JP4514150B2 - High strength steel plate and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength steel sheet suitable for transportation equipment and structural materials such as automobiles and a method for producing the same.

  As high-strength steel sheets having both excellent press formability and high strength, transformation structure strengthened steels such as composite structure steels and retained austenitic steels and precipitation strengthened steels are known. The composite structure steel mentioned here is a steel sheet in which martensite is dispersed in ferrite ground, and has a low yield ratio, high tensile strength, and excellent elongation properties, but the ferrite / martensite interface is the starting point of fracture. Thus, the hole expandability (stretch flangeability) is slightly inferior. Residual austenitic steel generates retained austenite in the structure, and this retained austenite induces transformation during processing deformation and exhibits excellent ductility, but also has the disadvantage of being inferior in hole expansibility. .

Precipitation strengthened steel uses precipitation strengthening by carbonitrides such as Ti and Nb, but coarse carbonitrides precipitate or crystal grains become finer, so that the press formability is a composite structure steel plate or Inferior to residual austenitic steel.
As techniques for improving the disadvantages of such conventional precipitation strengthened steels, Patent Documents 1 and 2 describe steel sheets containing Ti, Nb and Mo, and fine precipitates having a grain size of less than 10 nm in a matrix of a ferrite single phase structure. A steel sheet in which is dispersed is disclosed. Since this steel sheet has very fine precipitates, the precipitates do not serve as a base point for fracture. However, since Nb and Mo are expensive elements in the first place, there is a drawback that the manufacturing cost increases.

By the way, in the conventional continuous casting, for example, the average cooling rate in the middle part of the slab (1/4 t position of the slab of thickness t) is as small as about 0.1 ° C./sec. Such a coarse AlN did not contribute to the strengthening of steel. Therefore, AlN has little influence on the material characteristics, or rather has a possibility of deteriorating local deformation characteristics.
JP 2002-322539 A JP 2002-322540 A

  An object of the present invention is to solve the conventional problems and to provide a high-strength steel sheet excellent in press formability by effectively using AlN.

As a result of intensive studies, the inventors have obtained the following knowledge and completed the present invention.
That is, it was found that coarse AlN precipitation can be suppressed during slab cooling by setting the average cooling rate in the middle part of the slab to 3 ° C./sec or more. Thereafter, the slab is cooled to a low temperature and reheated or directly subjected to hot rolling, and further, by controlling the cooling after hot rolling, unprecipitated AlN is refined finely after hot rolling in the casting process. It can be deposited. Then, these composite nitrides are obtained by depositing AlN alone or nitride such as TiN, NbN, VN or the like, or carbide such as TiC, NbC, VC, or carbonitride such as TiCN, NbCN, VCN, etc. The composite carbonitride can be made very fine and dispersed in the ferrite, and it becomes possible to manufacture a high-strength steel sheet excellent in press formability with reduced alloy without deteriorating the local ductility. I found it.

The high-strength steel sheet of the present invention made based on the above knowledge is in mass%,
C: 0.001 to 0.25%, Si: 2.0% or less, Mn: 0.01 to 3%, P: 0.0010 to 0.1%, S: 0.0010 to 0.05%, N: 0.0020 to 0.010%, Al: 0.01 to 2.0%,
Further , if necessary, one or more of Ti: 0.001 to 0.5%, Nb: 0.001 to 0.5%, V: 0.001 to 0.5% can be represented by the formula ( 1) and a content that satisfies (2), and has a steel composition consisting of the balance iron and inevitable impurities,
The density of 1 × 10 ¹⁰ to 1 × 10 ¹⁵ particles / m of Al nitride, or composite nitride or composite carbonitride of Al and at least one of Ti, Nb, and V is 2 × 10 nm. ³ is present.
Ti / 48 + Nb / 93 + V / 51-When C / 12 <0,
Al / 27 ≧ 3.0 × (N / 14) (1)
Ti / 48 + Nb / 93 + V / 51-When C / 12 ≧ 0,
Al / 27 + (Ti / 48 + Nb / 93 + V / 51-C / 12) ≧ 3.0 × (N / 14) (2)

In the above-described invention, during the steel composition, Cr: 0.01 to 5%, Ni: 0.01 to 5%, Cu: 0.01 to 5%, Co: 0.01 to 5%, W: 0.00. 01-5%, Mo: 0.01-1.0% of one or more can be contained,
Further, in the steel composition, one or more of Zr, Hf, Ta can be contained alone or in total 0.001-1%,
Further, B: 0.0001 to 0.0050% can be contained in the steel composition,
The steel composition may further contain one or more of Mg, Ca, Y, and REM, either alone or in total of 0.0001 to 0.5%.

Moreover, the manufacturing method of the high strength steel plate of this invention is a manufacturing method of the high strength steel plate which manufactures the high strength steel plate in any one of Claims 1-5 from slab,
The slab in the middle of cooling after casting is cooled at a temperature range of 1400 ° C. or lower and 1000 ° C. or higher at an average cooling rate of 3 ° C./sec or higher at a position of 1/4 t of the thickness t of the slab, or 1100 ° C. Reheat to above,
Next, hot rolling is performed at a finishing temperature of 850 to 970 ° C., followed by cooling to a temperature range of 750 to 650 ° C. at an average cooling rate of 10 to 100 ° C./sec, and then winding at a temperature of 650 to 550 ° C. It is characterized by.

The high-strength steel sheet of the present invention has fine and high density dispersed precipitates of particles having a uniform particle size of 2 to 10 nm of composite nitrous acid or composite carbonitride of Al and Ti, Nb, etc. Compared to conventional precipitation-strengthened steel, it is possible to achieve both alloy saving and high strength of expensive additive elements such as Nb and Mo.
Further, the method for producing a high-strength steel sheet according to the present invention cools the slab at high speed, so that coarse AlN does not precipitate as in the case of conventional low-speed cooling, and there is little concern about deterioration of local deformation characteristics. . Further, since the slab is subjected to reheating or direct hot rolling and then wound at an appropriate temperature, the above-described fine composite nitride and the like can be efficiently dispersed and precipitated on the ferrite base.

The best mode for carrying out the present invention will be described below.
The high-strength steel sheet of the present invention has a structure in which the matrix is mainly composed of ferrite, and the ferrite has a particle size of 2 to 10 nm of a composite nitrous oxide or composite carbonitride of Al and Ti, Nb, V, or the like. The particles are present at a density of 1 × 10 ¹⁰ to 1 × 10 ¹⁵ particles / m ³ . The reason why the matrix is mainly composed of ferrite is that ferrite has the highest workability in the steel structure. The matrix can be a ferrite single phase, ferrite + pearlite or ferrite + bainite depending on the intended strength level.

The reasons for limiting the components of the high-strength steel sheet of the present invention will be described below.
C is an element that increases the strength. When the content of C exceeds 0.25%, ductility is insufficient and weldability is deteriorated when the retained austenite is not sufficiently formed. On the other hand, if C is less than 0.001%, the cost for refining will increase. Therefore, the range of C in the present invention is 0.001 to 0.00. 25%.

  Si is an element that is added to ensure the ductility by forming ferrite, but if added over 2.0%, the toughness and weldability deteriorate. Si is added for deoxidation, but if it is less than 0.005%, the deoxidation effect is not sufficient. Therefore, the range of Si is made 0.05 to 2.0%.

  Mn is necessary from the viewpoint of securing strength. If Mn is less than 0.01%, the strength is not satisfactory. On the other hand, if the amount of Mn added exceeds 3%, the hardenability is unnecessarily increased, resulting in an increase in excess strength, thereby resulting in insufficient ductility. Therefore, the range of Mn in the present invention is set to 0.01 to 3%.

  When P is contained in a large amount, it segregates to the grain boundary, so that the local ductility is deteriorated. In addition, the weldability is deteriorated. Therefore, the upper limit is made 0.1%. The reason why the lower limit is set to 0.001% is that a further reduction leads to a cost increase during refining in the steelmaking stage.

  S is an element that forms MnS to deteriorate local ductility and weldability. Therefore, the upper limit is made 0.05%. On the other hand, the reason why the lower limit is set to 0.001% is that, similarly to P, a further reduction leads to an increase in cost during refining in the steelmaking stage.

  N is an important element in the present invention in order to strengthen the steel by forming nitrides with Al, Nb, Ti, and V. For this purpose, the steel needs to contain 0.0020% or more of N. On the other hand, if the content exceeds 0.010%, solute nitrogen (sol.N) remains and ductility may be reduced, or it may cause blow holes during welding. Therefore, the range of N is made 0.0020 to 0.010%.

  Al is an important element for strengthening steel by precipitating Ti, Nb, V, composite nitride, and composite carbonitride. It is also important as a deoxidizer. For these purposes, it is necessary to add 0.01% or more of Al. On the other hand, even if Al is added excessively, the above effect is saturated and the steel is embrittled, so the upper limit was made 2.0%. Therefore, the range of Al is 0.01 to 2.0%.

  Ti, Nb, and V precipitate a fine composite nitride or composite carbonitride with Al to strengthen the steel. For this purpose, it is necessary to add 0.001% or more of any element. However, if added over 0.5%, the ductility is lowered or embrittled. Therefore, steel needs to contain 0.01 to 0.5% of any one or more of Ti, Nb and V. Steel consists of the balance inevitable impurities and iron. From the viewpoint of alloy saving, it is desirable to add 0.03% or less in total if it is 590 MPa class and 0.06% or less if it is 780 MPa class.

The Al described above must satisfy the following formula (1) when the contents of Ti, Nb, and V are Ti / 48 + Nb / 93 + V / 51−C / 12 <0.
Al / 27 ≧ 3.0 × (N / 14) (1)
That is, when the content of Ti, Nb, and V is Ti / 48 + Nb / 93 + V / 51−C / 12 <0, Ti, Nb, and V are combined with Al to precipitate a composite nitride. Since the effect contributing to the increase in strength is small, it is necessary to add Al in an amount that satisfies the formula (1).
On the other hand, when the contents of Ti, Nb, and V are Ti / 48 + Nb / 93 + V / 51−C / 12 ≧ 0, Al is added in an amount that satisfies the formula (2).
Al / 27 + (Ti / 48 + Nb / 93 + V / 51-C / 12) ≧ 3.0 × (N / 14) (2)
That is, when the content of Ti, Nb, and V is Ti / 48 + Nb / 93 + V / 51−C / 12 ≧ 0, a composite nitride having Ti, Nb, V and Al is precipitated. This is because the effect that contributes to the increase in strength is increased, and therefore, Al may be added in an amount satisfying the expression (2).
By satisfying the above formulas and applying the production method of the present invention, it is possible to achieve the high strength and high ductility that are the objects of the present invention.

In the present invention, the steel has a particle having a particle size of 2 to 10 nm of a composite nitrous oxide or composite carbonitride of Al alone or at least one of Ti, Nb, and V and Al. It must be present in density. As combinations with Al in the present invention, there are Al-Ti, Al-Nb, Al-V, Al-Ti-Nb, Al-Ti-V, Al-Nb-V, and Al-Ti-Nb-V. Particles having a particle size of 2 to 10 nm have a density of a composite nitride in which elements related to these combinations and nitrogen are combined, or a composite carbonitride in which elements related to these combinations are combined with carbon and nitrogen. It is necessary that 1 × 10 ¹⁰ to 1 × 10 ¹⁵ / m ³ exist. This is because if the particles of less than 2 nm are uniformly dispersed to a certain density or more, there is a concern that the curing allowance is large and the total elongation is greatly reduced, and if it exceeds 10 nm, a high reinforcing ability cannot be obtained.

In addition, as described above, the composite nitrous oxide or the composite carbonitride needs to exist in a density of 1 × 10 ¹⁰ to 1 × 10 ¹⁵ pieces / m ³ . This is because if it is less than 1 × 10 ¹⁰ pieces / m ³ , the steel cannot be sufficiently strengthened, while if it exceeds 1 × 10 ¹⁵ pieces / m ³ , the effect of increasing the strength is saturated.

  Steel is further Cr: 0.01-5%, Ni: 0.01-5%, Cu: 0.01-5%, Co: 0.01-5%, W: 0.01-5%, Mo : 0.01-1.0% of 1 type, or 2 or more types can be contained. These elements improve the hardenability and increase the strength of the steel. If it is less than 0.01%, these effects cannot be sufficiently expected. Even if Cr, Ni, Cu, Co, W is added in excess of 5% or Mo is added in excess of 1.0%, the effect of increasing the strength is saturated and the ductility is reduced. It becomes.

  The steel can further contain 0.001 to 1% of one or more of Zr, Hf, and Ta alone or in total. These elements form nitrides and refine crystal grains. For this purpose, it is necessary to add 0.001% or more, but even if added over 1%, the effect is saturated.

  The steel can further contain B: 0.0001 to 0.0050%. B is added to improve hardenability efficiently by adding a small amount, but 0.0001% or more of addition is necessary to exert this effect. However, if this amount increases excessively, there is a risk of hot cracking, and the effect is saturated, so the upper limit is made 0.0050%. Moreover, the hardenability improvement effect is easy to be exhibited by combined addition with Ti.

  The steel can further contain one or more of Mg, Ca, Y, and REM (rare earth elements) alone or in total 0.0001 to 0.5%. Mg, Ca, Y and REM improve the ductility by controlling the form of sulfides and oxides. For this purpose, it is necessary to add one or more of these elements alone or in total to 0.0001% or more. However, excessive addition deteriorates workability, so the upper limit was made 0.5%.

Below, the manufacturing method of the high strength steel plate concerning the present invention is explained.
When producing the high-strength steel sheet of the present invention, the slab that is being cooled after casting is cooled at a temperature range of 1400 ° C. or lower and 1000 ° C. or higher with an average cooling rate of 3 ° C./sec or higher. The average cooling rate here refers to the average cooling rate at a position of 1/4 t of the slab of thickness t. In the present invention, any method may be used as long as the cooling rate during casting can be higher than 3 ° C./sec. For example, in continuous casting, the slab thickness is reduced, in ingot casting, the size of the ingot is reduced, and the surface layer portion with a high cooling rate is cut out from a normal slab or ingot and used. good. In particular, when the continuous cast slab thickness is reduced, the thickness of the slab is preferably 100 to 30 mm. If the thickness exceeds 100 mm, the slab cannot be cooled at a large cooling rate. If it is less than 30 mm, the casting speed increases, causing fluctuations in the molten metal surface, breakout, etc., and stable casting becomes difficult. It is.

  Moreover, when the average cooling rate is less than 3 ° C./sec, coarse AlN is likely to precipitate because the cooling rate is low, and fine composite nitrides and composite carbonitrides of Al and Ti, Nb, and V are precipitated. This is because it becomes difficult.

  The slab after cooling can be directly subjected to hot rolling. Alternatively, when it is cooled to less than 1100 ° C., it can be reheated to 1100 ° C. or higher and 1300 ° C. or lower. If the reheating temperature is less than 1100 ° C, deformation resistance in hot rolling is high and there is a concern about precipitation of AlN. If it exceeds 1300 ° C, the generated scale becomes thick and the steel sheet has good surface properties. It is because it is not possible.

  Next, the slab is hot-rolled at a finishing temperature of 850 to 970 ° C. This is because if the finishing temperature is less than 860 ° C., the rolling load becomes large, or if the temperature is further lowered, it becomes (α + γ) two-phase region rolling, which may impair the shape of the plate. Thus, there is a concern that characteristics such as toughness deteriorate.

After the hot rolling finish, the temperature range from 750 to 650 ° C. is subsequently cooled at an average cooling rate of 10 to 100 ° C./sec, and then wound at a temperature of 650 to 550 ° C.
When the average cooling rate in the temperature range of 750 to 650 ° C. is less than 10 ° C./sec, AlN is excessively precipitated during cooling, and sufficient strengthening ability cannot be obtained. On the other hand, the production of AlN can be sufficiently suppressed by cooling to 100 ° C./sec, and there is no need to cool the steel plate at a higher speed. Therefore, an average cooling rate shall be 10-100 degrees C / sec.

By winding the steel sheet after rapid cooling at a temperature of 550 to 750 ° C., sufficient ferrite is ensured, and fine composite nitride or composite charcoal of Al and at least one of Ti, Nb, and V Nitride can be obtained. When the coiling temperature is less than 550 ° C., precipitation of composite nitride or the like is delayed, and a precipitate having a desired density cannot be secured. On the other hand, at 750 ° C. or higher, the size of the composite nitride or the like becomes coarse, and it becomes difficult to obtain sufficiently fine precipitates. Therefore, the winding temperature is set to 550 to 750 ° C.
By cooling after hot rolling under the above conditions, a high-strength steel sheet excellent in press formability in which fine composite nitrides and the like are precipitated in ferrite can be produced.

Hereinafter, the present invention will be described in detail based on examples.
Steels having chemical components shown in Tables 1 and 2 melted in a converter or a laboratory were cast. At this time, the cooling rate of the slab in the temperature range of 1400 to 1100 ° C. was changed as shown in Table 3. These slabs were subjected to hot rolling to produce high strength steel plates. Table 3 shows cooling conditions, rolling conditions, and material characteristics.

  Steel types CA and CB are comparative steels, and both have an Al content not satisfying the formula (1). For this reason, the density of the precipitates is small outside the range of the present invention, and the steel sheet is inferior in strength and elongation balance. Steel types CC, CD, and CE are high in at least one of additive elements such as Ti, Nb, Ni, and Mo beyond the scope of the present invention. For this reason, ductility was insufficient, cracks occurred frequently, and cold rolling was impossible.

Steel types A to L have chemical components within the scope of the present invention. However, in the processing numbers 3, 6, 10, 13, 17, and 19, the slab cooling rate was small outside the scope of the present invention. For this reason, since the density of the nitride particles is small outside the scope of the present invention, the steel cannot be made high strength and high ductility.
Moreover, since the heating temperature before the hot rolling was low in the processing numbers 8 and 16, the nitride was coarsened and the density was low, and the steel could not be made high strength and high ductility. It was.

  For the comparative steels as described above, the processing conditions of 1, 2, 4, 5, 7, 9, 11, 12, 14, 15, 18, 20 to 25 are within the scope of the present invention. Since it was appropriate, a large number of fine precipitates could be precipitated, and as a result, a high-strength steel sheet excellent in strength and elongation balance could be produced.

It is explanatory drawing which shows the method of this invention compared with the conventional method.

Claims (6)

In mass%
C: 0.001 to 0.25%, Si: 2.0% or less, Mn: 0.01 to 3%, P: 0.0010 to 0.1%, S: 0.0010 to 0.05%, N: 0.0020 to 0.010%, Al: 0.01 to 2.0%,
Further , if necessary, one or more of Ti: 0.001 to 0.5%, Nb: 0.001 to 0.5%, V: 0.001 to 0.5% can be represented by the formula ( 1) and containing in the range that satisfies (2), having a steel composition consisting of the balance iron and inevitable impurities,
The density of 1 × 10 ¹⁰ to 1 × 10 ¹⁵ particles / m of Al nitride, or composite nitride or composite carbonitride of Al and at least one of Ti, Nb, and V is 2 × 10 nm. high strength steel sheet characterized by the presence in ^3.
Ti / 48 + Nb / 93 + V / 51-When C / 12 <0,
Al / 27 ≧ 3.0 × (N / 14) (1)
Ti / 48 + Nb / 93 + V / 51-When C / 12 ≧ 0,
Al / 27 + (Ti / 48 + Nb / 93 + V / 51-C / 12) ≧ 3.0 × (N / 14) (2)   Further during the steel composition, Cr: 0.01-5%, Ni: 0.01-5%, Cu: 0.01-5%, Co: 0.01-5%, W: 0.01-5% Mo: 0.01-1.0% of 1 type or 2 types or more are contained, The high strength steel plate of Claim 1 characterized by the above-mentioned.   The high-strength steel sheet according to claim 1 or 2, further comprising 0.001 to 1% of one or more of Zr, Hf, and Ta alone or in total in the steel composition.   The high-strength steel sheet according to any one of claims 1 to 3, further comprising B: 0.0001 to 0.0050% in the steel composition.   The steel composition further comprises one or more of Mg, Ca, Y, and REM, individually or in total, 0.0001 to 0.5%. High strength steel sheet as described. A method for producing a high-strength steel sheet, wherein the high-strength steel sheet according to any one of claims 1 to 5 is produced from a slab,
The slab in the middle of cooling after casting is cooled at a temperature range of 1400 ° C. or lower and 1000 ° C. or higher at an average cooling rate of 3 ° C./sec or higher at a position of 1/4 t of the thickness t of the slab, or 1100 ° C. Reheat to above,
Next, hot rolling is performed at a finishing temperature of 850 to 970 ° C., and the temperature range from 750 to 650 ° C. is subsequently cooled at an average cooling rate of 10 to 100 ° C./sec, and then wound at a temperature of 750 to 550 ° C. The manufacturing method of the high strength steel plate characterized by the above-mentioned.

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