8 0.23 3 0.05 0.05 2.1 0.007 0.003 0.22 0.003 0.015 0.045 0.01 0.5 0.0013 0.02 10.9 0.18 4 0.03 0.1 2.5 0.008 0.004 0.043 0.005  - 0.06  - 0.7 0.0019 0.04 8.1 0.19 5 0.15 0.1 2.7 0.005 0.003 0.052 0.003 0.04  -  - 1.0 0.0021 0.03 11.7 0.31 6 0.08 0.5 2.1 0.009 0.003 0.35 0.007 0.02 0.03  - 0.8 0.0032 0.04 18.0 0.23 7 0.07 0.20 2.1 0.012 0.003 0.04 0.003 - - 0.04 0.9 0.0017 0.02 17.5 0.22 8 0.15 0.2 2.7 0.015 0.008 0.043 0.005  -  -  -  - 0.0012  -  - 0.35 9 0.11 0.3 2.4 0.015 0.008 0.043 0.005  -  - 0.04  -  -  -  - 0.30 10 0.18 0.2 1.8 0.011 0.005 0.038 0.004  - 0.05  -  -  -  -  - 0.32 Equation 1 = (Si / Mn + 150B) / Sb Equation 2 = C + Mn / 20 + Si / 30 + 2P + 4S Satisfying alloy design conditions of the present invention

River product Continuous Annealing Temperature (℃) Cooling rate (℃ / sec) Equation 3 calculated value Plating appearance Plating adhesion Tensile Strength (MPa) Hole expandability (%) Bending workability (mm) Bainite fraction (%) Remarks One CR 840 8 54.7 - - 1045 39 0 R 55 The present invention 2 GA 830 20 50.6 ○ ◎ 995 45 0 R 60 3 GA 855 20 30.8 ○ ◎ 830 68 0 R 45 4 GI 860 30 44.4 ○ ◎ 874 54 0 R 65 5 GA 810 20 62.8 ○ ◎ 1076 48 0 R 60 6 CR 820 10 62.1 - - 1012 60 0 R 55 7 CR 820 20 49.9 - - 982 54 0 R 45 8 GA 810 20 - Unplated X 1087 12 2 R 15 Comparative steel 9 GA 810 20 - ○ △ 990 8 2 R 10 10 GA 810 20 - Unplated △ 1040 7 2 R 10 Equation 3 = -5 LogCR + 25C-17Si + 40Cr + 13,000B * When the value calculated by Equation 3 is 30 or more, the manufacturing conditions of the present invention are satisfied.

As shown in Table 2, when the steel sheet is manufactured by the method of the present invention, the tensile strength of 800MPa or more with good surface characteristics and mechanical properties compared to the conventional comparative materials and good plating, weldability, bending workability and hole expansion properties It is possible to manufacture a high strength steel sheet having a. Invented steels consisted of a steel sheet composed of at least 40%, at least one selected from bainite and bainitic ferrite, with less than 25% ferrite and less than 35% martensite.

Claims (4)

By weight% C: 0.02 ~ 0.20%, Si: 1.5% or less, Mn: 1.5 ~ 3.0%, P: 0.001 ~ 0.10%, S: 0.010% or less, Sol.Al: 0.01 ~ 0.40%, N: 0.020% or less , Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, including Ti: 0.003-0.08%, Nb: 0.003-0.08%, Mo: 0.003-0.08% Composed of at least one selected and the remaining Fe and other unavoidable impurities, High strength excellent weldability in which Si, Mn, B, Sb, P, and S satisfy 5 <(Si / Mn + 150B) / Sb <20 and C + Mn / 20 + Si / 30 + 2P + 4S <0.27 Sheet steel. According to claim 1, wherein the structure of the steel sheet is at least one selected from bainite (Bainite) and Bainitic Ferrite (Bainitic Ferrite) is at least 40% and the rest of the ferrite and martensite phase is excellent weldability, characterized in that High strength steel sheet. By weight% C: 0.02 ~ 0.20%, Si: 1.5% or less, Mn: 1.5 ~ 3.0%, P: 0.001 ~ 0.10%, S: 0.010% or less, Sol.Al: 0.01 ~ 0.40%, N: 0.020% or less , Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, including Ti: 0.003-0.08%, Nb: 0.003-0.08%, Mo: 0.003-0.08% Composed of at least one selected and the remaining Fe and other unavoidable impurities, Reheating the slab of steel where Si, Mn, B, Sb, P, S satisfy 5 <(Si / Mn + 150B) / Sb <20 and C + Mn / 20 + Si / 30 + 2P + 4S <0.27 After, rolling and winding so that the finish rolling exit temperature is between Ar3 transformation point ~ 950 ℃, Cold rolling the wound hot rolled sheet at a reduction ratio of 40 to 80% after pickling, The cold rolled sheet thus obtained is continuously annealed at a temperature range of 740 ° C. to 860 ° C., and is -5 LogCR + 25C-17Si + 40Cr + 13,000B> 30 in the range of 3 to 150 ° C / s cooling rate (CR). Method of producing a high strength steel sheet excellent in weldability comprising the step of cooling continuously to a temperature of 250 ~ 600 ℃ at a cooling rate that satisfies the condition of 5 ℃ / min or more. According to claim 3, wherein the structure of the steel sheet is characterized in that the structure of the steel sheet is at least 40% or more selected from bainite and bainitic ferrite (40%) and the rest of the ferrite and martensite phase The manufacturing method of high strength steel sheet excellent in weldability.