JPH09256103A - Hot-rolled steel sheet and hot-rolled steel sheet member capable of obtaining a steel sheet member having a low yield ratio after hot forming - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot rolled steel plate capable of providing a steel plate member of sufficiently low yield ratio after hot forming and also to provide the hot rolled steel plate member. SOLUTION: This steel plate has a composition which consists of, by weight, 0.03-0.25% C, 0.01-1.0% Si, 0.50 2.0% Mn, <=0.05% P, <=0.03% S, <=0.08% sol.Al, <=0.01% N, and the balance Fe with inevitable impurities and further contains, if necessary, prescribed amounts of one or more elements selected from group A consisting of Cr, Ni, and Mo, group B consisting of Nb, Ti, and V, B, and Ca and in which Ceq represented by Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+ V/14 is regulated to 0.2-0.5. Further, the average hardness of ferrite grains of >=15μm is regulated to >=Hv110, and the difference between the maximum and the minimum hardness of these ferrite grains is regulated to >=Hv10, preferably >=Hv15.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet for hot forming and a hot-rolled steel sheet member used for automobile suspension members such as members and wheels having a low yield ratio, construction members and the like.

[0002]

2. Description of the Related Art Conventionally, molding of an automobile suspension member such as a member has been performed by cold working such as press molding. In this case, as a steel plate with a low yield ratio, ferrite
A hot rolled steel sheet (DP steel sheet) or the like having a martensitic structure is used, but since these steel sheets were developed for the purpose of improving formability (shape fixability) and impact resistance,
The yield ratio of the severely worked portions such as the corners becomes high, and the steel sheet member after forming is inferior in impact characteristics and material uniformity, and cannot be said to have sufficient quality.

Conventionally, hot rolled steel sheets for construction (JIS G 3101), rolled steel sheets for welded structures (JIS G 3106), etc. have been widely used as hot rolled steel sheets for construction. In order to improve the safety of steel, in particular, to reduce the yield ratio in order to improve the seismic resistance, a rolled steel material for building structures (JIS G 3136) has been enacted. When such a hot rolled steel sheet is used However, it is not uncommon for the yield ratio of a member after forming to become high. For example,
In the case of building materials, when hot-rolled steel sheet is formed into a tubular shape by cold working such as roll forming, the low green part of the opening is subjected to electrical low resistance welding to obtain a steel pipe, and then cold working is performed to form a square corner. The yield point rises due to the working effect during the forming by the cold working, and the yield ratio increases.

[0004]

Therefore, in order to suppress an increase in the yield ratio after processing, for example, JP-A-4-3233.
As disclosed in Japanese Patent Publication No. 19, a method of heating only a portion that has been subjected to severe cold working has been attempted, and a method of forming and working while hot is being studied, but a sufficient yield ratio reduction The reality is that the effect has not been obtained.

The present invention has been made in view of the above problems, and provides a hot-rolled steel sheet and a hot-rolled steel sheet member capable of obtaining a steel sheet member having a sufficiently low yield ratio by hot forming.

[0006]

The hot-rolled steel sheet of the present invention is, by weight%, C: 0.03 to 0.25%, Si: 0.01.
~ 1.0%, Mn: 0.50 to 2.0%, P: 0.0
5% or less, S: 0.03% or less, sol.Al: 0.08
% Or less, N: 0.01% or less, and Ceq = C
+ Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 Ceq is 0.2 to 0.5, and if necessary, Cr: 0.80% or less, Ni: 0.50% or less, Mo:
Group A consisting of 0.40% or less, Nb: 0.08% or less,
B consisting of Ti: 0.10% or less and V: 0.30% or less
Group, B: 0.0005 to 0.0050% and Ca: 0.
Containing one or more elements selected from 006% or less, the balance being Fe and inevitable impurities, the average hardness of ferrite particles of 15 μm or more is Hv110 or more, and the maximum and minimum hardness of these ferrite particles. Difference with hardness is H
v10 or more, preferably Hv15 or more.

The hot rolled steel sheet member of the present invention is formed by hot forming the hot rolled steel sheet at a temperature of 700 to 1000 ° C.

According to the hot-rolled steel sheet of the present invention, the ferrite grain size can be greatly increased during the hot forming process, whereby the yield ratio of the steel sheet member after hot forming can be greatly reduced. it can.

Factors for grain growth of ferrite grains of a member to be hot-formed include: (1) heat treatment in the case of working in a relatively low temperature region under the same working condition. The grain size of the rolled steel sheet base material is small, (2) the grain size distribution of the hot rolled steel sheet base material is wide, and (3) the hardness distribution of the hot rolled steel sheet base material is wide. Factor (1) above promotes grain growth because the driving force for grain growth increases because the grain size of the hot-rolled steel sheet base metal is small, and factor (2) causes re-heating and heat retention in the structure. The large grains in the grains cause the grain growth by eating the smaller grains.The factor of (3) is that the difference in hardness between the adjacent grains is the driving force, and the grains from low hardness to high hardness are The effect of increasing the particle size is brought about by facilitating the field transfer.

If the heating and holding temperature is high,
In place of the effects of factors (1) to (3), (4) a dynamic recrystallization process in which the grain size is determined only by the processing speed and strain rate.

In the region where the heating temperature changes from the low temperature region to the high temperature region, the effects of the factors (1) to (3) and the factor (4), the ferrite α → austenite γ transformation, and γ → α There is a possibility that the effect of reverse transformation will be complicatedly affected, and the grain size after hot forming will be controlled by the microstructure of the hot-rolled steel sheet base material to achieve a low yield ratio.

Based on the above metallurgical considerations, mainly the factors (2) and (3) above, and these factors and α → γ
In order to explore the possibility of utilizing the complex effects in the transformation and γ → α reverse transformation, the present inventors have investigated the average hardness of the hot-rolled steel sheet base metal, the hardness of ferrite grains in the base metal structure, the effect of processing temperature, etc. Through diligent research, the present inventors have come to discover the structural conditions of the hot rolled steel sheet of the present invention.

First, the reasons for limiting the components of the steel used in the present invention will be explained. C: 0.03 to 0.25% C is an effective element for increasing the strength of the steel sheet. 0.
If it is less than 03%, its effect is too small, while it is 0.25%.
If it exceeds, the weldability and other characteristics will deteriorate. Therefore, the lower limit is 0.03% and the upper limit is 0.25%.

Si: 0.01 to 1.0% Si has a deoxidizing effect on molten steel, and also improves strength and ductility by a solid solution strengthening effect in a cooling stage after hot working.
If it is less than 0.01%, such an effect is too small, while 1.
If it exceeds 0%, Si scale is likely to be generated in the hot rolling stage, and the steel sheet surface properties are deteriorated. Therefore, the lower limit is 0.01% and the upper limit is 1.0%.

Mn: 0.50 to 2.0% Like Si, Mn enhances strength by solid solution strengthening action upon cooling after hot working. If it is less than 0.50%, such an effect is too small, while if it exceeds 2.0%, a band-like structure is formed and ductility in the rolling direction is deteriorated. Therefore, the lower limit is 0.50% and the upper limit is 2.0%.

P: 0.05% or less, S: 0.03% or less P and S deteriorate the toughness, so the smaller the better, the upper limits of which are 0.05% and 0.03%, respectively.

Sol.Al: 0.08% or less Al is added as a deoxidizing agent for steel, but even if added in a large amount, its effect does not change and the cost increases, so the upper limit is set as solid solution Al. To 0.08%.

N: 0.01% or less N deteriorates ductility and weldability, so its upper limit is 0.0.
1%.

Ceq: 0.2 to 0.5 Ceq in the present invention is Ceq = C + Si / 24 + Mn /
6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 is an index, and the tensile strength of the steel sheet member formed by hot working the hot-rolled steel sheet base material is uniquely determined by this index. In the case of the present invention, when Ceq is less than 0.20, the high strength of the molded member cannot be secured, and when it exceeds 0.5, the tensile strength becomes saturated as compared with the increase in the yield strength. Since the yield ratio is further reduced by the effect of lowering the yield ratio by the above control, the lower limit of Ceq is set to 0.2 and the upper limit is set to 0.5%. In the invention of claim 1, Ni,
Although it does not contain Cr, Mo, or V, as will be described later, since the invention of claim 2 contains one or more of these elements, Ceq
Describes in a form including these elements.

The steel used in the present invention comprises, in addition to the above components, the balance Fe and unavoidable impurities, but one or more elements from the following Group A, Group B, B and Ca are required to enhance the strength. Can be further contained according to

Cr: 0.80% or less, Ni: 0.50%
Hereinafter, Mo: 0.40% or less of group A Cr, Ni, and Mo are effective in increasing the strength of steel by precipitation hardening and the like. It is particularly effective for improving the strength characteristics after reheating. However, if a large amount is added, the weldability deteriorates and the cost increases, so the upper limit of each element is Cr:
0.80%, Ni: 0.50% or less, Mo: 0.40%
The following is assumed. To obtain a particularly effective effect, Cr:
0.05% or more, Ni: 0.05% or more, Mo: 0.0
5% or more of content is desirable.

Nb: 0.08% or less, Ti: 0.10%
Below, V: 0.30% or less of group B Nb, Ti, and V are effective in increasing the strength of steel by precipitation hardening, etc., and even if forming is performed in a relatively high temperature range, the above (4) The effects of (2) and (3) are suppressed and the effects of (2) and (3) are brought about. However, if a large amount is added, any element deteriorates the weldability, so the upper limit of each element is set as described above.

B: 0.0005 to 0.0050% B is effective in increasing the strength by adding a trace amount, and
It has the effect of significantly increasing hardenability. If it is less than 0.0005%, the effect is too small, while if it is added in a large amount, B
As it produces compounds and deteriorates properties such as toughness,
The upper limit is 0.0050%.

Ca: 0.006% or less Ca has the effect of reducing the mechanical anisotropy and improving the ductility and weldability by controlling the morphology of sulfides from expanded to spherical. However, if a large amount is added, nonmetallic inclusions increase in the steel and the ductility decreases, so the upper limit is made 0.006%.

Next, the ferrite grains of the hot rolled steel sheet of the present invention will be described. In the hot rolled steel sheet of the present invention, 15 μ
The average hardness of ferrite particles of m or more is Hv110 or more,
And the hardness difference ΔH between the maximum hardness Hvmax and the minimum hardness Hvmin
v (Hvmax-Hvmin) is Hv10 or more, preferably 15 or more.

The reason why the ferrite grain size is limited to 15 μm or more is that the hardness and hardness difference of the ferrite grains having a grain size of less than 15 μm do not correlate much with the reduction of the yield ratio of the molded member. That is, it is considered that ferrite grains having a grain size of less than 15 μm are almost eaten by grains having a grain size of 15 μm or more during the temperature rise and disappear. On the other hand, the ferrite grains of 15 μm or more are important crystal grains which are influenced by the average hardness and hardness difference thereof and are either grains that grow by eating other grains or grains that are lost by being eaten by other grains.

The reason why the average hardness of ferrite particles of 15 μm or more is set to Hv110 or more is that if it is less than Hv110, even the ferrite particles of 15 μm or more do not satisfy the driving force necessary for grain growth.

Regarding the hardness difference (ΔHv) of ferrite grains, a hardness difference (hardness distribution) is required for grain growth even if the average hardness is Hv110 or more. It determines what is eaten away, which in turn determines the grain size of the final molded part and thus has a large effect on the yield ratio. To achieve a low yield ratio, at least ΔHv ≧
10, preferably ΔHv ≧ 15 is required.

In order for ferrite grains having different hardnesses to grow by eating other ferrite grains, it is necessary to move grain boundaries and a certain amount of ferrite is required. In addition, if the ferrite having a size of 15 μm or more that grows by eating other ferrite is extremely small, it will be difficult to obtain a desired large grain size. Therefore, the area ratio of ferrite in the structure is preferably 30% or more, and the ferrite having a grain size of 15 μm or more is preferably 30% or more.

By satisfying the conditions of the grain size and hardness of the ferrite grains, the ferrite grain size of the structure of the molded member is 10 to 20% larger than that in the case of using a normal hot-rolled steel sheet, and as a result, A low yield ratio can be achieved.

It should be noted that the average crystal grain size of ferrite of the hot-rolled steel sheet has little effect when hot forming is carried out in the temperature range described later, and it is considered that the factor (1) has little effect. . Further, the ferrite grains in the steel sheet of the present invention include not only ordinary polygonal ferrite, but also acicular ferrite produced by quenching, low-temperature winding, or the like, and ferrite grains containing solid solution carbon in a larger amount than usual or in supersaturation. May be. Further, the hot-rolled steel sheet may have a second phase structure, and any type thereof may be used.

Next, a suitable forming temperature for hot forming the hot rolled steel sheet of the present invention will be described. When the hot-rolled steel sheet of the present invention is used for hot forming, the working temperature range is preferably set to 700 to 1000 ° C. When the temperature is lower than this temperature range, recovery and grain growth are difficult to occur, it is difficult to effectively use the effects of the factors (2) and (3) described above, and as in the case of cold working, The yield ratio of the processed part is significantly higher than that of the flat plate part. Also, 100
In the temperature range exceeding 0 ° C, it becomes difficult to control the low yield ratio of the formed member by the hot-rolled steel sheet base material due to the effect of (4) dynamic recrystallization. Incidentally, prior to the heating, a mild cold working such as round tube forming may be performed as long as the structure of the hot rolled steel sheet is not significantly changed.

[0033]

EXAMPLES After melting and casting the various steels shown in Table 1, the slab was reheated to a temperature of around 1200 ° C.
The hot rolling was completed at the hot rolling finishing temperature of 80 ° C., and the obtained hot rolled steel sheet having a thickness of about 10 mm was cooled at the cooling rate (C
After cooling in (R), it was wound at the winding temperature (CT) shown in the table.

[0034]

[Table 1]

[0035]

[Table 2]

With respect to the obtained hot-rolled steel sheet, the hardness of ferrite particles of 15 μm or more was measured at 20 points using a micro Vickers measuring instrument, and the average value Hv and the difference ΔHv between the maximum hardness and the minimum hardness were determined. . These values are also shown in Table 2.

Further, using a hot rolled steel sheet as a base material, 800 to 9
After heating to 50 ° C., it was bent with a radius of 25 mm and air-cooled. Some hot-rolled steel sheets were cold-worked (Sample Nos. 30 and 31). Tensile test pieces were sampled from the forming part (corner part) of the bending-molded member, and mechanical properties such as tensile strength were examined by a tensile test. In addition, the average grain size of the ferrite grains in the molding site was measured. The results are also shown in Table 2.

FIG. 1 shows the CEQ (C
eq) and the yield ratio, the tensile strength, and the yield strength of the corner portion of the molded member are arranged. When the hardness difference ΔHv is about Hv15 or more within the range of the present invention, Ceq is 0.2.
It can be seen that the yield ratio is effectively reduced at .about.0.5. The element symbols in the figure represent the properties improving elements such as strength contained in the steel grades of Sample Nos. 23 to 29, and those marked "cold forming" are Samples Nos. 30 and 31.

Further, FIG. 2 shows sample No. 5 of steel type No. 3
16 is a graph in which the relationship between the hardness difference ΔHv and the yield ratio of the corner portion of the molded member is summarized for 16. When the average hardness of ferrite grains is about Hv130 or more within the range of the present invention, the yield difference is Hv10 or more. It can be seen that the ratio drops significantly. In particular, when ΔHv is Hv15 or more, the yield ratio drops to the minimum.

Further, FIG. 3 is a graph in which the relationship between the hardness difference ΔHv and the average ferrite grain size at the corners of the molded member for the same sample as in FIG. 2 is arranged. When the average hardness of ferrite grains is 100 or more, It can be seen that the average grain size increases when the hardness difference becomes Hv10 or more, which supports the decrease in the yield ratio.

Looking at the individual data in Table 2, the steel types
Nos. 1 and 5 are steel grades having Ceq outside the scope of the present invention, and steel grade No. 1 is lower than the scope of the present invention, so the tensile strength of the hot rolled steel sheet using this is also low, and sample No. 1 of the processed member Two
The tensile strength is as low as 420 N / mm ² . In addition, the steel type No.
No. 5, Ceq is higher than the range of the present invention, and the tensile strength of the hot rolled steel sheet using this is also high.
21 is also high, exceeding 550 N / mm ² .

Even if the steel composition satisfies the range of the present invention,
Sample Nos. 4, 8, 12, 16, and 19 are 1 of the base material of hot-rolled steel sheet.
Since the ΔHv of ferrite particles of 5 μm or more is outside the range of the present invention, the yield ratio is as high as 75% or more. In addition, steel composition and Δ
Although Hv is within the range of the present invention, the average hardness of ferrite particles of 15 μm or more is outside the range of the present invention.
As for No. 5, the yield ratio is as high as 75% or more. Sample No.
In Nos. 30 and 31, although the hot-rolled steel sheet satisfies the scope of the present invention, since bending was performed by cold working,
The yield ratio increased to over 80%.

Compared with these Comparative Examples, No. 3, 5-7, 9-11, 17, 18, 23- satisfying the steel composition of the present invention, the average hardness and the hardness difference of ferrite having a predetermined grain size. Two
In 9 (Example), the tensile strength of the molded member was 470 N / mm ²
Above, the yield ratio is 72% or less, and a high-strength steel plate member having a low yield ratio is obtained.

[0044]

As described above, according to the hot-rolled steel sheet of the present invention, the average hardness of ferrite particles having a predetermined steel composition of 15 μm or more is Hv110 or more, and the hardness difference is Hv10.
As defined above, ferrite grains grow during hot forming, work hardening is suppressed, and a steel plate member having a low yield ratio can be obtained. In particular, by setting the hardness difference to be Hv15 or more, it is possible to remarkably promote the lowering of the yield ratio. Further, by performing hot working of the hot rolled steel sheet at 700 to 1000 ° C., it becomes possible to effectively control the low yield ratio due to the growth of ferrite grains of the hot rolled steel sheet, and the yield ratio is 75. It is possible to easily obtain a hot-formed steel sheet member having a low yield ratio of not more than%.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between a carbon equivalent (CEQ = Ceq) of a hot-rolled steel sheet base material and a yield ratio of a corner portion of a forming member.

FIG. 2 is a graph showing a relationship between a hardness difference ΔHv of ferrite grains of a hot-rolled steel sheet base material and a yield ratio of a corner portion of a forming member.

FIG. 3 is a graph showing a relationship between a hardness difference ΔHv of ferrite grains of a hot-rolled steel sheet base material and an average ferrite grain size of corner portions of a forming member.

Claims (5)

[Claims] 1. C: 0.03 to 0.25 in% by weight.
%, Si: 0.01 to 1.0%, Mn: 0.50 to 2.
0%, P: 0.05% or less, S: 0.03% or less,
sol.Al: 0.08% or less, N: 0.01% or less, and Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5
Ceq represented by + Mo / 4 + V / 14 is 0.2 to 0.5, the balance is Fe and unavoidable impurities, the average hardness of ferrite particles of 15 μm or more is Hv110 or more, and the maximum of these ferrite particles is A hot-rolled steel sheet having a low yield ratio after hot forming, wherein the difference between the hardness and the minimum hardness is Hv10 or more. 2. In addition to the components described in claim 1, a group A consisting of Cr: 0.80% or less, Ni: 0.50% or less, Mo: 0.40% or less, in addition to the components described in claim 1, Nb. : 0.0
8% or less, Ti: 0.10% or less, V: 0.30% or less, Group B, B: 0.0005 to 0.0050% and Ca: 0.006% or less, one selected from The hot-rolled steel sheet according to claim 1, containing the above. 3. The difference between the maximum hardness and the minimum hardness of ferrite particles of 15 μm or more is Hv15 or more.
Hot-rolled steel sheet described in. 4. A hot rolled steel sheet member, wherein the hot rolled steel sheet according to claim 1, 2 or 3 is hot formed at a temperature of 700 to 1000 ° C. 5. The hot rolled steel sheet member according to claim 4, wherein the yield ratio is 75% or less.