JP2009509046A - A method for producing a high-strength bake-hardening cold-rolled steel sheet, hot-dip plated steel sheet, and cold-rolled steel sheet having excellent aging resistance. - Google Patents

Abstract

The present invention relates to a cold-rolled steel sheet, a hot-dip plated steel sheet, and a method for producing the same, which are used for an outer sheet material of an automobile.
The present invention provides a high-strength bake-hardened cold-rolled steel sheet and a hot-dip steel sheet that are excellent in bake hardenability and room temperature aging resistance, as well as in secondary work brittleness resistance. There is a purpose.
The present invention is by weight%, C: 0.0016-0.0025%, Si: 0.02% or less, Mn: 0.2-1.2%, P: 0.05-0.11%, S: 0.01% or less, Soluble Al: 0.08-0.12%, N: 0.0025% or less, Ti: 0-0.003%, Nb: 0.003-0.011%, It contains Mo: 0.01-0.1% and B: 0.0005-0.0015%, and is excellent in bake hardenability and room temperature aging resistance composed of the remaining Fe and other inevitable impurities. A high-strength bake-hardening type cold-rolled steel sheet and hot-dip plated steel sheet, and a method for producing the same, which are excellent in secondary work brittleness, are described.
[Selection] Figure 3

Description

  The present invention relates to a cold-rolled steel plate and a hot-dipped steel plate used for automobile outer plate materials and the like, and more particularly to an excellent bake hardenability, room temperature aging resistance and secondary work brittleness resistance. The present invention relates to a cold-rolled steel sheet having the above, a hot-dip steel sheet using the same, and a method for producing a cold-rolled steel sheet.

  Recently, by using a high-strength steel plate for a vehicle body for the purpose of improving the fuel consumption of an automobile and reducing the weight of the vehicle body, there has been an increasing demand for reducing the thickness of the plate and improving the dent resistance.

  Properties required for cold rolled steel sheets for automobiles include yield strength, tensile strength, good press formability, spot weldability, fatigue properties, and corrosion resistance.

  Among these, corrosion resistance is a characteristic recently required for extending the life of automobile parts.

  Such corrosion resistance improving steel sheets can be roughly classified into two types, electroplating type and hot dipping type.

  Steel plates for electroplating have better plating characteristics and better corrosion resistance than hot-dip plating materials, but the price of steel plates is very high compared to hot-dip plating materials. Therefore, there is a tendency to demand improvement in corrosion resistance of the material for hot dipping.

  Recently, most automotive materials, mainly steelworks in various countries, are producing hot-dip plating materials and supplying them to automobile companies. This ensures that even hot-dip plating materials have much better corrosion resistance than the previous level. There is a tendency for the use to increase as technology that can be developed continues.

  In general, steel sheets usually exhibit characteristics in which strength and workability are contradictory. As steels that can satisfy these two characteristics, there are two types of steels: cold-rolled steel sheets with complex structure and bake-hardened cold-rolled steel sheets.

  The above-mentioned composite steel can be easily manufactured in general, and as a material used for automobiles with a tensile strength of 390 MPa or higher, it is a factor that exhibits stretchability compared to high tensile strength. Although some elongation is high, the average r value indicating the press formability of automobiles is low, and an excessive amount of expensive alloy elements such as manganese and chromium are added, resulting in an increase in manufacturing cost.

  On the other hand, bake-hardening cold-rolled steel is a steel with a tensile strength of 390 MPa or less and has a yield strength close to that of a soft steel plate during press forming. Therefore, it has excellent ductility. It is attracting attention as a very ideal steel compared to conventional cold-rolled steel sheets, whose formability deteriorates as the strength increases.

  Bake hardening uses a kind of deformation aging generated by fixing dislocations generated in the process of deformation of carbon and nitrogen, which are interstitial elements dissolved in steel. When the amount is increased, the bake hardening amount is increased. However, due to the excessive amount of the solid solution elements, aging is caused at normal temperature and the formability is deteriorated. Therefore, it is very important to appropriately control the solid solution elements.

  As a method for producing a cold-rolled steel sheet having bake hardenability, there are a method using a batch (box) annealing method and a method using a continuous annealing method.

  Generally, a low carbon P-added aluminum-killed (Al-Killed) steel is simply wound at a low temperature, that is, a batch (box) using a low-temperature winding at a hot rolling coiling temperature range of 400-500 ° C. Steel having a bake hardening amount of about 40-50 MPa by annealing was mainly used.

  This is because it is easier to achieve both formability and bake hardenability by the batch (box) annealing method.

  On the other hand, in the case of P-added Al-Killed steel by the continuous annealing method, since a relatively fast cooling rate is used, bake hardenability is easy to secure, but on the other hand, there is a problem that formability deteriorates due to rapid heating and short-time annealing. Its use is limited only to automotive skins where workability is not required.

  Recently, with the rapid development of steelmaking technology, it is possible to control the proper amount of solid solution elements in steel, and by using Al-Killed steel plate to which strong carbonitride forming elements such as Ti or Nb are added. Bake-hardening cold-rolled steel sheets with excellent formability are manufactured, and their use tends to increase for automotive outer plate materials that require dent resistance.

  Patent Document 1 relates to Ti, Ti, and Nb composite-added ultra-low carbon cold-rolled steel sheets having C: 0.0005-0.015% and S + N content ≦ 0.05%, and Patent Document 2 includes C: 0. A production method for producing a steel having a bake hardening amount of about 40 MPa or more by using 010% or less of Ti-added steel is proposed.

  The method presented in the above-mentioned patent document provides bake hardenability while controlling the amount of Ti and Nb added or the cooling rate during annealing to appropriately control the amount of solute elements in steel and prevent deterioration of the material. It is. However, in the case of Ti or Ti, Nb composite added steel, in order to secure an appropriate bake hardening amount, strict control of Ti, nitrogen, and sulfur is required in the steel making process, which causes a problem of cost increase.

  In the case of the Nb-added steel, the workability deteriorates due to high-temperature annealing and the manufacturing cost increases due to the addition of special elements.

  On the other hand, Patent Document 3 and Patent Document 4 [Bethlehem Steel] have C: 0.0005-0.1%, Mn: 0-2.5%, Al: 0-0.5%, N: Bake hardening type using Ti-V ultra-low carbon steel with 0-0.04% Ti content controlled to 0-0.5% and V content 0.005-0.6% A method for producing a cold rolled steel sheet is disclosed.

  In general, V can lower the annealing temperature more stably than carbonitride-forming elements such as Ti and Nb. Therefore, VC, which is a carbide generated by V during hot rolling, can give bake hardenability by remelting even if the annealing temperature is controlled lower than that of Nb.

  However, although V forms a carbide such as VC, the remelting temperature is very low and it is not very useful for improving the moldability. Therefore, in the above patent document, Ti is added in an amount of about 0.02% or more. Formability is achieved.

  Therefore, the above-mentioned patent document has a problem that it is not only disadvantageous in terms of aging resistance, but also due to the large crystal grain size as well as an increase in manufacturing cost due to the addition of a large amount of Ti.

  On the other hand, methods for adding new alloy elements are presented in Patent Document 5, Patent Document 6, Patent Document 7, Patent Document 8, and the like.

  In the above-mentioned patent document 5, a method of increasing the BH property by adding Sn is proposed, and in patent document 6, by adding V in combination with Nb, stress concentration at the grain boundary is reduced and ductility is improved. A way to improve is presented.

  Patent Document 7 proposes a method for improving formability with Zr. Patent Document 8 adds Cr to minimize strength and work hardening index (N value) deterioration by adding Cr. A method for ensuring moldability is presented.

  However, the above technology only focuses on improving the bake hardenability or moldability, and is inevitably caused by the problem of deterioration of aging resistance due to the increase of bake hardenability and the high strength of bake hardened steel. No mention is made of problems such as secondary work embrittlement due to an increase in the P content added to.

  Generally, when bake hardenability increases, the normal temperature aging resistance deteriorates. In particular, according to the research results of the present inventor, solid solution carbon in steel exists as the P content added for increasing strength increases. It has been found that the secondary work brittleness deteriorates even in the bake hardened steel, and the deterioration degree becomes more serious as the P content increases.

  For example, when the P content added to produce a bake-hardened steel having a tensile strength of 340 MPa class is 0.07%, the DBTT (Ductile Brittle Transition Temperature), which is a criterion for judging secondary work brittleness, is an elongation ratio ( When a P content of about 0.09% is added in order to produce a high strength steel of −20 ° C. and 390 MPa class at 1.9 (Drawing Ratio), it can be seen that DBTT is very deteriorated at 0 to 10 ° C.

  Such steel materials are all steel materials to which about 5 ppm of B is added. Generally, when B is added, it is known that the secondary work brittleness resistance is improved, but since the P content is excessively large, B It is judged that there was a limit to the DBTT improvement.

  On the other hand, in order to improve the secondary work brittleness resistance, if B is added excessively from the current level, the secondary work brittleness resistance is deteriorated, so the addition amount is limited.

  Accordingly, since DBTT for preventing secondary work brittleness must be at least −20 ° C. or higher, it is necessary to examine new components or production conditions other than B even in bake hardened steel.

Japanese Patent Publication No. Sho 61-026757 Japanese Patent Publication No. 57-089437 US Pat. No. 5,556,485 US Pat. No. 5,656,102 Japanese Published Patent Publication No. 5-93502 Japanese Published Patent Publication No. 9-249936 Japanese Published Patent Publication No. 8-49038 Japanese Published Patent Publication No. 7-278654

  It is an object of the present invention to provide a high-strength bake-hardening type cold-rolled steel sheet excellent in bake hardenability and room temperature aging resistance and excellent in secondary work brittleness resistance and a hot dipped galvanized steel sheet using the same. There is.

  Another object of the present invention is to provide a method for producing a high-strength bake-hardening type cold-rolled steel sheet that is excellent in bake hardenability and normal temperature aging resistance and has excellent secondary work brittleness resistance.

The present invention is by weight%, C: 0.0016-0.0025%, Si: 0.02% or less, Mn: 0.2-1.2%, P: 0.05-0.11%, S: 0.01% or less, Soluble Al: 0.08-0.12%, N: 0.0025% or less, Ti: 0-0.003%, Nb: 0.003-0.011%, Mo: 0.01-0.1% and B: 0.0005-0.0015%, consisting of the remaining Fe and other inevitable impurities,
The amount of solute carbon in steel (C ^* ) satisfies the following relational expression (1),
[Relational expression 1]
C ^* [Amount of solid solution carbon present in grain boundaries (referred to as GB-C) + Amount of solid solution carbon present in crystal grains (referred to as GC)] = Total C (ppm) −C in NbC = 8-15ppm
[In the above formula (1), the amount of GB-C (the amount of solid solution carbon in the crystal grain boundary) is 5 to 10 ppm, and the amount of GC (the amount of solid solution carbon in the crystal grain) is 3 to 7 ppm]
After annealing, the crystal grain size is 9 or more in ASTM Number (hereinafter referred to as No.),
The bake hardening amount (BH) value and the aging index (AI) value satisfy the conditions of the following relational expressions (2) and (3), respectively.
[Relationship 2]
Bake hardening amount (BH) = 50− (885 × Ti) − (1589 × Nb) + (62 × Al)
[Relationship 3]
Aging index (AI) = 44− (423 × Ti) − (2119 × Nb) − (125 × Mo)
High strength seizure with excellent aging resistance, characterized in that the bake hardening amount (BH) is 30 MPa or more, the aging index (AI) value is 30 MPa or less, and the DBTT has an elongation ratio of 2.0 and −30 ° C. The present invention relates to a hardened cold rolled steel sheet and a hot dipped steel sheet using the same.

  Further, the present invention is by weight%, C: 0.0016-0.0025%, Si: 0.02% or less, Mn: 0.2-1.2%, P: 0.05-0.11%, S: 0.01% or less, Soluble Al: 0.08-0.12%, N: 0.0025% or less, Ti: 0-0.003%, Nb: 0.003-0.011 %, Mo: 0.01-0.1% and B: 0.0005-0.0015%, the remaining Fe and other inevitable impurities made of aluminum-killed (Al-Killed) steel at 1200 ° C. or more After homogenization heat treatment, finish hot rolling in a temperature range of 900-950 ° C, winding in a temperature range of 580-630 ° C, cold rolling at a reduction rate of 75-80%, then 770-830 Continuous annealing in the temperature range of ℃, and reduction ratio of 1.2-1.5% It relates to a method for manufacturing a high-strength bake hardening cold-rolled steel sheet excellent in aging resistance, characterized in that temper rolling.

  According to the present invention, it is possible to provide a high-strength bake-hardening cold-rolled steel sheet and a hot-dip steel sheet that are excellent in bake hardenability and room temperature aging resistance and also excellent in secondary work brittleness resistance.

  Hereinafter, the present invention will be described in detail.

In general, when carbon or nitrogen is added to steel, TiN, AlN, TiC, Ti ₄ C ₂ S _2, NbC, and the like are combined with precipitate forming elements such as Al, Ti, or Nb in the hot rolling stage. Carbon and nitrogen that could not be combined with such carbonitride-forming elements (hereinafter referred to as solute carbon or solute nitrogen) appear to exist in a solid solution state in the steel. Affects the bake hardenability or aging resistance.

  Nitrogen in particular has a very high diffusion rate compared to carbon, so that the deterioration of aging resistance is very fatal compared to the increase in BH property. Therefore, in general, nitrogen is tried to be removed as much as possible in the steel. In particular, Al or Ti is preferentially precipitated with nitrogen over carbon at a high temperature, so the influence of nitrogen in the steel on the BH property and aging resistance is Even if it is judged that there is little, there is no big problem.

  However, carbon is an essential element in steel, and its content determines the properties of steel. In the bake hardened steel to be proposed in the present invention, such a role of carbon is very important, and a small amount of solute carbon remains in the steel, thereby achieving bake hardenability and aging resistance at the same time.

  However, the influence on the bake hardenability and the aging resistance differs depending on the position where the solid solution carbon exists in the steel, that is, whether it exists in the grain boundary or in the crystal grain.

  That is, solute carbon that can be measured through an internal friction test is mainly solute carbon present in crystal grains, and since it is relatively free to move, it combines with mobile dislocations and affects aging characteristics.

  An item for evaluating such aging characteristics is an aging index, that is, AI (Aging Index).

  In general, when the AI value is 30 MPa or more, aging occurs before maintaining for 6 months at room temperature, and serious defects may occur during press working.

  However, the solid solution carbon existing in the crystal grain boundary is difficult to detect by a vibration testing method such as internal friction because it exists in the crystal grain boundary which is a relatively stable region.

  The solute carbon existing in the grain boundary is present at a relatively stable position, so that it hardly affects aging at low temperatures such as the AI test, but is activated under high temperature baking conditions. It affects the bake hardenability.

  Therefore, solid solution carbon in the crystal grains simultaneously affects aging and bake hardenability, but solid solution carbon existing in the crystal grain boundary only affects bake hardenability.

  However, since the grain boundary is a relatively stable region, all the solid solution carbon existing in the grain boundary does not affect the bake hardenability, and the amount of solid solution carbon normally existing in the grain boundary. It is reported that about 50% of this affects bake hardenability.

  Therefore, when appropriately controlling the existence state of such solid solution carbon, that is, when controlling so that the added solid solution carbon can be present in the grain boundary as much as possible in the crystal grain, Aging resistance and bake hardenability can be secured at the same time.

  For this reason, it is important to control the size of the crystal grains together with appropriate management of the amount of carbon added to the steel. This is because when the amount of added carbon is very large or small, it is difficult to ensure appropriate bake hardenability and aging resistance even if the position of the solid solution carbon is controlled.

  FIG. 1 shows the relationship between the bake hardening amount (BH) value and the aging index (AI value) according to the change in the crystal grain size as a result of research conducted by the present inventors.

  As shown in FIG. As the crystal grain size increases, that is, as the crystal grains become finer, the AI value relative to the BH value decreases significantly, and as a result, the BH-AI value (the value obtained by subtracting the AI value from the BH value) gradually increases and the aging resistance is excellent I understand.

  Based on the results shown in FIG. 1, the present inventor tried to reduce the size of the annealed plate crystal grains to an appropriate level or less in order to distribute as much solute carbon present in the steel as possible within the grain boundaries. .

  As a result of the inventor's research, in order to maximize the aging resistance while minimizing the deterioration of the bake hardenability, the crystal grain size is set to ASTM No. It turned out that it is preferable to control to 9 or more.

  On the other hand, even if a large amount of solute carbon is distributed in the grain boundary, it is necessary to strictly control the total carbon amount in the steel. This is because when the carbon content in the steel increases excessively, the amount of solid solution carbon present in the crystal grains increases in proportion to the total amount of carbon added even if the grain size becomes finer. This is because normal temperature aging resistance deteriorates due to an increase in the carbon content.

  In the present invention, in order to satisfy such a condition, the total carbon content is set to 16-25 ppm.

  However, when Nb is added as in the present invention, carbides such as NbC are precipitated by adding Nb, and the amount of solute carbon in the steel can be reduced.

  Therefore, in the case of Nb-added steel, the Nb / C precipitation ratio is determined by the Nb and carbon content in the steel, and the solid solution carbon in the steel has NbC precipitates and the remaining carbon exists in solid solution form. Affects bake hardenability and aging resistance.

  Therefore, it is understood that what is more important than the control of the added Nb or carbon content is more important to what level of solute carbon in the steel is managed.

  On the other hand, in order to ensure the aging resistance as described above, it is important that solute carbon is present in the crystal grain boundary from within the crystal grain.

  As a result of investigating the influence of solute carbon in steel capable of achieving both aging resistance and bake hardenability under the above conditions, the present inventors have found that the crystal grains are very fine with ASTM 9 or more as in the present invention. In contrast, the results shown in FIG. 2 were obtained.

  That is, as a result of investigating the bake hardenability of the Nb-added ultra-low carbon steel having fine crystal grains as shown in FIG. 2 due to the change in solute carbon, the bake hardening amount set in consideration of aging resistance is 30 to 30%. It was found that the amount of solid solution carbon within the grain boundaries satisfying 50 MPa was about 3 to 7 ppm.

  Moreover, it turned out that the total (Total) solid solution carbon amount except the NbC deposit deposited considering the Nb and carbon content added by this invention is about 8-15 ppm.

  Through such a result, a condition that can achieve both bake hardenability and aging resistance, that is, a relational expression (1) was obtained.

[Relational expression 1]
C ^* [Amount of solid solution carbon present in grain boundaries (referred to as GB-C) + Amount of solid solution carbon present in crystal grains (referred to as GC)] = Total C (ppm) −C in NbC = 8-15ppm

[In the above formula (1), the amount of GB-C (the amount of solid solution carbon in the crystal grain boundary) is 5 to 10 ppm, and the amount of GC (the amount of solid solution carbon in the crystal grain) is 3-7 ppm]

  In the above formula (1), “C in NbC” indicates the amount of carbon deposited in the form of NbC precipitates.

  That is, as shown in the relational expression (1), the total (total) dissolved carbon amount is about 8 to 15 ppm, and in the total (total) dissolved carbon amount, the GB-C amount (the dissolved carbon amount in the grain boundary). ) Was 5 to 10 ppm, and by controlling the GC content (the amount of solid solution carbon in the crystal grains) to 3 to 7 ppm, the bake hardenability and aging resistance required in the present invention could be secured. .

  On the other hand, in the present invention, in addition to Nb addition, the effect of AlN precipitates through Al addition was considered in order to ensure more stable bake hardenability and aging resistance.

  In general, in Ti-added steel, nitrogen precipitates mostly coarsely as TiN at a high temperature of 1300 ° C. or higher, so that the solid solution effect or the refinement of crystal grains cannot be greatly affected.

  However, when Ti is added at a very small amount of 30 ppm or less as in the present invention, Sol. AlN precipitation occurs due to Al.

  Such an AlN precipitate has an effect of removing solid solution nitrogen in the steel.

  As a result of various experiments using the steel of the present invention, the carbon content is very narrowly limited to 16 to 25 ppm, so that a bake hardened steel having BH properties and aging resistance is produced within a narrow range. It becomes like this.

  In the case of a customer, the bake hardened steel requires a high BH value and a aging resistance of 6 months or more, and therefore a technique for increasing the bake hardenability as much as possible without impairing the aging resistance is necessary.

  In this aspect, Al is very effective.

  That is, Sol. When Al is added in the range of 0.02 to 0.06%, which is a normal level, it simply serves to fix solute nitrogen, but when added in an amount of 0.08% or more, AlN precipitates are added. Becomes very fine and acts as a kind of barrier that hinders the growth of crystal grains during annealing recrystallization. The crystal grains become finer than Nb-added steel not containing Al, thereby exhibiting the effect of increasing the bake hardenability without changing the AI value. The following relational expression (2) is obtained from Sol. Sol. Affects the improvement of bake hardenability within the range of Al. The effect of Al addition is shown by a statistical method, and the range of components for showing the effect of such Al is Sol. Al was 0.08 to 0.12%.

[Relationship 2]
Bake hardening amount (BH) = 50− (885 × Ti) − (1589 × Nb) + (62 × Al)

  However, the carbon content, Sol. Even if the Al and Nb contents are controlled, the role of hot rolling coiling temperature is very important in Nb-added ultra-low carbon steel.

  That is, even if the BH property is improved and the aging resistance is improved by the refinement effect of the crystal grains using Nb as in the present invention, the crystal grains become coarse at the hot rolling stage if the coiling temperature is greatly increased. Therefore, the grain size is ASTM No. And the AI value exceeds 30 MPa, which is the upper limit required in the present invention.

  On the other hand, in the secondary work brittleness side, the molding of parts generally performed in an automobile company can obtain a desired shape by a plurality of repetitive press processes. That is, the secondary processing brittleness means that a processing crack is generated in the processing performed after the primary press processing.

  In such a crack, phosphorus (P) existing in the steel exists in the grain boundary and weakens the bonding force of the crystal grain, so that the fracture occurs around the grain boundary.

  In order to remove the secondary processing brittleness, it is preferable that basically no phosphorus (P) element is added. Usually, however, a solid solution element whose decrease in elongation is small compared to an increase in strength is phosphorus. There is an advantage that the cost is low.

  Therefore, in order to increase the strength of steel materials, it must basically be added. However, recently, in order to remove such secondary work brittleness, other materials can be used instead of phosphorus in order to remove such secondary processing brittleness. Research is also underway to increase the strengthening effect through solid solution elements.

  However, considering the results of research to date, it seems that phosphorus will continue to be used as a steel strengthening element for the time being.

  In such P-added steel, a solid solution element remains in the steel as in the case of bake-hardened steel in a method for improving the secondary work brittleness resistance, or B or the like is added to improve the position competition with phosphorus (site competition). studies to prevent secondary work brittleness by increasing the bonding force of the effect) or grain boundaries, or by lowering the coiling temperature below a certain temperature in the hot rolling stage to minimize phosphorus grain boundary diffusion. However, the reality is that this is not a complete solution.

  Therefore, in the present invention, Mo is considered in order to improve the secondary work brittleness resistance more stably. According to the research result of the present inventor, Mo is very advantageous in improving secondary work embrittlement resistance because it improves the bonding force of grain boundaries.

  In addition, Mo has an affinity for solute carbon in steel, so it is advantageous in aging resistance because it suppresses diffusion of solute carbon into dislocations when maintained at room temperature for a long time.

The following relational expression (3) shows such an effect of improving the aging resistance of Mo by a statistical method.
[Relationship 3]
Aging index (AI) = 44− (423 × Ti) − (2119 × Nb) − (125 × Mo)

  The present inventor has derived an optimum component range in order to appropriately utilize such characteristics of Mo and prevent deterioration of the material due to excessive addition of Mo.

  On the other hand, in order to further improve the secondary work brittleness resistance, the improvement of the secondary work brittleness is maximized by simultaneously applying the various methods that have been applied so far, namely, the appropriate addition of B and the optimization of the coiling temperature. Tried.

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

  Carbon (C) is an element representing solid solution strengthening and bake hardenability.

  When the carbon content is less than 0.0016%, the tensile strength is low due to the very low carbon content, and the absolute carbon content present in the steel is low even if the effect of refining the crystal grains by adding Nb is achieved. Curability cannot be obtained.

  In addition, the position competition effect between the solute carbon and P is lost, and the secondary work brittleness resistance side is extremely deteriorated.

  On the other hand, if it exceeds 0.0025%, the amount of solid solution carbon in the grain exceeds 3-7 ppm proposed in the present invention, and the bake hardenability becomes very high, but the room temperature aging resistance is not ensured, and the press Formability and ductility are reduced because stretcher strain occurs during molding.

  Silicon (Si) is an element that increases the strength. The strength increases as the amount added increases, but the ductility deteriorates significantly. Especially, it is an element that deteriorates the hot dipping property. Is advantageous.

  In the present invention, the addition amount is limited to 0.02% or less in order to prevent material deterioration including plating characteristic deterioration due to Si.

  Manganese (Mn) is an element that refines the particles without damaging ductility, completely precipitates sulfur in steel with MnS, prevents hot brittleness due to the formation of FeS, and strengthens the steel. In the present invention, when the Mn content is less than 0.2%, an appropriate tensile strength cannot be ensured, and when added over 1.2%, the strength increases rapidly due to solid solution strengthening. At the same time, the formability deteriorates, especially during the manufacturing process of hot-dip galvanized steel sheets, a large amount of oxides such as MnO are formed on the surface, which deteriorates the adhesion of the plating, and a lot of plating defects such as stripes occur. Since the quality is deteriorated, the amount added is preferably limited to 0.2 to 1.2%.

  Phosphorus (P) is a substitutional alloy element having the largest solid solution strengthening effect, and plays a role in improving the in-plane anisotropy and improving the strength.

  In addition, the result P of the inventor's research plays a role of promoting the development of a (111) texture that is advantageous for improving the average r value in the subsequent annealing stage by refining the crystal grains of the hot rolled sheet. In particular, it was confirmed that the bake hardenability tended to increase as the phosphorus content increased due to the position competition effect with carbon in the influence side of bake hardenability. However, when the phosphorus content is increased, there is a problem that the secondary work brittleness resistance deteriorates due to the weakening of the bonding strength of the grain boundaries.

  However, when the phosphorus content is less than 0.05%, the phosphorus content present at the grain boundaries is small, so the secondary work embrittlement resistance is improved, but the material improvement effect due to the grain refinement effect is weak. When the content exceeds 0.11%, the strength increases sharply compared to the improvement of moldability, and secondary processing brittleness causes P to segregate at grain boundaries and embrittle the material due to excessive addition of P amount. The risk of occurrence increases. Therefore, the P content is limited to 0.05-0.11%.

  Sulfur (S) is an element that must be precipitated with sulfides such as MnS at high temperatures to prevent hot brittleness due to FeS. However, when the content of S is excessive, the S remaining after precipitation with MnS may embrittle the grain boundary and cause hot embrittlement.

  Further, when the amount of S added is sufficient to completely precipitate MnS precipitates, if the S content is large, material deterioration due to excessive precipitates occurs, so it is preferable to limit the amount added to 0.01% or less. .

  Aluminum (Al) is usually added for deoxidation of steel, but in the present invention, the effect of refinement of crystal grains by AlN precipitation and the effect of improving bake hardenability are exhibited.

  That is, in the present invention, the grain refinement effect mainly uses NbC precipitates due to the addition of Nb. However, by further refinement of crystal grains by AlN precipitates, BH properties can be obtained without deterioration of aging resistance. It plays a role to improve.

  According to the relational expression (2), the more Al is added, the more advantageous the BH property.

  However, when other materials are taken into account, it is necessary to control the appropriate addition amount.

  That is, in order to show the effect of the present invention, Al must be added at least 0.08% or more.

  However, if the Al content exceeds 0.12%, the surface quality deteriorates due to the increase in oxidation inclusions during steelmaking along with the deterioration of formability, and the production cost increases due to excessive Al addition. The addition amount is preferably limited to 0.08-0.12%.

  Nitrogen (N) deteriorates the formability of steel by existing in a solid solution state before or after annealing, and aging deterioration is much larger than other interstitial elements, so it is necessary to fix it with Ti or Al. is there.

  When Nb is appropriately added together with a small amount of Ti as in the present invention, excessive addition of nitrogen leads to generation of solute nitrogen in the steel.

  Nitrogen generally has a much faster diffusion rate than carbon, and therefore when it is present in solid solution nitrogen, the deterioration in aging resistance at room temperature is very serious compared to solid solution carbon.

  In addition, since the yield strength is increased by the remaining solute nitrogen and the elongation and the r-value are deteriorated, it is necessary to limit the content to 0.0025% or less as in the present invention.

Ti is a carbonitride-forming element and forms nitrides such as TiN, sulfides such as TiS or Ti ₄ C ₂ S ₂ and carbides such as TiC in the steel.

  However, in the present invention, Ti is 0.003% or less and is added only to a level that fixes a small amount of nitrogen.

  The reason for presenting a minute amount of Ti content in the present invention is that the amount of Ti contained in various components added in order to satisfy the material properties in the operation of steelmaking during actual production, This is because Ti present in the previous steel output material can be included in the steel output material of the present invention when the steel output is performed several times at the same time.

  However, when Nb is controlled by a main element for improving the aging resistance as in the present invention, Ti addition is not necessary, and BH property is reduced when Ti is added, but the actual production conditions are considered. In the present invention, the Ti content is limited to a very small level of 0.003% or less.

  Nb is treated as a very important element together with Al and Mo presented as main components in the present invention.

  In general, Nb is a strong carbonitride-forming element, which fixes carbon present in steel with NbC precipitates and plays a role in controlling the amount of solute carbon in steel. In particular, the produced NbC precipitate is very fine compared to other precipitates in steel, and serves as a strong barrier to prevent grain growth during recrystallization annealing.

  That is, the effect of refinement of crystal grains by Nb in the present invention utilizes the effect of such NbC precipitates. However, the present invention is a technique for achieving bake hardenability with such solute carbon by leaving the solute carbon in steel.

  For this reason, it is necessary to appropriately control the amount of NbC precipitates in the steel and to leave the solid solution carbon within a range that minimizes the deterioration of the material.

  Therefore, in the present invention, in order to achieve the effect of refinement of crystal grains by NbC precipitates and to keep the solute carbon in the intragranular steel of about 3-7 ppm and to ensure bake hardenability and aging resistance at the same time, Nb The Nb content is preferably limited to 0.003-0.011% in consideration of the carbon content of 16-25 ppm proposed in the present invention.

  Mo is one of the very important elements considered in the present invention.

  Mo is dissolved in the steel and improves the strength or plays the role of forming a Mo-based carbide.

  In particular, the most important role of Mo, when present in a solid solution state, is to increase the bond strength of the grain boundary to prevent the grain boundary from being broken by phosphorus, that is, to improve the secondary work brittleness resistance, It is to improve the aging resistance by suppressing the diffusion of carbon by affinity with solute carbon. The relational expression (3) shows the aging resistance effect of Mo by a quantitative method. For this purpose, an appropriate range of Mo addition is necessary.

  That is, if Mo is less than 0.01%, the above effect cannot be obtained.

  Therefore, considering the production cost and the effect of contrasting the addition amount, it is preferable to limit the Mo content to a range of 0.01 to 0.1%.

  B is an interstitial element that is present in the steel and is dissolved in the grain boundary, or is combined with nitrogen to form a nitride such as BN.

  B is an element that has a very large influence on the material compared to the amount added, and it is necessary to strictly limit the amount added. That is, when a small amount of B is added to the steel, it segregates at the grain boundaries and improves the secondary work brittleness resistance.

  However, when it is added in a certain amount or more, material deterioration occurs that causes an increase in strength and a significant decrease in ductility, so an appropriate range of addition is necessary.

  In the present invention, the content of B is set to 0.0005 to 0.0015% in consideration of such characteristics and the ability of steel making for the current addition of B.

Hereinafter, the manufacturing method of the steel of this invention is demonstrated.
A steel slab (Slab) composed as described above is reheated at 1200 ° C. or higher at which the austenite structure before hot rolling can be sufficiently homogenized, and a temperature of 900-950 ° C. just above the Ar ₃ temperature. Finish hot rolling in range.

  When the slab temperature is less than 1200 ° C., the steel structure cannot be uniform austenite crystal grains, and mixed grains are generated, resulting in deterioration of the material.

  When the hot rolling finish temperature is less than 900 ° C., the upper (top) and lower (tail) portions and edges of the hot rolled coil become single phase regions, and the in-plane anisotropy increases and the formability deteriorates. Further, when the temperature exceeds 950 ° C., extremely coarse particles are generated, and defects such as orange peel are likely to occur on the surface after processing.

  In the production method of the present invention, it is important to appropriately control the winding temperature.

  When the coiling temperature is less than 580 ° C. in the present invention, the size of the crystal grains is refined and the secondary work embrittlement resistance is improved together with the aging resistance, but the degree of refinement of the crystal grains is too serious. This leads to an excessive increase in yield strength and deterioration of formability.

  On the other hand, when the coiling temperature is too high, the total content of solute carbon in the steel and the content of solute carbon present in the crystal grains and at the grain boundaries do not satisfy the above relational expression (1). The temperature is preferably controlled to 630 ° C. or lower.

  Thus, in the present invention, as an example of means for ensuring that the total content of solute carbon in the steel, the content of solute carbon present in the crystal grains and at the grain boundaries satisfies the above relational expression (1). The coiling temperature is controlled at 580 to 630 ° C.

  The steel that has been hot-rolled is pickled by a normal method and then cold-rolled at a cold rolling rate of 75-80%.

  The reason why the cold rolling rate is as high as 75% or more is to improve the formability, particularly the r value, together with the improvement of the aging resistance due to the crystal grain refinement effect required in the present invention.

  On the other hand, when the cold rolling rate exceeds 80%, the effect of refining the crystal grains is great, but due to the excessive rolling rate, the fineness of the size of the crystal grains becomes very large, resulting in the hardening of the material. The r value gradually decreases due to an excessive increase in the cold rolling rate.

  The steel that has been cold-rolled is subjected to continuous annealing in the normal temperature range of 770-830 ° C.

  Since Nb-added steel has a high recrystallization temperature compared to Ti-added steel, an operation at an annealing temperature of 770 ° C. or higher is required. That is, when the annealing temperature is less than 770 ° C., the yield strength increases due to the presence of unrecrystallized crystal grains, and the elongation and r value deteriorate.

  When the annealing temperature exceeds 830 ° C., the formability is improved, but ASTM No. which is the crystal grain size required in the present invention is the crystal grain size. Since it is smaller than 9, when the AI value is 30 MPa or less, the aging resistance deteriorates.

  For the purpose of ensuring normal bake hardenability and aging resistance at room temperature using the bake hardening type cold rolled steel plate produced by the above production method, it is slightly higher than the normal temper rolling ratio, 1.2 to 1.5. % Temper rolling.

  The reason why the temper rolling ratio is set to be slightly higher than 1.2% is to prevent normal temperature aging deterioration due to solute carbon in steel.

  However, when the temper rolling ratio is excessively increased to exceed 1.5%, even if the normal temperature aging resistance is improved, the temper rolling ratio is high, work hardening occurs, and the material deteriorates. In order to solve such problems, when producing hot dipped steel sheets using bake-hardening type cold rolled steel sheets, the plating adhesion deteriorates due to excessive temper rolling and peeling of the plating layer occurs. It is preferable to perform temper rolling at a temper rolling rate of 1.2 to 1.5%, which is an appropriate condition.

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

(Example)
The steel having the composition shown in Table 1 below was hot finish-rolled at a finish rolling temperature condition of 900-910 ° C. and then wound at a winding temperature of 610-630 ° C. After cold rolling at a cold rolling rate, continuously annealed at an annealing temperature of 800-820 ° C., hot-dip plated at a temperature of 450-470 ° C., and then alloyed in a temperature range of 500-530 ° C. Conditioned and rolled at a temper reduction ratio of 1.5%, measured DBTT at an elongation ratio of 2.0, which is an item for evaluating the BH value, AI value, crystal grain size, and secondary work brittleness of the final steel sheet, The results are shown in Table 2 below. Further, the amount of intragranular solid solution carbon was measured for the invention steel (6) shown in Table 1 below, and the results are shown in FIG.

  The amount of solute carbon in FIG. 3 is a value measured using an internal friction tester (Horizontal type, 10 KHz).

  As shown in Table 2, the intragranular solute carbon content of the invention steel (1-6) is 3.1-6.6 ppm, which is the range of the intragranular solute carbon content proposed by the present invention, 3-7 ppm. It can be seen that the above range is satisfied.

  As shown in FIG. 3, in the inventive steel, no solute nitrogen in the steel appears, and it can be seen that the amount of solute carbon is 3.1 ppm or more.

  This is because the added nitrogen precipitates as AlN precipitates due to Al added in a large amount and contributes to refinement of the crystal grains. Therefore, the solid solution carbon forms the NbC precipitate and remains of the solid solution carbon. This is because a part was present in the grains and measured.

  Such intragranular solid solution carbon is thought to have affected the bake hardenability.

  On the other hand, as shown in Table 2 above, the crystal grain size of the inventive steel (1-6) is ASTM No. The ASTM No. proposed by the present invention at 9.8-11.5 (average grain size 6.7-12.0 μm). It can be seen that all the conditions of 9 or more were satisfied.

  The crystal grains of the inventive steel (1-6) are fine as shown in Table 2 above, because fine AlN precipitates are formed in the steel by addition of Al content higher than the normal level, and during annealing recrystallization with NbC precipitates, This is because the growth of crystal grains was hindered.

  Therefore, the AI value which is an index indicating that the bake hardening amount is in the range of 38.1 to 47.9 MPa and shows normal temperature aging resistance by the refinement effect of the crystal grains and appropriate control of the solute carbon in the steel. Is 9.3-28.3 MPa, and it can be seen that the balance between BH property and normal temperature aging resistance is very excellent.

  On the other hand, the low AI value compared to the high bake hardening amount in the invention steel (1-6) is that the effect of delaying the solid solution carbon in the steel by the addition of Mo acted together with the effect of grain refinement by the AlN precipitates. it is conceivable that.

  Moreover, DBTT in elongation ratio 2.0 was -40 degreeC--60 degreeC also in the secondary process brittle side.

  On the other hand, in the case of the comparative steel (7), 0.0054% higher in carbon content than 0.0016-0.0025% presented in the present invention is added, and the hot rolling coiling temperature and the annealing temperature are presented in the present invention. It satisfies the condition, and the recrystallized grain size is ASTM No. 11.7 is very fine and satisfies the crystal grain size condition investigated in the present invention, and the DBTT characteristics and BH value are excellent, but the added carbon content is so high that the AI value is 30 MPa or more. It can be seen that the aging resistance is very deteriorated.

  The comparative steel (8), unlike the comparative steel (7), has a very low added carbon content of 0.0011%, and there is no solid solution carbon left after forming NbC precipitates. It can be seen that the crystallinity becomes coarse due to the decrease in the absolute carbon content, and the DBTT characteristics deteriorate.

  Comparative steel (9) is Sol. Steel with an Al content of 0.023%, lower than 0.08-0.12%, which is the condition proposed in the present invention, and 0.035% higher in Nb content than the condition proposed in the present invention. .

  Therefore, there is no crystal grain refinement effect and BH value increase effect due to AlN precipitates, and all the carbon added in the steel is precipitated by NbC due to the addition of a high Nb content and hardly shows bake hardenability. It can be seen that the effect of positional competition with phosphorus (P) is reduced due to the decrease in medium-solution carbon, and the DBTT characteristics are also deteriorated.

  Compared with the conditions presented in the present invention, the comparative steel (10) contains no Mo and cannot be expected to improve the secondary work brittleness resistance due to Mo.

  Further, in terms of manufacturing conditions, the coiling temperature was 720 ° C., which was higher than the temperature presented in the present invention, and the possibility that the movement of phosphorus (P) was activated was very high.

  Therefore, it can be seen that the comparative steel (10) is excellent not only in the absence of Mo but also in BH and AI properties due to an increase in the coiling temperature, but the DBTT property is very deteriorated.

  The comparative steel (11) had a Ti content of 0.035%, was added very high, had no solute carbon in the steel, and had a BH value and an AI value of 0.

  Further, B was not added at all, and secondary processing embrittlement due to phosphorus added by 0.071% could not be prevented.

  This is because even if Mo is added so as to conform to the component range of the present invention, there is no solid solution carbon advantageous for improving secondary work brittleness resistance, and an increase in the bonding strength of the grain boundaries due to B can be expected. Therefore, it is determined that the DBTT characteristic has deteriorated.

  The comparative steel (12) had a very high nitrogen content, although the other components sufficiently fulfilled the component range of the present invention.

  Nitrogen, unlike carbon, is an element that causes fatal problems not only in bake hardenability but also in terms of aging resistance.

  Even in the case of the comparative steel (12), since the amount of dissolved nitrogen in the grains is very high at 11.3 ppm due to excessive nitrogen addition, not only the BH value but also the AI value is very high, which is very fatal to aging resistance. I understand that.

It is a graph showing the influence of the size of a crystal grain on bake hardenability and an aging index. It is a graph showing the influence of the solute carbon in steel on bake hardenability. It is a graph showing the test result of the internal friction of invention steel (6).

Claims (3)

By weight%, C: 0.0016-0.0025%, Si: 0.02% or less, Mn: 0.2-1.2%, P: 0.05-0.11%, S: 0.01 %, Soluble Al: 0.08-0.12%, N: 0.0025% or less, Ti: 0-0.003%, Nb: 0.003-0.011%, Mo: 0 .01-0.1% and B: 0.0005-0.0015%, consisting of the remaining Fe and other inevitable impurities,
The amount of solute carbon in steel (C ^* ) satisfies the following relational expression (1),
[Relational expression 1]
C ^* [Amount of solid solution carbon present in crystal grain boundaries (referred to as GB-C) + Amount of solid solution carbon present in crystal grains (referred to as GC)] = Total C (ppm) −C in NbC = 8-15ppm
[In the above formula (1), the GB-C amount (the amount of solid solution carbon within the crystal grain boundary) is 5 to 10 ppm, and the GC amount (the amount of solid solution carbon within the crystal grain) is 3 to 7 ppm]
After annealing, the grain size is ASTM No. 9 or more,
The bake hardening amount (BH) value and the aging index (AI) value satisfy the conditions of the following relational expressions (2) and (3), respectively.
[Relationship 2]
Bake hardening amount (BH) = 50− (885 × Ti) − (1589 × Nb) + (62 × Al)
[Relationship 3]
Aging index (AI) = 44− (423 × Ti) − (2119 × Nb) − (125 × Mo)
High strength seizure with excellent aging resistance, characterized in that the bake hardening amount (BH) is 30 MPa or more, the aging index (AI) value is 30 MPa or less, and the DBTT has an elongation ratio of 2.0 and −30 ° C. Hardened cold rolled steel sheet. By weight%, C: 0.0016-0.0025%, Si: 0.02% or less, Mn: 0.2-1.2%, P: 0.05-0.11%, S: 0.01 %, Soluble Al: 0.08-0.12%, N: 0.0025% or less, Ti: 0-0.003%, Nb: 0.003-0.011%, Mo: 0 .01-0.1% and B: 0.0005-0.0015%, consisting of the remaining Fe and other inevitable impurities,
The amount of solute carbon in steel (C ^* ) satisfies the following relational expression (1),
[Relational expression 1]
C ^* [Amount of solid solution carbon present in grain boundaries (referred to as GB-C) + Amount of solid solution carbon present in crystal grains (referred to as GC)] = Total C (ppm) −C in NbC = 8-15ppm
[In the above formula (1), the GB-C amount (the amount of solid solution carbon within the crystal grain boundary) is 5 to 10 ppm, and the GC amount (the amount of solid solution carbon within the crystal grain) is 3 to 7 ppm]
After annealing, the grain size is ASTM No. 9 or more,
The bake hardening amount (BH) value and the aging index (AI) value satisfy the conditions of the following relational expressions (2) and (3), respectively.
[Relationship 2]
Bake hardening amount (BH) = 50− (885 × Ti) − (1589 × Nb) + (62 × Al)
[Relationship 3]
Aging index (AI) = 44− (423 × Ti) − (2119 × Nb) − (125 × Mo)
High strength seizure with excellent aging resistance, characterized in that the bake hardening amount (BH) is 30 MPa or more, the aging index (AI) value is 30 MPa or less, and the DBTT has an elongation ratio of 2.0 and −30 ° C. Hardened hot-dip galvanized steel sheet.   By weight%, C: 0.0016-0.0025%, Si: 0.02% or less, Mn: 0.2-1.2%, P: 0.05-0.11%, S: 0.01 %, Soluble Al: 0.08-0.12%, N: 0.0025% or less, Ti: 0-0.003%, Nb: 0.003-0.011%, Mo: 0 After homogenization heat treatment at 1200 ° C. or higher for aluminum-killed (Al-Killed) steel containing 0.01-0.1% and B: 0.0005-0.0015% and comprising the remaining Fe and other inevitable impurities , After hot rolling in a temperature range of 900-950 ° C., after winding in a temperature range of 580-630 ° C., cold rolling at a reduction rate of 75-80%, and then in a temperature range of 770-830 ° C. And temper rolling at a rolling reduction of 1.2-1.5% Method of producing a high strength bake hardening cold-rolled steel sheet excellent in aging resistance, characterized in that.

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