CN111020296A - Ni-Al-Si alloy system coated steel plate with wide hot forming heating process window and preparation and hot stamping forming process thereof - Google Patents

Abstract

The invention discloses a Ni-Al-Si alloy system coated steel plate with a wide hot forming heating process window and a preparation and hot stamping forming process thereof, wherein a liquid bath used for coating the steel strip contains the following raw materials in percentage by weight: 5-11% of Si, 0-4% of Fe and 0.05-5% of Ni, the balance of Al and impurities inherent in processing, and the thickness of the coating layer is 10-33 mu m.

Description

Ni-Al-Si alloy system coated steel plate with wide hot forming heating process window and preparation and hot stamping forming process thereof

Technical Field

The invention belongs to the technical field of steel preparation, and particularly relates to a Ni-Al-Si alloy system coated steel plate with a wide hot forming heating process window, and a preparation and hot stamping forming process thereof.

Background

In recent years, high strength and thinning, energy conservation and emission reduction are main development trends of the automobile industry, wherein hot stamping is a common mode for realizing high strengthening of products, and the high strength of the products is realized by combining heat treatment and high-temperature forming. The hot stamping products commonly used are mainly: the hot stamping product has the characteristics of high strength, complex shape, good formability, high dimensional accuracy, small rebound and the like.

Steel for hot stamping is classified into a bare plate and a steel plate with a plated layer in surface state according to service conditions of parts, and the steel for hot stamping with the plated layer is more and more emphasized because shot blasting after hot stamping can be omitted compared with the bare plate. The most commonly used steel is hot stamping steel with an aluminum or aluminum alloy coating and hot stamping steel with a zinc-based coating, and the zinc-based coating can generate cracks to a base body, so the steel is also the hot stamping steel with the aluminum or aluminum alloy coating at present.

Thermoformed parts are required to have not only high strength under complex load conditions, but also good ductility and toughness. However, since all the steel products obtained after hot forming and quenching are martensite structures, the strength is very high and can reach more than 1500MPa, but the toughness is insufficient, the elongation is only about 5 percent, and the three-point bending angle of the traditional hot forming steel products with Al-Si coatings is generally 45-55 degrees after heat treatment.

As the requirements for collision safety of automobiles become more stringent, hot-formed steels with higher and higher strength are gradually developed, and the strength thereof is increased mainly by adding C, Mn and other alloy elements. However, the strength is increased, the corresponding ductility and toughness are reduced, and the three-point bending performance (the test standard is referred to as VDA238-100) is an important index for evaluating the ductility and toughness of the hot-formed steel and plastic. Insufficient cold bending performance (bending limit angle) of materials or parts for automobiles can cause the parts to be easily broken early in the vehicle collision process, can not effectively absorb energy generated by collision, and is not beneficial to the safety of passengers. Therefore, it is necessary to develop a steel for hot forming of Al — Si plating and to study a production process and a hot forming process of a steel sheet to improve cold bending properties of an Al-Si plating sheet after hot forming.

Disclosure of Invention

The invention aims to provide a Ni-Al-Si alloy system coated steel plate with a wide hot forming and heating process window and a preparation and hot stamping forming process thereof.

The technical scheme adopted by the invention is as follows:

a Ni-Al-Si alloy system coated steel sheet with a wide hot forming heating process window, the liquid bath used to coat the steel strip comprising the following raw materials in weight percent: 5-11% of Si, 0-4% of Fe, 0.05-5% of Ni, the balance of Al and inevitable impurities, and the thickness of the coating layer is 10-33 μm.

The substrate of the Ni-Al-Si alloy system coated steel plate comprises the following chemical components in percentage by weight: c: 0.18-0.35%, Si: less than or equal to 0.5 percent, Mn: 0.5-1.5%, P: less than or equal to 0.020%, S: less than or equal to 0.010 percent, Cr: less than or equal to 0.50 percent, Al: 0.01-0.06%, Ti: less than or equal to 0.04 percent, Nb: less than or equal to 0.06%, V: less than or equal to 0.06 percent, Mo: less than or equal to 0.5 percent, B: less than or equal to 0.005 percent, and Nb + V is more than or equal to 0.02 percent, and the balance is Fe and inevitable impurities.

The invention also provides a preparation method of the Ni-Al-Si alloy system coated steel plate with the wide hot forming heating process window, which comprises the following steps: steel making → casting blank → hot rolling → acid pickling and cold rolling → annealing → coating.

Furthermore, the hot rolling laminar cooling adopts front-section cooling, and the main purpose is to ensure that the strip steel finishes phase change before entering a coiler, thereby eliminating the risk of coil collapse; side-spraying gas treatment is adopted in the laminar cooling section, and laminar cooling water accumulation on the upper surface of the strip steel is eliminated, so that the upper surface and the lower surface of the strip steel are uniformly cooled, and the risk of non-uniform decarburization is reduced; the hot rolling coiling temperature is 600-700 ℃, the high-temperature coiling aims to ensure that the steel coil is uniformly decarburized in the cooling process after being coiled, the coiling is carried out within the temperature range, and the thickness of the decarburized layer of the hot coil is 8-15 mu m.

In the annealing process, for a steel coil with the thickness of 1.6-2.5 mm, the annealing temperature is 780-820 ℃, the annealing time is 6-10 min, the heating section in the furnace is in a micro-oxidizing atmosphere, the soaking section is in a reducing atmosphere, the dew point of the heating section is controlled to be-30-10 ℃, and the dew point of the soaking section is less than or equal to-30 ℃; for the steel coil with the thickness less than 1.6mm, in the annealing process section, the annealing temperature is 750-800 ℃, the annealing time is 4-8 min, the heating section in the furnace is in a micro-oxidizing atmosphere, the soaking section is in a reducing atmosphere, the dew point of the heating section is controlled to be-30-10 ℃, and the dew point of the soaking section is less than or equal to-30 ℃.

The annealing is carried out according to the annealing temperature and the annealing time, and the main purpose of the annealing is to ensure that the matrix is subjected to uniform annealing and further eliminate a banded structure, and on the other hand, a uniform decarburized layer structure is obtained by combining high dew point control; the annealing temperature is not more than 820 ℃, the annealing time is not more than 10min, otherwise, on one hand, abnormal grain growth occurs, and the cold bending performance after hot forming is not good.

The dew point of the heating section in the furnace is controlled to be-30-10 ℃, and the main purpose is to obtain strip steel with a decarburized layer at the edge part, combine with the decarburization of a hot rolling product, and control the thickness of the full decarburized layer of the obtained finished product structure to be 25-40 μm and the thickness of the semi-decarburized layer to be 5-15 μm. The dew point is not controlled to be less than-30 ℃, otherwise, an ideal decarburized layer structure cannot be obtained, the dew point is not controlled to be more than 10 ℃, otherwise, the mechanical property of a finished part is influenced due to the excessively thick decarburized layer.

The invention also provides a hot stamping forming process of the Ni-Al-Si alloy system coated steel plate with the wide hot forming heating process window, for the coated steel plate with the thickness of less than or equal to 1.8mm, the temperature and the time are limited in ABCD coordinates, as shown in figure 11, A (2min, 950 ℃), B (5min, 950 ℃), C (13min, 880 ℃), D (3min, 880 ℃), wherein the time is an abscissa and the temperature is an ordinate.

Further, for a coated steel sheet having a thickness of 1.8mm or less, the temperature and time are limited to AA₁D₁D in the coordinate, A (2min, 950 ℃ C.), A as shown in FIG. 11₁(2.5min，950℃)、D₁(4.5min, 880 ℃) and D (3min, 880 ℃), thus obtaining the stamping part with the surface layer of the incompletely alloyed Al layer and still with higher roughness.

For coated steel sheets with a thickness greater than 1.8mm and not greater than 3.0mm, the temperature and time are defined within the EFGH coordinate, as shown in FIG. 11, E (2.5min, 970 ℃), F (7min, 970 ℃), G (13min, 900 ℃), H (4min, 900 ℃), with time on the abscissa and temperature on the ordinate.

Further, for a coated steel sheet having a thickness of more than 1.8mm and not more than 3.0mm, the temperature and time are as followsFruit limitation in EE₁H₁H coordinate, E (2.5min, 970 ℃), E (FIG. 11)₁(3.5min、970℃)、H₁(6min, 900 ℃) and H (4min, 900 ℃) to obtain the stamping part with the surface layer of the incompletely alloyed Al layer and still having higher roughness.

Further, the coating structure of the Ni-Al-Si alloy system coated steel plate after hot stamping forming sequentially comprises the following components from the base steel to the outside:

(1) interdiffusion layer Fe₃An Al layer;

(2) intermediate layer and intermetallic compound layer: the layer is FeAl₃The layer containing FeAl or tau₁Or τ₂Of discontinuous secondary phase intermetallic compounds, or Fe₂Al₅The layer containing FeAl or tau₁Or τ₂Discontinuous secondary phase intermetallic compounds of (a);

(3) an Al layer with incompletely alloyed surface layer, wherein certain Fe (not more than 1%) and Si are dissolved in the Al layer

(not more than 13%) and the thickness of the layer is less than or equal to 5 μm.

Compared with the prior art, the invention has the following advantages:

according to the technical scheme provided by the invention, after a small amount of Ni element is added into the aluminum-silicon coating liquid, under the condition of short heating time, the coating surface layer still has high roughness and good coating performance even if a small amount of incompletely alloyed Al exists, the hot forming process window can be effectively expanded, and compared with the traditional Al or Al alloy system coated steel plate, the hot forming heat preservation time can be reduced by 0.5-2 min; meanwhile, the hardness of the surface layer of the plating layer is low, and the abrasion of a welding joint in the spot welding process can be effectively reduced.

Meanwhile, the invention starts from the aspects of substrate components (by the fine crystal strengthening effect of Nb and V elements) and decarburization layer thickness control, so as to improve the cold bending performance of the product; the decarburized layer with a certain thickness is obtained through the control of a hot rolling and hot dipping annealing process, meanwhile, the decarburized layer in the original structure can be reserved to the maximum extent under low heat preservation time, and the cold bending resistance of the hot-formed part is further improved through the fine grain strengthening effect of Nb and V elements; meanwhile, dispersed and fine precipitates of Nb and V elements can become hydrogen traps for capturing hydrogen atoms, and the hydrogen-induced delayed cracking resistance is improved; compared with the cold bending angle of the traditional hot forming steel product, the cold bending angle can be improved by 10 degrees to 15 degrees, and the hydrogen-induced delayed cracking resistance time is improved by more than 10 times.

Drawings

FIG. 1 is a cross-sectional view showing a coating structure when Al or Al alloy plating layer without Ni component system is not completely alloyed with Al on the surface of the plating layer after hot pressing of a steel sheet coated with the Al or Al alloy plating layer;

FIG. 2 is a top view of the corresponding coating surface of the coating of FIG. 1;

FIG. 3 is a cross-sectional view showing a coating structure when Al is completely alloyed on the surface of a plated layer after hot pressing of an Al or Al alloy plated layer coated steel sheet not containing a Ni component system;

FIG. 4 is a top view of the corresponding coating surface of the coating of FIG. 3;

FIG. 5 is a cross-sectional view showing a coating structure when Al is not completely alloyed on the surface of a plated layer after hot pressing of an Al or Al alloy plated layer coated steel sheet containing a Ni composition system according to the present invention;

FIG. 6 is a top view of the corresponding coated surface of the coating of FIG. 5;

FIG. 7 shows a decarburized structure of the surface layer of a hot rolled coil after coiling at a coiling temperature of 650 ℃ and a thickness of 8.4 μm;

FIG. 8 shows an annealed microstructure at a coiling temperature of 650 ℃ +800 ℃ with a decarburized layer thickness of 32 μm;

FIG. 9 shows that the thermoforming holding time is too long and no decarburized layer is formed on the edge;

FIG. 10 shows that the decarburization layer is heated at 900 ℃ for +3min and the thickness of the decarburization layer after punching is 25 μm;

FIG. 11 is a hot forming process window for a Ni-Al-Si alloy system coated steel sheet.

Detailed Description

The present invention will be described in detail with reference to the following examples and drawings.

Firstly, the Ni alloy system hot-formed coated steel plate provided by the invention has the advantages that the cold bending property and the delayed cracking resistance are improved by controlling the decarburized layer and adding trace Nb, V and other alloy elements into the substrate. In the present example, an ingot of the desired composition is provided, and as an example, the composition of the reference substrate, and the composition of the substrate of the present invention are shown in table 1.

Table 1 Steel plate chemical composition, wt.%

Hot rolling, cold rolling and hot dip coating annealing are carried out on hot formed steel blanks selected from the same component system, and the thickness of a hot coil decarburization layer is 8-15 mu m through the heating furnace atmosphere and the rolling and cooling control technology (see figure 7 in detail). In the annealing process, the depth of the decarburized layer increases with the increase of the annealing temperature, the annealing time and the dew point of the annealing furnace, and as an example, the decarburized layer thickness of the product at different annealing temperatures, times and dew points is shown in Table 2, and FIG. 8 is a photograph showing the structure of the decarburized layer of the substrate after the annealing state in example 2.

TABLE 2 thickness of decarburized layer of raw material

Coating the annealed steel sheet with a coating bath composition A (according to the invention) comprising 5-11% Si, 0-4% Fe, 0.05-5% Ni, and the balance Al and unavoidable impurities, in a Ni alloy system; the plating bath component B (reference) is used for coating the steel plate without Ni alloy system, and the plating bath component B comprises 5-11% of Si, 0-4% of Fe, and the balance of Al and inevitable impurities.

The pre-coated sheets of different thickness and coating composition described above were hot stamped and table 3 shows the different parameters of the test and the coating structure results obtained.

FIGS. 1 and 2 show examples of a coating according to test condition 3 corresponding to the present invention, in which the heating time is short, incompletely alloyed Al is present in the coating, and the surface roughness is insufficient; FIGS. 3 and 4 show examples of the coating according to test condition 4 corresponding to the present invention, in which the heating time was increased, Al in the coating was completely alloyed, and the surface layer was relatively rough; fig. 5 and 6 show examples of coating layers according to test condition 1 corresponding to the present invention, according to the coating layer composition scheme provided by the present invention, when a certain amount of Ni is added to an Al or Al alloy system coated steel sheet, a large number of pores are generated in the surface layer when incompletely alloyed Al still exists in the coating layer during a short heating time, the surface layer of the coating layer is rough and has good painting performance, and the surface layer has low hardness due to incompletely alloyed Al, and the loss of a welded joint during welding is low.

TABLE 3 test conditions and results obtained

The part mechanics and cold bending angle of steel plates with thickness of 1.2mm, different substrate compositions and different raw material decarburized layer thicknesses after hot forming under different hot forming processes were tested, and table 4 shows the corresponding measurement results.

The group C is a base material (shown in figure 8) with an ideal decarburized layer obtained by carrying out hot rolling and annealing processes on the Nb-and V-containing component substrate, the decarburized layer on the surface layer of the substrate is reserved (shown in figure 10) under the condition of short hot forming heat preservation time, and the cold bending performance of a product is optimal at the moment of plating; if the product is coated by a steel strip without Ni alloy system, the roughness of the obtained coating is low (see figure 2) and is not beneficial to the subsequent coating performance, and if the steel strip coated by the Ni alloy system of the patent is used, the roughness is good (see figure 6) and can meet the coating requirement. For the coated steel strip without the Ni alloy system, the heat preservation time needs to be properly prolonged to improve the surface roughness, and the detection result of the group E shows that when the heat preservation time is improved, the depth of a decarburized layer is reduced or even disappears (see figure 9), and the cold bending angle is reduced; group D is a coil cold bending angle detection result which contains Nb and V components and has lower depth of a decarburized layer before heating, the depth of the decarburized layer before heating is small, and the thickness and the cold bending angle of the decarburized layer after corresponding hot forming are also lower; and the group F does not contain Nb and V, and the cold bending angle detection result after heating is that the coil with the lower decarburization layer thickness of the substrate before heating does not have the fine grain strengthening effect of Nb and V, and the cold bending performance is further reduced.

TABLE 4 test conditions and measurement results

Note: group C-E substrate compositions are the inventive substrate compositions in Table 1; group F substrate compositions are reference substrate compositions in Table 1

The test piece resistance to delayed cracking of the hot formed samples of steel plates with the thickness of 1.2mm and different substrate components under the same hot forming process is tested, and the test conditions are as follows: the product containing Nb and V components is soaked in 5mol/L hydrochloric acid solution under the stress condition of 1500MPa, and the corresponding measurement results are shown in Table 5, so that the delayed cracking resistance time of the product containing Nb and V components is obviously improved.

TABLE 5 delayed cracking Performance comparison

Therefore, according to the Ni alloy system hot stamping coated steel plate provided by the invention, when the hot forming heating time is shortened, the surface layer of the coating still has higher roughness and good painting performance, and has a wider heating process window; after the hot forming time is shortened, the decarburized layer obtained by hot rolling and annealing process control can be retained to the maximum extent after hot forming, and the hydrogen trap for capturing hydrogen atoms can be obtained by fine grain strengthening of Nb and V elements in the substrate and fine dispersed Nb and V precipitates, so that the cold bending performance and the delayed cracking resistance of the product can be greatly improved; meanwhile, the heating time is short, and the surface layer has incompletely alloyed Al, so that the hardness of the surface layer of the part can be effectively reduced, and the loss of a welding joint in the welding process can be effectively reduced.

The above detailed description of the Ni-Al-Si alloy system coated steel sheet for a wide hot forming heating process window and the preparation and hot press forming process thereof with reference to the embodiments is illustrative and not restrictive, and several embodiments can be enumerated in accordance with the limitations of the present invention, and thus, variations and modifications thereof without departing from the general concept of the present invention shall fall within the scope of the present invention.

Claims (10)

1. A Ni-Al-Si alloy system coated steel plate with a wide hot forming heating process window is characterized in that a liquid bath for coating a steel strip contains the following raw materials in percentage by weight: 5-11% of Si, 0-4% of Fe, 0.05-5% of Ni, the balance of Al and inevitable impurities, and the thickness of the coating layer is 10-33 μm.

2. The Ni-Al-Si alloy system coated steel sheet with wide hot forming heating process window according to claim 1, wherein the substrate of the Ni-Al-Si alloy system coated steel sheet comprises the following chemical composition in weight percent: c: 0.18-0.35%, Si: less than or equal to 0.5 percent, Mn: 0.5-1.5%, P: less than or equal to 0.020%, S: less than or equal to 0.010 percent, Cr: less than or equal to 0.50 percent, Al: 0.01-0.06%, Ti: less than or equal to 0.04 percent, Nb: less than or equal to 0.06%, V: less than or equal to 0.06 percent, Mo: less than or equal to 0.5 percent, B: less than or equal to 0.005 percent, and Nb + V is more than or equal to 0.02 percent, and the balance is Fe and inevitable impurities.

3. The method for preparing a Ni-Al-Si alloy system coated steel sheet having a wide hot forming heating process window according to claim 1, wherein the method for preparing comprises the steps of: steel making → casting blank → hot rolling → acid pickling and cold rolling → annealing → coating.

4. The preparation method of claim 3, wherein the hot rolling laminar cooling adopts front-stage cooling, and side-spraying gas treatment is adopted in the laminar cooling stage, and the hot rolling coiling temperature is 600-700 ℃; the thickness of the hot-rolled decarburization layer is 8-15 μm.

5. The preparation method according to claim 3, wherein for a steel coil with the thickness of 1.6-2.5 mm, the annealing temperature is 780-820 ℃, the annealing time is 6-10 min, the heating section in the furnace is in a micro-oxidizing atmosphere, the soaking section is in a reducing atmosphere, the dew point of the heating section is controlled to be-30-10 ℃, and the dew point of the soaking section is less than or equal to-30 ℃; for the steel coil with the thickness less than 1.6mm, in the annealing process section, the annealing temperature is 750-800 ℃, the annealing time is 4-8 min, the heating section in the furnace is in a micro-oxidizing atmosphere, the soaking section is in a reducing atmosphere, the dew point of the heating section is controlled to be-30-10 ℃, and the dew point of the soaking section is less than or equal to-30 ℃.

6. The hot press forming process of a Ni-Al-Si alloy system coated steel sheet with a wide hot forming heating process window according to claim 1, wherein for a coated steel sheet with a thickness of 1.8mm or less, the temperature and time are defined within ABCD coordinates, a (2min, 950 ℃), B (5min, 950 ℃), C (13min, 880 ℃), D (3min, 880 ℃), where time is abscissa and temperature is ordinate.

7. The hot press forming process of Ni-Al-Si alloy system coated steel sheet with wide hot forming heating process window according to claim 6, wherein for the coated steel sheet with thickness of 1.8mm or less, the temperature and time are limited to AA₁D₁D coordinate, A (2min, 950 ℃), A₁(2.5min，950℃)、D₁(4.5min, 880 ℃) and D (3min, 880 ℃), thus obtaining the stamping part with the surface layer of the incompletely alloyed Al layer and still with higher roughness.

8. The hot press forming process of a Ni-Al-Si alloy system coated steel sheet with a wide hot forming heat process window according to claim 1, wherein for a coated steel sheet with a thickness greater than 1.8mm and not more than 3.0mm, the temperature and time are defined within EFGH coordinates, E (2.5min, 970 ℃), F (7min, 970 ℃), G (13min, 900 ℃), H (4min, 900 ℃), with time on the abscissa and temperature on the ordinate.

9. The hot press forming process of Ni-Al-Si alloy system coated steel sheet with wide hot forming heating process window according to claim 8, wherein for the coated steel sheet with thickness more than 1.8mm and not more than 3.0mm, the temperature and time if limited to EE₁H₁In H coordinate, E (2.5min, 970 ℃), E₁(3.5min、970℃)、H₁(6min、900)、H(4min、900℃) And obtaining the stamping part with the surface layer of the incompletely alloyed Al layer and still having higher roughness.

10. The hot forming process of the Ni-Al-Si alloy system coated steel sheet with the wide hot forming heating process window according to any one of claims 6 to 9, wherein the coating structure of the Ni-Al-Si alloy system coated steel sheet after hot press forming is, in order from the base steel:

(1) interdiffusion layer Fe₃An Al layer;

(2) intermediate layer and intermetallic compound layer: the layer is FeAl₃The layer containing FeAl or tau₁Or τ₂Of discontinuous secondary phase intermetallic compounds, or Fe₂Al₅The layer containing FeAl or tau₁Or τ₂Discontinuous secondary phase intermetallic compounds of (a);

(3) the surface layer is an incompletely alloyed Al layer, Fe and Si are dissolved in the Al layer in a solid solution mode, and the thickness of the Al layer is less than or equal to 5 mu m.

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