JP5097712B2 - High-strength thin steel sheet having excellent plating characteristics and elongation characteristics - Google Patents

Description

  The present invention relates to a thin steel plate mainly used for an automobile inner plate. More specifically, the present invention relates to a deep-working thin steel sheet having a tensile strength of 440 MPa or more and having excellent plating characteristics and elongation characteristics, and a method for producing the same.

  Recently, high-strength steel plates for deep processing, which are mainly used for steel plates for automobile inner plates, have rapidly increased their applications because of their excellent formability. This is because such a deep-working high-strength steel sheet not only has high strength but also has excellent elongation characteristics and the possibility of forming defects during processing is extremely low.

  Such properties are closely related to the recent demand for steel sheets with higher strength and superior formability to actively respond to changing customer demands and preferences in the automotive industry. However, increasing the strength of steel sheets for automobiles leads to deterioration of formability, so it is very difficult to satisfy both strength and formability at the same time, and higher-dimensional manufacturing techniques are required.

  On the other hand, steel plates for automobiles are required to have a clean surface by plating the steel plate surface from an environmental aspect. However, it is already well known that it is difficult to ensure a clean steel plate surface due to impurity elements added for increasing the strength of the steel plate.

  Generally, in order to improve the strength and formability of a steel sheet, it is a high-purity steel in which impurities in the steel are minimized and is a strength improving element (solid solution strengthening element such as Mn, P, Si, etc.) and a workability improving element. Usually, it is produced by adding (carbonitride-forming elements such as Ti and Nb). However, due to the characteristics of steel materials, it is not easy to ensure strength and formability at the same time, and strength-enhancing elements (Mn, Si, etc.) added to improve strength are plated with Mn and Si-based oxides during the annealing process. There are many difficulties in producing a plated steel sheet with a clean surface because it elutes on the surface to reduce the surface properties of the plated steel sheet. After all, the alloy element added for improving the strength usually acts as an element that hinders workability and plating characteristics.

  Usually, in order to produce a thin steel sheet for deep working, the amount of interstitial solid solution elements such as C and N is reduced to 50 ppm or less in the steel making process in order to ensure good formability, and it is a carbonitride forming element separately. It is common to use an extremely low carbon IF (Interstitial Free) steel to which Ti, Nb or the like is added alone or in combination.

  As described above, there is a conventional technique for a manufacturing method of a deep-working thin steel sheet developed mainly by a Japanese blast furnace company in order to manufacture a steel sheet for high strength and high workability having excellent plating characteristics. The parent patent for the method of manufacturing the thin steel sheet for deep processing using the IF steel mentioned above is the world's first Ti-added steel in Yawata, the predecessor of the current NSC, including the Nb-added steel of Armco, USA, and the improved Ti addition of NSC Steel, KSC Ti-Nb composite added steel, etc. In addition to the above parent patents, it is well known that many related patents have been filed worldwide, with some limitations on the components, composition method and production conditions.

  In addition, these common points are generally produced by adding 0.01 to 0.07% of a carbonitride-forming element such as Ti or Nb to ultra-low carbon steel to ensure workability. However, in this case, since there is no interstitial solid solution element that strengthens the grain boundaries in the steel, there is a problem that secondary work brittleness occurs and the work characteristics are not greatly improved.

  On the other hand, the grain boundary strength is further weakened by the solid solution strengthening elements (P, Mn, Si, etc.) added to improve the strength, but conventional techniques for improving this include Patent Document 1, Patent Document 2, Patent. There are Literature 3, Patent Literature 4, and Patent Literature 5. The above prior art improves the above problems by adding about 5 to 10 ppm of B. However, Mn, Si and B are added to the surface of the steel sheet during annealing, so that Mn and Si-based oxides are eluted, so that the characteristics of the plated steel sheet are remarkably deteriorated and it is difficult to produce a plated product with a clean surface. .

Japanese Published Patent Publication No. 5-009587 Japanese Published Patent Publication No. 5-279798 Japanese Published Patent Publication No. 5-214487 Japanese Published Patent Publication No. Hei 6-057373 Japanese Published Patent Publication No. Hei 7-179946

  The present invention is for improving the above-described conventional problems. By appropriately controlling the alloy elements, the present invention has a high strength for deep machining that has excellent strength in plating and elongation while having high strength of 440 MPa or more. The purpose is to provide a thin steel sheet and a manufacturing method.

The present invention for achieving the above object is, in mass% , C: 0.01% or less, Si: 0.3% or less, Mn: 0.03-0.2%, P: 0.15% or less, S: 0.003 to 0.015%, Sol. Al: 0.1-0.4%, N: 0.01% or less, Ti: 0.003-0.01%, Nb: 0.003-0.04%, B: 0.0002-0.002 %, Mo: 0.05% or less, Cu: 0.005 to 0.2%, Cr: 0.05 to 0.5%, Sb: 0.02 to 0.1%, remaining Fe and other inevitable Plating comprising 75% or more of MnS, CuS, (Mn, Cu) S precipitates comprising impurities and containing MnS, CuS, (Mn, Cu) S precipitates, and having a size of 20 nm or less among the above precipitates The present invention relates to a high-strength thin steel sheet having excellent characteristics and elongation characteristics.

Moreover, this invention is the mass% , C: 0.01% or less, Si: 0.3% or less, Mn: 0.03-0.2%, P: 0.15% or less, S: 0.003- 0.015%, Sol. Al: 0.1-0.4%, N: 0.01% or less, Ti: 0.003-
0.01%, Nb: 0.003-0.04%, B: 0.0002-0.002%, Mo: 0.05% or less, Cu: 0.005-0.2%, Cr: 0.00. Re-heat steel slab consisting of 05-0.5%, Sb: 0.02-0.1%, remaining Fe and other inevitable impurities, and hot finish rolling at 880 ° C. or higher which is an austenite single phase region After completion, the steel sheet is wound at 700 ° C. or less, cold-rolled at a rolling reduction of 65% or less, and then continuously annealed in a temperature range of 780 to 830 ° C. It is about the method.

  According to the present invention, the thin steel plate can ensure high strength with a tensile strength of 440 MPa or more and excellent elongation characteristics. In addition, there is an effect that it is possible to provide a high-strength thin steel sheet that suppresses the surface elution of oxide and is excellent in plating characteristics without surface defects.

  Hereinafter, the present invention will be described in detail.

  While the present inventor is studying a method for solving the problem of surface defects due to the addition of Si and Mn, when Sb is appropriately added, the oxide is suppressed from being concentrated and coarsened on the steel sheet surface. I found out that I can do it. That is, adding Sb prevents the oxide from moving to the grain boundary, significantly improves the possibility of surface defects due to Si and Mn, and ensures excellent plating characteristics even when Si and Mn are added. Can do.

  The present invention also relates to Sol. It is characterized in that deep workability can be stably secured even at a relatively low annealing temperature by appropriately controlling Al. That is, Sol. Al affects the formation behavior of Ti and Nb-based precipitates, which are carbonitrides, and coarsens the size of the precipitates to develop a {111} series texture and Ti and Nb compared to normal IF steel. Even if a small amount is added, better processability can be secured.

  Therefore, the present invention provides a steel plate with Sol. By appropriately adding Al, it is possible to improve the deep workability and secure a steel sheet having excellent formability. First, the composition range of the steel component of the present invention will be described.

  C: 0.01% or less is preferable. C is an interstitial solid solution element that inhibits the formation of {111} texture that is advantageous for workability in the process of forming the texture of the steel sheet during cold rolling and annealing. In addition, when a large amount of C is contained, the amount of Ti and Nb, which are carbonitride forming elements, must be increased, which is economically disadvantageous. Therefore, the C content is 0%. It is preferable to limit to 0.01% or less.

  Si: 0.3% or less is preferable. The Si is a solid solution strengthening element, which is advantageous in terms of strength improvement. However, since Si-based oxides are eluted on the surface during annealing to deteriorate the plating surface characteristics, it is preferable to add as little as possible. In order to secure the target strength, the Si content is preferably limited to 0.3% or less.

  Mn: 0.03 to 0.2% is preferable. The Mn is known as an element that precipitates solute S in steel as MnS and prevents hot shortness due to the solute S. In the present invention, the Mn and S contents are controlled organically, and the Mn content is controlled to 0.03 to 0.2% so that very fine MnS can be precipitated. The direction has been greatly improved. If the content is less than 0.03%, it is difficult to ensure the above effects, whereas if it exceeds 0.2%, coarse MnS precipitates may be formed and the aging resistance may be deteriorated. Since it is high, the Mn content is preferably limited to 0.03 to 0.2%.

  P: 0.15% or less is preferable. The above-mentioned P is a typical solid solution strengthening element added to increase the strength together with Mn. In the Ti-Nb-based component system which is the steel of the present invention, not only the increase in strength but also due to grain refinement and grain boundary segregation, etc. A {111} texture is developed that favors the r value. If it exceeds 0.15%, the brittleness greatly increases as the elongation decreases rapidly, so the P content is preferably limited to 0.15%.

  S: 0.003 to 0.015% is preferable. If the S content is less than 0.003%, the amount of MnS, CuS, (Mn, Cu) S precipitates is small, and the precipitates are very coarse, which is likely to impair strength and aging resistance. . On the other hand, if it exceeds 0.015%, the content of solute S increases, which greatly impairs ductility and formability, and there is a risk of red heat embrittlement. Therefore, the S content is limited to 0.003 to 0.015%. It is preferable.

  Sol. Al: 0.1 to 0.4% is preferable. Sol. Al plays a role of ensuring stable deep workability even at a relatively low annealing temperature while maintaining the amount of dissolved oxygen in the steel sufficiently low. That is, the Sol. Al coarsens the size of (Ti, Nb) C precipitates and promotes recrystallization by acting to inhibit the recrystallization suppression effect of P, and develops a {111} series texture. In the present invention, the Sol. Al affects the formation behavior of Ti and Nb-based precipitates, which are carbonitrides, and roughens the size of the precipitates so that better processing can be achieved even if a small amount of Ti and Nb is added compared to normal IF steel. Sex can be secured. When the content is less than 0.1%, the above effect cannot be ensured. On the other hand, when the content exceeds 0.4%, the cost increases and the operability of continuous casting decreases. The content of Al is preferably limited to 0.1 to 0.4%.

  N: 0.01% or less is preferable. When the N is present in a solid solution state, the workability is greatly reduced, and if it exceeds 0.01%, the addition amount of Ti and Nb for fixing with precipitates must be increased. It is preferable to limit it to 01% or less.

  Ti: 0.003 to 0.01% is preferable. If the Ti content is less than 0.003%, the remaining nitrogen that is not precipitated by AlN cannot be effectively precipitated, so that an aging phenomenon may occur and the processing surface may be inferior. high. On the other hand, when it exceeds 0.01%, Ti-based oxide is eluted in the surface layer during plating and deteriorates the plating surface characteristics. Therefore, the Ti content is preferably limited to 0.003 to 0.01%.

  Nb: 0.003 to 0.04% is preferable. If the Nb content is less than 0.003%, the solid solution elements (Mn, Si, etc.) present in the steel cannot be effectively controlled and the workability can be deteriorated. When it exceeds 0.04%, not only an increase in manufacturing cost but also unprocessed solid solution Nb can return workability and be inferior. Therefore, the Nb content is preferably limited to 0.003 to 0.04%.

  B: 0.0002 to 0.002% is preferable. B is a grain boundary strengthening element, and is a useful element that can improve fatigue characteristics of spot welds and prevent P grain boundary brittleness. When the content is less than 0.0002%, it is difficult to ensure the above effect, but when it exceeds 0.002%, the workability is drastically reduced and the surface properties of the plated steel sheet are deteriorated. The B content is preferably limited to 0.0002 to 0.002%.

  Mo: 0.05% or less is preferable. The Mo is an element that improves the secondary work brittleness resistance and plating properties, and when the content exceeds 0.05%, the above effect is greatly reduced and economically disadvantageous. The content is preferably limited to 0.05% or less.

  Cu: 0.005 to 0.2% is preferable. The Cu increases the strength of the steel sheet. When the content is less than 0.005%, it is difficult to ensure the target strength in the present invention. On the other hand, when the content exceeds 0.2%, Cu is returned. The system precipitates are coarsened and are not so advantageous in terms of improving the strength, and the manufacturing cost increases. Therefore, the Cu content is preferably limited to 0.005 to 0.2%.

  Cr: 0.05 to 0.5% is preferable. The Cr acts as an element for improving elongation properties by forming Cr-based carbide (CrC) during annealing and precipitating solute C present in the steel. If this content is less than 0.05%, sufficient CrC cannot be precipitated and the workability is inferior. On the other hand, if it exceeds 0.5%, it is disadvantageous in terms of economy. Is preferably limited to 0.05 to 0.5%.

  Sb: 0.02 to 0.1% is preferable. The Sb is a very important component in the present invention, and is a necessary component to be added in order to ensure excellent plating characteristics. The Sb improves plating characteristics by preventing Si and Mn oxides from being eluted on the steel sheet surface during annealing.

  That is, after hot rolling, the Sb segregates mainly at the crystal grain boundary, blocks the movement path of Mn and Si oxides through the crystal grain boundary, and ensures excellent plating characteristics. When the content is less than 0.02%, there is almost no effect of suppressing the passage of Mn and Si oxides. On the other hand, when the content exceeds 0.1%, excessive Sb exists in a solid solution state and inhibits the elongation characteristics of steel. Therefore, the Sb content is preferably limited to 0.02 to 0.1%.

  The steel sheet of the present invention contains MnS, CuS, (Mn, Cu) S precipitates, and contains 75% or more of MnS, CuS, (Mn, Cu) S precipitates having a size of 20 nm or less among the above precipitates. . When the size of the precipitate exceeds 20 nm, the strength cannot be greatly ensured, and even when the amount of the precipitate is less than 75%, the target strength in the present invention cannot be ensured. Therefore, it is preferable to limit the precipitate having a size of 20 nm or less to 75% or more.

  In the steel having the above composition, Ti, Al, and N further have a relationship of 5.2 ≦ (Ti / 3.42N) + (Al / 1.92N) ≦ 21.1 and the Nb, Cr, and C 1.2 ≦ (Nb / 7.75C) + (Cr / 4.3C) ≦ 12.1 is satisfied, and Mn, Cu and S are 6.7 ≦ (Mn / 1.7S) + (Cu / 1.96S) ≦ 14.6 can be satisfied.

  That is, in the present invention, the relational expressions of Ti and Nb are presented in order to ensure excellent aging resistance, drawability, elongation, and plating characteristics. The relational expressions of the present invention will be described below. .

  It is preferable that Ti, Al and N satisfy the relationship of 5.2 ≦ (Ti / 3.42N) + (Al / 1.92N) ≦ 21.1.

  N added to the steel is usually precipitated with TiN and AlN to improve the workability of the steel. Therefore, if the contents of Ti and Al are not sufficient, an aging phenomenon due to solute N occurs, and there is a high possibility that the squeezability is reduced. However, when the solid solution Ti in the steel is more than a certain amount, there is a high possibility that the stretchability during processing is lowered and the plating characteristics are also lowered. That is, when the above relational expression is less than 5.2, an aging phenomenon is likely to occur and the drawability is deteriorated. On the other hand, when it exceeds 21.1, the stretchability is lowered and the plating characteristics are deteriorated. The above relational expression is preferably limited to 5.2 to 21.1.

  It is preferable that the Nb, Cr, and C satisfy the relationship of 1.2 ≦ (Nb / 7.75C) + (Cr / 4.3C) ≦ 12.1.

  The above relational expression is an empirical formula for stably ensuring deep workability and stretchability. When the value is less than 1.2, the scavenging of C in the steel is incomplete and the squeezability is reduced. On the other hand, if it exceeds 12.1, the solid solution Nb and Cr contents in the steel increase, and not only the manufacturing cost increases but also the stretchability greatly decreases. It is preferable to limit.

  In the present invention, in order to control the size of MnS, CuS, (Mn, Cu) S precipitates, the above relational expression of Mn, Cu, S is presented. The explanation is as follows.

  It is preferable that the Mn, Cu and S satisfy the relationship of 6.7 ≦ (Mn / 1.7S) + (Cu / 1.96S) ≦ 14.6.

  The steel sheet of the present invention that satisfies the above relational expression contains 75% or more of MnS, CuS, (Mn, Cu) S precipitates having a size of 20 nm or less, and ensures the target strength in the present invention. it can. When the value of the above relational expression is less than 6.7, there is almost no precipitation effect, and it is difficult to ensure the target strength in the present invention, whereas when it exceeds 14.6, a large amount of coarse precipitates are present. The above relational expression is preferably limited to 6.7 to 14.6 in order to form and inhibit strength improvement.

  Hereinafter, it demonstrates in detail with respect to the manufacturing method of the thin steel plate which has steel comprised as mentioned above.

  First, after the steel slab having the above composition is reheated, the hot finish rolling is finished at 880 ° C. or more which is an austenite single phase region.

  When the hot finish rolling temperature is less than 880 ° C, the hot finish rolling temperature is preferably limited to 880 ° C or higher because there is a high possibility that the hot finish rolling temperature is a two-phase region that is not an austenite single-phase region.

  Thereafter, the film is wound at 700 ° C. or lower and cold-rolled at a cold reduction rate of 65% or lower. When the coiling temperature exceeds 700 ° C., the precipitate is very coarse and contributes little to improving the strength. Therefore, the coiling temperature is preferably limited to 700 ° C. or less.

  In addition, when cold rolling is performed at the above rolling reduction, the r value, which is an evaluation index of workability, can be increased. On the other hand, when it exceeds 65%, the work roll trouble (Roll) is high and work troubles occur. Therefore, the cold rolling reduction is preferably limited to 65% or less.

  Next, the cold-rolled steel sheet is continuously annealed in a temperature range of 780 to 830 ° C. When the annealing temperature is less than 780 ° C., the elongation characteristics are likely to be deteriorated. When the annealing temperature exceeds 830 ° C., there is a very high risk of problems such as strip-through properties in operation due to high temperature annealing. In addition, the annealing temperature is preferably limited to 780 to 830 ° C. in order to increase the possibility of surface elution of Si and Mn oxides and deteriorate the plating characteristics.

  Moreover, the steel of the present invention that has been continuously annealed can be alloyed by a conventional method.

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

[Example]
A steel slab composed as shown in Table 1 below was reheated at 1200 ° C. and finish hot rolled at 910 ° C., which is higher than the Ar ₃ transformation point. Next, after rolling under the production conditions shown in Table 2 below, cold rolling was performed, and the steel was heated to a soaking temperature at a rate of 10 ° C./second at the continuous annealing temperature shown in Table 2 below and maintained for 40 seconds.

The obtained annealed plate was processed with a standard specimen according to the ASTM standard (ASTM E-8 stan-dard) in order to examine the mechanical properties. Coupons tensile tester (INSTRON Corporation, Model 6025) yield strength by utilizing tensile strength, elongation, plastic anisotropy index _{(r m} values), measured in-plane anisotropy index (△ r value) did.

Here, r _m = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4, Δr = (r ₀ -2r ₄₅ + r ₉₀ ) / 2. Evaluation of secondary work brittleness resistance (DBTT) is a method of measuring a ductile-brittle transition temperature (DBTT) by dropping a weight on a side of a cup molded under a processing ratio of 1.9. evaluated.

  The plating characteristics were evaluated by the powdering grade by measuring the peel width (mm).

  Table 2 shows the mechanical properties, secondary work brittleness resistance and plating characteristics of the steels of the present invention and comparative steels.

上記表１及び２で示したように、本発明の成分範囲を満たす発明鋼（１〜５）を利用して本発明の製造方法により製造された発明材（１〜９）の場合、７５％以上の２０ｎｍ以下の微細析出物を形成して引張強度４４０ＭＰａ以上の高強度を確保し、伸び率３４％以上、塑性異方性指数（ｒ値）１．８８以上等比較鋼対比伸び特性が２〜３％向上された。また、−４０℃以下の耐２次加工脆性を確保し、剥離幅も４〜５ｍｍで比較鋼対比優れたメッキ特性を確保した。

As shown in Tables 1 and 2 above, in the case of the inventive material (1-9) manufactured by the manufacturing method of the present invention using the inventive steel (1-5) satisfying the component range of the present invention, 75% The above fine precipitates of 20 nm or less are formed to ensure a high strength of a tensile strength of 440 MPa or more, an elongation of 34% or more, a plastic anisotropy index (r value) of 1.88 or more, etc. Improved by ~ 3%. Moreover, the secondary process brittleness resistance of -40 degrees C or less was ensured, and the peeling width was 4-5 mm, and the plating characteristic excellent in comparison steel was ensured.

  However, in the case of the comparative materials (10 to 13) manufactured using the comparative steels (1 and 2) that do not satisfy the component range of the present invention, a large amount of Si and Mn which are solid solution strengthening elements are added. Although the target tensile strength was ensured by the invention, the composition range of the present invention was not satisfied and an inferior elongation characteristic was exhibited. Further, the comparative steel was Sb-unadded steel, and the comparative materials (10 to 13) showed inferior plating characteristics from Si and Mn added in a large amount.

Claims (2)

In mass%, C: 0.01% or less, Si: 0.3% or less, Mn: 0.03-0.2%, P: 0.15% or less, S: 0.003-0.015%, Sol. Al: 0.1-0.4%, N: 0.01% or less, Ti: 0.003-0.01%, Nb: 0.003-0.04%, B: 0.0002-0.002 %, Mo: 0.05% or less, Cu: 0.005 to 0.2%, Cr: 0.05 to 0.5%, Sb: 0.02 to 0.1%, and the remaining Fe and other inevitable Comprising MnS, CuS, (Mn, Cu) S precipitates, 75% or more of MnS, CuS, (Mn, Cu) S precipitates having a size of 20 nm or less among the precipitates,
The Ti, Al, and N satisfy the relationship of 5.2 ≦ (Ti / 3.42N) + (Al / 1.92N) ≦ 21.1, and the Nb, Cr, and C satisfy 1.2 ≦ (Nb / 7.75C) + (Cr / 4.3C) meet the relationship of ≦ 12.1,
A high-strength thin steel sheet having excellent plating characteristics and elongation characteristics in which Mn, Cu and S satisfy the relationship of 6.7 ≦ (Mn / 1.7S) + (Cu / 1.96S) ≦ 14.6 . In mass%, C: 0.01% or less, Si: 0.3% or less, Mn: 0.03-0.2%, P: 0.15% or less, S: 0.003-0.015%, Sol. Al: 0.1-0.4%, N: 0.01% or less, Ti: 0.003-0.01%, Nb: 0.003-0.04%, B: 0.0002-0.002 %, Mo: 0.05% or less, Cu: 0.005 to 0.2%, Cr: 0.05 to 0.5%, Sb: 0.02 to 0.1%, remaining Fe and other inevitable After reheating the steel slab made of impurities and finishing the hot finish rolling at 880 ° C. or higher, which is an austenite single phase region, winding at 700 ° C. or lower and cold rolling at a rolling reduction of 65% or lower, Continuous annealing at a temperature range of 780 to 830 ° C,
The Ti, Al, and N satisfy the relationship of 5.2 ≦ (Ti / 3.42N) + (Al / 1.92N) ≦ 21.1, and the Nb, Cr, and C satisfy 1.2 ≦ (Nb / 7.75C) + (Cr / 4.3C) meet the relationship of ≦ 12.1,
The Mn, Cu and S is 6.7 ≦ (Mn / 1.7S) + (Cu / 1.96S) method of producing a high strength thin steel sheet excellent in coating properties and elongation properties satisfying the relationship ≦ 14.6 .