JP2015513608A - High strength bake hardenable low density steel and method for producing the same - Google Patents

Abstract

The present invention relates to a high-strength bake-hardening low density steel and a method for producing said steel.

Description

  The present invention relates to a high strength bake hardenable low density steel and a method for producing the same.

In an ongoing effort to reduce vehicle carbon emissions, the steel industry is working with automakers to enable carbon emissions to be reduced without affecting steel processability and finished product safety. We are continuously striving to gain. In order to meet future CO ₂ emission regulations, the fuel consumption of automobiles must be reduced. One way to reduce this is to reduce the weight of the car body. Low density and high strength steel can contribute to this. When low density steel is used with the same thickness, the weight of automobile parts is reduced. A problem with known high strength steels is that when forming sheets into car parts, the formability is impaired due to the high strength of the material.

  With normal high-strength steel such as duplex stainless steel, a thinner sheet can be used, which makes it possible to reduce the weight. However, thinner parts will negatively affect other properties such as stiffness, crash resistance and dent resistance. To eliminate these negative effects, the effect of weight reduction is lost, but the thickness of the steel is increased, or this is also undesirable, but the shape of the part must be changed.

  An object of the present invention is to provide a low density steel having both high strength in a finished part and excellent formability before forming an automobile part.

  It is also an object of the present invention to provide a high strength steel having excellent stiffness and dent resistance.

The objects of the present invention can be achieved by providing a ferritic steel strip or sheet comprising the following components in weight percent:
C (total carbon content) is 0.01% or less,
Si is 0.5% or less,
Mn is 1.0% or less,
Al 5% to 10% upper limit,
N is 0.010% or less,
Ti is 0.019% or less,
Nb is 0.08% or less,
Zr is 0.1% or less,
V is 0.1% or less,
S is 0.01% or less,
P is 0.1% or less,
Optionally, 5-50 ppm of B, with iron and inevitable impurities as the balance.
here,
C (solute carbon content) = C (total carbon content)
・ Minimum [X, Y]
・ Maximum [Z, 0]
・ (12/93) × Nb
・ (12/91) × Zr
・ (12/51) × V
Here, X, Y, and Z are
X = 2 × 12 / (2 × 32) × S,
Y = 2 × 12 / (4 × 48) × (Ti−48 / 14 × N),
Z = 12/48 x (Ti-48 / 14 x N-4 x 48 / (2 x 32) x S),
And where
Minimum [X, Y] = the smaller value of X and Y,
Minimum [X, Y] = 0 if Y is negative,
Maximum [Z, 0] = 0 and the larger value of Z,
C (solute carbon content) is at least 0.0005 (5 ppm).

  Unless otherwise noted, all percentages are percent by weight. In order to avoid misunderstandings, the above formulas are correctly interpreted when entered in a commercial spreadsheet program, such as Microsoft Excel. For example, one skilled in the art understands the atomic mass of carbon (12) and Nb (93) in this formula, so 12/93 × Nb is interpreted correctly as (12/93) × Nb. This is the same for the other numerical values in the formula (with changes to be made). Therefore,

である。

It is.

  The steel according to the invention has a chemical composition adapted to allow the steel to contain carbon (solute carbon content) in the solid solution after annealing and optional galvanizing steps. The carbon in this solid solution allows the steel to be bake hardenable, for example in a paint baking cycle. Automotive parts are formed from steel obtained from a mill, and the components are painted and baked after they are formed into their final shape.

  In addition, the steel according to the present invention combines good formability before forming automotive parts, that is, before paint baking operation, and high strength after paint baking operation.

  The inventors have found that for steels that are bake hardenable in paint baking cycles, at least 5 ppm of solute carbon (solute carbon content) must be present in the steel. If the amount of solute carbon is small, the effect is negligible or cannot be reproduced.

  The level of solute carbon may also not exceed the critical upper value. This is because steel is preferably not naturally aged. Natural aging is the spontaneous aging of supersaturated solid solutions at room temperature, including spontaneous changes in the physical properties of the steel, which are transported to the customer after hot or cold rolling or after the final heat treatment, e.g. before processing the strip Occurs when held at ambient temperature during storage or at customer storage. This natural aging includes changes in mechanical properties that are considered undesirable because they lead to unpredictable variations in processability during the formation of automotive parts. Also, the surface quality can be adversely affected by the formation of so-called Luder-lines. Moreover, when the carbon level in a solid solution is too high, the moldability before baking hardening may fall.

  For that reason, a maximum of 50 ppm solute carbon is preferred. A more preferred maximum is 40 ppm solute carbon (ie 0.004%).

  In one embodiment of the invention, the solute carbon content is at least 0.0010 (10 ppm) and / or at most 0.0030 (30 ppm). This achieves a stable process and reproducible properties.

  Nitrogen, especially free nitrogen (ie, nitrogen in solid solution) is undesirable in steelmaking but is unavoidable. Titanium can optionally be added to bind nitrogen to TiN. Large amounts of aluminum in the steel can also ensure that all nitrogen is bound. This means that the matrix is substantially free of nitrogen in the solid solution.

  Optionally, boron is added to the steel. Its presence is not essential, but may help to reduce the tendency of secondary products to become brittle. If added, a minimum of 5 ppm boron is required.

  In one embodiment of the invention, the manganese content is at least 0.1%. In another embodiment, the aluminum content is at least 6% and / or at most 9%, preferably at most 8%.

  The steel is preferably calcium treated. The chemical composition may therefore also contain an amount of calcium suitable for calcium treatment.

  In the steel according to the invention, the amount of carbon in the solid solution is controlled by the addition of microalloying elements (Ti, Nb, V, Zr) in combination with an excellent control of the total carbon content in the steel.

  The amount of Ti or Nb must be strictly controlled. If there is too much titanium or niobium, it will combine with carbon to form carbides or carbon sulfides in the presence of sulfur. As a result, solute carbon cannot be obtained, and bake hardenability cannot be obtained.

The amount of carbon in the solid solution according to the present invention is calculated by subtracting the precipitate comprising carbon from the total carbon content (C_total) as follows.
C (solute carbon content) = C (total carbon content)
・ Minimum [X, Y]
・ Maximum [Z, 0]
・ (12/93) × Nb
・ (12/91) × Zr
・ (12/51) × V,
Here, X, Y, and Z are
X = 2 × 12 / (2 × 32) × S,
Y = 2 × 12 / (4 × 48) × (Ti−48 / 14 × N),
Z = 12/48 x (Ti-48 / 14 x N-4 x 48 / (2 x 32) x S),
And where
Minimum [X, Y] = the smaller value of X and Y,
Minimum [X, Y] = 0 if Y is negative,
Maximum [Z, 0] = 0 and the larger value of Z.

  See above in the specification for the interpretation of these formulas. The addition of Ti is beneficial for bonding with nitrogen, but is not strictly necessary. Up to 0.019% Ti can be added to the steel, and it can be combined mainly with nitrogen to form TiN, and the amount of solute carbon can be controlled secondary. The titanium content should be 0.019% or less, for example 0.018% or 0.015% at the upper limit, or even 0.012% at the maximum.

  When titanium is added as an alloying element, the preferred minimum value of titanium content is 0.005%. A preferred minimum value when Nb is added is 0.008%. Preferred minimum values when V and Zr are added are 0.002 and 0.004, respectively.

According to a preferred embodiment, the composition of the ferritic steel according to the invention has the following basic composition (weight percent):
C (total carbon content) is 0.01% or less,
Si is 0.5% or less,
Mn is 1.0% or less,
Al 5% to 10% upper limit,
N is 0.010% or less,
Ti is 0.019% or less,
Nb is 0.08% or less,
Zr is 0.1% or less,
V is 0.1% or less,
S is 0.01% or less,
P is 0.1% or less,
Optionally, 5 to 50 ppm of B, and iron and unavoidable impurities as the balance. In this composition, no titanium has been added to the steel, and the titanium present is an unavoidable impurity.

Titanium first forms TiN as an alloying element or as an inevitable impurity. If excess nitrogen is present, the remaining nitrogen combines with aluminum. If there is an excess of titanium, the remaining titanium will form Ti ₄ C ₂ S ₂ until all of the titanium is consumed. The factor Minimum [X, Y] calculates how much carbon is consumed by the formation of Ti ₄ C ₂ S ₂ after all the free nitrogen has combined with TiN. If the calculation results in a negative value for Y, the factor is set to zero.

In the absence of titanium, TiN or Ti ₄ C ₂ S ₂ is not formed and Minimum [X, Y] is zero. The factor Maximum [Z, 0] determines how much carbon binds to titanium after participating in the formation of TiN and Ti ₄ C ₂ S ₂ .

  The other three factors are involved in the formation of NbC, ZrC and VC, and together with the factors Minimum [X, Y] and Maximum [Z, 0] determine the amount of solute carbon in the steel.

  By adding no or only a small amount of titanium and / or by adding a certain amount of Nb, there will be sufficient solute carbon available for bake hardening. By controlling the level of solute carbon below 50 ppm, preferably below 40 ppm, the steel according to the invention can be bake hardened and is resistant to natural aging.

The method for producing a ferritic steel strip according to the second aspect comprises:
Preparing a steel slab or thick strip by continuous casting, or by thin slab casting, or by belt casting, or by strip casting,
Optionally reheating the steel slab or strip at a reheating temperature of at most 1250 ° C., if desired;
Hot rolling the slab or thick strip and ending the hot rolling process at a hot rolling finish temperature of at least 850 ° C .;
Coiling the hot rolled strip at a coiling temperature of 550-750 ° C .;
A method comprising is provided.

  In a preferred embodiment, the coiling temperature is at least 600 ° C. and / or the hot rolling finish temperature is at least 900 ° C.

The hot rolled strip is then
Cold rolling the hot rolled strip at a cold reduction of 40-90% to produce a cold rolled strip;
Annealing the cold-rolled strip at a peak metal temperature of 700-900 ° C. in a continuous annealing process;
Optionally galvanizing strips annealed in hot dip galvanization or electrogalvanization or heat-to-coat processes;
It can be further processed by a process comprising:

  Hot rolled strips are usually pickled and cleaned prior to the cold rolling process. In one embodiment, the peak metal temperature in the continuous annealing process is at least 750 ° C, preferably at least 800 ° C.

  In one embodiment, the cold reduction is at least 50%.

  In one embodiment, the thickness of the hot rolled strip is 1-5 mm and / or the thickness of the cold rolled strip is 0.4-2 mm.

  In one embodiment of the present invention, the hot-rolled strip is annealed in a continuous annealing process and optionally galvanized in a hot dip galvanizing process. Annealing can also involve a so-called heat-to-coat cycle. In a heat-to-coat cycle, the hot-rolled steel is reheated to a temperature sufficient to perform hot dip galvanizing, but not as high as a normal continuous annealing step. During reheating, the carbon that can precipitate during the slow cooling of the hot-rolled coil after hot rolling is again made into a solid solution. After annealing and / or galvanization, the steel must be rapidly cooled to avoid precipitation of carbon in the solid solution. If this galvanized steel sheet is used to produce automotive parts or other products by molding, followed by painting and baking, paint baking will surely increase the strength associated with the paint baking cycle. .

  The invention will now be further illustrated by the following non-limiting examples.

  Steel was produced and processed into cold rolled steel sheets having a thickness of 1 mm. The hot rolled strip had a thickness of 3.0 mm. The chemical composition of the steel is shown in Table 1.

  Steel was produced by casting the slab and reheating the slab at temperatures up to 1250 ° C. This temperature is the maximum temperature. This is because excessive grain growth can occur at higher reheating temperatures. Finishing temperature during hot rolling is 900 ° C., coiling temperature is 650 ° C., followed by pickling, cold rolling (67%), continuous annealing at 800 ° C. peak metal temperature, Plating. Steel 3a also contained 16 ppm B.

  The results shown in Table 2 clearly show that the presence of solute carbon in amounts up to 14-24 ppm or 31 ppm results in an increase of about 40 MPa on work hardening and the basic strength of steel. The inventors have found this effect at solute carbon levels of 5-50 ppm.

Claims (15)

In weight percent
C (total carbon content) is 0.01% or less,
Si is 0.5% or less,
Mn is 1.0% or less,
Al 5% to 10% upper limit,
N is 0.010% or less,
Ti is 0.019% or less,
Nb is 0.08% or less,
Zr is 0.1% or less,
V is 0.1% or less,
S is 0.01% or less,
P is 0.1% or less,
If desired, 5-50 ppm of B, and the balance iron and inevitable impurities,
Comprising
C (solute carbon content) = C (total carbon content)
・ Minimum [X, Y]
・ Maximum [Z, 0]
・ (12/93) × Nb
・ (12/91) × Zr
・ (12/51) × V,
Here, X, Y, and Z are

And
Minimum [X, Y] = the smaller value of X and Y,
Minimum [X, Y] = 0 if Y is negative,
Maximum [Z, 0] = 0 and the larger value of Z,
A ferritic steel strip or sheet having a C (solute carbon content) of at least 0.0005 (5 ppm).   The steel according to claim 1, wherein the upper limit of C (solute carbon content) is 0.0050 (50 ppm).   The steel according to claim 1 or 2, wherein Mn is at least 0.1%.   Steel according to any one of claims 1 to 3, wherein Al is at least 6% and / or 9% at the upper limit, preferably 8% at the upper limit.   The steel according to claim 1, wherein C (total carbon content) is at least 0.0010% (10 ppm).   C (solute carbon content) is at least 0.0010% (10 ppm) and / or 0.0040% (40 ppm) at the upper limit, preferably 0.0030% (30 ppm) at the upper limit. The steel according to any one of 5.   The steel according to any one of claims 1 to 6, wherein N has an upper limit of 0.005% (50 ppm).   The steel according to any one of claims 1 to 7, wherein Si has an upper limit of 0.2%. The steel according to any one of claims 1 to 8, wherein the specific gravity of the steel is 6800 to 7300 kg / m ³ . A method of manufacturing a ferritic steel strip,
Preparing a steel slab or thick strip by continuous casting, or by thin slab casting, or by belt casting, or by strip casting,
Optionally reheating the steel slab or strip at a reheating temperature of at most 1250 ° C., if desired;
Hot rolling the slab or thick strip and ending the hot rolling process at a hot rolling finish temperature of at least 850 ° C .;
Coiling the hot rolled strip at a coiling temperature of 550-750 ° C .;
Comprising a method. The hot rolled strip carbon,
Reheat in a continuous annealing step, optionally followed by hot dip galvanizing, followed by rapid cooling, or reheated in a heat to coat step, followed by hot dip galvanizing, rapid cooling Item 11. The method according to Item 10. A method of manufacturing a ferritic steel strip, comprising:
Cold rolling the ferritic steel strip according to claim 10 at a cold reduction of 40 to 90% to produce a cold rolled strip;
Annealing the cold rolled strip at a peak metal temperature of 700-900 ° C. in a continuous annealing process;
Optionally galvanizing the annealed strip in a hot dip galvanizing or electrogalvanizing, or heat to coat process;
Comprising a method.   13. A method according to claim 12, wherein the peak metal temperature in the continuous annealing process is at least 750 ° C, preferably at least 800 ° C.   14. A method according to any one of claims 11 to 13, wherein the cold rolling reduction is at least 50%.   15. The method according to any one of claims 10 to 14, wherein the hot rolled strip has a thickness of 1 to 5 mm and / or the cold rolled strip has a thickness of 0.4 to 2 mm.