JP3945373B2 - Method for producing cold-rolled steel sheet with fine grain structure and excellent fatigue characteristics - Google Patents

Description

【００４３】
【表２】

【００４４】
【表３】

【００４５】
【表４】

【００４６】
No.1〜３は、表１に示したＡ鋼を二相域温度に保持する時間を変えて組織の変化を確認したものである。保持時間が30秒のNo.1の場合はマルテンサイトを得るためのγへのＣの濃化量を確保できず、60秒以上保持したNo.2, ３の場合と比較してTSが不足している。従って、TSを確保するためにも二相域温度で60秒以上確保すべきであることが分かる。
【００４７】
No.2とNo.4〜８は、熱延後の巻取り温度を変えたものである。疲労限のTS比（FL／TS）を見ると、巻取り温度が 650〜750 ℃のところで高いレベルが得られて疲労特性に優れている。これは巻取り時に、TiＣやNbＣが析出してフェライト粒が強化され、これに伴って疲労亀裂発生と亀裂伝播が抑制されるためである。従って、巻取り温度は 650〜750 ℃が望ましい。
【００４８】
No.2とNo.9，10は、（Ａ₁ ＋50）〜（Ａ₁ ＋20）℃の二相域に保持する温度を変えたものであるが、高温で保持するものほどマルテンサイトが少なくなり、代わってベイナイトが増加した。二相域の高温部での存在比はγ＞αであるが、この時のγは低温域のγに較べると炭素および合金成分の濃縮が少ないためマルテンサイト変態を起こし難かったものと考えられる。
【００４９】
No.11 〜13は、焼鈍温度を変えたものであるが、Ａ₃ 点が 841℃であるＡ鋼を900 ℃で焼鈍すると、得られた粒径は粗大化して強度が低下した。一方、800 ℃で焼鈍すると再結晶が完遂していないため加工組織がそのまま残存していた。この点、Ａ₃ 点直上で焼鈍したNo.11 の発明例は、結晶粒の粗大化も加工組織の残存も認められなかった。
【００５０】
No.14 は、鋼種Ａとは成分の異なる鋼種Ｂを、二相域での保持処理無しとしたものである。この条件では、パーライトの生成により、TSが低く疲労特性が悪化した。
No.15, 16 は、熱間圧延の圧延終了時から巻取りまでの冷却速度を20℃/sと30℃/sにしたものであるが、いずれの場合もパーライトの生成が抑えられてTiＣが析出したため、FL／TSが高く、良好な疲労特性が得られた。
No.17, 18 は、（Ａ₁ ＋20）℃からＭs 点以下までの冷却速度を変化させたものであるが、冷却速度が遅いNo.18 は、マルテンサイトが得られずにTSおよび疲労特性が低下した。このことから、二相域すなわち（Ａ₁ ＋20）℃から少なくともＭs 点までの冷却速度は５℃/s以上が好適であることが分かる。
No.19 は、（Ａ₁ ＋50）℃までの冷却速度を下げたものであるが、冷却速度の低下により必要以上の粗大化を招いて結晶粒が大きくなった。そのために粒径の均質性が悪化して、穴拡げ性の低下を招いた。このことから、二相域までの冷却速度は５℃/s以上とするのが適当であることが分かる。
【００５１】
No.20 はNbのみを添加した場合、No.21 はTiのみを添加した場合、No.22 はＡ鋼ベースにCrを添加した場合、さらにNo.23 はTiおよびNbの添加がない場合である。なお、これらはいずれもNo.7と同じ製造条件で製造した。
No.20およびNo.21 は、TiまたはNbの効果によってTiＣまたはNbＣが析出し、これにより再結晶温度の上昇が発現して、結晶粒の微細化と疲労特性の向上がもたらされた。
No.22は、オーステナイト安定化元素であるCrを添加したことにより、マルテンサイト分率が向上し、若干穴拡げ率は低下したものの、強度の向上がもたらされた。
この点、No.23 は、TiＣやNbＣが生成しないため、焼鈍時に結晶粒が粗大化し、強度の低下を招いた。
【００５２】
【発明の効果】
かくして、本発明によれば、微細粒組織を有し、疲労特性に優れた高張力冷延鋼板を、製造設備の大幅な改造を伴うことなしに安定して製造することができ、産業上極めて有用である。
【図面の簡単な説明】
【図１】 Ａ₁ ＝700 ℃、Ａ₃ ＝855 ℃に調整した鋼組成において、Ti，Nb添加量を種々に変更した場合のTi，Nb添加量と再結晶温度との関係を示した図である。
【図２】 637.5＋4930｛Ti^* ＋ (48/93)・[%Nb] ｝＞Ａ₁ の条件下において、Ａ₃ を種々に変化させた場合におけるＡ₃ と再結晶温度Ｔreとの関係を示した図である。
【図３】 疲労試験片の形状・寸法を示した図である。[0001]
BACKGROUND OF THE INVENTION
The present invention is a cold-rolled steel sheet suitable for use in automobiles, home appliances, and machine structural steel, particularly a high-tensile cold-rolled steel sheet having a fine grain structure and excellent fatigue strength and stretch flangeability. It is about.
[0002]
[Prior art]
Steel materials used for automobiles, home appliances, and mechanical structural steel plates are required to have excellent mechanical properties such as strength, workability, and fatigue characteristics (also referred to as durability). As a means for comprehensively improving these mechanical properties, it is effective to refine the structure. Therefore, many production methods for obtaining the microstructure have been proposed so far.
[0003]
As a means for refining the structure, a large reduction rolling method is conventionally known. As such a large rolling reduction method, for example, a technique has been proposed in which a large reduction is applied to austenite grains to promote γ-α strain-induced transformation to refine the structure (for example, Patent Document 1 and Patent Document 2). ).
Moreover, the case where the controlled rolling method and the controlled cooling method are applied is also known (for example, Patent Document 3).
[0004]
In addition, with regard to the material steel, at least a part of the steel structure is made of ferrite, and the transformation point (Ac₁Point) Increase the temperature to the above temperature range or follow this temperature increase with Ac₁Hold for a certain period of time in the temperature range above the point, once part or all of the structure is reversely transformed into austenite, ultrafine austenite grains appear, and then cooled to an isotropic with an average crystal grain size of 5 μm or less There has been proposed a technique of making a structure mainly composed of a typical ferrite crystal grain (for example, Patent Document 4).
[0005]
On the other hand, as means for improving workability and fatigue characteristics required for high-strength steel sheets, the presence of hard second phase (mainly martensite) that plays a role in inhibiting the propagation of fatigue cracks and the control of precipitates are controlled. It is also known to carry out, and a technique for obtaining a steel sheet having excellent fatigue characteristics and workability by applying both precipitation strengthening and structure strengthening to a hot-rolled steel sheet has been proposed (for example, Patent Document 5).
In this technology, the hard second phase suppresses crack propagation and improves fatigue characteristics, and at the same time, the precipitate strengthens the soft ferrite phase and the hardness difference between the second phase and ferrite is reduced. Is dispersed, so that the hole expandability, that is, stretch flangeability is improved.
[0006]
All of the above techniques are techniques in the hot rolling process. In order to reduce the body weight of automobiles in consideration of environmental issues, it is effective to apply high-strength steel to reduce the thickness, but the higher the strength, the greater the control of the structure. Since more alloying elements are added, generally a larger rolling load is required, making it difficult to produce a hot-rolled steel sheet having a small thickness. For such production reasons, there is a great demand for cold-rolled steel sheets for high-strength thin materials. However, for cold-rolled steel sheets, there are hardly any techniques for refining crystal grains in a normal cold rolling-annealing process.
[0007]
[Patent Document 1]
JP-A-53-123823 (Claims)
[Patent Document 2]
Japanese Patent Publication No. 5-65564 (Claims)
[Patent Document 3]
JP 63-128117 A (Claims)
[Patent Document 4]
JP-A-2-301540 (Claims)
[Patent Document 5]
JP-A-5-179396 (Claims)
[0008]
[Problems to be solved by the invention]
The present invention has been developed in view of the above-mentioned present situation, and it is possible to make a fine grained cold rolled steel sheet used as a steel sheet for automobiles, home appliances, and mechanical structures, and at the same time, it is fine with excellent fatigue characteristics. It aims at proposing the advantageous manufacturing method of the cold-rolled steel plate which has a grain structure.
[0009]
[Means for Solving the Problems]
Now, as a result of intensive studies to achieve fine graining on the cold-rolled steel sheet, the inventors have adjusted the alloying elements appropriately to adjust the recrystallization temperature and A of the steel sheet.₁ And A_Three The inventors have obtained the knowledge that a fine grain structure can be obtained by controlling the transformation temperature and accurately controlling the recrystallization annealing temperature after the cold rolling and the subsequent cooling step.
In addition, by optimizing the cooling and winding temperature in the hot rolling process, it is possible to control the precipitation of Ti and Nb, and to obtain a cold rolled steel sheet having fine grains and excellent fatigue characteristics. Obtained knowledge.
The present invention is based on the above findings.
[0010]
  That is, the present inventionThe summary of is as follows.
1.C: 0.03-0.16%,
    Si: 2.0% or less,
    Mn: 3.0% or moreunder,
    Ti: 0.01-0.2% and / or Nb: 0.01-0.2%,
    Al: 0.01 to 0.1%,
    P: 0.1% or less,
    S: 0.02% or less and
    N: 0.005% or less
And C, Si,Mn,A steel material containing Ti and Nb satisfying the following formulas (1), (2), and (3), with the balance being Fe and inevitable impurities, is heated to 1200 ° C or higher, and then Ar is heated._ThreeAfter the hot rolling is finished at a point or more, the steel sheet is wound in a temperature range of 750 ° C. or lower and 650 ° C. or higher, pickled, cold-rolled, and then the temperature A obtained by the following formula (6)_Three(℃) or more, (A_Three+30) Recrystallization annealing at (° C) or less, and then the temperature A determined by the following equation (5)₁Cool to + 50 ° C at a rate of 5 ° C / s or higher, then (A₁+50) to (A₁A method of producing a cold-rolled steel sheet having a fine grain structure and excellent fatigue characteristics, characterized by holding at +20) ° C for 60 seconds or more and then cooling to at least the Ms point at a rate of 3 ° C / s or more.Law.
                                Record
     637.5 + 4930 {Ti^* ＋ (48/93) ・ [% Nb]} ＞ A₁              --- (1)
    A_Three <860 --- (2)
    [% Mn] ≧ 1.3 --- (3)
  However,
    Ti^* = [% Ti] − (48/32) ・ [% S] − (48/14) ・ [% N] --- (4)
    A₁ ＝ 727 ＋ 14 [% Si] −28.4 [% Mn]                         --- (Five)
    A_Three = 920+ 612.8 [% C]²− 507.7 [% C] + 9.8 [% Si]^Three
          − 9.5 [% Si]²+68.5 [% Si] +2 [% Mn]²− 38 [% Mn]
          + 102[% Ti] +51.7 [% Nb] --- (6)
    [% M] is the content of M element (mass%)
2. % By mass
    C: 0.03 ~ 0.16 %,
    Si : 2.0 %Less than,
    Mn : 3.0 %Less than,
    Ni : 3.0 %Less than,
    Ti : 0.01 ~ 0.2 % And / or Nb : 0.01 ~ 0.2 %,
    Al : 0.01 ~ 0.1 %,
    P: 0.1 %Less than,
    S: 0.02 % And below
    N: 0.005 %Less than
And C, Si, Mn, Ni, Ti and Nb Is (one two Three)' Each of the formulas is contained within the range that satisfies each, and the balance is Fe And steel material that becomes a composition of inevitable impurities, 1200 After heating above ℃ A r _Three Hot rolling is finished at the point or more, then 750 ℃ or less, 650 Winding in the temperature range above ℃, pickling, and after cold rolling, (6) ' Temperature A determined by the formula _Three (° C) or more, (A _Three + 30 ) Recrystallization annealing at (° C) or less, then (Five)' Temperature A determined by the formula ₁ + 50 Up to 5 ℃ / s Cool at the above speed, then (A ₁ + 50 ) ~ (A ₁ + 20 ) In the temperature range of ℃ 60 Hold for more than a second, then at least M s 3 ℃ to the point / s A method for producing a cold-rolled steel sheet having a fine grain structure and excellent fatigue characteristics, characterized by cooling at the above speed.
                                Record
     637.5 + 4930 { Ti ^* + (48/93) ・ [% Nb] }> A ₁               --- (1)
    A _Three < 860                                              --- (2)
    [% Mn] + [% Ni] ≧ 1.3                                    --- (3) '
  However,
    Ti ^* = [% Ti] − (48/32) ・ [% S ] − (48/14) ・ [% N ]          --- (Four)
    A ₁ = 727 + 14 [% Si] − 28.4 [% Mn] − 21.6 [% Ni]              --- (Five)'
    A _Three = 920 + 612.8 [% C ] ² − 507.7 [% C ] + 9.8 [% Si] ^Three
          − 9.5 [% Si] ² + 68.5 [% Si] +2 [% Mn] ² − 38 [% Mn]
          + 2.8 [% Ni] ² − 38.6 [% Ni] + 102 [% Ti] + 51.7 [% Nb]      --- (6) '
    Also, [% M ] Is M element content (% by mass)
[0011]
3. In 1 or 2 above,Steel materialButIn addition, by mass%
    Mo: 0.1% or less and
    Cr: 0.1% or less
Contains one or two selected fromThe manufacturing method of the cold-rolled steel plate which was excellent in the fatigue characteristic which has the fine-grain structure characterized by becoming the composition which carries out.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be specifically described below.
First, the reason why the composition of steel is limited to the above range in the present invention will be described. Unless otherwise specified, “%” in relation to ingredients means mass%.
C: 0.03-0.16%
C is not only an inexpensive strengthening component but also an element useful for generating a low-temperature transformation phase such as martensite. However, if the content is less than 0.03%, the effect of addition is poor. On the other hand, if the content exceeds 0.16%, ductility and weldability deteriorate, so C is limited to a range of 0.03-0.16%.
[0013]
Si: 2.0% or less
Si, as a solid solution strengthening component, effectively contributes to improving strength while improving the strength-elongation balance, but excessive addition deteriorates ductility, surface properties, and weldability, so the Si content is 2.0. % Or less. From the above viewpoint, the preferable range of the Si amount is 0.01 to 1.0%.
[0014]
Mn: 3.0% or moreDownOr( Mn : 3.0 % Or less +Ni: 3.0% or less)
  Both Mn and Ni are austenite stabilizing elements, and A₁ , A_Three It contributes to refinement of crystal grains through the action of lowering the transformation point, and has the action of increasing the strength-ductility balance through the action of promoting the formation of the second phase. However, addition of a large amount hardens the steel and, on the other hand, deteriorates the strength-ductility balance, so in either case the content was limited to 3.0% or less.here, Ni When adding Mn Need to be added in combination.
  Mn also has an effect of detoxifying harmful solid solution S as MnS, so it is preferably contained in an amount of 0.1% or more. Ni is preferably contained in an amount of 0.01% or more in order to obtain the effect of Ni described above.
[0015]
Ti: 0.01-0.2% and / or Nb: 0.01-0.2%
As will be described later, Ti and Nb are elements that finely disperse and precipitate in the ferrite phase and contribute to the improvement of formability such as stretch flangeability and fatigue characteristics. In order to obtain this effect, it is necessary to contain Ti and Nb in an amount of 0.01% or more, and they may be added alone or in combination.
Further, by adding Ti and Nb, TiC, NbC and the like are precipitated, and there is an effect of increasing the recrystallization temperature of the steel sheet. However, in both cases, if it exceeds 0.2%, not only the effect is saturated, but also the amount of precipitates increases and the ductility of the ferrite is reduced, so each of them was included at 0.2% or less.
[0016]
Al: 0.01 to 0.1%
Al acts as a deoxidizer and is an effective element for improving the cleanliness of steel, and it is desirable to add it in the deoxidation process. Here, if the amount of Al is less than 0.01%, the effect of addition is poor. On the other hand, if it exceeds 0.1%, the effect is saturated, and rather the production cost is increased, so Al is limited to the range of 0.01 to 0.1%.
[0017]
P: 0.1% or less
P is an element effective in achieving high strength at a low cost without causing a significant decrease in ductility. On the other hand, a large amount causes deterioration in workability and toughness, so the P content is 0.1% or less. Limited to. In addition, when the request | requirement with respect to workability and toughness is severe, since it is preferable to reduce P, in this case, it is desirable to set it as 0.02% or less.
[0018]
S: 0.02% or less
S is not only a cause of hot cracking during hot rolling, but also exists as inclusions such as MnS in the steel sheet and causes deterioration of ductility and hole expansion workability, so it is desirable to reduce it as much as possible, but 0.02% In the present invention, the content is set to 0.02% or less.
[0019]
N: 0.005% or less
Nitrogen not only causes aging deterioration, but also causes yield elongation, so it is desirable to reduce it as much as possible. However, since it is acceptable up to 0.005%, in the present invention, it is limited to 0.005% or less.
[0020]
The basic components have been described above. However, in the present invention, other elements described below can be appropriately contained.
One or two selected from Mo: 0.1% or less and Cr: 0.1% or less
Both Mo and Cr can be included as a reinforcing component as required. However, addition of a large amount deteriorates the strength-ductility balance on the contrary, so it is desirable to contain each at 0.1% or less. In order to sufficiently exhibit the above-described action, it is preferable to contain 0.01% or more of Mo and Cr.
[0021]
  Although the proper component composition range has been described above, in the present invention, it is not sufficient that each component simply satisfies the above composition range, and C, Si, Mn, TiFor Nb and Nb, the following formulas (1), (2), and (3) need to be contained within a range satisfying each of them.
                                Record
     637.5 + 4930 {Ti^* ＋ (48/93) ・ [% Nb]} ＞ A₁              --- (1)
    A_Three <860 --- (2)
    [% Mn] ≧ 1.3 --- (3)
  However,
    Ti^* = [% Ti] − (48/32) ・ [% S] − (48/14) ・ [% N] --- (4)
    A₁ ＝ 727 ＋ 14 [% Si] −28.4 [% Mn]                         --- (Five)
    A_Three = 920+ 612.8 [% C]²− 507.7 [% C] + 9.8 [% Si]^Three
          − 9.5 [% Si]²+68.5 [% Si] +2 [% Mn]²− 38 [% Mn]
          + 102[% Ti] +51.7 [% Nb] --- (6)
    [% M] is the content of M element (mass%)
  further, Ni In the case of containing (3) , (Five) , (6) Instead of the formula: (3) ' , (Five)' , (6) ' It is necessary to make it contain in the range which satisfies a formula.
    [% Mn] + [% Ni] ≧ 1.3                                    --- (3) '
    A ₁ = 727 + 14 [% Si] − 28.4 [% Mn] − 21.6 [% Ni]              --- (Five)'
    A _Three = 920 + 612.8 [% C ] ² − 507.7 [% C ] + 9.8 [% Si] ^Three
          − 9.5 [% Si] ² + 68.5 [% Si] +2 [% Mn] ² − 38 [% Mn]
          + 2.8 [% Ni] ² − 38.6 [% Ni] + 102 [% Ti] + 51.7 [% Nb]      --- (6) '
[0022]
The above A₁ , A_Three Respectively, the Ac of steel₁Transformation temperature (℃), Ac_ThreeThis is a predicted value of the transformation point temperature (° C.), and is a component regression equation derived from the inventors' detailed basic experiments. This predicted temperature (° C.) is particularly suitable when applied at a heating rate of 2 ° C./s or more and 20 ° C./s or less.
[0023]
  The above (1), (2), (3), (3) ' The reasons for limiting the expressions will be described in order.
  Equation (1) is a condition that defines the amount of Ti and Nb added, and is based on the following findings.
  In general, it is known that when Ti and Nb are added, TiC, NbC and the like are precipitated, and the recrystallization temperature of the steel sheet is increased. Therefore, when the relationship between the addition amount of Ti and Nb and the recrystallization temperature Tre was investigated in detail, the recrystallization temperature was calculated by the above formula (6) when more than a certain amount of Ti and Nb was added.Or (6) ' formulaA calculated by_Three Was found to be equivalent to
[0024]
In FIG.₁ = 700 ° C, A_Three The following shows the results of examining the relationship between the Ti and Nb addition amounts and the recrystallization temperature when the Ti and Nb addition amounts are variously changed in the steel composition adjusted to 855 ° C. Here, the recrystallization temperature Tre was determined by performing continuous annealing in a laboratory with various heating temperatures, measuring hardness, and observing the structure. Moreover, Ti addition amount is Ti as an effective Ti amount for precipitating TiC.^* The amount of Nb added is expressed as (48/93) · [% Nb] in terms of Ti, and the relationship between the amount of Ti and Nb added and the recrystallization temperature is expressed.
According to the figure, 637.5 + 4930 {Ti^* + (48/93) · [% Nb]} is 700 ° C or A₁ Above the recrystallization temperature Tre is around 855 ° C., ie A_Three It can be seen that it rises quickly and saturates.
[0025]
Next, in Fig. 2, 637.5 + 4930 {Ti^* ＋ (48/93) ・ [% Nb]} ＞ A₁ Under the conditions of A_Three A (when fluctuating by changing C, Si, Mn, Ni, etc.)_Three The result of investigating the relationship between recrystallization temperature Tre is shown.
As shown in the figure, 637.5 + 4930 {Ti^* ＋ (48/93) ・ [% Nb]} ＞ A₁ Under the conditions, the recrystallization temperature Tre is A_Three Is equivalent to
[0026]
Although this reason is not necessarily clear, it can be considered as follows.
That is, Ti and Nb are added, and the recrystallization temperature rises due to the pinning force of these fine carbonitrides.₁ If recrystallization is not possible in the following ferrite (α) region, the non-recrystallized processed α (ferrite + austenite (γ)) will be at the two-phase region temperature, giving priority to high dislocation density parts, non-uniform deformation parts, etc. At the nucleation site, competition between recrystallized α nucleation from the processed α and α → γ transformation nucleation occurs. At this time, since the driving force of the γ transformation is larger, the γ nuclei are generated one after another in preference to the recrystallization α nucleation and occupy the preferential nucleation site.
[0027]
Strain (dislocation) is consumed by the atomic rearrangement in this γ transformation, and only the processed α having a low dislocation density remains, and recrystallization of the processed α becomes more difficult. A_Three It is not until the gamma single-phase region is exceeded that the strain is completely eliminated, and apparently recrystallization is completed. This means that the recrystallization temperature is A_Three Is considered to be a mechanism that saturates.
Note that the α → γ transformation at this time nucleates from the processed α (many preferential nucleation sites), so that the γ grains at a high temperature at which recrystallization is completed are refined. Therefore, the recrystallization temperature is set to A for refining high-temperature γ grains during annealing._Three Therefore, in the present invention, Ti and Nb satisfying the formula (1) are added.
[0028]
Next, equation (2) is expressed as A_Three It is a condition that prescribes.
As described above, if the expression (1) is satisfied, A_Three Is substantially at the recrystallization temperature, A_Three It is necessary to perform recrystallization annealing at the above temperature. Where A_Three When the temperature was 860 ° C. or higher, the recrystallization annealing temperature had to be applied at a higher temperature, and the γ grain growth was severe, and the crystal grain size was increased. Therefore, A_Three <860 ° C must be satisfied.
[0029]
  Next, (3), (3) 'The equation is a condition that defines the amount of Mn or Ni, that is, the amount of austenite stabilizing element added.
  As the austenite stabilizing element increases, the ferrite start line in the CCT diagram shifts to the low temperature side, which increases the degree of supercooling during the γ → α transformation in the cooling process after annealing and causes α to form fine nuclei. , Α crystal grains can be refined. To get this effect, at least MnContent of 1.3 % Or more Ni When containing multiple MnAnd the total content of Ni and 1.3% or more,[% Mn] ≧ 1.3 (%) Or [% Mn] + [% Ni] ≧ 1.3 (%) is required.
  YoMore preferably[% Mn] ≧ 1.5 (%) Or The range is [% Mn] + [% Ni] ≧ 1.5 (%).
[0030]
  Next, manufacturing conditions will be described.
  The steel adjusted to the above-mentioned preferred component composition is melted in a converter or the like, and is made into a slab by a continuous casting method or the like. After heating this steel material to 1200 ℃ or higher, Ar_ThreeAfter the hot rolling is finished at the point or more, and then rolled up in a temperature range of 750 ° C. or lower, 650 ° C. or higher, pickled, cold-rolled, and the above formula (6)Or (6) ' formulaTemperature A required by_Three(° C) or more, (A_Three+30) (° C) or less, then recrystallized annealing, then formula (5)Or (Five)' formulaTemperature A required by₁Cool to + 50 ° C at a rate of 5 ° C / s or more (A₁+50) to (A₁After holding for 60 seconds or more in a temperature range of +20) ° C., cool to at least the Ms point at a rate of 3 ° C./s or more. Here, the Ms point indicates the temperature at the start of martensitic transformation, and can usually be obtained by the following equation (7).
  Ms point (° C) = 561−474 × [% C] −33 × [% Mn ”−17 × [% Ni]
                −17 × [% Cr] −21 × [% Mo] --- (7)
    However, [% M] is the content of M element (mass%)
[0031]
In the above process, when the heating temperature of the slab, which is a steel material, is less than 1200 ° C, TiC and the like are coarsened without being sufficiently dissolved, and the effect of increasing the recrystallization temperature and suppressing grain growth in the subsequent recrystallization annealing process Since the effect becomes insufficient, the heating temperature of the slab was set to 1200 ° C. or higher.
In addition, the finish rolling temperature is set to avoid the band-like structure, to ensure homogeneity and to improve stretch flangeability._ThreeThe temperature needs to be higher than the point.
[0032]
In the winding process after the end of hot rolling, the coil is cooled to 650 to 750 ° C. and wound, during which γ → α transformation is caused and TiC and NbC are finely precipitated. Here, when the coiling temperature is less than 650 ° C, the desired precipitates mainly composed of Ti and Mo carbides cannot be obtained. On the other hand, when the coiling temperature exceeds 750 ° C, pearlite transformation proceeds and is necessary for Ti and Mo carbides. Since carbon is taken, the desired precipitate cannot be obtained.
In addition, after finishing rolling, it is desirable to cool at a rate of 20 ° C./s or more in order to suppress precipitation of pearlite and finely precipitate TiC and NbC until the coiling temperature.
[0033]
Next, after removing the oxidized scale on the surface of the hot-rolled sheet by pickling, it is subjected to cold rolling to obtain a cold-rolled steel sheet having a predetermined thickness. Here, pickling conditions and cold rolling conditions are not particularly limited, and may be according to a conventional method.
The rolling reduction during cold rolling is desirably 40% or more from the viewpoint of increasing the number of nucleation sites during recrystallization annealing and promoting the refinement of crystal grains. On the other hand, if the rolling reduction is increased too much, Since operation becomes difficult due to work hardening, the upper limit of the rolling reduction is preferably about 90% or less.
[0034]
  Next, the obtained cold-rolled steel sheet is represented by the above formula (6).Or (6) ' formulaTemperature A shown in_Three(℃) or more, (A_Three+30) Heat to below (° C.) and perform recrystallization annealing.
  In the steel material of the present invention whose components are adjusted as described above, A_Three Is equivalent to the recrystallization temperature._Three If the temperature is less than 1, recrystallization is insufficient. On the other hand, (A_Three When the temperature exceeds +30) (° C.), the growth of γ grains during annealing is intense, and fine TiC and NbC precipitates precipitated during winding after hot rolling are coarse, which is inappropriate.
[0035]
In the present invention, as will be described later, the manufacturing conditions are defined for the purpose of controlling the cooling rate from the annealing temperature to obtain a structure having a second phase such as ferrite and martensite as the main phase. Here, the fine TiC or NbC plays an important role in improving the strength (precipitation strengthening) by uniformly dispersing and precipitating in the main phase ferrite. Thereby, the strength difference between the second phase (martensite) and the main phase is reduced to improve the molding. Further, in the situation where fatigue failure occurs, the initial crack generation is suppressed by strengthening the ferrite itself, and even if the fatigue crack is generated, its propagation is suppressed and a significant improvement in fatigue characteristics is brought about.
[0036]
This recrystallization annealing is preferably performed in a continuous annealing line, and the annealing time for continuous annealing is preferably about 10 seconds to 120 seconds at which recrystallization occurs. This is because recrystallization is insufficient in a time shorter than 10 seconds, and a structure that remains stretched in the rolling direction remains, so that sufficient ductility may not be ensured. This is because the crystal grains are coarsened and a desired strength may not be obtained.
[0037]
Next, the temperature (A₁ Cool to +50) ° C at a rate of 5 ° C / s or more (A₁ +50) to (A₁ After holding for 60 seconds or more in a temperature range of +20) ° C., cool to at least the Ms point at a rate of 3 ° C./s or more.
In order to achieve high strength and good fatigue characteristics, it is effective to have a hard second phase such as martensite with ferrite as the main phase as described above. On the other hand, as in the present invention, when recrystallized during recrystallization annealing, since it is a fine grain, the C diffusion distance between the intragranular → grain boundary is short, and so-called baking is difficult to enter. In particular, in a component system with few quenching promoting elements such as Mn, Mo, Cr, it is necessary to cool at a high cooling rate to the Ms point or lower in order to secure a desired hard second phase such as martensite.
[0038]
Therefore, in the present invention, after recrystallization annealing is performed under the above conditions, the temperature is the low temperature side temperature of the ferrite-austenite two-phase region (A₁ +20) to (A₁ By maintaining the temperature in the temperature range of +50) ° C, the carbon and alloy elements are concentrated in austenite to improve the hardenability, making it easier to obtain a hard phase such as martensite even under relatively mild cooling conditions, and fatigue characteristics. It will improve.
Here, the holding temperature is (A₁ When the temperature is lower than +20) ° C., the austenite fraction becomes too small and sufficient martensite is not formed, so that it is difficult to obtain good fatigue characteristics. The holding temperature is the temperature (A₁ If the temperature exceeds +50) ° C., the ferrite fraction is low, the concentration of C in the austenite becomes insufficient, sufficient martensite is not formed, and it is difficult to make fatigue characteristics sufficiently good.
Therefore, after recrystallization annealing, (A₁ +20) to (A₁ It was decided to keep it in the temperature range of +50) ° C.
Also, if the holding time in this temperature range is less than 60 seconds, the transformation from austenite to ferrite may not be completed, resulting in austenite lacking carbon and alloy elements, and it is difficult to obtain martensite even after cooling. Therefore, the holding time was set to 60 seconds or more.
In addition, as the holding method in the said temperature range, what is necessary is just to carry out suitably by making cooling gradually slow cooling, letting it cool, or adding heating positively. In addition, the holding time is (A₁ +20) to (A₁ +50) Means the time of substantial residence in the temperature range of ° C.
[0039]
After recrystallization annealing, (A₁ When the cooling rate to +50) ° C. is less than 5 ° C./s, the degree of supercooling in the austenite-ferrite transformation is small, and the ferrite becomes coarse before reaching the temperature range. Therefore, after recrystallization annealing (A₁ The cooling rate to +50) ° C. was set to 5 ° C./s or more.
Further, after holding in the temperature range, the martensite is formed by cooling to the Ms point or less. In the production method of the present invention, after annealing, holding is performed in a low temperature region of a two-phase region to promote C concentration in austenite.₁ Even if the cooling rate from +20) ° C. to at least the Ms point is a relatively slow cooling rate of 3 ° C./s or more, martensite can be formed and the fatigue characteristics can be improved. When the cooling rate is less than 3 ° C./s, it is difficult to improve the fatigue characteristics even if the intermediate holding is performed as described above. Therefore, (A₁ The cooling rate from +20) ° C. to below the Ms point was 3 ° C./s or more.
[0040]
【Example】
A slab having the component composition shown in Table 1 was heated under the conditions shown in Table 2 and hot-rolled according to a conventional method to obtain a 4.0 mm thick hot-rolled sheet. All finish rolling temperatures are Ar._ThreeIt was above the point. This hot-rolled sheet is pickled and cold-rolled (rolling ratio: 60%) to form a 1.6 mm-thick cold-rolled sheet, and then subjected to recrystallization annealing under the conditions shown in Table 3 in a continuous annealing line. And made a product plate.
Table 4 shows the results of examining the structure, strength, fatigue characteristics and formability of the product plate thus obtained.
[0041]
In addition, the structure | tissue was investigated using the optical microscope or the electron microscope about the rolling direction cross section of the steel plate, and also measured the average crystal grain diameter.
Further, the strength (tensile strength TS) was measured by a tensile test using a JIS No. 5 test piece collected from the rolling direction of the steel sheet.
Further, the fatigue characteristics were obtained by a double-bending repeated bending test using a test piece of JIS Z 2275 (a flat plate bending fatigue test method for a metal flat plate) having a dimensional shape shown in FIG. At this time, the point at which 10 million cycles were reached was defined as the fatigue limit FL.
In addition, as an evaluation method for formability, a hole expansion test defined in JFS-T1001 was adopted. That is, using a punch diameter of 10mmφ, the initial hole (hole diameter: D) with a clearance of 12 ± 1.0%₀ mm) and then expanding the hole with a conical punch with an apex angle of 60 °, the hole diameter D where the crack penetrated the plate thickness₁(mm)
λ = {(D₁ -D₀ ) / D₀ } × 100%
The hole expansion rate λ obtained in (1) was evaluated.
[0042]
[Table 1]

[0043]
[Table 2]

[0044]
[Table 3]

[0045]
[Table 4]

[0046]
Nos. 1 to 3 confirm the change in the structure by changing the time for holding the steel A shown in Table 1 at the two-phase region temperature. In the case of No. 1 with a holding time of 30 seconds, the amount of C enriched in γ for obtaining martensite cannot be secured, and TS is insufficient compared with the cases of No. 2 and 3 holding for 60 seconds or more. is doing. Therefore, it is understood that 60 seconds or more should be secured at the two-phase temperature in order to secure TS.
[0047]
No. 2 and Nos. 4 to 8 have different coiling temperatures after hot rolling. Looking at the fatigue limit TS ratio (FL / TS), a high level is obtained when the coiling temperature is 650 to 750 ° C., and the fatigue characteristics are excellent. This is because TiC and NbC are precipitated and the ferrite grains are strengthened at the time of winding, so that fatigue crack generation and crack propagation are suppressed. Therefore, the winding temperature is preferably 650 to 750 ° C.
[0048]
No. 2 and No. 9, 10 are (A₁ +50) to (A₁ The temperature maintained in the two-phase region of +20) ° C. was changed, but the one held at a higher temperature had less martensite and increased bainite instead. The existence ratio in the high temperature part of the two-phase region is γ> α, but it is considered that γ at this time was less likely to cause martensitic transformation because the concentration of carbon and alloy components was less than that in the low temperature region. .
[0049]
Nos. 11 to 13 are obtained by changing the annealing temperature._Three When steel A having a point of 841 ° C. was annealed at 900 ° C., the obtained particle size became coarse and the strength decreased. On the other hand, when annealed at 800 ° C., the recrystallization was not completed and the processed structure remained as it was. This point, A_Three In the invention example No. 11, which was annealed just above the point, neither coarsening of crystal grains nor remaining of the processed structure was observed.
[0050]
No. 14 is a steel type B having a different composition from that of steel type A, without the retention treatment in the two-phase region. Under these conditions, TS was low and the fatigue characteristics deteriorated due to the formation of pearlite.
Nos. 15 and 16 have the cooling rate from the end of hot rolling to the winding up to 20 ° C / s and 30 ° C / s. In either case, the formation of pearlite is suppressed and TiC is reduced. As a result of precipitation, FL / TS was high and good fatigue characteristics were obtained.
No.17 and 18 are (A₁ Although the cooling rate was changed from +20) ° C. to the Ms point or lower, No. 18 with a slow cooling rate was not able to obtain martensite, and TS and fatigue characteristics were deteriorated. From this, the two-phase region, ie (A₁ It can be seen that the cooling rate from +20) ° C. to at least the Ms point is preferably 5 ° C./s or more.
No.19 is (A₁ The cooling rate was lowered to +50) ° C., but the decrease in the cooling rate resulted in unnecessarily coarsening and increased crystal grains. Therefore, the homogeneity of the particle diameter deteriorated, and the hole expandability was lowered. From this, it is understood that the cooling rate to the two-phase region is appropriately 5 ° C./s or more.
[0051]
No. 20 when Nb only is added, No. 21 is when only Ti is added, No. 22 is when Cr is added to the steel A base, and No. 23 is when Ti and Nb are not added. is there. All of these were produced under the same production conditions as No. 7.
In No. 20 and No. 21, TiC or NbC precipitated due to the effect of Ti or Nb, which caused an increase in recrystallization temperature, resulting in refinement of crystal grains and improvement in fatigue characteristics.
In No. 22, the addition of Cr, an austenite stabilizing element, improved the martensite fraction and slightly increased the hole expansion rate, but improved strength.
In this respect, No. 23 did not produce TiC or NbC, so the crystal grains became coarse during annealing, leading to a decrease in strength.
[0052]
【The invention's effect】
Thus, according to the present invention, a high-tensile cold-rolled steel sheet having a fine grain structure and excellent fatigue characteristics can be stably produced without significant modification of the production equipment. Useful.
[Brief description of the drawings]
[Figure 1] A₁ = 700 ° C, A_Three FIG. 5 is a graph showing the relationship between the amount of Ti and Nb added and the recrystallization temperature when the amount of Ti and Nb added is variously changed in the steel composition adjusted to 855 ° C.
[Fig.2] 637.5 + 4930 {Ti^* ＋ (48/93) ・ [% Nb]} ＞ A₁ Under the conditions of A_Three A with various changes_Three It is the figure which showed the relationship between recrystallization temperature Tre.
FIG. 3 is a diagram showing the shape and dimensions of a fatigue test piece.

Claims (3)

% By mass
C: 0.03-0.16%,
Si: 2.0% or less,
Mn: 3.0% or less under,
Ti: 0.01-0.2% and / or Nb: 0.01-0.2%,
Al: 0.01 to 0.1%,
P: 0.1% or less,
S: 0.02% or less and N: 0.005% or less, and C, Si, Mn, Ti and Nb are contained within the ranges satisfying the following formulas (1), (2) and (3) respectively, and the balance is Fe and After heating the steel material, which has an inevitable impurity composition, to 1200 ° C or higher, the hot rolling is finished at ₃ points or more of Ar, and then rolled up in the temperature range of 750 ° C or lower and 650 ° C or higher, and pickled. After cold rolling, recrystallization annealing is performed at the temperature A ₃ (° C.) or higher and (A ₃ +30) (° C.) or lower obtained by the following equation (6), and then the temperature obtained by the following equation (5). Cool to A ₁ + 50 ° C at a rate of 5 ° C / s or higher, and then hold at a temperature range of (A ₁ +50) to (A ₁ +20) ° C for 60 seconds or more, then at least 3 ° C / s or more to the Ms point. A method for producing a cold-rolled steel sheet having a fine grain structure and excellent fatigue characteristics, characterized by cooling at a speed.
Record
637.5 + 4930 {Ti ^* + (48/93) ・ [% Nb]}> A ₁ --- (1)
A ₃ <860 --- (2)
[% Mn ] ≧ 1.3 --- (3)
However,
Ti ^* = [% Ti] − (48/32) ・ [% S] − (48/14) ・ [% N] --- (4)
A ₁ = 727 + 14 [% Si] -28.4 [% Mn ] --- (Five)
A ₃ = 920 + 612.8 [% C] ² − 507.7 [% C] + 9.8 [% Si] ³
−9.5 [% Si] ² +68.5 [% Si] +2 [% Mn] ² −38 [% Mn]
+ 102 [% Ti] +51.7 [ % Nb] --- (6)
[% M] is the content of M element (mass%) % By mass
C: 0.03 ~ 0.16 %,
Si : 2.0 %Less than,
Mn : 3.0 %Less than,
Ni : 3.0 %Less than,
Ti : 0.01 ~ 0.2 % And / or Nb : 0.01 ~ 0.2 %,
Al : 0.01 ~ 0.1 %,
P: 0.1 %Less than,
S: 0.02 % And below
N: 0.005 %Less than
And C, Si, Mn, Ni, Ti and Nb Is (one two Three)' Each of the formulas is contained within the range that satisfies each, and the balance is Fe And steel material that becomes a composition of inevitable impurities, 1200 After heating above ℃ A r _Three Hot rolling is finished at the point or more, then 750 ℃ or less, 650 Winding in the temperature range above ℃, pickling, and after cold rolling, (6) ' Temperature A determined by the formula _Three (° C) or more, (A _Three + 30 ) Recrystallization annealing at (° C) or less, then (Five)' Temperature A determined by the formula ₁ + 50 Up to 5 ℃ / s Cool at the above speed, then (A ₁ + 50 ) ~ (A ₁ + 20 ) In the temperature range of ℃ 60 Hold for more than a second, then at least M s 3 ℃ to the point / s A method for producing a cold-rolled steel sheet having a fine grain structure and excellent fatigue characteristics, characterized by cooling at the above speed.
Record
637.5 + 4930 { Ti ^* +  (48/93) ・ [% Nb] }> A ₁ --- (1)
A _Three <  860 --- (2)
[% Mn] +  [% Ni] ≧  1.3 --- (3) '
However,
Ti ^* =  [% Ti] −  (48/32) ・ [% S ] −  (48/14) ・ [% N ] --- (Four)
A ₁ =  727 + 14 [% Si] − 28.4 [% Mn] − 21.6 [% Ni] --- (Five)'
A _Three =  920 +  612.8 [% C ] ² −  507.7 [% C ] +  9.8 [% Si] ^Three
−  9.5 [% Si] ² +  68.5 [% Si] +2 [% Mn] ² −  38 [% Mn]
+  2.8 [% Ni] ² −  38.6 [% Ni] + 102 [% Ti] + 51.7 [% Nb] --- (6) '
Also, [% M ] Is M element content (% by mass) The steel material according to claim 1 or 2 , further in mass%,
Mo: 0.1% or less and
Cr: A method for producing a cold-rolled steel sheet having a fine grain structure and excellent fatigue characteristics, characterized by having a composition containing one or two selected from 0.1% or less.

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