JPH04272155A - High strength hot-rolled steel sheet and production thereof - Google Patents

Abstract

PURPOSE:To relieve rolling load at the time of hot finish rolling and to obtain a hot-rolled sheet having a bainite-based structure, >=80kgf/mm<2> tensile strength and very small anisotropy of stretch flanging property. CONSTITUTION:A steel slab chiefly contg., by weight, 0.08-0.20% C, 1.50-3.50% Mn, 0.50-1.50% Cr and 0.01-0.10% Al is hot-rolled at a finishing temp. between the Ar3 point and the Ar3 point +70 deg.C. Within 6sec after this hot rolling, cooling is started at a proper cooling rate and coiling is carried out at a proper coiling temp. to produce a hot-rolled steel sheet.

Description

Detailed Description of the Invention [0001] [Industrial Application Field] The present invention relates to an anisotropic stretch flange material suitable for relatively lightly processed strength members such as automobile bumpers and door guard bars. The present invention proposes a high-strength hot-rolled steel sheet having an extremely low tensile strength of 80 Kgf/mm2 or more, and a method for manufacturing the same. Steel plates used as strength members for automobiles are required to have various properties in order to reduce the weight of the vehicle and improve safety by further increasing the strength. Furthermore, in recent years, from the viewpoint of economic efficiency, high-strength hot-rolled steel sheets, which are advantageous in terms of cost, have attracted attention in place of conventionally used cooling steel sheets, and the demand for them has been increasing. The main properties required for high-strength hot-rolled steel sheets are:
■ There should be little variation in materials. ■ Small in-plane anisotropy of various mechanical properties. ■ Good spot weldability. ■ As a manufacturing issue, harsh hot rolling conditions are not required. etc. [Prior Art] The most common high-strength hot-rolled steel sheet known to date is a so-called low-alloy steel made by adding about 0.2 wt% or less of Nb, Ti, or V to low-C steel. There is high tensile strength steel (HSLA steel). Although this HSLA steel is relatively easy to manufacture, its high yield ratio makes it difficult to freeze the shape after forming, and because it is a low-alloy steel, it is difficult to obtain a tensile strength of 80 Kgf/mm2 or more. . Furthermore, in addition to the above, there are dual phase steels (DP steels) that have a two-phase mixed structure of ferrite and martensite, and so-called TRIP steels (Transformation industrial steels) that utilize retained austenite.
ced plasticity steel) are also known. However, in the case of the above DP steel, 80Kgf/
In order to obtain a tensile strength of mm2 or more, the manufacturing conditions become harsh, making it difficult to manufacture, and it also requires the addition of large amounts of alloying components, which may lead to shape defects during the manufacturing process. , there will be variations in materials, and an increase in costs will also be inevitable. On the other hand, TRIP steel shows good ductility, but its tensile properties are greatly affected by the phase fraction of the steel, mainly the amount of retained austenite, so it is extremely difficult to make the material uniform in the width and length directions of the steel strip. becomes difficult, and moreover, 0.2 wt% or more,
Desirably, it is essential to add about 0.4 wt% of C,
There are problems such as poor spot weldability under normal conditions. In addition, these DP steels and TRIP steels are also inferior in stretch flangeability and impact resistance after forming, which is a major problem. [0004] Due to the above reasons, bainite steel, which is mainly composed of bainite, is currently attracting attention because it has a high yield ratio but has excellent stretch flangeability and impact resistance after forming. Problems to be Solved by the Invention This invention aims to advantageously solve the above-mentioned problems by optimizing the component composition and manufacturing conditions, and does not require harsh hot rolling and cooling conditions. Moreover, it has a tensile strength of 80K, which is better than conventional products against stretch flangeability, especially cracking in the C direction, of the final product.
The purpose of the present invention is to propose a high-strength hot-rolled steel sheet mainly having a bainite structure of gf/mm2 or more and a method for manufacturing the same. Regarding bainitic steel, the applicant company previously proposed a high-strength hot-rolled steel sheet with good workability and a method for producing the same having a C--Mn--Nb--B composition system in Japanese Patent Application No. 1-192713. However, although the hot-rolled steel sheet proposed by this proposal has various good properties, since it is Nb-added steel,
The rolling load is high and the load on the rolling mill is large;
Because it is necessary to rapidly cool the product immediately after hot rolling, and because most of the hot rolling process is carried out in the unrecrystallized austenite region, the structure of the final product is stretched in the rolling direction, which poses problems particularly in workability in the C direction. There are still some issues that need to be resolved. [Means for Solving the Problems] The present invention is capable of reducing the rolling load during hot rolling by not containing Nb but containing Cr and, if necessary, B. Rolling in the austenite region can be minimized, thereby suppressing ferrite transformation by reducing ferrite formation sites due to a reduction in deformation bands.
This is based on the discovery that it is effective in relaxing the cooling conditions after hot rolling and improving the anisotropy of the material, and the summary is as follows. ■ As the first invention, C: 0.08wt
% or more and 0.20wt% or less, Mn: 1.50wt% or more and 3.50wt% or less, C
r: 0.50wt% or more and 1.50wt% or less, Al
:0.01wt% or more, 0.10wt% or less, P:0
．． A high-strength hot-rolled steel sheet containing S: 0.010 wt% or less and S: 0.001 wt% or less, with the balance being iron and inevitable impurities. ■ As a second invention, in addition to the component composition of the first invention of (■), B: 0.0005 wt% or more 0.0
A high-strength hot-rolled steel sheet containing 10 wt% or less. [0009] As a third invention, as a fourth invention, steel slabs prepared to have the component composition of the first invention as raw material and the component composition as shown in
After hot rolling in a temperature range from the transformation point to the Ar3 transformation point + 70°C, cooling was started within 6 seconds at a cooling rate (V) ℃/S calculated by the following formula (1), and the following formula (2) was applied. A method for producing a high-strength hot-rolled steel sheet, characterized by winding at a winding temperature (T) calculated at °C. (V) ≧15+ 50×[C]-3×[Mn]-2
× [Cr]-15 × [B] … (1
) 513-65×[C]-9×[Mn]-7[Cr
]≧(T) >325-30×[C]-6.5[Mn
]-5×[Cr]
... (2) Here, the symbol in [ ] is the element symbol and represents the content (wt%) of that component in the steel. [Operation] First, the experimental results that formed the basis of this invention will be described below. Two types of steel slabs having the compositions shown in Table 1 were hot-rolled under almost the same conditions conforming to the present invention (slab dimensions and finished dimensions are also the same), and a plate thickness of 2.3 mm with almost equal tensile strength was obtained. A hot rolled sheet was manufactured. [0010] [Table 1] [0011] Regarding the above two types of steel, the rolling load for each finish rolling stand during hot rolling was measured. FIG. 1 shows the relationship between the finish rolling stand No. and the rolling load ratio of Nb-added steel (A steel) and Cr-added steel (B steel...no Nb added). The rolling load ratio here is the rolling load of Nb-added steel/rolling load of Cr-added steel. As is clear from FIG. 1, the rolling load ratio increases in the latter stages of finish rolling, indicating that the rolling load of Cr-added steel is smaller than that of Nb-added steel. Next, side bend elongation (
L direction, C direction) were measured. The results are shown in Table 2. [Table 2] [0015] Here, the side bend elongation is the test piece dimensions: length: 200 mm, width: 40 mm,
Thickness: 2.3 mm, distance between fulcrums: 150 mm, according to the schematic diagram showing the side bend test method in Figure 2.
Assuming that the gauge distance: l0 = 50 mm, the gauge distance: l1 at the time when cracking occurred due to bending was measured, and calculated from the following formula. Side bend elongation (%) = (l1-l0)/l0×
As is clear from Table 2, there is almost no difference in side bend elongation between the L direction and the C direction in steel B with Cr addition (no Nb addition), but in steel A with Nb addition. The value in the C direction has decreased significantly. In this way, by adding Cr and not adding Nb, the rolling load in finish rolling can be reduced compared to the case of adding Nb,
In-plane anisotropy for bending can be significantly improved. [0017] The reason for limiting the composition range of steel and manufacturing conditions will be described below. ■ Steel composition C: 0.08 wt% to 0.20 wt% If the C content is less than 0.08 wt%, the transformation rate of γ→α increases, making it difficult to form a bainite-based structure. Therefore, the target tensile strength cannot be obtained. On the other hand, 0.
If it exceeds 2 wt%, spot weldability will deteriorate. Therefore, its content is 0.08wt% or more and 0.20wt
% or less. Mn: 1.50 wt% to 3.50 wt% Mn is a component that suppresses ferrite transformation and facilitates the appearance of bainite in the cooling process after hot rolling, and is an important component in this invention. If the content is less than 1.5 wt%, high strength cannot be obtained, and if the content exceeds 3.5 wt%, no improvement in material quality commensurate with the cost of addition will be observed. Therefore, its content is set to 1.5 wt% or more and less than 3.5 wt%. [0019] Cr: 0.50wt% to 1.50wt%
Cr is an important component in this invention. By containing it within an appropriate range, ferrite transformation and pearlite transformation are significantly suppressed, and a bainite-based structure can be easily obtained. That is, the content is 0.50wt
%, the effect cannot be obtained, and conversely, if it is less than 1.
If it exceeds 50 wt%, the effect will be saturated and the material quality will not improve enough to justify the cost of addition. Therefore, its content is set to 0.50 wt% or more and 1.50 wt% or less. [0020] Al: 0.01wt% to 0.10wt%
Al is necessary for cleaning steel, and in particular, improving the cleanliness is essential for aiming at high strength. This effect can be obtained when the content is 0.01 wt% or more, but 0.1
If the content exceeds wt%, it is not preferable because it causes surface defects due to alumina clusters. Therefore, its content is 0.01wt% or more and 0.10wt%
The following shall apply. P: 0.010 wt% or less P is a harmful component in the present invention. If it is contained in a large amount, it causes deterioration of spot weldability and deterioration of bending workability, especially in the C direction, due to the formation of ferrite bands due to center segregation. These phenomena become noticeable when the content exceeds 0.01wt%, so the upper limit of the content is set to 0.0%.
It is assumed to be 1wt%. S: 0.001 wt% or less S, like P, is a harmful component. If it is contained in a large amount, it causes deterioration of spot weldability and deterioration of stretch flangeability due to the formation of MnS. In this invention, since a relatively large amount of Mn is contained, it is particularly important to reduce S. Since the deterioration of the above properties becomes noticeable when the content exceeds 0.001wt%, the upper limit of the content is set at 0.0%.
01wt%. [0023] B: 0.0005wt% ~ 0.010
wt%B is a component that facilitates the appearance of a bainite-based structure, and this effect can be obtained when the content is 0.0005wt% or more, but when it exceeds 0.010wt%, the effect is just saturated. However, cracks are more likely to occur during hot rolling. Therefore, its content is 0.0005w
t% or more and 0.010 wt% or less. [0024] Manufacturing conditions A hot-rolled sheet having a bainite-based structure is manufactured using a steel slab that satisfies the above-mentioned composition range.
Although the hot rolling conditions are not significantly different from those in general, some restrictions are necessary. Finish rolling temperature: Ar3 transformation point ~ Ar3 transformation point +7
If the 0°C finish rolling temperature exceeds the Ar3 transformation point +70°C, the final bainite structure will become coarse, which is unfavorable in terms of ductility. Furthermore, when the temperature is below the Ar3 transformation point, a structure stretched in the rolling direction appears significantly, and workability, particularly in the C direction, deteriorates. Therefore, the finish rolling temperature is Ar
3 transformation point or more and Ar3 transformation point +70°C or less. Time until the start of cooling after hot rolling: within 6 seconds Regarding the time until the start of cooling after hot rolling, in this component system, it is not necessary to rapidly cool immediately after hot rolling. However, if the time exceeds 6 seconds, ferrite transformation progresses and the target tensile strength cannot be obtained. Therefore, the time after hot rolling until the start of cooling is within 6 seconds. Cooling rate after hot rolling (V) °C/S:
(V): 15 + 50 × [C] - 3 × [Mn]
-2×[Cr]-15×[B] … 1
Here, the element symbol in [ ] represents the content (wt%) of that component in the steel. In terms of cooling after hot rolling, it is better to keep the cooling rate low from the perspective of preventing variations in material quality and disturbances in the shape of the steel sheet, but if it is too slow, pearlite will appear, making it difficult to achieve the target bainite structure and strength. I won't be able to do it. Therefore, in order to obtain a bainite-based structure and strength, the cooling rate (V) °C/S needs to be calculated using the above formula (1). Here, formula (1) was obtained according to the following procedure. Continuous cooling transformation behavior was investigated for several types of steels whose content satisfies the range of this invention, and the minimum cooling rate at which pearlite transformation does not occur, the so-called upper critical cooling rate, was determined. It was found that there is a correlation between this upper critical cooling rate and C, Mn, Cr, and B among the contained components, and the upper critical cooling rate was subjected to multiple regression using C, Mn, CR, and B. [0028] Winding temperature (T) °C: 513 - 65 × [C
]-9 × [Mn]-7 [Cr]≧T>
325-
30 [C]-6.5 [Mn]-5×[Cr]...
(2) Here, the symbol in parentheses is an element symbol and represents the content (wt%) of that component in the steel. The winding temperature is
This is important for this invention; when the upper limit of the above formula (2) is exceeded, pearlite appears, and below the lower limit, martensite appears. Therefore, in order to obtain the target bainite-based structure, strength, and workability, the winding temperature range (T)° C. needs to be calculated using the above formula (2). Here, equation (2) was obtained according to the following procedure. Continuous cooling transformation behavior was investigated for several types of steels whose content satisfies the invention range, and the upper critical cooling rate was first determined. Next, it was cooled at a rate higher than the upper critical cooling rate and wound up at various temperatures, and the upper limit temperature at which pearlite does not appear and the lower limit temperature at which martensite does not appear were determined. It was found that there is a correlation between these temperatures and C, Mn, and Cr among the contained components, and the upper limit temperature at which pearlite does not appear and the lower limit temperature at which martensite does not appear are determined by C, Mn. , and Cr. [0029] In this invention, the fraction of steel structure is not particularly determined, but in order to ensure strength and good stretch flangeability, it is desirable that the bainite fraction in the steel structure is 85% or more. [Example] Steel slabs prepared with a total of 11 types of compositions, 6 types of compatible steels of the present invention and 5 types of comparative steels, shown in Table 3, were
Hot rolled sheets with a thickness of 2.3 mm were produced by hot rolling under various conditions, and the tensile properties, side bend elongation (L and C directions), structure, etc. of the obtained hot rolled sheets were investigated. [Table 3] [0032] Here, the tensile test was conducted using a JIS No. 5 test piece in the usual manner, and the side bend elongation was
This was done in the same manner as described above. These hot rolling conditions and investigation results are summarized in Table 4. [Table 4] [0034] As is clear from Table 4, all of the examples conforming to the present invention have a tensile strength of 80 Kgf/mm2 or more, and have a difference in side bend elongation values in the L and C directions. The value is also good. Furthermore, in these conforming examples, the strength of the spot welds, which was separately investigated, was also good, and no troubles such as inability to roll occurred during hot rolling. On the other hand, in the comparative example, sample No. 16 was C
Since the amount was as high as 25 wt%, the strength of the spot welded portion was significantly deteriorated, and in sample No. 18, the steel became hard during hot rolling, making it difficult to roll, resulting in shape defects. [0036] According to the present invention, by optimizing the component composition including Cr and the manufacturing conditions, the load on the rolling mill is reduced and the tensile strength is increased to 80 Kgf/mm2.
The above-mentioned high-strength cooling steel sheet can be easily produced, and the hot-rolled sheet obtained by this invention has high strength and excellent stretch flangeability, and has extremely small anisotropy, such as bumpers and other strength members of automobiles. It can be advantageously used for door guard bars, etc.

[Brief explanation of the drawing]

[Figure 1] Finish rolling stand No. in hot rolling and Nb
It is a graph showing the relationship between rolling load ratios of additive steel and Cr-added steel.

FIG. 2 is a schematic diagram showing a side bend test method.

Claims (4)

[Claims] [Claim 1] C: 0.08wt% or more 0.20w
t% or less, Mn: 1.50wt% or more 3.50wt
% or less, Cr: 0.50wt% or more 1.50wt%
Below, Al: 0.01wt% or more 0.10wt%
Hereinafter, a high-strength hot-rolled steel sheet containing P: 0.010 wt% or less and S: 0.001 wt% or less, with the balance being iron and inevitable impurities. [Claim 2] C: 0.08wt% or more 0.20w
t% or less, Mn: 1.50wt% or more 3.50wt
% or less, Cr: 0.50wt% or more 1.50wt%
Below, B: 0.0005wt% or more 0.010wt
% or less, Al: 0.01wt% or more 0.10wt%
Hereinafter, a high-strength hot-rolled steel sheet containing P: 0.010 wt% or less and S: 0.001 wt% or less, with the balance being iron and inevitable impurities. [Claim 3] C: 0.08wt% or more 0.20w
t% or less, Mn: 1.50wt% or more 3.50wt
% or less, Cr: 0.50wt% or more 1.50wt%
Below, Al: 0.01wt% or more 0.10wt%
Hereinafter, a steel slab containing P: 0.010 wt% or less and S: 0.001 wt% or less, with the remainder being iron and unavoidable impurities, is used as a raw material, and the finish rolling temperature is set to Ar3 transformation point or higher Ar3 transformation point +70 After hot rolling in a temperature range below ℃, the cooling rate (V) is calculated by the following formula (1) within 6 seconds. Cooling is started at ℃/S, and the winding temperature is calculated by the following formula (2). (T) A method for producing a high-strength hot-rolled steel sheet, characterized by winding at °C. (V) ≧15+ 50×[C]-3×[Mn]-2×
[Cr]-15 × [B] … (1)
513-65×[C]-9×[Mn]-7[Cr]≧
(T) >325-30×[C]-6.5[Mn]-
5×[Cr]
... (2) Here, the symbol in [ ] is the element symbol and represents the content (wt%) of that component in the steel. [Claim 4] C: 0.08wt% or more 0.20w
t% or less, Mn: 1.50wt% or more 3.50wt
% or less, Cr: 0.50wt% or more 1.50wt%
Below, B: 0.0005wt% or more 0.010wt
% or less, Al: 0.01wt% or more 0.10wt%
Hereinafter, a steel slab containing P: 0.010 wt% or less and S: 0.001 wt% or less, with the remainder being iron and unavoidable impurities, is used as a raw material, and the finish rolling temperature is set to Ar3 transformation point or higher Ar3 transformation point +70 After hot rolling in a temperature range below ℃, the cooling rate (V) is calculated by the following formula (1) within 6 seconds. Cooling is started at ℃/S, and the winding temperature is calculated by the following formula (2). (T) A method for producing a high-strength hot-rolled steel sheet, characterized by winding at °C. (V) ≧15+ 50×[C]-3×[Mn]-2×
[Cr]-15 × [B] … (1)
513-65×[C]-9×[Mn]-7[Cr]≧
(T) >325-30×[C]-6.5[Mn]-5
× [Cr]
... (2) Here, the symbol in [ ] is the element symbol,
It represents the content (wt%) of the component in the steel.