CN108463570B - Ultrahigh-strength steel sheet having excellent chemical conversion treatability and hole expansibility, and method for producing same - Google Patents

Ultrahigh-strength steel sheet having excellent chemical conversion treatability and hole expansibility, and method for producing same Download PDF

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CN108463570B
CN108463570B CN201680073487.3A CN201680073487A CN108463570B CN 108463570 B CN108463570 B CN 108463570B CN 201680073487 A CN201680073487 A CN 201680073487A CN 108463570 B CN108463570 B CN 108463570B
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steel sheet
hole expansibility
chemical conversion
ultra
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CN108463570A (en
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徐石宗
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Posco Holdings Inc
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Posco Co Ltd
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
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    • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
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Abstract

The present invention relates to an ultra-high strength steel sheet used as an automobile steel sheet and the like, and provides an ultra-high strength steel sheet excellent in chemical conversion treatability and hole expansibility, the steel sheet comprising, in wt%: carbon (C): 0.08 to 0.2%, silicon (Si): 0.05 to 1.3%, manganese (Mn): 2.0 to 3.0%, phosphorus (P): 0.001 to 0.10%, sulfur (S): 0.010% or less, aluminum (Al): 0.01 to 0.1%, chromium (Cr): 0.3 to 1.2%, boron (B): 0.0010 to 0.0030%, titanium (Ti): 0.01 to 0.05%, nitrogen (N): 0.001 to 0.01%, and the balance Fe and other unavoidable impurities, wherein Ti and N satisfy [ relational expression 1] 3.4. ltoreq. Ti/N.ltoreq.10, Mn, Si and Cr satisfy [ relational expression 2] 1.0. ltoreq. Mn/(Si + Cr), and the contents of Mn, Si and Cr in the surface layer (from the surface to 0.1 μm in the thickness direction) satisfy [ relational expression 3] 0.7. ltoreq. Mn/(Si + Cr).

Description

Ultrahigh-strength steel sheet having excellent chemical conversion treatability and hole expansibility, and method for producing same
Technical Field
The present invention relates to an ultra-high strength steel sheet used as an automobile steel sheet and the like, and more particularly, to an ultra-high strength steel sheet excellent in chemical conversion treatability and hole expansibility, and a method for manufacturing the same.
Background
In recent years, as for steel sheets for automobiles, use of ultra-high strength steel materials has been expanding for the purpose of regulation of fuel efficiency for global environment conservation and securing collision stability for passengers. In order to manufacture such ultra-high strength steel, it is difficult to secure sufficient strength and ductility only by conventional steel materials using solid solution strengthening or steel materials using precipitation strengthening.
Therefore, Phase Transformation reinforced steels have been developed which utilize a Phase Transformation structure to improve strength and ductility, and examples of such Phase Transformation reinforced steels include Dual Phase steels (hereinafter referred to as DP steels), Complex Phase steels (hereinafter referred to as CP steels), Transformation Induced Plasticity steels (hereinafter referred to as TRIP steels), and the like.
Among the transformation-strengthened steels, DP steel is a steel type in which hard martensite is finely and homogeneously dispersed in a soft ferrite to ensure high strength and ductility, and CP steel is a steel type including two or three phases among ferrite, martensite, and bainite and including precipitation hardening elements such as Ti and Nb for strength improvement. TRIP steel is a steel grade that secures strength and ductility by processing finely and homogeneously dispersed retained austenite at normal temperature to cause martensitic transformation.
Patent document 1, which relates to a method for manufacturing a steel sheet having excellent formability by controlling the amount of retained austenite of the steel sheet, is a typical technique of the TRIP steel. Patent document 2 discloses a method for producing a high-strength steel sheet having excellent press formability by controlling the alloy composition and the microstructure of the steel sheet, and patent document 3 discloses a steel sheet containing 5% or more of retained austenite and having excellent workability, particularly local ductility.
In view of the fact that the steel sheet for automobile is being accelerated to increase the strength, it is actually difficult to ensure sufficient elongation even if it is desired to ensure elongation by using various transformation structures as described above.
In particular, the proportion of ultra-high strength steel used as a collision-resistant member is increasing with regulations for enhancing collision characteristics for ensuring passenger safety, but even if the yield strength is high, there is a problem that breakage is likely to occur by impact at the time of collision or energy cannot be smoothly absorbed.
Therefore, in order to smoothly absorb energy without breaking the ultrahigh-strength steel and to suppress cracks generated in the flange portion during complicated processing such as bending and roll forming, it is necessary to improve hole expandability.
A representative steel material having excellent hole expandability is a hot-rolled high-burring steel material, and a great deal of effort is put on minimizing the difference in hardness between phases in order to improve the hole expandability. Conventionally, as for the basic structure of high-flange steel, there have been disclosed a scheme in which nanosized fine precipitates are applied to a ferrite single phase, a scheme in which precipitates are applied to a bainite single phase or a composite structure of ferrite and bainite, and the like, and efforts for similar morphology have been made also for cold-rolled steel sheets
In addition, in order to simultaneously secure the ultra-high strength and the hole expansibility, it is necessary to develop CP steel using both the transformed structure and the precipitates. However, CP steel has a disadvantage of being more varied than ductility and hole expansibility depending on the composition of the phase (phase), and sufficient studies on an appropriate phase fraction and manufacturing range have not been completed so far, and thus the necessity of research and development is increasing.
In addition, although it is necessary to add a large amount of solid-solution strengthening elements, i.e., alloying elements such as Si and Cr, for the purpose of stably securing a hard phase and reducing the difference in hardness between phases, there is a problem that steel containing Si or Cr forms surface oxides during annealing and is difficult to remove the oxides during pickling after annealing, thereby deteriorating chemical conversion treatability of a final product.
Therefore, it is required to develop a technology capable of ensuring chemical conversion treatability while ensuring ultra-high strength and hole expansibility.
(patent document 1) Japanese laid-open patent publication No. 1994-145892
(patent document 2) Japanese patent laid-open publication No. 2704350
(patent document 3) Japanese patent laid-open publication No. 3317303
Disclosure of Invention
Technical problem to be solved
An object of one aspect of the present invention is to provide an ultra-high strength steel sheet excellent in chemical conversion treatability while securing excellent hole expansibility by optimizing the composition of alloy components and manufacturing conditions, and a method for manufacturing the same.
Technical scheme
One aspect of the present invention provides an ultra-high strength steel sheet excellent in chemical conversion treatability and hole expansibility, comprising in weight%: carbon (C): 0.08 to 0.2%, silicon (Si): 0.05 to 1.3%, manganese (Mn): 2.0 to 3.0%, phosphorus (P): 0.001 to 0.10%, sulfur (S): 0.010% or less, aluminum (Al): 0.01 to 0.1%, chromium (Cr): 0.3 to 1.2%, boron (B): 0.0010 to 0.0030%, titanium (Ti): 0.01 to 0.05%, nitrogen (N): 0.001 to 0.01%, and the balance Fe and other unavoidable impurities,
the Ti and N satisfy the following relational expression 1, the Mn, Si, and Cr satisfy the following relational expression 2, the contents of Mn, Si, and Cr in a surface layer (from the surface to 0.1 μm in the thickness direction) satisfy the following relational expression 3, and the steel sheet has a yield ratio of 0.8 or more.
[ relational expression 1]
3.4≤Ti/N≤10
[ relational expression 2]
1.0≤Mn/(Si+Cr)
[ relational expression 3]
0.7≤Mn*/(Si*+Cr*)≤Mn/(Si+Cr)
Another aspect of the present invention provides a method for manufacturing an ultra-high strength steel sheet having excellent chemical conversion treatability and hole expansibility, the method comprising the steps of: preparing a steel material satisfying the composition and composition relationship; hot rolling and cold rolling the steel to manufacture a cold-rolled steel sheet; annealing the cold-rolled steel plate at 800-850 ℃; rapidly cooling the cold-rolled steel sheet subjected to the annealing heat treatment to a range from Ms (martensite transformation start temperature) to Bs (bainite transformation start temperature), and then maintaining the cooled steel sheet; after the maintaining, cooling at a speed of 10-50 ℃/min; and removing a surface oxide of the cold-rolled steel sheet after the cooling;
the maintaining step is performed during a period of time satisfying the following relational expression 5, and the removing step is performed under a condition satisfying the following relational expression 6.
[ relational expression 5]
300≤4729+71C+25Mn-16Si+117Cr-20.1T+0.0199T2≤500
[ relational expression 6]
(HCl concentration. times. HCl temperature)/(1.33 + Mn +7.4Si +0.8Cr) × (47+2.1Mn +13.9Si +4.3Cr) ≥ 1
Advantageous effects
According to the present invention, an ultra-high-strength steel sheet can be provided which has an ultra-high strength of 1GPa or more in tensile strength and a yield ratio of 0.8 or more, and which has excellent hole expansibility so that flange cracks are not generated at the time of forming, and which has not only excellent energy absorption ability at the time of collision but also excellent chemical conversion treatability.
Best mode for carrying out the invention
The present inventors have conducted extensive studies to obtain an ultra-high strength steel sheet having high tensile strength and excellent hole expansibility and chemical conversion treatability, and have confirmed that a steel sheet having desired physical properties can be provided when a structure fraction suitable for the purpose is secured by optimization of alloy composition and manufacturing conditions, and have completed the present invention.
The present invention will be described in detail below.
Preferably, the alloy composition of the ultra-high strength steel sheet excellent in chemical conversion treatability and hole expansibility according to one aspect of the present invention includes: carbon (C): 0.08 to 0.2%, silicon (Si): 0.05 to 1.3%, manganese (Mn): 2.0 to 3.0%, phosphorus (P): 0.001 to 0.10%, sulfur (S): 0.010% or less, aluminum (Al): 0.01 to 0.1%, chromium (Cr): 0.3 to 1.2%, boron (B): 0.0010 to 0.0030%, titanium (Ti): 0.01 to 0.05%, nitrogen (N): 0.001 to 0.01%, and the composition ratio of the Ti, N, and the Mn, Si, and Cr is controlled as appropriate.
First, the reason why the alloy composition and the composition relationship of the ultrahigh-strength steel sheet provided in the present invention are controlled will be described in detail below. At this time, the content of each component represents weight% unless otherwise specifically stated.
C：0.08～0.2％
Carbon (C) is an important element for ensuring strength in the phase transformation structure steel.
Therefore, it is preferable to contain 0.08% or more of C, and when the content of C is less than 0.08%, the tensile strength of 1GPa or more cannot be ensured. On the other hand, when the content of C exceeds 0.2%, hole expansibility and formability are deteriorated, so that there is a problem that not only press formability and roll formability are deteriorated but also spot weldability is deteriorated.
Therefore, in the present invention, the content of C is preferably limited to 0.08 to 0.2%.
Si：0.05～1.3％
Silicon (Si) is an element that improves both the strength and elongation of the steel material, and has the effect of suppressing carbide formation during austempering.
In order to sufficiently obtain the above-mentioned effects, it is preferable to add 0.05% or more of Si, but when the content of Si exceeds 1.3%, a large amount of oxide is generated in the annealing heat treatment step, and it is difficult to remove the oxide in the pickling step, thereby significantly reducing the chemical conversion treatability and causing a problem of causing defects. Further, there is a problem that the annealing temperature, which needs to be raised to ensure an appropriate two-phase fraction, becomes high, which causes a load on the annealing furnace.
Therefore, in the present invention, the content of Si is preferably limited to 0.05 to 1.3%.
Mn：2.0～3.0％
Manganese (Mn) is an element having a very large solid solution strengthening effect.
When the Mn content is less than 2.0%, it is difficult to secure the strength desired in the present invention, while when the Mn content exceeds 3.0%, problems such as deterioration of weldability and increase of cold rolling load are likely to occur. Further, there is a problem that a large amount of annealed oxide is formed, and chemical conversion treatability is deteriorated.
Therefore, in the present invention, the Mn content is preferably limited to 2.0 to 3.0%.
P：0.001～0.10％
Phosphorus (P) is an element having an effect of reinforcing steel.
When the content of P is less than 0.001%, the above-mentioned effects cannot be secured, and when P is controlled to be extremely small in the steel-making process, the production cost is increased, which is not preferable. On the other hand, if the content of P is too large and exceeds 0.10%, grain boundary segregation deteriorates impact characteristics, and causes brittleness of steel, which is not preferable.
Therefore, in the present invention, the content of P is preferably limited to 0.001 to 0.10%.
S: 0.010% or less
Sulfur (S) is an impurity element in steel, and hinders ductility, hole expansibility, and weldability of steel, so it is preferable to minimize the content of sulfur.
When the content of S exceeds 0.010%, MnS is formed to remarkably reduce hole expandability and the possibility of hindering ductility and weldability of the steel sheet becomes high, so it is preferable to limit the content of S to 0.010% or less.
Al：0.01～0.1％
Aluminum (Al) is an effective element that bonds with oxygen in steel during a steel-making process to perform a deoxidation effect and promotes carbon distribution in austenite during phase transformation together with Si.
For this reason, it is preferable to add Al of 0.01% or more, but if the Al content exceeds 0.1%, there is a problem that the surface quality of the billet is lowered and the manufacturing cost is increased.
Therefore, in the present invention, the content of Al is preferably limited to 0.01 to 0.1%.
Cr：0.3～1.2％
Chromium (Cr) is a component added to improve hardenability of steel and to ensure high strength, and in the present invention, chromium is an effective element for retarding transformation of ferrite and inducing formation of bainite.
When the content of Cr is less than 0.3%, it is difficult to secure the above-described effects, and on the other hand, when the content of Cr exceeds 1.2%, not only the above-described effects are saturated, but also the strength of the hot rolled material is excessively increased, so that a load at the time of cold rolling is increased, and the manufacturing cost is greatly increased. Further, there is a problem that the formation of an annealed oxide during annealing heat treatment makes it difficult to control the pickling process, and the chemical conversion treatability is seriously deteriorated.
Therefore, in the present invention, the content of Cr is preferably limited to 0.3 to 1.2%.
B：0.0010～0.0030％
Boron (B) is an effective element that inhibits transformation of austenite to ferrite and increases the fraction of bainite during cooling in annealing.
When the content of B is less than 0.0010%, it is difficult to secure the above-described effects, and on the other hand, when the content of B exceeds 0.0030%, the above-described effects are not only saturated due to grain boundary segregation of B, but also concentrated on the surface during annealing heat treatment, thereby causing a problem of deterioration of chemical conversion treatability.
Therefore, in the present invention, the content of B is preferably limited to 0.0010 to 0.0030%.
Ti：0.01～0.05％
Titanium (Ti) is an element added for the purpose of improving strength and removing nitrogen (N) present in steel.
When the content of Ti is less than 0.01%, it is difficult to secure the above effect, while when the content of Ti exceeds 0.05%, not only the effect is saturated but also a process defect such as clogging of a nozzle may be caused in the continuous casting process.
Therefore, in the present invention, the content of Ti is preferably limited to 0.01 to 0.05%.
N：0.001～0.01％
Nitrogen (N) is a typical invasive solid solution strengthening element, similar to C. Since N is an element that is normally mixed from the atmosphere, it is necessary to control N in a degassing step of a steel-making step.
When the content of N is less than 0.001%, excessive degassing treatment is required, which causes a problem of increase in production cost, while when the content of N exceeds 0.01%, precipitates such as AlN and TiN are excessively formed, thereby causing a problem of reduction in high-temperature ductility.
Therefore, in the present invention, the content of N is preferably limited to 0.001 to 0.01%.
In addition, Ti and N in the alloying elements of the steel sheet of the present invention preferably satisfy the compositional relationship represented by the following relational expression 1.
[ relational expression 1]
3.4≤Ti/N≤10
(in the above-mentioned relational expression 1, Ti and N represent the weight contents of the respective elements.)
When the ratio of Ti/N is less than 3.4, the addition amount of Ti is insufficient compared to the amount of dissolved N, so that NB or the like is formed by the remaining N, and the strength increasing effect by the addition of B is reduced, thereby resulting in a reduction in strength. On the other hand, when the ratio of Ti/N exceeds 10, the denitrification cost increases, and the possibility of causing nozzle clogging or the like in the continuous casting step becomes high.
At the same time, it is preferable that Mn, Si, and Cr in the alloying elements satisfy a composition relationship represented by the following relational expression 2, and that the contents of Mn, Si, and Cr in the steel surface layer (from the surface to 0.1 μm in the thickness direction) satisfy the following relational expression 3.
[ relational expression 2]
1.0≤Mn/(Si+Cr)
[ relational expression 3]
0.7≤Mn*/(Si*+Cr*)≤Mn/(Si+Cr)
(in the above-mentioned relational expressions 2 and 3, Mn, Si and Cr represent the weight contents of the respective elements, and in the above-mentioned relational expression 3, Mn, Si and Cr represent the average values of the analyzed values of the GDS components from the surface to 0.1 μm in the thickness direction, respectively.)
In order to ensure the chemical conversion treatability of the ultra-high strength steel sheet of the present invention, it is necessary to control the relational expressions 2 and 3, and when the value of the relational expression 2 is less than 1, a very dense Si and Cr oxide layer is formed during the annealing heat treatment, so that even if the final pickling process is strengthened, there is a problem in that it is difficult to remove the oxide layer. Further, when the oxide layer is removed under the condition that the concentration of the acid and the temperature are excessively high, relatively fragile grain boundaries are preferentially eroded, thereby there is a problem that the workability and fatigue characteristics of the steel are remarkably reduced.
Further, when the value of the relational expression 3 indicating the component concentration on the surface of the steel sheet after the final pickling process is less than 0.7, there is a problem that formation of phosphate crystals is inhibited as Si oxide, Cr oxide, or a concentrated layer having poor chemical conversion treatability remains on the surface layer, grain boundary, or the like of the steel sheet. On the other hand, when the value of relation 3 exceeds the Mn/(Si + Cr) ratio, the Mn-based oxide formed in the extremely surface layer is oxidized at the time of initial temperature rise in the annealing heat treatment step, and then is partially reduced under the reducing atmosphere in the annealing furnace, or is selectively coarsened at the initial time, and thus cannot be completely removed in the final pickling step, and therefore, there is a problem that variation occurs in forming phosphate crystals, and the chemical conversion treatability is deteriorated.
In addition to the above alloy components, the ultra-high strength steel sheet of the present invention may contain one or more of Nb, Mo, V, and W in the following amounts.
Nb：0.01～0.05％
Niobium (Nb) is a typical precipitation strengthening element, and is an element added to improve the strength of steel and to refine crystal grains.
When the content of Nb is less than 0.01%, it is difficult to sufficiently secure the above effects, and on the other hand, when the content of Nb exceeds 0.05%, not only the manufacturing cost is excessively increased, but also excessive precipitates are formed, so there is a possibility that ductility is significantly reduced.
Therefore, in the present invention, when Nb is added, the content of Nb is preferably limited to 0.01 to 0.05%.
Mo, V and W: respectively accounts for 0.01 to 0.20 percent
Molybdenum (Mo), vanadium (V), and tungsten (W) are elements that function similarly to Nb, and when the contents of molybdenum, vanadium, and tungsten are less than 0.01% respectively, it is difficult to sufficiently ensure the effect of improving the strength of steel and refining crystal grains, while when the contents of molybdenum, vanadium, and tungsten exceed 0.20% respectively, the manufacturing cost may be excessively increased compared to the strength effect.
Therefore, in the present invention, when Mo, V and W are added, the content is preferably limited to 0.01 to 0.20% respectively.
Further, the Nb, Mo, V, and W preferably satisfy the following relational expression 4.
[ relational expression 4]
0.01≤Nb+0.2(Mo+V+W)≤0.05
(in the above-mentioned relational expression 4, Nb, Mo, V and W represent the weight contents of the respective elements.)
When the relationship among Nb, Mo, V, and W is less than 0.01, it is difficult to obtain the grain refinement and precipitation strengthening effect, while when the relationship among Nb, Mo, V, and W exceeds 0.05, the production cost may be excessively increased compared to the above effect, which is not preferable.
The remaining component of the present invention is iron (Fe). However, since impurities which are not required are inevitably mixed from the raw materials or the surrounding environment in a general steel manufacturing process, the impurities cannot be excluded. These impurities are well known to those skilled in the art of conventional steel making processes and therefore not all of them will be specifically referred to in this specification.
The ultrahigh-strength steel sheet proposed in the present invention preferably contains, in terms of area fraction, 50 to 80% of martensite or tempered (tempered) martensite, 10 to 30% of bainite, less than 5% of retained austenite, and the balance ferrite as a fine structure. This is to maximize the fraction of tempered martensite and bainite, to reduce the ferrite fraction that causes a large difference in hardness between phases (phases), and to suppress voids (void) formed at the time of punching.
More preferably, the resin composition contains 50 to 80% of a martensite phase containing tempered martensite, and when the fraction of the martensite phase containing tempered martensite is less than 50%, there is a problem that it is difficult to ensure an ultrahigh strength with a desired tensile strength of 1GPa or more and to ensure hole expansibility. On the other hand, if the fraction of the martensite phase including tempered martensite exceeds 80%, there is a problem that the strength excessively increases to deteriorate the hole expandability.
Further, when the fraction of bainite is less than 10%, the hole expansibility is drastically reduced due to an excessive increase in strength, and it is difficult to secure a desired value (tensile strength (MPa) × Hole Expansibility (HER)) of 40000 or more, while when the fraction of bainite exceeds 30%, it is difficult to secure ultra high strength.
An important structure for ensuring the desired hole expandability in the present invention is tempered martensite formed at a high temperature, and the tempered martensite can be obtained in a large amount by slowly cooling only in a region lower than the martensite transformation start temperature during cooling.
Further, when the fraction of retained austenite in the microstructure exceeds 5%, there is a problem that delayed fracture resistance is deteriorated, and therefore, it is preferable to limit the fraction of retained austenite to 5% or less.
The fraction of ferrite is not particularly limited, but in order to ensure hole expansibility, ferrite is preferably included at a fraction of 20% or less.
The ultrahigh-strength steel sheet of the present invention having the above-described fine structure has a tensile strength of 1GPa or more and ensures a value of (tensile strength (MPa) × Hole Expansibility (HER)) of 40000 or more, thereby ensuring hole expansibility to such an extent that flange cracking does not occur and molding can be performed at the time of press molding or roll molding.
When the value of (tensile strength (MPa) × Hole Expansibility (HER)) is less than 40000, there is a problem that it is difficult to secure an ultrahigh strength although hole expansibility is excellent, or that hole expansibility is deteriorated although an ultrahigh strength can be secured, and flange cracks are generated at the time of molding or energy absorption capability at the time of collision is deteriorated.
In addition, the yield ratio of the ultrahigh-strength steel sheet of the present invention is 0.8 or more, and when the yield ratio is decreased to less than 0.8, the fraction of ferrite is generally increased, which is not only undesirable because of the decrease in strength but also the deterioration in hole expansibility.
As described above, the ultra-high strength steel sheet of the present invention having excellent tensile strength of 1000MPa or more and hole expansibility has an advantage that it can be press-formed and roll-formed and has excellent impact characteristics.
Such an ultra-high strength steel sheet of the present invention may be a cold-rolled steel sheet or a hot-dip galvanized steel sheet.
Next, a method for producing an ultra-high strength steel sheet excellent in chemical conversion treatability and hole expansibility according to another aspect of the present invention will be described in detail.
Preferably, first, a steel material satisfying the above alloy composition and composition relationship is prepared, and then hot rolled and cold rolled to manufacture a cold rolled steel sheet.
In this case, the cold rolling and the hot rolling may be performed by a conventional method for manufacturing an ultra high strength steel sheet, and may be, for example, the hot rolling and the cold rolling conditions in the method for manufacturing the CP steel. However, the present invention is not limited thereto.
Preferably, the cold-rolled steel sheet manufactured as described above is subjected to a step of annealing heat treatment at 800 to 850 ℃.
When the temperature during the annealing heat treatment is less than 800 ℃, the fraction of the ferrite structure exceeds 20%, and it is difficult to secure desired ultra-high strength and hole expansibility, while when the temperature during the annealing heat treatment exceeds 850 ℃, although hole expansibility is improved, the amount of surface oxides or concentrates of Si, Mn, Cr, B, and the like generated during high temperature annealing is greatly increased, and even if an acid pickling process is performed thereafter, the surface oxides or concentrates remain on the surface, and chemical conversion treatability is deteriorated.
The annealing heat treatment is performed in an annealing furnace under a reducing atmosphere consisting of hydrogen and nitrogen, and in this case, the atmosphere in the furnace is preferably controlled so that the dew point temperature is-35 to-50 ℃. When the dew point temperature exceeds-35 ℃, elements having a large oxygen affinity, such as Mn, Si, Cr, and B, contained in the steel are easily formed as surface oxides or concentrates, and thus, even if the pickling process is performed thereafter, there is a possibility that they cannot be easily controlled and remain. In addition, it may be attached to the roller in the furnace and then grown, thereby having a possibility of causing a dent defect. On the other hand, when the dew point temperature is lower than-50 ℃, the production cost is greatly increased, which is not preferable.
It is preferable that the cold-rolled steel sheet subjected to the annealing heat treatment as described above is rapidly cooled to a certain cooling-finish temperature range and then maintained.
At this time, the cooling end temperature range is preferably defined as Ms (martensite start temperature) to Bs (bainite start temperature), and is preferably maintained within the temperature range for a certain time. In performing the maintenance, it is preferable to perform during a period of time represented by the following relational expression 5.
[ relational expression 5]
300≤4729+71C+25Mn-16Si+117Cr-20.1T+0.0199T2≤500
(in the above-mentioned relational expression 5, Mn, Si, Cr and C represent the weight contents of the respective elements, and in the above-mentioned relational expression 5, T represents the rapid cooling termination temperature (. degree. C.) and the unit of the value thus obtained is sec.)
The relational expression 5 specifies the transformation time of bainite (in seconds (sec)) based on the alloy composition and the rapid cooling temperature, and when the value of the relational expression is 300 seconds or less, there is a possibility that the fraction of bainite formed exceeds 30%. In this case, the fraction of martensite or tempered martensite is relatively decreased, and there is a problem that it is difficult to simultaneously secure the ultrahigh strength having the tensile strength of 1GPa or more and the hole expansibility. On the other hand, if the value of the relational expression exceeds 500 seconds, the fraction of martensite including tempered martensite exceeds 80%, and the strength excessively increases, thereby causing a problem of deterioration in hole expansibility.
The rapid cooling is preferably performed at a rate of 100 to 600 ℃/min, and when the rate of rapid cooling is less than 100 ℃/min, the fraction of ferrite and pearlite in the microstructure increases, so that the ultrahigh strength and hole expansibility desired in the present invention cannot be ensured. On the other hand, when the rate of rapid cooling exceeds 600 ℃/min, the hard phase may excessively increase, so that there is a possibility that ductility may decrease, and a problem of poor shape or the like may be caused.
Preferably, after the rapid cooling and maintenance as described above, cooling is preferably performed at a slower cooling rate (slow cooling). In this case, the cooling rate is preferably 10 to 50 ℃/min, and when the cooling rate is less than 10 ℃/min, a proper fraction of martensite cannot be secured, and thus it is difficult to secure a desired ultra-high strength, while when the cooling rate exceeds 50 ℃/min, the fraction of primary (fresh) martensite increases more than tempered martensite, and strength is excessively increased, and thus the hole expansibility is deteriorated.
Preferably, in the present invention, the steel sheet after the slow cooling as described above is subjected to a process for removing the annealed oxide formed on the surface layer, i.e., a post-pickling process.
The post-pickling step is performed by pickling in a heated hydrochloric acid solution tank, and then washing with water and drying.
More specifically, in the post-pickling step of the present invention, it is important to control the concentration of the acid and the temperature of the acid, and specifically, the concentration of the hydrochloric acid may be adjusted according to the alloy composition of the steel, but is preferably controlled to be 5 to 20%. When the concentration of hydrochloric acid is less than 5%, there is a possibility that oxides cannot be completely removed, and on the other hand, when the concentration of hydrochloric acid exceeds 20%, the erosion by acid is fast and selective erosion to grain boundaries is rapidly generated, so that there is a problem that the workability and fatigue characteristics of the material are deteriorated. The temperature of the hydrochloric acid also needs to be adjusted according to the alloy composition of the steel, but is preferably controlled to be 50-80 ℃. When the temperature of hydrochloric acid is lower than 50 ℃, the reactivity is low and there is a problem that it is difficult to remove the oxide, while when the temperature of hydrochloric acid exceeds 80 ℃, the reactivity becomes too fast and the possibility of occurrence of selective etching becomes high.
When the post-pickling step is performed under the conditions described above, the concentration of hydrochloric acid and the temperature of hydrochloric acid preferably satisfy relational expression 6 expressed as a relationship with the alloy composition as described below.
[ relational expression 6]
(HCl concentration. times. HCl temperature)/(1.33 + Mn +7.4Si +0.8Cr) × (47+2.1Mn +13.9Si +4.3Cr) ≥ 1
(in the above relational expression 6, Mn, Si, Cr and C represent the weight contents of the respective elements.)
When the value of the relational expression 6 is less than 1, pickling property may be reduced, so that there is a problem that the annealed oxide cannot be completely removed at a specified hydrochloric acid concentration and hydrochloric acid temperature. That is, only when the value of the relational expression 6 is 1 or more, the surface oxide of the steel sheet can be easily removed.
Further, the post-pickling step is preferably performed within 5 to 15 seconds, and when the pickling time is less than 5 seconds, the pickling cannot be completely performed, while when the pickling time exceeds 15 seconds, there is a problem that productivity is lowered.
In the ultrahigh-strength steel sheet of the present invention in which the step of removing the surface oxide is completed as described above, the contents of Mn, Si, and Cr in the surface layer (up to 0.1 μm in the thickness direction from the surface) satisfy relational expression 3 as described above, and excellent chemical conversion treatability can be ensured.
Detailed Description
The present invention will be described more specifically with reference to examples. However, it should be noted that the following examples are only for illustrating the present invention to describe the present invention in more detail, and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the contents recited in the claims and reasonably derived therefrom.
(examples)
Cold-rolled steel sheets were produced by hot-rolling and cold-rolling billets having alloy compositions shown in table 1 below under normal conditions, and then were annealed and cooled under conditions shown in table 2 below, followed by a post-pickling process, thereby producing each cold-rolled steel sheet.
Then, the phosphate treatability (chemical conversion treatability) and the surface GDS analysis results of each of the produced cold rolled steel sheets are shown in table 3 below.
In this case, the GDS analysis measures the concentration of the element in the depth of 0.1 μm from the surface to the thickness direction of the cold-rolled steel sheet to be manufactured, and calculates the average value.
In addition, the chemical conversion treatability was evaluated using a degreasing agent CHEMKLEEN #177 from Pittsburgh Plate Glass (PPG), a washing liquid conditioner (rintse conditioner) as a surface conditioner, and a phosphate solution prepared from four kinds of Chemfos #740A, Chemfos #740R, chemfil Buffer (Buffer) and Accelerator (Accelerator). For the qualification of the chemical conversion treatability, the surface was magnified 1000 times by a Scanning Electron Microscope (SEM), and the generation of non-attached sites and the formation of non-attached sites were measuredThe amount of phosphate deposited is determined by whether or not 2 to 3g/m is satisfied2To decide.
Further, the microstructure fraction and the mechanical physical properties (yield strength, tensile strength, elongation, and Hole Expanding Ratio (HER)) of each cold-rolled steel sheet were measured, and the results thereof are shown in table 4 below.
At this time, for the fine structure, an SEM photograph at 3000 times was taken, and then the area fraction of each phase was measured by an image analyzer (image analyzer). Further, the fraction of retained austenite is measured by peak intensity (peak intensity) of austenite by XRD.
The tensile test was evaluated at a rate of 10 mm/min using JIS No. 5 test pieces.
The Hole Expansibility (HER) is based on the JFST 1001 and 1996 specifications.
The results of the tensile test and the HER test described above are shown as the average values after three tests.
[ Table 1]
Figure GDA0001696188250000151
Figure GDA0001696188250000161
Figure GDA0001696188250000171
(in said Table 1, the contents of B and N are expressed in the unit of "ppm".
In table 1, comparative steels 3, 4, 5, 9 and 10 are comparative steels, which are shown in table 1 because the production conditions of table 2 below do not satisfy the present invention. )
[ Table 2]
Figure GDA0001696188250000172
Figure GDA0001696188250000181
Figure GDA0001696188250000191
(in Table 2, the rapid cooling temperatures of inventive steels 1 to 11 and comparative steels 1 to 10 all satisfied the range of Ms to Bs.)
[ Table 3]
Figure GDA0001696188250000192
(in Table 3, Mn, Si, and Cr represent the average values of the GDS component analysis values from the surface to 0.1. mu.m.)
[ Table 4]
Figure GDA0001696188250000193
Figure GDA0001696188250000201
(in Table 4, B represents bainite, M represents martensite, F represents ferrite, and γ represents retained austenite.
In addition, YS represents yield strength, TS represents tensile strength, El represents elongation, and HER represents hole expansibility, and the physical property relational expression represents a value (tensile strength (MPa) × Hole Expansibility (HER)). )
As shown in tables 1 to 4, in the case of invention examples 1 to 11, which all satisfy the alloy composition and the production conditions proposed in the present invention, the chemical conversion treatability satisfies the standard, so that it can be confirmed that the chemical conversion treatability is excellent.
Further, as bainite and martensite are formed at an appropriate fraction, the tensile strength is 1GPa or more, and the value of (tensile strength (MPa) × Hole Expansibility (HER)) is 40000 or more while satisfying a yield ratio of 0.8 or more.
The above results show that the inventive steel of the present invention has not only excellent chemical conversion treatability but also excellent hole expansibility.
On the other hand, in comparative examples 1 to 10, it was confirmed that any one or more of the alloy composition and the production conditions deviate from the present invention, and that the desired physical properties of the present invention were not satisfied in any of them.
In comparative example 1 and comparative example 2, the contents of C and Mn in the alloy composition do not satisfy the present invention, and the desired tensile strength of 1GPa or more is not secured, so that the value of (tensile strength (MPa) × Hole Expansibility (HER)) is less than 40000. Further, the yield ratio is also less than 0.8 without satisfying the present invention.
In comparative examples 3 and 4, since the step of maintaining after rapid cooling (relational expression 5) in the production conditions does not satisfy the present invention, an excessive bainite phase is formed and a desired fraction of martensite cannot be secured, and thus the value of (tensile strength (MPa) × Hole Expansibility (HER)) is less than 40000.
In comparative example 5, since the value of the relational expression 5 exceeds 500, which is a step of maintaining after rapid cooling in the production conditions, the martensite fraction exceeds 80% to deteriorate the hole expansibility, and thus the value of (tensile strength (MPa) × Hole Expansibility (HER)) is less than 40000.
In comparative examples 6 and 7, since excessive amounts of Si and Cr were added, respectively, relational expression 2 showing the relationship between the above components did not satisfy the present invention, and the chemical conversion treatability was deteriorated.
In comparative example 8, since excessive amounts of Si and Cr were added, relational expression 2 did not satisfy the present invention, and since the post-pickling step (relational expression 6) did not satisfy the present invention, the chemical conversion treatability was deteriorated.
The alloy compositions of comparative examples 9 and 10 satisfied the present invention, but did not satisfy the post-pickling step (relational expression 6), and therefore oxides remained on the surface of the pickled steel, which failed to satisfy relational expression 3 of the present invention, and the final chemical conversion treatability was deteriorated.
As can be seen from the above, the ultrahigh-strength steel sheet having the ultrahigh strength, the excellent hole expansibility and the chemical conversion treatability, which are desired in the present invention, can be manufactured only when the alloy composition and the manufacturing conditions proposed in the present invention are all satisfied, particularly, when the alloy composition and the manufacturing conditions are all satisfied with the relational expressions 1 to 6.

Claims (10)

1. An ultra-high strength steel sheet excellent in chemical conversion treatability and hole expansibility, comprising in weight%: carbon (C): 0.08 to 0.2%, silicon (Si): 0.05 to 1.3%, manganese (Mn): 2.0 to 3.0%, phosphorus (P): 0.001 to 0.10%, sulfur (S): 0.010% or less, aluminum (Al): 0.01 to 0.1%, chromium (Cr): 0.3 to 1.2%, boron (B): 0.0010 to 0.0030%, titanium (Ti): 0.01 to 0.05%, nitrogen (N): 0.001 to 0.01%, and the balance Fe and other unavoidable impurities, wherein the Ti and N satisfy the following relational expression 1, the Mn, Si and Cr satisfy the following relational expression 2, the contents of Mn, Si and Cr in a surface layer which is a region ranging from the surface to 0.1 [ mu ] m in the thickness direction satisfy the following relational expression 3, and the steel sheet has a yield ratio of 0.8 or more,
[ relational expression 1]
3.4≤Ti/N≤10
[ relational expression 2]
1.0≤Mn/(Si+Cr)
[ relational expression 3]
0.7≤Mn*/(Si*+Cr*)≤Mn/(Si+Cr)，
In the above-mentioned relational expressions 1 to 3, Ti, N, Mn, Si, and Cr represent the weight contents of the respective elements, and in the above-mentioned relational expression 3, Mn, Si, and Cr represent the average values of the GDS component analysis values from the surface to 0.1 μm in the thickness direction, respectively.
2. The ultra-high strength steel sheet excellent in chemical conversion treatability and hole expansibility according to claim 1, further comprising niobium (Nb): 0.01 to 0.05%, molybdenum (Mo): 0.01 to 0.20%, vanadium (V): 0.01 to 0.20% and tungsten (W): 0.01 to 0.20% of at least one of them, and satisfies the following relational expression 4,
[ relational expression 4]
0.01≤Nb+0.2(Mo+V+W)≤0.05，
In the above relational expression 4, Nb, Mo, V and W represent the weight contents of the respective elements.
3. An ultra-high strength steel sheet excellent in chemical conversion treatability and hole expansibility according to claim 1, wherein a microstructure of the steel sheet comprises, in terms of area fraction, 50 to 80% of martensite or tempered martensite, 10 to 30% of bainite, less than 5% of retained austenite, and the balance ferrite.
4. An ultra-high strength steel sheet excellent in chemical conversion treatability and hole expansibility according to claim 1, wherein the steel sheet has a value of 40000 or more (tensile strength x hole expansibility), the unit of the tensile strength is MPa, and the hole expansibility is HER.
5. The ultra-high strength steel sheet excellent in chemical conversion treatability and hole expansibility according to claim 1, wherein the steel sheet is a cold-rolled steel sheet or a hot-dip galvanized steel sheet.
6. A method for manufacturing an ultra-high strength steel sheet having excellent chemical conversion treatability and hole expansibility, the method comprising the steps of:
preparing a steel material comprising, in weight%: carbon (C): 0.08 to 0.2%, silicon (Si): 0.05 to 1.3%, manganese (Mn): 2.0 to 3.0%, phosphorus (P): 0.001 to 0.10%, sulfur (S): 0.010% or less, aluminum (Al): 0.01 to 0.1%, chromium (Cr): 0.3 to 1.2%, boron (B): 0.0010 to 0.0030%, titanium (Ti): 0.01 to 0.05%, nitrogen (N): 0.001 to 0.01%, and the balance of Fe and other unavoidable impurities, wherein Ti and N satisfy the following relational expression 1, and Mn, Si, and Cr satisfy the following relational expression 2;
hot rolling and cold rolling the steel to manufacture a cold-rolled steel sheet;
annealing the cold-rolled steel plate at 800-850 ℃;
rapidly cooling the cold-rolled steel plate subjected to annealing heat treatment to a range of Ms-Bs, and then maintaining the cold-rolled steel plate, wherein Ms is a martensite phase transformation starting temperature, and Bs is a bainite phase transformation starting temperature;
after the maintaining, cooling at a speed of 10-50 ℃/min; and
removing surface oxides of the cold-rolled steel sheet after the cooling;
the maintaining step is performed during a period of time satisfying the following relational expression 5, the removing step is performed under a condition satisfying the following relational expression 6,
[ relational expression 1]
3.4≤Ti/N≤10
[ relational expression 2]
1.0≤Mn/(Si+Cr)
[ relational expression 5]
300≤4729+71C+25Mn-16Si+117Cr-20.1T+0.0199T2≤500
[ relational expression 6]
(HCl concentration. times. HCl temperature)/(1.33 + Mn +7.4Si +0.8Cr) × (47+2.1Mn +13.9Si +4.3Cr) ≥ 1,
in the relational expressions 1 to 6, Ti, N, Mn, Si, Cr and C represent the weight contents of the respective elements; in the relation 5, T represents the rapid cooling termination temperature and is represented by 4729+71C +25Mn-16Si +117Cr-20.1T +0.0199T2The values obtained are in seconds and the rapid cooling end temperature is in degrees celsius.
7. The method for producing an ultra-high strength steel sheet excellent in chemical conversion treatability and hole expansibility according to claim 6,
the steel further comprises niobium (Nb): 0.01 to 0.05%, molybdenum (Mo): 0.01 to 0.20%, vanadium (V): 0.01 to 0.20% and tungsten (W): 0.01 to 0.20% of at least one of them, and satisfies the following relational expression 4,
[ relational expression 4]
0.01≤Nb+0.2(Mo+V+W)≤0.05，
In the above relational expression 4, Nb, Mo, V and W represent the weight contents of the respective elements.
8. The method for manufacturing an ultra-high strength steel sheet having excellent chemical conversion treatability and hole expansibility according to claim 6, wherein the annealing heat treatment step is performed at a dew point temperature of-35 to-50 ℃.
9. The method for producing an ultra-high strength steel sheet excellent in chemical conversion treatability and hole expansibility according to claim 6,
the rapid cooling step is carried out at a cooling rate of 100-600 ℃/min.
10. The method for manufacturing an ultra-high strength steel sheet excellent in chemical conversion treatability and hole expansibility according to claim 6, wherein the contents of Mn, Si and Cr in a surface layer of the ultra-high strength steel sheet, which is a region ranging from the surface to 0.1 μm in a thickness direction thereof, satisfy the following relational expression 3,
[ relational expression 3]
0.7≤Mn*/(Si*+Cr*)≤Mn/(Si+Cr)，
In the above-mentioned relational expression 3, Mn, Si, and Cr represent the weight contents of the respective elements, and Mn, Si, and Cr represent the average values of the GDS component analysis values from the surface to 0.1 μm in the thickness direction, respectively.
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