JPH09170048A - High strength hot rolled steel plate for working, excellent in fatigue characteristic and bore expandability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel plate for working, excellent in fatigue characteristics and bore expandability. SOLUTION: This hot rolled steel plate has a chemical composition consisting of, by weight, 0.05-0.20% C, 0.1-2.0% Si, 0.5-2.5% Mn, 0.005-0.1% P, <=0.01% S, <=0.1% Al, and the balance Fe with inevitable impurities and further containing, if necessary, one or more kinds among prescribed amounts of Nb, Ti, V, Cr, Mo, B, Ni, Ca, REM, and Cu. Moreover, this steel plate has a microstructure consisting of fine polygonal ferrite and bainite of <=10μm average grain size, and further, laminar cementite exists in 5-50% of grain boundaries and also its average length is regulated to 0.6-7μm.

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 working which has excellent fatigue characteristics and hole expandability, and more particularly to a hot-rolled steel sheet which can be suitably used as a material for underbody members for automobiles.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of improving the safety of automobiles and improving fuel efficiency, the high strength and thinning of steel sheets for automobiles have been widely promoted. Among the steel sheet processing members for automobiles, reducing the weight of wheels and underbody members that are unsprung members is an extremely effective means for improving the fuel efficiency of automobiles, so high strength for the purpose of weight reduction has been studied. There is. Of the properties required for the steel sheets that are the raw materials for these members, fatigue strength is important as well as hole expandability. In the case of a thin steel plate, since it is formed by pressing or the like, it is not desirable to increase the strength more than necessary from the viewpoint of workability. Therefore, a steel sheet having the same strength and more excellent fatigue properties is desired. As such a steel plate,
For example, it is known that a so-called DP steel having a composite structure of ferrite and martensite has excellent fatigue properties. Further, in JP-A-3-82708, high fatigue characteristics are obtained by controlling the amounts of C and P, and in JP-A-5-179397, fatigue strength is improved by adding Nb and Ti. As described above, some developments have already been carried out for steel sheets having excellent fatigue properties as they are.

[0003]

However, in many of the actual underbody parts, there is a portion to which shearing such as punching is applied, and the fatigue life of this shearing edge determines the fatigue strength of the entire member. There are quite a few decisions to make. Since the fatigue strength of such a part is greatly affected by the properties of the sheared edge, even if the fatigue strength of the material is high, the fatigue strength after processing is not always high. A material having high notch sensitivity may rather lower the fatigue strength.

Up to now, in order to prevent the fatigue strength of the sheared edge from being lowered, measures such as coining of punched holes have been mainly taken on the machined surface, but improvement by the material itself is desired. It was The present invention has been made in view of the above problems, and provides a high-strength hot-rolled steel sheet for working, which is excellent in fatigue characteristics and hole expandability.

[0005]

The high-strength hot-rolled steel sheet for working of the present invention has a chemical composition of wt% and C: 0.05 to 0.2.
0%, Si: 0.1 to 2.0%, Mn: 0.5 to 2.5%,
P: 0.005-0.1%, S: 0.01% or less, Al:
0.1% or less, and if necessary, the first group Nb: 0.01 to 0.3%, Ti: 0.01 to 0.3%,
V: 0.01 to 0.5% Second group Cr: 0.05 to 1.0%, Mo: 0.05 to 1.0%,
B: 0.0005 to 0.01%, Ni: 0.05 to 2.0% Third group Ca: 0.0005 to 0.0050%, REM: 0.005 to 0.15% Fourth group Cu: 0.05 to 2.0% 4 A hot-rolled steel sheet containing at least one component selected from at least one of the groups, the balance being Fe and inevitable impurities, the microstructure of which is fine polygonal ferrite having an average grain size of 10 μm or less and bainite. The plate-like cementite is present in 5 to 50% of the grain boundaries, and the average length thereof is set to 0.6 to 7 μm or less.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION First, the reasons for limiting the components of the chemical composition of the high strength hot rolled steel sheet for working of the present invention will be explained. All units are% by weight.

C: 0.05 to 0.20% C is an element which is effective for strengthening steel, and is particularly necessary for forming plate-like cementite and bainite at grain boundaries. For this purpose, it is necessary to add 0.05% or more,
On the other hand, if excessively added, ductility deteriorates remarkably and weldability also deteriorates, so the upper limit is made 0.20%.

Si: 0.1 to 2.0% Si is not only a very effective solid solution strengthening element for improving the tensile strength, but also accelerates the formation of polygonal ferrite and forms an appropriate amount of cementite at the grain boundaries. There is an action to generate. In order to exert these effects, addition of 0.1% or more is required. On the other hand, if added excessively, not only the welded portion becomes brittle and the low temperature toughness is caused, but also the surface quality deteriorates due to the formation of oxide scale, so the upper limit is made 2.0%.

Mn: 0.5 to 2.5% Mn is also a solid solution strengthening element and is effective in improving tensile strength, and is an element necessary for suppressing the formation of coarse pearlite and forming bainite. Is. In order to exert this effect effectively, it is necessary to add 0.5% or more,
If added in excess, ductility is reduced and weldability is impaired, so the upper limit is made 2.5%.

P: 0.005-0.1% P has the effect of improving the strength of steel, but excessive addition deteriorates workability and toughness. Therefore, the lower limit is 0.00
5 and the upper limit is 0.1%.

S: 0.01% or less Since S is an impurity element, it is desirable that the content be as small as possible, and it is regulated to 0.01% or less in order to improve hole expandability.

Al: 0.1% or less Al is added for deoxidation, but if it exceeds 0.10%, alumina-based inclusions increase and workability deteriorates. It is 10% or less.

The hot rolled steel sheet of the present invention contains the above components,
The balance consists of Fe and unavoidable impurities, but in order to further improve the material properties, one or more components selected from at least one of the following first to fourth groups may be added in addition to the components described above. Can be included.

First group Nb: 0.01 to 0.3%, Ti: 0.01 to 0.3% V: 0.01 to 0.5% Nb, Ti and V are precipitation strengthening elements and have strengths. Not only is it necessary for increasing the temperature, but it also has the effect of coexisting with Mn to affect the transformation structure after hot rolling and facilitate the production of low-temperature transformation products. Further, the structure is made finer, the stretch flangeability is improved, the hardness of the heat-affected zone after welding is prevented from lowering, and it is useful for improving the fatigue strength. For each element, if the content is less than 0.01%, the action is too small, while excessive content causes an increase in the yield ratio and a decrease in ductility, so the upper limits are 0.3% for Nb and Ti, and 0. It was set to 5%.

Second group Cr: 0.05 to 1.0%, Mo: 0.05 to 1.0% B: 0.0005 to 0.01%, Ni: 0.05 to 2.0% Cr, Mo, B and Ni improve the hardenability and advantageously act to generate a desired structure. If it is less than the lower limit value of each element, such an action is too small, while if it exceeds the upper limit value of each element, the action is saturated and the cost is increased.
Ni is also effective in preventing cracking of the slab when Cu is added.

Group 3: Ca: 0.0005 to 0.0050%, REM: 0.005 to 0.15% Ca, REM (rare earth element) has the effect of improving ductility, especially stretch flangeability, through morphology control of sulfides. For each element, if the content is less than the lower limit, the effect is too small, while if it is contained excessively, the effect is saturated and the cost increases, so the upper limit was set for each element.

Fourth group Cu: 0.05 to 2.0% Cu can increase the strength by precipitating after winding, and also has the function of imparting corrosion resistance to the steel sheet. If it is less than 0.05%, such an effect is too small, while if it is excessively contained, the effect is saturated and the cost increases, so the upper limit was made 2.0%.

The microstructure of the hot-rolled steel sheet according to the present invention containing the above components is composed of fine polygonal ferrite and bainite having an average grain size of 10 μm or less.
Plate-like cementite is present in ˜50%, and the average length thereof is set to 0.6 to 7 μm or less.

Structure A mixed structure of polygonal ferrite and bainite is used. It is difficult to secure the strength with a single ferrite phase when a strength of 490 N / mm ² or higher is to be obtained. For this reason, a hard second phase is required, but sufficient strength cannot be obtained with lamellar pearlite as the second phase, stretch flangeability is also insufficient, and with martensite (including retained austenite), elongation This is because the flangeability is extremely poor.

The average particle size of polygonal ferrite is 10 μm.
The reason why the particle size is suppressed to m or less is that by setting the average particle size to 10 μm or less, the stretch flangeability can be improved, and the effect of grain boundary cementite described later can be maximized. is there.

Next, the limitation of the abundance and size of the plate-like cementite at the grain boundaries will be described. The inventor has
Upon completion of the present invention, the fatigue properties of the punched holes of high-strength hot-rolled steel sheets having various metal structures consisting of ferrite and bainite were investigated, and even if the ferrite / bainite mixed structure was the same, the fatigue properties of the punched parts were investigated. There are steel sheets that are excellent and steel sheets that are not so excellent, and as a result of observing the punched portions of these steel sheets in detail, in the steel sheets that have excellent fatigue properties in the punched portions, the burr height is low and the occurrence of large voids is small, so the fracture surface On the other hand, it was found that the steel plate, which is smooth but has poor fatigue properties in the punched part, has high burrs and a large number of large voids, resulting in a large fracture surface roughness. Voids and burrs generated during punching are considered to be the starting points of cracks even during hole expansion forming, and are therefore considered to adversely affect stretch flangeability.

The fracture surface produced by punching is smooth,
When the microstructure of the steel sheet with good fatigue properties in the punched part was observed in detail, plate-like cementite was observed at the grain boundaries. It is considered that this plate-shaped cementite promotes the occurrence of cracks during punching, suppresses the height of burrs, and smoothes the fracture surface, but on the other hand, such plate-shaped cementite is Since it also serves as a starting point of cracks during local deformation, it is generally considered that stretch flangeability is deteriorated.

However, as a result of observing the existence state of the plate-like cementite, the inventors have found that the plate-like cementite having an appropriate size is present between the fine polygonal ferrites or at the grain boundaries between the fine polygonal ferrites and bainite. It has been found that, if present, the fatigue characteristics of the punched part can be improved with almost no deterioration in hole expandability. That is, 5 to 50% of the grain boundaries, preferably 10 to
By occupying 30% by plate-like cementite having a length of 0.6 to 7 μm, preferably 1 to 5 μm, both excellent fatigue properties and hole expandability can be obtained. If the grain boundary occupancy of the plate-like cementite is less than 5% and the length is less than 0.6 μm, the effect of promoting crack initiation during punching is not sufficient, and the smoothness of the punched fracture surface deteriorates, resulting in a rough surface with a large fracture surface. Burrs cause cracks to occur during hole expanding and under load, and eventually the hole expandability and fatigue strength deteriorate. On the other hand, when the grain boundary occupancy exceeds 50% and the length exceeds 7 μm, even if a smooth fracture surface is obtained during punching, the plate-like cementite exposed on the fracture surface during hole expansion is directly cracked. Since it will be the starting point and the propagation will be promoted,
Hole expandability deteriorates.

The reason why the presence of the plate-like cementite in the predetermined range does not adversely affect the hole expandability and the fatigue strength is not clear, but it can be considered as follows. That is, when there is almost no plate-like cementite at the grain boundaries or when the size thereof is small even if it exists,
Since cracks are less likely to occur during shearing such as punching, the roughness of the punched fracture surface after processing and the height of the burr exceed a certain limit, and during hole expansion, these cracks preferentially over plate-like cementite. As a result, the hole expandability deteriorates. On the other hand, when a fine plate-like cementite within the scope of the present invention is present in an appropriate amount at the grain boundary, this promotes the occurrence of cracks during punching, the punching fracture surface becomes smooth, the occurrence of burrs is suppressed, and the fracture surface Since the roughness and burrs are minute, the hole expandability does not deteriorate. Moreover, since the plate-like cementite itself is small and minute, it is unlikely to be a starting point of crack generation during hole expansion processing, and not only the hole expandability but also the fatigue strength after processing does not deteriorate. However, when the plate-like cementite is large or when the ratio of grain boundaries is high, the shearing workability is good, and therefore the burr height is low and the fracture surface becomes smooth, but this cementite itself cracks during hole expansion processing. Since this is the starting point of the generation, the hole expandability comes to deteriorate.

[0025]

EXAMPLE Steels having the chemical composition shown in Table 1 were melted in a vacuum melting furnace and rough rolled to obtain a slab having a thickness of 30 mm. Next, this slab is heated to 1150 to 1300 ° C., rolled to 3.0 mm in the finish rolling temperature range of 780 to 920 ° C., cooled to a temperature corresponding to the winding temperature (300 to 700 ° C.), and at that temperature. After holding for 30 minutes, the furnace was cooled.

[0026]

[Table 1]

The hot-rolled steel sheet thus obtained was subjected to JIS No. 5 tensile test, hole expansion test, and plane bending test of punched holes. The hole expansion ratio λ is 60 ° when punching holes of 10 mmφ.
Hole diameter d when a burr is in contact with the die side and is expanded until a crack penetrates the plate thickness using a conical punch
λ = ((d _b −d _i ) / d _i ) × 100% using _b and the initial hole diameter d _i . Also, the fatigue strength of punched holes is
Make a punching hole of 8 mmφ in the center of the plane bending test piece,
A test was performed at a frequency of 40 Hz with both stress ratios R = −1, and stress that does not break after 10 ⁷ cycles was evaluated as a fatigue limit σ _W. Table 2 shows the test results. For the samples in Table 2, the average length 1 of the plate-like cementite at the grain boundary, TS (tensile strength) * λ, and fatigue strength σ _W / TS
Fig. 1 shows a graph summarizing the relationship with.

[0028]

[Table 2]

As is apparent from Table 2 and FIG. 1, in the example samples having the steel composition of the present invention, the microstructure, the proportion of the plate-like cementite present in the grain boundaries, and the size satisfy the requirements of the present invention. It can be seen that the tensile strength is as high as 580 N / mm ² or more, and that it has excellent hole expandability and fatigue strength. For example, in the examples, the tensile strength is the lowest (58
In sample No. 2 of 0 N / mm ² ), λ is extremely high at 97%, and σ _W / TS is as good as 0.44. Also,
Sample No. 17, which has the maximum tensile strength (821 N / mm ² ), also has λ of 66% and σ _W / TS of 0.47.

On the other hand, the comparative sample Nos. 9 and 11 having components other than the present invention and having a structure of only ferrite or ferrite + pearlite have tensile strengths of 410 to 420 k.
The value was as low as gf / mm ^2, and the average grain size d of polygonal ferrite and the average length l of plate-like cementite at grain boundaries were large, so λ was 88% or less for low strength. Also,
Comparative sample Nos. 10 and 12, which are components other than the present invention, have high strength, but λ and σ _W / TS are very low. Also,
Comparative sample Nos. 6, 14, 16, 18 having d and l larger than the range of the present invention even if they have the components and structure of the present invention
Then, λ is low for strength, and σ _W / TS is also low in general. On the other hand, in Comparative Example Samples Nos. 19 and 21, which have the components and structure of the present invention, but the average length 1 of the plate-like cementite and the abundance ratio X at the grain boundaries are lower than the range of the present invention, the strength is relatively low. λ is low and σ _W / TS is very low.

[0031]

As described above, according to the hot-rolled steel sheet of the present invention, not only the components but also the structure and the morphology of cementite in the grain boundaries are defined in a specific range.
^It has a high strength of ² or more and combines excellent hole expandability and fatigue characteristics.

[Brief description of the drawings]

FIG. 1 Average length l of plate-like cementite at grain boundaries
Is a graph showing the relationship between TS (tensile strength) * λ and fatigue strength σ _W / TS.

Claims (2)

[Claims] 1. A chemical composition in% by weight, C: 0.05 to.
0.20%, Si: 0.1 to 2.0%, Mn: 0.5 to 2.
5%, P: 0.005 to 0.1%, S: 0.01% or less, Al: 0.1% or less, a hot rolled steel sheet comprising the balance Fe and unavoidable impurities, and having a microstructure Is composed of fine polygonal ferrite with an average particle size of 10 μm or less and bainite, has plate-like cementite in 5 to 50% of the grain boundaries, and has an average length of 0.6 to 7 μm or less. High-strength hot-rolled steel sheet for machining with excellent hole expandability. 2. In addition to the component of claim 1, the following first to first
A high-strength hot-rolled steel sheet for working, which contains one or more components selected from at least one of the four groups and has excellent fatigue properties and hole expandability. First group Nb: 0.01 to 0.3%, Ti: 0.01 to 0.3%,
V: 0.01 to 0.5% Second group Cr: 0.05 to 1.0%, Mo: 0.05 to 1.0%,
B: 0.0005 to 0.01%, Ni: 0.05 to 2.0% Third group Ca: 0.0005 to 0.0050%, REM: 0.005 to 0.15% Fourth group Cu: 0.05 to 2.0%