CN110423945B

CN110423945B - Zinc-containing coating layer hot forming component with tensile strength of more than 1800MPa and excellent cold bending performance and preparation method thereof

Info

Publication number: CN110423945B
Application number: CN201910773261.XA
Authority: CN
Inventors: 崔磊; 晋家春; 刘永刚; 詹华; 谷海容; 李子涛; 王蕾; 陈乐�; 王伟峰; 肖洋洋; 董标
Original assignee: Maanshan Iron and Steel Co Ltd
Current assignee: Maanshan Iron and Steel Co Ltd
Priority date: 2019-08-21
Filing date: 2019-08-21
Publication date: 2020-10-27
Anticipated expiration: 2039-08-21
Also published as: CN110423945A

Abstract

The invention discloses a hot forming component with a zinc-containing coating layer and excellent cold bending performance, wherein the tensile strength of the hot forming component is more than 1800MPa, and a preparation method of the hot forming component, a substrate of the hot forming component with the zinc-containing coating layer comprises the following chemical components in percentage by weight: c: 0.29-0.35%, 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%, Nb: 0.01-0.06%, V: 0.01-0.06%, Mo: less than or equal to 0.5 percent, and the balance of Fe and inevitable impurities; the hot forming component containing the zinc coating layer disclosed by the invention has excellent mechanical property, corrosion resistance and welding property.

Description

Zinc-containing coating layer hot forming component with tensile strength of more than 1800MPa and excellent cold bending performance and preparation method thereof

Technical Field

The invention belongs to the technical field of hot forming steel, and particularly relates to a zinc-containing coating hot forming component with tensile strength of more than 1800MPa and excellent cold bending performance and a preparation method thereof.

Background

The automobile lightweight technology is one of key technologies adapted to the trends of safety, energy conservation and environmental protection of modern automobiles, and more automobile body parts use the thermoforming technology (such as automobile bumpers, anti-collision beams, A columns, B columns, door anti-collision bars and the like) at present. The hot forming technology is a new technology for producing the ultrahigh-strength automobile parts by dividing forming and strengthening into two steps, and the produced parts have the advantages of ultrahigh strength, high forming precision, no resilience and the like.

With the continuous development of hot forming technology, the amount of hot stamping steel is increasing. The hot formed steel currently in wide use on the market is 1500MPa strength grade. Compared with a non-plated plate, the hot forming steel with the plated layer is more and more applied because protective atmosphere is not needed during heating, shot blasting and oil coating treatment are not needed after forming, the high-temperature friction coefficient is small, and the service life of a die is long. At present, Al-Si coatings are the most widely applied, and Zn-based coatings (GI, GA) have sacrificial anode protection effect compared with Al-Si coatings, and are the hot spots of research at home and abroad at present.

On the other hand, not only high strength but also good ductility and toughness are required for hot-formed parts under complicated load conditions. However, since all the steel products obtained after hot forming quenching have a martensite structure, the strength is very high, but the toughness is insufficient, the strength can reach 1500MPa or more, the elongation is only about 5%, and the three-point bending angle of the hot formed steel product with a coating after heat treatment is generally 45 ° to 55 °.

As the requirements for collision safety of automobiles become more stringent, hot formed steels of higher and higher strength are gradually developed, and the strength of hot formed steels 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 important index for evaluating the ductility and toughness of the hot-formed steel is the three-point bending performance (the test standard is referred to as VDA 238-100). 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 Zn-plated sheet and to study a production process and a hot forming process of a steel sheet to improve cold bending property after hot forming of the Zn-plated sheet.

Disclosure of Invention

The invention aims to provide a zinc-containing coating layer hot forming member with excellent cold bending performance with the tensile strength of more than 1800MPa and a preparation method thereof, and the zinc-containing coating layer hot forming member has excellent mechanical property, corrosion resistance, cold bending performance and welding performance.

The technical scheme adopted by the invention is as follows:

a hot forming component of a zinc-containing coating layer with excellent cold bending performance and tensile strength of more than 1800MPa grade comprises the following chemical components in percentage by weight: c: 0.29-0.35%, 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%, Nb: 0.01-0.06%, V: 0.01-0.06%, Mo: less than or equal to 0.5 percent, and the balance of Fe and inevitable impurities.

Further, the substrate of the hot forming member comprising the zinc coating layer preferably comprises the following chemical components in percentage by weight: c: 0.29-0.33%, Si: 0.18 to 0.26%, Mn: 0.80-1.10%, P: less than or equal to 0.010 percent, S: less than or equal to 0.003 percent, Cr: 0.17-0.22%, Al: 0.042-0.048%, Nb: 0.032-0.038%, V: 0.033 to 0.036%, Mo: 0.08-0.14%, and the balance of Fe and inevitable impurities.

The matrix metallographic structure of the hot forming component containing the zinc coating layer is a ferrite and pearlite structure before hot forming, and the grain size of the matrix metallographic structure is less than or equal to 12 micrometers; after hot forming, the steel has a complete martensite structure, and the original austenite grain size is less than or equal to 10 mu m.

The invention also provides a preparation method of the hot forming member containing the zinc coating layer with excellent cold bending performance and tensile strength of more than 1800MPa, which comprises the following steps: smelting, continuous casting, hot rolling, acid washing, annealing, hot dip galvanizing, on-line alloying, trimming and hot forming.

In the hot rolling process, a casting blank is heated in a heating furnace in weak oxidizing atmosphere for 120-180 min, and the air-fuel ratio is 0.8-1.0; and the discharging temperature of the casting blank is 1000-1230 ℃.

In the hot rolling process, a CVC rolling mill is adopted for rolling in finish rolling, and an edge heater is started to ensure that the hot rolled strip steel is uniform in shape and uniform in organization along the width direction; the width of the thickness-reduced area of the hot-rolled edge is controlled within 25mm of a single side, so that the uneven decarburized area of the edge caused by edge reduction can be cut off in the subsequent edge cutting process;

the finish rolling temperature is 870-920 ℃, and finish rolling is carried out at the temperature to ensure that the strip steel can be cooled to the target coiling temperature after passing through a laminar cooling section;

the front-section laminar cooling is adopted, so that the phase change of the strip steel is completed before the strip steel enters a coiler, and the risk of coil collapse is eliminated; 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 coiling temperature is 650-700 ℃, uniform decarburization is ensured in the cooling process after coiling, coiling is carried out within the temperature range, and the thickness of the decarburization layer of the hot-rolled coil is 15-25 μm; the thickness of the hot rolled coil is less than or equal to 2.5 mm.

In the annealing process, the surface of a steel plate is degreased and cleaned, then annealing is carried out in an annealing furnace filled with a reducing atmosphere, the temperature of an annealing section is 750-820 ℃, the annealing time is 4-8 min, the dew point temperature in the annealing furnace is controlled to be less than or equal to minus 30 ℃, a band-shaped structure can be further eliminated through annealing to obtain a band steel with a uniform structure, the annealing temperature is not more than 820 ℃, the annealing time is not more than 8min, otherwise, abnormal growth of crystal grains occurs, and the cold bending performance after hot forming is not good.

Further, the reducing atmosphere is 5-15% of H₂+85％～95％N₂。

In the hot dip galvanizing process, the hot dip galvanizing temperature is as follows: 440-480 ℃; hot dip coating time: 3-10 s; the thickness of the obtained coating is 5-15 mu m on one side, and the corrosion resistance is reduced due to the excessively thin coating; the plating layer is too thick and the cost is increased.

In the on-line alloying process, the iron content in the plating layer after on-line alloying is 10-20%, and the plating layer is preferably alloyed to a certain degree through the adjustment of the on-line alloying process, so that the heating temperature and time for hot forming are reduced; but the alloying degree cannot be too high, if the iron content of the initial plating layer is more than or equal to 20 percent, the alloying degree of the plating layer is too high in the subsequent hot forming process, and a plating layer structure with certain corrosion resistance cannot be obtained; the alloying degree can not be too low, if the iron content of the initial plating layer is less than or equal to 10%, the alloying degree of the plating layer is insufficient in the subsequent hot forming process, the melting point of the plating layer is low, the plating layer is easy to melt and stick to a roller during hot forming, and the practical production is not facilitated.

In the trimming process, the double trimming amount of the finished product is 25-50 mm, and the complete cutting of the edge decarburized area of the hot coil caused by the reduction of the edge thickness is ensured.

In the hot forming process, the heating temperature is 850-900 ℃, preferably 860-890 ℃, and is as low as possible under the condition of ensuring that the matrix can be completely austenitized; on one hand, the alloying degree of the plating layer is reduced, so that the corrosion resistance of the plating layer is increased; on the other hand, the decarburized layer on one side of the substrate close to the coating ensures a certain thickness, so that the cold bending performance after forming is improved; but the heating temperature cannot be less than 850 ℃, otherwise, the matrix structure cannot be completely austenitized in the heating process, so that the martensite structure cannot be obtained in the quenching process, and the strength of the formed part cannot be ensured; the heating temperature cannot be higher than 900 ℃, otherwise, the alloy coating melts and volatilizes in the heating process, which is not beneficial to the protection of the coating; on the other hand, the excessive heating temperature can cause the excessive alloying degree of the plating layer, thereby reducing the corrosion resistance of the plating layer;

the heating time is 2-5min, and the heating time is as short as possible under the condition of ensuring that the matrix can be completely austenitized; on one hand, the alloying degree of the plating layer is reduced, so that the corrosion resistance of the plating layer is increased; on the other hand, the decarburized layer on one side of the substrate close to the coating ensures a certain thickness, so that the cold bending performance after forming is improved; combining the heating temperature range, the heating time cannot be less than 2min, otherwise, complete austenitizing cannot be realized; the heating time cannot be more than 5min, otherwise, austenite grains become coarse, so that the cold bending performance of finished parts is influenced; the too long heat preservation time can cause the alloying degree of the plating layer to be too high, thereby reducing the corrosion resistance of the plating layer; on the other hand, the thickness of the decarburized layer becomes thinner in the process of heat homogenization as the heating time is longer, thereby affecting cold bending property and welding property.

Transfer time: 7-10 s; the transfer time cannot be less than 7s, otherwise, the plating layer is not completely solidified after being discharged from the furnace, and the risk of liquid-induced crack brittleness can occur in the stamping forming process; the transfer time also cannot be more than 10s, because over 10s, the microstructure transformation such as ferrite and pearlite occurs, which results in insufficient martensite content in the final part structure, and thus the mechanical property requirement cannot be achieved.

Maintaining the pressure for 4-10 s; on the basis of the process, the pressure maintaining time is not longer than 4s, otherwise, the temperature of the part taken out of a mold is higher, the part is cooled and deformed when being cooled in air, the pressure maintaining time is not longer than 10s, and otherwise, the production rhythm is influenced.

The cooling speed is more than or equal to 25 ℃/s, and the matrix structure is a complete martensite structure after the part is formed.

In the hot forming process, the dew point in the heating furnace is less than or equal to minus 45 ℃, and if the dew point is less than minus 45 ℃, the formed part has the risk of delayed cracking.

And (5) carrying out laser cutting treatment after the component is taken out of the die.

According to the preparation method of the hot forming member containing the zinc coating layer with the tensile strength of more than 1800MPa and excellent cold bending performance, the hot rolling coil containing the decarburized layer of 15-25 mu m is obtained after hot rolling, and the hot forming member is obtained after annealing, galvanizing and hot forming of the hot rolling coil, has the tensile strength of more than 1800MPa, and has excellent cold bending performance, welding performance and corrosion resistance.

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

1) on the basis of improving the content of C, alloy elements such as Cr, Nb, V, Mo and the like are added in a compounding way, and the addition of C mainly enables the strength of hot formed products to meet the condition that Rm is more than or equal to 1800 MPa; due to the addition of alloy elements, on one hand, an austenite phase region is expanded, and the austenitizing temperature is reduced; on the other hand, the crystal grains are refined to obtain a matrix with uniform and fine tissues; moreover, alloy elements are fully precipitated in the processes of raw material preparation and hot forming, and dispersed and finely distributed precipitates are obtained, so that the delayed cracking resistance is improved;

2) in the preparation process, the thickness of the hot coil decarburization layer is 15-25 μm by controlling the atmosphere in the heating furnace and TMCP;

3) the mechanical properties of the steel strip with the Zn coating which is not thermoformed are as follows: the yield strength is 350MPa to 600MPa, the tensile strength is 500MPa to 800MPa, and the elongation is more than or equal to 15 percent;

4) the thickness of the decarburized layer of the steel strip with the Zn coating is reduced along with the increase of heating time after hot forming, and the cold bending angle is reduced along with the reduction of the decarburized layer;

5) the yield strength of the member or part obtained after hot forming is more than 1250MPa, the tensile strength is more than 1800MPa, the elongation A80 is more than or equal to 4.5 percent, and the cold bending angle is more than or equal to 55 degrees;

6) the Gamma phase proportion in the plating layer after hot forming is more than or equal to 15 percent, and the corrosion resistance is superior to that of the aluminum-silicon plating layer and the bare board;

7) after hot forming, the welding performance is superior to that of an aluminum-silicon coating product.

Drawings

FIG. 1 is a microstructure of a hot-rolled sheet containing a zinc coating layer of example 3 after annealing;

FIG. 2 is a microstructure view of a hot formed component comprising a zinc coating layer of example 3;

FIG. 3 is a graph of the morphology of a hot formed component and a coating comprising a zinc coating layer of example 3, wherein A is α -Fe and B is phase;

FIG. 4 is a polarization curve of a thermoformed component containing an Al-Si plating of a thermoformed component containing a zinc-containing coating layer and a thermoformed component without a plating layer in example 3;

FIG. 5 is a performance result of cycle corrosion 240h for a hot formed component comprising a zinc coating layer in example 3;

FIG. 6 is a range of weldability for a hot formed member containing a zinc coating layer in example 3;

FIG. 7 is a graph of performance results of cycle corrosion 240h for a hot formed component containing an Al-Si coating layer in comparative example 1;

fig. 8 is a weldability range of a hot formed member containing a zinc coating layer in comparative example 1.

Detailed Description

The present invention will be described in detail with reference to examples.

The chemical composition and weight percent of the substrate of the thermoformed component comprising the zinc-containing coating layer in each example are shown in table 1.

TABLE 1 chemical composition (wt%) of substrate of thermoformed component comprising zinc coating

The thermoformed components containing the zinc coating layer in each example were prepared using the following steps: smelting, continuous casting, hot rolling, acid washing, annealing, hot dip galvanizing, on-line alloying, trimming and hot forming.

Wherein, the preparation process parameters and the performances before the hot forming are respectively shown in tables 2 and 3, and the hot forming process parameters and the performances are respectively shown in tables 3 and 4.

TABLE 2 preparation Process parameters before thermoforming

TABLE 3 Properties of the Steel sheet or strip before Hot Forming

TABLE 4 thermoforming Process parameters

TABLE 5 post thermoforming mechanical Properties

The heating temperature for hot forming and the holding time for heating in example 1 were changed to the values shown in table 6, respectively, and the comparative examples (%) of the resulting hot formed member containing a zinc coating layer were as shown in table 6.

TABLE 6 phase ratio as a function of heating temperature and time

Examples	Heating temperature of	Holding time min	Proportion of gamma-phase%
				1	860	5	24.63
2	880	5	8.04
				3	890	3	40.44
4	890	4	23.07
				5	890	5	7.27
6	890	6	1.17
				7	890	7	0.00

As can be seen from Table 6, under the same holding time conditions, the phase decreased as the heating temperature increased; under the condition of the same heating temperature, the phase difference is less as the heat preservation time is prolonged. The existence of the decarburized layer of the hot forming member with the zinc coating layer prepared by the invention does not influence the mechanical property of the member and is beneficial to the cold bending property of the member, and as shown in Table 7:

TABLE 7 relationship between properties after thermoforming and decarburized layer

Full decarburized layer mum	Yield strength/MPa	Tensile strength/MPa	Elongation/percent	hardness/HV 1	Cold bend angle °
						5	1288	1965	5.2	581	55
10	1306	1914	5.9	599	58
						15	1292	1938	5.4	588	62

The microstructure of the hot-formed member containing the zinc coating layer obtained in example 3 before and after hot forming is shown in fig. 1 and 2, respectively, and it can be seen from fig. 1 and 2 that the decarburized layer thickness of the annealed strip steel is 25 μm, the decarburized layer thickness after hot forming is 15 μm, and it can be seen from fig. 3 that the plating r phase ratio after press forming is about 25%.

The polarization curve of the thermoformed article without the plated layer of the thermoformed article having the zinc-containing coating layer obtained in example 3, the thermoformed article containing the Al — Si plated layer in comparative example 1, is shown in fig. 4, and it can be seen from the graph that the electrode potential of the Zn-containing plated layer is lower than that of the base body and the aluminum-silicon plated layer, and therefore, the Zn-containing plated layer has a sacrificial anode protecting effect in a corrosive environment, thereby improving the corrosion resistance.

The corrosion performance and the weldability of the hot-formed member including the zinc coating layer obtained in example 3 are shown in fig. 5 and 6, respectively, and it can be seen from fig. 5 that corrosion occurs only in the zinc plated portion and no corrosion occurs in the substrate; as can be seen from FIG. 6, the weldability window of the hot forming member containing the zinc coating layer is more than or equal to 1, and the welding requirements of industrial production are met.

Comparative example 1

Otherwise as in example 3 except that the surface of the hot formed member was coated with Al-Si, the heating temperature and time at the time of hot forming were 930 ℃ and 5min, respectively, and the performance results of the cycle etching 240h of the obtained hot formed member containing the Al-Si coating layer are shown in FIG. 7, from which it can be seen that the etching had corroded the substrate through the coating. The weldability range of the hot-formed member containing the Al-Si coating layer obtained in this comparative example is shown in fig. 8, from which it can be seen that: although the weldability range is more than or equal to 1, the weldability range is less than that of the hot forming component containing zinc coating layer.

The above detailed description of the zinc coated hot formed member excellent in cold bending property with tensile strength of 1800MPa class or more and the method for manufacturing the same with reference to the examples is illustrative and not restrictive, and several examples can be cited within the limits of the invention, so that changes and modifications without departing from the general concept of the invention shall fall within the scope of the invention.

Claims

1. The hot forming component of the zinc-containing coating layer with excellent cold bending performance and tensile strength of more than 1800MPa is characterized in that a substrate of the hot forming component of the zinc-containing coating layer comprises the following chemical components in percentage by weight: c: 0.29-0.35%, 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%, Nb: 0.01-0.06%, V: 0.01-0.06%, Mo: less than or equal to 0.5 percent, and the balance of Fe and inevitable impurities;

the preparation method of the hot forming member containing the zinc coating layer with excellent cold bending performance and tensile strength of more than 1800MPa comprises the following steps: smelting, continuous casting, hot rolling, acid washing, annealing, hot dip galvanizing, on-line alloying, trimming and hot forming; hot rolling to obtain a hot rolled coil containing a decarburized layer of 15-25 mu m;

in the annealing process, the surface of a steel plate is degreased and cleaned, then annealing is carried out in an annealing furnace filled with a reducing atmosphere, the temperature of an annealing section is 750-820 ℃, the annealing time is 4 min-8 min, and the dew point temperature in the annealing furnace is controlled to be less than or equal to minus 30 ℃;

in the hot dip galvanizing process, the hot dip galvanizing temperature is as follows: 440-480 ℃; hot dip coating time: 3-10 s; the thickness of the obtained coating is 5-15 mu m on one side;

in the on-line alloying process, the iron content in the plating layer after on-line alloying is 10-20%;

in the hot forming process, the heating temperature is 850-900 ℃, and the heating time is 2-5 min; the transfer time is 7-10 s; maintaining the pressure for 4-10 s; the cooling speed is more than or equal to 25 ℃/s;

in the hot forming process, the dew point in the heating furnace is less than or equal to minus 45 ℃.

2. The hot-formed member containing a zinc-containing coating layer excellent in cold bending property with a tensile strength of 1800MPa class or more according to claim 1, wherein the substrate of the hot-formed member containing a zinc-containing coating layer is composed of the following chemical components in percentage by weight: c: 0.29-0.33%, Si: 0.18 to 0.26%, Mn: 0.80-1.10%, P: less than or equal to 0.010 percent, S: less than or equal to 0.003 percent, Cr: 0.17-0.22%, Al: 0.042-0.048%, Nb: 0.032-0.038%, V: 0.033 to 0.036%, Mo: 0.08-0.14%, and the balance of Fe and inevitable impurities.

3. The hot-formed member with the zinc-containing coating layer, which has excellent cold bending performance and tensile strength of 1800MPa or more according to claim 1, is characterized in that in the hot rolling process, a casting blank is heated in a heating furnace for 120-180 min, and the air-fuel ratio is 0.8-1.0; and the discharging temperature of the casting blank is 1000-1230 ℃.

4. The hot-formed member having a zinc-containing coating layer excellent in cold formability with a tensile strength of 1800MPa or more according to claim 1, wherein in the hot rolling process, the finish rolling is performed by a CVC rolling mill and an edge heater is turned on; the width of the thickness-reduced area of the hot-rolled edge part is controlled within 25mm of a single edge; the finishing temperature is 870-920 ℃; adopting front-section laminar cooling; the coiling temperature is 650-700 ℃; the thickness of the decarburized layer of the hot-rolled coil obtained after coiling is 15-25 μm.