JP4452191B2 - Manufacturing method of high-stretch flange-formable hot-rolled steel sheet with excellent material uniformity - Google Patents

Description

  The present invention is a method for producing a high workability and high strength steel sheet suitable for structural use in the fields of automobiles, transportation equipment, etc., particularly excellent in burring workability and stretch flangeability suitable for automobile underbody parts and material uniformity. The present invention relates to a method for producing a Ti-added high-strength steel sheet that is excellent in the quality of steel.

  Conventionally, since a rolling temperature, a rolling speed, and a cooling rate during hot rolling are different in the width direction and the longitudinal direction, it is difficult to obtain a uniform material over the entire length. Therefore, extremely fine control is required in the hot rolling process. However, performing fine control is one of the factors that reduce yield and the amount of sheet passing per unit time.

  The hot-rolled steel sheet containing Ti has a problem that the strength changes greatly due to a slight change in the cooling rate in the ROT (run-out table), and high operation accuracy is required. This is because in the hot rolling process, the plate passing speed is usually slow until the top reaches the winder, and after the top is wound, the operation is performed with the plate passing speed increased. This is because the material characteristics change in the longitudinal direction in accordance with the change of the plate passing speed in the case of a material having a large dependency.

  Steel sheets excellent in material uniformity and stretch flange formability are disclosed in Patent Documents 1 to 7 and the like.

  However, Patent Document 7 improves the material uniformity by performing an additional heat treatment after hot rolling, and does not mention the material variation in the As hot state. In addition, Patent Documents 1 and 2 disclose techniques for improving the material uniformity of Ti-added high workability and high-strength steel sheets, but both regulate the Si content to be 0.05% or less. It cannot be applied to many steel materials. Patent Document 3 relates to a relatively low strength steel plate having a tensile strength of 540 MPa or less, and is not applicable to a high strength steel type having a strength higher than 540 MPa. Patent Document 4 is an invention related to a Ti-added high-strength hot-rolled steel sheet that is excellent in stretch flangeability as in the present invention, and is similar in that it can contain 1% or more of Si. However, although the example satisfying Mn-Si> 0.9 is only steel type B in Table 1, the finishing temperature in Table 2 is 890 ° C., which is outside the scope of the present invention. Patent Document 4 only mentions that there is little variation in the hole expansion value, and does not mention the stability of the strength due to fluctuations in the cooling rate.

  In Patent Documents 5 and 6, W and / or Mo are essential. In the present invention, it is not always necessary to add these elements.

  In Patent Document 5, in the case of adding Mo alone, the present invention and the components overlap, but both contain Cr and are different from the components of the present invention.

In Patent Document 6, only No. 6 in the example satisfies the C and Si amounts of the present invention, but contains W and is different from the components of the present invention.
JP 2003-138343 A JP 2000-282150 A JP2003-89847 JP 2004-244651 A JP 2002-322541 A JP 2003-321735 A JP 2003-321734 A

  An object of the present invention is to provide a high-strength hot-rolled steel sheet having a tensile strength of 540 MPa or more, which has excellent stretch flangeability and excellent material uniformity in a coil.

  The present inventors added 0.06% or more of Si having excellent effects such as acceleration of precipitation strengthening by suppressing precipitation of cementite, ensuring hole expandability and solid solution strengthening, and does not contain Cr and W, With respect to high workability and high strength hot-rolled steel sheet having a strength of 540 MPa or more, research was conducted on a production method with little material change even if the cooling rate in the ROT in the hot rolling process was changed. As a result, when the relationship between Mn and Si content is satisfied, it has been found that the variation in material in both the width direction and the longitudinal direction becomes small regardless of the cooling rate on the ROT.

  That is, the gist of the present invention is as follows.

(1) By mass%, C: 0.01 to less than 0.08%, Si: 0.06 to 2.0%, Mn: 0.96 to 3.0%, P ≦ 0.10%, S ≦ 0.01%, Al: 0.05 to 0.3%, N ≦ 0.01% , Ti: 0.01 to 0.20%, Mn-Si> 0.9%, and satisfying the formula (1), the remaining temperature when hot rolling the steel consisting of Fe and inevitable impurities Is a method of producing a hot-rolled steel sheet with high stretch flangeability and excellent material uniformity with a tensile strength of 540 MPa or more, characterized by winding at a temperature of 900 ° C or higher and winding at 400 to 600 ° C.

  According to the present invention, it is possible to provide a high-strength hot-rolled steel sheet having excellent stretch flangeability and a stable material in the coil.

  The reasons for limiting the chemical components of steel in the present invention will be described below.

  C (carbon): C is required to be at least 0.01% or more in order to secure a tensile strength of 540 MPa class or more, but is preferably 0.02% or more. However, if it is 0.08% or more, the workability such as the hole expansion rate as an evaluation of stretch flangeability deteriorates, so the upper limit was made less than 0.08%, but preferably 0.06% or less.

  Si (Si): Si is an element that suppresses the formation of cementite and indirectly increases precipitation strengthening, and also has the effect of preventing end face damage during punching by keeping C in the matrix phase in solid solution. In order to ensure high burring properties (hole expansion properties), 0.06% or more of Si is essential. However, if it exceeds 2%, the chemical conversion processability deteriorates, so the upper limit is made 2%, but it is preferably 1.5% or less.

  Mn (manganese): Mn is effective as a solid-solution strengthening element, and by increasing the uniformity of the material by lowering the transformation temperature and causing phase transformation in the wound state after hot rolling, at least 0.96% is necessary. However, since the segregation zone is formed in the steel by microsegregation and the workability is deteriorated, the upper limit is set to 3.0%, preferably 2.0% or less.

  Mn-Si: The difference between Mn and Si is most important in the present invention. Si suppresses the precipitation of cementite to ensure hole expansion and at the same time increases the g / a transformation temperature, facilitates phase transformation on the ROT, and increases the cooling rate dependency of the material. On the other hand, Mn has the effect of lowering the g / a transformation temperature, and has the effect of reducing the dependence of the material on the cooling rate on the ROT. For this reason, by making Mn-Si more than a predetermined value, the transformation on ROT is suppressed so that the phase transformation proceeds after winding, so that the variation of the material with respect to the cooling rate is reduced. ing. Mn-Si> 0.9 is set as an index for preventing material fluctuations within the range of the cooling rate (20-100 ° C./s) that can be changed in the hot strip mill, but preferably Mn-Si> 1.4.

P (phosphorus): P is preferably as little as possible, and if it exceeds 0.1%, the workability and weldability are adversely affected, so 0.1% is made the upper limit.
S (sulfur): S is preferably as less as possible because it is an impurity, and it is easy to combine with Ti and reduce the amount of TiC produced that contributes to high strength, so 0.01% is made the upper limit.
Al (aluminum): Al is added as necessary for deoxidation of molten steel. If added too much, non-metallic inclusions are increased and elongation is deteriorated, so the content is made 0.3% or less. The lower limit is set to 0.005% because it is inevitably contained from refractories.
N (nitrogen): N is preferably as little as possible, and it is preferable to combine with Ti to form coarse TiN in the steel, thereby expanding the hole and reducing the strength, so the upper limit is made 0.01%.
Ti (titanium): Ti binds to C in ferrite and precipitates as TiC to ensure strength, so at least 0.01% or more is necessary. However, as the amount of Ti increases, the slab reheating temperature needs to be higher, which increases the cost.

The formula (1) is a formula showing the ratio of C to the metal element that forms MC type carbide. If C is excessive compared to the metal element, a hard phase such as cementite or pearlite is formed in the steel and deteriorates the hole expanding property. Preferably, it is 1.1 or less. Further, if the metal element is excessive, the growth of MC type carbide is accelerated, the precipitates become coarse and the strength is lowered, so 0.75 or more, preferably 0.9 or more. Nb (niobium), Mo (molybdenum), and V (vanadium), like Ti, are elements that combine with C during hot rolling and contribute to securing the strength through refinement effects and precipitation strengthening. Although it may be added, the alloy cost is high although the precipitation strengthening ability is smaller than Ti, and the precipitate does not remelt unless the slab reheating temperature is raised, so Nb and V are 0.1%, The upper limit of Mo is 0.2%.

Next, the reasons for limiting the production method of the present invention will be described.
In the present invention, when hot-rolling a slab obtained by casting molten steel whose components have been adjusted so as to achieve the desired component content, the heating temperature is preferably 1150 ° C. or higher. This is because when the temperature is lower than 1150 ° C., precipitation constituent elements such as Ti are not sufficiently re-dissolved, and thus precipitation strengthening becomes insufficient. In order to sufficiently exhibit precipitation strengthening, it is desirable that the heating temperature is 1200 ° C. or higher.

  In the present invention, in addition to the case where the slab is reheated after cooling, the slab may be directly sent to a hot rolling mill as it is. Further, it is possible to manufacture by continuous hot rolling in which rough bars are heated and kept warm, or the rough bars are welded to each other and finish-rolled.

  The hot rolling finishing temperature is 900 ° C or higher. If it is less than 900 ° C, TiC precipitates in austenite due to distortion caused by hot rolling, and a strength of 540 MPa or more cannot be obtained. Preferably it is 930 degreeC or more.

  Although there is no particular upper limit, it is desirable that the temperature be set to 1000 ° C. or less because scale soot may occur during operation. When the winding temperature is less than 400 ° C., precipitation after winding becomes insufficient, precipitation strengthening cannot be obtained, and martensite which is harmful to hole expandability may be generated. On the other hand, if the coiling temperature exceeds 600 ° C., precipitates effective for precipitation strengthening such as TiC are coarsened and the strength cannot be secured, and solid solution C in the steel is precipitated as TiC or cementite, and the C concentration in the steel is Reduced and causes end face damage. Therefore, the winding temperature is set to 400 ° C. or more and 600 ° C. or less.

The reason why the tensile strength of the steel sheet of the present invention is defined as 540 MPa or more is that a low-strength steel sheet can achieve a certain level of elongation and hole expansibility without applying the present invention.
The upper limit is not particularly defined, but it is preferably 980 MPa or less in order to obtain sufficient hole expandability.

  A steel having the chemical composition of steel No. 1-9 shown in Table 1 was subjected to a machining formaster test simulating hot working and cooling history in a hot strip mill. Table 1 also shows the value of Mn-Si. The steels having these compositions were tested under the conditions of a rolling finishing temperature FT in the hot rolling process of 950 to 900 ° C. and a winding temperature CT of 400 ° C. to 600 ° C. The cooling rate CR of the portion corresponding to water cooling on the run-out table was changed in the range of 10 ° C / s to 100 ° C / s.

  Table 2 shows the Vickers hardness Hv (load 5 kgf) after testing each of the nine steel types with these heat patterns. Table 3 shows the difference between the maximum value and the minimum value of Vickers hardness at each winding temperature CT in the range of this experiment.

[Table 1]

[Table 2]

[Table 3]

  In steel types with Mn-Si <0.9 (steel No.4, No.7, No.8) such as 1.0% Si-1.0% Mn steel, hardness varies greatly depending on the cooling rate CR, but Mn-Si> 0.9. In steel types satisfying the relationship (steel No. 2, No. 3, No. 5, No. 6, No. 9 to No. 15), the cooling rate dependency is small. Further, in the steel types satisfying Mn-Si> 1.4 (steel No. 3, No. 6, No. 11, No. 12), the range of the cooling rate CR in this experiment (10 ° C./s to 100 ° C. / In s), it can be seen that Hv is within the range of 10 or less (guaranteed range of standard sample). On the other hand, when Mn and Si do not satisfy this relationship, it can be seen that the hardness varies greatly depending on the cooling rate CR.

Claims (1)

% By mass
C: 0.01 to less than 0.08%,
Si: 0.06-2.0%
Mn: 0.96-3.0%
P ≦ 0.10%,
S ≦ 0.01%,
Al: 0.005-0.3%
N ≦ 0.01%,
Ti: 0.01 ~ 0.20% included,
And when hot-rolling steel that satisfies Mn-Si> 0.9% and satisfies the formula (1), the balance being Fe and inevitable impurities, the finishing temperature is set to 900 ° C. or more, and 400 to 600 A method for producing a hot-rolled steel sheet with high stretch flange formability and excellent material uniformity with a tensile strength of 540 Mpa or more, characterized by winding at ℃.