JP5228062B2 - High strength thin steel sheet with excellent weldability and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength thin steel sheet having a tensile strength of about 800 MPa or more, which is mainly used for building materials, home appliances and automobiles, and a method for producing the same. The present invention relates to a high-strength thin steel sheet having bending workability and a hole expansion ratio (HER) and a method for producing the same.

  Recently, steel sheets for automobiles have been required to have higher strength to improve fuel economy and durability. From the viewpoint of collision safety and passenger protection, high-strength steel sheets of about 800 MPa or more are used for vehicle structure and reinforcement. Has increased. However, increasing the strength of the steel sheet causes a decrease in forming processability and weldability, and therefore development of a material that complements this is required. In response to such demands, various composite structure steel plates such as ferrite-martensite duplex steel plates and TRIP steel plates using transformation-induced plasticity of retained austenite have been developed so far.

  For example, Japanese Patent Application Laid-Open No. 6-145892 (Patent Document 1) proposes a method of manufacturing a steel sheet that is excellent in formability by controlling the chemical components and the amount of retained austenite of the steel sheet. In Japanese Patent No. 2660644 (Patent Document 2) and Japanese Patent No. 2704350 (Patent Document 3), by controlling the chemical components and the microstructure of the steel sheet, a high strength steel sheet with good press formability is obtained. A manufacturing method is proposed. Japanese Patent No. 3317303 (Patent Document 4) proposes a steel sheet containing 5% or more of retained austenite and excellent in workability, particularly local elongation. However, most of these technologies have been developed to improve ductility, and they are important measures for bending workability, hole expansion rate or weldability, which are important when actually processing parts. Have not been fully considered.

  Among the characteristics required for steel sheets, spot weldability is the most important characteristic for vehicle body structures and reinforcing materials in which high strength steel sheets of 800 MPa or more are mainly used. Steel plates used as structural or reinforcing materials serve to protect passengers by absorbing collision energy at the time of collision, and if the strength of the spot weld is not sufficient, it is broken at the time of collision and sufficient collision absorption I can't get energy. As a technique related to high strength steel sheets in consideration of weldability, there is Japanese Patent Laid-Open No. 2003-193194 (Patent Document 5), but there is a problem that the weldability actually required in the market cannot be satisfied.

  Japanese Patent Application Laid-Open No. 2005-105367 (Patent Document 6) proposes a technique that ensures weldability and ductility with a steel of 780 MPa or more. Thus, when manufacturing a high-strength steel sheet of 800 MPa or more in the actual process, the cold rolling property is greatly reduced due to the high strength of the hot-rolled sheet that is an intermediate material, and the rapid heat treatment conditions must be applied during the annealing heat treatment. Therefore, there is a problem that operability is greatly reduced. The above-mentioned Japanese Patent Application Laid-Open No. 2005-105367 does not sufficiently study this.

Japanese Unexamined Patent Publication No. 6-145892 Japanese Patent No. 2660644 Japanese Patent No. 2704350 Japanese Patent No. 3317303 Japanese Unexamined Patent Publication No. 2003-193194 Japanese Unexamined Patent Publication No. 2005-105367

  The present invention has been made to solve the above-mentioned problems of the prior art, and in producing a high-strength thin steel sheet having a tensile strength of 800 MPa or more, the plating property, weldability, bending workability, and hole expansion rate are improved. An object is to provide an excellent steel sheet. Moreover, it aims at providing the manufacturing method which can ensure the operativity of such a steel plate.

  According to one configuration of the present invention, by weight, C: 0.02 to 0.20%, Si: 1.5% or less, Mn: 1.5 to 3.0%, P: 0.001 to 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%, and Sb: 0.001-0. 10%, at least one selected from the group consisting of Ti: 0.003 to 0.08%, Nb: 0.003 to 0.08%, and Mo: 0.003 to 0.08% And Si, Mn, B, Sb, P, and S contain 5 <(Si / Mn + 150B) / Sb <20 and C + Mn / 20 + Si / 30 + 2P + 4S <0.27. Provide a steel sheet to fill.

According to another configuration of the present invention, C: 0.02 to 0.20%, Si: 1.5% or less, Mn: 1.5 to 3.0%, P: 0.0. 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%, and Sb: 0.001-0. 10%, at least one selected from the group consisting of Ti: 0.003 to 0.08%, Nb: 0.003 to 0.08%, and Mo: 0.003 to 0.08% And Si, Mn, B, Sb, P, and S contain 5 <(Si / Mn + 150B) / Sb <20 and C + Mn / 20 + Si / 30 + 2P + 4S <0.27. After reheating the slab of the steel sheet to be filled, rolling and winding so that the temperature on the exit side of finish rolling is between Ar ₃ transformation point and 950 ° C., after pickling the wound hot-rolled sheet Cold rolling at a rolling reduction of 40-80%, and The obtained cold-rolled sheet is subjected to continuous annealing in a temperature range of 740 ° C. to 860 ° C., and a condition of −5 LogCR + 25C-17Si + 40Cr + 13,000B> 30 in a range of a cooling rate (CR, Cooling Rate) of 3 to 150 ° C./s. Provided is a method for producing a steel sheet including a step of cooling at a cooling rate satisfying the above condition to a temperature of 250 to 600 ° C. and then cooling at a cooling rate of 5 ° C./min or more.

  The steel sheet produced according to the present invention contains 40% or more of one or more selected from the group consisting of bainite and bainitic ferrite, and the rest is ferrite and martensite phase. .

  According to the present invention, while having a high strength of about 800 MPa or more, a steel plate excellent in plating property, weldability, bending workability and hole expansion rate, and ensuring the operability of such a steel plate. The manufacturing method which can be provided can be provided.

  The steel sheet of the present invention is in weight percent, C: 0.02 to 0.20%, Si: 1.5% or less, Mn: 1.5 to 3.0%, P: 0.001 to 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%, and Sb: 0.001-0. 10%, at least one selected from the group consisting of Ti: 0.003 to 0.08%, Nb: 0.003 to 0.08%, and Mo: 0.003 to 0.08% And Si, Mn, B, Sb, P, and S contain 5 <(Si / Mn + 150B) / Sb <20 and C + Mn / 20 + Si / 30 + 2P + 4S <0.27. Fulfill.

Moreover, the method of manufacturing the said steel plate is 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%, and Sb: 0.001-0. 10%, at least one selected from the group consisting of Ti: 0.003 to 0.08%, Nb: 0.003 to 0.08%, and Mo: 0.003 to 0.08% And Si, Mn, B, Sb, P, and S contain 5 <(Si / Mn + 150B) / Sb <20 and C + Mn / 20 + Si / 30 + 2P + 4S <0.27. After reheating the slab of the steel sheet to be filled, rolling and winding so that the temperature on the exit side of finish rolling is between Ar ₃ transformation point and 950 ° C., after pickling the wound hot-rolled sheet Cold rolling at a rolling reduction of 40-80%, and The obtained cold-rolled sheet is subjected to continuous annealing in the temperature range of 740 ° C. to 860 ° C., and cooling satisfying the condition of −5 Log CR + 25C-17Si + 40Cr + 13,000B> 30 in the range of the cooling rate (CR) of 3 to 150 ° C./s. And cooling at a cooling rate of 5 ° C./min or more after cooling to a temperature of 250 to 600 ° C. at a rate.

  The steel sheet may contain 40% or more of one or more selected from the group consisting of bainite and bainitic ferrite, and the rest may be ferrite and martensite phase.

  Hereinafter, the present invention will be described in detail.

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

  Carbon in steel is an element added to strengthen the transformation structure. However, if the amount exceeds 0.20%, the hole expansion rate and weldability are lowered, and if the amount is less than 0.02%, it is difficult to ensure the strength.

  Silicon (Si) is preferably 1.5% or less.

  Si in steel is an element that can be used effectively to improve strength, but it not only induces surface scale defects in terms of surface properties, but also reduces the surface properties of plated steel sheets and also reduces the chemical conversion processability. In many cases, however, the content is usually limited to 1.0% or less. However, due to the recent progress in plating technology, it has become possible to produce the steel without any problem even when the content in steel is about 1.5%. Limited to 5% or less.

  Manganese (Mn) is preferably 1.5 to 3.0%.

  Mn in steel is an element having a very large effect of solid solution strengthening, and at the same time promotes formation of a composite structure containing ferrite and martensite. When the content is less than 1.5%, it is difficult to ensure the target strength in the present invention, and when it exceeds 3.0%, problems such as weldability and hot rollability are likely to occur.

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

  P in the steel is an element having an effect of strengthening the steel plate. If the content is less than 0.001%, not only the effect cannot be secured, but also causes a problem of manufacturing cost, while if added excessively, press formability may deteriorate and steel brittleness may occur. There is sex.

  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. When the content exceeds 0.01%, there is a high possibility of inhibiting the ductility and weldability of the steel sheet.

  The soluble aluminum (Sol. Al) is preferably 0.01 to 0.4%.

  Soluble Al in steel is an effective component that combines with oxygen in steel to cause deoxidation, and distributes carbon in ferrite to austenite to improve martensite hardening ability. When the content is less than 0.01%, this effect cannot be ensured. On the other hand, when the content exceeds 0.4%, this effect is not only saturated but also the production cost may increase.

  Nitrogen (N) is preferably 0.020% or less.

  N in steel is an element that stabilizes austenite, and when it exceeds 0.020%, the safety of austenite is greatly increased, which may hinder the formation of bainite, which is a microstructure to be formed by the steel of the present invention. There is.

  Chromium (Cr) is preferably 0.3 to 1.5%.

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

  Boron (B) is preferably 0.0010 to 0.0060%.

  B in the steel is an element that delays the transformation of austenite to pearlite during the cooling process during annealing, and is added as an element that suppresses ferrite formation and promotes bainite formation. However, if the content of B is less than 0.0010%, it is difficult to obtain the above effect, and if it exceeds 0.0060%, excessive B may be concentrated on the surface, resulting in deterioration of plating adhesion. There is sex.

  Antimony (Sb) is preferably 0.001 to 0.1%.

Sb in steel is an essential element to be added in order to ensure excellent plating characteristics in the present invention. Sb is excellent in suppressing surface thickening of oxides such as MnO, SiO ₂ , Al ₂ O ₃ and reducing surface defects, and suppressing the increase in temperature and coarsening of the surface concentrate due to hot rolling process changes. There is an effect. When the Sb content is less than 0.001%, it is difficult to ensure the above effect, and even if the addition amount continues to increase, such an effect does not increase greatly, but the manufacturing cost and processability Therefore, it is preferable to limit the Sb content to 0.001 to 0.1%.

  In the present invention, the steel having the above composition is selected from Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%. One type or two or more types can be added to increase the strength and reduce the particle size.

  When the lower limit is less than 0.003%, the addition amount of Ti, Nb and Mo is difficult to ensure the effect of increasing the strength and reducing the grain size, and the upper limit is 0.08%. If it exceeds 1, ductility may be greatly reduced due to an increase in production cost and excessive precipitates.

  In the present invention, in addition to the above elements, the balance is composed of Fe and other inevitable impurities.

  According to the present invention, it is preferable to satisfy Formulas 1 and 2 of Si, Mn, B, Sb, P, and S when designing an alloy of a steel sheet having the above component range.

(Formula 1)
5 <(Si / Mn + 150B) / Sb <20

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

  Formula 1 is an empirical numerical value of the component relationship that can ensure the surface quality. That is, Mn, Si, and B in the steel are elements having a characteristic of forming a concentrate on the surface during the annealing operation, and the plating characteristics decrease as the concentration of these elements increases. On the other hand, Sb plays a role in hindering the grain boundary diffusion of the above-described surface-enriched elements, and thus is very advantageous from the aspect of surface quality. For example, when the value calculated by Equation 1 has a value between 5 and 20, it means that good surface quality can be ensured.

  On the other hand, Formula 2 is an empirical value obtained for the component relationship that can ensure weldability. That is, the elements C, Mn, Si, P, and S in the steel serve to increase the carbon equivalent, and as is well known, the higher the carbon equivalent, the worse the weldability. When spot welding, which is a welding method that is mainly performed when the steel of the present invention is used, conditions are set such that no welding failure occurs by repeating the experiment, a formula 2 is established. If the value calculated by Equation 2 exceeds 0.27, it means that the possibility of poor welding is increased.

  The steel sheet of the present invention preferably contains 40% or more of at least one selected from the group consisting of bainite and bainitic ferrite, and the remainder is preferably a ferrite and martensite phase. Ferrite and martensite are preferably 25% or less of ferrite and 35% or less of martensite.

  Hereinafter, a method for producing a steel plate composed as described above with a cold-rolled steel plate will be described in detail.

Hot rolling is performed after reheating the slab containing the components by the alloy design method described above. In the finish rolling by hot rolling, it is preferable to perform rolling so that the temperature on the outlet side is between Ar ₃ transformation point and 950 ° C. That is, when the hot finish rolling temperature is lower than the Ar ₃ transformation point, the hot deformation resistance is likely to be increased rapidly, which may cause manufacturing problems. If it exceeds 950 ° C., the oxide scale is too thick. Not only does this occur, the steel sheet is likely to be coarsened.

  The hot-rolled sheet manufactured by the above method is cold-rolled after pickling.

  The rolling reduction in cold rolling is preferably 40 to 80%. When the rolling reduction is less than 40%, the driving force of recrystallization is weakened, which may cause a problem in obtaining good recrystallized grains, and when the rolling reduction exceeds 80%, the rolling load suddenly increases. To increase.

  The cold-rolled sheet obtained above is continuously annealed, and the annealing temperature is preferably 740 ° C to 860 ° C. If the temperature during continuous annealing is less than 740 ° C., the risk of non-recrystallized grains increases, and if it exceeds 860 ° C., there is a possibility that the plateability becomes poor due to the formation of large grains and high-temperature annealing operation.

  After the continuous annealing, the cooling is continuously performed to a temperature of 250 to 600 ° C. at a cooling rate in which the value calculated by the following Equation 3 exceeds 30 within a cooling rate (CR) of 3 to 150 ° C./s. After that, it is gradually cooled at a cooling rate of 5 ° C./min or more. By continuous annealing under the above conditions, a high-strength thin steel sheet having good plating properties with a tensile strength of 800 MPa or more, weldability, and hole expansion rate can be easily produced.

(Formula 3)
-5 LogCR + 25C-17Si + 40Cr + 13,000B> 30
Where CR is the cooling rate

  When the cooling rate is lowered to less than 3 ° C./s after continuous annealing, ferrite or pearlite is formed, and it is difficult to ensure the target strength in the present invention. On the other hand, if it is too high at 150 ° C./s or more, a hard phase such as martensite is excessively formed, and not only the bending workability and the hole expansion rate are greatly deteriorated, but also the plateability is deteriorated due to the shape defect during operation. Therefore, it is preferable to cool in the above-mentioned cooling rate (CR) range of 3 to 150 ° C./s.

  Further, in order to achieve the excellent bending workability and hole expansion rate that are the characteristics of the steel of the present invention, the value calculated by Equation 3 must be applied at a cooling rate exceeding 30. That is, when the value calculated by Equation 3 is less than 30, it is difficult to obtain 40% or more of the bainite or bainitic ferrite phase to be obtained as a preferable microstructure in the steel of the present invention. When the bainite series structure is 40% or more, it is possible to produce a product having a high strength of about 800 MPa or more, which is a feature of the steel of the present invention, and excellent bending workability and hole expansion rate.

  On the other hand, it is preferable that the cooling end point temperature at the time of cooling is between 250 and 600 ° C. When the cooling end point temperature is less than 250 ° C, the risk of a large amount of martensite increases, and when it exceeds 600 ° C, a soft phase such as ferrite or pearlite is formed in large quantities, making it difficult to achieve the target material. This is because.

  The above manufacturing method can be applied not only to cold-rolled steel sheets but also to plated products such as hot-dip galvanized (GI) materials and alloyed hot-dip galvanized (GA) materials.

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

  As shown in Table 1 below, a slab having the component composition of the present invention is extracted by heating to a temperature of 1200 ° C, and then hot-rolled and manufactured under conditions of a finish rolling temperature of 900 ° C. It was used and rolled at a cold reduction of 55%. A continuous annealing heat treatment was performed at the annealing temperature and cooling conditions shown in Table 2 (CR), and in the case of a plated product, the product was manufactured by hot dip galvanizing (GI) and alloying hot dip galvanizing (GA). The conditions and alloying treatment time applied during continuous annealing are as follows.

- annealing furnace _{Atmosphere: N 2 -10% H 2 O} ( dew point -32 ° C.)
-Annealing furnace heating rate: 3 ° C / sec
-Annealing time: 90 sec
-Plating temperature: 460 ° C
-Alloying time: 24 sec (for GA products)

  As shown in Table 2, the plating characteristics (appearance and adhesion) and material (tensile strength, hole expansion rate, and bending workability) were measured, and the results were shown together with comparative steel.

  In Table 2, the appearance of plating is indicated by ◯ when unplated or does not contain other plating defects, and the name of the defect is specified when a plating defect occurs.

  The evaluation of plating adhesion in Table 2 is as follows. The plated plate was cut at 20 mm x 50 mm, subjected to a bending test with the plated plate, then stretched again, attached with a tape at the bent position, and the plating layer removed from the plated plate. The width was evaluated according to the following criteria.

◎: No removed plating or removed plating width within 1 mm ○: Removed plating width within 1 to 3 mm △: Removed plating width within 3 to 5 mm X: Removed plating width 5 mm or more

  In Table 2, the hole expansion rate (HER) is determined by drilling a hole having a diameter of 10 mm in a specimen having a size of 120 × 120 mm and then using a punch having a forming portion angle of 60 degrees until a crack is generated. And the ratio of the expanded holes is calculated as compared with the initial hole having a diameter of 10 mm. In Table 2, the bending workability was evaluated by measuring the radius (mm) of the smallest punch that does not break by performing a bending test with a test piece using a 90-degree V-shaped punch.

  As shown in Table 2 above, when producing a steel sheet by the method of the present invention, it has better surface properties and mechanical properties than existing comparative steels, and has a tensile strength of about 800 MPa or more, good plating properties, It is possible to manufacture a high-strength thin steel sheet having weldability, bending workability, and hole expansion rate.

  Invention steel contains 40% or more of one or more selected from the group consisting of bainite and bainitic ferrite, and the structure of the steel sheet is composed of 25% or less of ferrite and 35% or less of martensite.

Claims (6)

In mass %, C: 0.02 to 0.20%, Si: 1.5% or less, Mn: 1.5 to 3.0%, P: 0.001 to 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%, and Sb: 0.001-0. 10%, at least one selected from the group consisting of Ti: 0.003 to 0.08%, Nb: 0.003 to 0.08%, and Mo: 0.003 to 0.08% wherein, and consist of balance of Fe and other unavoidable impurities,
A high strength thin steel sheet excellent in weldability, wherein the Si, Mn, B, Sb, P and S satisfy 5 <(Si / Mn + 150B) / Sb <20 and C + Mn / 20 + Si / 30 + 2P + 4S <0.27.   2. The high strength excellent in weldability according to claim 1, wherein the structure of the steel sheet contains 40% or more of one or more selected from the group consisting of bainite and bainitic ferrite, and the remainder is a ferrite and martensite phase. Thin steel plate. The high-strength thin steel sheet excellent in weldability according to claim 1 or 2 , comprising a hot-dip galvanized (GI) layer or an alloyed hot-dip galvanized (GA) layer on the surface of the steel sheet. In mass %, C: 0.02 to 0.20%, Si: 1.5% or less, Mn: 1.5 to 3.0%, P: 0.001 to 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%, and Sb: 0.001-0. 10%, at least one selected from the group consisting of Ti: 0.003 to 0.08%, Nb: 0.003 to 0.08%, and Mo: 0.003 to 0.08% wherein, and consist of balance of Fe and other unavoidable impurities,
After reheating the slab of the steel plate 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, Rolling and winding so that the temperature on the outlet side is between Ar ₃ transformation point and 950 ° C.,
The picked hot-rolled sheet is pickled and then cold-rolled at a rolling reduction of 40 to 80%, and the obtained cold-rolled sheet is continuously annealed in a temperature range of 740 ° C to 860 ° C. After cooling to a temperature of 250 to 600 ° C. at a cooling rate satisfying the condition of −5 Log CR + 25C-17Si + 40Cr + 13,000B> 30 in a cooling rate (CR) range of −150 ° C./s, cooling at a cooling rate of 5 ° C./min or more. The manufacturing method of the high strength thin steel plate excellent in weldability including the step to do.   5. The high strength excellent in weldability according to claim 4, wherein the structure of the steel sheet contains 40% or more of at least one selected from the group consisting of bainite and bainitic ferrite, and the remainder is a ferrite and martensite phase. Manufacturing method of thin steel sheet. The method for producing a high-strength thin steel sheet excellent in weldability according to claim 4 or 5 , further comprising a step of hot dip galvanizing (GI) or alloying hot dip galvanizing (GA).