JP5192704B2 - High strength steel plate with excellent strength-elongation balance - Google Patents

Description

  The present invention relates to a high-strength steel plate, and more particularly to a high-strength steel plate that serves as a base material (material) of a hot-dip galvanized high-strength steel plate or an alloyed hot-dip galvanized high-strength steel plate.

  Structural members such as members and pillars that play a role in absorbing energy at the time of collision as automobile framework members are from the viewpoint of improving safety or from the viewpoint of reducing the weight of the vehicle body for the purpose of improving fuel efficiency as a countermeasure for environmental problems. There is a demand for higher strength. In addition, structural members are also required to have improved rust prevention properties, such as hot-dip galvanized high-strength steel sheets that have been hot-dip galvanized on the surface of high-strength steel sheets, and alloyed hot-dip zinc alloys that have been alloyed after hot-dip galvanizing Plated high strength steel plates are used as structural members.

  By the way, in addition to strength and rust prevention properties, such a structural member material is required to have workability (elongation) in order to be molded into a desired shape of the structural member. However, when the strength of the steel sheet is increased, the workability deteriorates, so that both strength and workability (improvement of strength-elongation balance) are required.

  As a technique for improving workability when increasing the strength of a hot-dip galvanized steel sheet, Patent Document 1 describes that the metallographic structure of the steel sheet is a mixed structure containing a low temperature transformation generation phase mainly composed of martensite in a ferrite base. It is described that it is good. However, the strength of the steel sheet disclosed in this document is about 600 MPa, and a further increase in strength is required.

On the other hand, Patent Document 2 describes a high-tensile alloyed hot-dip galvanized steel sheet having a strength of 800 MPa or more with improved formability. This document describes that 0.4% or more of Si is added to increase the strength of the steel sheet and to make the metal structure of the steel sheet into a two-phase structure of ferrite and martensite. However, attention is not paid to the relationship between Si and the strength-elongation balance, and the strength-elongation balance sometimes deteriorates.
Japanese Examined Patent Publication No. 62-40405 Japanese Patent Laid-Open No. 9-13147

  This invention is made | formed in view of such a condition, The objective is to improve the workability (especially elongation) of a high strength steel plate, and to provide the high strength steel plate excellent in strength-elongation balance. is there.

  The inventors of the present invention have intensively studied to achieve both strength and workability (particularly elongation) of the steel sheet. As a result, the present inventors have found that the Si content should be increased and the Si content should be appropriately controlled according to the amount of other alloy elements, thereby completing the present invention.

That is, the high-strength steel sheet according to the present invention that has solved the above problems is mass%, C: 0.03 to 0.2%, Si: 0.5 to 2.5%, Mn: 1 to 1. 3.0%, Cr: 0.01-0.5%, Mo: 0.01-0.5%, Al: 0.02-0.15%, the balance being Fe and inevitable impurities, And it has a gist in the point that the amount of Si satisfies the following formula (1).
α-4.1 ≦ [Si] ≦ α-2.4 (1)
However,
α = 6.9 × ([C] + [Mn] / 6 + [Cr] / 5 + [Mo] / 4) ^1/2
In the formula, [] indicates the amount (% by mass) of each element contained in the steel plate.

The above-mentioned problem is mass%, C: 0.03-0.2%, Si: 0.5-2.5%, Mn: 1-3.0%, Cr: 0.01-0.5%, Mo: 0.01 to 0.5%, Al: 0.02 to 0.15%, Ti: 0.15% or less (not including 0%), Nb: 0. Containing at least one selected from the group consisting of 15% or less (not including 0%) and V: 0.15% or less (not including 0%), the balance being Fe and inevitable impurities, and the Si Even a high-strength steel sheet whose amount satisfies the following formula (2) can be solved.
β-4.1 ≦ [Si] ≦ β-2.4 (2)
However,
β = 6.9 × ([C] + [Mn] / 6 + [Cr] / 5 + [Mo] / 4 + [Ti] / 15 + [Nb] / 17 + [V] / 14) ^1/2
In the formula, [] indicates the amount (% by mass) of each element contained in the steel plate.

  In the high-strength steel sheet of the present invention, as other elements, B: 0.01% or less (not including 0%), Ca: 0.01% or less (not including 0%), Cu: 0.5 % Or less (not including 0%), Ni: 0.5% or less (not including 0%), and the like.

  A hot-dip galvanized layer or an alloyed hot-dip galvanized layer may be formed on the surface of the high-strength steel plate of the present invention.

The metal structure of the high-strength steel sheet is composed of a mixed structure of ferrite and a low-temperature transformation generation phase, and the area ratio (S _C ) of the intermediate stage transformation structure in the low-temperature transformation generation phase is 0.3 or less. Is preferred.

  According to the present invention, the strength of the steel sheet can be increased by increasing the Si content, and the workability (particularly the elongation) of the steel sheet can be increased by appropriately controlling the Si content according to other alloy elements. it can.

  The high-strength steel sheet of the present invention contains 0.5 to 2.5% Si. Si is an element that has a large solid solution strengthening ability and acts to increase the strength. Further, when the Si content is increased, the ferrite fraction is increased, and the bainite transformation is suppressed in the low-temperature transformation generation phase, so that a martensite structure is easily obtained. Accordingly, the metal structure of the steel sheet becomes a composite structure of ferrite and martensite, and high strength and good elongation (workability) can be achieved. Si is 0.5% or more, preferably 0.6% or more, more preferably 0.7% or more. However, if it is contained excessively, Si scale is generated during hot rolling, the surface properties of the steel sheet are deteriorated, the chemical conversion treatment property and plating adhesion of the steel sheet are also reduced, and non-plating occurs. Further, when Si is excessive, an austenite phase is hardly obtained during annealing, and therefore, a mixed structure of ferrite and martensite is difficult to be generated. Therefore, Si is 2.5% or less, preferably 2.3% or less, more preferably 2.1% or less.

Although the high-strength steel sheet of the present invention contains Si in the range of 0.5 to 2.5%, in the present invention, the Si content is a low-temperature transformation generation phase among the contents of alloy elements other than Si. It is important to control in accordance with the content of alloying elements that affect the process. The inventors made various steel plates with different chemical components, and studied the relationship between the chemical components of the steel plates and the mechanical properties (that is, strength-elongation balance). As a result, the Si content contained in the steel and the low temperature were reduced. It was found that the mechanical properties of the steel sheet can be improved by appropriately controlling the balance with the content of the alloying elements that affect the transformation generation phase.

  The low-temperature transformation generation phase includes (a) martensite, and (b) martensite defined by “Steel Bainite Photobook-1” (Araki et al., Japan Iron and Steel Institute, 1992, P.1-2). It means a structure that combines the intermediate stage transformation structure of ferrite (that is, bainite and pseudo-pearlite).

The alloy elements that affect the low-temperature transformation generation phase are C, Mn, Cr, Mo, Ti, Nb, and V, and the steel sheet does not contain at least one element selected from the group consisting of Ti, Nb, and V. In this case, the Si content in the steel needs to satisfy the following formula (1). When the steel sheet contains at least one element selected from the group consisting of Ti, Nb and V, the Si in the steel The content must satisfy the following formula (2).
α-4.1 ≦ [Si] ≦ α-2.4 (1)
β-4.1 ≦ [Si] ≦ β-2.4 (2)
However,
α = 6.9 × ([C] + [Mn] / 6 + [Cr] / 5 + [Mo] / 4) ^1/2
β = 6.9 × ([C] + [Mn] / 6 + [Cr] / 5 + [Mo] / 4 + [Ti] / 15 + [Nb] / 17 + [V] / 14) ^1/2
In the formula, [] indicates the amount (% by mass) of each element contained in the steel plate.

  C, Mn, Cr, and Mo are elements that affect the formation of a low-temperature transformation generation phase, but if the Si content is small relative to the C, Mn, Cr, and Mo content, the effect of adding Si is exhibited. On the other hand, when the Si content is large, the effect of adding Si is saturated, and in any case, the mechanical properties (strength-elongation balance) tend to deteriorate.

  The Ti, Nb, and V are elements that act to suppress the formation of intermediate-stage transformation structures and produce martensite among the low-temperature transformation formation phases. On the other hand, if the Si content is small, the effect of adding Si is not exhibited, while if the Si content is large, the effect of adding Si is saturated, and in any case, the mechanical properties (strength-elongation balance) tend to deteriorate. Show.

  This is apparent from FIG. 1 shown in the embodiment described later. FIG. 1 shows the relationship between the value obtained by subtracting the α value or β value from the Si content in steel and the mechanical properties (strength-elongation balance). As is apparent from FIG. ] -Α ”or“ [Si] -β ”is controlled within the range of −4.1 to −2.4, 17000 MPa% or more can be achieved.

The lower limit of the above formula (1) is preferably the following formula (1a), more preferably the following formula (1b). On the other hand, the upper limit of the formula (1) is preferably the following formula (1c), more preferably the following formula (1d).
α-4.0 ≦ [Si] (1a)
α-3.65 ≦ [Si] (1b)
[Si] ≦ α−2.55 (1c)
[Si] ≦ α-2.60 (1d)

The lower limit of the formula (2) is preferably the following formula (2a), more preferably the following formula (2b). On the other hand, the upper limit of the above formula (2) is preferably the following formula (2c), more preferably the following formula (2d).
β-4.0 ≦ [Si] (2a)
β-3.8 ≦ [Si] (2b)
[Si] ≦ β−2.55 (2c)
[Si] ≦ β-2.60 (2d)

  The steel sheet of the present invention contains C, Mn, Cr, Mo, and Al as basic elements as long as the above formula (1) or (2) is satisfied. The appropriate range of each element and the reason for its limitation are as follows.

C: 0.03-0.2%
C is an element necessary for securing a strength of 590 MPa or more. C is an element that affects the amount and form of the low-temperature transformation product phase and improves elongation. Therefore, C is 0.03% or more, preferably 0.04% or more, more preferably 0.05% or more. However, when C is too much, weldability tends to be lowered. Therefore, C is 0.2% or less, preferably 0.18% or less, more preferably 0.17% or less.

Mn: 1 to 3.0%
Mn is an element that stabilizes the austenite in the soaking process and influences the strength of the low-temperature transformation generation phase generated in the cooling process by changing the amount of solid solution C in the austenite. Mn acts as a solid solution strengthening element and strengthens ferrite. Therefore, Mn is 1% or more, preferably 1.4% or more, more preferably 1.6% or more. However, if Mn is excessive, not only melting becomes difficult, but also the elongation and weldability are adversely affected. Therefore, Mn is 3.0% or less, preferably 2.7% or less, more preferably 2.5% or less.

Cr: 0.01 to 0.5%
Cr is an element that enhances the hardenability of the steel sheet and promotes the formation of martensite in the low-temperature transformation generation phase, and effectively acts to increase the strength of the steel sheet. Therefore, Cr is 0.01% or more, preferably 0.03% or more, more preferably 0.05% or more. However, even if Cr is excessively contained, the effect is saturated and the cost is increased. Therefore, Cr is 0.5% or less, preferably 0.4% or less, more preferably 0.35% or less.

Mo: 0.01 to 0.5%
Mo is an element having the same action as Cr. Therefore, Mo is 0.01% or more, preferably 0.02% or more, more preferably 0.03% or more. However, even if Mo is excessively contained, the effect is saturated and the cost is increased. Therefore, Mo is 0.5% or less, preferably 0.45% or less, more preferably 0.40% or less.

Al: 0.02-0.15%
Al is an element to be included for deoxidation. Therefore, Al is 0.02% or more, preferably 0.04% or more, more preferably 0.05% or more. However, if the amount is too large, the amount of oxide inclusions increases and the surface properties of the steel sheet deteriorate. Therefore, Al is 0.15% or less, preferably 0.13% or less, more preferably 0.11% or less.

  The balance of the steel sheet of the present invention is Fe and inevitable impurities. Examples of inevitable impurities include P, S, N, and a playing card element. Preferred ranges for P, S, and N are as follows.

  When P is excessive, weldability tends to deteriorate. Therefore, P is preferably 0.03% or less, more preferably 0.025% or less, and still more preferably 0.023% or less.

  When S is excessive, sulfide inclusions increase and the strength of the steel sheet tends to deteriorate. Accordingly, S is preferably 0.01% or less, more preferably 0.007% or less, and still more preferably 0.005% or less.

  N is an element for strengthening the steel by precipitating nitrides in the steel. However, if N is excessively present, a large amount of the nitride is precipitated, which may cause deterioration of elongation. Therefore, N is preferably 0.01% or less, more preferably 0.008% or less, and still more preferably 0.0070% or less.

  In addition to the above basic elements, the steel sheet of the present invention may include, as necessary, at least one element selected from the group consisting of (a) Ti, Nb and V, (b) B, (c). Ca, (d) Cu and / or Ni may be contained. The preferable range in the case of containing such an element and the reason for the limitation are as follows.

(A) Ti: 0.15% or less (not including 0%), Nb: 0.15% or less (not including 0%), V: 0.15% or less (not including 0%) At least one element selected from the group consisting of Ti, Nb, and V is an element that suppresses the formation of an intermediate stage transformation structure.

In particular, Ti is an element that strengthens steel by forming precipitates such as carbides and nitrides in the steel. Ti also effectively acts to refine crystal grains and increase yield strength. However, when Ti is excessively contained, a large amount of carbide precipitates on the grain boundary, and the local elongation decreases. Therefore, Ti is 0.15% or less, preferably 0.13% or less, more preferably 0.1% or less. The lower limit of Ti is not particularly limited, but in order to effectively exhibit such effects, Ti is preferably contained in an amount of 0.01% or more, more preferably 0.015% or more, and further preferably 0.02% or more. It is. Note that Ti also has an effect of increasing the solid ductility balance of the steel sheet by suppressing the formation of an intermediate stage transformation structure in the cooling process by solid solution in the steel. In order to exhibit such an effect effectively, it is preferable to contain so that the following formula (3) may be satisfied. More preferably, the following expression (3a) should be satisfied.
[Ti]> 3.43 × [N] (3)
[Ti] ≧ 4 × [N] (3a)
However, [] shows content of each element.

  Nb and V are elements that refine crystal grains and increase the strength without impairing toughness. These elements, similarly to Ti, have the effect of solid solution in the steel, suppressing the formation of intermediate stage transformation structure during the cooling process, and increasing the balance of ductility of the steel sheet. However, even if it contains excessively, the effect will be saturated and cost will become high. Accordingly, Nb is 0.15% or less, preferably 0.13% or less, more preferably 0.1% or less, and V is 0.15% or less, preferably 0.13% or less, more preferably 0.1% or less. % Or less. The lower limit of Nb and V is not particularly limited, but Nb is preferably 0.01% or more, more preferably 0.02% or more, still more preferably 0.03% or more, and V is 0 The content is preferably 0.01% or more, more preferably 0.02% or more, and still more preferably 0.03% or more.

  Ti, Nb, and V may each be contained alone or in combination. In the case of containing two kinds of elements, the total may be 0.3% or less, and in the case of containing three kinds of elements, the total may be 0.45% or less.

(B) B: 0.01% or less (excluding 0%)
B is an element that enhances hardenability and improves the strength of the steel sheet. Moreover, the hardenability at the time of the accelerated cooling after rolling is controlled by containing together with Mo, and the strength-toughness balance of a steel plate is optimized. However, B hardly affects the generation of the intermediate stage transformation structure, and does not affect the optimum Si amount described above. However, if contained in excess, the toughness of the steel sheet deteriorates, so B is preferably 0.01% or less. More preferably, it is 0.05% or less. The lower limit of B is not particularly limited, but may preferably be 0.0005% or more.

(C) Ca: 0.01% or less (excluding 0%)
Ca is an element that spheroidizes the form of sulfide in steel and improves workability. However, even if it exceeds 0.01%, the effect is saturated and it is economically useless. Therefore, Ca is preferably 0.01% or less, and more preferably 0.005% or less. The lower limit of Ca is not particularly limited, but may be preferably 0.0005% or more.

(D) Cu: 0.5% or less (not including 0%) and / or Ni: 0.5% or less (not including 0%)
Cu and Ni are both solid solution strengthening elements and are elements having an action of improving the strength of the steel sheet. It is also an element that improves the corrosion resistance of the steel sheet. However, even if Cu is contained in excess of 0.5% and Ni is contained in excess of 0.5%, the effect is saturated and the cost is increased. Therefore, Cu is preferably 0.5% or less, more preferably 0.4% or less, and still more preferably 0.3% or less. Ni is preferably 0.5% or less, more preferably 0.4% or less, and still more preferably 0.3% or less. Cu and Ni may each be contained alone or in combination. Cu and Ni are elements that promote the formation of martensite in the low-temperature transformation generation phase. However, if Cu and Ni are within the above ranges, the effect is negligible. It does not affect.
In addition to the above elements, the steel sheet of the present invention may contain, for example, Mg, Co, Zn, As, Se, Zr, Sn, Sb, Ta, W, and REM in a total of about 0.5% or less. . Even if these elements are contained, the effects of the present invention are not impaired, and depending on the content, delayed fracture resistance and aging resistance may be improved.

  The steel sheet of the present invention is characterized in that its chemical components are specified. For example, a hot dip galvanized layer or an alloyed hot dip galvanized layer may be formed on the surface of the steel sheet.

  The metal structure of a steel plate should just be comprised with the mixed structure of a ferrite and a low temperature transformation production | generation phase.

  The ferrite fraction and the low-temperature transformation generation phase in the metal structure are not particularly limited, and may be determined according to the balance between strength and elongation required for the steel sheet. That is, when the ferrite fraction increases, the strength decreases, but the elongation tends to improve. When the fraction of the low-temperature transformation generation phase increases, the strength improves, but the elongation tends to decrease. Accordingly, the ferrite fraction is preferably 90% or less, more preferably 80% or less, still more preferably 70% or less, preferably 10% or more, more preferably 20% or more, and further preferably 30. % Or more. The fraction of the low temperature transformation product phase is preferably 90% or less, more preferably 80% or less, still more preferably 70% or less, preferably 10% or more, more preferably 20% or more, and further Preferably it is 30% or more.

In the high-strength steel sheet of the present invention, the area ratio of the intermediate-stage transformation structure in the low-temperature transformation generation phase (area ratio of the intermediate-stage transformation structure / area ratio of the low-temperature transformation generation phase; hereinafter may be referred to as _SC value) is 0. .3 or less is preferable. By suppressing the formation of the intermediate stage transformation structure, the hard martensite structure increases, and in particular, the workability (elongation) can be improved. As a result, the mechanical properties (strength-elongation balance) can be further improved. The S _C value is more preferably 0.28 or less, and still more preferably 0.27 or less.

  The metal structure of the steel plate may be observed at a magnification of 3000 times using a scanning electron microscope (SEM) at the center of the plate thickness.

  The high-strength steel sheet of the present invention has a tensile strength (TS) of about 590 to 1270 MPa and a good balance between strength and elongation (TS × El is 16000 MPa% or more). It can be preferably used as a material for an automobile member that plays a role of absorbing energy at the time of collision as a member or a pillar.

  The plate thickness of the high-strength steel plate of the present invention is not particularly limited, but the balance between the tensile strength and the strength and elongation can be ensured even when the thickness is about 1 to 3 mm.

  The high-strength steel sheet of the present invention satisfies the requirements specified above, and the production conditions are not particularly limited, but for example, the following conditions may be adopted.

  A slab having the above component composition is hot-rolled, wound up at 700 ° C. or lower, pickled as necessary, and then cold-rolled, and then at an Ac1 point or higher in an annealing line or a continuous hot-dip galvanizing line. After soaking at the temperature, cooling may be performed at an average cooling rate of 1 ° C./second or more.

  The hot rolling may be performed according to a conventional method, but the heating temperature may be about 1000 to 1300 ° C. in order to ensure the finishing temperature and prevent the austenite grains from becoming coarse. The finishing temperature of hot rolling is set to 800 to 950 ° C. so as not to form a texture that hinders workability, and the average cooling rate up to the winding start temperature after finishing rolling is 30 to 120 in order to suppress the formation of pearlite. Centigrade / second may be used.

  The winding temperature is preferably 700 ° C. or lower. When this temperature is exceeded, the scale formed on the surface of the steel sheet becomes thick and the pickling property deteriorates. The lower limit of the coiling temperature is not particularly limited, but if it is too low, the low temperature transformation generation phase is excessively generated, the steel sheet becomes too hard, and the cold rollability is lowered. Therefore, the lower limit of the winding temperature is preferably 250 ° C, more preferably 400 ° C.

  After hot rolling, if necessary, pickling is performed according to a conventional method, followed by cold rolling. The rolling reduction is preferably 15% or more. In order to reduce the rolling reduction to less than 15%, it is necessary to reduce the thickness of the steel sheet in the hot rolling process, and if the thickness is reduced in the hot rolling process, the length of the steel sheet becomes longer. Sex is reduced.

  After the cold rolling, the steel sheet may be heated and held in a ferrite-austenite two-phase region or an austenite single-phase region at an Ac1 point or higher in a continuous annealing line or a continuous hot dip galvanizing line, and soaking may be performed.

  The soaking temperature may be set to Ac1 point or higher. However, in order to improve the workability by making the metal structure at the time of heating a mixed structure of ferrite and austenite and reliably generating martensite, the temperature is about 50 ° C. or higher. It is preferable to perform a soaking treatment at a high temperature. Specifically, it is about 780 ° C. or higher. The upper limit of the soaking temperature is not particularly limited, but is set to 900 ° C. or less from the viewpoint of preventing coarsening of austenite grains.

  The holding time at the time of soaking is not particularly limited, and may be, for example, about 10 seconds or more.

  After soaking, a high-strength steel sheet (cold-rolled steel sheet) can be obtained if the average cooling rate to room temperature is cooled at 1 ° C./second or more. When the average cooling rate is less than 1 ° C./second, a pearlite structure is formed during cooling, and this remains as the final structure, causing deterioration of workability (elongation). The average cooling rate is preferably 5 ° C./second or more. Although the upper limit of the average cooling rate is not particularly defined, it is preferable to set the upper limit of the average cooling rate to about 50 ° C./second in view of easy control of the steel sheet temperature and equipment cost.

  In order to produce a hot-dip galvanized high-strength steel sheet in which hot-dip galvanizing is formed on the surface of the high-strength steel sheet, after soaking in the above-mentioned conditions in a continuous hot-dip galvanizing line, the plating bath temperature (400 to 500 ° C, After cooling to an average cooling rate of 1 ° C./second or more, preferably 440 to 470 ° C., hot dip galvanization may be performed. When the average cooling rate is less than 1 ° C./second, a pearlite structure is formed during cooling, which remains as the final structure and causes deterioration in workability (elongation). The average cooling rate is preferably 5 ° C./second or more. Although the upper limit of the average cooling rate is not particularly defined, it is preferable to set the upper limit of the average cooling rate to about 50 ° C./second in view of easy control of the steel sheet temperature and equipment cost.

  The composition of the plating bath is not particularly limited, and a known hot dip galvanizing bath may be used. The Al content in the plating bath is preferably 0.05 to 0.2%. Al is an element that acts to control the alloying rate of the hot dip galvanized layer. When the steel plate is immersed in a hot dip galvanizing bath containing Al, the surface of the steel plate (that is, between the steel plate and the hot dip galvanized layer). An Fe—Al metal layer is formed at the interface), and the steel plate and zinc can be prevented from immediately alloying. However, if the Al content is less than 0.05%, the Fe—Al alloy layer is too thin. Therefore, when the steel sheet is immersed in a plating bath, the alloying of the steel sheet and zinc tends to proceed immediately. Therefore, before the alloying is completed to the plating surface in the alloying process, the Γ phase grows greatly, and the powdering resistance (plating peeling resistance) is lowered. The Al content is more preferably 0.07% or more. However, if the Al content exceeds 0.2%, the Fe—Al alloy layer becomes too thick, so that alloying of Fe and Zn is hindered in the alloying process, and alloying of the hot dip galvanized layer is delayed. Therefore, in order to proceed with alloying, it is necessary to lengthen the alloying line or separately perform an alloying treatment at a high temperature. The Al content is more preferably 0.18% or less.

  After hot dip galvanization, the austenite in the steel sheet is transformed into martensite by cooling to room temperature at an average cooling rate of 1 ° C./second or more, and a mixed structure mainly composed of ferrite and martensite can be obtained. When the cooling rate is less than 1 ° C./second, martensite is hardly generated, and pearlite or an intermediate stage transformation structure may be generated. The average cooling rate is preferably 10 ° C./second or more.

  In order to manufacture an alloyed hot-dip galvanized high-strength steel plate in which alloyed hot-dip galvanized plating is formed on the surface of the high-strength steel plate, after hot-dip galvanizing under the above conditions, about 400 to 750 ° C. The alloying treatment may be performed by heating to about ° C. The heating means in the case of alloying treatment is not particularly limited, and various conventional methods (for example, gas heating, induction heater heating, etc.) can be used.

  After the alloying treatment, a mixed structure mainly composed of ferrite and martensite can be obtained by cooling to room temperature at an average cooling rate of 1 ° C./second or more.

The area ratio of the intermediate stage transformation structure occupying the low-temperature transformation product phase of high-strength steel sheet (S _C value) to 0.3 or less, if a high strength steel sheet is cold-rolled steel sheet, after soaking, cooling to room temperature In this case, the steel sheet may be cooled so that the time for which the temperature of the steel sheet passes through the temperature range of 500 ° C. to 300 ° C. is 120 seconds or less. In the case of a hot dip galvanized steel sheet, after the hot dip galvanization, when the steel sheet is cooled to room temperature, it may be cooled so that the time for which the temperature of the steel sheet passes through the temperature range of 500 ° C. to 300 ° C. is 120 seconds or less. Similarly, in the case of the alloyed hot-dip galvanized steel sheet, when cooling to room temperature before and after the alloying treatment, the steel sheet is cooled so that the time for passing the temperature range from 500 ° C to 300 ° C is 120 seconds or less. do it. By setting the time for passing through the temperature range to be 120 seconds or less, the generation of intermediate stage transformation structure can be suppressed.

  The steel sheet of the present invention is excellent in workability despite its high strength, and therefore can be suitably used as a material for automobile parts having a complicated shape. Examples of the automobile parts include front and rear side members, A / B pillar inner / reinforce, sill inner, and cross members.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.

  Steel having the chemical composition shown in Table 1 or Table 2 (the balance being Fe and inevitable impurities) was melted in a converter and continuously cast to obtain a slab. The obtained slab was heated and held at 1250 ° C., and this was hot rolled to a finishing temperature of 900 ° C. and a reduction rate of about 99% to a thickness of 2.4 mm, and then cooled at an average cooling rate of 50 ° C./sec. Winding was performed at 480 ° C. to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet was pickled and then cold-rolled to a thickness of 1.2 mm at a reduction rate of about 50% to obtain a cold-rolled steel sheet.

  From the chemical composition of the steel sheet shown in Table 1 or Table 2, the range of the Si content was calculated using the above formula (1) or (2). The results are shown in Table 1 or Table 2 below. Further, a value obtained by subtracting the α value or β value from the Si content (% by mass) contained in the steel (“[Si] −α” or “[Si] −β”, where [Si] is in the steel) Table 1 or Table 2 below also shows the Si content.

  The obtained cold-rolled steel sheet was subjected to the following treatment on an annealing line or a continuous hot-dip galvanizing line to obtain a cold-rolled steel sheet, a hot-dip galvanized steel sheet, or an alloyed hot-dip galvanized steel sheet.

  No. shown in Table 3 below. 1-6 are cold-rolled steel plates that have been soaked, heated to 830 ° C. in an annealing line, held at this temperature for 30 seconds, soaked, and then cooled to room temperature at an average cooling rate of 15 ° C./s. It has been cooled.

  No. shown in Table 3 below. 7-11 and no. 14 is a hot dip galvanized steel sheet. The cold rolled steel sheet is heated to 830 ° C. by a continuous hot dip galvanizing line, held at this temperature for 30 seconds, soaked, and then cooled to a plating bath temperature at an average cooling rate of 15 ° C. / It is cooled in seconds and immersed in a plating bath for 3 seconds to form a hot dip galvanized layer on the surface of the cold rolled steel sheet. As the plating bath, a hot dip galvanizing bath having an Al content of 0.13% is used, and the plating bath temperature is 450 to 470 ° C. After plating, it was cooled to room temperature at an average cooling rate of 10 ° C./second.

  No. shown in Table 3 below. Nos. 12 to 13 are hot dip galvanized steel sheets. After forming a hot-dip galvanized layer on the surface of the cold rolled steel sheet under the same conditions as in No. 7, the average cooling rate in the temperature range from the plating bath temperature to 300 ° C. is 30 ° C./second, and the average cooling rate is 10 ° C. from 300 ° C. to room temperature. Cooled at / sec.

  No. shown in Table 4 below. 15-24 and no. Nos. 28 to 36 are galvannealed steel sheets. After forming a hot-dip galvanized layer on the surface of the cold-rolled steel plate under the same conditions as No. 7, heat to 540-560 ° C., hold at this temperature for about 10 seconds, and alloy it to the surface of the cold-rolled steel plate A hot dip galvanized layer is formed. After the alloying treatment, it was cooled to room temperature at an average cooling rate of 10 ° C./second.

  No. shown in Table 4 below. Nos. 25 to 27 and No. 5 shown in Table 5 below. Nos. 37 to 46 are also galvannealed steel sheets. After alloying the hot dip galvanized layer under the same conditions as in No. 15, the average cooling rate in the temperature range from 500 ° C. to 300 ° C. was set to 30 ° C./sec, and the cooling was performed from 300 ° C. to room temperature at an average cooling rate of 10 ° C./sec .

Next, for the obtained cold-rolled steel sheet, hot-dip galvanized steel sheet, or alloyed hot-dip galvanized steel sheet after soaking, the metallographic structure at the center of the thickness of the base steel sheet was measured using a scanning electron microscope (SEM). The area ratio of the ferrite and the low-temperature transformation generation phase (intermediate stage transformation structure and martensite) was measured respectively. The measurement results are shown in Tables 3 to 5 below. Also, calculated area ratio of the intermediate stage transformation structure occupying the low-temperature transformation product phase (S _C value), the results shown in Tables 3-5 to.

  In addition, from the obtained cold-rolled steel sheet, hot-dip galvanized steel sheet, or alloyed hot-dip galvanized steel sheet after soaking, a JIS No. 5 test piece was cut out and subjected to a tensile test to obtain tensile strength (TS) and elongation (El). It was measured. The results are shown in Tables 3 to 5 below. In addition, TS made the range which is 590 Mpa or more and less than 1270 Mpa pass, and made the thing remove | deviating from this range unacceptable.

  Further, TS × El was calculated, and the strength-ductility balance was evaluated. The results are shown in Tables 3 to 5 below. In addition, the steel plate having a strength of 590 MPa (590 MPa or more and less than 780 MPa) has an elongation (El) of 28% or more, and the steel plate having a strength of 780 MPa (780 MPa or more and less than 980 MPa) has an elongation (El) of 20% or more and 980 MPa (980 MPa). The above (less than 1180 MPa) steel sheet has an elongation (El) of 15% or more, and the 1180 MPa class (1180 MPa or more) steel sheet has an elongation (El) of 9% or more.

  Further, FIG. 1 shows the relationship of “[Si] -α” or “[Si] -β” shown in Table 1 or 2 and TS × E1 shown in Tables 3 to 5. In FIG. 1, □ indicates the results of examples (No. 1 to 5, No. 7 to 11, No. 15 to 24) using steel types A1 to A20, and ▲ indicates the steel types C1 to C6 (No. 14, No. 14). .33-36) shows the result of the example. Moreover, ■ shows the result of the example (No. 12-13, No. 25-27, No. 37-46) which optimized the cooling rate using steel types A9-10, A17, A19-30.

  Moreover, about the obtained hot dip galvanized steel plate and alloyed hot dip galvanized steel plate, the presence or absence of non-plating was observed visually and the plating property was evaluated. The results are shown in Tables 3 to 5 below.

  As is apparent from Tables 1 to 5, it can be seen that any steel sheet is composed of a mixed structure of ferrite and a low-temperature transformation generation phase. In particular, no. 20 (steel type A16) and No. 34 (steel type C4) is an example in which chemical components other than Si are substantially the same. No. 34 has a low Si content, so that the intermediate stage transformation structure fraction in the low temperature transformation generation phase is high. Since No. 20 has a large Si content, the intermediate-stage transformation structure fraction in the low-temperature transformation generation phase is low. Therefore, the strength class is almost the same level, but there is a difference in elongation. No. 20 is superior in mechanical properties (strength-elongation balance).

  On the other hand, no. 28 to 32 are examples in which the chemical component deviates from the range defined in the present invention, and the strength-ductility balance is poor. In particular, no. No. 28 (steel type B1) is an example in which the amount of C is small, and sufficient strength cannot be secured. No. 29 (steel type B2) is an example in which the amount of Si is large, and the ferrite fraction becomes too high, and sufficient strength is not obtained. Further, Si scale is generated on the surface of the steel sheet, the surface properties are deteriorated, and the plating property is lowered. No. 30 (steel type B3) is an example in which the amount of Mn is small, and the amount of solid solution is small, so the strength is low. Moreover, since a part of low temperature transformation production | generation phase produces | generates as an intermediate | middle stage transformation structure, elongation (El) has fallen. No. 31 (steel type B4) is an example with a large amount of Mn, which has high strength but poor elongation. No. 32 (steel type B5) is an example having a large amount of Ti and has a sufficient strength, but a large amount of carbides are precipitated on the grain boundaries, and the local elongation is remarkably reduced.

  As is apparent from FIG. 1, the mechanical properties of the steel sheet (strength-elongation balance) can be achieved by appropriately controlling the balance between the Si content contained in the steel and the content of the alloying element that affects the low temperature transformation generation phase. ) Can be improved. In particular, No. 1 shown in Table 3. 12-13 and No. 1 shown in Table 4. Nos. 25 to 27 and Table 5 No. Looking at the results of 37 to 46 (■ in FIG. 1), the elongation can be increased particularly by lowering the intermediate stage transformation structure fraction in the low temperature transformation formation phase, and the mechanical properties (strength-elongation) of the steel sheet. It can be seen that the balance can be further improved.

  On the other hand, the balance of the alloy elements affecting the Si content and the low-temperature transformation generation phase contained in the steel is deteriorated, and the “[Si] -α” value or the “[Si] -β” value is −4.1 to 4.1. If it is out of the range of -2.4, the strength-ductility balance is deteriorated. For example, no. 20 (steel type A16), No. 33 (steel grade C3), and 34 (steel type C4) is an example in which the chemical components other than Si are almost the same, but as the amount of Si increases, the mechanical properties (strength-elongation balance) increase, and the amount of Si is appropriately controlled. . 20 indicates a very good strength-elongation balance.

FIG. 1 is a diagram illustrating the relationship between the value of “[Si] −α” or the value of “[Si] −β” and TS × El.

Claims (8)

% By mass
C: 0.03-0.2%,
Si: 0.5 to 2.5%
Mn: 1 to 3.0%,
Cr: 0.01 to 0.5%
Mo: 0.01 to 0.5%,
Al: 0.02 to 0.15% is contained,
The balance is Fe and inevitable impurities, and the amount of Si satisfies the following formula (1) ,
The metal structure is composed of a mixed structure of ferrite and a low-temperature transformation phase composed of martensite, bainite, and pseudopearlite,
The area fraction of the low-temperature transformation generation phase in the metal structure is 10 to 90%, the area fraction of the ferrite is 10 to 90%, and
The product of strength TS (MPa) and elongation El (%) satisfies 17000 or more, characterized in that the area ratio of bainite and pseudo pearlite in the low-temperature transformation generation phase in the metal structure is 0.3 or less. High-strength cold-rolled steel sheet with excellent strength-elongation balance .
α-4.1 ≦ [Si] ≦ α-2.4 (1)
However,
α = 6.9 × ([C] + [Mn] / 6 + [Cr] / 5 + [Mo] / 4) ^1/2
In the formula, [] indicates the amount (% by mass) of each element contained in the steel plate. % By mass
C: 0.03-0.2%,
Si: 0.5 to 2.5%
Mn: 1 to 3.0%,
Cr: 0.01 to 0.5%
Mo: 0.01 to 0.5%,
Al: 0.02 to 0.15% is contained,
As other elements,
Ti: 0.15% or less (excluding 0%),
Containing at least one selected from the group consisting of Nb: 0.15% or less (excluding 0%) and V: 0.15% or less (not including 0%),
The balance is Fe and inevitable impurities, and the amount of Si satisfies the following formula (2) ,
The metal structure is composed of a mixed structure of ferrite and a low-temperature transformation phase composed of martensite, bainite, and pseudopearlite,
The area fraction of the low-temperature transformation generation phase in the metal structure is 10 to 90%, the area fraction of the ferrite is 10 to 90%, and
The product of strength TS (MPa) and elongation El (%) satisfies 17000 or more, characterized in that the area ratio of bainite and pseudo pearlite in the low-temperature transformation generation phase in the metal structure is 0.3 or less. High-strength cold-rolled steel sheet with excellent strength-elongation balance .
β-4.1 ≦ [Si] ≦ β-2.4 (2)
However,
β = 6.9 × ([C] + [Mn] / 6 + [Cr] / 5 + [Mo] / 4 + [Ti] / 15 + [Nb] / 17 + [V] / 14) ^1/2
In the formula, [] indicates the amount (% by mass) of each element contained in the steel plate. The high-strength cold-rolled steel sheet according to claim 1 or 2, which is obtained by cold rolling, soaking, and cooling to room temperature. The high-strength cold-rolled steel sheet according to any one of claims 1 to 3, further comprising B: 0.01% or less (not including 0%) as another element. The high-strength cold-rolled steel sheet according to any one of claims 1 to 4 , further comprising Ca: 0.01% or less (not including 0%) as another element. Still other elements, Cu: 0.5% or less (not including 0%) and / or Ni: more of claims 1 to 5 are those containing 0.5% or less (not including 0%) High strength cold-rolled steel sheet according to crab. On the surface of the high strength cold rolled steel sheet according to any one of claims 1 to 6 high-strength cold-rolled steel sheet hot-dip galvanized layer is formed. High strength on the surface of the cold-rolled steel sheet, high-strength cold-rolled steel sheet galvanized alloy layer is formed of any one of claims 1-6.

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