JP2005154813A - Compound structure steel plate having excellent bore expandability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a compound structure steel plate which is appropriate for automobile structural members requiring working, such as bore expansion, and has excellent bore expandability. <P>SOLUTION: The compound structure steel sheet consists, by mass%, 0.01 to 0.2% C, ≤2.0% Si, ≤3.0% Mn, ≤0.08% P, ≤0.03% S, 0.01 to 0.1% Al, ≤0.01% N and the balance substantially iron, and is composed of a structural form in which the ferrite grains of the main phase are coated by coupling of the grains of a hard second phase. The compound structure steel sheet satisfies the following formula (1) when the overall circumferential length of a piece of the ferrite grain is defined as L and the length of the hard second phase existing around the ferrite grains as L<SB>2</SB>: L<SB>2</SB>≥0.7×L...(1). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a composite structure steel plate excellent in hole expansibility, and more particularly to a composite structure steel plate suitable for a structural member such as an automobile.

  In recent years, in the automobile industry, with the background of environmental problems and the like, reduction in weight of an automobile body by applying a high-strength steel sheet has been studied. The properties required for automotive structural materials are required to have various workability as well as strength and resistance to breakage that determines material life. Fatigue fractures and the like are based on hard non-metallic inclusions in the steel, and cracks develop, leading to fractures. Therefore, measures such as improving cleanliness are taken.

  A similar fracture behavior has been confirmed in a composite structure steel plate composed of a hard second phase and a soft main phase. Moreover, the hole expansion workability is requested | required of the steel plate which has high intensity | strength and is used as a structural member. This hole expansion process exhibits crack growth that is qualitatively similar to fatigue fracture. Therefore, in the composite structure steel plate, the hole expandability is remarkably lowered due to the increase in the ratio of the hard second phase accompanying the increase in strength and the hardness thereof. For this reason, the present condition is that the application member (use) is restricted when using it as a structural member, although a composite structure steel plate has the characteristic superior to a single phase structure steel plate.

  A structure morphology control method for imparting high workability to a steel plate having such a hard second phase has been proposed. For example, Patent Document 1 discloses a structural form of main phase ferrite and second phase (bainite, pearlite). Patent Document 2 discloses a method in which main phase polygonal ferrite and second phase (bainite) are finely dispersed. Patent Document 3 discloses a method in which plate-like cementite is finely dispersed at the grain boundaries of main phase (polygonal ferrite). , Respectively.

However, these are all methods for finely dispersing the second phase, and are inventions that have been made with a focus on suppressing the occurrence of cracks during hole expansion. For this reason, it has not been studied to suppress crack propagation during hole expansion processing, and the hole expandability is not necessarily good.
JP-A-5-51646 Japanese Patent Laid-Open No. 5-98353 JP-A-9-170048

  This invention is made | formed in view of this situation, Comprising: It aims at providing the composite structure steel plate excellent in hole expansibility suitable for the automotive structural member which needs processes, such as hole expansion.

  In the composite steel sheet, the inventors of the present invention suppress the occurrence of cracks and suppress the development of cracks in order to significantly improve the hole expandability, which was insufficient with the prior art, and to solve the above problems. I found it important. Furthermore, as a result of repeatedly investigating the influence of the hard second phase on hole expansibility using the hard second phase and paying attention to the structure of the hard second phase, It has been found that the cracks can be prevented from occurring and progressing by continuously distributing the ferrite grain boundaries.

This invention is made | formed based on such knowledge, and is mass%, C: 0.01-0.2%, Si: 2.0% or less, Mn: 3.0% or less, P: 0.08% or less, S: 0.03% or less, Al: 0.01 to 0.1%, N: 0.01% or less, the balance being substantially made of iron, and grains of hard second phase When the structure forms a structure in which the ferrite grains of the main phase are coated by joining, the entire circumference of one ferrite grain is L, and the length of the hard second phase existing around the ferrite grain is L ₂ Further, the present invention provides a composite structure steel plate excellent in hole expansibility, characterized by satisfying the relationship of the following formula (1).
L ₂ ≧ 0.7 × L (1)

  Further, the present invention is the above composite steel sheet, Cr: 1% or less, Mo: 1% or less, V: 1% or less, Ni: 1% or less, B: 0.01% or less, Ti: 0.3% Hereinafter, the composite structure steel plate excellent in hole expansibility containing 1 or more types out of Nb: 0.3% or less is provided.

  The composite structure steel plate according to the present invention is excellent in hole expansion workability because generation of cracks during hole expansion processing is suppressed and the progress of cracks is also suppressed.

Hereinafter, details of the invention will be described.
First, the component composition will be described.
The composite structure steel plate according to the present invention is mass%, C: 0.01 to 0.2%, Si: 2.0% or less, Mn: 3.0% or less, P: 0.08% or less, S: It is 0.03% or less, Al: 0.01 to 0.1%, N: 0.01% or less, and the balance is substantially made of iron. Furthermore, Cr: 1% or less, Mo: 1% or less, V: 1% or less, Ni: 1% or less, B: 0.01% or less, Ti: 0.3% or less, Nb: 0.3% or less One or more of them may be contained.

C: 0.01 to 0.2%
C is an element necessary for generating a certain amount of the hard second phase and ensuring the strength of the hard second phase, and for that purpose, it is necessary to contain 0.01% or more. On the other hand, if the content exceeds 0.2%, the workability and weldability are remarkably deteriorated, which is not suitable for the automotive steel plate targeted by the present invention. Therefore, the C content is set to a range of 0.01 to 0.2%. Since it is possible to improve the hole expandability by increasing the strength of the hard second phase and the amount necessary to occupy the ferrite grain boundary and strengthen the continuity of the hard second phase The amount is desirably 0.1% or more.

Si: 2.0% or less Si is an effective element for ensuring strength and stably obtaining a low-temperature transformation phase. However, if its content exceeds 2.0%, surface property deterioration due to red scale, plating Problems such as poor adhesion and poor chemical conversion treatment become prominent and are not suitable as practical steel sheets. Therefore, the Si content is set to 2.0% or less.

Mn: 3.0% or less Generally, Mn is effective in preventing S from hot cracking of slab by precipitating S in steel as MnS. In the present invention, in order to form a hard second phase, it is desirable to add a certain amount of Mn that improves hardenability. However, if the Mn content exceeds 3.0%, not only the slab cost is significantly increased, but also the workability is deteriorated. Therefore, the Mn content is 3.0% or less. In order to improve the hardenability, the Mn content is preferably 0.3% or more, and more preferably 0.8% or more.

P: 0.08% or less P is an element effective for securing strength and stably obtaining a low-temperature transformation phase. However, if its content exceeds 0.08%, the secondary work brittleness resistance is deteriorated. This causes harmful effects such as Moreover, when it is set as a galvanized steel plate, the alloying processability falls. Therefore, the P content is 0.08% or less.

S: 0.03% or less S reduces hot workability and increases the hot cracking susceptibility of the slab. If the content exceeds 0.03%, the workability deteriorates due to the precipitation of fine MnS. . Therefore, the S content is set to 0.03% or less.

Al: 0.01 to 0.1%
Al contributes to the deoxidation of steel and has a role of fixing unnecessary solid solution N in the steel as nitrides. This effect is not sufficient if the Al content is less than 0.01%, and even if it exceeds 0.1%, an effect commensurate with the content cannot be obtained. Therefore, the Al content is in the range of 0.01 to 0.1%.

N: 0.01% or less It is not desirable for N to remain in a solid solution state from the viewpoint of aging, and its content is preferably as small as possible. If the N content exceeds 0.01%, ductility and toughness deteriorate due to the presence of excess nitride. Therefore, the N content is 0.01% or less.

Cr, Mo, V: 1% or less respectively Cr, Mo, V are ferrite-forming elements and are effective for obtaining a composite structure of ferrite phase + low-temperature transformation phase, and can be added as necessary. . However, if the content exceeds 1%, an effect commensurate with the cost cannot be obtained. Therefore, when Cr, Mo, V is added, the content is 1% or less.

Ni: 1% or less Ni has an effect of improving hardenability and toughness without impairing weldability, and can be added as necessary. However, even if the content exceeds 1%, an effect commensurate with the cost cannot be obtained. For this reason, when adding Ni, the content is made 1% or less.

B: 0.01% or less B is an element effective for improving the hardenability, and can be added as necessary in order to stably generate a low-temperature transformation phase. However, since an effect commensurate with the cost cannot be obtained even if B is added in excess of 0.01%, the content is made 0.01% or less when B is added.

Ti and Nb: each 0.3% or less Ti and Nb have a function of forming nitrides and fixing N. By fixing N with Ti or Nb instead of Al, the crystal grains are refined and the toughness is improved. Therefore, these elements may be added as necessary. However, even if these contents exceed 0.3%, an effect commensurate with the cost cannot be obtained. Therefore, when these are added, the contents are each set to 0.3% or less.

  In the composite steel sheet of the present invention, in addition to the above components, the balance may be substantially iron, and inevitable impurities and other trace elements within a range that does not impair the function and effect of the invention are allowed. .

Next, the organization will be described.
The composite structure steel plate of the present invention has a structure form in which a grain boundary of a ferrite phase is covered with a hard second phase. The occupation ratio (coverage ratio) must satisfy the following formula (1).
L ₂ ≧ 0.7 × L (1)
Where L: the entire circumference of one ferrite grain,
L ₂ : Length of the hard second phase existing around the ferrite grains

  The present invention is intended to greatly improve the hole expansibility of a composite structure steel sheet by adopting a structure in which ferrite grains are coated with a hard second phase. Conventionally, a composite structure steel sheet has been widely used for automobile steel sheets and the like because it exhibits an excellent strength-ductility balance. Furthermore, as described above, attempts have been made to improve hole expansibility using the hard second phase. However, in the prior art, sufficient improvement of the hole expansion characteristic has not been achieved.

  The present inventors paid particular attention to the structure of the hard second phase, prepared systematic samples in which the structure of the hard second phase was changed, and observed crack propagation under an electron microscope. As a result, we found that the structure of the hard second phase plays an extremely important role in the crack initiation and propagation behavior during hole expansion. That is, the crack generated at the interface between the hard second phase and the ferrite phase, which is found in the prior art, penetrates after passing through the ferrite phase. However, when the ferrite phase grains are coated with the hard second phase, the phase interface having a large hardness difference is increased, and the local stress that is the source of cracks is relieved. Moreover, it has been found that the cracks that have progressed are retained in the hard second phase, the cracks do not easily propagate, and the hole expandability is improved, leading to the present invention.

  An example of the microstructure according to the present invention is shown in FIG. 1 together with conventional steel. The steel of the present invention has a completely different structure from that of the conventional steel, that is, a ferrite phase particle (a portion that appears white in FIG. 1B) due to a hard second phase mainly composed of martensite (a portion that appears black in FIG. 1B). ) Having a grain morphology coated with grains; However, in the present invention, local hard phase discontinuities may inevitably exist.

  Next, the state in which the ferrite phase of the hard second phase of the present invention is coated will be described in detail. The coating of the ferrite grains with the hard second phase refers to a form in which a hard second phase in contact with the ferrite grains exists and between adjacent ferrite grains is occupied by the hard second phase grains. However, it is not necessary that the entire periphery of the ferrite grain is covered. At this time, the coverage may be 70% or more as shown in the formula (1), and more preferably 85% or more.

  The observation of the structure of the composite structure steel plate and the measurement of the ferrite grain coverage can be carried out by the following method, similarly to the normal structure observation. That is, first, a sample for tissue observation is cut out from an arbitrary position of a specific part (part other than the unsteady part such as the vicinity of the edge) of the steel sheet, and the position excluding the plate surface is taken as the observation part. At this time, in the observation from the cross-sectional direction, after burying and polishing for cross-sectional observation, and corroding with an etching solution appropriate for revealing the composite structure, a quarter position of the plate thickness may be observed. . Further, in the observation from the plane, the surface may be polished by 1/4 of the plate thickness, and similarly observed after corrosion. At this time, an optical microscope can be used as the observation device, but when the hard second phase is fine, it is preferable to use an electron microscope. After capturing about 10 fields of view using such an observation apparatus, the coverage of ferrite grains by the hard second phase can be confirmed by capturing and analyzing image data in the image analysis apparatus. The hard second phase is a low-temperature transformation phase such as martensite, bainite, and retained austenite, and includes one or more of these.

Such a composite structure steel plate can be manufactured by box-annealing a hot-rolled sheet at a temperature of 600 ° C. or higher and Ac ₃ points + 70 ° C. or lower and then continuously annealing at a temperature in the (α + γ) region. The temperature during the box annealing is preferably 650 ° C. or more and Ac ₃ points or less, and more preferably 650 ° C. or more and Ac ₁ point or less. The temperature during the continuous annealing is important for obtaining a low-temperature transformation phase essential for the present invention. In the α single phase region, a low temperature transformation phase cannot be obtained, and in the γ region, it is initialized to a structure controlled to ferrite + band-like pearlite by a hot-rolled sheet annealing process.

  The steel sheet of the present invention is mainly a hot-rolled steel sheet and a zinc-based plated steel sheet, but may be a cold-rolled steel sheet. The hot dip galvanized steel sheet may be subjected to an alloying treatment. Also. These hot dip galvanized steel sheets may be further subjected to organic coating treatment after plating. In the present invention, when the slab is hot-rolled, it may be rolled after being heated in a heating furnace, or may be directly rolled without being heated, and the obtained hot-rolled steel sheet is normalized or annealed. Processing may be performed.

  In a cold-rolled steel sheet, the cold pressure rate may be about 40 to 80% within a normal range. However, the higher the cold pressure ratio, the finer ferrite grains can be obtained, which is advantageous from the viewpoint of annealing time and final hole expansibility.

  The manufacturing method which can obtain the steel plate which concerns on this invention is not restricted above. Regardless of the manufacturing method, even if it is a cold-rolled structure, if a structure form as shown in FIG. Sex is obtained.

After melting steel numbers 1 to 10 shown in Table 1, slabs were produced by continuous casting. After heating these slabs to 1200 ° C, they are hot-rolled at a finishing temperature of normal operation (Ar ₃ points or more), wound at a winding temperature of 550 to 600 ° C, and then subjected to box annealing under the conditions shown in Table 2 And the continuous annealing was given and the hot-rolled steel plate was manufactured. The hot-rolled steel sheet thus obtained was pickled and then partially passed through a continuous hot dip galvanizing line. These steel sheets were examined for the ferrite grain coverage, tensile strength, and hole expansibility by the final microstructure and the hard second phase. In Table 1, No. 1-No. No. 7 satisfies the composition range of the present invention. 8-No. 10 is outside the composition range of the present invention.

  Table 2 shows these results. FIG. 2 shows the relationship between the coverage ratio and the hole expansion ratio for those satisfying the composition range of the present invention. As shown in Table 2 and FIG. 2, the example of the present invention satisfying the composition of the present invention and the ferrite grain coverage by the hard second phase satisfies the formula (1) is more than the comparative example (conventional steel). It was confirmed that the hole expandability was greatly improved. On the other hand, in the comparative example that does not satisfy the coverage condition, the hole expandability was not improved.

  The composite structure steel plate of the present invention is suitable for automobile structural members that require processing such as hole expansion.

The photograph which shows an example of the microstructure of the conventional structure steel and the composite structure steel plate by this invention. Explanatory drawing which shows the relationship between the coverage of a ferrite grain, and a hole expansion rate.

Claims (2)

In mass%, C: 0.01 to 0.2%, Si: 2.0% or less, Mn: 3.0% or less, P: 0.08% or less, S: 0.03% or less, Al: 0 .01-0.1%, N: 0.01% or less, the balance being substantially made of iron, and the structure of the ferrite phase of the main phase is covered by joining the grains of the hard second phase , the entire circumferential length of one of the ferrite grains L, and the length of the hard second phase present around the ferrite grains is taken as L _2, and satisfies the following relationship (1) , A composite steel sheet with excellent hole expandability.
L ₂ ≧ 0.7 × L (1)   1% of Cr: 1% or less, Mo: 1% or less, V: 1% or less, Ni: 1% or less, B: 0.01% or less, Ti: 0.3% or less, Nb: 0.3% or less The composite structure steel plate excellent in hole expansibility of Claim 1 containing the seed | species or more.

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